Predicting the effects of forest fuel treatments is difficult and uncertain — it is unclear whether the treatments are more helpful to forest health or streamflow. According to new research by disturbance ecohydrologist Ryan Bart and his colleagues at the Sierra Nevada Research Institute (SNRI), the answer is both, though not at the same time.
Fuel treatments for forest management include prescribed burns, tree thinning and pruning, for example, each of which are done to reduce fire risk and severity. Bart recently conducted two studies to examine the effect of these fuel treatments in the Sierra Nevada on both the streamflow in rivers and overall forest health of the trees.
“The focus of these two papers was trying to understand how and when water made available from fuel treatments is allocated to forest health and/or streamflow,” Bart said. “Many studies have investigated these issues separately, but they have rarely been examined in tandem. This is important because fuel treatments cannot provide full hydrologic benefits to both forest health and streamflow simultaneously. They can be allocated to forest health or to streamflow, or partially to both.”
The first study, titled “Do fuel treatments decrease forest mortality or increase streamflow? A case study from the Sierra Nevada,” was coauthored with SNRI research scientist Mohammed Safeeq and published in the journal Ecohydrology. The findings show that during periods of drought, fuel treatments are likely to benefit existing trees rather than streamflow. This is because in times of stress, the trees access the water first. So, while streamflow suffers, there is less mortality among trees.
“When trees are water stressed, they are very good at grabbing any additional water in the soils. They get first access,” Bart said.
Bart and Safeeq used remote sensing and streamflow data from 2002 to the present, gathered in the Kings River Experimental Watersheds, to show that fuel treatments sometimes reduced forest mortality during the California drought and sometimes increased streamflow, but not both at the same time.
“Fuel treatments provide hydrologic benefits in both cases, but the level and type of benefit depends on how wet the conditions are,” Bart said.
Trees are not as stressed when water is plentiful, meaning water can make its way downstream. During wetter periods, streamflow benefits more from fuel treatments.
In the second study, published in Hydrological Processes, Bart and Safeeq worked with professors Martha Conklin and Roger Bales on a modeling study to showcase the conditions that favor forest health benefits versus streamflow benefits.
“We are excited about the findings in these two studies. Having a better understanding of how secondary benefits from fuel treatments are allocated can ultimately help facilitate the funding and implementation of fuel treatments in the Sierra Nevada,” Bart said. “This research provides a key piece to the puzzle.”