Collaborative Research: Hydrological tipping points and desertification of semi-arid woodlands

干旱造成的森林死亡率最近有所增加，现已引起全球关注。该项目旨在提高生态学家预测半干旱森林生态系统中大量树木死亡影响的能力。这项研究的重点是piñon-juniper林地。在美国西南部，气温升高和降水减少加剧了干旱的严重程度，降低了树木的健康，并引发了整个地区广泛的树木死亡。这项研究的动机是先前的研究结果，表明piñon松树的死亡意外地导致林地变得更热、更干燥，可能改变控制未来植物生长和生态系统恢复的环境条件。这些令人惊讶的结果挑战了树木死亡会导致土壤水分可用性增加的预期。这项工作将为木本植物在大量树木死亡后调节水资源有限的半干旱生态系统中的具体作用提供关键数据。该模型将澄清特定森林在多大程度上容易受到干旱和气候变化的影响，并将有助于确定增加脆弱性的关键环境因素。一名博士后研究员、两名研究生和几名本科生研究员将在两个拉美裔服务机构接受培训和支持。这项研究将向区域自然资源管理者通报气候影响，并通过两所学校的STEM重点K-12教育项目，为阿尔伯克基地区和德克萨斯州圣马科斯学校的学生开展活动。该项目将测试一个总体假设，即半干旱生物群落中的树木密度可能代表一个临界点，即由于干旱导致的死亡率导致树木密度下降可能引发系统不可逆转的干燥，并过渡到一个新的生态系统状态。该项目将解决：1)解释为什么植物有效土壤水分和剩余的杜松和小piñon松树功能在piñon死亡后下降的具体机制（水力再分配减少，能量平衡改变，或疏水凋落物输入导致的入渗减少）；如果有的话，哪个过程占主导地位；2)如果存在特定的树木密度阈值或物种特定的群落组成，有助于树木死亡后的干旱化；3)在半干旱林地中是否存在促进树木死亡后干旱化的土壤结构和/或降水制度？ 4)这些受干扰林地恢复到以前状态或发展到新的生态系统状态的可能性有多大？实验设计集中在9个大地块上，其中3个不受干扰的地块将作为对照，3个地块将所有piñon松树围起来，3个地块将所有杜松围起来，这将使研究小组能够检查piñon和杜松在调节这些林地水文方面的特定物种作用。测量包括剩余物种的水力再分配和水流量的响应，土壤湿度剖面和地表能量平衡成分，这些将用于约束一个陆地表面模型，即修改为包括水力再分配的社区土地模型（CLM 4.5）。这项工作将极大地提高预测半干旱生物群落中针叶树死亡如何以及为什么会引发干旱化的能力，促进我们对半干旱系统中树木死亡与水文之间联系的理解，并在生态系统尺度上量化水力分布的作用。

Forest mortality caused by drought has recently increased and is now of global concern. This project is designed to improve the ability of ecologists to predict the impact of significant tree die-off in semi-arid, forested ecosystems. The study focuses on piñon-juniper woodlands. In the Southwest U.S., where they occur, higher temperatures and decreased precipitation have increased drought severity, reducing tree health, and triggering widespread tree death across the region. This research is motivated by previous findings showing that piñon pine mortality unexpectedly leads to woodlands becoming hotter and drier, potentially altering the environmental conditions that control future plant growth and ecosystem recovery. These surprising results challenge the expectation that tree mortality would lead to increased soil water availability. This work will contribute key data on the specific roles of woody plants in regulating water availability in water-limited, semi-arid ecosystems following mortality of large numbers of trees. The modeling will clarify the extent to which specific forests are vulnerable to drought and changing climate, and will help to identify key environmental factors that increase vulnerability. One postdoctoral researcher, two graduate students, and several undergraduate researchers will receive training and support at two Hispanic-American Serving Institutions. This study will inform regional natural resource managers of climate effects and develop activities for students from Albuquerque-area and San Marcos, TX schools through STEM focused K-12 education programs at both institutions. The project will test the overarching hypothesis that tree density in semi-arid biomes may represent a tipping point, such that a decrease in tree density due to drought-induced mortality can trigger a potentially irreversible drying of the system, and transition to a new ecosystem state. The project will address: 1) specific mechanisms that explain why plant-available soil moisture and remaining juniper and small piñon pine tree function decreases following piñon mortality (decrease of hydraulic redistribution, altered energy balance, or reduced infiltration due to hydrophobic litter inputs); and which process, if any, is dominant; 2) if there are specific tree density thresholds or species-specific community compositions that facilitate aridification following tree mortality; 3) if there are soil structure and/or precipitation regimes in semi-arid woodlands that facilitate aridification following tree mortality and 4) what is the likelihood that these disturbed woodlands will return to their previous state or progress to a new ecosystem state. The experimental design is centered on nine large plots, three with no disturbance that will act as controls, three in which all piñon pines are girdled, and three in which all juniper are girdled, which will allow the research team to examine species-specific roles of piñon and juniper in regulating the hydrology of these woodlands. Measurements include the response of hydraulic redistribution and sapflow in the remaining species, soil moisture profiles, and surface energy balance components, which will be used to constrain a land surface model, the Community Land Model (CLM 4.5), modified to include hydraulic redistribution. The proposed work will greatly enhance the ability to predict how and why coniferous mortality in semi-arid biomes may trigger aridification, advance our understanding of the links between tree mortality and hydrology in semi-arid systems, and quantify the role of hydraulic distribution at the ecosystem scale.