Bonsai growth – Single Limiting Factors

I would like to share with you my thoughts on the implications of Limiting Factors for tree growth. The basic theory is that the optimum rate of growth of a bonsai will be limited by the one resource that is in the least supply. Obviously different trees require different balance of the essential factors but the basic theory remains. To recap the factors are water, light and the wide range of nutrients.

Leibigs Law “states that plant growth is limited by a single resource at any one time; only after that resource is increased to the point of sufficiency can another resource increase plant growth”*

We need to be mindful of this when choosing soils for re-potting, when watering and fertilising, and when choosing a place for our trees in the garden. For instance a light demanding tree can be well watered and fertilised but without adequate sun to photosynthesize, will always fall behind its own maximum potential.

It is interesting to see how plants respond to a Limiting Factor: “One of the major mechanisms by which plants adjust to resource imbalance is by allocating new biomass to the organs that acquire the most strongly limiting resource”*. This means that they can grow more leaf that root or vice verse – overall making the plant out of the balance that is so critical for quality bonsai.

Consider how a bonsai will respond to our attempts to improve growth with an oversupply of some nutrients without addressing the one that is missing. A plant will ‘spend’ Carbon to acquire Nitrogen and ‘spends’ Nitrogen to acquire Carbon. Essentially the plant can compensate to access the limiting factor, but again the cost is clearly a reduction from its maximum growth potential.

This was reinforced for me recently when I was fortunate enough to listen to an expert in hydroponics (the growing of plants without soil) speak and was very impressed with the results achieved by supplying the exact nutrient requirements of each species of plant (chemical analysis of leaves used to determine what the Limiting Factor was in each case).

One side effect is unexpected: “Individuals within a species tend to increase leaf and root longevity in response to nutrient stress.”* Starving a plant of some nutrients makes them tougher in the long term. I have been experimenting with this by holding back the Spring time feeding regime by a few extra weeks with the result so far showing no difference in tree health, and trees that (anecdotally so far) seem more resilient.

*All quotes taken from: Chapin, Bloom, Field, Waring (1987) Plant Response to Multiple Environmental Factors, Bioscience Vol.37 No.1

Oak Bonsai – how to make them drought tolerant

I have just been reading the latest scientific report on the development of cells in oak, and it is clear that there is some useful information here for Bonsai.

The research* suggests that trees which have abundant water when laying down earlywood (formed during the springtime flushing) causes the trees to create larger vessel diameter in the conductive tissue of the stem. Conversely less available water at this time causes a smaller diameter of vessel to form.

Later in the season the larger vessels need more water to fill them and to allow the natural transpiration process to ‘pull’ water up from the roots to the leaves, if the volume of water is not high enough them the leaves do not receive the water and will suffer scorch in hot sun. Smaller earlywood vessels need less water to active the ‘pump’ and can more efficiently provide water to the leaves, making them more drought tolerant in high summer.

So water less in spring and you will need to water less in summer.

However, the side effect of this is that for a tree in development we want the largest diameter growth on the stem and so can directly effect the annual increment by watering heavily in spring – more wood is created at this time than any other time of year as the late-wood (laid down in summer) is shown not to be effected by climatic conditions but by overall tree health.

Now the research paper was specifically on oak so I will have to find out if the same applies to other species, but it does seem to be a clever evolutionary trick, and I imagine similar mechanisms exist in other species.

Interesting stuff.

* Price (2015) Shake in Oak: an evidence review; Forestry Commission Research report

New series: why trees grow the way they do, a photo essay…. part one

I am a firm believer that if you understand how a tree grows naturally and then incorporate that into you Bonsai the final image will be inherently more convincing. I was reinforced in this view when an otherwise good tree was downgraded in judging because it looked like a Pine – the only problem was it was a Hawthorn!

So I want to talk about the differences between tree species. As this is quite a large topic I will break each blog into one key difference to help keep it simple – building up into a hopefully useful source of reference for everyone. So to begin:

What makes trees grow upwards?

Trees are fundamentally divided into two categories, they will either be Phototropic or Geotropic.

Phototropic trees are ones that are influenced by light. Trees need light to photosynthesize and any leaves that cost more energy to grow than they can produce are useless and the tree will shed them, if too many leaves are in the shade and get cast then eventually that branch will also be dropped. If the majority of light comes from one side then the tree will develop a bend or lean in that direction. Most broadleaved trees fall into this categoryP1000489. The photo shows one large old beech dominating a smaller one which has always grown under its canopy but with adequate side light to keep it alive, however it has developed a persistent angle of growth so it is now at 45 degrees, only eventually developing a more upright tip when out from under the crown of the dominant tree.

P1000500Geotropic trees are influenced by gravity. In any situation they will grow directly upwards. They can still loose branches if they don’t receive enough light but this can result in a straight tree with branches only on a single side. If a Geotropic tree gets blown over by the wind it will direct itself up again. Most conifers fit in this group.

Apologies for the poor photo here but if you look closely you can see that the stem blew over many years ago to lay along the floor, the angle of bend suggests that the now vertical stem was previously a branch, but as the closest part of the tree to vertical it has taken over as the leader, and then straightened to perfectly upright. A Phototropic tree in this situation would not have grown upwards but come towards the camera where the source of sunlight can be found.

To apply this to Bonsai a tree with a lean or twist needs it to be ‘explained’. Plantings involving more than one tree P1000493need to be consistent in how the trees interact. Look at this twin group, the tree on the right, if taken as a separate tree, would look ridiculous but when added together with its neighbour forms a single consistent canopy and looks interesting and acceptable, the deformation in the trunk and its one-sidedness is ‘explained’ by the dominant neighbour and can be enjoyed. Contrast this with a pair of Western Hemlock (below) just coming into bud. The photos show the overall image first, then a couple of angled shots to show how the large tree dominants the smaller one. Hemlock are strongly Geotropic so if I bent the trunk of the small one it would not have been ‘true’ instead it now gives the impression of having grown through the crown of the larger one, or the larger one over-topping the small one threateningly, or they coexist in harmony of mother embracing a child – you are left to choose, but you will not get distracted by a false image. These will rapidly fill out into a single canopy and start to look very interesting.

P1000504 P1000505 P1000506

If you still needed any convincing that a Bonsai tree needs to be grown to match its true nature, but think that maybe you could not really spot something that wasn’t true then take a look at these two photos and see which one strikes you as correct:

P1000491       P1000492

In each image, when you look at the small trees in the background as well as the large dominant trees in the foreground there are trees growing directly vertical and some that have a lean. So does your instinct tell you this is a flat site with leaning trees or a slope with vertical trees??? Answer in part two…

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