Despite high annual rainfall, tropical forests can suffer severe droughts. These droughts are expected to become more frequent and extreme with climate change. On regional scales, that is, in the range of several hundreds of kilometres, the sensitivity of tropical tree species to drought is evident: species distributions are strongly related to gradients of dry season length and annual rainfall.
On local scales, tree species are often associated with specific habitats such as sand or clay soils, swamps, slopes or hilltops. While these habitats often vary in soil moisture levels, they also vary in the availability of various nutrients. We therefore need to look at the moisture variation at smaller spatial scales within these habitats if we want to understand the role of soil moisture in shaping species distributions.
Our team of researchers from Germany and Panama set out to resolve how soil moisture shapes species distributions in a 50 hectare research plot on Barro Colorado Island, Panama, one of the most studied tropical forests in the world. Within the 50-ha plot, we focused on 200 seedling plots of 1m2 where the fate of thousands of naturally regenerating seedlings had been followed for over twenty years.
At the seedling plots we measured dry season soil water potential, commonly described as the energy plants need to expend to take up water from the soil. We then determined the responses of 62 different species to soil moisture, quantified in terms of relative growth rates and first-year mortality rates. Finally, we tested if those responses were related to the species’ distribution along the soil moisture gradient.
The result was surprising: while species were most abundant on the side of the soil moisture gradient where they grew fastest, species distributions were not related to mortality rates on the moisture gradient. We had expected that moisture variation would strongly impact mortality of the fragile first-year seedlings and that differences in mortality among species would directly influence species abundances, whereas growth differences would only have an indirect effect.
We decided to further investigate these results by looking beyond the responses of individual species and instead compare groups of species with each other. This revealed a competitive advantage for species that were more common on the wet end of the moisture gradient: these wet distributed species grew faster in wet sites compared to dry distributed species.
What could explain our results? In addition to quantifying responses of species to soil moisture, we also quantified the effect of seedling size on growth and mortality. Not surprisingly, size was a strong predictor of mortality: taller seedlings had a better chance of survival, and were thus more likely to contribute to the species distribution in later life stages. Here is where growth comes into play: seedlings of species that grew faster at particular soil moisture levels outgrew the vulnerable early seedling stage faster. As a consequence, fewer seedlings died and the respective species became more abundant at that particular part of the moisture gradient. Thus, growth indirectly shaped species distributions by affecting mortality risk.
Our findings highlight the importance of studying multiple demographic processes to improve our understanding of how species may respond to climate change. Exact shifts in rainfall patterns in the tropics are still highly uncertain, but are most likely characterized by more extreme conditions such as droughts and floods. The future composition and diversity of tropical forests will likely be highly vulnerable to how these climatic changes will play out.
Stefan Kupers, German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, Germany