After a varied 'rainy season' where some areas have had reasonable rainfall, while others have had almost nothing, the leaves on many tree species are turning yellow, gold and red. Have you ever wondered why some trees remain green while others change colour and shed their leaves as soon as the dry season begins? We need to look at the plant physiology (structure and function) to understand what we see. Many folk will understand the basic workings of plants from their school biology lessons but the intricate dynamics of plant community interactions with their environment are fascinating. So let us start with the real basics.
All plants need some moisture and the soil type will have a marked effect on available moisture. Bear in mind that clay soils will hold moisture for longer than sandy soils, which drain faster. The heavier, loam soils along rivers and drainage lines will also hold moisture for longer while shallow soils on ridges will dry out faster. Areas with protruding base rock will normally indicate shallow soils. Soils with poor grass cover will catch less water and also dry out faster.
Among plant communities there are species that flower and bear foliage during our 'winter' months rather than during the 'normal' wet season. These species often provide food for various animals throughout the year and are normally associated with sources of moisture.
Examples are the 'potato bush' - Phyllanthus reticulatus - the small flowers of which smell like 'baked potato', normally in low lying areas, during the winter months. The berries and foliage of this species are eaten by animals. Others are the tree-parasite plants such as various Viscum spp that provide fruit and the Tapianthus spp which provide flowers and fruit for certain bird species during the dry months. Notable is the 'anaboom', that used to be called Acacia albida which has been changed to Faidherbia albida because it bears spines instead of thorns. This species comes into leaf during the winter months and is bare during summer an almost complete reversal to the norm thus giving much needed shade during the dry season along watercourses. There are many other species active during the dry season.
Many plant species have various methods of cutting down 'transpiration' (loss of water) from their leaves thus allowing them to retain their leaves while other species are bare. Some species have particularly 'oily' leaf content which allows them to remain evergreen. However, none of these plants are immune from severe desiccation.
Plant structure and Function
Bear in mind, as a general rule, that 50 percent of most plants is underground in terms of 'biomass' - living matter. Some much less and some much more to this rule - for example a carrot and a cabbage. The underground portion we refer to as 'roots' and these are the 'foundations' of the plants. It seems incredible, when looking at the size of some trees, that there can be so much growth underground. A good illustration of this fact was given when the roots of a single, large marula tree were unearthed and found to cover an area of five hectares - and that did not take into account how deep they penetrated the soil.
As an indication of root depth, the roots of a 70 metre 'gum tree' were found 80 metres down a borehole. So it is not that hard to accept that plants are generally 50 percent underground. Animals often lick or chew soil in search of required minerals, where these are lacking in their diet. This cannot be construed as 'feeding' as it provides no 'food energy'. It is just the same as us taking mineral supplements. The only way food can be manufactured from the minerals is through 'photosynthesis' - a process in plant leaves combining chlorophyll, minerals, water and sunlight. To do this the plant must have light and must be able to obtain the water and the minerals.
You cannot pass fine salt through tissue paper - but make a salt solution in water and it will easily pass through the paper. Catch it in a saucer and dry it out and you have your salt again. (By the way, if you want to try and obtain salt in the bush, dissolve the fine ash from 'hardekool' or 'mopani' in water, filter it and evaporate it to leave a small amount of salt). Anyhow, the point is that root tips cannot 'feed' or take in the minerals from the soil except in solution. There has to be moisture present. No moisture - no feeding! The roots pass the water and minerals up the stem and along the branches to the leaves.
The major route for this transport is through the 'cambium' layer - between the wood and the bark. The inner wood is mainly for support. Where trees are ring-barked or sustain massive bark damage the supply route is disrupted and the leaves dependant on that supply route die. The minerals and some of the water are used in photosynthesis but the majority of the water, which is a transport medium, is passed out or 'transpired' through small holes (stomata) in the leaves. Test this by tying a plastic bag over a bunch of leaves still attached and growing on a tree.
You will catch quite a good amount of water from certain species and, except from certain plants with a bitter or toxic 'sap', can obtain a little drinking water by this means. The plant must balance intake with output. If it didn't transpire it would 'drown' in excess water. As the soil moisture dries up the plant is unable to take in enough water, and it would pump itself dry by continued transpiration. When the amount of water taken in becomes less than that pumped out, the plant reacts by closing the stomata and withdrawing the nutrients (including the green chlorophyll) from the leaves back down into the trunk and roots for storage.
Hence the reason for elephants and other animals ringbarking and digging up plants to feed on the root stock late in the season - to get to the main source of nutrients. As the leaves are deprived of chlorophyll they turn yellow and red with the remaining carotene and other chemicals, eventually to turn brown and die. During this process a 'corky' seal is formed at the leaf base where it is attached to the plant. This seal prevents any loss of moisture back through the leaf attachment. In this state the dead leaf is easy to detach and ready to be blown off by the August winds as part of the nutrient cycle.
Should any damage to the bark or branch occur before this process has taken place, the nutrients will not be withdrawn from the leaf and the cork seal will not be formed. The dead leaves from the damaged plant will remain firmly attached to the stem. Thus, cut leaves retain their nutrients the same as grass that is cut while green or subjected to a killing frost prior to the 'withdrawal' process. Dead leaves sticking to the plant will indicate some previous damage - an easy way to spot human or animal activity.
Reasons that certain trees may lose their leaves or 'turn colour' sooner than others can be varied but all will be to do with lack of moisture. They may be growing in a drier patch of soil. They may have smaller root systems. They may be diseased and less efficient. Various species of tree will differ from others. Termite mounds will provide islands of clay soil with improved nutrients and moisture.
As you travel around try to work out the condition and state of the underground part of plants and how this is expressed above ground in the state of the leaves. What ever the cause, enjoy the brief 'Autumn colours' and gentle, pastel shades (even the smoky haze!) of our 'winter' - and take time to marvel at the intricate life of trees and the rest of Creation.