The disproportionate sensitivity of herbaceous legumes to freezing: exploration of responses to freezing timing and severity under controlled conditions

Background/Question/Methods

: In northern temperate regions, the results of snow removal experiments conducted in herbaceous plant communities have suggested that leguminous forbs may be more sensitive to freezing than non-leguminous forbs and grasses. However, snow removal does not provide precise control over freezing severity, nor can it be used to assess freezing responses in early spring when snow is absent. Our objective was therefore to examine the relative freezing sensitivities of herbaceous legumes from these regions in the context of variation in freezing timing (season) and severity under controlled conditions. We collected legumes (4 native and 5 non-native species) and non-legumes from old field and tallgrass prairie communities and established them in pots and soil-plant mesocosms. We incubated the plants outdoors prior to and after the freezing treatments and covered them with a thin layer of hay. Freezing was administered in controlled environment chambers set to 0°, -5° or -10° C, and freezing treatments were conducted in winter or spring (a control treatment remained outdoors for experiment’s duration). The plants were harvested in mid-summer (July). Prior to harvest, mortality and height were assessed periodically. Following harvest, we separated the plants into belowground, aboveground, and reproductive structures, which were dried and weighed.

Results/Conclusions

: The non-native legume species (Lotus corniculatus, Securigera varia, Trifolium repens, and T. pratense), as well as three of the four native legumes (Desmodium paniculatum, Lespedeza capitata, and L. hirta) decreased in aboveground biomass with increasing freezing severity in both seasons relative to the control. All of these species exhibited more severe freezing responses than the non-leguminous species. Additionally, L. hirta and S. varia experienced complete mortality in response to -10° C in the spring. The remaining native species, D. canadense, only decreased in biomass in response to spring freezing at -10° C, which was comparable to the non-leguminous species. Overall, these results confirm disproportionate sensitivity of legumes in these plant communities and indicate that the legumes are most sensitive to freezing in the early spring, at a time when they are not insulated from cold air temperatures by snow cover. Increased spring temperature variability with climate change could therefore lead to disproportionate damage to this important plant functional group in herbaceous plant communities.