Groundwater-forest interactions during drought in temperate forests across scales

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An estimated 3 trillion trees cover 42 million km2 worldwide - equivalent to the combined areas of North and South America. Societies around the globe rely on forests for water supply and provision of food, medicine, and forest products, as well as other recreational, cultural, and aesthetic benefits. Economically, tropical and temperate forests provide ecosystem goods and services totaling an estimated $23.32 trillion/yr worldwide. Drought - and changes in drought regimes - have the potential to disrupt this provisioning of forest services through reduced forest growth and increased vegetation stress and mortality. Groundwater, when and where available, may buffer specific portions of forest from the consequences of changing drought regimes. Strategies to adapt to drought impacts on forests requires improved understanding and forecasting of patterns, consequences, and feedbacks between drought response and groundwater. The overarching goal of this research is to evaluate groundwater as an ecosystem attribute that alleviates adverse impacts of changing drought regimes on temperate forests. This project will develop a suite of complementary drought detection tools to observe and link tree and forest response to drought, as influenced by groundwater. We will use simulation models to place our measurements in the context of a range of scenarios. This study will help society to better address forest and water resource challenges in the 21st century through dissemination of results to a broad array of stakeholders including researchers, forest and water resource managers, and the public. As drought regimes continue to change and impact forests, it is critical to identify and evaluate ecosystem attributes that alleviate drought stress and to develop novel methodologies to monitor drought response. Traditional methods rely on limited water stress observations of individual trees and stand level vegetation metrics derived from coarse resolution satellite images. Synthesis of such data dilutes species-specific responses and does not provide the necessary datasets to reveal mechanisms linking drought impacts on forests across spatial scales. Through the proposed work, multiple lines of evidence will be used to quantify species-specific tree and forest response to drought as buffered by groundwater. Specifically, this research proposes to investigate drought response across gradients of groundwater depth using: (1) tree ring chronologies to provide historic tree response and quantify the influence of groundwater on tree growth and drought vulnerability; (2) tree sway measurements from accelerometers to continuously monitor water stress and tree physiological response to drought; and, (3) hyperspectral remote sensing to spatially map species-specific foliar drought response. These novel methods for monitoring drought response will be aggregated with traditional complementary datasets to develop a suite of numerical models for simulating groundwater-tree interactions to mechanistically represent field observations. This project will provide a basis for understanding the role of groundwater in conferring drought resistance at the tree, transect, and landscape scales in temperate forests.