14.1 Thermal environment and growth responses

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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. As a rough guide atmospheric temperatures decrease by about 1°C for each 2° increase in latitude, or for each 100 m increase in altitude, but there is considerable variation in this relationship. A crop such as potato, which has its origin in tropical high altitudes, is now grown widely in temperate low-altitude regions of the world where temperature conditions are similar. How-ever, 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, while 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.

Average monthly air temperatures, which take account of seasonal changes, are much more relevant to plant growth than mean annual values. Some idea of the temperature range in Australia can be seen by comparing the alpine areas, where the average monthly maximum for July (midwinter) is 5°C and the average monthly minimum is –5°C, with the north coast of Western Australia where the average monthly maxi-mum for January (midsummer) is 40°C and the average monthly minimum is 27°C. While average temperatures may provide a useful base for studying plant development, extreme conditions must also be taken into account, particularly in relation to sensitive stages of development.

Air temperatures greater than 50°C have been exceeded in all states of Australia except Tasmania and the ACT. The frequency of heatwave temperatures in Australia is generally greater to the north and away from the coast. At Marble Bar (in the northwest) summer temperatures may exceed 40°C for several weeks at a time and on one occasion lasted from November through to February.

Frosts are widespread in the winter and spring months in southern Australia and extend northwards along the Eastern Highlands of New South Wales to the Darling Downs in Southern Queensland and into central Australia. In the south-eastern highlands of Australia there is a median frost period of 200 days and the lowest temperature recorded in this area was –23°C at Charlottes Pass.

Temperature extremes are not so great in the agricultural regions of New Zealand, which has a more maritime climate, although in some regions (Otago) the frost season lasts 250 days and there is permanent snow in more mountainous regions above 2000 m.

Unseasonal weather and temperature extremes will of course influence distribution of native plant species, as well as limiting growth of crop plants.