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 agoref 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.
By contrast the vascular system in the leaf of a flowering plant is much more sophisticated with many different vein patterns across speciesref 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’).
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 patternsref. 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:
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 leavesref, 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
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 yearsref , the scale leaves on Thuja plicata live on average 8 yearsref, 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 Cupressaceaeref, Pinaceaeref and Podocarpaceaeref 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.