Topographic controls on temperate forest structure and evapotranspiration in complex terrain

Background/Question/Methods

Topographic gradients have long been recognized as important determinants of forest structure and function, but how they will mediate forest response to climate change remains uncertain. In the complex terrain of the Appalachian Mountains, topography generates hillslope-scale microclimate conditions that are often decoupled from regional patterns. In this study, we integrate observations of forest structure and function by examining the covariance of forest canopy height, land surface temperature (LST), and evapotranspiration (ET) along topographic gradients in Shenandoah National Park. LST and ET measurements were acquired from ECOSTRESS during the 2018-2019 growing seasons (May 1^st - September 30^th), and forest canopy height was acquired from a combination of GEDI and airborne LiDAR collections. Relationships were examined between canopy height, LST, and ET and a suite of terrain, climate, and edaphic variables.

Results/Conclusions

Preliminary results from two watersheds Shenandoah National Park (38.8 km² total), spanning an elevation gradient of ~700 meters, suggest that elevation is the dominant control on forest canopy height. Canopy height decreased with elevation (r²=0.29), but the strength of this relationship differed by aspect, with the strongest negative relationships observed on south and southwest-facing slopes. Spatial patterns in ET were evident, with generally higher ET at lower elevations and on convex slopes near hydrologic flowpaths. Temporal analyses of the ECOSTRESS Daily ET product were limited by lack of data – only one scene provided >75% coverage of Shenandoah during the 2018-2019 growing seasons. This suggests that, as with many remotely sensed data products, ECOSTRESS may be of limited utility for time series analyses in cloudy regions. Nighttime LST revealed patterns of both cold air drainage to valley bottoms and decreasing LST at higher elevations (>800 m), which may have important implications for ecosystem carbon balance in a warming climate. Future integration of ECOSTRESS and GEDI data with Landsat-derived vegetation indices and LST over a wider range of climatic conditions will provide further insights into the spatial and temporal dynamics of plant physiological response to hydroclimate variability in mountainous terrain.