The Roles of Time and Disturbance Regimes in Savanna Plant Communities

Abstract

Despite growing recognition that Earth’s biodiverse grasslands and savannas require long periods of time to accumulate their diversity, the idea that grasslands can be as ancient as forests (i.e., the old-growth grassland concept) is still not widely accepted. Consequently, biodiversity of tropical grasslands and savannas receives far less attention than tropical forests in studies of land-use change and environmental gradients. My dissertation addresses the ways that savanna plant diversity is shaped over long periods of time by fire and herbivores and over short periods of time by land-use change at the global, community, and organismal levels. In my first chapter, as a global test of the old-growth grassland concept, I performed a meta-analysis to compare plant species richness of old-growth grasslands and secondary grasslands (i.e., grasslands recovering after destruction by agriculture and afforestation). I found that old-growth grasslands had 37% more species compared to secondary grasslands, and that secondary grasslands need at least a century to recover their former richness. This analysis highlighted the paucity of studies on land-use change in tropical savannas, and so for my second chapter, I quantified the effects of land use on savanna plant communities in India. I sampled four land uses (old-growth savanna, tillage agriculture, fallows, and tree plantations) stratified across a 1500 mm precipitation gradient. I found that tillage agriculture and tree planting have consistent negative effects on old-growth savanna plant diversity across the precipitation gradient. These findings underscore an urgent need to recognize expanding agriculture and afforestation as existential threats to fire- and grazer-maintained tropical savanna biodiversity. In my third chapter, I studied how fire and grazing have shaped grass functional traits of 337 native Texas grasses. I found that fire and grazing have resulted in Texas grasses evolving strategies to either promote fire or promote grazing. Results highlight the legacy of Pleistocene megafauna and the undeniable role of fire in shaping the grass flora of Texas. In conclusion, my dissertation provides evidence to recognize fire and grazing as ancient forces shaping savanna biodiversity. Maintenance of these endogenous disturbance regimes in old-growth savannas and limiting land-use change should be a conservation priority.