Revisiting the two-layer hypothesis: theoretical insights on the coexistence of alternative functional rooting strategies across rainfall and edaphic gradients

Background/Question/Methods

Two models are usually invoked to explain tree-grass coexistence in savanna ecosystems: the ‘two-layer’ hypothesis and the ‘demographic bottleneck hypothesis.’ The two-layer model argues that trees and grasses differ in rooting depth, with grasses exploiting soil moisture in shallow layers and trees using deeper water. The lack of clear differences in maximum rooting depth between these two functional groups has caused this model to fall out of favor. The alternative model, the demographic bottleneck hypothesis, suggests that trees and grasses occupy overlapping rooting niches, and that stochastic events such as fires and droughts result in episodic tree mortality at various life stages, thus preventing trees from otherwise displacing grasses, at least in mesic savannas. Two potential problems with this view are: 1) we lack data on functional rooting profiles in trees and grasses, and these profiles are not necessarily reflected by differences in maximum or physical rooting depth, and 2) subtle, difficult to detect differences in rooting profiles between the two functional groups may be sufficient to result in coexistence in many situations. Savannas occur across broad edaphic and climatic gradients. Is it possible that the relative strength of the niche separation and bottleneck mechanisms varies systematically across these gradients? This possibility was tested theoretically by examining the conditions under which three alternative rooting profiles (with equal rooting depth and biomass) were able to coexist. Plant uptake models were coupled with a soil moisture dynamics model under conditions that mimicked a rainfall gradient and two soil texture templates.

Results/Conclusions

The results indicate that coexistence mechanisms based on rooting niche differentiation are more viable under some conditions than others. This suggests that a full understanding of tree-grass coexistence may require a hybrid model that incorporates both deterministic niche mechanisms and stochastic bottleneck mechanisms.