Thresholds of Vegetation Change Following N Deposition in Southern California Ecosystems

该项目将研究人为氮（N）沉降对入侵的一年生草引起的植被类型转换阈值的影响。 入侵植物已经极大地改变了美国西部的许多半干旱生态系统，但氮沉降可能会加剧这个问题。这一努力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，以确定阈值N沉积和火灾不可逆地改变生态系统，从沙漠到森林。氮沉降的影响因生态系统与主要氮排放源的接近程度、气象条件以及土壤物理和化学条件而异。该地区的氮排放主要来自内燃机的硝酸盐，其次来自农业的铵。该项目建议确定氮沉降的来源，描述大气传输的N，量化跨景观的沉积速率，并随后确定植被的N供应率。生物地球化学，水文，火灾，景观过渡模型，然后将被用来显示如何升高的N和波动的降水影响草生物量，火灾循环，植被变化在不同的生态系统types.Methods：该研究将有两个主要组成部分。首先，它将描述在南加州空气盆地的不同生态系统的氮沉降。水文，碳，氮循环将耦合，因为它们一起控制N的运动，和植被生长的年际变化。研究人员将研究从1982年到现在模拟的氮沉降和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预计的氮沉积，比较与三种城市发展情景相关的可能影响。其次，他们将检查植被组成沿着氮沉积梯度和重新采样历史植被地块，以确定植物多样性的变化，并使用遥感检测大规模的植被变化。研究人员将测量土壤中的氮输入，周转和损失，并在沙漠，森林和灌木生态系统中进行施肥实验，以确定增加氮负荷下的变化阈值。植被变化将使用火灾和景观过渡模型进行建模。这些结果将与大气模型相结合，以显示2018年几种氮沉降情景对植被的影响。智力优势：这项研究的相关性将远远超出南加州，因为城市化和氮沉降的过程正在世界各地发生，以及从干旱到中温带的一系列生态系统。这项工作将是为数不多的耦合地球化学和景观过渡模型，以显示如何N沉积，降水变化和外来入侵促进非线性变化，在不同的生态系统类型。通过比较四种生态系统类型，每种生态系统类型对氮沉降的响应都有自己的阈值和突发特性，这将促进我们对复杂性理论的理解。更广泛的影响：该研究将研究由不断变化的人类人口统计和人口增长引起的排放影响的景观的复杂动态。对植物入侵及其进一步影响的控制并不为人所知，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计划合作，教育公众和学校的孩子关于氮沉积，组织一个区域会议的氮沉积的土地管理者，立法者和科学家，并与联邦，州和地方机构的栖息地保护规划工作的敏感物种受到空气污染和入侵物种。 该项目将为多样化的学生群体提供研究机会，并将通过由跨学科研究团队领导的高级研讨会课程来加强。

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.