Abstract
Author(s): Roche, James W; Ma, Qin; Rungee, Joseph; Bales, Roger C | Abstract: We assessed the response of densely forested watersheds with little apparent annual water limitation to forest disturbance and climate variability, by studying how past wildfires changed forest evapotranspiration, and what past evapotranspiration patterns imply for the availability of subsurface water storage for drought resistance. We determined annual spatial patterns of evapotranspiration using a top-down statistical model, correlating measured annual evapotranspiration from eddycovariance towers across California with NDVI (Normalized Difference Vegetation Index) measured by satellite, and with annual precipitation. The study area was the Yuba and American River watersheds, two densely forested watersheds in the northern Sierra Nevada. Wildfires in the 1985-2015 period resulted in significant post-fire reductions in evapotranspiration for at least 5 years, and in some cases for more than 20 years. The levels of biomass removed in medium-intensity fires (25- 75% basal area loss), similar to magnitudes expected from forest treatments for fuels reduction and forest health, reduced evapotranspiration by as much 150-200 mm yr-1 for the first 5 years. Rates of recovery in post-wildfire evapotranspiration confirm the need for follow-up forest treatments at intervals of 5-20 years to sustain lower evapotranspiration, depending on local landscape attributes and interannual climate. Using the metric of cumulative precipitation minus evapotranspiration (P-ET) during multi-year dry periods, we found that forests in the study area showed little evidence of moisture stress during the 1985-2018 period of our analysis, owing to relatively small reliance on interannual subsurface water storage to meet dry-year evapotranspiration needs of vegetation. However, more-severe or sustained drought periods will push some lower-elevation forests in the area studied toward the cumulative P-ET thresholds previously associated with widespread forest mortality in the southern Sierra Nevada.
Highlights
Understanding and predicting how forests respond to disturbance is important for managing source-water areas, in semiarid climates, which have a high ratio of evapotranspiration to precipitation
In the analysis of water-balance changes from wildfire, we focused on evapotranspiration (ET) patterns in the Yuba and American River watersheds (Figure 1), two densely forested basins with significant annual runoff and multiple downstream services that depend on that runoff, yet high potential for severe wildfire and disruption of those services
Evapotranspiration values peak in the Yuba and American at 700–800 mm year−1 around the 1,100–1,200 m elevation, whereas precipitation peaks at elevations closer to 1,600–2,000 m (Figure 2)
Summary
Understanding and predicting how forests respond to disturbance is important for managing source-water areas, in semiarid climates, which have a high ratio of evapotranspiration to precipitation. This is an immediate concern where a combination of a warming climate and past management has contributed to: (i) high wildfire extent and intensity (McKenzie et al, 2004; Westerling, 2006; North et al, 2015a), (ii) drought-related forest mortality. Saksa et al (2019) reported a significant reduction in evapotranspiration following fuel treatment in a densely forested central Sierra Nevada area but no significant reduction in a comparable but more water-limited southern Sierra area. Forest regrowth following disturbance is quite variable (Tague et al, 2013; Roche et al, 2018; Tague and Moritz, 2019)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
