Doctoral Dissertation Research: Tree-Grass Coexistence in Semiarid Savannas

University Of Arizona, Tucson AZ

Investigators

Abstract

Savannas are ecosystems with a background of grass and intermittently distributed trees and shrubs. They are home to a large and fast growing proportion of the world's population and most of the livestock production. Savannas are particularly susceptible to disturbance and climate change and the alteration of these ecosystems has a significant impact on regional and global biogeochemical cycles. A large body of prior research has revealed that the spatial pattern and abundance of woody and herbaceous plants may be influenced by one or many factors, but the vegetation dynamics of savannas is still not well understood. Most studies have only focused on the effects of the invasion of woody plants on herbaceous species at a local scale. Only a handful of studies have looked at tree-grass interactions and assessed the impacts of disturbance on different successional stages of savannas. In addition, there has been a lack of quantative analyses in prior studies. This doctoral dissertation research project therefore is designed to address the question, "What is the key factor that determines the abundance of woody and herbaceous plants at different successional stages within semiarid savannas across environmental gradients?" The primary objective of this project is to investigate the vegetation dynamics of savannas during the 21-years period of 1984 to 2005 over a large and well-documented experimental range located within a semiarid environment of southern Arizona. This study will integrate field data, current and historical remote sensing spectral information, and a geographic information system (GIS)-based analysis to unravel the spatial and temporal dynamics of vegetation in semiarid savannas. The study will extract quantitative data from three elements of the environment: (1) Biotic factors (woody and herbaceous green biomass measured right after the summer monsoons), ( 2) Abiotic factors (precipitation, topography, and soils), and (3) Disturbance (grazing and fire). The doctoral candidate will search for valid and statistically significant correlations between field measures of green biomass and the satellite spectral data. These correlations will permit estimatation of green biomass over large areas and over time. Furthermore, the project will separate woody and herbaceous green biomass from total green biomass based on the phenological difference of these two life forms. The student will develop spatially explicit surfaces of precipitation using geostatistical interpolation, integrate topography, soils, and grazing using GIS techniques, and characterize burned areas from historical satellite images. By isolating and controlling for each of these variables, this study will be able to derive key factor(s) influencing semiarid savanna ecosystems at different successional stages. This doctoral dissertation research project will enhance basic understand the ecological processes operating within semiarid savanna ecosystems over large spatial extents. Specifically, this study will have direct impact on capabilities to map, monitor, and understand woody plant encroachment and its effect on ecosystem health. Results of the study should also better inform range management and begin to reveal the influence of management decisions on carbon balance at local, regional and global scales. The study will have a broader impact by providing new insight into college ecology curriculum and will benefit society at large by making the research outcomes available to natural resource managers through the Internet in order to help bridge the gap between science and application. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.