High sensitivity of herbaceous legumes to freezing: insights from a multi-year snow removal study

Reductions in snow cover resulting from winter climate change are anticipated to increase the intensity of freezing exposure for overwintering herbaceous plants in many northern temperate regions. We examined whether herbaceous legumes, a key functional group responsible for increasing soil nitrogen inputs, are more sensitive to freezing than non-leguminous plants in northern temperate plant communities. We conducted snow removal experiments for three years in three herbaceous plant communities (a fallow area, an old field and a restored prairie) and recorded plant cover and biomass responses. For all three years, there were substantial decreases in legume biomass in response to snow removal for the fallow area (reductions of 70%, 90% and 50%; primarily Melilotus albus and M. officinalis) and old field (reductions of 50%, 60% and 50%; primarily Trifolium pratense and Lotus corniculatus), whereas there were no consistent significant reductions in biomass across years for the other herbaceous functional groups (graminoids and non-leguminous forbs). In contrast, in the restored prairie, dominated by the native legume Desmodium canadense, there were no consistent snow removal effects on the biomass of legumes or any other functional groups across years, although snow removal did reduce the percent cover of this species early in the growing season. Our results suggest that increased soil freezing via reductions in snow cover can have a stronger negative effect on some legumes than on other functional groups in northern temperate herbaceous plant communities, potentially reducing soil nitrogen inputs.