Due to the reality that resources in nature are never unlimited (a fact that human society doesn’t always seem to grasp
), all organisms including trees, have to balance their lives between growth, maintenance, storage, reproduction and defense in order to be successful. Nutrient-rich areas hold both advantages and disadvantages in store for the trees that grow there. On the one hand these trees have ample nutrients at their disposal to flourish and grow quickly. On the other hand, the same nutrients make their leaves very tasty and attract high densities of browsers. Luckily, due to the high nutrient resources at their disposal, they can afford to sacrifice some of their growth potential to create physical defenses in the form of thorns. This is not a luxury however but a necessity. Any tasty tree would quickly perish in an area with high browser densities, if it didn’t have sufficient defenses in place. Some trees are so tasty and popular with animals that they even have thorns on their trunks. A good example from Kruger is the ever-present Knob-thorn (Acacia nigrescens) whose bark is very popular with browsers…
Knob thorn stem covered in thorns
Still, even the thorniest tree can’t escape the onslaught from herbivores completely. The constant evolutionary arms-race has ensured that most browsers have developed ways to cope with these defenses. Some have long and though tongues to probe for leaves in-between thorns. Others have learnt to eat only the youngest, softer parts of branches before the thorns can harden. Elephants don’t even seem to mind thorns sometimes!
It seems that the main function of thorns is not to prevent browsing completely but rather to minimize the impact during the time spent by an animal at a specific tree. It takes more time to pick out leaves between the thorns and buys the tree time to activate its next defense mechanism... An ingenious adaptation of some trees is to pump unpleasant chemicals into their leaves when it is being devoured by an animal. After a few bites, the leaves start to turn nasty and the animal rather moves on to the next tree. It has to move quickly and preferably up-wind however, as the original tree also releases hormones in the air which drifts down-wind to neighbouring plants, warning them of the impending attack!
Sweet thorn branchlet with thorns
Some trees develop spines (modified stems or branchlets) instead of thorns (modified stipules). This is a clever way of preventing excess sacrifices of potential photosynthesizing leaves by making its branches (which would have grown anyway) sharp and painful! Good examples of this are the Sickle-bush (Dichrostachys cinerea) and Spike-thorns (Gymnosporia spp.).
Spike thorn branch with spines
Of course, a large portion of Kruger is not nutrient-rich. So what kind of defenses do trees in these areas rely on? Trees growing in nutrient-poor areas tend to grow very slowly, obviously due to the lack of nutrients at their disposal. Therefore they don’t have the luxury of sacrificing any growth-potential towards building thorns. Luckily for them, they usually don’t need to, because fewer nutrients mean less tasty leaves and fewer browsers. Still, when the risk of losing precious leaves or branchlets is high, these plants also have to find a way to protect themselves. Plants in nutrient-poor areas usually focus on chemicals for defense as it is less expensive to maintain than thorns. Take for instance the Gifblaar (Dichapetalum cymosum), a shrub that sprouts its new leaves in the late dry season when most of the bush is devoid of greenery. To protect itself from herbivores in its nutrient-poor habitat, its leaves contain potent toxic chemicals. Wild animals know perfectly well to avoid this plant but it is deadly to cattle in the northern regions of South Africa.
In nutrient-poor, broad-leaved areas, the main herbivores are sometimes insects, especially larvae of butterflies and moths. Some plants such as the Sandpaper Raisin (Grewia flavescens), have found it worthwhile to produce small bristle-like hairs on its leave surfaces to discourage caterpillars from walking and feeding on it.
A further indirect way for trees to protect themselves in nutrient-poor areas is by keeping most of their obtained nutrients and growth-material below-ground, away from the hungry mouths of herbivores. Hence, many plants that you see in sandy areas are actually just the “tip of the iceberg” as their roots and underground storage tubers are much larger than the above-ground branches and leaves.
Next time we will be looking at grass ecology and its links to the 4 drivers of savannas, including the impact on grazers.
References:
Herms, D.A. & Mattson, W.J. ‘The dilemma of plants: To grow or defend’, Quarterly Review of Biology 67 (1992), 283–335. http://dx.doi.org/10.1086/417659
Scholes, Robert and Walker, B.H. An African Savanna: Synthesis of the Nylsvley study, Chapter 15: Plant-Herbivore Interactions. Cambridge-UK: Cambridge University Press, 2004. Print.
van Wyk, van Heerden, and van Oudtshoorn. Poisonous Plants of South Africa. Pretoria: Briza Publications. 2002. Print
