Exponential growth of ‘‘snow molds’’ at sub-zero temperatures: an explanation for high beneath-snow respiration rates and Q10 values

Numerous studies have demonstrated exceptionally high temperature sensitivity of the beneath-snow respiratory flux in cold-winter ecosystems. The most common, but still untested, explanation for this high sensitivity is a physical one based on the observation that water availability in soils increases exponentially as soils warm from -3 to 0 C. Here, we present evidence for a biological hypothesis to explain exponential kinetics and high Q10 values as beneath-snow soils warm from -3 to 0 C during the early spring in a high-elevation subalpine forest. First, we show that some of the dominant organisms of the beneath-snow microbial community, ‘‘snow molds’’, exhibit robust exponential growth at temperatures from -3 to -0.3 C. Second, Q10 values based on growth rates across the temperature range of -2 to -0.3 C for these snow molds vary from 22 to 330. Third, we derive an analytical equation that combines the relative contributions of microbial growth and microbial metabolism to the temperature sensitivity of respiration. Finally, we use this equation to show that with only moderate snow mold growth (several generations), the combined sensitivities of growth and metabolism to small changes in beneath-snow soil temperature, create a double exponential in the Q10 function that may explain the extremely high (*1 9 10) Q10 values observed in past studies. Our biological explanation for high Q10 levels is supported by several independent studies that have demonstrated build up of microbial biomass under the snow as temperatures warm from -2 to 0 C.