Abstract
Fuel-reduction treatments are commonly employed throughout the western United States to improve forest health and/or decrease the risk of wildland fires. Periods of prolonged drought and high temperatures increase both the risk of wildland fires and the stress on water resources. Forest managers may mitigate the risk of wildland fires by increasing fuel-reduction treatments but the subsequent effect on forest hydrology and water resources is not well understood. Of particularly interest to water resources is the effect on snowpack accumulation and melt timing, which is impacted by forest cover. As part of a Comprehensive Forest Landscape Restoration Program (CFLRP), four sites were selected in the Hat Creek Basin of Lassen National Forest to study the hydrologic effects of two common fuel-reduction strategies, forest thinning and group selection.
In this work, we document the in-canopy meteorological and subsurface hydrologic response to thinning treatments through a period of extreme drought and above-average precipitation with little snow in a site located in the rain-snow transition zone of the southern Cascades, near the Sierra Nevada of California. When compared to near-complete removal of trees to form mid-size (< 1 ha) openings, thinning was found to have a modest and predictable impact on in-canopy meteorology, including lower nighttime minimum temperatures during the critical summer months and higher wind speeds. Relative to the unaltered control, thinning and clearing had great impact on soil moisture storage, with delayed timing of annual soil moisture decline and increased minimum soil water storage by the end of summer. The start of soil moisture depletion was strongly tied to the magnitude of the winter precipitation, with dry years inducing soil moisture depletion much earlier in the season in the dense control stand. During average or above average water years, the soil moisture storage capacity was fully “topped off” by winter rains for all treatments. The transition from snow to rain also eliminates late spring soil moisture replenishment, further stressing unthinned forests.