Freezing tolerance of herbaceous legumes within southwestern Ontario: evidence of disproportionate freezing sensitivity
Abstract
Legumes (Fabaceae) represent a diverse and ecologically significant plant taxon; most described legumes form mutualisms with diazotrophic rhizobia, potentially fixing substantial quantities of nitrogen within habitats where they are well-established. Stressors causing lethal or sub-lethal impacts in legumes or rhizobial symbionts may therefore impact the nitrogen dynamics of such habitats. In recent decades, variability of winter temperatures, precipitation, and soil freeze-thaw cycling has increased in temperate regions. Without adequate snow cover to insulate roots and shoot bases, herbaceous plants will likely be exposed to more frequent or severe freezing. In southwestern Ontario, a pattern of disproportionate freezing sensitivity relative to other herbaceous species has been observed for several herbaceous legumes. To determine the extent and generalizability of this pattern, I tested responses in situ using a three-year snow removal experiment within intact temperate plant communities with existing herbaceous legume populations. I conducted a snow removal experiment utilizing transplanted legumes to better understand the impacts across a greater variety of herbaceous legumes, particularly native species, while controlling for size and age. In situ experimentation revealed that while responses were species dependent (even within genera), most herbaceous legumes responded more negatively to increased freezing exposure than other herbaceous species, with significant decreases in cover, abundance, and aboveground biomass. I then investigated the impacts of freezing severity and timing on legumes relative to non-legumes using controlled environment chambers. The use of controlled environment chambers highlighted more substantial reductions in biomass in response to increased freezing severity and spring rather than winter freezing. Finally, I assessed the influence of rhizobial associations on freezing tolerance in four legumes using inoculation treatments and nutrient solutions differing in nitrogen content, and the prevalence of cyanogenesis among the legumes studied prior. Rhizobial associations increased freezing tolerance in terms of survival and growth in a species-specific manner rather than consistently. Many species which were disproportionately susceptible to freezing were predominantly acyanogenic. These findings suggest many herbaceous legumes may experience disproportionate impacts relative to non-legumes with future changes in freezing exposure, potentially causing significant alterations to plant communities and ecosystem nitrogen dynamics.