The Physical Environment
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Biogeography of the Earth

Transpiration

Transpiration is the loss of water from plant leaves. Water exits the leaf through stomata, which are tiny pore spaces in the leaf. The rate of transpiration depends on air temperature and solar radiation. As pointed out earlier, transpiration is a cooing process for plants when temperatures or incident light rise too high and cause heating of the plant. Low humidity, often aided by windy conditions, creates a vapor gradient between the plant and the air. This too induces transpiration. Soil factors are important control over transpiration. If the pore space between soil particles are too large the soil will have poor or low soil capillary. That is, the rate of water rise is too low for plants to extract water from the soil and maintain proper moisture supply. Low soil capillary results from soil drying too. Figures 12.13a and b below indicate seasonal changes in plant transpiration. During the moist season, ample soil water is available to line soil particles to aid the movement of soil water upwards to the root zone.

However, during the dry season (Figure 12.13b), a dry layer of soil develops beneath the root zone inhibiting the upward movement of capillary water causing a capillary lag. The plant ultimately wilts as it cannot extract enough water to meet the increasing demand for water during warm seasons. A small soil moisture reserve will inhibit transpiration too. 

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