Why are large milkwood trees so scarce in Cape St Francis?
Think of the Cape coast and what tree springs to mind? For most, it would be the white milkwood tree (Sideroxylon inerme). From Kommetjie on the Cape Peninsula to the shores of Algoa Bay and beyond, dune thickets and forests are invariably dominated by milkwood trees. Yet this is not the case here in Cape St Francis where kershout (Pterocelastrus tricuspidatus) is the most conspicuous denizen of our dune thickets. Why is this so?
Kershout (left) and milkwood (right). Note the distinctive orange fruits and oval-shaped leaves of kershout. Milkwood has leaves with prominent veins that produce milky sap when detached from the branch.
The answer is simple – fire. Let me explain.
Kershout is better adapted to fire than milkwood. It is capable of resprouting vigorously after fire.
Every plant that we monitored after the fierce fire of January 2016 resprouted from buds at the base of the plant. On the other hand, almost 20% of milkwoods were killed outright by that fire and those that did survive also resprouted from basal buds, but at a much slower rate than the kershout.
Kershout (left) resprouting vigorously 16 months after the January 2016 fire in Cape St Francis. At the same time, milkwood (right) had produced far fewer sprouts.
A key feature of kershout’s ability to tolerate fire by strong resprouting, is that mature plants are multi-stemmed. This enables the plant to rapidly occupy space in the postfire environment and outcompete more slower growing plants such as milkwood – hence the hare (kershout) and the tortoise (milkwood).
But, as in everything in nature, every strategy adopted by a species has implications. The cost to kershout of investing so many resources in all those stems is a limited height. Imagine, in the photo below of a mature, multi-stemmed kershout, if all that wood was allocated to a single stem: the plant would be three times its present height!
Mature kershout (left) and milkwood (right) trees in dune forest. Note the multi-stemmed kershout with its numerous, almost vertical stems and dense canopy shading out any competitors. Milkwood, on the other hand, has few basal stems and grows much taller and spreads more widely than kershout.
Thus, the hare (kershout), while rapidly occupying space by strong post-fire sprouting, is – in the long-term (more than 50 years at least) – outcompeted by the taller and more widely spreading canopy of milkwood (the tortoise), which overtops the kershout, denying it light.
We can conclude then, that dune thickets and forest dominated by kershout have been subjected to relatively frequent fire whereas those dominated by milkwood have been fire-free for centuries or more.
Here in Cape St Francis, our dune landscapes have likely been swept by fierce fires driven by the frequent westerly gales that the area experiences. This fire regime has benefitted kershout at the expense of milkwood. Only in Seal Bay nature reserve, tucked as it is in the corner of the bay and thus protected from the wind-driven fire path to the north, do we find mature milkwoods.
The warmer and drier conditions predicted for our region because of human-induced climate change will likely result in a higher frequency of intense wildfires. These conditions will favour the fire-loving kershout and limit the prevalence of the iconic milkwood, which will remain dominant only in sites that free from even the wildest of veld fires. Such is the fate of the tortoise in the rapidly changing world of today.
An ancient milkwood killed by an unusually intense wildfire that burnt through dune forest at Franskraal near GanBay in the Cape.