Category Archives: How Trees Grow

Conifer Leaves

I’ve been planning a post on this subject for a while because conifers have always been a bit scary to me from a bonsai point of view – they don’t seem as forgiving or obvious in terms of their growth behaviour. This was one of those subjects which ended up being a lot more interesting and complex than I was expecting – once I hit 3000 words for this post I realised I needed to separate things out! So below is a *summary* overview of conifer leaves, and detail on the three different types of conifer leaves are in separate posts: conifer needle leaves, conifer scale leaves and conifer flat leaves.

But let’s start from the start. What are conifers? Strictly speaking they are any of the species in the family Pinopsida also known as Pinales or Pinophyta (for a reminder review the previous post on The kingdom Plantae and where trees fit in), that is to say, the Pinophytes. Pinophytes are cone-bearing plants, hence the name conifers. They include six different families:

Araucariaceae (including monkey puzzles and the Wollemi pine)
Cupressaceae (including cypress, juniper, redwood, Cryptomeria japonica)
Pinaceae (including pines, cedar, spruce, hemlock, larch & fir)
Podocarpaceae (mainly southern hemisphere evergreens including Buddhist Pine), including Phyllocladaceae (celery pines from New Zealand)
Sciadopityaceae (Japanese umbrella pine is the only member in this family)
Taxaceae (yews) including Cephalotaxaceae (Japanese plum yew)

So why do these families have different leaves to those of angiosperms/flowering plants? It’s because gymnosperms (including conifers) and angiosperms diverged in their evolutionary paths 350 million years ago^ref and as a result they have evolved with key genetic differences. These are exposed in leaves in five key areas:

Venation – the structure of the vascular system which transports water through the leaf and products of photosynthesis back into the tree (ie. its ‘veins’) (and thus determines the possible leaf shapes)
Stomata – the distribution, density and effectiveness of the pores on the leaf which allow air in and water vapour/oxygen out
The photosynthetic apparatus – how the cells in the leaf are arranged to perform photosynthesis and which reactions are used
Heteroblasty – the phenomenon of ‘extreme variation in leaf morphology during plant development’ or in other words, leaves being completely different on young plants versus old plants of the same species (trees which have different juvenile and mature foliage) – although this also exists in angiosperms the versions in conifers are unique genetically
Resin canals – the ducts in conifer leaves & stems containing secondary metabolites

Starting with venation, the vascular system of conifers (which performs water & sugar transport) has only one single vein or two parallel veins per leaf, running up its centre. This is shown in the examples of conifer leaf cross-sections below – purple shows the xylem (water transport) and the blue shows the phloem (sugar sap transport). (2), (10) and (14) have two parallel sets of veins and (5) & (12) have a single, larger vein.

(2) *Abies holophylla,* (5) *Larix kaempferi,* (10) *Cedrus deodara,* (12) *Picea smithiana,* (14) *Pinus tabuliformis*
https://bmcecolevol.biomedcentral.com/articles/10.1186/s12862-020-01694-5

By contrast the vascular system in the leaf of a flowering plant is much more sophisticated with many different vein patterns across species^ref and the average vein length per area in an angiosperm leaf is 2 to 5 times higher than in conifer leaves.^ref Some examples of angiosperm leaf venation are below – you can see veins branching and extending to every part of the leaf and this is one of the advantages that allow angiosperms to create larger leaves (hence the name ‘broadleaf’).

https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.12253

The vascular structure of conifer leaves limits how much water can be delivered to their outer edges. From the vascular bundle/s, ‘transfusion tissues’ or specialised cells conduct water and photosynthates to and from the margins.^ref Their conducting capacity is limited, which in turn limits how wide a leaf can become. In layman’s terms, because conifer leaves have basic water piping, they can’t grow too wide – which affects the size and shape that conifer leaves can take.

Conifer leaf shape categorisation is inconsistent across the literature, and you may see different descriptions such as awl-shaped, sabre-shaped or even intermediate (a catch-all for anything which doesn’t fit). A reasonable set of descriptions has been created by Paul Fantz at the North Carolina State University. But at the end of the day most conifer leaves fit into one of three types – flat, scale or needle leaves. A nice study was done in Iran which produced line drawings of the main three types of conifer leaf, which you can see below (and here). On the top is a flat leaf of Taxus baccata (yew), on the bottom left is a scale leaf (and stem) of Cupressus sempervirens (italian yew) and on the bottom right is a needle leaf of Juniperus communis (common juniper). Due to their shapes, each type of leaf is a little bit different in terms of how they perform in a given environment, and you can learn more about this in my posts about each type: flat, needle, scale. The fact that the same tree produces foliage of more than one type is covered below in the section about heteroblasty.

Now let’s consider the stomata on conifer leaves (to learn or remind yourself about stomata you can read my stomata post). Whilst conifers have the same basic structure for their stomata, with one guard cell on either side, they differ from angiosperms in their arrangement and effectiveness.

Conifer stomata develop at the base of each leaf, meaning that they grow out in longitudinal bands as the leaf emerges, whereas angiosperm stomata develop at multiple points on a leaf, resulting in more variation in their patterns^ref. In needle species they are arranged around all sides of the leaf (with a few exceptions), in scale leaf species they appear in the grooves between scales and stem, and in flattened leaf species they appear mainly on the bottom of the leaf. Below is an image of the stomata from a Picea species, showing them arranged in lines:

https://www.scielo.br/j/abb/a/MjNwf9Bw3VW3jbzJxKVFgJt/?lang=en#ModalFigf2

Stomata in conifers have a couple of other characteristics – often they are ‘sunken’ or set into the layers of the leaf, as well as filled with wax plugs.^ref This massively reduces the gas exchange capacity of the leaves – one study found that gas exchange was only 35% compared to species without wax plugs. Their conclusion was although this blocks the stomata and reduces photosynthesis, it may have been an advantage during wetter periods of earth’s history by keeping the pores free of water. The wax plugs also prevent fungal intrusion – which is more of a risk for conifers with long-lived leaves. Finally a less open stomata also reduces water loss. This allows conifers to survive in drier areas and to stay alive for longer with minimal water – hence they are now found in more extreme environments where angiosperms can’t survive. Below is a sunken stomata from a Tsuga canadiensis on the left and a Cryptomeria japonica stomata full of wax on the right.

Next we need to look at one of the most important attributes of a leaf – its photosynthetic apparatus and performance.

Whilst conifer leaves photosynthesise about 30% less effectively than angiosperm leaves^ref, they live and photosynthesise on average 50% longer when compared to angiosperm evergreens – and obviously much longer (around 300%) when compared to deciduous angiosperm leaves.^ref So overall conifers need to invest less resources to generate their energy, since each leaf works for longer periods. Where angiosperm leaves have a ‘live fast, die young’ lifestyle, conifer leaves are more ‘slow and steady wins the race’.

One surprising fact I came across while researching this post was that conifer seedlings can actually grow in the dark. They are able to synthesise chlorophyll and create the photosynthetic apparatus without light, and these are ready to work as soon as the plant is illuminated- although the amount of chlorophyll produced is lower than if the seedling has been illuminated.^ref This makes sense since seedlings may often germinate in low light conditions on a forest floor.

Like angiosperms, conifers can have different shade and sun leaves (this is known as ‘heterophylly’). In Abies alba (silver fir) sun leaves are on average longer, have thicker cuticles, more photosynthesising palisade mesophyll cells, fewer spongy mesophyll cells and more stomata than shade leaves, as well as significantly higher photosynthetic performance.^ref By contrast shade leaves contain 3 times more chlorophyll content and 2.5 times more carotenoids than sun leaves. Even the arrangement of sun and shade leaves look quite different – see the image below showing sun leaves on the left and the shade leaves on the right.^ref

Source: https://onlinelibrary.wiley.com/doi/full/10.1111/pce.13213

Another factor which determines the photosynthetic performance of a leaf is its age. Except for the few deciduous conifers, conifer leaves can last anywhere from one to 45 years, although the latter is unusually long. The data is scattered across many papers but to provide some examples, the majority of pine needles live for 2-8 years^ref , the scale leaves on Thuja plicata live on average 8 years^ref, and flat yew leaves also live up to 8 years.^ref Needle leaves live longer at higher elevations and with poorer conditions in general (such as lack of water).^ref

Which brings me to the topic of heteroblasty, or trees which have obviously different juvenile and mature leaves. It’s a well noted phenomenon in bonsai circles that certain junipers have needle leaves when young and scale leaves when older. It turns out that heteroblasty is observed in Cupressaceae^ref, Pinaceae^ref and Podocarpaceae^ref and results from what is called a ‘phase change’ in the shoot apical meristem. This is when the growing tips change to produce different organs – so instead of producing buds that become juvenile leaves, they produce buds which become mature leaves – and eventually buds which become reproductive organs as well. This phase change is relatively stable, so once a meristem produces mature foliage, it will continue to do so. It is also position specific – so the lower branches may retain juvenile foliage even when the rest of the tree has mature foliage.^ref

One explanation for heteroblasty is that it’s a useful way for plants to deter herbivores or other environmental hazards that exist for smaller, younger plants. New Zealand has a high number of heteroblastic plants (200 species), and academics have proposed that the unusual branching form in juvenile trees which is specific to the area has specifically developed to deter large ratite birds like emus and moa.^ref 10 such species were found which changed their leaves and branches once they surpassed 3m in height (the maximum bite-height of the ratites). However since there are no more moas, it’s hard to prove the theory, which is apparently hotly debated.^ref

Phase changes are controlled by genes and plant growth regulators, which change their expression when a meristem has undergone a certain number of cell divisions.^ref This was demonstrated by showing that mature flowering meristems, when rooted as cuttings, also flowered and so retained their mature state. This is why position matters when it comes to heteroblasty and only meristems which have reached the mature phase will produce mature foliage.

Since phase change to a juvenile state is desirable for plant cloning, there are studies which have considered how to maintain juvenility or reverse it in mature plants. One method for delaying phase change is to ‘hedge’ – what you and I would call pruning – presumably because this removes the apical meristem programmed for the new phase and reverts to meristems lower down the plant which haven’t changed phase. Another is to apply stress to a plant by starving it, dehydrating it or exposing it to heavy metals.^ref

The final and fifth familiar attribute of conifer leaves that differs from angiosperms is that they are almost all resinous. Conifer resins are mostly terpenes made up of linked isoprene elements (C₅H₈) and are conducted through leaves (as well as some cones and wood) through resin canals. 30,000+ different terpene structures produced by conifers have been identified – some of which are used to produce various products including turpentine, printing inks, soap, plastic, fireworks, and tar. The effect of resinous leaves is to deter insects (Farjon, 2008) and microbes.^ref Resin doesn’t feature too much in bonsai (other than when you’re cleaning your branch cutters), but the resin does provide a defensive benefit to your trees which is probably better than many of the chemicals that are sold for the purpose.

Anyway what does it all mean for bonsai? (Thank god I hear you say – it only took her 2000 words!!)

Well let’s start by acknowledging that conifer leaves are quite different from those of angiosperms. Their vascular system dictates that the leaves take one of the three forms – needle, scale or flattened, and aside from the few deciduous conifers, in general their leaves are designed to stay on the tree for much longer than most angiosperms. This means you’re not going to get the same level of leaf turnover on your coniferous bonsai as you would with your angiosperms, and your styling decisions need to be more carefully made and executed. It is going to take longer to fix a mistake on a conifer.

Similarly, their photosynthetic rate is not as high as an angiosperm, so in many cases a conifer is not going to be able to achieve the same growth rates as an angiosperm unless they have a lot of light, although there are some more fast-growing species. As per the previous point, conifers are less forgiving of poor styling decision.

Depending on its leaf type, your different conifers will prefer different conditions (full sun for needle, humid and less sunny for flattened), but you should also be thinking about how to cultivate the types of leaves you want to see on your tree. Sun needles appear denser and better for bonsai, so shading a fir or a pine is probably not a great idea. Similarly making use of short shoots with their increased leaf numbers is important (see my post on shoots).

Species which display needle-scale leaf heteroblasty are a special case as usually you want them to take on mature scale foliage which is preferred in bonsai. To do this, lower, older branches (with the juvenile form) will eventually need to be removed, and you shouldn’t prune the apical stem of these species until they have reached the mature foliage phase. Or sidestep the juvenile phase altogether by taking cuttings of mature foliage which should stay mature unless they are seriously stressed.

A final point would be to say that although conifers all fall under Pinopsida (etc) they have a much longer evolutionary path than angiosperms and more divergence between them, so lumping them all together into one post is not really comparing apples with apples (hehe). So have a look at the other posts which spawned from this one to dive into a bit more detail: conifer needle leaves, conifer scale leaves and conifer flat leaves.

Tropical Bonsai

We recently had guest speaker Amelia Williams talk to members of Twickenham bonsai club about tropical bonsai. She has moved entirely to keeping tropicals & sub-tropicals, to the extent that her back garden is full of her ex-non-tropical bonsai trees which are now planted as a foliage bed! This post references some of Amelia’s talk, if you would like to read more she has written several articles for Swindon Bonsai Club.

So what is a tropical tree? At a basic level it’s a tree which lives in the tropics. The tropics are the region of the Earth between the Tropics of Cancer and Capricorn^ref, shown on the map below. The Tropic of Cancer is a line at 23.4° N where the sun is overhead during the northern summer solstice, and the Tropic of Capricorn is the line at 23.4° S when the sun is overhead during the southern summer solstice; the sun is never fully overhead at locations outside of these lines.^ref In the middle of this region is the Equator where there is always 12 hours of daylight.

https://en.wikipedia.org/wiki/Tropics#/media/File:World_map_indicating_tropics_and_subtropics.png

The first thing you will notice from the map is that the tropics and the subtropics cover a large proportion of land mass on Earth (36%^ref), and aside from being where 40% of the world’s population and over 50% of its children live, it’s also home to 80% of the planet’s terrestrial species and over 95% of its corals and mangroves.^ref

It’s estimated that over 40,000 tree species live in the tropics, compared to only 124 in temperate Europe.^ref These species have distinct ranges – that is, species in the Americas are different to those in Africa which are different to those in the Indo-Pacific. So tropical tree species offer bonsai enthusiasts a huge opportunity to diversify our collections and explore trees we may never have known about before. Add in the sub-tropics where trees can handle a wider temperature variation, and the selection becomes absolutely enormous.

Tropical trees live fast and die young, growing twice as fast as trees from temperate and boreal regions and living on average 40% shorter lives (around 200 years). Research shows a relationship between high temperatures, fast growth rates and shorter lifespan for trees which is most evident in the tropics. It’s particularly the case where annual mean temperatures exceed 25.4°C and trees die much earlier than in cooler places. Below is a map showing the longevity of 3,343 tree populations across 438 species worldwide – the darker dots are locations of the oldest trees, none of which fall into the tropical region.^ref

https://www.pnas.org/doi/full/10.1073/pnas.2003873117 (note: each dot reflects the average data for 3 or more trees in a location)

How did the researchers work out the age of all these trees? They used tree rings – the science known as dendrochronology. Despite what you might have heard, tropical trees do produce annual growth rings, not based on seasonal growth like in temperate areas, but instead on limiting environmental factors such as water shortage during the dry season or root anoxia in flooded forests during the wet season.^ref

The good news is that tropical trees grow quickly, which is awesome for bonsai and helps us get nicely shaped trees faster, just as long as we provide the tree with the environment it needs. So what is that environment?

A defining attribute of the tropical environment is its weather. The temperature in the tropics doesn’t vary much, ranging between 25^oC and 28^oC all year round^ref. It never gets cold in this region and it certainly never gets frost. As you get closer to the Equator, the annual cycle of the Earth’s angle of rotation has a smaller and smaller effect, so these locations don’t change a lot in terms of their distance to the sun. Since the distance to the sun doesn’t vary that much, neither does the temperature.

The same applies to daylight. This never varies from 12 hours at the Equator, and even on the edges of the tropics the daylight period in winter is only 3 hours less than in summer (see the daylight chart for Alice Springs, on the Tropic of Capricorn). Compare this to London where the difference is 8 hours between summer and winter.

The amount of sunlight a tropical tree actually receives of course depends on its habitat and position within that habitat. In Costa Rica a study found that understory plants in a tropical forest received only 1-2% of the total light available, and that up to 77% of the light they did receive was from ‘sunflecks’ (spots of light which make their way to the forest floor through gaps in leaves).^{ref1, ref2} So whilst trees which occupy the forest canopy or live in a wet-dry tropical desert environment may require intense light for 12 hours a day, there are plenty of tropical species which can thrive in shady positions as well. As with any plant care, it’s all about understanding where your tree naturally thrives and trying to emulate that environment.

As well as constant temperatures, tropical plants receive a lot of rainfall. Two thirds of annual global rainfall occurs in the tropics and sub-tropics^ref with different patterns in different zones. While the Earth’s rotation has less of an effect on temperature in this region, it has a greater effect on weather systems, which occur more spontaneously in the tropics than elsewhere.^ref

The equatorial zone has high monthly precipitation (60mm or more) and annual precipitation of 2m or more. In this zone are many of the tropical rainforests, where there are often dry, humid mornings and rain in the afternoon. Outside of this there are ‘Dry and Wet’ regions with lower rainfall and distinct dry and wet seasons which depend on position relative to the Equator. Finally some areas of the sub-tropics are categorised as monsoon zones, where there is higher temperature variation (for example going down to 13^oC in Chittagong in January), but also periods of dry and periods of significant rainfall (known as the monsoon).^ref Your tropical tree will be expecting a lot of water at some point in the season!

What all of this means is that to keep tropical bonsai in non-tropical areas we need to create a suitable environment, with four main attributes: (1) a stable, high temperature, no drafts or strong temperature variations and definitely no frost, (2) a consistent level of light between 10.5-13.5 hours (with intensity depending on the species) (3) high humidity and (4) a good watering regime. For anyone who doesn’t live in a tropical location, this means keeping them indoors, in a house or heated conservatory, near a window with some sunlight and away from drying drafts or wind.

In their natural home many tropical species will be used to temperatures above 18^oC and up to 28^oC, but Amelia Williams recommends no less than 12^oC and for Ficus it should be above 15.5^oC. This is the lower limit for temperature for your tree, ideally it should be higher, so room temperature of around 20^oC with maybe some heat from the sun during the day should work well.

If your tree is in a conservatory or even in a window, it might get quite hot during the summer. Medium heat stress in trees is thought to be transitory and doesn’t result in long-term damage (although it does increase net energy use), but “long or exceedance of heat tolerance thresholds leads to irreversible damage to the photosynthetic biochemistry and leaf tissues”.^ref One study found in Phaseolus vulgaris that very hot conditions (over 40^oC) photosynthesis declined rapidly and the cost of respiration exceeded the energy from photosynthesis at around 43^oC. Damage to the leaf and death of cells and chloroplasts was visible from 48^oC.^ref This means that a tree which is extremely heat stressed may need to regrow leaves to recover. However even in this situation they need to have enough water. Trees under heat stress which have enough water keep their stomata closed and minimise water loss, but when they don’t they actually open up their stomata. This causes more water loss through transpiration and the possible death of the tree.^ref So it’s super important at high temperatures to keep your trees well watered.

Sunlight can be a difficult commodity to provide your trees all year round, but a protected window position (and occasional rotation of the pot) will give it the best light, which can be augmented by artificial light if you want to more accurately emulate a tropical photoperiod. Don’t worry if you have no natural light though, it is possible to use entirely artificial light, your trees just won’t be as vigorous. A study on houseplants in Uzbekistan found that those with no natural sunlight required artificial light of a minimum of 2000 lux per day and ideally 5000 lux.^ref

Taking your tree outside in the summer is an option, but be careful taking one from behind the protection of UV-filtered glass straight into a hot sun. Model plant Arabidopsis was found to take 8 days to synthesise and accumulate maximal levels of sinapate esters in its leaves, which act as a UV sunscreen.^ref Instead give the tree some shade or protection for a period while it acclimatises (this actually applies to any plant you keep inside for part of the year).

Humidity can be provided by regular spraying (or misting if you have the facility), but Amelia Williams uses humidity trays. These are trays or dishes filled with a layer of pumice which the bonsai pot sits upon. Water is added to the tray and evaporates up to the foliage, the roots also detect the humidity below (and can sneak out of the holes in the pot in search of this water).

As a watering regime, in the UK once a week is enough in winter, twice a week in spring/autumn and daily in summer, but as with all bonsai watering, this needs to be considered based on each tree’s needs. Water well in hot or dry weather to minimise heat stress.

Other tips from Amelia when keeping tropical bonsai – as with all bonsai use soils with good aeration and differing particle grades (see Bonsai growing medium), repot when actively growing, and avoid ‘cold’ soil substrates or substrates which don’t warm up easily or quickly (such as grit).

Choosing which tropical or sub-tropical bonsai you are going to start with is probably the hardest part, there are just so many options!

On the angiosperm (flowering tree) side of the fence, there are many, many options. Acacia, Diospyros, Eucalyptus, Ficus, Adansonia (Baobab), Bougainvillea – the list goes on – but if you are in the UK or a place with similar weather, Amelia Williams’ articles on Swindon Bonsai’s website give you lots of great suggestions – in this one she has a list. A quick tour of the websites of bonsai sellers located in the tropics yields a lot of the same species we see in Europe – Japanese pines and junipers. But there is inspiration out there – here are some specific African bonsai styles, tropical bonsai from Indonesia by Gede Merta, a bonsai farm in China and one in Florida (strictly Florida is sub-tropical so this owner takes his trees inside during cold weather). You don’t even need to stick to trees, as there are also tropical non-trees which look amazing as bonsai – such as these ‘Rambo form’ Adeniums from Thailand (note that Adeniums have toxic sap which was used to create poisoned arrow heads^ref).

If you’re a conifer fan there are fewer options, since angiosperms (flowering plants) have dominated in the tropics since the Cretaceous period. One exception is the Podocarpaceae family which thrives in nutrient-poor soils in these environments, including bogs.^ref Tropical species are found in the Podocarpus genus including Podocarpus macrophyllus or Buddhist Pine. They are also found in the Dacrycarpus genus for example Dacrycarpus dacridioides is apparently a ‘popular bonsai subject’ according to this website, although it’s not mentioned anywhere else I can find online. Other tropical gymnosperms include some members of Araucariaceae such as Agathis or Kauri trees, Araucaria cunninghamii the Hoop Pine and the sub-tropical Araucaria heterophylla Norfolk Island Pine. Two pine species can also be found in the tropics, Pinus merkusii from South-East Asia and Pinus hondurensis from Mexico.

The main constraint you will have in adding tropical or sub-tropical trees to your bonsai collection is probably going to be accessing plants or seeds, so let your imagination fly and see what works for you (I admit to being side-tracked while writing this post by this UK Adenium seller and now await my first packet of seeds!)

Root-Shoot Connections (aka sectional growth) – when will pruning one kill the other?

Sometimes in a bonsai context it’s said that specific branches are connected to specific roots – often in discussions about pruning and carving. For example it may be suggested that pruning a specific branch will kill an associated root, or vice versa.

As I’ve learned over the last 6 months researching this site, when it comes to trees – ‘it depends’.

The effect of pruning branches or roots on the rest of the tree comes down to its ‘plumbing’ – that is, the way in which the xylem (water) sap and the phloem (sugar) sap flow around the tree. That plumbing is laid down as new shoots and other organs develop – each new organ has a connection to a vascular bundle with xylem and phloem ‘pipes’. These pipes (in reality different types of cells which connect to each other), then connect to the vascular system in a branch, then in the trunk, then to the roots.

Trees can have what is called ‘sectorial’ growth in one or both of these systems. Phloem appears to be more sectorial than xylem – there is less of it, it only runs around the outside of the trunk in a thin layer and it has fewer connections between cells than xylem. Since roots are dependent on phloem from leaves, this would suggest that roots might be more likely to die from a branch being cut than the other way around.

Xylem is a different system, and the way the xylem cells of a particular species are structured determines how sectorial that species is – trees with more connections between their xylem cells are less so (because water has more routes it can travel to reach an organ).^ref

If you’ve read the post on xylem, you’ll know that all gymnosperms/conifers (and some angiosperms) accumulate water-conducting xylem rings over time and have many layers conducting at once. This type of wood is called diffuse porous. Some angiosperms have a different strategy – they regrow their conducting xylem every year and only use that one outer layer for water transport. These trees are called ring porous.

It may then be obvious to you that ring porous species are more sectored than diffuse porous species. This was confirmed in one study using dye injections into xylem vessels – diffuse porous Acer saccharum, Betula papyrifera, and Liriodendron tulipifera had dye show up in more leaves than ring porous species Castanea dentata, Fraxinus americana, and Quercus rubra.^ref This is presumably because in diffuse porous trees there are more water conducting cells and more options for water to travel – it is less likely to get cut off.

Trees which have more isolated root – leaf paths include Quercus, Fraxinus, Prunus, Ulmus, Cotoneaster, Crataegus, Sorbus, Populus, Salix, some Acer and Olive.^{ref1 ref2} If you prune their roots, there is a higher risk of removing a xylem sap flow path to certain leaves and vice versa. Interestingly, if you look at anecdotal reports of ‘summer branch drop’ where trees drop their branches for no obvious reason, the species most susceptible to it appear to be these trees – Quercus, Fraxinus, Populus, Salix and Ulmus are all known for this phenomenon.^ref This implies that a sector has died – perhaps due to embolism (air gaps in the xylem cells) – and the branch has dropped off as a result. The same could happen to your bonsai trees of this species, either by root pruning or by underwatering. Fellow bonsai enthusiasts have reported this in Salix (willow) and Morus (mulberry – also a ring porous species). The upside of this behaviour is the survival of the tree – since the death of one part of it doesn’t cause the death of the rest of it.

Trees which are more integrated include all gymnosperms/conifers and these have more uniform water distribution.^ref Therefore they should be less susceptible to losing sectors due to root pruning or uneven watering. But once you’ve reached the point where they aren’t getting enough water overall (due to overly aggressive root pruning) or energy overall (due to overly aggressive leaf pruning), the tree is more likely to die since it is less able to keep one part alive separately to the others.

Note that trees may also drop branches for ‘economic’ reasons, when they don’t get enough return on investment from that branch, but that’s a post for another day.

Live Veins on Bonsai – do they exist?

Most bonsai enthusiasts will have come across the term ‘live veins’ in the context of bonsai. Live veins are areas of living bark surrounded by deadwood. They are often seen on juniper bonsai, where a section of bark twists around the tree in a dramatic contrast to the white deadwood (Sierra juniper are particularly amazing). But how does this actually work and is it a ‘vein’?

The bark layer on a tree contains the phloem, which is responsible for transporting photosynthates (sugars) and other molecules around the tree – it sits just at the base of the bark next to the sapwood. As new plant organs develop, a connected line of phloem cells is created so that sugars can be transported from these organs (if they are leaves) or to them (if they are sugar consuming organs like roots).

Phloem cells in the leaves connect to phloem cells in the branch, then to phloem cells in the trunk. They are long tubular cells with the main connection point for sap flow at the end of the tube, and minor connections in the sides. Below is an vertical image of sieve tubes of Cercidiphyllum japonicum (Katsura tree) – you can see the sieve tubes in blue, and their connections at a diagonal in dark blue. The brown cells are companion cells which help the sieve tubes to function. In this example there are some connections between the sides of tubes, but most of the connections are end to end.

https://scholarlypublications.universiteitleiden.nl/access/item%3A2951200/view

Phloem & sugars preferentially flow along this natural end-to-end route. One research study looked at what happens when you block a phloem path by girdling. It was observed that initially sugar flow to roots from that branch stopped, then resumed partially by finding another route (probably laterally through the sides of the phloem cells), then the tree grew new phloem and resumed sap flow.^ref

So what does this mean for bonsai? Basically – live veins (or more accurately, ‘live strips’) of bark can supply sugars to roots as long as they have phloem connections to sugar producers (leaves) and to sugar consumers (roots). What is really important is that we work with the orientation of the phloem cells when creating deadwood. Cutting across the grain of the phloem would sever the sap connections and be a form of ringbarking. Instead leaving a strip which goes along the grain of the phloem will provide a leaf to root connection. The phloem tubes will always be aligned lengthwise along a branch or on the trunk – ie. heading down to the roots. The variation you might see is that some phloem & bark spirals around the trunk and some goes straight down. This should be obvious from the bark pattern.

It’s also important to ensure there is enough foliage at the top of the live vein to meet the needs of the tree (or scope to grow more foliage). It’s useful to know that sugar from a leaf is prioritised for use local to that leaf. Leaves provide sugars for the developing shoot apex nearest to them, and flowers or fruit on the same branch; so from an energy perspective, as soon as it can be, a branch is self-sustaining. Lower leaves on a branch typically are the ones exporting sugars to the roots.^ref This might be useful when thinking about deadwood creation and possible options for live veins.

If you are aggressive with your live vein creation, and remove a lot of bark, it’s likely that some roots will die. One way to minimise this is to maintain a reasonable bark/phloem coverage around the base of the tree, and to start the deadwood further up.

One final word – there isn’t really any such thing as ‘finding’ a live vein. All phloem/bark is live until you create deadwood above or on it. It’s more about creating the deadwood and leaving the live vein (or ‘live strip’) behind.

Growing trees from seed

At this time of year (December) I like to grow from seed, just to give me something green to watch.

Ginkgo & dawn redwood seeds growing — Ginkgos & Dawn Redwood growing on a desk in my study (Dec 2022)

To many people the very idea of growing an actual tree from seed to the point that it can be used for bonsai seems completely ridiculous. If, like me, you’ve come to the hobby as a ‘mature adult’ (ahem), you might be doing calculations to work out if anything you grow from seed now will ever make it to a decent size in your lifetime.

But you don’t actually need that long to grow some species to a size that’s suitable for bonsai, and if you are a fan of mame (very small bonsai) it’s eminently achievable. You will need at least 5 years up your sleeve realistically but it’s a nice project to have on the boil while you’re working away at your desk dreaming of retirement. If you choose a so-called pioneer species (trees which establish first in clearings), it will grow very quickly. Such species in Europe include birch, alder, willow, poplar and rowan^ref, in the Pacific north-west these would include Douglas fir, western hemlock, western redcedar and Pacific silver fir^ref. From personal experience, larch grow extremely quickly as well, particularly if they’re in the ground. More info on thickening the trunk of a tree as quickly as possible is in Thickening the Trunk.

This cedar was started from seed about 4 years ago – it’s about 15cm high now. It’s not going to win any bonsai competitions but I like having it in my garden and the nebari is gnarly. Cedar have quite dense short-needled foliage so they don’t need too much work to look decent.

Another way to use seed-grown trees is in small bonsai forests – when they are seedlings they can be positioned very close together in groups without disturbing their roots. When they grow their roots intertwine and their trunks become close which makes for a nice aesthetic. Japanese maple and Ginkgo are good for this.

The advantage of growing from seed is obviously that you can influence the shape of the tree from the very beginning – and in the case of coniferous species which don’t readily backbud (particularly Pinus), this helps get the branch structure right and allows the introduction of trunk movement. The key is not putting the tree in a bonsai pot until its trunk is at the desired thickness as this will basically stop the tree growing. Grow it on in a normal pot or in the ground until it gets the trunk size you want, shape the trunk as you go, then proceed as per the instructions for shaping bonsai.

A key point to understand when growing from seed is that seeds come from sexual reproduction between male and female gametes (in plants defining this is more complicated than in animals due to the two generations involved in reproduction but that’s a subject for another post). Sexual reproduction mixes up the genes of both parents in the offspring (actually it mixes up the genes of all four grandparents^ref), which means that you cannot grow named varieties from seed – as named varieties always have specific genetics.

You probably don’t want to grow from seed if your goal is to have flowering or fruiting trees, or trees with cones. Flowers only develop when a plant reaches sexual maturity which can take decades in some species. This means seed growing realistically is best for foliage trees. Clonal propagation is better for flowering trees (cuttings or micropropagation) since it uses the parent plant (including its age) as the starting point.

The other thing I have noticed is that seed packets and advice online appears to vary widely and can’t all be correct. There are many factors which have evolved to help seeds be more successful in germinating and it’s beyond the scope of this post to cover them all. But whenever trying to germinate a seed, always take a look at Google Scholar to see if any research has been done on the seed in question. One also needs to consider where the tree lives – for example Swamp or Bald Cypress lives in warm, humid areas of the Americas where the temperature doesn’t go below 5 degrees C – why on earth would they need cold stratification for several months? And in fact – they don’t.^ref

Many seeds exhibit dormancy once they have dried out, this is a state of suspended animation which “improves survival by optimizing the distribution of germination over time”.^ref The plant growth regulator abscisic acid (“ABA”) is involved in initiating and maintaining this state – according to one study, it prevents seeds from absorbing water which is a key mechanism required to break dormancy.^ref The seed coat is known to be a site of ABA synthesis^ref and the removal of it can enable germination to proceed, in the presence of water (I recently did this with some wisteria seeds and they immediately germinated).

Some hard-coated seeds use their coat as the barrier to water entry, and can be helped along by nicking the coat with a knife, sanding down the end of it or otherwise allowing water to enter (known as scarification). For really hard seeds, acid is recommended! In this study seeds soaked in concentrated sulphuric acid for 3h showed the highest germination (but 4h was too long).^ref On the other hand, gibberellic acid is also known to induce germination. It is used in in-vitro micropropagation to break seed dormancy so a seed can be used as an explant (the material from which to propagate new plants) (Johnson, 2020). Gibberellic acid can be purchased from suppliers to the hydroponics/plant micropropagation trade. Some seeds have embryonic dormancy (ie. they are prevented from germinating due to substances occurring within the embryo or seed itself)^ref – this is harder to get around, without destroying the seed.

It’s also useful to find out if your particular seed requires light – in one study all 8 taxa included had significantly improved germination when exposed to white light, and in some red light was sufficient.^ref

The next section is not based on science at all, but reflects my personal experience of growing trees from seed:

Ginkgo – a very easy tree to grow! The hardest part is finding a female tree – they are unpopular because of the stink that the ginkgo fruit creates as it rots. The fruit ripens all summer and then falls from the tree apparently ready to germinate – although the later they fall, the better they will germinate^ref. I put them in the fridge for a couple of months and then plant them in some potting compost, this results in quite a high germination rate. I’ve had similar success just putting them into a pot over the winter and they eventually emerged in the late spring.
Cedar (cedrus) – also very easy once you’ve managed to get plump seeds from a cone. Find a semi-falling-apart cone and extract the seeds – you’ll see which ones look healthy and which don’t. Put them in the fridge in a bag with a bit of moist paper towel, wait a month or so and they will start germinating. Then you can pot them into little pots to grow on. My interest in cedar came from seeing this amazing video by George Omi, wiring a blue atlas cedar as he was taught by his father in the 1950s: https://georgeomi.wordpress.com/2016/10/26/my-bonsai-video/
Oak – oak trees are the easiest plant to grow, they have all the food they need in their acorn, they tolerate heat, drought, frost and being treated badly and they backbud fantastically when cut back. I make oak groups planted on mounds & rocks which are really easy and look nice even after just a couple of years. To germinate – take your acorn, stick it in a pot over winter, keep giving it some water occasionally and hey presto, an oak tree will grow.
Horse Chestnut – the same as oak. Stick in a pot, wait. A tree will emerge. A bit more tricky to make good bonsai but I have a couple of nice mame.
Crab Apple – a common choice for bonsai but note that seeds will reflect genetic variation and will probably be different from the tree that the seed came from. Extract the seeds, pop them in the fridge and plant out a month or so later.
Japanese Maple – also easy to grow. Gather fresh seeds from the tree in autumn, put them in the fridge in a bag with some moisture for 3-4 months. They should start to germinate in the fridge and at this point bring them out, plant them up and let them grow. Note as per above you cannot be guaranteed of similar characteristics as the parent tree due to the seed being a mix of it’s grandparents’ genes.
Dawn Redwood – I’ve grown dawn redwood straight from a fresh seed pod I collected in Richmond Park in London – no refrigeration was required (although the germination rate was quite low).
Hinoki Cypress – if you’re lucky enough to find one with a seed cone, give it a shot, I found they germinated surprisingly easily.

Anyway, if you have the winter blues, get some green in your life and grow some trees from seed!

Bark (Cork Cambium)

Weirdly the definition of bark seems to be variable depending on what book or article you read. As my main reference for this post I’m using Romero’s “Bark: Structure and Functional Ecology” accessible via a free account on JSTOR here.

According to Romero, bark is all the tissues on a tree outside the vascular cambium – that is everything from (and including) the phloem outwards. The inner bark is simply the phloem (both the conducting layer and the non-conducting layer). The outer bark collectively is known as the ‘ritidome’. This is where a diagram is needed! This is the best one I could find (from the University of Vigo website).

https://mmegias.webs.uvigo.es/02-english/2-organos-v/guiada_o_v_tsecundario.php

The ritidome contains another meristem within the tree – the cork cambium. The cork cambium (called the phellogen) works similarly to the vascular cambium – it has a layer of stem cells which create layers of differentiated cells. In the top diagram there is one phellogen, a pale beige line. On its left is the phelloderm – this layer is not always created depending on the species but if present it contains living cells. On its right is the phellem, or cork, this is the thickest layer and these cells become suberised/lignified (impregnated with suberin or lignin) so they become the corky bark texture we are familiar with. All of these ‘phello’ layers together are known as the periderm. A microscope image of these layers in an old Pinus sylvestris are shown in the image below:

https://link.springer.com/book/10.1007/978-3-319-73524-5

What’s interesting is that multiple periderms can develop over the life of the tree. A new periderm will develop on the inside of the old one, pushing that periderm layer to the outside. These aren’t always continuous either, and are affected by the structure of rays and growth rings within the phloem, which is why old bark has more character. Periderms can be shed, or retained, depending on the species. The pattern of a tree’s bark is genetically determined by the structure of the phellem cells which are produced, and by the location of successive periderms. Smooth bark can come from a single periderm and continuous shedding, while rough bark is created when the periderm has structural fractures or constraints – for example due to the development of rays (radial lines of cells in the phloem).

See this image of old bark from ‘The Plant Stem – A Microscopic Aspect’ by Schweingruber & Börner. It shows how the bark splits apart as the xylem and sapwood layers expands from the inside of the tree.

Bark is made up of quite different materials from the wood or foliage of the tree, with considerably more mineral compounds (such as ash). Both the inner and outer bark contain so-called ‘extractives’ (organic substances which can be dissolved in solvents, such as polyphenols, alcohols, resin acids, vitamins, alkaloids, pigments including flavonoids, terpenes, steroids and essential oils) as well as suberin, lignin and cellulose. Bark chemistry in general is poisonous and indigestible, representing a good barrier to herbivores or insects. As the inner bark is living tissue, it can produce its own metabolites as a defence mechanism, whereas outer bark is dead tissue and relies on its physical structure and the substances impregnated into its cells to repel invaders.

Bark helps trees reduce water loss, prevents pathogens entering and provides a protective layer to protect the living tissue underneath from mechanical and heat/cold damage. It provides a flexible covering for the tree which can absorb the stresses of bending and twisting, and prevent cracking of the trunk.

As bonsai enthusiasts, bark is a key part of the look of our trees and we want to encourage interesting bark with good character. Since the cells of bark are renewing from the inside, the only way to modify the appearance and texture of bark in a natural way is to manipulate the periderm – as mentioned above, this causes fractures and divergence of the growth habit of the phellem. Harry Harrington has a video showing exactly this on a young black pine – he wires the tree so that the wire interrupts the shape of the periderm and forces the phellem to grow in a twisted habit.

https://www.youtube.com/watch?v=f0fe6v7X0MQ

I’m a bit nervous by the suggestion to leave the wire in as this seems like it would then cut through the phloem and ultimately the xylem. Whilst the twisted shape should leave continuous conducting cells for both, I’d be concerned at how much water and photosynthate conducting would be reduced. If possible to remove the wire I think I would.

The Endosphere

Although it might sound like we’re veering into science fiction territory, the endosphere is actually part of a plant’s microbiome, like the rhizosphere and the phyllosphere. It is the community of microbes which live inside the plant itself – that is, between and in its cells. It’s only in the last few decades that research on the endosphere has accelerated – this has found that in fact a wide variety of microbes including bacteria and fungi live inside plants for at least a part of their lifecycle.^ref They are known as endophytes – and some of these are symbiotic whilst others can be pathogens.

Endophytes are found throughout the plant, in leaves, roots and stems, in spaces between cells as well as within cells themselves; the greatest number are found in roots, then leaves, then fruit/flowers. The types of microbes in residence depends on the microenvironment in each part of the plant, the specific physical and chemical characteristics in each environment attract different microbes.^ref

To enter the plant in the first place, microbes come from outside, through the root tips and hairs, through stomata and trichome pores in leaves, fruit & flowers, through holes in the stem made by insects, or by producing enzymes which break down plant cell walls to create an opening. Often these microbes are present in the rhizosphere or phyllosphere, and they migrate into the plant for all or part of their lifecycle.^ref Usually they live between cells, but some examples of bacteria and fungi entering plants cells have also been found. Endophytes can be transmitted vertically (from mother plant to seed), and horizontally (from the outside environment).^ref

Of all the spheres, the endosphere is the hardest to study, so there isn’t a huge amount of research which demonstrates what endophytes actually do when they are inside plants and how the host plant might benefit. Some findings are that endophytes are able to detect Reactive Oxygen Species (“ROS”) and may be able to help plants fight high ROS levels (eg. acting as an anti-oxidant).^ref Others have found endophytic fungi which produce the plant growth regulators gibberellic acid and indole acetic acid (auxin), and that this contributes to greater root & shoot mass.^{ref1 ref2} One study found an endophyte which conferred resistance to Dutch Elm Disease in vitro^ref. Finally a large number of endophytes associated with trees have been found to produce Taxol^ref, the best-selling cancer drug ever manufactured^ref and this promises to be a way for greater volumes of the drug to be created.^ref So like bacteria & fungi across the microbiome, these microbes appear to be pop-up pharmacies within the tree.

The endosphere probably doesn’t need to be your prime concern from a bonsai perspective. Like the other components of the tree’s microbiome, you want to foster a healthy one, which benefits the tree, and not an unhealthy one. Doing this mainly involves not killing them off!

The Phyllosphere

The phyllosphere is the community of microbes which live in and on a plant’s leaves. I had no idea that this even existed before writing the section for this website about the microbiome. Of course, if you think about it for a microsecond, it must! Our world has more microbes than anything else by several orders of magnitude, so, there must be microbes in a tree’s leaves. But the phyllosphere has been less publicised due to the intense interest in the rhizosphere (root microbiome) and in its beneficial microbes which can help plants grow by manipulating the soil and root environment.

The phyllosphere is different to the rhizosphere in that its main microbial members are bacteria and not fungi, although fungi are present, along with some archaea. It has been estimated that there are 1 million -10million bacterial cells per cm² of leaf surface.^ref And worldwide, the phyllosphere is an important microbiome, with a possible 10²⁶ cells! But it’s a relatively hostile environment, with fluctuating temperature & humidity and limited nutrients on the leaf surface. The shape and structure of the leaf at a microscopic level provides a range of microhabitats for bacteria, including the bases of trichomes, stomata, hydathodes (leaf pores), grooves along the veins, epidermal cell junctions, and cuticle depressions.^ref A study into tree phyllospheres found 129 bacterial species were significantly associated with the gymnosperms including Armatimonadetes, Actinobacteria, Bacteroidetes, Acidobacteria, TM7, TM6, Deltaproteobacteria, OD1, Fusobacteria, and FBP and 79 with the angiosperms including Chlamydiae, Proteobacteria, Gammaproteobacteria, Alphaproteobacteria, and Firmicutes.^ref

Bacteria on a leaf surface, from: https://www.ethlife.ethz.ch/archive_articles/090915_blattleben_kw/index_EN.html

What determines the microbial mass and mix on leaves is a combination of different factors, including the nitrogen content of leaves, the specific leaf area (related to carbon availability), wood density and seed mass^ref and the largest part of the variation seen between phyllospheres comes down to the host species. Conifers have a different phyllobiome than other species, for example they have less ice nuclei active bacteria (bacteria which can cause ice crystals to form) and they have Frankiaceae which is involved in nitrogen fixing in the soil.^ref Location also plays a role, with urban trees displaying a different phyllosphere makeup – correlated to ultrafine particulate matter and black carbon on the leaves.^ref

Bacteria usually require an available carbon source. You might be surprised to know that similar to roots, leaves also produce exudates (substances they exude into the environment). These include a wide range of carbon compounds, such as carbohydrates, amino acids, organic acids, and sugar alcohols, primarily products of photosynthesis, as well as proteins, oils, secondary metabolites and mucilage.^ref These carbon sources are not the only ones – the Methylobacterium species can use methanol exuded from the leaf from the breakdown of pectin as its only carbon source.^ref One of the bacterial families found on birch – Rhodospirillaceae – is able to photosynthesise, removing the dependence on leaf carbon sources. Another study discovered that certain phyllosphere bacteria can use diesel for their carbon source!^ref

Similarly, bacteria in the rhizosphere produce a range of substances just like they do in the rhizosphere – biosurfactants which reduce surface tension, degrade hydrocarbons and improve moisture levels and dissolved nutrients on the leaf surface, plant growth regulators which open up the leaf cells and cause them to leak nutrients, enzymes which help break down nutrients and protect the bacteria from solar radiation, and phytotoxins (if the bacteria is a pathogen).^ref

The benefits of phyllosphere microbes to their host are similar to those in the rhizosphere – for example Acetic Acid Bacteria have been found to perform nitrogen fixation within the needles of Pinus flexilis^ref, others confer resistance to Bursaphelenchus xylophilus-induced pine wilt disease^ref, some phyllosphere fungi produce zeatin, a cytokinin (plant growth regulator)^ref and others auxins, some also produce anti-freeze proteins which lower the freezing temperatureon the leaf.^ref Bacteria are implicated in the bioremediation of harmful chemicals or pollutants^ref, improved tolerance to stress, production of proteins which trigger the plant to mount defences against pathogens as well as those which attract populations of beneficial fungi.^ref

So, just like the rhizosphere, the phyllosphere is a very active place with many microorganisms playing different roles and constantly interacting in a dynamic ecosystem. What this means for bonsai is that there likely are organisms in the foliage which benefit your plant. Similar to the advice in general around the microbiome, applying fungicides, anti-bacterials and chemical pesticides can kill phyllosphere organisms so avoiding this is a good idea.

Rauhs Model

Rauh’s model represents the Cupressaceae family (cypress, juniper & redwood), some Araucariaceae, the Pinaceae family including most Pinus species, the Podocarpaceae family as well as angiosperms such as oak, maple and ash. It is a very common model for trees we encounter in bonsai.

The architecture according to Rauh’s model includes a monopodial trunk (one which continues to extend, and does not terminate) which grows rhythmically (on a seasonal cycle) and so develops tiers of branches, the branches themselves morphogenetically identical with the trunk (ie. they develop in the same way). Because the branches are identical, the trunk can be less dominant in this form and another stem can take over if the trunk is removed or damaged. Flowers and reproductive organs are always lateral and without effect on the growth of the shoot system.^ref Often these are on short shoots.

https://horizon.documentation.ird.fr/exl-doc/pleins_textes/divers20-09/09318.pdf

An essential feature of Rauh’s model is that branches develop mainly by ‘prolepsis’, from dormant lateral buds close to the resting terminal bud.^ref Prolepsis in this context means ‘the discontinuous development of a lateral from a terminal meristem to establish a branch, with some intervening period of rest of the lateral meristem’. So basically there is a gap or period of dormancy before the bud extends to form a branch. Whilst this might seem obvious to European readers, actually this mode of development is not what happens in other parts of the world, particularly the tropics, where continuous growth occurs, and this difference creates differences in the tree architectures visible in those different places.

It was noted in one study that Apple trees follow Rauh’s model during their juvenile phase but a different one during their reproductive phase (ie. their flowers terminate shoots and affect the branching after this point).^ref

Massarts Model

This architectural model is associated with many conifer families including Abies, Picea, Sequoia, Metasequoia, Cedrus, Taxodium, Taxus, Cephalotaxus, Ginkgo & Ilex aquifolium. The pattern for this architecture is a vertical, dominant trunk with rhythmic growth and which consequently produces regular tiers of branches at levels established by the growth of the trunk meristem. Branches are plagiotropic (horizontal) either by leaf arrangement or symmetry. The position of flowers/cones is not significant in the definition of the model (which means they don’t terminate the branches and have any effect on the structure).

One study in Indonesia looked at rainfall ‘stemflow’ and ‘throughfall’ (basically how much water runs off the tree into the ground causing erosion) and found that the Massart’s model tree (Pterocarpus indicus) had the highest leaf surface area and caused the least erosion from water runoff.^ref However the study used angiosperms with broadleaves and not gymnosperms with needles.