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14.1 - Thermal environment and plant heat budgets

Temperatures vary with latitude, altitude, size of land mass (and position within that land mass), atmospheric conditions (cloud cover and air movement) and local topography. Atmospheric temperatures decrease by about 1°C for each 2° increase in latitude, or for each 100 m increase in altitude. Temperatures in high altitudes near the equator can be similar to low-land temperatures further from the equator. The potato, which has its origin in tropical high altitudes, is now grown widely in low-altitude regions of the world where temperature conditions are similar. However, temperature is not the only concern in this expansion of a crop plant to new growing regions, as a change in latitude and altitude may require some adaptation to changing photoperiod and radiation levels.

Seasonal variation in temperature is greatest near the poles, and small near the equator. At high altitudes the increased solar radiation which can result in rapid local heating is balanced by greater night radiation losses. The most stable temperatures occur under oceanic conditions in the tropics.

Survival outside the active growth range of all plants is nevertheless dependent on developmental stage, duration of temperature stress and degree of acclimation. Terrestrial plants have evolved life cycles that match seasonal necessity, which at low temperatures necessitates winter dormancy. Within the dynamic range of plant–temperature relations, where short-term responses are readily reversible, phenology and productivity have been successfully linked with various measures of accumulated heat (thermal time), and predictive models validated.