Seedling survival is key to explaining savanna tree distributions in Africa

Author Sally Archibald discusses newly published article: “Browsing is a strong filter for savanna tree seedlings in their first growing season” by Archibald, Twine, Mthabini, & Stevens.

This research found that establishment limitation through early browsing is an under-recognised constraint on savanna tree species distributions

When trying to explain why an organism is found in one place rather than another people generally first think about climate: species are expected to only be found in certain suitable climatic conditions. If climate controls species’ distributions, then as climates change species will flourish in different regions from where they are currently found. Some of the most important global change science questions are focused on trying to work out what parts of the world will be suitable for different species (identify their “climatic envelopes”), and whether they are likely to be able to adjust their ranges to track their required conditions.

However, for many organisms, especially in the tropics, climate is only one of many factors controlling where they can live. Species that cannot tolerate frequent fire, for example, will be excluded from environments that burn often, even if the climate is totally suitable. Sometimes, even if the adults of a species can be successful in a particular environment, if the juveniles are unable to survive, then the species ranges will be limited.

Two month old seedlings growing at our experimental site – Colophospermum mopane in the foreground. Photo: Sally Archibald.

The term “environmental filtering” describes the process by which environmental conditions exclude certain species from an environment. The word ‘filter’ is a useful way to conceptualise how of all the possible organisms that could reach a particular environment, some end up dying before they can reproduce.

We suspected that there was a strong environmental filter associated with browsing of small tree seedlings in African savannas. Seedlings are very vulnerable when they first germinate – they have one small shoot above-ground, and if this is eaten by a herbivore, plants might not have resources or capacity to recover and regrow. We expected that trees growing in places with many herbivores would have evolved capacity to survive these “early herbivory” events, but that trees that were less likely to be eaten (either because they grow in places without herbivores, or because they are not palatable and avoided by browsers) would die when browsed at a very young age.

We grew tree seedlings from four different plant groups and from plants with distributions ranging from very low to very high rainfall in Africa. In high rainfall environments herbivore densities are lower, and the chance of a small seedling being eaten by a browser is lower. We imitated the effect of browsers experimentally  by clipping the above-ground biomass of these small seedlings at various ages after germination, to find out what age they would survive this damaging event.

Interestingly, young plants from low-rainfall environments were much better at surviving and regrowing after being damaged: it took about 3 months for a seedling from a low-rainfall environment to have a 80% chance of survival, but about 6 months for a seedling from a high-rainfall environment. Once we knew the probability of a species surviving being browsed we were able to work out what density of herbivores would act as an ‘environmental filter’ for this species.

Using impala browsing intensities as a demonstration we showed that some tree species would be excluded at much lower numbers of impala than others (24 impala per km2 for Acacia caffra from a high rainfall environment compared with 49 impala per km2 for Combretum collinum from a low rainfall environment). These results help to explain why some species that are perfectly able to grow in particular climates are not found there: they are being excluded by the high numbers of browsing animals. It also helps to explain why we often find new tree seedlings of different species in an environment after an extreme drought: when the browser numbers go down it increases the range of species that can survive and grow from seed. i.e. instead of droughts filtering species from environments they can actually act to increase the range of organisms that can survive because they take away the environmental filter caused by browsers.

Two browsing impala (Aepyceros melampus). Photo: Jan Venter.

This research demonstrates that “climate envelopes” are tools that are too simplistic to explain tree distributions in tropical ecosystems, and that understanding and predicting early seedling survival will be key to explaining changes we observe in our ecosystems under global change.

Sally Archibald University of Witwatersrand, Johannesburg, South Africa

Read the full article in Journal of Ecology: Browsing is a strong filter for savanna tree seedlings in their first growing season

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