Declining water yield from forested mountain watersheds in response to climate change and forest mesophication

Abstract

Climate change and forest disturbances are threatening the ability of forested mountain watersheds to provide the clean, reliable, and abundant fresh water necessary to support aquatic ecosystems and a growing human population. Here, we used 76 years of water yield, climate, and field plot vegetation measurements in six unmanaged, reference watersheds in the southern Appalachian Mountains of North Carolina, USA to determine whether water yield has changed over time, and to examine and attribute the causal mechanisms of change. We found that annual water yield increased in some watersheds from 1938 to the mid-1970s by as much as 55%, but this was followed by decreases up to 22% by 2013. Changes in forest evapotranspiration were consistent with, but opposite in direction to the changes in water yield, with decreases in evapotranspiration up to 31% by the mid-1970s followed by increases up to 29% until 2013. Vegetation survey data showed commensurate reductions in forest basal area until the mid-1970s and increases since that time accompanied by a shift in dominance from xerophytic oak and hickory species to several mesophytic species (i.e., mesophication) that use relatively more water. These changes in forest structure and species composition may have decreased water yield by as much as 18% in a given year since the mid-1970s after accounting for climate. Our results suggest that changes in climate and forest structure and species composition in unmanaged forests brought about by disturbance and natural community dynamics over time can result in large changes in water supply.

Climate-driven disturbances in the San Juan River sub-basin of the Colorado River

“…Even as the ESMs project increasing precipitation and temperature, we see the variability in responses through the cascade of scale and land cover variability. This finding is consistent with other studies that observed that variability (forest cover composition and topography) in the area and size of forest-shrub conversion can buffer responses of streamflow or evapotranspiration shifts from climate change (Winkler et al, 2014;Caldwell et al, 2016).…”

“…Studies pointing towards increased streamflow also broadly found evapotranspiration from the canopy decreased, leading to an increase in runoff. However, work based on observations across scales and encompassing multiple disturbances indicates that the regrowth potential for understory, such as shrubs used in this study, is high and that the regrowth is a major controlling factor for water availability and direction of change for evapotranspiration and runoff (Caldwell et al, 2016;Biederman et al, , 2015Brown et al, 2014;Pribulick et al, 2016). Moreover, ecologists project that global forest covers are expected to decline and be replaced with species and understory compositions that are more water intensive.…”

“…Effects may also be impacted by the type of post-disturbance regrowth and extent of disturbance that occurs, such as increases or a difference in shrubs, grasses, forbs coverage, or differences in the composition of tree species that replace the forests (Buma and Livneh, 2017). For instance, water-yield declines have been associated with changes in species compositions of forests in the southern Appalachian Mountains (Caldwell et al, 2016). A recent study indicates that soil water residence time is a key factor in water availability post-disturbance owing to limited (or enhanced) evapotranspiration processes (Buma and Livneh, 2017).…”

“…In addition, we investigated the aridity effect upon water availability under forest disturbances using a one-cell analysis. Some studies have suggested an aridity effect, whereby basins with less than 500 mm annual precipitation will see streamflow decrease and vice-versa (Guardiola-Claramonte et al, 2011;Adams et al, 2012), although this finding is not supported in all work (Brown et al, 2010;Caldwell et al, 2016). For the single cell, we considered climate change by adding +3 • C (warm) and +6 • C (hot) to the temperature time series and changing precipitation by 20 or −20 %.…”

Classifying forest inventory data into species-based forest community types at broad extents: exploring tradeoffs among supervised and unsupervised approaches

“…For example, in many temperate forest communities in the eastern United States (U.S.), recent decreases in the abundance of oak species have been associated with impacts on biodiversity, wildlife habitat, and water quantity (Fralish 2004;Nowacki and Abrams 2008;Hanberry 2013;Hiers et al 2014;Caldwell et al 2016). Because changes in tree species composition can affect forest functions in these ways, characterizing species composition of existing forest communities is important for understanding the functions of those communities (Tierney et al 2009; Thompson et al 2013).…”