Ecosystem level N and P effects on carbon assimilation and growth by savanna shrubs

Background/Question/Methods: A common perspective holds herbivory, fire, climate and soil nutrients as critical determinants of tree establishment dynamics, and therefore of the structure and function of savanna ecosystems. A major limitation of such competing single-factor explanations, however, is that they ignore the possibility that savanna ecosystems may be governed by interactions between two or more of these factors. In this regard, it may be reasonable to consider savannas as prototypical examples of complex adaptive systems: they are inherently complex, highly dynamical, and may be disproportionately influenced by interactions between fundamental factors. For example, fire causes volatilization of nutrients such as nitrogen, which, in turn, may act to constrain the buildup, recycling and availability of nutrients to plants during post-fire recovery. In turn, soil nutrients may influence both plant productivity and herbivory rates, two factors that interact to determine fuel loads, and thus ultimately may feed back upon fire frequency and intensity. This feedback is but one of a number of complex causal interactions that can be hypothesized from published mechanisms. From this perspective, our focus is on the factors that individually, or in combination, determine the rate of post-fire plant recovery. Our emphasis on rate of plant growth differs from earlier treatments of this problem, which have emphasized plant biomass, and the different factors that may influence the size of static biomass pools.

Results/Conclusions: We have established a novel landscape-level experimental platform in the savanna of the Kruger National Park, South Africa, in which we examine the influence of each of these factors, and their potential interactions, during the post-fire recovery period. Here we find that high photosynthetic output and high growth rates of savanna shrubs on nutrient and water treatment plots suggest an increase in woody biomass in high rainfall and high resource savannas, with high productivity and resilience to herbivory. Overall plant production is most strongly co-limited by water and nitrogen, and less sensitive to phosphorus. Increased growth rates under these conditions greatly increase the probability of a shrub escaping the size class most sensitive to fire. Carbon assimilation and growth decrease under low rainfall/nutrients, perpetuating a strong disturbance-driven system, where fire and herbivory greatly influence plant response. The management of savanna systems within designated natural areas relies on our understanding of these fundamental constraints on the system. Plant strategies and responses to changes in these regimes determine the future dynamics of savannas and our stewardship of these intricate systems.