The symbiosis between legumes (family Fabaceae) and rhizobia can provide substantial soil N inputs in ecosystems where legumes are abundant. Stressors that adversely affect legumes can thus strongly affect ecosystem nitrogen dynamics. While exposure to freezing can reduce the survival or subsequent growth of plants, herbaceous (i.e., non-woody) species often benefit from snow cover, which can minimize freezing exposure over winter. However, with decreased snow cover and increased temperature variability, the shoot bases and root systems of herbaceous plants may experience more frequent and intense freezing in some regions over the next century. As a follow-up to previous anecdotal observations, we used controlled snow removal experiments in intact old field and prairie communities to examine the question of whether herbaceous legumes are disproportionately susceptible to freezing relative to other herbaceous plants. Snow removal was performed for thirty 1m2 plots which were paired with thirty corresponding ambient snow plots, and we selected the plots to target three non-native old field legume species and one native prairie legume species. The following growing season, we examined percent cover and abundance of the plants non-destructively, and biomass was harvested in late summer.
Results/Conclusions
We found that increased freezing exposure via snow removal significantly reduced the biomass of the non-native herbaceous legumes (Lotus corniculatus, Melilotus spp., Trifolium pratense) when compared to the control, and there were disproportionate impacts of freezing on legumes relative to other functional groups in the old field plots. In contrast, snow removal did not significantly decrease the native legume (Desmodium canadense) compared to the control plots, and in the prairie plots, snow removal significantly decreased graminoid biomass and significantly increased the biomass of non-leguminous forbs. Percent cover and abundance data showed similar trends to those seen in biomass data.
These findings suggest that the disproportionately low freezing tolerance of non-native legumes in northern temperate regions may result from their seeds originating from southern ecotypes introduced for agriculture or erosion control, whereas native legume species may not be disproportionately sensitive to freezing.