Shifting Savanna Stability: Assessing Semi-Arid Grassland Dynamics Via Experimental Manipulations of Precipitation, Soil Fertility, Fire Disturbance, and Non-Native Herbivore Impacts

Abstract

Understanding how factors such as precipitation, nutrients, disturbances, and herbivory alter herbaceous cover in a savanna is imperative for conserving biological diversity and critical savanna ecosystem functions. This study investigates 1) how herbaceous plant productivity and community composition within a semi-arid savanna will respond to interactions of prescribed fire, drought, and nutrient addition (DroughtNet Experiment), and 2) varied levels of prescribed fire energy interact with mammalian wildlife herbivory to effect herbaceous plant productivity and community composition within a semi-arid savanna (Joint Fire Science Project). Separate experimental research projects were established at the Texas A&M AgriLife Research Station in Sonora, Texas on the Edward’s Plateau in a semi-arid savanna ecosystem.

The objective of the DroughtNet experiment was to contribute to an international research network aimed at assessing how varied ecosystems respond to precipitation-limitations as predicted by future climate models. By constructing rainfall exclusion shelters, conducting prescribed fires, and adding nitrogen supplements full-factorial effects of each treatment on local vegetation were examined for a 2-year period. Fire had the most significant treatment effect on ANPP, drastically reducing standing biomass 1- year post burn, though above-ground biomass recovered within 2 years. All treatments had effects on diversity at different time-steps throughout the experiment. Drought had the most lasting impact on community composition even though higher than historically normal precipitation events occurred during the 3-year sampling period. It was expected that interactively drought, nutrient addition, and fire would be drivers of biomass loss, though that was not manifest in the data collected for this study. While this may be an artefact of unusual recent precipitation patterns, it may alternatively indicate the considerable resilience of semi-arid savanna vegetation to water-limitations and other natural disturbances.

In a separate study, I experimentally examined the effects of varying levels of prescribed fire energy interacting with mammalian wildlife herbivory on herbaceous plant productivity and community composition within a semi-arid savanna. Results indicated that high fire energy did not reduce vegetation diversity or productivity; however, combined grazing pressures of both native and non-native mammalian herbivores substantially reduced above-ground biomass.

These results will provide insight to how land managers can react to changing climate patterns and nutrient cycles, implement more effective burning regimes, and manage for native and non-native herbivores depending on their desired alternative outcomes.