Plant overwintering strategies Jason Zarnowski

Over-wintering Success Plants, not just animals, have adapted for success in cold climates. As with many evolutionary traits, there is not “best answer”. Common stressors: Low temperatures (as low as -65°C in Siberia) Desiccation Mechanical Stress Success of plants depends on overcoming these two factors

Herbaceous Annuals - One strategy is to simply die. - Surviving winter is an energy expensive process. - Annuals produce hearty seeds that remain dormant until conditions become optimal for germination. Herbaceous Annuals

Herbaceous Perennials - Perennials regenerate year after year. - Root stock stays protected below ground. - New plant produced from existing roots or bulbs. Herbaceous Perennials

Woody-stemmed plants Acclimation to the cold. “…process by which plants each year become tolerant to subfreezing temperatures without sustaining injury,” Marchand. Not the process of producing “anti-freeze”. Plants tissues can undergo “super cooling”. Process by which tissues are colder than freezing point without forming ice. Starts chain reaction of flash freezing that released latent heat. This causes marked rise in temperature around plant stem. Cytoplasmic water migrates from area of greater energy (cytoplasm) to area of lesser energy (ice forming in extracellular space).

Woody-stemmed plants Causes intercellular solute concentration to increase. Lowers freezing point. Formation of ice, not low temperatures, causes cell injury and death. Even non hardy plants can survive -196°C in liquid nitrogen. Water is solidified without being oriented into ice crystals “vitrification”.

Shows ice formation in extracellular space without perforating cell wall. Woody-stemmed Plants

Woody-stemmed plants Slow cooling key to even heartiest plant species. Rapid cooling which occurs occasionally in nature, can cause water within cells to freeze resulting in cell death. Water is trapped in the cell. Water expands nearly 7% when it becomes ice.

Acquiring Freeze Tolerance Two step process that begins in late summer, early fall. First stage linked to end of growing season. Translocation of biochemical compounds. Simple sugar Hormone Second stage linked to first “hard frost”.

Acquiring Freeze Tolerance Alteration of plant membrane also vital. Membrane structure and permeability altered. Lipid content increased for freeze tolerance. Decreased saturation of membrane lipids. Crystallization point of membrane depressed. Lipids exhibit higher flexibility when unsaturated. Increased permeability of membrane offers little resistance to water exiting the cell. Freeze tolerance exerts strong selective pressure for many species. Tolerance often within a few degrees of average minimum temperature in species northern limits.

Desiccation Dry winter winds often cause severe desiccation to plants exposed above snowpack at timberline. Greatest water loss in most plants is on calm, sunny days. Water loss directly proportional to water vapor in outside air. Increased leaf temperature leads to increased evapotranspiration rate.

Desiccation Leaf temperatures can be as high as 20°C above ambient temperature due to insolation. Stomates, cuticle, and boundary layer of air offer resistance to water vapor loss. Wind removes this layer of air increasing water vapor loss potential, but also lowers leaf temperature thus counteraction loss of air layer.

Mechanical Stress Snow accumulated on branches can be substantial. As much as 3000 kg in spruce uplands of Finland. High winds carrying ice particles have affect of a ‘sand blaster’. Can abrade bark accelerating water loss.

Thermogenic Plants Members of the Araceae family. Bloom in late February, early March. Inflorescence can be 15°-35°C above ambient temperature. Large starch reserves and high metabolic rate create heat as a by-product.

Conclusion Strategy for surviving winter not a “one size fits all” approach. For example, deciduous vs. evergreens. Different strategies work equally well for different species. Adapting to cold climates is an active process.