The Cost of Protection: Frost Avoidance and Competition in Herbaceous Plants
Abstract
Perennial herbaceous plants in regions that experience winter freezing must survive using belowground structures that can tolerate or avoid frost stress. Soil and plant litter can insulate plant structures from frost exposure, but plants must invest into growth to penetrate through these layers to reach the surface in the spring. The overall goal of my thesis was to test the hypothesis that the protection of overwintering clonal structures by soil or plant litter (frost avoidance) comes at the expense of subsequent reduced growth and competitive ability in absence of freezing stress. I first explored this trade-off with a suite of experiments using plants with bulbs and stem tubers - storage-focused organs that are typically located below the soil surface. Seven plant species were subjected to different burial and frost exposure treatments (via snow removal) to disentangle the relationship between frost avoidance and the cost of organ depth. I then examined frost avoidance trade-offs for species with shallow bud placement (rhizomes). Rhizome fragments of six species were subjected to different soil depth and litter cover treatments combined with frost exposure treatments. There was a general trend of increased growth with depth under snow removal (increases soil freezing), but decreased growth with depth under ambient snow cover. These results were consistent with the mortality and growth trends observed for the species in controlled environment freezing trials. Responses to litter thickness were more variable. I also examined the freezing responses of mature plants within a self-assembled, old field community over three separate winters using snow removal. Species responses were pooled based on recruitment, organ of perennation, and life form (bud placement). Snow removal decreased total plant cover, primarily in species with shallow bud recruitment. Snow removal responses also varied based on recruitment depth and organ of perennation. These are the first studies to explore the trade-off between frost avoidance and competitive ability with growing depth in herbaceous species. In northern temperate regions, the balance of this trade-off may be altered by future increases in soil freezing intensity caused by declining snow cover and increased temperature variability in a warmer climate.