Savanna woodlands

From Tropical Topics newsletter, No. 71, December 2001, produced by Stella Martin at the Queensland Environmental Protection Agency. Download the PDF to read the whole issue.

Life is tough for plants living in the seasonally dry tropics. Soils are poor and for half the year the land is parched and prone to fires while for the other half it is inundated with water. Only plants which have been able to adapt to this punishing regime can grow here, having developed certain characteristics to make this possible.

While trees in the rainforest tend to have spreading surface roots to make the most of nutrients available on the forest floor, those in savanna lands generally have deep root systems, to reach deep reserves of water. Some trees concentrate their resources in the early stages of growth on developing a deep and massive tap root.

Once obtained, water must be used economically. The thick bark on many tropical woodland trees, apart from giving protection from fire, can help to conserve moisture. Leaves, however, are a major ‘leak’ due to transpiration — the evaporation of water through the leaf pores (stomata). To minimise water loss, the leaves of many tropical woodland trees have a leathery texture, with a tough, thick, surface cell layer and are sometimes hairy or woolly. The stomata are often sunken below the surface. Leaves tend to hang down at angles, thus minimising exposure to the sun and encouraging convectional cooling.

The grey-green colouring or pale undersides of some leaves also helps to reflect heat away. Grasses and many other small plants simply disappear during the Dry, some dying after setting seeds and others persisting underground as dormant roots and tubers. It is fairly common for trees to also shed their leaves and ‘play dead’.

However, not all trees take this option — deciduous (leafshedding), semi-deciduous and evergreen trees co-exist throughout the savanna woodlands. It seems that these trees are simply using different survival strategies. It is as if they make the choice between investing energy into producing a strong, long-lasting product or numerous poor-quality disposable ones.

Kapok trees are deciduous

Studies have shown that the ‘construction costs’ to a tree for production of deciduous leaves are lower than the costs of producing evergreen leaves. Evergreen leaves need more built-in defences, such as a tough structure or toxins, to prevent damage from leaf eating animals over their relatively long life-span.

These attributes are largely lacking in the disposable deciduous leaves. On the other hand, deciduous leaves photosynthesise more efficiently because they contain more nitrogen and are usually larger than evergreen leaves. The extra nitrogen makes them more attractive to leaf-eaters, but allows them to ‘feed’ more energy into the plant to compensate for their short working life. Evergreen leaves, conversely, repay the tree’s investment by photosynthesising, albeit more slowly, for a full 12 months a year.