Thresholds of Vegetation Change Following N Deposition in Southern California Ecosystems

本项目将研究人为氮沉降对入侵一年生草引起的植被类型转换阈值的影响。入侵植物极大地改变了美国西部许多半干旱的生态系统，但氮沉降可能会加剧这一问题。这项工作的概念框架是基于复杂系统理论，包括建模非线性，非平衡动力学，以确定氮沉降和火灾不可逆地改变生态系统的阈值，从沙漠到森林。氮沉降的影响随生态系统与主要氮排放源的接近程度、气象条件和土壤理化条件的变化而变化。该地区的氮排放主要来自内燃机作为硝酸盐，其次来自农业作为铵。该项目旨在确定氮沉降的来源，描述氮的大气运输，量化景观间的沉降速率，从而确定对植被的氮供应速率。然后将使用生物地球化学、水文、火灾和景观过渡模型来显示N升高和降水波动如何影响不同生态系统类型中的草生物量、火灾循环和植被变化。方法：研究将有两个主要组成部分。首先，它将描述南加利福尼亚空气盆地不同生态系统的氮沉积。水文、碳和氮循环是耦合的，因为它们共同控制着氮的运动和植被生长的年际变化。研究人员将研究1982年至今模拟的氮沉降与植被响应之间的关系。他们还将模拟未来由于预计的城市和农业土地利用变化而导致的氮沉降，比较与三种城市发展情景相关的可能影响。其次，他们将沿着N沉积梯度研究植被组成，重新采样历史植被样地，以确定植物多样性的变化，并利用遥感技术检测大尺度植被变化。研究人员将测量土壤的氮输入、周转和损失，并在沙漠、森林和灌木生态系统中进行施肥试验，以确定氮负荷增加下的变化阈值。植被变化将使用火灾和景观过渡模型进行模拟。这些结果将与大气模型相结合，以显示2018年几种N沉降情景对植被的影响。智力价值：由于城市化和氮沉积的过程正在世界各地发生，这项研究将远远超出南加州的范围，并涉及从干旱到湿润的一系列生态系统。这项工作将是为数不多的将生物地球化学和景观过渡模型结合起来的研究之一，以显示N沉积、降水变化和外来入侵如何促进不同生态系统类型的非线性变化。该研究将通过比较四种生态系统类型来促进我们对复杂性理论的理解，每种生态系统类型都有自己的阈值和紧急特性来响应氮沉积。更广泛的影响：该研究将研究由人口结构变化和人口增长引起的排放对景观的复杂动态影响。对植物入侵的控制及其进一步影响尚不清楚，因此这项研究的结果将对区域规划工作和排放法规产生影响。该项目的教育和推广部分包括与当地项目合作，教育公众和学校儿童关于氮沉积的知识，为土地管理者、立法者和科学家组织一次关于氮沉积的区域会议，并与联邦、州和地方机构合作，为易受空气污染和入侵物种的敏感物种进行栖息地保护规划。该项目将为不同的学生群体提供研究机会，并将由跨学科研究团队领导的高级研讨会课程加以加强。

This project will examine the influence of anthropogenic nitrogen (N) deposition on thresholds of vegetation-type conversion caused by invasive annual grasses. Invasive plants have dramatically altered many semiarid ecosystems of the western United States, but N deposition may exacerbate the problem. The conceptual framework for this effort is based on complex systems theory, including modeling non-linear, non-equilibrium dynamics to determine thresholds at which N deposition and fire irreversibly change ecosystems ranging from desert to forest. The impacts of N deposition varies as a function of ecosystem proximity to major N emission sources, meteorology, and soil physical and chemical conditions. N emissions in this region originate primarily from internal combustion engines as nitrate, and secondarily from agriculture as ammonium. This project proposes to identify sources of N deposition, describe atmospheric transport of N, quantify rates of deposition across landscapes, and subsequently determine the N supply rate to vegetation. Biogeochemical, hydrological, fire, and landscape transition models will then be used to show how elevated N and fluctuating precipitation affect grass biomass, the fire cycle, and vegetation change in the different ecosystem types.Methods: The research will have two major components. First, it will describe N deposition to different ecosystems in the southern California air basin. The hydrological, carbon, and nitrogen cycles will be coupled since together they control the movement of N, and interannual variability in vegetation growth. Researchers will examine the relationships between modeled N deposition and vegetation response from 1982 to the present. They will also model N deposition expected in the future as a result of projected urban and agricultural land use changes, comparing possible impacts associated with three urban development scenarios. Second, they will examine vegetation composition along N deposition gradients and resample historic vegetation plots to determine changes in plant diversity, and use remote sensing to detect large-scale vegetation changes. Researchers will measure N inputs, turnover, and losses in soils, and conduct fertilization experiments in desert, forest, and shrubland ecosystems to determine thresholds of change under increased N loading. Vegetation change will be modeled using fire and landscape transition models. These results will be coupled with the atmospheric model to show the effect on vegetation for several N deposition scenarios in 2018.Intellectual Merit: This research will have relevance far beyond southern California since the processes of urbanization and N deposition are occurring around the world, and to a range of ecosystems from arid to mesic. This effort will be one of the few to couple biogeochemical and landscape transition models to show how N deposition, variation in precipitation, and exotic invasions promote non-linear changes in different ecosystem types. The research will promote our understanding of complexity theory by comparing four ecosystem types, each responding with its own threshold and emergent properties to N deposition. Broader Impacts: The research will examine complex dynamics of landscapes impacted by emissions caused by changing human demographics and population growth. The controls on plant invasions and their further impacts are not well known, so the results of this research will have implications for regional planning efforts and for emissions regulations. The educational and outreach components of the project include working with local programs to educate the public and school children about N deposition, organizing a regional conference on N deposition for land managers, legislators, and scientists, and working with federal, state, and local agencies on Habitat Conservation Planning efforts for sensitive species subject to air pollution and invasive species. The project will provide research opportunities for a diverse student population, which will be augmented by advanced seminar courses led by an interdisciplinary research team.