Forest water use is increasingly decoupled from water availability even during severe drought

Abstract

Context: Key to understanding forest water balances is the role of tree species regulating evapotranspiration (ET), but the synergistic impact of forest species composition, topography, and water availability on ET and how this shapes drought sensitivity across the landscape remains unclear.Objectives: Our aims were to quantify (1) the effect of forest composition and topography including elevation and hillslope gradients on the relationship between ET and water availability, and (2) whether the relationship has changed over time.Methods: We used remotely sensed Landsat and MODIS ET to quantify forest ET across the Blue Ridge ecoregion of the southeastern USA. Then quanti ed metrics describing ET responses to water availability and trends in responses over time and assessed how these metrics varied across elevation, hillslope, and forest composition gradients.Results: We demonstrated forest ET is becoming less constrained by water availability at the expense of lateral ow. Drought impacts on ET diverged along elevation and hillslope gradients, and that divergence was more pronounced with increasingly severe drought, indicating high elevation and drier, upslope regions tend to maintain ET rates even during extreme drought. We identi ed a decoupling of ET from water availability over time, and found this process was accelerated at higher elevations and in areas with more diffuse-porous trees.Conclusions: Given the large proportion of forests on the landscape distributed across high elevation and upslope positions, reductions in downslope water availability could be widespread, amplifying vulnerability of runoff, the health of downslope vegetation, and aquatic biodiversity.

Changes in Blue/Green Water Partitioning Under Severe Drought

“…The contrasting results of our study with McQuillan et al. (2022) highlight the importance of local and regional factors in determining hydrologic response to drought.…”

“…However, for Walshes Creek we also concluded that reductions in downslope water subsidies can strongly affect locations that are accustomed to receiving them (hence the reduction in midslope LAI in Figure 6a2), although the drought was not severe enough to induce substantial water limitation in our simulated riparian zone. The contrasting results of our study with McQuillan et al (2022) highlight the importance of local and regional factors in determining hydrologic response to drought.…”

“…These results contrast with the reductions in ET we simulated in upslope areas in the Walshes Creek catchment, likely because energy (as opposed to water) limitation was more prevalent across the McQuillan et al. (2022) study region. However, for Walshes Creek we also concluded that reductions in downslope water subsidies can strongly affect locations that are accustomed to receiving them (hence the reduction in midslope LAI in Figure 6a2), although the drought was not severe enough to induce substantial water limitation in our simulated riparian zone.…”

“…A recent study using remotely sensed data demonstrated increased upslope water use during drought in the Southern Appalachian Mountains (USA), which reduced downslope subsidies such that lowland ET decreased (McQuillan et al., 2022). These results contrast with the reductions in ET we simulated in upslope areas in the Walshes Creek catchment, likely because energy (as opposed to water) limitation was more prevalent across the McQuillan et al.…”

Serviços hidrológicos em região de transição Cerrado-Mata Atlântica

“…A proteção do solo em florestas ripárias é um serviço importante para reduzir o assoreamento de corpos hídricos (Tambosi et al, 2015). E, apesar da proteção do solo através do dossel florestal ser importante para redução do fluxo de sedimentos também em posições superiores no relevo (Bacellar, 2005;Li et al, 2019), ao diminuir o fluxo, diminui-se também a quantidade de água que percorre e abastece os trechos mais inferiores do relevo, o que poderia impactar a provisão de água (Garcia, 2018;McQuillan et al, 2022). Desta forma, as diferenças encontradas são positivas, à medida que a menor proteção do solo na posição superior garante o abastecimento de água para as posições mais inferiores do relevo, e que a maior proteção do solo na posição inferior garante a maior proteção dos corpos hídricos.…”