Inter-annual rainfall variability supports coexistence of savanna tree and shrub species with dimorphic rooting strategies

Background/Question/Methods

Rainfall climatology and its impacts on soil moisture dynamics are key determinants of the abundance and structure of vegetation in savannas. Most theories of savanna function have focused on rooting depth and water-use tradeoffs between woody plants and grasses. We present field data on the individual- and community-level root distribution of trees and shrubs indicating that there is a diversity of rooting strategies among woody plant species.  Working at three sites spanning a rainfall gradient on the Botswana Kalahari Transect, we excavated the root systems of individual trees and shrubs and recorded the location, length, diameter and mass of all coarse (greater than 2 mm diameter) roots. We used our field data to develop and parameterize a model of soil moisture and transpiration to compare the fitness of individuals with deep and shallow root systems. The model is forced by a long-term rainfall simulation, making it possible to analyze how climatic variability at a range of time scales affects the relative fitness of individuals employing different rooting strategies.

Results/Conclusions

Of four species excavated, we found that two (Terminalia sericia and Acacia mellifera) have shallow, laterally-extensive roots. The cumulative distribution of root biomass with depth for these species was best described by an exponential distribution, while the other two species (Boscia albitrunca and Ochna pulchra) had deeper roots with a cumulative depth distribution following a gamma distribution. The deeper-rooted species had a significantly higher root sinuosity, defined as the ratio of root length to straight line distance between root origin and root end.  They also demonstrated differences in the rate and pattern by which root diameter tapered. We found that deep- and shallow-rooted species coexisted at both the most arid and most mesic sites. Our model results predict that shallow-rooted individuals will experience greater variability in transpiration at short (daily to weekly) time scales, but that deep-rooted individuals will experience greater variability at long (seasonal to inter-annual) time scales. Shallow-rooted trees had higher total transpiration  in dry to average years, suggesting that anomalously rainy years support the coexistence of species. Our results indicate that deep- and shallow-rooted species may respond differently to changes in rainfall climatology, and that vegetation models based on a single, average root profile may not correctly predict the dynamics of the woody plant community.