Snow cover variability in a forest ecotone of the Oregon Cascades via MODIS Terra products

Snowcover pattern and persistence have important implications for planetary energy balance, climate sensitivity
to forcings, and vegetation structure, function, and composition. Variability in snow cover within mountainous
regions of the Pacific Northwest, USA is attributable to a combination of anthropogenic climate change and climate
oscillations. However, snowcovered areas can be heterogeneous and patchy, requiring more detailed mapping
of snow trends to understand their potential influences on montane forests. We used standard dailyMODIS
snow products (MOD10A1.5) to investigate the 15-year record (2000–2014) of snow cover in the predominant
forest ecotone of the Oregon Western Cascades. We modeled the ecotone using field data from the H.J. Andrews
Experimental Forest, and only considered forested MODIS Terra pixels located within the mapped ecotone of a
five-county region. Three snow cover metrics were developed using both binary and fractional snow cover
data: mean ecotone snow cover percent, number of snow covered days during the melt season, and day of
snow disappearance. Snow cover and depletion dates exhibited large interannual variability and no significant
linear trends. This variability is likely influenced by the preceding wintertime states of the Pacific Decadal Oscillation
(PDO) and the El Niño/Southern Oscillation (ENSO), which tend to covary. We improve and generalize
existing methods for power analysis of trend estimation and quantify the number of uninterrupted observations
of the snowmetrics thatwould be needed to distinguish trends of different magnitudes fromnoise variance, taking
possible autocorrelation into account. Sensitivity analyses of the results to some of our heuristic choices are
conducted, and challenges associated with optical remote sensing of snow in a dense montane forest are
discussed.

Keywords: Snowmelt metrics, Depletion curves, Western hemlock-true fir ecotone, Generalized least squares regression, Trend power analysis,
PDO/ENSO, Pacific Northwest