Investigation of Environmental Factors Driving Shifts in Vegetation Health and Functional Traits Across the East River and Taylor Watersheds
Nicola Falco1* (firstname.lastname@example.org), Nicholas Bouskill1, Brian E. Enquist2, Boris Faybishenko1, Cesar Hinojo Hinojo2,5, Lara Kueppers3, Michelle Newcomer1, Sebastian Uhlemann1, Haruko Wainwright4, Marshall Worsham3, Kenneth Williams1, Eoin Brodie1
1Lawrence Berkeley National Laboratory, Berkeley, CA; 2University of Arizona, Tucson, AZ; 3University of California–Berkeley, Berkeley, CA; 4Massachusetts Institute of Technology, Cambridge, MA; 5Universidad de Sonora, Hermosillo, Sonora, Mexico
Forest decline due to pulse and press disturbances—including bark beetles, temperature increase, earlier snowmelt, exceptional droughts, and wildfire—have been observed in mountainous ecosystems, affecting species diversity and composition, associated functional traits, productivity, and mortality. To better understand how forested regions of watersheds will respond to future perturbations, it is critical to quantify and characterize such ecosystem changes. This work performed a multidecadal analysis of satellite data spanning 35 years to capture changes in vegetation health and plant traits that occurred over the East River and Taylor watersheds in the Upper Colorado River Basin. Landsat images were used to compute a multiyear trend of vegetation health based on normalized difference vegetation index (NDVI). Also used was a new remote sensing index for leaf mass per area (iLMA) recently developed to map the spatiotemporal variation in LMA.
Covariability analysis with geomorphological and climatic properties shows that the response to disturbance in trees, shrubs, and meadow plants is heterogeneous across the elevation and geological gradients. Specifically, decline in productivity was observed in lower subalpine regions (2800–3200 m)—areas of transition between water and energy limitation—where increasing drought stress may be more evident. Areas of decreasing productivity are also associated with south-facing slopes. Among tree species of Aspen, Engelmann spruce, Subalpine fir, and Lodgepole pine, about 45–65% of forest area in the East River was associated with decrease in canopy health, with Aspen forest being affected to a greater extent. Increased productivity was observed at higher elevations spanning subalpine and alpine regions (3400–3600m). LMA shows an average loss of about 5g*m-2yr-1, mostly driven by changes in evergreen forests, and declining LMA with elevation. Bedrock properties also show heterogeneous relationships to plant productivity. For example, forests have shown an increased productivity at higher elevation over shale and granodiorite bedrock, unlike those on alluvium that followed an average negative NDVI trend.
Further, time-series clustering and correlation analysis of meteorological forcing across the domain shows the importance of climatic variables, such as air temperature, measured and computed across 17 meteorological stations. This supports the hypothesis of changing water and energy balance being linked to increased productivity at higher elevations. Climate-driven changes in vegetation performance directly impacts ecosystem biogeochemical processes and microbial catalysts that alter, for example, the retention of soil nutrients, and may underlie multidecadal changes in nitrogen export observed in the East River.