ABSTRACT:

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.

ABSTRACT:

Forests modify snow accumulation and ablation rates, and overall snow storage amounts and durations, with multiple processes acting simultaneously and often in different directions. To synthesize complex forest-snow relations and help guide near-term management decisions, we present a decision tree model based on a hypothesized hierarchy of processes and associated variables that predict forest effects on snow storage. In locations with high wind speeds, forests enhance snow storage magnitude and duration relative to open areas. Where wind speeds are low, and winter and spring air temperatures are colder, forests diminish snow storage magnitude but enhance duration. Where air temperatures are warmer, forests diminish both magnitude and duration. Forest structure and aspect are secondary influences that shift the net effect of forest on snow storage. We apply the model to map the influence of forests on snow storage under historic and warming climate conditions across the western United States, but this model is applicable in any region with forests and snow. The decision tree model provides practitioners a first-step evaluation to guide management decisions that consider where and how forests can be managed to optimize in-situ water storage alongside other objectives, such as reducing wildfire fuels. This framework also articulates geospatial hypotheses, in order of anticipated importance, to be tested in future investigations of forest-snow-climate relations.

The data and code included herein are described in Dickerson-Lange, et al. 2021, Ranking forest effects on snow storage: a decision tool for forest management, Water Resources Research. The repository contains all input data, model code, and results.