Tag Archives: Age

Old Trees

A premise in bonsai is that the best bonsai look like old trees. In my opinion a lot of ‘best’ bonsai look more like fantasy trees from a Studio Ghibli film, and not at all like real trees. I live near Richmond Park in London which has 1300 veteran trees, of which 320 are considered ‘ancient’ (in the third and final stages of their lives)ref and I know that they have reverse taper, weird branching, knots, breaks and all sorts of attributes which bonsai rules would aim to avoid. So what do we know about old trees and how they actually look?

The Woodland Trust Ancient Tree Inventoryref identifies the following general characteristics for ancient trees:

  • Crown that is reduced in size and height
  • Large girth in comparison to other trees of the same species
  • Hollow trunk which may have one or more openings to the outside
  • Stag-headed appearance (dead branches in the crown)
  • Fruit bodies of heart-rot fungi growing on the trunk
  • Cavities on trunk and branches, running sap or pools of water forming in hollows
  • Rougher or more creviced bark
  • An ‘old’ look with lots of character
  • Aerial roots growing down into the decaying trunk

For species-specific attributes check out the Ancient Tree Inventory websiteref which outlines specific attributes for eleven of the most common UK species (Oak, Ash, Beech, Yew, Sweet Chestnut, Alder, Hornbeam, Scot’s Pine, Hawthorn, Field Maple & Lime).

Another studyref outlines some of the expected characteristics of ancient and veteran trees as “a hollowing trunk, holes and cavities, deadwood in the canopy, bark loss and the presence of fungi, invertebrates and other saproxylic organisms.” And citizen submitted recordings of tree measurements across the UK in the same study showed that “ancient trees have larger girths in general than veterans, which in turn are larger than notable trees.”

Trouet in her book Tree Story says that in old trees the top of the tree has ‘caught up’ with the bottom, so the trunk becomes more columnar, whereas a middle-aged tree is more tapered. She also says that branches thicken. This perhaps goes against the bonsai edict of taper above all else.

Another study reports that mature trees have only short shoots – these have smaller leaves and more foliage per shoot that more immature long shoots. Read more in shoots.

So to create a bonsai which looks old, you want it to have a very wide, columnar but hollowing trunk, rough or deeply grooved bark, holes, cavities, dead & broken branches, a compact canopy with deadwood, small leaves on short shoots, and ideally some fungi and a busy community of invertebrates.

Reducing leaf size

In bonsai a small leaf is preferred, because this give the impression of the proper scale of the tree. But how small do leaves need to be? Let’s take the beech tree out the back of my house. It’s about 25m tall, with a 75cm diameter trunk, and its leaves are 8-10cm long. If you were to actually scale this down to a generous bonsai size of 50cm tall, it would have a trunk of only 1.5cm and leaves of 2mm long!! Which is obviously ridiculous. But even if we can’t get a bonsai tree down to the precise scale of its full-sized siblings, we do want to reduce the leaf size to make the tree look more realistic.

The first thing to say, if it isn’t already obvious, is that you can’t shrink a tree’s leaves – they have to grow small in the first place, or be prevented from growing as large as they could.

Achieving the former is all about selecting a small-leaved variety of tree. Many species have small-leaved varieties which lend themselves much better to bonsai than their large-leaved siblings. Unfortunately if you are selecting a variety with small leaves (vs a species) you will need to use a vegetative form of reproduction to obtain your tree – a graft, a cutting or an air layer. I’ve had some success collecting seeds from small-leaved Japanese maples, which sometimes pass their diminutive leaves to their progeny.

If you happen to have the opportunity to analyse a prospective bonsai tree’s genome, you’ll prefer to choose haploid trees (with just one set of chromosomes) and avoid polyploid trees (with more than two sets of chromosomes) – as can be seen in this image of different ploidy ginkgos, the leaves are much larger for trees with more replicated genetic material. Unfortunately determining ploidy requires a sample of your tree, a flow cytometer and some lab skills most of us lack!


Achieving the latter (preventing the leaves from growing large) basically involves disrupting the leaves as they are growing to stunt them before they grow to their full size.

Ennos (2016) reports that ‘thigmomorphogenesis’ – mechanical perturbation by the wind, results in smaller leaves. This study on Ulmus americana seedlings found that total leaf size was reduced by 40% – but only when they were exposed to the highest level of ‘flexures’ (a proxy for wind).ref Another study which I can’t access behind a paywall is summarised as finding “in needle-shaped leaves the elongation of the leaves is inhibited”ref. Researchers think that mechanical perturbation of plants triggers the production of ethylene, and its cross-talk with auxin, both plant growth regulators. So putting your trees in a windy position may result in smaller leaves (and shorter internodes). But be aware this will also increase transpiration so they will need more water.

The other mode for leaf size reduction is to starve the tree of resources when it is making leaves, in one form or another. This leaves less energy available for leaf production leading to smaller leaves. Various forms of defoliation achieve this, such as:

  • partial or full foliage removal, forcing the tree to use up resources growing a new flush of leaves
  • bud pinching, which is personally the best way I’ve seen to reduce leaf size on deciduous trees
  • maintenance pruning – cutting off leaves when they exceed a certain size – so that new leaves are grown and only the ones below a certain size remain
  • note – the above should not be used on conifers

On conifers, pruning back the candles to a few needles at the base will apparently trigger another flush of budding, and due to depleted resources the needles will not grow as longref (since leaf size is apparently not very interesting commercially, there really is little research on how to achieve it).

Another technique is to deprive the tree of fertiliser until it has leafed out. I think this might weaken the plant over the long term but it’s apparently popular for Japanese maple enthusiasts (for their trees, not for them!)

Finally, it’s important to balance leaf size reduction techniques with the tree’s energy requirements because reduced leaf area will reduce photosynthesis.

How big should a bonsai trunk be?

It’s a how-long-is-a-piece-of-string question because the trunk on a bonsai doesn’t exist in isolation, it exists relative to the foliage, nebari and pot. Because trees undergo secondary thickening however, their trunks expand with every year. So, older trees have thicker trunks.

For another post I found this data below. It shows mass rather than volume, but you can see that as trees get older and bigger, their mass skews to the trunk, which ends up being 80%+ of the total mass of the tree. Whereas at the beginning of the tree’s life, on the left hand side of the chart, the leaf mass exceeds the stem mass.


So in general if we want to emulate older trees, our bonsai needs to be weighted towards a fat trunk (and main branches). Note also that the root mass doesn’t go below 20% – the main contribution to mass in a root are the big structural roots which are largest within a metre or so of the trunk. So this gives an indication of how big a nebari should be.

But as mentioned above the trunk exists relative to the canopy so what do we know about the ratio between the two? One measure which is used in forestry is the live crown ratio which is used as an indicator of tree health. The live crown ratio is the vertical length of the foliage as a percentage of the total tree height.ref Some studies have measured crown ratios for different species (usually in managed forests):

  • A stand of coast redwoods: between 30%-50%ref
  • Douglas Fir: in the 80% range for 20y old trees, down to the 40% for 40y old trees and up to 60% range for 450y old trees
  • Turkey oak: between 20%-50%

Also interesting is the crown radius to trunk diameter. A study measured this for 22 different species including both angiosperms and gymnosperms and came up with equations that represent the ‘allometric types’ for each species – that is an equation that describes how a tree’s dimensions change over time.ref For example for common beech (Fagus sylvatica) they found (see table 5) that the following equation could be used to calculate the crown radius given a particular trunk diameter:

ln(crown radius) = 0.0111 + 0.4710 x ln(trunk diameter) ; (note crown radius is in m and trunk diameter in cm)

if we have, for example, a 1m wide trunk, you could calculate the crown radius as follows: 0.0111 + 0.4710 x ln(100) = 2.180 so crown radius = e2.180 = 8.85m – this actually then gives a crown diameter of 2 x 8.85 = 17.8m. So an old beech which has achieved a 1m wide trunk could have a nearly 18m crown diameter – which means the trunk is about 5.5% of the width of the crown.

Because I love a bit of excel, I took the data for the rest of the species to work out the trunk/crown diameter ratio for each of them based on a 1m trunk – and here is the answer:

So for most species a 1m trunk will be between 4% and 10% of the width of the canopy. I couldn’t resist looking up Auracaria cunninghamii to see why it was different – it looks like the canopy habit is quite narrow which increases the trunk/canopy ratio#.

If you have Douglas fir, this study found that “the vertical distribution of branch volume shifted toward the upper-crown with increasing tree age”ref The mechanisms at work include self-pruning, branches dying and falling off and then adventitious branches growing in the spaces. As they included a picture you can see it makes quite an obvious difference to the look of the tree.


That’s just one species though – the shape of old trees is going to be to a certain extent genetically determined so different species will have a different mature look in terms of their shape and branch distribution.

Conventional bonsai wisdom says a tree needs to have good taper in order to look old. This means it is thicker at the bottom than at the top. But tree-ring researcher and dendrologist Valerie Trouet in her book Tree Story says otherwise. She says “once height growth has stopped in an older tree, then the upper part of the stem will start to catch up, it’s girth increasing year after year, and the stem will gradually take on a more columnar, rather than tapered, look….the tree’s limbs also continue to thicken; branches and roots of old trees often are quite sizable.”

What we are trying to achieve with bonsai is small trees which look like mature, large ones in nature. So the size of the trunk, whether it has taper or not, needs to be in proportion with the canopy and the roots, and the branches should start anywhere from the 20% to the 50/60% of the total tree height mark and be in proportion to the trunk as in the table above.

There are more attributes which make a tree look old, to learn more check out this post: Old Trees.