Transpiration

I’ve talked about transpiration in quite a few different posts on this site, but a recent thread on http://bonsainut.com caused me to think maybe I should have a post dedicated to it, so here goes…

Transpiration is the evaporation of water from the leaves of a tree. It’s actually a critical process for trees, because excess transpiration is one of the few ways in which a tree can die; so-called ‘hydraulic failure’ has been identified as the most prominent cause of tree death.^ref Hydraulic failure – the failure to access enough water to replace water lost mainly through evaporation – causes cell death, xylem failure and a fatal reduction in photosynthesis. So it’s really important for bonsai practitioners to understand this process.

The main driver of transpiration is not – as you might think – to cool the leaves (although this is one reason for it). In fact transpiration is a by-product, or ‘cost’, of photosynthesis, and it happens because of the way that leaves obtain carbon dioxide. You may already know that plants have small pores called ‘stomata’ which open up to let air – and CO₂ – inside the leaf. But you might not have known that gaseous CO₂ in air needs to be dissolved in water before it can be accessed by chloroplasts and used for photosynthesis (this is explained in Vogel (Chapter 5 – ‘Leaking Water’). This means that water needs to be available on the surfaces inside the leaf – which means that when stomata open up, this water is subject to evaporation.

Vogel says that “only if the relative humidity is 100 percent will water not be lost…[and]…if the leaf’s temperature is above that of the surrounding air, then water can be lost even at that humidity.” He also says that for every gram of CO₂ used by a leaf for photosynthesis, it’s estimated that 125 grams of water is lost.

Smith et al (Chapter 4.10 Movement of Water & Minerals) explain that this evaporation causes a constant flow of water known as the ‘transpiration stream’. As water evaporates from the leaf cells, pressure in those cells is reduced, and this negative pressure causes water from the xylem to move into the space, due to strong mutual attraction between water molecules. That in turn pulls more water behind it and so on. This hydraulic mechanism is responsible for pulling water all the way up the tree from the roots. Actually this process is fundamental to the health of the tree, maintaining cell turgor (stiffness), transporting nutrients, metabolites & growth substances synthesised in the roots throughout the tree, and providing a source of water for the phloem stream which flows in the opposite direction providing energy to the tree. When there is enough water available, all of this works perfectly – when there isn’t, problems arise.

The extent of evaporation from the leaves of a tree is determined by several different factors, which can be divided into environmental, tree-specific physical factors and tree-specific response factors.

The main environmental factor which drives transpiration is the ‘vapour pressure deficit’ (“VPD”) – this is the “difference between the amount of moisture in the air and how much moisture the air could potentially hold when it’s saturated.”^ref VPD is a function of both heat and humidity, and provides a measure of how powerful the evaporative force of the air is with any combination of these.^ref

Occasionally while writing articles for this blog, I end up in the world of cannabis cultivation. Maybe because they are very motivated to keep their crops vigorous, cannabis growers and their equipment suppliers sometimes have the best data and charts out there! This is just such an occasion, see below for an excellent chart from ‘Ceres Greenhouse Solutions’ showing the VPD for a given temperature and humidity (you can download a copy here). The VPD is low in the blue section and high in the red.

https://ceresgs.com/wp-content/uploads/2018/10/VPD-Bioengineering-Chart.pdf

What you will notice is that the relationship between humidity and temperature isn’t exactly linear. Also, VPD increases with higher temperature and lower humidity. Since a higher vapour pressure deficit means there is more ‘pull’ on the water in leaves, increasing temperature and decreasing humidity both increase transpiration – and they reinforce each other, so dry and hot is a high transpiration combination.

Another environmental factor is wind. One study found that wind actually improves water use efficiency, because whilst it does increase transpiration, it also increases CO₂ uptake, and the net effect is greater water use efficiency and not less.^ref But for the purpose of this article, wind does increase transpiration.

Coming onto tree-specific physical factors – these are all the attributes that relate to the size, shape, position and structure of the tree. In general the more foliage a tree has, the more it will transpire – so a large broadleaf tree will transpire significant amounts on a hot day – in one study they found a large canopy tree in the tropics (Eperua purpurea) transpired up to 1180 litres per day!^ref By comparison in the same study, smaller (presumably more shaded) trees transpired a lot less. Thomas (Chapter 2: Leaves the food producers) gives the following figures: “<100L/day in conifers, 20-400 L/day in eucalypts and temperate trees such as oaks, reaching perhaps 500 L/day in a well-watered palm and as high as 1200 litres per day in specimens of Eperua purpurea growing out of the top of the Amazonian rainforest canopy.”

The chart below shows the daily transpiration rate during the growing season for a sessile oak tree in Turkey which measured 18.5m x 34.5m – this maxed out at 160 kg/day (effectively 160L).

https://www.ewra.net/ew/pdf/EW_2017_59_34.pdf

As well as the volume of foliage, trees have different stomatal size and density (number of stomata in a given area) which are determined by genetics as well as environmental factors (such as intensity of light and VPD to which they are exposed when developing).^ref1 ^ref2 Low stomatal area (ie. density x size) will result in lower transpiration when compared to a tree with higher stomatal area. These researchers measured stomatal area for 737 plant species across 9 forests and at the lower end of the spectrum conifers such as Cunninghamia lanceolata (0.2%) and Picea koraiensis (0.4%) had 100 times less stomatal area than angiosperms such as Viburnum betulifolium (23.77%) or Quercus serrata (21.74%). You can download all their data here. Basically the more stomatal area which is open to the air, the more transpiration there will be.

Many trees have wax plugs in their stomata which reduce their efficiency, and transpiration at the same time. To copy a piece from my article on needle leaves, wax deposits in Sitka spruce stomata reduce transpiration by two thirds but photosynthesis by only one third.^ref One study found that 81% of the species they looked at contained such plugs and that wax plugs are particularly numerous in conifers.^ref

Another factor is the level of transpiration via bark. This isn’t due to stomatal opening but simply due to partial permeability of bark to air – also genetically determined and due to the presence of ‘lenticels’ – small channels which allow passage of water and air for the metabolism of living cells in the bark. One study on Pinus halupensis found that “Bark transpiration was estimated to account for 64–78% of total water loss in drought-stressed trees, but only for 6–11% of the irrigated trees.”^ref This is because bark transpiration is passive and unmanaged, unlike leaf transpiration which can be somewhat controlled by the tree (see below).

Also relevant for individual trees is their position relative to other trees and the sun. Shade will reduce the temperature at the leaf surface and reduce transpiration, a mass of trees together along with undergrowth may increase humidity, also reducing transpiration. A tree standing alone or above others will be exposed to higher temperatures and lower humidity, thus increasing transpiration. Different areas on a single tree will be exposed to different combinations of these factors as well, so rates of transpiration will differ even from leaf to leaf on a given tree.

The final category of attributes which determine transpiration relates to the trees’ ‘behaviour’. That is, how they react to different environmental conditions. As we all know trees may be sessile but they are also incredibly dynamic and can adjust a wide range of parameters of their own biology. The main issue they need to address in this case is losing too much water, which could lead to death. As a result, they change their physiology to manage evaporation as well as water intake at the start of the transpiration stream.

To manage evaporation, trees adjust their stomata based on water availability, changing their ‘stomatal conductance’ to reduce transpiration if not enough water is available.

They do this in a couple of different ways – ‘passively’ and ‘actively’.^ref The passive mechanism is where lower water pressure within leaves causes guard cells around the stomata to lose their stiffness, which has the effect of reducing the stomatal aperture. The active mechanism relies on the tree producing abscisic acid (ABA) – this “triggers efflux of anions and potassium via guard cell plasma membrane ion channels, resulting in decrease of turgor pressure in guard cells and stomatal closure”.^ref

A study on Metasequoia glyptostroboides found that in most conditions of water availability the passive mechanism was in play, and it wasn’t until prolonged or severe water stress was experienced that the active ABA-mediated mechanism came into play.^ref The article explains that different gymnosperm species use different combinations of these passive and active processes to manage a lack of water availability by reducing transpiration. Angiosperms by contrast use a more sophisticated and more recently evolved version of the active process, mediated by ABA.^ref

Thomas says that stomata usually close when it is “too cold or dark for photosynthesis” or when the leaves are in danger of losing too much water and wilting”. The consequence of stomatal closing is an associated reduction in photosynthesis – so when a tree is drought stressed, it won’t be generating energy at the same rate as when it was healthy. A study measuring photosynthesis versus stomatal conductance for Pinus radiata (see in the chart below) found there was a roughly linear relationship, as the stomatal conductance increased, so did photosynthesis.

https://www.researchgate.net/publication/8690701_Variation_in_foliar_13C_values_within_the_crowns_of_Pinus_radiata_trees

There are several other ways that trees manage their transpiration – by adjusting their root conductance (ability to draw in water), changing their leaf expansion so that there are fewer/more leaves which are smaller/larger in area, pointing exposed leaves downwards during hot periods of the day, changing the root/shoot ratio to match water source to water use and by operating a daily cycle of metabolism which optimises transpiration (eg. increasing their root hydraulic conductance at night when there is lower evaporation, and ‘filling up’ to deal with higher transpiration during the day).^ref So they are very much active participants in responding to and controlled their transpiration rate.

But what does it all mean for bonsai? The first thing is, if your tree has plenty of water availability, transpiration should not become a problem, but you need to remember that up to 95% of water use is evaporation^ref so trees need a lot more water than you might expect. The best way to avoid issues associated with excess transpiration is to supply your trees with all the water they need. This is achieved by regular and sufficient watering, and by using a medium which has some water retention to avoid drought stress – but is also well-draining. A well-draining medium allows you to water more often without the risk of waterlogging roots or creating conditions for pathogens to take hold.

Also – a tree’s ability to handle water loss varies widely depending on the species – Thomas gives the examples of eucalypts and alder as species which cannot control transpiration effectively, and some oaks as species which can. So each tree in your collection will be different.

But let’s consider all the factors explained above that increase transpiration: high vapour pressure deficit (high temperature and/or low humidity), wind, lots of foliage, high stomatal area, clean (unwaxed) stomata, passive bark & leaf evaporation, a sunny/solitary/high position, and a lack of water availability to the roots which activate stomatal closure.

Some of these are adjustable for bonsai. If it’s going to be a hot, dry, windy day then your trees are going to transpire a lot more than normal and if their roots can’t keep up, you need to improve their environment; newly collected and recently root-pruned trees or trees in particularly small or shallow pots will be most affected. You can help them by providing shade (reducing the temperature), increasing humidity, and moving them out of the wind – and obviously by watering. For a temporary period, on a very hot day, it might even make sense to sit pots in water (do not do this for an extended period).

Transpiration can also be a problem in the winter as trees do continue to transpire, albeit at lower levels, even if they are deciduous. As such, they do need water to be available which means you need to keep an eye on moisture levels in pots. If they get dry, water them. If the medium is frozen, this will lock up water and can have a dehydrating effect so in this case you need to also water, ideally when it’s above freezing. Mulch is suggested to avoid hydraulic failure for trees in the ground^ref, a similar approach can be used for bonsai in pots, to reduce freezing and make more water available to roots. Even at night it is not the case that transpiration completely stops – typically it is 5% – 15% of daytime rates.^ref

Balancing the amount of foliage with the roots when repotting or pruning is another important way to help your trees manage their transpiration rates, so that there is enough root mass to meet transpiration demands. Root pruning in the heat of summer should be avoided unless a comparable foliage reduction takes place. If you’ve gone a bit far with the root pruning, use the approaches above – provide some shade, increase the humidity and maintain a watering regime. This is where the bagging method for collected trees comes from – it reduces transpiration by increasing humidity and can be used for trees struggling to recover from a severe root prune.

Anti-transpirant is a product that some bonsai aficionados use. This does what it says on the tin – it is a “film-forming complex of polyethylenes and polyterpenes that when applied to foliage will reduce the moisture vapor transmission rate”^ref The active substance is derived from conifer resins. In reducing transpiration these products also reduces photosynthesis, which is a consideration. I’m personally not a fan of disrupting a plant’s natural processes in this way, and successful use of the product depends on the individual tree and product selected (read more here).

Hopefully you can see from all of the above that transpiration is an extremely important concept to understand as a bonsai geek, but one which can be managed, as long as you are aware of the factors at play. Here’s to helping our trees avoid hydraulic failure!

What does frost do to bonsai trees?

Living in London means that even though spring has blossomed forth, there is still a chance of frost all the way through to the end of May. And many of us will have learned the hard way that frost and/or sub-zero temperatures can seriously damage our trees.

One of the key risks of frost and extreme cold is the problem of ice. Ice formation pulls water from plant cells, causing dehydration, which can be just as lethal as dehydration from underwatering.^ref Ice masses which form and thaw can deform and damage cell membranes, as well as buds and other plant organs, and ice in trees’ xylem vessels can cause embolisms (air bubbles) to form, cutting off the water flow above the bubble.

Another problem is that low temperatures make photosynthesis dangerous due to an excess of energy which ends up as damaging reactive oxygen species (“ROS”). This is why some trees convert green chloroplasts to bronze chromoplasts in cold weather – read more here, and others shed their leaves altogether to eliminate the problem.

Plants which are ‘hardy’ or ‘frost-hardy’ have mechanisms to resist the effect of ice – either by ‘avoidance’ such as supercooling (lowering a liquid to below freezing point without freezing), or ‘tolerance’ – using biochemical changes and physical adaptations allowing the tolerance of ice in their tissues.^ref Some pines have evolved specific mesophyll cells which allow the ice into the intracellular spaces without deforming the key structures within the needles – a physical adaptation. Oaks and ring porous trees regrow their conducting xylem every year, since embolisms during winter make last seasons’ xylem ineffective.

Crucially, since many cold-hardiness mechanisms rely on biochemical changes within the plant, they require time, so that the relevant proteins, enzymes and other metabolites can be synthesised in sufficient quantities to have the desired effect in plant cells. This process is called ‘cold acclimation’. Smith et al (2010) give the example of rye. 50% of non-acclimated rye plants will die at -6^oC, but after spending 2 days at 4^oC, they can go down to -21^oC before 50% of plants will die. The cold acclimation process is what’s known as an ‘epigenetic’ process – where environmental triggers such as shortening days and reducing temperatures turn on genes – in this case known as COR (cold-regulated genes).^ref These then produce the proteins which are used to create the cold-hardy biochemical changes.

There is an equal and opposite process known as cold deacclimation. Smith et al (2010) say that above a temperature of 10^oC cold hardiness is rapidly lost, which is why a spring frost at the end of May can be so damaging.

To complicate matters, shoot hardiness is not the same thing as root hardiness.^ref Studies have shown that roots are less hardy than shoots, even when exposed to identical temperature acclimation treatments.^ref For example, the stems of Pyracantha are hardy to -25.6^oC, whereas mature roots are hardy to -18.8^oC and young roots are hardy to -6.1^oC.^ref This is an important insight for bonsai because the scale of our trees and the fact they are suspended in pots means their young roots are particularly vulnerable. A symptom of winter damage to young roots is when a tree flushes later than normal, and has retarded growth during the season.^ref It may not kill the tree, but it will certainly give it a handicap for the next season.

So what does it all mean for bonsai?

For me the main risk boils down to root damage. Those lovely fine roots we work hard to encourage during the growing season are more vulnerable to frost than any other part of the tree, and unfortunately due to the size, shape and positioning of most bonsai pots, they are very exposed to cold temperatures. In one study it was found that container-grown trees were subjected to temperatures down to -15^oC when the night-time temperature reached -30^oC, whereas the soil only went down to -6^oC.^ref And since surfaces cool faster than air when it’s cold, resting up against a cold pot surface is the last place a young root wants to be! When the temperature gets really cold, mature roots can be damaged as well, which could be fatal to the tree.

Sure – leaves can be damaged by frost as well – mainly when deciduous trees have leafed out expecting above-zero temperatures and then a frost comes along. Most will deal with this and should regrow their leaves – there will be an energy penalty which will reduce the overall energy they have to devote to the growing season, and they won’t look great in the meantime, but it shouldn’t be fatal if the stems and roots are still healthy. But certainly to keep your trees looking and growing their best, you want to avoid deciduous trees leaves being exposed to frost if at all possible (see below for some strategies).

To be able to manage and prevent frost damage, we need to know when a frost will happen. This starts with monitoring the temperature during winter. Importantly, when you hear or see a temperature forecast for a location, it is a forecast for the air temperature. The temperature of the ground is often several degrees lower. The UK Met Office says “As a general rule of thumb, if the air temperature is forecast to fall between 0 °C and 4 °C on a night with little or no cloud and light winds, then you need to bear in mind there may be a frost outside in the morning. The closer it is to zero, the greater the chance of seeing frost. If the air temperature is forecast to be below zero, then the risk of seeing frost is much higher.”^ref This goes for surfaces like pots as well. Wind and cloud cover reduce the chance of frost at a given temperature.

The actual temperature which will damage or kill roots is species-specific, so there’s no hard and fast rule but knowing where a tree comes from should give you an indication. In general the species common in the boreal forests such as many Pinaceae and Betula pendula will cope best with freezing temperatures. Some ornamental tree ‘killing temperatures’ are provided in this list.

So what should you do to help your trees defend themselves against frost damage?

The first thing to do is to let them be exposed to the cold over time, before it freezes. Give them time to activate their COR genes and establish cold acclimation. This includes not putting them in a polytunnel or shed or wrapping them up until they’ve had some time in colder temperatures (but not freezing).

It’s also useful to have actual temperature data from the location where your pots are going to be over winter. This can be achieved using a digital thermometer/hygrometer which records the temperature and humidity at regular intervals – there are many wireless/bluetooth-enabled options now which are very reasonably priced (I just bought 3 for £35, one for my garden bonsai bench, one for my allotment bed and one for my allotment greenhouse). This will enable you to monitor what the temperature does relative to the forecast so you can better predict the forecast in your actual location – and you can see which locations might provide better winter protection. If you see the temperature approaching zero you can act to protect your trees.

Then, adopting a form of overwintering system could be beneficial. Somewhat counter-intuitively, this involves watering everything well, without overwatering.^ref The reason for this is to avoid dehydration. Then providing a physical barrier to the cold, effectively a ‘tree duvet’. This can move the frost surface away from the pot edges. You could use an insulating blanket of some kind, but do some research – one study found that ‘microfoam thermoblanket’ made a difference, but clear, black or white poly did not.^ref The covering would need to have insulating properties, and not get saturated with water such that it would then freeze anyway. You could also move your trees into a polytunnel, garden shed or greenhouse, with the walls and air inside acting as a form of insulation. If you use a digital thermometer you can monitor these to ensure they aren’t getting too cold – note that polytunnels have been found to get cold enough to damage roots.^ref

The absolute best is to provide some form of heat so that the temperature can’t go below a certain point – in practice this could be moving your pots closer to your house, using a (lightly) heated propagation bed, or putting them in a (lightly) heated greenhouse or outhouse. You don’t want them to think it’s spring so there shouldn’t be too much heat, just enough to keep the temperature above freezing. Be aware that in a warmer environment such as a shed your trees will deacclimate earlier as well, so make sure they don’t go back outside into a frost as they may have lost their cold acclimation.

Male Tree Flowers

Leave a Reply

Spring has sprung (finally) in the UK and now is the time that many trees flower, so they can pollinate the next generation with enough time for seeds to develop before winter. Plant reproduction is quite a complicated multi-stage process, but for the purposes of this post it’s enough to know that pollen is the vehicle for the male gamete (the plant equivalent of sperm). It is produced by anthers in angiosperms (flowering plants) and by male cones in gymnosperms (including conifers).

It turns out that pollen production can happen in different places. In some trees, both male and female parts are within the same flower (known as ‘perfect’ or ‘complete’ flowers). In others, they are separate flowers or cones on different parts of the same branch, shoot or tree – known as monoecious trees. In yet others they are separate flowers or cones on single-sex trees – known as dioecious.^ref

Whilst female flowers/cones and the fruit associated with them are usually obvious, the pollen-bearing male flowers/cones are often hard to spot and sometimes people don’t even realise they are there (it was only about 2 years ago that I realised that oak trees even had flowers!). After releasing their pollen, male reproductive parts usually drop off and disappear from view, while the female flower/fruit/cone persists as the seeds develop. So it’s definitely worth appreciating male tree flowers when you see them, as they don’t hang around for long.

In angiosperms, many male tree flowers take the form of catkins. A catkin is an “elongated cluster of single-sex flowers bearing scaly bracts and usually lacking petals.”^ref Male catkins appear on oak, chestnut, alder, birch, hazel, poplar, aspen, hornbeam, walnut and willow – a good overview of these is on the UK Tree Guide website and some examples are shown below. There are female catkins as well, but these don’t contain pollen, which is one way to tell them apart.

Some trees have flowers that look more like what we learn about at school, so-called ‘perfect’ flowers, with both male and female parts. Examples of trees with this type of flower structure include maples, hawthorn, lime, horse chestnut and many fruit trees including apple, cherry, pear and plum (see below).

Gymnosperms do not have flowers at all, instead they have male and female cones (also called strobili). 98% of gymnosperms use the wind for pollination, with the male pollen cones releasing their pollen into the wind to find its way to females.^ref Because this is a bit of a hit and miss approach, gymnosperms produce a *lot* of pollen – one study estimated Juniper pollen production to be up to 532 billion pollen grains per tree!^ref These vast quantities of pollen create pollen clouds, which you can see in a video on youtube here.

Another nifty thing is that female conifer cones produce a ‘pollination drop’ – this is a drop of liquid which sits on the surface of the cone to catch wind-borne pollen. As soon as pollination takes place, the drop is quickly retracted back into the cone.^ref And you’ll notice that often male pollen cones are positioned at the top of a tree and in open positions along the stem (ie. not covered by leaves), to give their pollen the best chance of going far and wide.^ref

Male cones on gymnosperms are smaller and less conspicuous than females, but, I think they’re still beautiful and worthy of our appreciation. Here are some examples of gymnosperm male cones:

Conifer species which have leaflets have a sort of tree-within-a-tree approach – such as the cypress below. – it has browny-pink male pollen cones on the ends of the shoots towards the back of the leaflet, and green developing female cones on the ends of the shoots at the tip of the leaflet.

There are more great images including scanning electron microscope images of gymnosperm pollen in this research paper.

But what if the trees in your bonsai collection don’t have any flowers or cones at all? Unfortunately this is a sign that they haven’t yet reached their reproductive phase. It’s hypothesised that a plant’s transition to its reproductive phase happens after a certain number of cell divisions have taken place.^ref If you keep pruning the new growth off, your tree may never reach a reproductive phase, since it may never achieve the number of cell divisions required. The only way around this is to let your tree grow, use a mature tree to begin with, or use grafted material which comes from mature trees.

In the meantime, make sure to take a good look at your trees and the trees around you, and appreciate the underrated male tree flowers when they make an appearance.

Bonsai Pinetum Species List

Below is a shopping list if you’re wanting to create a bonsai pinetum. You may want to explore alternative species to include, if so the Gymnosperm database is a fantastic resource.

ARAUCARIACEAE

Monkey puzzle tree (Araucaria araucana) and/or Wollemi pine (Wollemia nobilis and/or Kauri (Agathis australis)

CUPRESSACEAE

Basic (5 genera): Hinoki cypress (Chamaecyparis obtusa), Dawn redwood (Metasequoia glyptostroboides), Japanese cedar/sugi (Cryptomeria japonica), Sabina Juniper (Juniperus sabina), and Thuja (eg. Thuja occidentalis)

Intermediate (10 genera): as above plus Giant redwood (Sequoiadendron giganteum), Coast redwood ( Sequoia sempervirens), Swamp cypress (Taxodium distichum), Italian/Mediterranean cypress (Cupressus sempervirens) and an Oriental arborvitae (Platycladus orientalis).

Extensive (17 genera): as above plus Chinese Coffin Tree (Taiwania cryptomerioides), Tasmanian Cedar/Pencil Pine or King Billy Pine (Athrotaxis cuppresoides/Athrotaxis selaginoides), Rottnest Island Pine or Oyster Bay Pine (Callitris preisii, Callitris rhomboidea), Chinese Fir (Cunninghamnia lanceolata) , Chilean Cedar (Austrocedrus chilensis), Incense Cedar (Calocedrus decurrens) and Chinese Swamp Cypress (Glyptostrobus pensilis)

Complete (25 genera): as above plus Diselma, Fitzroya, Libocedrus, Microbiota, Papuacedrus, Tetraclinis, Thujopsis and Widdringtonia.

PINACEAE

Basic (5 genera): Scot’s or Japanese Black or Japanese White Pine (Pinus sylvestris/thunbergii/parviflora), Abies koreana (Korean fir), Cedrus atlantica (Atlantic cedar), Engelmann spruce (Picea engelmannii), European Larch (Larix decidua)

Extensive (9 genera plus extras): as above plus Eastern white pine (Pinus strobus), Mountain Hemlock (Tsuga mertensiana), Golden larch (Pseudolarix amabilis), Douglas fir (Pseudotsuga menziesii) and Yunnan youshan (Keteleeria evelyniana). Consider also Pinyon pine (Pinus monophylla)

Complete: all 11 genera: as above plus Cathay silver fir (Cathaya argyrophylla) and Bristlecone hemlock (Nothotsuga longibracteata)

PODOCARPACEAE

Basic: Buddhist Pine (Podocarpus macrophylla)

Extended: as above plus Celery Top Pine (Phyllocladus alpinus or Phyllocladus asplectiifolius), Chilean plum yew (Prumnopitys andina),

+ if you are a true collector and willing to track down seeds or specimens in collections near you, try for a Kahikatea (Dacrycarpus dacrydioides), Creeping Strawberry Pine (Microcachrys tetragona) and Rimu (Dacrydium cupressinum)

SCIADOPITYACEAE

Japanese Umbrella Pine (Sciadopitys verticillata)

TAXACEAE

Basic: Common Yew (Taxus baccata) or Japanese Yew (Taxus cuspidata),

Intermediate: as above plus Japanese Plum Yew (Cephalotaxus harringtonii)

Extended: as above plus White Berry Yew (Pseudotaxus chienii) and Japanese Nutmeg Yew (Torreya nucifera)

+ for collectors Stinking Cedar (Torreya taxifolia)

Creating a Bonsai Pinetum

A pinetum is an arboretum, or collection of trees, dedicated to conifers. There is a fabulous pinetum at RHS Wisley in the UK (my ‘About Me’ pic was taken there), and the UK National Pinetum at Bedgebury Forest has a collection of 12,000 specimen trees.

Whilst very few of us have the space to create a full-sized pinetum, the wonderful thing about bonsai is that you can create your own miniature version. There are only six conifer families, and within those, 68 genera, some of which would be impossible or at least extremely difficult to procure. So you could have a very respectable and representative bonsai pinetum with around 50 trees. A mame-sized bonsai pinetum might even fit on a single table!

Only a small number of conifer species are common bonsai subjects, so embarking on this project would require some creativity – there won’t be online tutorials or examples for many of these species. Some may be completely hopeless for bonsai (most of the Araucariaceae family for example), others may require conditions that you just can’t provide, but along the way I’m sure you would find a few that make excellent bonsai and give you something unique and different for your collection.

If you want to jump straight to the shopping list here it is, otherwise read on to find out about the trees in the list and where they fit in the different conifer families. For beginners to taxonomy, you start with a family, then a genus (or genera if there is more than one genus), then a species. So for example for Scot’s Pine Pinus sylvestris, Pinaceae is the family, Pinus is the genus, Sylvestris is the species.

Family 1: Araucariaceae

You have a few different options for your representative tree/s from Araucariaceae as it has three genera (agathia, araucaria & wollemia) which cover a range of different forms. The most well-known in Europe would be the monkey puzzle tree (Araucaria araucana) but you could also include a Wollemi pine (Wollemia nobilis) which are available to buy, albeit at a cost. Neither of these are the easiest of bonsai subjects as they have a very regimented architecture with whorled branches, however they do have the advantage of being frost hardy. Wollemia nobilis also does backbud, and grows as a multi-stem.

An alternative could be an Agathis, also known as a Kauri tree. The New Zealand Agathis australis is the third largest known conifer after the giant and coast redwoods^ref, depending on where it has been grown it may or may not be frost hardy. The New Zealand Bonsai Association has a Kauri forest on their native species web page.

If you live in the southern hemisphere, many of these options will be easier to find and will be happy outside.

Family 2: Cupressaceae

The Cupressaceae conifer family is a lot easier to cover in your pinetum as it contains 25 genera (listed below) and 152 species.^ref You can read about the leaves of many Cupressaceae species in my post on conifer scale leaves. It would be appropriate to include several members of this large family, as many are known as bonsai subjects anyway. An easy selection of five from different genera could include a Hinoki cypress (Chamaecyparis obtusa), a Dawn Redwood (Metasequoia glyptostroboides), a Japanese Cedar/Sugi (Cryptomeria japonica), a Sabina Juniper (Juniperus sabina), and a Thuja (also called arborvitae or cedars, although they are not true cedars).

Expanding to ten specimens across ten genera would allow the addition of five other reasonably easy to procure and grow species: Giant Redwood (Sequoiadendron giganteum), Coast Redwood ( Sequoia sempervirens), Swamp Cypress (Taxodium distichum), Italian/Mediterranean Cypress (Cupressus sempervirens) and an Oriental Arborvitae (Platycladus orientalis).

If you’re a purist and want to include more genera from Cupressaceae, some excellent options would be the Chinese Coffin Tree (Taiwania cryptomerioides), Tasmanian Cedar/Pencil Pine or King Billy Pine (Athrotaxis cuppresoides/Athrotaxis selaginoides), one of the Australian Callitris species such as Rottnest Island Pine or Oyster Bay Pine (Callitris preisii, Callitris rhomboidea), Chinese Fir (Cunninghamnia lanceolata) , Chilean Cedar (Austrocedrus chilensis), Incense Cedar (Calocedrus decurrens) and the Chinese Swamp Cypress (Glyptostrobus pensilis). You may need to grow these from seed, depending on where you live.

To fully represent Cupressaceae you’d also need to add the remaining eight genera, Diselma, Fitzroya, Libocedrus, Microbiota, Papuacedrus, Tetraclinis, Thujopsis and Widdringtonia. Many of these are specific to small or remote locations and/or endangered, but you may come across them while travelling, or while visiting full-sized pinetums or botanic gardens.

Family 3: Pinaceae

This family is a stalwart of the bonsai hobby, containing a massive (for conifers) 11 genera and 232 different species, including the eponymous pines, which account for more than half of these. You can read about the leaves of most Pinaceae in my post on conifer needle leaves. For your first pinetum Pinaceae, consider the UK native Scot’s Pine (Pinus sylvestris), Japanese black pine (Pinus thunbergii) or Japanese white pine (Pinus parviflora) – and perhaps a Pinus strobus which is a separate subgenus within Pinus.

For something unique, you could also include the only single-needled pine, Single-leaf piñon or Pinus monophylla. It’s actually a pine which has fused needles (five of them) which appear as one, and this results in its needles being very fat.

A top 5 selection from Pinaceae would also include species from the fir (Abies), true cedar (Cedrus), spruce (Picea) and larch (Larix) genera, all of which have species which are relatively easy to source and grow, at least in Europe (and most are extremely frost hardy). Let’s make it top 6 to include another well-known genus, the hemlock (Tsuga).

The 7th and 8th Pinaceae specimens could be a beautiful Golden larch (Pseudolarix amabilis the only species in its genus), and the classic Pacific north-west representative the Douglas fir (Pseudotsuga menziesii). The Douglas fir is not actually a fir, it’s a ‘false hemlock’, one of seven species of Pseudotsuga.

Most of these trees are well-known, can be procured either as plants or seeds, and could be relatively easily added to your collection. But the Pinaceae family also includes some extremely rare genera. Cathaya has only one species, Cathaya argyrophylla, or Cathay silver fir, which has a similar history to the famous Dawn redwood – thought to be a fossil but then ‘discovered’ in a small living stand in China in 1946.^ref Cathaya was also discovered in China in 1938, but the discovery was not recognised as a new species until the 1950s.^ref It is endangered with less than 1000 mature individuals in its native habitat.^ref

Bristlecone hemlock or Nothotsuga is also a genus with only one species (Nothotsuga longibracteata) which comes from China, where it is near-threatened and “populations are highly fragmented, with some consisting of just a few scattered individuals”.^ref Unfortunately Cathaya and Nothotsuga are probably out of reach for your pinetum unless you have access to seeds via a botanic garden, or live in China.

The 11th and final genus in Pinaceae is also relatively unknown outside of its native region of China, Taiwan & Vietnam – Keteleeria or ‘Yunnan youshan’ has three species, of which Keteleeria evelyniana can be found as seed. So with Keteleeria evelyniana you can still have an unusual Pinaceae in your collection without having to raid your nearest full-sized pinetum.

Family 4: Podocarpaceae

Podocarpaceae is another large family within the conifers, with 172 species across 20 genera – so it is larger than Cupressaceae in terms of size but less well known in Europe and North America.^ref This may be because podocarps are mainly found in tropical and subtropical mountain habitats^ref, which has resulted in their leaves being quite different to other conifers (read more in my post on conifer flat leaves). It also results in trees from Podocarpaceae being a little harder to obtain in Europe.

There are three main groups within Podocarpaceae which could be represented in your pinetum – these are known as the ‘prumnopityoid clade’, the ‘dacrydioid clade’ and the ‘podocarpoid clade’.

The first group includes Phyllocladus – the so-called celery pines which come from Australasia. A somewhat hardy species includes Phyllocladus alpinus which I note can be purchased in the UK from Bluebell Nurseries, and seeds for other species such as Phyllocladus aspleniifolius are also available online. Depending on where you live these will need protection from hard frosts. Phylloclade species don’t have true leaves, aside from very small, almost invisible ones when they are seedlings. Instead they have ‘phylloclades’ which are photosynthetic flattened stems^ref. So, of course you must have one of these interesting plants in your collection!

Also in the first group is the Prumnopitys genus from Polynesia and South America. Prumnopitys andina is known as the Chilean plum yew, and was the International Dendrology Society’s Tree of the Year in 2017. They produced a comprehensive report about this tree which you can read online, and which lists locations where they have been planted outside of Chile. It appears to be seasonally available in a small number of plant nurseries in the UK. I would have included a photo but no decent creative commons images were available, so I’ll add one when I get the chance to find a Chilean plum yew for myself.

The second group (the dacrydioid clade) includes the Dacrycarpus genus, of which Dacrycarpus dacrydioides is a species known and loved in New Zealand as ‘Kahikatea’ from the Maori. It’s the tallest tree in New Zealand, and apparently has a hardy form which can be bought from this provider in the UK. Below is one as a bonsai from the New Zealand bonsai association:

Also in this image is another member of the dacrydioid clade – which happens to be (according to Farjon) the smallest known conifer in the world. Microcachrys tetragona, or the Strawberry Pine, is from Tasmania. Bonsai enthusiast Diana Jones explains in the Newsletter of the Australian Plants as Bonsai Study Group: “Growing on the top of Mt. Wellington is a large tree, about 10m in diameter, called Microcachrys tetragona or creeping strawberry pine. Few people notice it because it is only about 5cm high, being negatively geotropic. This causes a few problems when making it into an attractive bonsai, because in a pot, it just flops.” Nevertheless I think her specimen has a certain charm, and she is definitely one tree ahead of me in the bonsai pinetum stakes.

And one can’t leave this group without mentioning Dacrydium cupressinum, also from New Zealand and called variously red pine, red spruce or ‘Rimu’. My favourite podcaster has covered this species on his blog In Defense of Plants where he says the fleshy cones of this tree are an essential part of the diet of the endangered kākāpō bird. Availability of the ‘fruit’ (not really since it’s a conifer – it’s a female cone) triggers breeding for the kākāpō so it’s seen as critical to their survival.^ref The tree itself is a wonderful tree with long dangly stems so I’d love to have one even without a kākāpō.

But if you cannot find Kahikatea, Tasmanian creeping strawberry pine or a kākāpō-infested Rimu, the third group in Podocarpaceae (the Podocarpoid clade) is going to be much easier to represent in your pinetum, because it contains the oft-seen bonsai species the Buddhist pine, or Podocarpus macrophyllus (also called Kusamaki in Japanese). If you are going to have at least one specimen Podocarp, this is likely to be the easiest one to obtain. I have a couple in my London garden (in the ground) but after the last brutal winter I think they would prefer to be indoors, and they are sold as indoor bonsai in the UK.

There are various other Podocarpus species available at nurseries such as Podocarpus salignus or willow-leaved podocarp – it’s probably a good idea to do a local search to see which species are available in your area.

Podocarpus salignus (willow-leaved pine)

Family 5: Sciadopityaceae

Sciadopityaceae has only one genus (Sciadopitys) and within that only one species, the Japanese umbrella pine or Sciadopitys verticillata. This tree is an endemic Japanese evergreen conifer, with relatively slow growth rates, used in gardens and construction in Japan.^ref1,ref2

Unfortunately since this is the only representative of one of the six conifer families, you really do need one in your collection if you are to truly represent all the conifers. This tree isn’t very easy to propagate, and is also quite expensive to buy (at least in the UK), not only that, it doesn’t appear very commonly as a bonsai. I haven’t had any luck growing it from seed (and they were expensive) so I think your best best is keeping an eye on nurseries and waiting until you see one at a reasonable price.

*Sciadopitys_verticillata* at Pinetum Blijdestein, The Netherlands

Family 8: Taxaceae

Finally we come to the yews, the Taxaceae family which has six genera and 28 species. Many will be familiar with its most prominent genus, Taxus, containing the Common yew Taxus baccata. According to the gymnosperm database, most members of Taxus look pretty much the same, so to save yourself money and time, I’d suggest simply finding the yew that is easily available in your area. For me Taxus baccata are a dime a dozen, you also see Japanese Yew Taxus cuspidata used in bonsai.

I believe you should also have a Japanese plum yew in your collection. Cephalotaxus is a genus with 11 species, which used to be considered its own family, but DNA testing revealed it really belonged in Taxaceae.^ref This tree has yew-like leaves and small plum-like fleshy cones which start green and then move through red and dark purple colouration^ref. It’s not commonly seen as a bonsai but it’s not difficult to propagate and I have seen them available at plant nurseries in the UK. One would assume adopting a similar styling approach to yew would work with this tree.

Japanese Yew at Matthei Botanical Gardens (*Taxus cuspidata*)

The remaining genera in Taxaceae are a lot rarer and more difficult to include in any non-tropical pinetum. The New Caledonia Yew Austrataxus spicata is the only southern hemisphere Taxaceae, and thrives in the very unusual habitat of the island, based on ultramafic rock containing chrome and nickel and not much else in the way of nutrients.^ref

The Catkin Yew Amentotaxus is a threatened genus with six species found in China, India, Laos, Vietnam and Taiwan.^ref One nursery in the UK sells Amentotaxus argotaenia var. argotaenia as a pot-grown specimen to be taken indoors during winter (but when I checked they were out of stock).

The White Berry Yew, Pseudotaxus chienii, is the only species in the Pseudotaxus genus, and is also native to China.^ref It has white arils instead of the red arils of Taxus baccata. Cited as rare, it is nevertheless available from some suppliers.

The final genus of Taxaceae and of this pinetum article, is a really interesting one called Torreya which I think deserves a place representing this family alongside the more familiar trees mentioned above.

Torreya nucifera was the International Dendrology Society’s Tree of the Year (2019) and has a full report write-up available online here. This tree is known as the Nutmeg Yew, or in Japanese ‘kaja’ and ‘kaya’ and oil from its seeds (not nuts!) have been used for tempura cooking oil. As a widely cultivated tree, there is availability of Torreya from plant nurseries (eg. here) although the article I linked to says they do not like cool summers so a protected position or a pot may be needed.

Another option from Torreya if you can find one, and have the budget, is the alarmingly named Stinking Cedar or Torreya taxifolia. This is a rare species native to Florida USA, now protected in the Torreya State Park in Florida. One can be acquired for £50 per 2L pot in the UK here.

The end (of this post, but the start of your conifer collection?)

So there you have it, a set of suggestions for creating your own mini-pinetum using bonsai trees across the six conifer families. If you decide to take on the challenge, I’d love to see your efforts – tag me on Facebook (Bonsai-Science) or twitter (@BonsaiScience). Here’s another link to the shopping list, to get you started.

Thanks to the University of Oxford Department of Plant Sciences Conifer Database and the fantastic Gymnosperm database for source material for this article.

Pathogens – nasty tree microbes

Leave a Reply

A pathogen is a microorganism such as a virus, bacterium, oomycete (water mould) or fungus which causes disease and/or death.^ref Examples you might have heard of include Dutch Elm disease (caused by the fungus Ophiostoma ulmi), Horse Chestnut bleeding canker disease (caused by the bacterium Pseudomonas syringae v. aesculi), Ash dieback (caused by the fungus Hymenoscyphus fraxineus), Sudden Oak Death (caused by the oomycete Phytophthora ramorum) and mosaic viruses in vegetables.

The groups of pathogens causing forest tree diseases and their prevalence is shown in the chart below. 85% of these are fungi and the remainder are divided among bacteria, viruses, nemotodes (microscopic worms) and oomycetes. Oomycetes appear similar to fungi but are actually not genetically similar, and are classed in the Chromista kingdom.^ref

Link to source article here; original sources Butin, 1995; Capretti and Ragazzi, 2010; Manion, 1991; Tainter and Baker, 1996

Note that as they are not microorganisms, insects are not classified as pathogens, although they can also cause significant damage and death to trees. Insect damage is a subject for another post, but for this one, their main role is in introducing pathogens to trees by delivering microbes on them into wounds they create (this is called being a pathogen ‘vector’).

Every living thing is in a battle for survival so that its genes can be passed on to future generations. This means that both trees and pathogens are constantly evolving to outsmart each other. But fungi and bacteria can also be beneficial and even necessary for trees, so you can’t just try to kill off every fungus or bacteria as this will destroy the ones that trees actually want and need.

The other thing to keep in mind is that plants have a completely different ‘lifestyle’ to animals in the sense that they are plastic and can grow new tissues, synthesise new compounds and respond to pathogens in a very different way to humans. As noted elsewhere, in response to wounds, trees ‘don’t heal they seal’ and they don’t have an immune system in the same way as animals.

But, there are three ways to help your trees avoid damage from pathogens. The first is to prevent them from being exposed to pathogens in the first place. The second is to remove any pathogens which do take hold. The third is to bolster your trees’ defences so they can fight off the pathogen and its effects.

How can you prevent exposure to pathogens? A tree’s first line of defence is its bark, leaf cuticle, and the pectin and lignin in cell walls, which are physical barriers which prevent pathogens from entering its cells.^ref Since they can’t run away, this is the main way that trees avoid exposure. But in bonsai we do a heck of a lot of pruning, which unfortunately breaks the physical barrier, leaving the tree vulnerable.

To minimise the chance of nasty microbes attacking your tree as a result of pruning there are some steps you can take. Firstly, practice excellent hygiene and make sure your pruning tools are disinfected regularly, particularly when moving between trees. But be careful that the disinfecting method you use doesn’t damage your tools (carbon steel is particularly vulnerable). Soap and water can work (but dry off the water), or you could use an antimicrobial oil such as tea tree oil (from Melaleuca alternifolia) or oregano oil (from Origanum vulgare) – not only are these antimicrobial^ref1,^ref2 but they also protect steel from corrosion.^ref1,ref2

Another approach is to prune during the wintertime. The latter helps because in general most living things are more active in warmer weather – including pathogens. Pruning in winter reduces the likelihood that a pathogen will enter a wound before the tree can seal it off.^ref Also many fungi and oomycetes prefer a moist environment, so when pruning try to avoid leaving the wound wet. Angling pruning wounds towards the sun can also be beneficial, since sunlight has disinfecting as well as drying properties (although probably not so much during the wintertime in higher latitudes).^ref

Contrary to some advice, wound sealants have not been shown to reduce bacterial or fungal infection on tree wounds.^ref This is because a tree wound is not sterile so any sealant can seal pathogens in as well as out. One wonders whether applying an antimicrobial oil to a wound might work, but I cannot find any studies looking into this. Recently I tried applying raw linseed oil to the cut ends of various crabapple branches I was trying to propagate, but these ended up with large communities of mould on them regardless (possibly because I was using high humidity which is perfect for fungal growth).

One interesting pathogen avoidance method is to decouple the seasonal timing between the host plant and the pathogen vector (vector means the delivery method of the pathogen, often an insect). One study found that Dutch Elm disease was avoided by trees which flushed early, since they were not as susceptible to infection after this point.

The second way to help your trees avoid pathogens is to remove them once there. This is tricky since by definition a pathogen is microscopic and impossible to see with the naked eye. You can’t go and squash every bacterium on your tree! It is possible to kill pathogens using antibiotic and/or antifungal substances, but effectiveness varies depending on what you need to remove and usually involves unpleasant and toxic chemicals (such as glyphosate) which also kill good microbes.

Biological control is an alternative to chemicals, this means finding another organism which feeds on or somehow damages the pathogen in question. An example of biological control is the introduction of the Myxoma virus into Australia to control the rabbit population. In trees, the fungus Trichoderma is used as a biological control agent, and it is found in bacterial inoculants for plants such as this one. Species of Trichoderma have been found to be effective against Armillaria root rot (also known as the dreaded honey fungus) and pine pitch canker.^ref

The final way to help your trees is to bolster the natural processes they use to resist the negative effects of pathogens. Plants produce a huge range of substances which have defensive effects, and can detect pathogens with surprising speed and specificity. When a pathogen is detected by a plant, it first activates a specific ‘pattern-triggered immunity’ response which is believed to be sufficient to defend against a wide range of pathogens.^ref This is known as ‘basal resistance’. A second line of defence detects the so-called ‘effectors’ – substances that pathogens create to avoid the pattern-triggered response. This second ‘effector-primed immune response’ causes cell death at the site, limiting the spread of the pathogen^ref and is known as the ‘hypersensitive response’. Plants also synthesise a wide range of defensive compounds such as resins in conifers, terpenoids or essential oils, saponins and flavonoids (of which 9,000 are known).^ref1,ref2 These help them deter pathogens by making their cells poisonous or unpalatable.

So ensuring your plant is not stressed by lack of water, light or nutrients is one way to help it have the resources to defend itself. Another way is to provide it with beneficial microbes such as mycorrhizal fungi and beneficial bacteria. One study found that providing the bacteria Bacillus cereus to tomato plants enhanced their resistance to pathogens by activating the plant growth hormones salicylic and jasmonic acids. Cultivating a healthy rhizosphere (root microbiome) which supports your tree’s health can be achieved by using a well-aerated soil mix and by not constantly repotting. Repotting risks losing the microbiome which a tree has built up over time, for this reason I always try to add back in some of the previous soil when I repot.

So what should you be doing as a bonsai enthusiast to avoid any nasty pathogens ruining your great work? I’d suggest three things. The first is good pruning practices – minimising wounds, avoiding pruning wounds becoming wet or humid, and vigilance in disinfecting tools and pots. The second is to keep your trees vigorous and healthy – particularly before doing large-scale pruning or defoliation. Give your trees lots of water, nutrients and sunlight to help them bolster their defences. And finally help your trees out with the addition of beneficial fungi & bacteria; products containing these can be found online.

Conifer flat leaves

Leave a Reply

Flat leaves are found across the conifer families including Taxus, Cephalotaxus, and even in Pinus (check out Pinus krempfii) as well as many of the Araucariaceae and Podocarpaceae families. Commonly known trees with this leaf shape include all varieties of yew, swamp cypress, dawn redwood and coast redwood. Here are some examples:

Conifer flat leaves are relatively inefficient from a photosynthesis point of view, since water and photosynthates have to travel further to reach the vascular bundle/s.^ref As a result, they have better photosynthetic efficiency in humid, low light environments^ref and are more common where it is wet or tropical. In these areas their greater leaf surface can help them attract the energy they need without drying out due to having more stomatal openings on their surface.

The primarily southern hemisphere family Podocarpaceae is super interesting and not one that I have worked with much from a bonsai perspective, although buddhist pine (Podocarpus macrophyllus) is a species commonly found as an indoor bonsai in the UK. (It is hardy down to just below -10°C so could be an outdoor bonsai as well). Podocarpaceae are interesting from a foliage perspective, as they have evolved a wide range of different leaf shapes which in some cases have become similar to angiosperms and take the forms of flattened leaves with modifications that allow them to grow larger. Below are some examples from which it’s clear that this family has found a workaround for the venation constraints of others in the conifers.

Podocarpaceae: (a) *Retrophyllum*, (b) *Dacrycarpus*, (c) *Falcatifolium*, (d) *Acmopyle*, (e) *Podocarpus*, (f) *Nageia*, (g) *Prumnopitys*, (h) *Phyllocladus* and (i) *Sundacarpus*
https://royalsocietypublishing.org/doi/10.1098/rspb.2011.0559#RSPB20110559F1

One study mapped the structure of Podocarpaceae leaves and created a cross-sectional image for each sub-family, their results are shown below (apologies for the poor resolution but the original paper wasn’t great to start with). You can see that although most species only have one vascular bundle, they also have various mechanisms to get substances to and from it – including the orange ‘organised accessory transfusion’ cells and ‘pitted thick-wall mesophyll’ cells. Interestingly leaf (f) from the image above – the largest leaf – is also shown below and it’s the only one to have multiple vascular bundles (N and O) which is similar to angiosperm leaves.

https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1096-0031.2011.00381.x

Whilst the majority of Podocarpaceae are flat-leaved, quite a few members of Araucariaceae have flat leaves as well. The famous Wollemi Pine or ‘pinosaurus’ Wollemia nobilis has flat leaves, as does the equally well-known monkey puzzle tree Araucaria araucana and various others in the family. These trees don’t tend to be common bonsai subjects as their leaves are quite large and observing the Wollemi pine in my garden, they don’t have much variation in their form so wouldn’t be that easy to style.

Also unusual among conifers is the Phyllocladus genus. Plants in Phyllocladus don’t have true leaves at all – any leaves they develop are non-synthetic and ephemeral – ie. they quickly drop off. Instead Phyllocladus use their stems, which have developed a leaf-like flattened form, to photosynthesise.^ref These are called phylloclades – see below for some examples from New Zealand:

https://www.nzpcn.org.nz/site/assets/files/0/31/258/qqq_culyybpynqhf_gevpubznabvqrf_gnarxnun_pngxvaf.400×400-u0c0i1s1q90f1.jpg

I know they look identical to leaves – and they contain all the same components including vascular bundles and photosynthesising palisade and mesophyll cells. The nuances of why they are not leaves but flattened branches instead are based on the way they develop and branch, and the relationships between organs – if you want to read the details refer to this excellent paper. But sometimes they may get lumped in with flat leaved conifers.

Which brings me to Taxus or yew. This is a very common bonsai subject, with its own family, Taxaceae. Yew leaves are famous for two things. Firstly, along with every other part of the yew except for its aril (the red ‘berry’), yew leaves are renowned for being extremely toxic. They contain ‘taxine alkaloids’ of which only a small amount is needed to bring on “dizziness, nausea, vomiting, diffuse abdominal pain, cardiac arrest, respiratory paralysis and death” in all animals including humans^ref. Secondly, they are one of the few conifers which are known to do just fine in shady positions, although they are also happy in full sun as their leaves adjust to different light levels.^ref

Taxus have a unique stomatal feature called ‘papillose’ cells^ref which can only be described as ‘pimply’ (see below) – basically the entire surface of the leaf has tiny protrusions as part of the cuticle structure. The presence of these pimples is one of the ways of identifying a Taxus leaf. I couldn’t find any clear explanation as to the role of these papillae, except perhaps to provide some level of water-repellence.^ref Taxus are known to harbour endophytes (microbes) in their leaves which help combat pathogens^ref so it could be that the nooks and crannies created by papillae are a nice home for endophytes.

https://www.researchgate.net/profile/Balkrishna-Ghimire-3/publication/260093980_Leaf_anatomy_and_its_implications_for_phylogenetic_relationships_in_Taxaceae_s_l/links/54879ac60cf268d28f07262f/Leaf-anatomy-and-its-implications-for-phylogenetic-relationships-in-Taxaceae-s-l.pdf?origin=publication_detail

Anyway, what of all this is relevant for bonsai? It sort of depends on which flat-leaved species that you have. Many will be suited to humid and/or low-light environments, so keep an eye on your watering and don’t let them dry out – they will probably appreciate a misting every now and then. Full sun may not be necessary – or may even be over the top depending on your location – but as always find out what a given species needs as there are lots of quirky members of this club with their own unique requirements.

Conifer needle leaves

I’ll be honest, I find conifer needles to be quite weird. As someone raised in the southern hemisphere, true pine trees were something we had at Christmas and that was about it (this is true because the most common Christmas tree found in Australia is Pinus radiata). But actually, needles really are just plain old leaves. They contain all the same components as ‘normal’ leaves, like an epidermis, photosynthesising mesophyll cells, green chlorophyll pigment, xylem/phloem, stomata and so on. They just happen to have pushed the ‘leaf’ form to the extreme, ending up extremely long, thin and tough.

Within conifers, what are *called* needle leaves are found across most of the families, including Araucariaceae (Norfolk Island pine, Cook pine, Hoop pine), Cupressaceae (juniper, Thuja, Cryptomeria japonica), Pinaceae (pines) and Podocarpaceae (Platycladus).

At this point though I think it’s important to point out that the Pinaceae family is the least similar to all the other conifer families, having diverged from them very early in evolutionary history. Look at the ‘family tree’ of conifers below and you can see that Pinaceae including pine, cedars, larch, fir and spruce, has been evolving separately for the longest of any conifer family. This means that ‘needle leaves’ in Pinaceae are not the same thing at all as needle leaves in other families, and they probably need to be treated as two separate sub-categories.

https://www.researchgate.net/figure/Plastid-based-phylogeny-of-the-conifers-and-relatives-inferred-from-ML-for-15-17_fig3_228683987

This is why it can be confusing to understand what people are talking about when they refer to needle leaves in conifers. Needle leaves in Pinaceae are obvious – they are long, thin, spiky, tough, 3-dimensional and in mature foliage form in clusters known as fascicles which are actually short shoots (see my post on shoots for more on these). What you and I would call pine/fir/spruce needles – as per these examples:

Needle leaves in other families are a bit more ambiguous. Sometimes they are referred to as ‘awl-shaped’ or ‘sabre-shaped’ and often they have a needly element but also a scale-leafy element. Needle leaves in these families, such as Juniperus and Cupressus, often mature into scale leaves. Here are a few examples of non-Pinaceae needle leaves:

As is obvious, these are different to Pinaceae needles and it’s not just the leaf shape and configuration – one major difference is that these species usually have a photosynthetic stem which pines definitely do not have. You can sort of see how these leaves could change to become scale leaves by the way they are attached to the stem in alternating pairs – if they just shrink and get closer to the stem you could see a mature scale leaf emerge.

For the sake of the rest of this post, I’m going to focus on Pinaceae needle leaves, since these are persistent needles whereas most of the needle leaves on other families are juvenile (although there are some exceptions).

One of the key attributes of Pinaceae needle leaves is that they have a 3-dimensional profile – usually with a quadrangular, triangular or semicircular cross-section, as shown in the images below (1,2,3 & 5 needle pines):

https://www.flickr.com/photos/146824358@N03

Anatomically, Pinaceae needles are like other conifer leaves in only having one or two vascular bundles for water & sugar transport. Their stomata are arranged more or less evenly around the needle and appear in lines. But needles are unique in having an unusual type of mesophyll cell (a cell used for photosynthesis). Instead of cylindrical palisade cells lined up under the epidermis like other plants, needles have frilly looking ‘arm palisade parenchyma’ (see below). These have very lignified (woody) cell walls which intrude into the cells and it is this feature which is thought to provide needles with their extreme cold resistance.

Pinaceae needles are actually *the* most frost-hardy leaves of all. Mountain pine needles survive temperatures down to −93°C and can still perform gas exchange (oxygen for cell respiration) even when their needles are frozen.^ref They are also highly resistant to herbivory, due to being tough, spiky, full of toxic resins and not very nutritious. And they live anywhere from 2 to 45 years (Bristlecone pine has the record).^ref The needle is one tough leaf!

Pine foliage is also heteroblastic – which means it has one type of foliage during its juvenile phase, which lasts 1-3 years, and a different type during its mature phase. Juvenile needles don’t appear on fascicles, instead developing directly on the stem, and their profile is more flattened – although it’s not super noticeable until you know to look.^ref Mature foliage develops in fascicles (bundles) and is the familiar three dimensional profile – they fall off as a group when the fascicle falls off. See examples below – Pinus cembroides juvenile on the left and adult on the right of both frames.

https://onlinelibrary.wiley.com/doi/full/10.1002/ece3.8611

Juvenile and mature needles have different properties with juveniles having 3x the photosynthetic capability of mature needles. This is believed to be able to provide such species with a ‘fast return’ strategy whilst establishing and a ‘slow return stress-resistant’ strategy when older.^ref

Surprisingly needle leaves actually have better photosynthetic performance than other conifer leaves – despite that fact that Pinaceae have lost key genes required for photosynthesis in other plants.^ref Their improved performance is believed to be because water has to travel further than in non-needle leaves, but there is likely also a genetic factor which hasn’t been discovered yet.^ref Needle leaves take a couple of years to reach their full photosynthetic capacity, but once they do, from then on it reduces with age. One study found that for spruce leaves this declines linearly from the 2 year point, reducing to 30% or less of the maximum photosynthetic capacity by the 6 year point.^ref

When photosynthesising, trees need to deal with transpiration, which is the evaporation of water from the stomata in their leaves and is the main driver of their water requirements. Needle-leaved trees have a massive advantage in this domain, as many species have wax deposits in their stomata which reduce transpiration. It has been found that wax deposits in Sitka spruce stomata reduce transpiration by two thirds but photosynthesis by only one third.^ref Unfortunately these waxes are degraded by pollution, so needle leaved trees can dry out in high pollution areas.^ref

Since needle leaves can generate more energy from the light they have, and from a wider range of sun angles, they can survive in poorer light conditions than flattened-leaved species. Along with their cold resistance and ability to minimise transpiration, this is why the boreal forest comprises mainly Pinus, Abies and Picea species, which all have needle leaves.^ref

Finally a note on buds and how needle leaves develop on a tree. Leaf buds on mature needle-leaved species form as part of a shoot rather than individually, and these are usually determinate, which means that everything is formed inside the bud. So before bud break the leaf primordia (baby needles) are sitting inside the bud. Below are some cross-sections of long shoot pine buds (which you might know as candles) and a spruce bud (on the right). The brackets show where short shoot buds are located, and within these the baby leaves are waiting to emerge with the shoot.

Contrary to some advice, needles do all their extension in their first year of growth. After this they replace or add to their phloem annually, but not their xylem.^ref This means there may be some thickening of needles, but no lengthening after the first year. They may be shorter with increased dryness and poverty of the soil.^ref Eventually needles fall off along with their fascicle and the other needles in their group. However, pines have been shown to retain their needles up to twice as long if they have been defoliated (eg. by insects).^ref

What’s the impact of this all for the bonsai enthusiast? Firstly your needle-leaved species are going to be tough, they will cope with reduced water, poor soil, wind, rain and freezing temperatures. They definitely do not want to be inside.

Secondly as per conifer leaves in general, needle leaves are not as plastic or regenerative as angiosperms – they are part of a shoot and form in the bud, so there aren’t as many styling options as you find with angiosperms or even flat or scale-leaved conifers. And they have a more relaxed time frame than angiosperms do – needles may stick around for a long time – usually this will be 2-4 years in most species and low-medium elevations but can be a lot higher. So your styling decisions can’t be completely redone on an annual basis – a better approach for needle-leaves is a gradual evolution towards a vision. These trees suit bonsai practitioners with patience and a slow, thoughtful approach.

Bonsai folk like small leaves and in the case of needles, short needles. This is achieved in one of two ways. The first way is that long shoots (candles) are completely broken off early in the growing season. This forces the tree to activate dormant foliage buds at the base of the shoot, which don’t have the time or resources to develop full length needles. I’m not sure whether breaking the part of the candle off (as is also advocated) would also reduce needle size since this retains the active short shoots at the base of the candle to still develop. This practice might have a slightly different effect of creating more short shoots with more needles, so giving denser foliage, rather than shorter needles. The second way to reduce needle size is to starve the tree of water and nutrients, but I’d say manipulating candles would be better for the health of the tree. Obviously to add branches to a tree you need to leave the long shoots in place to develop, as these are what create the long-term framework.

To finish off, I just have to share one of the brilliant images created by Gerhard Vicek who does microscope cross-sections of plants – below is a cross-section of a Cedrus atlantica needle. The beautiful staining he does of his samples makes the different cell types really clear. There are epidermal cells (in red) on the outside, below these the unusual frilly arm palisade cells (in green). Then you have the transfusion cells in a brown & white ring, which move water to the outside of the needle and sugars to the centre. In the centre you can see two vascular bundles with the tiny xylem & phloem cells. Truly his images are art for the bonsai science nerd!

Please visit Gerhard Vicek’s’s website for more great microscope images of trees: foto-vision.at

Conifer scale leaves

Leave a Reply

Scale leaves are a curious form of conifer leaves which cover up the stem in interlocking patterns. I believe they are called ‘scale’ because they look a bit like fish scales in the way they overlap but I have not found an authoritative source which confirms this. Scale leaves appear primarily in the Cupressaceae family – including junipers, various cypresses, Arborvitae/Thuja, redwoods & Callitris, as well as in the Podocarpaceae family including Dacrydium and Acmopyle. Sometimes the scale leaf form is the mature foliage, while the juvenile foliage takes a needle form (see my post on conifer needle leaves).

Some examples of scale foliage leaflets are below:

You’ll note that I called them ‘leaflets’. The actual leaves are the individual scales that you see in the images, which all combine to create a larger leaflet which is actually a short shoot. The leaves are wrapped around and connected to the stem of the shoot underneath.

Scale leaves are usually in opposite pairs, and depending on the species can have main or ‘facial’ scales and lateral scales with slightly different anatomy. Below is a scanning electron microscope image of Thuja occidentalis leaves which demonstrates these two scale types.

https://digitalcommons.conncoll.edu/cgi/viewcontent.cgi?article=1038&context=arbbulletins

Probably the most distinctive attribute of scale-leaved species is the leaf pattern. Each species has a distinct cross-sectional profile, with different leaf shapes and configurations, these are what ultimately create the three-dimensional shape of the leaflet.

Great work was done on this by some Iranian researchers, who created the following cross-section drawings which I have matched to images of the species in each drawing. These show how the scale leaves attach to each stem, the positions of vascular bundles, resin ducts (large holes) and stomata (which are mostly present in the grooves indicated by ‘S’).

As can be seen in the drawings, scale leaves are very simple, usually with a single vascular bundle (other than Juniperus excelsa above which has none), palisade and spongy mesophyll cells for photosynthesis and the darker transfusion cells which move water to and from the stem.^ref

In terms of their performance, there actually isn’t a lot of information out there comparing scale leaves with needle or flat leaves. One study found that Thuja leaves were about on par with pine needles in terms of photosynthesis^ref and another found that juvenile needle leaves of Juniperus sabina outperformed its scale leaves^ref. Many studies seem to conflate needle and scale conifer leaves and talk about both of them having strong performance in high sun, low humidity situations. There must be some benefit, because quite a few species ‘graduate’ to scale foliage as they age, but I haven’t found any research explaining what that benefit may be.

One weird and wonderful variation of the scale leaf is the unusual ‘axial’ leaf of the coast redwood. Most people associate the coast redwood (Sequoia sempervirens) with a flat-leaved leaflet as shown on the left – and in fact this type of leaf makes up 95% of the leaf surface area of these trees. But 5% is made up of the axial-leaved ‘twiglet’ on the right. These leaves are optimised to absorb water, having much less waxy coating than the flat leaves and contribute up to 30% of the water requirement of the tree. Which can be high, given the size of a coast redwood!

https://theconversation.com/redwood-trees-have-two-types-of-leaves-scientists-find-a-trait-that-could-help-them-survive-in-a-changing-climate-179812

So what are the bonsai implications?

Since scale leaves are associated with older trees, they are preferred for bonsai. If your tree is still in its juvenile phase, you need to let it grow as it’s believed that the trigger for changing phase to mature foliage is the number of meristem cell divisions.^ref Pruning the leader on these trees will keep them permanently in a juvenile state, so let the tree grow until it develops mature foliage, then you will need to use all branches & foliage *after* this point to style your tree.

Also remember that scale leaves usually appear on short shoots, which abscise (fall off) as a unit. But don’t worry because usually there will be a bud waiting at the base of the short shoot to replace the one which fell.

Finally if you look at how scale leaves connect to the stem, I believe that the technique of pinching leaves (and stem) off instead of pruning with secateurs or scissors would leave less dead material on the tree. Cutting straight through a stem is always going to sever one or more scale leaves and cause them to die and go brown. The pinching technique is when you use your fingertips, and pull the stem gently so it breaks at a natural breaking point between leaves.

Why do some conifer leaves go bronze in winter?

Leave a Reply

You may have noticed that certain conifer species’ leaves go a different colour over the winter without dying. This phenomenon is most associated with Buxus (box) and Cryptomeria japonica (Japanese cedar) which take on a range of bronze, reddy-brown colours, as shown in the examples below:

*Cryptomeria japonica spiralis* (from my garden, with frost)

Although it may look a bit alarming, these leaves will go green again as the temperatures warm up. The reason for the colour change is that in these species, sun-exposed leaves at low temperatures shift their pigmentation to protect themselves from excess energy.

During the winter, evergreen trees face the challenge that the sun can be relatively strong, but the temperatures low. Many of the biological processes needed to convert light into sugars rely on enzymes, which don’t work efficiently at very low temperatures. Although it’s possible for plants to acclimate to lower temperatures^ref, beyond a certain point the rate of photosynthesis starts to outpace the downstream processes, and excess energy and reactive oxygen species (“ROS”) accumulate. This can permanently damage Photosystem II (part of the photosynthetic apparatus in plants), so a mechanism is needed to avoid that.

The solution adopted by Cryptomeria japonica is to transform some of its chloroplasts into chromoplasts.^ref

The part of the plant cell which photosynthesises is the chloroplast, which is a self-contained organelle with its own DNA known as a ‘plastid’ (and also, in conifers, paternally inherited). Normally it synthesises the green pigment chlorophyll. But plastids can also be ‘chromoplasts’, instead of synthesising chlorophyll, they synthesise coloured carotenoids^ref which aside from being different colours, also act as anti-oxidants, removing dangerous excess oxygen levels from plant tissues. The pigments found in winter Cryptomeria foliage include Rhodoxanthin (found in Taxus baccata arils), Zeaxanthin (found in marigolds) and Antheraxanthin (found in dandelions).^ref These are all carotenoids which are known to safely dissipate excess energy during photosynthesis.^ref

Red *Taxus* arils coloured by Rhodoxanthin (image: Kew Gardens)

So during winter when temperatures fall, green chloroplasts are converted into coloured chromoplasts^ref, and vice versa^ref when the temperature rises. The effect of this temporary transformation is to reduce photosynthesis and to increase the plant’s anti-oxidant levels, so that excess energy is dissipated safely and the photosynthetic apparatus is not damaged.

Aside from being observed in Cryptomeria^ref and Buxus^ref, there is also anecdotal evidence that the same process happens in Juniperus^ref and Thuja (see below for one of my Thuja bonsai – the white bits are frost and the foliage is a distinctly reddy-brown colour). Interestingly these are all ‘flattened’ forms of conifer leaves, and colour-changing behaviour is rarely observed in needle-type leaves, which may instead deal with the problem of winter sun by plugging up their stomata to reduce the rate of photosynthesis. One exception is Pinus contorta ‘Chief Joseph’ which has golden needles in the winter and green ones in the summer.

The conversion of chloroplasts to chromoplasts is also responsible for the colouring of fruit and flowers, but it’s *not* what makes leaves coloured in the autumn (that would be ‘gerontoplasts’^ref).

It seems that the conifer species which can do this are all members of Cupressaceae. So if you see pine needles going brown, that is probably the needles dying (which they do naturally after a certain period of time) unless it is a rare form. But if your Cryptomeria, Juniperus or Thuja develops a winter ‘blush’, don’t worry, it will probably come back all green when the temperature warms up.