Abstract:
Savanna ecosystems cover 20 % of the terrestrial surface and are particularly vulnerable to environmental fluctuations that are predicted to increase with climate change. Resilience, as the degree of recovery following a disturbance, has traditionally been described using biodiversity or productivity estimates. However, this can be challenging due to the large variations in structure and function of ecosystems, resulting in differences in energy use efficiencies. Thermodynamic metrics such as maximum entropy production (MEP) and energy density provide better methods to study resilience in ecosystems, as they place metabolic processes from differences in structural variations in relation with available energy. This research investigates resilience of three sites in a longleaf pine ecosystem with varying levels of biodiversity, land use history and soil water holding capacity. Changes in understory phenology within the system were explored in response to fire and drought using the Normalized Difference Vegetation Index (NDVI) and eddy covariance (EC) techniques. Energy use efficiency was estimated using radiative, metabolic and overall MEP ratios, as well as energy densities from carbon inputs and exports, to quantify resilience to drought and temperature extremes across sites. In this study the understory at the mesic site recovered more rapidly from prescribed fire but was more affected by drought compared to the xeric site, indicating lower adaptation to decreases in soil moisture. Sites with greater levels of agricultural land use history had roughly 10 % lower energy use efficiencies and relied on stored energy four times more often compared to sites with greater plant functional diversity, which resulted in delayed recovery from drought. More structurally complex sites exhibited an adaptive capacity to prolonged temperature extremes through increasing energy storages by ~150 %, by decreasing metabolic demands. These results suggest that systems that matured with natural environmental variability developed strategies to adapt to disturbances by conserving energy, whereas agricultural legacy reduced energy use efficiency and decreased resilience to disturbances. This study provides the tools to identify ecosystem resilience as functions of ecosystem structure and resource efficiency using thermodynamic metrics. These tools can be applied to other global ecosystems to recognize vulnerability to future changes in climate.
