Professor Western University London, Ontario, Canada
Legumes (Family: Fabaceae) represent a diverse and ecologically significant plant group. Most described legumes form mutualistic associations with diazotrophic rhizobia, potentially fixing substantial quantities of nitrogen within habitats where they are abundant. Stressors causing lethal or sub-lethal impacts in the legume host or rhizobial symbiont may impact the nitrogen dynamics of such habitats. In recent decades, variability in winter temperatures and soil freeze-thaw cycling have increased in temperate regions. Without adequate snow cover to insulate roots and shoot bases, herbaceous plants are more likely to be exposed to more frequent and severe freezing. In northern temperate regions of North America, a trend of disproportionate freezing sensitivity relative to other herbaceous species has been observed for several herbaceous legumes. To determine the extent and generalizability of this trend, and possible mechanisms, a suite of investigations were conducted. To test responses in situ, a snow removal experiment was conducted using intact temperate plant communities with existing herbaceous legume populations over three years. A subsequent snow removal experiment was conducted utilizing transplanted legumes in two of the aforementioned communities to better understand the impacts across a broader variety of herbaceous legumes, particularly native species, while controlling for size and age uniformity. To determine the impacts of freeze severity and timing, controlled environment chambers were utilized to administer freezing stress. To investigate the influence of rhizobial associations on legume freezing tolerance, plants were assigned to a range of nitrogen levels and inoculation treatments (either compatible rhizobia or none) and frozen in controlled environment chambers.Results from the in situ experiments revealed that while responses were species dependent (even within genera), most of the herbaceous legumes responded more negatively to increased freezing exposure than other herbaceous species, with significant decreases in percent cover, abundance, and aboveground biomass. The experiments under controlled conditions highlighted the importance of freezing severity and timing for the responses of legumes, with more significant reductions in biomass both in response to increased freeze severity and freezing in the spring, rather than winter freezing. Overall, these findings suggest that herbaceous legumes may experience more pronounced impacts relative to non-leguminous species due to changes in freezing exposure in the coming decades. These responses have the potential to cause plant community alterations and significant changes to ecosystem nitrogen dynamics.
