Remote sensing-based monitoring of snow cover dynamics and its influence on vegetation growth in the Middle Atlas Mountains

Abstract: Endowed with diverse vegetation covers, the Middle Atlas Mountains receive substantial snowfall in winter, which contributes to plant hydration during the growing season. Using remotely-sensed imagery, this study aims to assess the trends in snow cover variations between 2000 and 2015, analyse the local influence of snowpack persistence on vegetation growth and evaluate the significance of this effect by comparing snow-fed and non-snow-fed sites. Trend analyses with Mann-Kendall and Ordinary Least-Squares (OLS) methods showed that, at the regional level, maximum Snow Cover Area (SCA) did not significantly change. However, fine scale analysis of SCA and Snow Cover Duration (SCD) along elevations and among basins revealed a pattern of increase in the mid-elevations, particularly in the range 2250 - 2500 m and the ocean-facing Sebou basin. This tendency was supported by significantly larger maximum SCAs in spring for Sebou and the region as a whole. With lower certainty, SCA and SCD also flagged a non-significant decrease in the low altitudes, particularly in the more continental south-facing zones of Oum Er-Rbia basin. Kendall Rank correlation between SCD and vegetation growth, as approximated with the Plant Phenology Index (PPI), indicated that, while non-prominent in the low elevations, plant sensitivity to snow increased in the mid-elevations, either positively, mainly in the wetter and more fertile windward plateaus, or negatively, essentially on ridges, complex terrains and on the drier and highly-drained Moulouya slopes. Forests were the most benefitting from snow, while shrublands appeared the most adversely impacted. Above a certain altitudinal threshold between 2000 and 2500 m, snow influence became drastically negative for all the vegetation types. Finally, using Kruskall-Wallis test and Mann–Whitney U pair-wise comparisons, the gradual strengthening of snow effect with duration was highlighted for a sample of shrublands located in the range 2000 - 2250 m in the rain-shadowed Moulouya basin.