Sunday 9 September 2007

No Sweat !

“Do trees sweat”? The answer is yes, in their own way.

Trees use the evaporation of moisture to cool themselves when moisture is easily available.

Moisture in the leaves comes up from the roots through the trees vascular system.

This process is both passive in the form osmosis and active with cells moving the water along like a vegetative bucket brigade. This all works well in a particular set of limited circumstances. For instance, when ground water is available and the tree is in relatively

dry air. Only then can the tree release tremendous amounts of water. Some estimates run as high as 350 gallons per day with optimum conditions. 95% of this water is lost to the atmosphere in the process called evapotranspiration. Trees don’t exist to emit water vapor into the air and cool the globe in the process. However this seemingly altruistic process does occur as a result.

Transpiration uses water to move minerals from the root zone to the leaves where they are combined in the process of photosynthesis to create sugars. When trees release moisture into the atmosphere it does indeed cool the leaf. This is important because the process of photosynthesis in most trees occurs in a relatively narrow temperature range. This range is between 15’c and 25’ c or 59 ‘f and 77’f. Within this small range of temperatures the whole process works in an optimum fashion. Too low a temperature and the process is not efficient. At high temperature it begins to slow down as carbon dioxide becomes a limiting factor. Evapotranspiration works to keep the leaf in this optimum range.

Leaves are also able to dissipate heat directly into the atmosphere using a process called convection. If the leaf is hotter than the surrounding atmosphere heat will waft away from the leaf on air currents. Typically a leaf in sun light is slightly hotter than the surrounding atmosphere. This temperature gradient in itself allows moisture to evaporate from the leaf even if the relative humidity in the surrounding air is high. Trees are not immune to loosing more water than they can replenish from the roots. This causes a water deficit. If the tree is able to replenish its supply of moisture before the start of the next day it is able to continue growing even though this situation does limit the overall growth of the tree. This condition is called a daily water deficit. If the tree is not able to re-supply its water needs it will become desiccated and die.

Tree leaves regulate the loss of water vapor through small openings in the leaves called stomata.

These specially modified groups of cells are found on the outer surface of the leaf, top and bottom. As moisture exits the leaf it creates a humid boundary layer around the leaf. Wind moves the moisture away and allows the leaf to continue transpiring. If the wind speed becomes too great the leaf will close the stomata to reduce moisture loss also reducing the cooling effect losing moisture creates. For a more detailed explanation of tree water relations contact an I.S.A. Certified Arborist.

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