Warming and rainfall redistribution effects on linkages between plant functional traits and ecosystem processes in oak savanna

Mark G. Tjoelker, Texas A&M University

Co-Investigators: David D. Briske and Astrid Volder, Texas A&M University

Abstract

The proposed research seeks to understand how plant functional traits mediate tree-grass interactions in response to climate change in oak savanna. The objectives are to: 1) evaluate the independent and combined effects of annual rainfall distribution and warming on the function, growth, and competitive interactions of dominant tree and grass species, and 2) investigate the linkages between plant traits and ecosystem processes, including coupled carbon, nitrogen, and water fluxes. Our rationale is that integrated studies of physiological mechanisms, tree-grass interactions, and their consequences for ecosystem processes are central to predicting the effects of climate change on savanna ecosystems.

The overarching hypothesis is that trait-mediated plant responses to seasonal changes in temperature and soil water availability will govern tree-grass interactions, which drive savanna ecosystem processes under altered climate change scenarios.

Research will be conducted in southern oak savanna where post oak (Quercus stellata), little bluestem (Schizachyrium scoparium, a C₄ grass) and an invasive evergreen, eastern redcedar (Juniperus virginiana), are the dominant species. Since March 2004, annual rainfall distribution (control, 40% of summer rainfall redistributed to autumn and spring) and warming treatments (ambient, +1.5 °C) have been applied in factorial combination. Eighty 2x2 m plots were established beneath permanent rainout shelters with overhead irrigation and infrared lamps and subjected to annual rainfall redistribution and continuous warming. Evaluation of species responses and tree-grass dynamics in monoculture and treegrass mixtures provides insight into competition and processes controlling savanna ecosystems.

The proposed research will monitor plant growth, water potential, and CO₂ fluxes at the leaf, plant, and plot level as a function of season, soil water availability, warming, and species mixture. Ecosystem processes include responses of soil CO₂ flux, total and available N, litter decomposition and mineralization, and fine root production and turnover. Leaf, plant, and plot level responses will be related to environmental drivers (i.e. temperature, soil water content, solar radiation) in the global change scenarios.

This project seeks to reduce scientific uncertainty about potential effects of warming and rainfall redistribution on the structure and functioning of terrestrial ecosystems (NICCR Focus 1) by providing novel information on the dynamics of savanna tree-grass interactions and linkages with ecosystem processes. The proposed study will inform modeling efforts and contribute to a predictive framework for savanna and grassland systems by providing a mechanistic interpretation of contrasting functional traits, tree-grass interactions, and water and N fluxes under future climate change scenarios.