Vegetation density has been increasing over the past 30 years at mid-elevation southern Sierra mixed-conifer catchments and northern California coastal conifer catchments, as shown from Landsat mean annual values of the Normalized Difference Vegetation Index (NDVI). Forest vegetation in these regions is being maintained or even increased during the recent drought, despite record low precipitation levels statewide and heavy tree mortality at lower elevation Sierra pine-dominated forests. The increase in vegetation density suggests an increase in forest water use, exacerbating the effects of the drought on runoff and water supply. Between 1984 and 2015, mean annual NDVI increased in the southern Sierra from 0.47 to 0.56 (+19%) in the Providence Creek basin (1730-2115m elevation) and from 0.53 to 0.62 (+17%) in Bull Creek basin (2145-2490m), while NDVI increased along the northern California coast from 0.75 to 0.88 (+17%) in the Caspar Creek basin (40-300m). Previous research has shown an exponential relationship exists between NDVI and measured annual evapotranspiration (ET), indicating even greater increases in ET than NDVI over the three decades. This increase in vegetation has also led to more competition for plant available water, and can be used to explain the large tree mortality occurring in the pine forests at lower elevations, which did not occur during previous drought periods. Preliminary results of a hydro-ecological model, RHESSys, suggest intensive vegetation treatments in these regions can reduce evapotranspiration and modify the catchment-scale water balance. Forest management strategies for implementation of thinning and fuels treatments to reduce forest density may be effective in limiting vegetation mortality during future drought events.
Saksa, P.C.; Safeeq, M.; Dymond, S. (2016): Implications of Increasing Forest Density and Vegetation Water Demand on Drought Impacts in California Montane Forest. Fall Meeting, American Geophysical Union, December 2016. Abstract H43G-1529..