Do herbaceous legumes exhibit disproportionate freezing sensitivity relative to their non-leguminous neighbors? Results from a field experiment using legume transplants

Samuel Rycroft, Department of Biology, University of Western Ontario, London, ON, Canada and Hugh Henry, Biology, University of Western Ontario, London, ON, Canada

Background/Question/Methods
Most described legumes (family: Fabaceae) form symbioses with nitrogen-fixing rhizobia. These associations increase soil nitrogen inputs, potentially increasing ecosystem fertility where legumes are abundant. Stressors which affect legumes can therefore impact ecosystem nitrogen dynamics. A common stressor in northern temperate regions is freezing, which can cause mortality or have sub-lethal effects (e.g., reduction in subsequent growth). Herbaceous (i.e., non-woody) species can benefit from snow cover, as it minimizes freezing exposure over winter. Reductions in snow cover, such as those resulting from increased temperature variability, can thus cause plants to experience more severe and frequent freezing. Prior findings suggest herbaceous legumes may be disproportionately susceptible to freezing relative to other functional groups, but these results were based on a limited number of naturally occurring legume species. We explored the generality of this trend by transplanting 5 old field legume species and 4 prairie legume species into their respective community types and subjecting them to snow removal treatments (to increase the soil freezing intensity) versus ambient snow (1 m2 plots, n=10). The following spring and summer we examined percent cover and plant abundance non-destructively and harvested biomass later in the summer to correspond with peak biomass.
Results/Conclusions
The biomasses of old field legumes (Lotus corniculatus, Melilotus officinalis, Trifolium pratense, Trifolium repens) decreased significantly in response to snow removal, and these decreases exceeded the snow removal effect on the non-leguminous forbs. There was no significant snow removal effect on graminoid biomass. For the prairie species, there was no significant snow removal effect on Desmodium canadense biomass, but there were significant reductions in biomass for the other three transplanted legume species (Desmodium paniculatum, Lespedeza capitata, Lespedeza hirta). These reductions in biomass in response to snow removal exceeded those of the non-leguminous prairie forbs, and there was no significant effect on graminoid biomass. These findings suggest that the high freezing susceptibility of herbaceous legumes in northern temperature regions may be a broader phenomenon, and thus may have important implications for changes in ecosystem nitrogen cycling in response to climate change and increased variability in winter conditions.