Impact of altered precipitation distribution and warming on tree and grass life
forms in oak savanna

P.I./institution: David D. Briske; Texas A&M University

Co-P.I./institution: Mark. G. Tjoelker; Texas A&M University

Summarize what activities are being carried out in the project: The over all experimental design, includes four replicate rainfall exclusion shelters for each of the two simulated precipitation regimes (an ambient and a dry summer – wet spring treatment). Each shelter includes 10 2 x 2, m plots, two each of five plant species combinations. One set of plots will be heated with infrared lamps suspended over the plots to simulate global warming while the other plots will be exposed to ambient temperatures. The five species combinations within the plots are monocultures of each of the three species plus oak-grass and juniper-grass combinations. This will require 760 plants of each of the two woody species and 980 grass plants to complete the experimental design.

Project Abstract: This research project will explore the effects of altered precipitation distribution and climate warming on grass and tree life forms to establish an ecological basis to interpret the response of savanna structure and function to relevant global change scenarios. Scenario I is a Dry summer/Wet spring that possesses the greatest potential for savanna conversion to juniper woodland. Scenario II is a Dry summer/Wet spring accompanied by warming (ca. 2-3 °C) which we anticipate will either 1) maintain savanna structure, or 2) contribute to woodland conversion. We hypothesize that the expression of phenological and physiological trait plasticity, and the effectiveness of soil water partitioning will represent the fundamental ecological mechanisms underpinning life form responses. We will investigate the dominant oak (Quercus stellata), dominant C4 grass (Schizachyrium scoparium), and an aggressive evergreen invader, (Juniperus virginiana) within the Post Oak savanna of Texas. Precipitation distribution will be altered with permanent rainout shelters and warming will be simulated with infrared lamps in an experimental garden on the Texas A&M University campus. The proposed research will provide knowledge of the ecological mechanisms establishing the relative responses of grass and tree life forms to relevant global change drivers in the south central region of NIGEC.

Summary (200 words or less), in “layman’s terms,” of what will be done and why. This research project will investigate the ecological mechanisms that establish the response of the key grass and tree species to altered precipitation distribution and warming in a southern oak savanna. Greater understanding of the impact of global change drivers on key plant species will have important implications to land use patterns, agricultural productivity, and natural resource conservation and management.

How will the project contribute to one or more of the critical questions relevant to society, the regional NIGEC center and the global change community?
The proposed research will investigate the effects of altered precipitation distribution and warming on contrasting life forms to establish an ecophysiological basis to interpret the response of savanna structure and function to relevant global change scenarios. This will provide a mechanistic basis for the interpretation and prediction of growth form responses to various global change scenarios. Experimental simulation of both precipitation distribution and warming in a field setting represents a unique and potentially valuable research protocol to evaluate the response of terrestrial vegetation to various global change scenarios.

For third-year continuation proposals, what are the “next-step” strategic questions to be addressed?
Third year funding will enable us to conduct a second complete year of data collection addressing plant growth form responses to precipitation redistribution, simulated warming, and their interactive effects. We anticipate that the effects of resource partitioning mechanisms and their differences among species will become amplified and that these data will more clearly define the responses of these contrasting growth forms to global change drivers. In the second year of data collection, plants will likely have become acclimated to the precipitation and temperature treatments and exhibit greater competitive intensity because of their larger size. Based on first year responses, sampling strategies and related experiments will be designed to test the mechanistic basis of physiological acclimation to temperature and water availability. The second year data set will permit us to examine both above and below-ground traits and their temporal dynamics for more than one year. New strategic approaches include examination of plot-level dynamics (growth and gas exchange) and competition for water and nutrients that are only fully realized as longer-term responses to global change drivers. The second year data set will validate plant responses to global change drivers in terms of growth and survival responses of individual plants in competitive mixtures. Third year funding will optimize returns on the initial investment in construction costs of the experimental infrastructure.