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树木大量死亡的影响的能力。研究的重点是皮农-杜松林地。在美国西南部，在干旱发生的地方，气温升高和降水减少加剧了干旱的严重程度，降低了树木的健康状况，并引发了整个地区的树木死亡。这项研究的动机是以前的研究结果表明，皮农松死亡率意外地导致林地变得更热和更干燥，可能会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供应增加的预期。这项工作将提供关键数据，说明木本植物在大量树木死亡后，在水有限的半干旱生态系统中调节水供应方面的具体作用。建模将澄清特定森林易受干旱和气候变化影响的程度，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人员将在两个西班牙裔美国人服务机构接受培训和支持。这项研究将告知气候影响的区域自然资源管理人员，并通过这两个机构以STEM为重点的K-12教育计划，为来自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。该项目将检验一个总体假设，即半干旱生物群落的树木密度可能是一个临界点，干旱导致的死亡导致树木密度下降，可能引发系统的不可逆转的干旱，并过渡到一个新的生态系统状态。 该项目将处理：1）解释为什么植物可用的土壤水分和剩余的杜松和小皮农松树功能下降后皮农死亡的具体机制（减少水力再分配，改变能量平衡，或减少渗透，由于疏水性凋落物输入）;和过程，如果有的话，是占主导地位的; 2）是否存在特定的树木密度阈值或物种特异性群落组成，以促进树木死亡后的干旱化; 3）半干旱林地中是否存在土壤结构和/或降水状况，从而促进树木死亡后的干旱化; 4）这些受干扰的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。实验设计集中在九个大的地块上，三个没有干扰，将作为对照，三个所有的皮农松都被环绕，三个所有的杜松都被环绕，这将使研究小组能够研究皮农和杜松在调节这些林地水文方面的物种特异性作用。测量包括水力再分配和液流在其余的物种，土壤水分剖面和表面能量平衡组件，这将被用来限制土地表面模型，社区土地模型（CLM 4.5），修改，包括水力再分配的响应。拟议中的工作将大大提高能力，以预测如何以及为什么针叶树死亡率在半干旱生物群落可能会引发干旱化，推进我们的理解之间的联系，树木死亡率和水文在半干旱系统，并量化的作用，水力分布在生态系统尺度。

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.