Seeing the Forest Without the Trees: Long-Term Influence of Wildfire on the Ecohydrology of a Mountain Watershed

Abstract:

The Western United States relies heavily on forested mountain watersheds for its water supply. These same watersheds are nearly uniformly managed using fire suppression, which has persisted over the past century. The dense vegetation resulting from fire suppression has the potential to increase the risk of catastrophic fires, damage ecosystems, and reduce water yield from these watersheds. Despite the ubiquity of fire suppression, the hydrologic effects of restoring the pre-European fire regime in forested watersheds are unknown, as the vast majority of water-fire studies focus on severe, infrequent events. The Illilouette Creek Basin, within Yosemite National Park, has been allowed to revert to a natural fire regime over the last forty years. The results presented here form part of the first hydrological analysis of the effects of restoring a natural fire regime in a Western U.S. watershed.

An aerial photography record dating from the time when the new fire regime was instituted shows that the repeated fires in the Illilouette Basin have converted many forested areas into meadows, shrublands, and wetlands. Preliminary measures of soil moisture and plant water potential during summer suggest that the changes in vegetation type are associated with changes in plant water availability, allowing the aerial photography record to be interpreted in hydrological terms. With reduced leaf area to support transpiration and interception of winter snows, the new vegetation cover in the Illilouette Basin is hypothesized to increase water storage and yields, and may be associated with observed increases in the runoff ratio from the Illilouette Basin.