Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Biology

Supervisor

Henry, Hugh A. L.

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.

Summary for Lay Audience

Non-woody plants that live in areas with seasonal freezing temperatures must survive under the ground over winter. Plants can avoid freezing stress by overwintering deep within the soil, under dead plant material, and under snow, all of which act as insulation. However, plants deep in the soil have the added cost of growing to reach the soil surface in spring. In this project, I explored the strength and generality of this trade-off using a range of species. I first used seven plant species adapted for high storage and deep growth. They were planted at different times and exposed to different depths and levels of frost stress. Generally, plants grew more if they overwintered deep when winter temperatures were severe, but grew more if they overwintered shallow when winter soil temperatures were milder. I then studied six species with belowground stems near the soil surface. They were planted at different times and exposed to different depths, cover of dead plant material, and levels of frost stress. For half of the species deep soil placement was a cost when winter soil temperatures were mild, and the response to the thickness of dead plant matter cover was highly variable. Finally, I exposed plants in a mature plant community to freezing stress and compared how the responses varied among species with different types of belowground structures. Freezing stress reduced plant growth, with tap-rooted species being the most sensitive. These trade-offs with respect to the depth of overwintering are particularly important to consider in the context of future changes in winter soil temperatures caused by climate warming.

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