Doctor of Philosophy
Henry, Hugh A.L.
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.
Summary for Lay Audience
Legumes can increase soil nitrogen through associations with bacteria called rhizobia. Factors that can cause stress for legumes or rhizobia may impact nitrogen cycling. One important factor for plants in northern temperate regions is freezing stress. In recent years, winter temperature and snowfall variability have increased in these regions. Plant species with roots and other structures near the soil surface may experience more frequent or severe freezing. Evidence suggests non-woody legumes may be more vulnerable to freezing than non-legumes. Higher freezing sensitivity in legumes may impact soil nitrogen and plant performance within affected areas. I examined freezing responses of legumes and non-legumes using snow removal experiments in established plant communities. I also examined legumes and non-legumes using experiments in controlled environment chambers. Field experiments showed that many legumes exposed to increased freezing responded worse than non-legumes in the following growing season. Chamber experiments showed that the timing of freezing was more influential for legumes than non-legumes for survival and growth. Spring freezing caused the most negative responses. I then investigated potential mechanisms behind the responses I observed in previous experiments. I grew four species of legumes with or without rhizobia and with different amounts of nitrogen. I assessed the responses of the legumes to different freezing temperatures. I also investigated cyanogenesis (a biochemical defense pathway which is present in many legumes and other plants) to see if this too may impact freezing tolerance given prior findings on decreased freeze tolerance in select legumes. In the rhizobia experiment, responses were more specific to the legume species studied. Most legumes studied were not cyanogenic. My findings suggest many legumes may be more vulnerable to future changes in winter conditions than non-legumes. This may influence species composition and abundance as well as nitrogen cycling in affected habitats.
Rycroft, Samuel L., "Freezing tolerance of herbaceous legumes within southwestern Ontario: evidence of disproportionate freezing sensitivity" (2023). Electronic Thesis and Dissertation Repository. 9917.
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