In the northeastern United States, climate projections suggest that winter snow cover will become more variable, leaving longer periods of time with little to no snow cover. Because removing snow cover removes insulation, reductions in snow cover may result in colder soils. For shallowly rooted plants, such as the spring ephemeral Thalictrum dioicum, colder soils could result in increased over-winter mortality and damage to storage organs, thus potentially affecting both growth and reproduction. In order to determine how changes in winter snow cover affect this plant species, we performed a three-year snowfall manipulation experiment in a common garden setting. In order to simulate projected snowfall patterns, we reduced snow cover after every accumulation greater than 10 cms. We measured plant vital rates (ex: growth, survival, fecundity) and soil temperature and soil moisture in both control and reduced conditions. We analyzed individual level responses to changes in snow cover, and assessed projected population level responses using population matrix models. In long-lived perennials, survival of reproductive adults typically drives population dynamics. Therefore, we performed a Life Table Response Experiment (LTRE) to determine how differences in the vital rates contributed to differences in the projected population growth rate.
Results/Conclusions:
Snow cover reduction generally resulted in significantly colder soils in both degree and duration, but did not affect soil moisture in early spring. There was no difference in survival across the treatments in any of the years; however, reproduction was lower in the reduced snow cover treatment in the first and third years (year 1: p=0.0525, year 3: p=0.0244). Using matrix models we calculated the projected population growth rate (λ) for each treatment across 2 yearly transitions. In both models, λ>1 in the control treatment, suggesting population growth, whereas λ<1 were observed in the snow reduction treatment, indicating population decline. Elasticity analyses suggested that survival of the seed bank in both the control and snow reduction treatments had the greatest relative contribution to the projected population growth rates. Also, the elasticities for fecundity of small reproductive individuals were much greater in the control than in the snow reduction treatments. Life Table Response Experiments concluded that differences in the fecundity of small reproductive individuals had the greatest effect on differences in λ, suggesting that reductions in fecundity, not survival, are more likely to harm these populations as snow cover declines.