This project will provide a toolkit and quantitative support for land/water conservation management plans to promote the sustainability and resilience of riparian forest ecosystems in arid and semi-arid landscapes. This work will focus on drought-prone ecosystems, where prolonged dry periods affect riparian habitat quantity and quality, thereby limiting their role as thermal and moisture refugia for many threatened and endangered (T&E) species including songbirds and amphibians. Drought stress affects the extent, functioning, and sustainability of riparian habitats for T&E species, which are of great management concern on Department of Defense (DoD) bases. However, there are currently limited tools available for developing sustainable, long-term riparian habitat management plans that are responsive to changes in the mean state and variability of climate. This project will detect and assess the responses of sensitive riparian forests to drought stress over recent decades, and will generalize these responses through modeling of a warming/drying climate punctuated by variable rainfall. The research will be carried out within three DoD bases in the Southwest, but will be widely applicable to other drought-prone lands, which is critical because the spatial extent and severity of drought is increasing across the USA.

This project combines field-based forest assessments, remote sensing, dendro-isotopic analysis, and numerical modeling of surface-subsurface hydrology. This unique combination of methods enables a multi-pronged assessment of trends and thresholds of riparian vegetation responses to water stress, as well as overall forest health and functioning of the riparian forest ecosystem. Specifically, the research team will undertake detailed field surveys of riparian forests on each of three bases that will document species composition, canopy characteristics, and forest health. These data will be combined with any existing forest inventory information that is available for each site to assess trends in forest health. The remote sensing analysis comprises spectral analysis of vegetation indices and mixing models from Landsat and higher resolution datasets, which quantify forest extent, water status, and health over a time series of several decades. The dendro-isotopic work involves coring key forest plant species and extracting tree-ring cellulose to assess water source signatures of water usage (oxygen isotopes) on seasonal and annual timescales over the last several decades. It will also investigate the corresponding physiological responses of these vegetation species to water limitation (carbon isotopes). All of these historical metrics will be integrated to develop a set of water stress indicators (WSIs) that can be analyzed for trends and thresholds of past forest responses to drought. Then, to generalize the understanding of riparian habitat vulnerability to climate, researchers will simulate hydrologic responses (soil moisture depth profiles and water table elevation) to historical rainfall events and inter-event periods. The research team will also create ensembles of stochastic simulations forced by various climate change scenarios over multiple decades, using the existing modeling framework. Finally, researchers will integrate the project data and modeling results to investigate the expected responses riparian forests in drought-prone lands to future water stresses.

This research will generate new understanding of the role riparian forests in providing thermal and moisture refugia to T&E species under varying conditions of climatic forcing. Researchers will demonstrate how water availability (in the form of soil moisture depth profiles and water table elevation) varies with climate forcing, how multiple species within riparian forests cope with water stress, and how climate-controlled changes in riparian forest extent and functioning affect their provision of vital ecosystem services in drought-prone landscapes. This information will support conservation management through identification of key metrics that can be monitored on DoD bases, probabilistic assessment of likely manifestation of ‘the new normal’ in terms of key climate state variables, and recommendations for management plan creation. This work will support management plans designed to cope with prolonged drought periods and to generate maximum ecosystem benefit from infrequent, short-lived rainfall events.