The effect of trees on snowpack: live versus dead lodgepole pines in subalpine forest Dylan Brown Winter Ecology Spring 2014 Mountain Research Station, University of Colorado Boulder

Introduction Mountain pine beetles are increasing the number of dead trees in our forests Dead trees with no remaining needles intercept less snow and solar radiation than live trees (Pugh and Small, 2011) Live trees emit long-wave (IR) radiation and therefore increase the temperature directly underneath the tree (Viglietti et al., 2009)

Question Q: How do live and dead trees differ in their influences on the snowpack stability? To answer this question I compared: snow depth isolated column tests slab size weak layer size and type

Methods Changing Variables: snow depth, stability, and type of layers, grain size Constant variables: Aspect: NE, elevation: 3000m (9800 ft.), slope: 27°, species: Pinus contorta (lodgepole pine), moderate canopy cover Variables that are more difficult to control: air temperature, new snow, incoming solar radiation

Methods Measurements taken directly next to trees; temperature measured at surface, middle, and bottom; snowpack layers described, isolated column test, and slab size Analysis: t-test between live and dead tree: snow depths, slab size, and weak layer size. Chi-square analysis for comparing the compression tests

Results – Snow Depth Dead Tree mean z = cm Live Tree mean z = 79.5cm T-test (2 tailed, paired) p value =

Results - Stability Dead Trees – Columns easily failed at the depth hoar layer Live Trees – Columns were more difficult to get to fracture Chi-square analysis: p – value:

Results – Slab Size Mean Dead tree slab size = 78.25cm Mean Live tree slab size = T-test (2 tailed, paired) p value =

Results – Weak Layers Dead Trees showed larger depth hoar/faceting layers (avg cm) with larger grain sizes

Results – Weak Layers Live Trees showed smaller faceting layers (avg cm) with smaller grain sizes T-Test (2 tailed, paired) p – value =

Discussion Live and dead trees have significantly different influences on the snowpack in their immediate surroundings Snow depth, Stability, Slab size, weak layer size Dead trees create a less stable snowpack Larger weak layers and slabs Both can still act as trigger points (Viglietti et al., 2009) Future Research: Temperature differences, different terrain, different species

References Pugh, E., & Small, E. (2011). The impact of pine beetle infestation on snow accumulation and melt in the headwaters of the Colorado River. Ecohydrology, 5, Retrieved February 13, 2014, from Viglietti, D., Letey, S., Motta, R., Maggioni, M., & Freppaz, M. (2009). Snow and avalanche: the influence of forest on snowpack stability. International Snow Science Workshop, n/a. Retrieved February 13, 2014, from pdf