TEMPERATURE

Factors influencing air temperature and soil temperature at local scales

1. Color of soil surface

2. Soil moisture content

3. Soil texture

4. Plant cover

5. Snow cover

6. Topography

Cold air drainage/frost pockets (inverted treelines)

Effects of concavity or convexity on treeline

IMPORTANCE OF TEMPERATURE TO PLANTS

1. TRANSPIRATION (consider under water)

Generally, increase in air temperature increases transpiration rate.

2. EFFECTS OF TEMPERATURE ON PHOTOSYNTHESIS AND RESPIRATION

Responses vary according to species, local population, and physiological state:

a. Species

e.g. minimum temperature for net photosynthesis is 15 to 18E C (59-64EF) for date palm, melon and sorghum but is -2 to -8E C (18 to 18E F) for many plants in the arctic tundra.

e.g. maximum temperatures for net photosynthesis are as low as 10E C (40E F) for some understory herbs in boreal forests, but prickly pear can grow at temperatures of over 56E C .

b. Population

e.g. balsam fir's optimum temperature for photosynthesis decreases 4EC from elevations of 700 m to 1500 m.

c. Physiological state of the plant or plant part

e.g. "sun leaves" generally have higher optimum temperatures for photosynthesis than do "shade leaves."

3. THERMOPERIODISM (response of plants to daily fluctuations in temperature).

Thermoperiod = the difference between day and night temperatures.

The thermoperiod has a major influence on growth rates of some tree species (e.g. Engelmann spruce). Greater thermoperiod generally favors greater growth rate.

4. STIMULATING EFFECTS OF TEMPERATURE

a. Dormancy

For example in evergreen conifers shoot dormancy is induced by sequence of:

i. short days which causes "acclimation" or "hardening" of shoots.

ii. hard frost causes further dehydration.

iii. very low winter temperatures (e.g. - 5 to -30E C) causes "true dormancy" (this severe winter chill is required before dormancy can be broken in the spring).

iv. longer spring days plus warmer spring temperatures stimulate the bud burst (shoot growth); sometimes "temperature sums" (number of hours above a threshold temperature) can be used to predict bud burst.

b. Stratification of seeds

c. Stimulation of flowering

5. DIRECT HEAT INJURY

a. Stem girdle of tree seedlings

b. "Sunscald" = rupture and cracking of bark due to rapid increase in temperature.

ADAPTATIONS TO PREVENT DIRECT AND INDIRECT

HEAT INJURY (considered further under the topic of water)

i.. Small leaf surface area

ii. Vertical orientation of leaf blades

iii. Whitish color of leaves

iv. Dense hair covering of leaves

v. Thick corky bark

vi. Leaf habit (e.g. deciduous during season of high temperature and low moisture availability)

6. DIRECT COLD INJURY

a. Freezing of water inside plant

b. Cold lesions = cracks in bark due to rapid change in temperature

c. Frost heaving