Do fuel treatments decrease forest mortality or increase streamflow? A case study from the Sierra Nevada (USA)

Reduced forest mortality and increased streamflow have been promoted as hydrologic benefits of forest fuel treatments in water-limited systems. These benefits, however, are often quite variable because they both rely upon the use of water made available through tree-scale reductions in evapotranspiration. In this study, we examined whether forest mortality benefits and streamflow benefits from fuel treatments offset one another, because the allocation of unused water for one benefit implies less availability of that water for the other benefit. To do this, we took advantage of two paired-watershed experiments in the Kings River Experimental Watersheds located in the southern Sierra Nevada. The fuel treatments, which included mechanical thinning and prescribed fire, began in 2012 and coincided with the 2012-2016 California drought and related forest mortality event. We found that in the higher-elevation Bull watersheds, fuel treatments decreased forest mortality but had no effect on annual streamflow. In the lower-elevation Providence watersheds, we observed the opposite result, as fuel treatments had no effect on forest mortality but depending on the model, increased annual streamflow at 95% and 90% confidence. The results suggest that the water made available from fuel treatments followed different hydrological pathways through the watersheds, with the water being taken up by neighbouring vegetation and decreasing water stress in Bull, and the water contributing to streamflow in Providence. These findings suggest that fuel treatments in water-limited systems may not provide full hydrologic benefits to both forest mortality and streamflow concurrently in a given watershed.