John Muir Institute for the Environment, University of California, Davis, California, 95616, USA.
Department of Plant Sciences, University of California, Davis, California, 95616, USA.
Ecol Appl. 2017 Sep;27(6):1888-1900. doi: 10.1002/eap.1575. Epub 2017 Jul 21.
Winter snowpack in dry montane regions provides a valuable ecosystem service by storing water into the growing season. Wildfire in coniferous montane forests has the potential to indirectly affect snowpack accumulation and ablation (mass loss) rates by reducing canopy cover, which reduces canopy interception of snow but also increases solar radiation and wind speed. These counteracting effects create uncertainty regarding the canopy conditions that maximize post-fire snowpack duration, which is of concern as montane regions across the western United States experience increasingly warm, dry winters with below-average snowpack. The net effect of wildfire on snowpack depth and duration across the landscape is uncertain, and likely scale dependent. In this study, I tested whether intermediate levels of wildfire severity maximize snowpack depth by increasing accumulation while slowing ablation, using gridded, repeated snow depth measurements from three fires in the Sierra Nevada of California. Increasing fire severity had a strong negative effect on snowpack depth, suggesting that increased ablation after fire, rather than increased accumulation, was the dominant control over snowpack duration. Contrary to expectations, the unburned forest condition had the highest overall snowpack depth, and mean snow depth among all site visits was reduced by 78% from unburned forest to high-severity fire. However, at the individual tree scale, snowpack depth was greater under canopy openings than underneath canopy, controlling for effects of fire severity and aspect. This apparent paradox in snowpack response to fire at the stand vs. individual tree scales is likely due to greater variation in canopy cover within unburned and very low severity areas, which creates smaller areas for snow accumulation while reducing ablation via shading. Management efforts to maximize snowpack duration in montane forests should focus on retaining fine-scale heterogeneity in forest structure.
高山干旱地区的冬季积雪通过储存水分来支持植被生长，起到了重要的生态系统服务作用。针叶林高山火灾通过减少林冠覆盖间接影响积雪的积累和消融（质量损失）速率，这降低了林冠对雪的截留，但也增加了太阳辐射和风速。这些相互抵消的影响使得火灾后积雪持续时间的林冠条件最大化的不确定性增加，这是一个关注点，因为美国西部的高山地区正经历着越来越温暖、干燥的冬季，积雪量低于平均水平。野火对景观中积雪深度和持续时间的净影响是不确定的，而且可能依赖于规模。在这项研究中，我通过增加积累和减缓消融来测试中度野火严重程度是否通过增加积累而最大化积雪深度，方法是使用加利福尼亚内华达山脉的三次火灾的网格化、重复的积雪深度测量数据。增加火灾严重程度对积雪深度有强烈的负面影响，这表明火灾后消融的增加，而不是积累的增加，是控制积雪持续时间的主要因素。与预期相反，未燃烧的森林条件具有最高的整体积雪深度，在所有现场访问中，与未燃烧的森林相比，高严重度火灾的平均积雪深度减少了 78%。然而，在个体树木尺度上，林冠下的积雪深度比林冠开口处的积雪深度大，这是控制火灾严重程度和方位影响的因素。在林分和个体树木尺度上，火灾对积雪的响应存在这种明显的悖论，这可能是由于未燃烧和低严重度地区的林冠覆盖变化更大，这在减少消融的同时，通过遮荫来减小了积雪积累的面积。在高山森林中为最大化积雪持续时间而进行的管理工作应侧重于保留森林结构的精细尺度异质性。
