Cold environments: arctic and alpine examples

Temperature gradients are found on all known planets. Earth and Mars both have ice caps at the poles, although Earth has only water ice, whereas Mars appears to have both water ice and carbon dioxide ice. High mountain tops have similar extremes of cold, and so the vegetation of arctic and alpine regions is usually treated jointly as arctic–alpine vegetation (Figure 4.27). These environments share a number of environmental characteristics that put stress on plants (Billings and Mooney1968, Savile1972): (1) low winter temperature, (2) low summer temperature, (3) short growing season, (4) strong winds, (5) long photoperiod, (6) low light intensity, (7) low soil nitro-gen, and (8) low precipitation. The flora of the arctic and alpine regions is drawn from many different plant families, but all species must be able to cope with these environmental conditions.

AKLA VIK3 m102 mm 120100

80

6040200DNOSAJJM

M

J F

–10–20–30–4010020 °C –9.1

A BAKER LAKE 4m 150 mm 120100

80

6040200DNOSAJJM

MJ F

–10–20–30–4010020 °C –11.9

A GODTHAAB 27 m 736 mm 120100

80

6040200DNOSAJJM

M

6040200DNOSAJJM

M

6040200DNOSAJJM

M

6040200DNOSAJJM

M

J F

–10–20–30–4010020 °C –3 .6A MURMANSK 46 m

386 mm 120100

80

6040200DNOSAJJM

M

6040200DNOSAJJM

M

J F

–10–20–30–4010020 °C –13 .8

A

Figure 4:27 Distribution of polar and high mountain tundra ecosystems and representative climatic conditions. Mean monthly temperatures are indicated by the line and mean precipitation for each month is shown by the bars. Station elevation, mean annual temperature, and mean precipitation appear at the top of each climograph (from Archibold 1995).

Low temperatures appear to be the overriding factor. Life as we know it requires liquid water and, if the water is frozen, metabolic processes cannot occur. Further, when ice crystals form in tissues, cell walls and membranes are ruptured (for a graphic demonstration, take a piece of soft plant tissue like a fruit, freeze it, thaw it, and note how the tissues are softened and how much water leaks out). The exact temperature at which the liquid content of cells will freeze depends upon the concentration of solutes. In some cases cell fluids may be supercooled, meaning that they can be cooled to a temper-ature below freezing without ice crystals forming. Solute concentra-tions can be increased by the accumulation of soluble carbohydrates, polyols, amino acids, polyamines, and water-soluble proteins (Larcher 2003). These and other aspects of cold tolerance are sum-marized in Figure4.28.

The predominant effect of low temperature is the reduction in acquisition of resources through lower rates of photosynthesis and nutrient uptake (Woodward and Kelly 1997) – this is why cold is considered a regulator. While the exact mechanisms will vary, it should again be evident that chemical reactions, in general, are a function of temperature and, furthermore, that most enzymes have certain tem-perature ranges within which they function most efficiently. While the cold conditions can be modified by increasing the temperature of tissues, or reducing the temperature of optimum enzyme function, there is no way to avoid the basic laws of chemistry. Plants can avoid cold stress by growing only during warm conditions, or they can toler-ate cold, say, by increasing the concentrations of compounds within cells that act as anti-freeze, but both of these strategies have costs.

Freezing avoid ance Freezing toler ance

S urviv al of second ary effects of frost S urviv al of freezing events

Frost surviv al

Protection ag ainst winter drought

Tempor al frost exclusion (se ason al timing)

Extr aorg an freezing and tr ansloc ated freezing in tissue sp aces

Toler ance to hypoxi a under ice sheets and compressed snow Resist ance to mech anic al str ain c aused by frost, ice

and snow lo ad, and snow pressure

Figure 4:28 How plants survive frost events and winter stress (from Larcher2003).

Stress avoidance usually incurs the high costs of a greatly reduced growing season. Even brief cold periods can damage new shoots.

‘‘During the most intensive phase of elongation growth, most plants can scarcely become hardened at all, and are therefore extremely sensitive to low temperatures’’ (Larcher2003, p. 387). The shoots of arctic and alpine plants must therefore be able to tolerate frosts even in the summer. Creeping and rosette growth forms are less exposed to damage from cold; their meristems are protected in at least three ways: they remain close to ground that is heated by the sun, they are less exposed to wind, and the dense (and often hairy) foliage may trap warm air (Archibold1995). Some arctic plants also have deep antho-cyanin pigmentation, which appears to increase rates of absorption and therefore the temperatures of tissues. Savile (1972) suggests that deeply pigmented plants can even extend the growing season by absorbing enough light to commence growth while still buried under snow in the spring. A few arctic and alpine plants also have flowers that track the sun and are shaped like parabolic reflectors (e.g., Dryas integrifolia, Papaver radicatum). The higher temperatures that are produced by this combination are thought to both attract pollinating insects and enhance maturation of the seeds by main-taining higher temperatures in the tissues of the ovary (Kevan1975).

Although many arctic plants have vivid flowers, nearly all have vigorous means for vegetative reproduction to maintain growth when the seed set is unreliable.

Stress tolerance incurs the cost of constructing anti-freezing com-pounds, and their possible interference with other aspects of cellular metabolism (Loehle1998a). As a consequence of such constraints, the maximum photosynthetic rates of arctic and alpine plants are at best only a half of those of plants in warmer areas (Figure4.29).

Woodward (1987) recognized five temperature limits for survival of different physiognomic classes of vegetation: evergreen broadleaf (chilling sensitive, 10 8C), evergreen broadleaf (frost sensitive, 0 8C), evergreen broadleaf survival (15 8C), deciduous broadleaf survival

Figure 4:29 Temperature response of the maximum photosynthetic rate by biome.

Amaxis a general maximum photosynthetic rate that cannot be exceeded in any climate and has been defined from the temperature responses of CO2fixation, O2

fixation, and high-temperature enzyme inactivation. The rates for biomes were derived from data referenced in Woodward and Smith 1994 (from Woodward and Kelly1997).

(40 8C) and boreal (<40 8C). Some species can withstand temper-atures in excess of50 8C.

Abrasion may be a less-appreciated consequence of cold. Savile (1972) observes that: ‘‘The most serious form of winter injury to arctic plants is unquestionably that due to abrasion by wind-driven snow particles’’ (p. 15). Rather than the large soft flakes of snow typical of the temperate zone, much of the arctic snow consists of small, hard, and sharp crystals which, when driven by winter gales, are strongly abrasive. Valleys and the lees of hills provide shelter where dwarf phanaerophytes are found, but each winter the new shoots may be trimmed back by winter gales. Woody plants may be abraded an entire meter above the snow.

These effects of wind abrasion may further illustrate why Raunkiaer placed emphasis upon the location of meristems in his physiognomic classification of plants (recall Figure2.8). Traits that are interpreted as adaptations to cold may in fact be adaptations specifically to reduce abrasion. Many genera of arctic plants (e.g., Empetrum, Salix, Vaccinium, Arctostaphylos) have a prostrate, creeping growth form in spite of being woody plants. Others (e.g., Draba, Diapensia, Cassiope, Saxifraga) form densely packed shoots. Grasses and sedges tend to grow in tussocks where dead tissue provides protection to new shoots. Any variation in topography will modify exposure to wind speed and snow depth (Johnson and Billings1962, del Moral 1983) with consequent changes in species composition (Figure 4.30). While areas that are buried by snow are protected from abrasion, they also have a very short growing season, and tend to have very few species (e.g., Viola glabella, Luzula campestris, Ranunculus eschscholzii, and Carex spectabilis in the Olympic Peninsula of western North America (del Moral1983)).

Extreme environments such as cliffs may have arctic plants beyond what is considered their normal geographic range. These disjunct species presumably reflect distributions that were once

Figure 4:30 Alpine vegetation patterns associated with environmental gradients in the Rocky Mountains (from Archibold 1995).

more extensive during the ice age. The cliff creates a cold and wet environment that simulates some arctic conditions. It may also sim-ply provide a refuge from competition with better-adapted temperate zone species. In eastern North America, for example, disjunct pop-ulations of arctic and alpine plants are found along the north shore of Lake Superior (Figure4.31) and on sea cliffs in Nova Scotia.