Evaporative loss from canopy interception (Ec), defined as the difference between gross rainfall and throughfall, is an important component of the hydrologic budget in forested ecosystems. Many studies have demonstrated that forest structure affects Ec and that Ec tends to be positively related to leaf area index and stand density. The longleaf pine (Pinus palustris) savanna ecosystem is characterized by an open canopy, low basal area, and sparse sub-canopy and is dependent upon frequent fire to maintain its structure. When fire is excluded from the ecosystem, a dense sub-canopy of fire-intolerant woody species develops. Our objective was to compare Ec in a longleaf pine savanna in southwestern Georgia under two different fire regimes to determine the effects of prescribed fire on Ec loss. We used climate stations and throughfall collectors to measure gross rainfall and estimate Ec at two sites, one mesic and one xeric. At each site, we collected throughfall weekly in stands representing two fire treatments: long term frequent prescribed fire (2-year return interval) and 14 years of fire exclusion. We also estimated canopy storage for each site and treatment by regressing estimated Ec and gross rainfall for individual rain events across a range of event magnitudes.
Results/Conclusions
Rainfall was 1122 and 1177 mm yr-1 at the mesic and xeric sites, respectively for 2015. Canopy interception was extremely variable at both sites, regardless of gross rainfall, but was generally lower at the mesic site, and lowest in the prescribed fire stands at both sites. Ec was highest in the xeric stand with fire exclusion, averaging ~14% of gross annual rainfall (161 mm/yr), whereas interception was ~5% (59 mm/yr) in xeric stands treated with frequent prescribed fire. In all stands, including those treated with fire exclusion, <1 mm of precipitation was adequate to produce measurable throughfall, likely due to openings that persist in the canopy despite fire exclusion. These results demonstrate that fire regime can have a substantial impact on forest hydrologic budgets and these findings improve our understanding of how fire regime affects specific components of evapotranspiration in the longleaf pine ecosystem. This research has important implications for informing land management priorities in the southeastern U.S., particularly in light of recent regional policy disputes over both prescribed fire and water resources.