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Influence of within-stand tree spatial arrangement on snowpack distribution and ablation in the Sierra Nevada, CA


Much of the Sierra Nevada forests are overstocked with too many trees, are at risk of burning in stand-replacing wildfires due to the fuel loading, contain almost complete canopy coverage over the landscape and intercept a substantial portion of the snowpack, and in need of thinning. New research shows that forest thinning patterns allowing for forest structures comprising of clumps of varying density, single individual trees, and forest gaps are what the historic stands in Sierra Nevada were comprised of. In addition to returning the forest to historic stand structure, forest thinning will also allow for greater amounts of snowpack to reach the forest floor and persist longer into the season before complete meltout. This study set out to see if this new thinning spacing pattern would affect snow accumulation or ablation rates as compared with the current standard even-tree spacing. Data from four snow surveys, distributed light/temperature sensors, hemispherical photographs, and a small distributed network of snow-depth sensors were analyzed from three different forest treatments in the Sierra Nevada. The first treatment consisted of variable thinning, which reduced the stems per acre by 80%, the basal area per acre by 40%, and created a heterogeneous forest structure with small gaps and clumps of trees of varying density. The second forest treatment reduced the stems per acre and the basal area per acre stand down to similar levels as the variable thinning, however the residual trees were left to replicate a homogenous, even spacing. An unthinned control provided a comparison to pre-treatment conditions. Snow-survey data showed that approximately 10-20% more snow accumulated in the two thinning treatments than in the control. Differences in snow depth between the two thinning treatments were statistically insignificant. Snow-survey results also indicated that the variable treatments kept snow on the ground longer than the even treatments in the 2013 water year, which had a peak snow water equivalent (SWE) of 17.9 cm, and melted out at approximately the same time as the control units. In 2014 the peak SWE recorded was only 6.3 cm, both the variable and even treatments melted out at the same time, and the control units kept a steady snowpack for approximately 1-3 days longer. Data from snow-surveys, distributed light/temperature sensors and snow-depth sensors showed that more snow accumulated in the open than at the drip edge and under the forest canopy, and that melt rates are faster in the open than under the canopy. Our data showed that the variable treatment held the snowpack longer and exhibited a slower melt rate than did the even treatments indicating a possibility to retain substantial amounts of snow in the mountains longer if this thinning structure were to be implemented on a landscape level.

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