Climate forcings and the nonlinear dynamics of grassland ecosystems

Abstract

The nonlinear interaction of climate forcings and ecosystem variables is instrumental in creating the temporal and spatial heterogeneity of grasslands. Ecosystem processes are a product of these interactions and vary in sensitivity to them across time. How forcings aggregate and shape ecosystem responses is an important aspect of grassland states and defines how they respond to changes in environmental conditions. Characterizing the relationship between climate drivers and ecosystem variables helps sharpen analysis of ecosystem flux dynamics during the growing season and identifies likely deviations from mean functioning. To address the question of how climate forcings and ecosystem variables interact to shape seasonal water and carbon dynamics in grasslands, this thesis is split into two analysis chapters. The first (Chapter 3) is a characterization of water and carbon flux responses to variable precipitation timing and magnitude. Particular focus is placed on temporal sensitivity to inputs, seasonality in water flux dynamics, and the linkage between precipitation, soil moisture, evapotranspiration, and potential evaporation. Chapter 4 extends International Panel on Climate Change (IPCC A1B) regional climate scenario projections for the Central Plains of the United States to assess mesic grassland responses. The specific focus is assessing the ecosystem response to increased precipitation variability, increased potential evaporation, and earlier growing season onset. Effects of these forcings are shaped by simulations of constant and seasonally-varying water-use efficiency to assess the role of vegetation on grassland carbon assimilation, and also to explore species-specific responses at the Konza Prairie in North Central Kansas, USA. Results from both chapters show variation in seasonal sensitivity of fluxes to precipitation, with varying relationships between drivers, variable conditions, and fluxes. This research provides for a better understanding of ecosystem processes and provides assessment of the magnitude and extent that forcing variation has on grassland function. Results from the second chapter show increased seasonal water and carbon flux variability and increased frequency of water stress conditions. Vegetation responses suggest climate change will impact species and habitat compositions through changing environmental conditions and partitioning of carbon assimilation periods. This illustrates potential effects to grassland functioning and growing season dynamics.