Climate change and soil temperature response across a montane to alpine gradient, Niwot Ridge LTER
Soil temperature is one of the key determinants of carbon flux, nutrient availability, decomposition rates, and primary productivity in high-elevation and high-latitude ecosystems. Global climate models predict that as air temperatures rise there will be a corresponding increase in soil temperature and a longer snow-free season. For ecosystems where temperature is a primary limitation to growth and metabolism, like boreal, arctic, subalpine, and alpine regions, increases in soil temperature are expected to increase N mineralization and subsequent nitrate and ammonium runoff, increase respiration and the release of CO2, and alter plant community composition. Paradoxically, Groffman et al. (2001) have shown experimentally that less snow in a warmer climate may lead to colder soils. Here, we examine whether soil temperatures in seasonally snow-covered areas correlate with air temperatures over an elevational gradient spanning subalpine forests to high alpine in the Front Range of the Colorado Rockies, Niwot Ridge LTER. This area is typified by short growing seasons where the snow free period typically lasts less than 4 months. Soil and air temperature, soil moisture and precipitation were measured at three sites since 2000. At two sites, growing degree days and the frost free period were calculated using a base of 0C and -3C from 1951 to the present. Spatial variation in soil temperature was examined by measuring hourly temperatures at 19 locations using dataloggers buried 5 cm from 2004-2008. Snow depth above the soil temperature loggers was measured approximately twice per month for the same period. Our results show that soil temperature during the winter months is not correlated with air temperature when snow depths are greater than about 50 cm. When snow depths exceed 50 cm, the insulating ability of the snowpack de-couples soil temperatures from air temperatures. Even when air temperatures are less than -30C, the insulating ability of the snowpack coupled with geothermal heat flux from the ground results in soil temperatures right around 0C. Conversely, warmer air temperatures and less snow result in colder soil temperatures. Colder soil temperatures can affect microbial diversity, growth and mineralization rates, and plant diversity and distribution.