Elevation-dependent influence of snow accumulation on forest greening

Abstract

Rising temperatures and declining water availability have influenced the ecological function of mountain forests over the past half-century. For instance, warming in spring and summer and shifts towards earlier snowmelt are associated with an increase in wildfire activity and tree mortality in mountain forests in the western United States 1,2 . Temperature increases are expected to continue during the twenty-first century in mountain ecosystems across the globe 3,4 , with uncertain consequences. Here, we examine the influence of interannual variations in snowpack accumulation on forest greenness in the Sierra Nevada Mountains, California, between 1982 and 2006. Using observational records of snow accumulation and satellite data on vegetation greenness we show that vegetation greenness increases with snow accumulation. Indeed, we show that variations in maximum snow accumulation explain over 50% of the interannual variability in peak forest greenness across the Sierra Nevada region. The extent to which snow accumulation can explain variations in greenness varies with elevation, reaching a maximum in the water-limited midelevations, between 2,000 and 2,600 m. In situ measurements of carbon uptake and snow accumulation along an elevational transect in the region confirm the elevation dependence of this relationship. We suggest that mid-elevation mountain forest ecosystems could prove particularly sensitive to future increases in temperature and concurrent changes in snow accumulation and melt.Recent studies have documented a shift from energy to water limitation across forested ecosystems of western North America 5,6 . This transformation has reversed the response of these ecosystems to increases in temperature where before the early 1990s increases in air temperature increased terrestrial carbon uptake. Following the early 1990s an apparent shift from energy to water limitation resulted in reduced carbon uptake with increased temperature and coincident decreases in water availability 5 . In the western United States, increases in regional spring-summer temperatures and earlier snowmelt since the mid-1980s strongly correlate with increases in forest wildfire activity 1 and increases in tree mortality rates 2 . A consistent message has emerged from these studies: the combined effects of increases in temperature and decreases in water availability over the past half-century have impacted the ecological function of mountain forests.The sensitivity of mid-latitude mountain forests to water availability and the associated importance of snowmelt water has been well documented at the plot scale [7][8][9][10] . However, the effects of variations in snowpack accumulation on vegetation activity

Temperature and Snow-Mediated Moisture Controls of Summer Photosynthetic Activity in Northern Terrestrial Ecosystems between 1982 and 2011

“…Positive influences of deeper winter snowpack on subsequent peak summer photosynthetic activity and carbon uptake have been well documented in alpine forests in the western United States, where typically more than half of the annual precipitation falls as snow [32,33]. These mountain forests rely heavily on snowmelt water even late into the growing season and variations in maximum snow accumulation can explain over 50% of the interannual variability in peak forest photosynthetic activity [33].…”

“…This is because the (monthly) resolution of the meteorological forcing data is quite coarse. However, the timing of snowmelt is closely associated with the amount of snow accumulated in the winter snowpack [33] and thus our scPDSI estimates may still incorporate the effect of the widespread reduction in spring snow cover observed at northern latitudes [36].…”

“…These mountain forests rely heavily on snowmelt water even late into the growing season and variations in maximum snow accumulation can explain over 50% of the interannual variability in peak forest photosynthetic activity [33]. Earlier growing seasons in these snow-dominated environments correlate with shallower snowpacks and result in reduced rather than increased growing season productivity because of insufficient spring soil moisture recharge to sustain summer growth [32].…”

“…The potential influence of changes in winter snowpack on summer soil moisture status and vegetation productivity has not received much attention, despite the well-known controls of snow dynamics on the hydrology and phenology of northern ecosystems [1,29,30]. Local studies have shown that reductions in winter snowpack and earlier spring snowmelt can lead to severe summer soil moisture deficits and productivity losses in boreal and alpine regions [31][32][33], where snow is typically the main source of soil water recharge. A positive association has also been observed between deeper snowpack and summer NDVI in central Siberia and parts of North America [34,35].…”

Mountain runoff vulnerability to increased evapotranspiration with vegetation expansion

“…Widespread increases in subalpine tree growth, tree-line altitude, and species distribution with elevation have been reported with recent climate trends in California and elsewhere, implying that rapid vegetation shifts are possible (14)(15)(16). Time series of Sierra Nevada forest greenness indicate a transition from water limitation at low elevation to cold limitation at high altitude, implying that upper elevation ET is sensitive to warming (17). Nonetheless, the extent to which annual montane ET is currently temperature-limited, as well as the sensitivity of large-scale ET to vegetation redistribution, remain largely unquantified.…”