Doctoral Dissertation Research: Tree-Grass Coexistence in Semiarid Savannas

热带稀树草原是一种以草和间断分布的树木和灌木为背景的生态系统。 它们是世界人口的一个大的和快速增长的比例和大部分牲畜生产的家园。 热带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然没有得到很好的理解。 大多数研究仅关注木本植物入侵对局部草本植物的影响。 只有少数研究着眼于树木和草的相互作用，并评估干扰对稀树草原不同演替阶段的影响。 此外，在以往的研究中缺乏定量分析。 因此，本博士论文研究项目旨在解决的问题，“是什么决定了丰富的木本和草本植物在不同的演替阶段在半干旱稀树草原跨环境梯度的关键因素？“该项目的主要目标是调查1984年至2005年21年间在亚利桑那州南部半干旱环境中的一个大型和有据可查的实验范围内稀树草原的植被动态。 这项研究将整合现场数据，当前和历史的遥感光谱信息，和地理信息系统（GIS）为基础的分析，以解开在半干旱稀树草原植被的空间和时间动态。 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），（3）干扰（放牧和火灾）。 博士生将寻找有效的和统计上显着的相关性之间的实地测量的绿色生物量和卫星光谱数据。 这些相关性将允许估计大面积和长时间的绿色生物量。 此外，该项目将根据木本和草本绿色生物量的物候差异，从总绿色生物量中分离这两种生命形式。 学生将使用地质统计插值开发降水的空间显式表面，使用GIS技术整合地形，土壤和放牧，并从历史卫星图像中描述烧毁区域。 通过对这些变量的分离和控制，本研究将能够获得在不同演替阶段影响半干旱稀树草原生态系统的关键因子。本博士论文研究项目将加强对大空间范围内半干旱稀树草原生态系统生态过程的基本理解。 具体而言，这项研究将直接影响到测绘，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。 研究结果还应更好地为牧场管理提供信息，并开始揭示管理决策对地方、区域和全球范围碳平衡的影响。 这项研究将产生更广泛的影响，为大学生态学课程提供新的见解，并通过互联网向自然资源管理人员提供研究成果，以帮助弥合科学与应用之间的差距，从而使整个社会受益。 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的研究生涯。

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.