Collaborative Research: Integrating Plant Hydraulics with Climate and Hydrology to Understand and Predict Responses to Climate Change

干旱通过减少树木生长和增加包括火灾和虫害在内的多种原因造成的死亡率，威胁着国家森林资源的健康。该项目将确定在当前和未来的气候情景下，美国西部的森林在何时何地最容易受到干旱导致的死亡的威胁。植物的干旱反应将根据基于水流物理学的水供需理论进行预测。当植物进行光合作用时，它们也会通过气孔蒸发失去水分，气孔吸收叶片中的二氧化碳。流失的水分通过被动的抽吸过程得到补充，该过程将水分从土壤中抽到根部，并进入充满水的木质部管。干旱导致的这种水运输的失败限制了可能的供水量，从理论上讲，这与气孔关闭导致的需水量减少有关。因此，长期干旱降低了生长和运输能力，造成持续的代谢压力，根据“慢性压力假说”增加了死亡率。一个计算机模型将把供需理论与干旱压力的气候和景观原因结合起来，包括降水、地下水水文和大气湿度。该模型和慢性应力假设将在温室、人工林和自然林分进行测试，重点是西部山间的树木。对该地区森林的预测将基于气候预测。预报将为土地管理决策提供信息。该模型的教育版本将说明气候和地下水水文对植物健康的综合作用。该项目以区域为重点，对一种可在国家和全球应用的方法进行试验。目标是确定和描述植物的关键水力特征，以准确预测它们的气体交换、血管性能、生产力和死亡风险将如何应对气候变化。背景是西部山区（美国）的森林，干旱已经造成大量木本植物死亡。该项目的核心是土壤-植物-大气连续体模型，该模型应用数学方法研究土壤水分流向植物木质部。关键的研究问题包括气候和水文是否能够准确地驱动该模型，木质部再填充对血管恢复的重要性，以及是否存在血管功能障碍的慢性应激阈值，以及随之而来的二氧化碳吸收和生产力的降低是否与死亡风险可重复相关。慢性压力假说的效用是，水力学可以从气候耦合模型中预测出来，而确切的近距离死亡原因可能是可变的，难以预测。综合模型和慢性压力理念将在温室、研究花园和自然环境中进行严格测试，包括测试其根据已知气候输入“预测”该地区山地木本植物当前水力状况的能力。该模型将预测威胁西部森林所需的干旱的严重程度，并预测该地区未来气候情景的直接影响。预报将为森林管理决策提供信息。教育模型版本将促进对植物-气候联系的理解。研究版本将通过网站提供，因为基本方法与分析干旱对任何地区森林的影响有关。

Droughts threaten the health of the nation's forest resources by reducing tree growth and increasing mortality from multiple causes including fires and insect attack. This project will determine when and where forests are most at risk from drought-induced mortality in the western USA under current and future climate scenarios. Plant drought responses will be predicted from a theory of water supply and demand that is based on the physics of water flow. When plants are photosynthesizing they are also losing water by evaporation through the stomatal pores that take up CO2 in their leaves. The lost water is replenished by a passive wicking process that pulls water from the soil into roots and up dead, water-filled xylem tubes. Drought-induced failure of this water transport limits the possible rate of water supply, which by theory is associated with reduced water demand caused by closure of stomatal pores. Consequently, prolonged droughts reduce growth and transport capacity, creating persistent metabolic stresses that increase mortality by the "chronic stress hypothesis". A computer model will combine the supply-demand theory with climatic and landscape causes of drought stress, including precipitation, groundwater hydrology, and atmospheric humidity. The model and chronic stress hypothesis will be tested in greenhouse, plantation, and natural stands, focusing on trees of the intermountain west. Predictions for the region's forests will be made from climate projections. Forecasts will inform decisions on land management. Educational versions of the model will illustrate the combined roles of climate and groundwater hydrology on plant health. The project is a regionally focused trial of an approach that can be applied nationally and globally. The goal is to identify and characterize the key hydraulic traits of plants necessary for accurate predictions of how their gas exchange, vascular performance, productivity, and risk of mortality will respond to climate change. The context is forests of the mountain west (USA), where droughts have already caused significant woody plant mortality. The backbone of the project is a model of the soil-plant-atmosphere continuum that applies mathematical approaches for soil water flow to plant xylem. Key research questions include whether the model can be accurately driven by climate and hydrology, the importance of vascular recovery via xylem refilling, and whether there is a chronic stress threshold of vascular dysfunction and consequent reduction in CO2 uptake and productivity that is reproducibly associated with mortality risk. The utility of the chronic stress hypothesis is that hydraulics can be predicted from the climate-coupled model, whereas the exact proximal cause of death is likely variable and less predictable. The integrated model and chronic stress idea will be tested rigorously in greenhouse, research garden, and natural settings, including tests of its ability to "hindcast" the current hydraulic status of montane woody plants in the region from known climate inputs. The model will predict the severity of droughts required to threaten western forests, and forecast the immediate impacts of future climate scenarios for the region. Forecasts will inform forest management decisions. Educational model versions will promote understanding of the plant-climate linkage. Research versions will be available via website, because the basic approach is relevant for analyzing drought impacts on forests of any region.