Doctoral Dissertation Research: Tree-Grass Coexistence in Semiarid Savannas

稀树草原是以草和间歇性分布的树木和灌木为背景的生态系统。它们是世界上很大比例且快速增长的人口和大部分畜牧业生产的家园。稀树草原特别容易受到干扰和气候变化的影响，这些生态系统的改变对区域和全球生物地球化学循环产生重大影响。大量的前期研究表明，木本植物和草本植物的空间格局和丰富度可能受到一个或多个因素的影响，但稀树草原的植被动态仍未得到很好的了解。大多数研究只集中在局部尺度上木本植物入侵对草本植物的影响。只有少数研究着眼于树与草的相互作用，并评估了干扰对稀树草原不同演替阶段的影响。此外，以往的研究缺乏量化分析。因此，这一博士论文研究项目旨在解决这样一个问题：“决定半干旱稀树草原不同演替阶段跨环境梯度木本植物和草本植物丰度的关键因素是什么？”该项目的主要目标是在亚利桑那州南部半干旱环境中的大范围和有充分记录的实验范围内，调查1984至2005年21年间热带草原的植被动态。这项研究将结合野外数据、当前和历史的遥感光谱信息，以及基于地理信息系统(GIS)的分析来揭示半干旱稀树草原植被的时空动态。这项研究将从环境的三个要素中提取定量数据：(1)生物因素(夏季季风过后立即测量的木本和草本绿色生物量)，(2)非生物因素(降水、地形和土壤)，以及(3)干扰(放牧和火灾)。博士生将在绿色生物量的实地测量和卫星光谱数据之间寻找有效的和统计上显著的相关性。这些相关性将使估计大面积和长期的绿色生物量成为可能。此外，该项目将根据木本植物和草本植物两种生命形式的物候差异，将这两种生物量从总的绿色生物量中分离出来。学生将使用地统计插值法开发空间清晰的降水表面，使用地理信息系统技术整合地形、土壤和放牧，并从历史卫星图像中确定烧毁区域的特征。通过对每个变量的分离和控制，本研究将能够得出影响半干旱稀树草原生态系统不同演替阶段的关键因素(S)。本博士论文研究项目将增强对半干旱稀树草原生态系统在大空间范围内运行的生态过程的基本认识。具体地说，这项研究将对绘制、监测和了解木本植物入侵及其对生态系统健康的影响的能力产生直接影响。研究结果还应更好地为范围管理提供信息，并开始揭示管理决策对地方、区域和全球尺度碳平衡的影响。这项研究将产生更广泛的影响，为大学生态课程提供新的见解，并通过互联网将研究成果提供给自然资源管理者，从而帮助弥合科学和应用之间的差距，从而造福整个社会。作为博士论文研究改进奖，该奖项还将提供支持，使有前途的学生建立一个强大的独立研究生涯。

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.