Lecture Notes


  1. Skunk cabbage is endothermic, makes ring of melted snow around it.

  2. Many plants can survive extremely cold temperatures if cooled slowly.

  3. Killing temperature of plants in the summer is around freezing, but by september, is below freezing.

  4. Acclimation related to freezing stages?

    1. Two stages of exotherms (release of heat) seen in rapidly cooled twig.

    2. Double peak

      1. Intercellular and xylem

      2. Interpores and cell walls

    3. Later Peak

      1. Intracellular freezing and cell death.
  5. First peak - extracelllular freezing.

    1. Water between cells freezes, drawing water out of the cell, increasing concentration of salts in cell, lowering freezing point of cytoplasm. Allows for vitrification (glass formation) which prevents damaging crystals.
  6. Second peak

    1. Inside of cells freeze, twig dies.
  7. Acclimation is sometimes a two-stage process.

    1. Cessation of growth, accumulation of sugars or other hormones or cryoprotectants.

      1. Abscisic acid - hormone that halts growth, increases membrane permeability to draw water out.
    2. Second stage acclimation is associated with intracellular reorganization (movement of photosystems)

  8. Two stage hardening less apparent in boreal trees (phytochrome system)

    1. Day length cues and cool temperatures are enough to signal hardening.

    2. Length of day is perceived with phytochromes.

      1. Phytochrome Pr is sensitive to red (650-680 nm), Pfr is sensitive to far-red (710-740 nm).

      2. Active form sensitive to far-red is Pfr.

      3. When Pfr gets lots of far-red, gets converted to Pr. When Pr gets a lot of red light (in the daytime), gets converted to Pfr.

      4. Conversion of Pr to Pfr in the daylight triggers clorophyll synthesis and leaf expansion.

      5. When the days are shorter near winter, you get more Pr, gearing the plant up for winter

    3. Temperature mediates transition between Pfr and Pr.

      1. Warm temperatures shift Pfr to Pr. (Complicates things)
  9. Does winter temperature limit species’ range?

    1. In some species, but in other species the killing temperature is far below the lowest temperatures in the area.

    2. Realized niche is the same as fundamental niche for the dominant species, where the only limiting thing is the winter temperature, not competing species.

    3. Other species have a smaller realized niche, range is limited by competition.

    4. Explanation for freeze tolerance being a lot lower than actual temperatures could be that cold hardiness is not costly, another one could be selection for glaciation periods earlier in recent history.

    5. However there is geographic variation in hardiness, which suggests it is costly.

  10. Pinus nigra Arnold (Black Pine), Kreyling study was common garden study, all plants subjected to 1.6 C warming and 42 day drought.

    1. Found origin affects coldhardiness, killing temperature higher in southern italy and france (genetic differences), where it’s warmer and nonmountainous.

    2. Drought increases cold hardiness.

    3. Low moisture increases cold hardiness.

    4. Warmness in winter allows bark beetles to be more common, which ultimately kills trees even if they are more cold hardy. So climate change decreases tree survival.

  11. Bark beetle population explosion in last 20 years has lead to vastly more dead trees and vastly more wild fires.

  12. Species whose geographic limits are at the edge of their cold tolerance are the ones expected to change with climate change. Species whose cold tolerance is well below the average winter temps not expected to be significantly affected by climate change.

Water balance in winter

  1. Three questions

    1. How stressful is winter in terms of desiccation?

    2. Is primary water loss through stomata or cuticle?

    3. Are cold winds worst or calm sunny days?

      1. Intuition is that wind would be worse. However for pine needles this may not be the case.
  2. Stomata are easiest paths to lose water. Cuticle has a covering to prevent water loss. If air is humid, not much moisture will leave. If air is dry, there will be more evaporation. Gradient is most important.

  3. Warmer leaves have much higher vapor pressure than cold air. Liquid leaves much faster when leaves are warm.

    1. Sun heats leaves up, wind cools them down via convection.
  4. Resistance to water loss or temperature loss is measured in seconds/cm

    1. Cuticle (200-1000 seconds/cm) has greater resistance than open stomates (2-20 seconds/cm).
  5. To reduce water loss needles sink stomata, reduce surface area, become thicker and rounder.

  6. Two opposing effects of wind

    1. Removes moist air from boundary layer around leaf, increases moisture gradient and increases water loss.

    2. Removes warm air from boundary layer surrounding leaf

      1. Eliminates temp gradient and cools leaf

      2. Reduces vapor pressure inside leaf.

  7. Wind overall cools leaves.

  8. Effect of wind is different in winter.

    1. In summer with open stomata, boundary layer is important (resistance is similar to open stomata, wind increases transpiration).

    2. In winter, with closed stomata, boundary layer protection is less important, drying effect of wind on movement through cuticle is more important.

  9. Is water transport possible in winter?

    1. Must have continuous flow of water, if you break the tension anywhere, there is a stop in the upward pull.

    2. Freezing can lead to bubbles or cavitation, breaking tension in xylem.

      1. Evergreen trees have big advantage against deciduous trees. Deciduous trees have bubble formation, so water transport is stalled.

      2. Evergreen trees have self-sealing tracheid segments, which hold water, maintaining pressure, preventing bubble formation.

    3. Water can increase in branches during thaws.

      1. Evergreens have less water loss than birch, but all have some transport during thaws. Birch may have narrower tubes to prevent bubbling.
  10. Overwintering conifers use stored water.

    1. Stored water in bark and xylem may be sufficient to last much of the winter.

    2. Do leaves/needles absorb water?

      1. Positive vapor pressure only from saturated air in snow.
    3. Marchand experiment

      1. Cut off supply from roots, allow snow to cover trunk, cover nothing, or portion of needles.

      2. First tree tested for how much moisture it absorbed through the needles from the snow.

      3. Found that storage is critical, absorption through leaves is not.

Evergreen advantage of early photosynthesis

  1. Why be an evergreen?

    1. In milder climates, photosynthesis is possibel during several thaws.

      1. Not possible in boreal forests.
    2. Earlier photosynthesis.

      1. Much lower risk of losing a season of growth due to late freeze.
    3. Every once in a while, deciduous tree will leaf out when it’s a late freeze, fooling it and killing it. Evergreen will not do this. Also vulnerable to branch death because of snow storm. Evergreen can’t get weighed down.

  2. Deciduous trees do photosynthesis in bark

    1. Cutting branches off from other parts of the tree still results in starch production. Girdling is constraining the tree so that it cannot produce anything.

    2. At low temps bark photosynthesis can use most of the CO2 the tree produces by respiration. Recycled energy. When it’s warm, respiration is much greater, so need photosynthesis from leaves to keep up.

  3. Conifer Adaptations

    1. Cold-temperature photosynthesis.

    2. Torus-valves in xylem prevent bubbling.

    3. Downward sloped branches reduce snow damage.

Wind stress near treeline

  1. Ice Erosion is a greater threat than snow.

  2. Rime ice forms on branches. Rime is when moist air hits a nucleator on a branch.

  3. Waves of mortality/germination at treeline.

  4. Krumholtz is trees growing along the ground to limit exposure to wind.