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

干旱造成的森林死亡最近有所增加，现在是全球关注的问题。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。这项研究的重点是皮尼翁-杜松林地。在发生干旱的美国西南部，更高的气温和更少的降水增加了干旱的严重性，降低了树木的健康，并在整个地区引发了广泛的树木死亡。这项研究的动机是之前的研究结果表明，皮尼翁松树的死亡出人意料地导致林地变得更加炎热和干燥，潜在地改变了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将导致土壤水分可获得性增加的预期。这项工作将有助于在大量树木死亡后，木本植物在调节缺水、半干旱生态系统中的水可获得性方面所发挥的具体作用的关键数据。该模型将阐明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。一名博士后研究人员、两名研究生和几名本科生研究人员将在两所拉美裔美国人服务机构接受培训和支持。这项研究将向地区自然资源管理者通报气候影响，并通过这两所学校以STEM为重点的K-12教育项目，为来自阿尔伯克基地区和圣马科斯和德克萨斯州学校的学生开展活动。该项目将测试最重要的假设，即半干旱生物群中的树木密度可能代表一个临界点，因此，由于干旱导致的死亡导致的树木密度下降可能引发系统不可逆转的干燥，并过渡到新的生态系统状态。该项目将致力于：1)解释为什么植物有效土壤水分和剩余的刺柏和小松树功能在小松树死亡后降低的具体机制(水力再分配减少、能量平衡改变或疏水凋落物输入导致入渗量减少)；如果有的话，哪一个过程是主导的；2)是否有特定的树木密度阈值或特定物种的群落组成，促进在树木死亡后的干旱；3)如果半干旱林地存在促进树木死亡后干旱的土壤结构和/或降水制度，4)这些受干扰的林地恢复到以前的状态或进展到新的生态系统状态的可能性是什么。实验设计以九大块地为中心，其中三块没有任何干扰，将作为对照，三块所有的皮尼翁松树都被环扎，三块所有的刺柏都被环扎，这将使研究团队能够检查皮尼翁和刺柏在调节这些林地的水文中特定物种的作用。测量包括剩余物种中水力再分配和萨普流的响应、土壤水分剖面和地表能量平衡成分，这些将被用来约束陆面模型--社区陆地模型(CLM 4.5)，该模型经过修改以包括水力再分配。拟议的工作将极大地提高预测半干旱生物群中的针叶死亡如何以及为什么会引发干旱的能力，促进我们对半干旱系统中树木死亡与水文学之间的联系的理解，并量化生态系统尺度上的水力分配的作用。

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.