After decades of fire suppression in the Sierra Nevada, a program was initiated in the late 1960's and early 1970's to restore the natural fire regime in Yosemite and Sequoia-Kings Canyon National Parks; under this program, for the past ~50 years, fires ignited by lightning have been allowed to burn. This natural experiment has afforded scientists a unique opportunity to document changes in biodiversity and the landscape's hydrological response as a result of the transition to a more natural fire regime. A recent paper in Environmental Research Communications, by Scott Stephens and his colleagues, summarizes some of the most important findings from research carried out at two field sites: the Illilouette watershed in Yosemite NP and the Sugarloaf watershed in Sequoia-Kings NP.
Figure 1 of Stephens et al. (2021) showing the study areas.
The first set of studies reviewed by the authors examined the relationship between fire severity and vegetation. Research in these two regions has found that, of the total area burned, only ~15% of it was from high-severity fire; in contrast, in fire-excluded regions, high-severity fire accounted for 27% of the total area burned. A possible explanation for this result is that surface fuels and tree density are significantly lower in Illilouette and Sugarload watersheds compared to similar regions in the Sierra Nevada where fire has been supressed.
The second set of studies focused on biodiversity. As the authors note, pyrodiversity (i.e., variability in the fire regime) leads to biodiversity. For example, increasing pryodiversity is positively associated with increasing diversity in bees, birds, bats, and plants. A reasonable explanation for this result is that variation in fire behavior (e.g., fire severity and size) will lead to an increase in habitat heterogeneity that can support a wider range of plants and animals. Moreover, differences in fire severity will also create different, time-dependent successional pathways, suggesting that temporal heterogeneity in the fire regime is also important in supporting biodiversity.
Finally, the authors reviewed studies examining the hydrological effects of managing wildfires. Under the more natural fire regime, areas dominated by meadows and shrubland in the Illilouette basin grew at the expense of areas dominated by the conifer forest, which decreased from 82% to 62%. This vegetation conversion is thought to have important effects on soil moisture. Ecohydrological models suggest that a decrease in forest area leads to a decrease in snowpack sublimation and evapotranspiration. Moreover, repeat photography revealed that the snowpack is thinner and less persistent under trees relative to meadows and shrublands. In other words, for a variety of reasons, forest cover can lead to drier soils and, thus, potentially drier fuel. In contrast, the establishment of shrublands and meadows can lead to wetter soils, an increase in fuel moisture, and higher streamflows.
Figure 1 of Stephens et al. (2021) showing the study areas.
The second set of studies focused on biodiversity. As the authors note, pyrodiversity (i.e., variability in the fire regime) leads to biodiversity. For example, increasing pryodiversity is positively associated with increasing diversity in bees, birds, bats, and plants. A reasonable explanation for this result is that variation in fire behavior (e.g., fire severity and size) will lead to an increase in habitat heterogeneity that can support a wider range of plants and animals. Moreover, differences in fire severity will also create different, time-dependent successional pathways, suggesting that temporal heterogeneity in the fire regime is also important in supporting biodiversity.
Finally, the authors reviewed studies examining the hydrological effects of managing wildfires. Under the more natural fire regime, areas dominated by meadows and shrubland in the Illilouette basin grew at the expense of areas dominated by the conifer forest, which decreased from 82% to 62%. This vegetation conversion is thought to have important effects on soil moisture. Ecohydrological models suggest that a decrease in forest area leads to a decrease in snowpack sublimation and evapotranspiration. Moreover, repeat photography revealed that the snowpack is thinner and less persistent under trees relative to meadows and shrublands. In other words, for a variety of reasons, forest cover can lead to drier soils and, thus, potentially drier fuel. In contrast, the establishment of shrublands and meadows can lead to wetter soils, an increase in fuel moisture, and higher streamflows.
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