Eastern Cascades Forest-Snow Observations 2019-2021

Forest thinning and gap creation are being implemented across the western United States of America (USA) to reduce wildfire and forest mortality risk as the climate warms. The Eastern Cascades in Washington, USA, is in a transitional zone between maritime and continental climate conditions and represents a data gap in observations describing the relationship between forest density and snowpack. We collected three years of snow observations across a range of forest densities to characterize how forest management efforts in this region may influence the magnitude and duration of snow storage. Observations indicate that peak snow storage magnitude in small gaps ranges from the same to over twice that observed in unburned forest plots in the Eastern Cascades. However, differences in snow duration are generally small. Across all Eastern Cascade sites and years, we observed a median difference of snow storage lasting 7 days longer in gaps as compared to nearby forest plots. A notable exception to this pattern occurred at one north-facing site, where snow lasted 30 days longer in the gap. These observations of similar snow storage duration in the Eastern Cascades are attributed to minimal differences in canopy snow interception processes between forests and gaps at some sites, and to higher ablation rates that counterbalance the higher snow accumulation in the gaps at other sites. At the north-facing site, more snow accumulated in the gap, and ablation rates in the open gap were similar to the shaded forest due to the aspect of the site. Thus, snow storage duration was much longer in the gap. Together, these data suggest that prescriptions to reduce forest density through thinning and creating gaps may increase the overall amount of snow storage by reducing loss due to sublimation and melting of canopy-intercepted snow. However, reducing forest density in the Eastern Cascades is unlikely to buffer climate-induced shortening of snow storage duration, with the possible exception of gap creation in north-facing forests. Lastly, these observations fill a spatial and climatic data gap and can be used to support hydrological modeling at spatial and temporal scales that are relevant to forest management decisions.

随着气候变暖，美国（USA）西部正在实施森林疏伐与林隙营造措施，以降低野火风险与森林死亡率。美国华盛顿州东部喀斯喀特山脉地处海洋性气候与大陆性气候的过渡带，其现有观测数据存在缺口——尚未有观测能够完整描述森林密度与积雪储量（snowpack）之间的关联关系。我们针对梯度分布的森林密度样地开展了为期三年的积雪观测，以表征该区域森林管理措施对积雪储量规模及持续时长的影响。观测结果显示，东部喀斯喀特山脉区域小型林隙的峰值积雪储量，与未过火的森林样地持平，甚至可达后者的两倍以上。不过，积雪持续时长的差异总体较小。综合所有东部喀斯喀特山脉样地及观测年份的数据，我们观测到林隙内积雪的持续时长较周边森林样地长7天（以差值中位数计）。但该规律存在一个显著例外：在一处朝北的样地中，林隙内的积雪持续时长较周边森林样地长30天。本次观测中，东部喀斯喀特山脉区域的积雪持续时长总体差异不大，这一现象可归因于两点：其一，部分样地内森林与林隙的冠层积雪截留过程差异极小；其二，部分样地内林隙的积雪消融速率更高，抵消了林隙内更高的积雪积累量。在该朝北样地中，林隙内的积雪积累量更高，而由于该样地的坡向，开阔林隙的积雪消融速率与遮阴森林相近，因此林隙内的积雪持续时长显著更长。综合来看，通过疏伐与营造林隙来降低森林密度的森林经营措施，可通过减少冠层截留积雪的升华与消融损失，提升整体积雪储量。不过，在东部喀斯喀特山脉区域通过降低森林密度，不太可能缓解气候变化导致的积雪持续时长缩短问题——唯有朝北森林营造林隙这一情形可能例外。此外，本次观测填补了该区域在空间与气候维度上的观测数据缺口，可用于支撑与森林经营决策相匹配的时空尺度水文模拟工作。