THE EXPANSIVE TREE

Trees need a lot of water to grow and transport vital substances and also to cool down in summer. When it gets hot and we humans start to sweat, we give off heat with our sweat, which is mainly water. Although trees do not sweat, their cooling system is also based on water, as the constant evaporation (release of water in the form of steam) through their leaves or needles protects them from overheating. In addition, trees can produce some of their own food by absorbing carbon dioxide from the air using sunlight and converting it into sugar and starch using water. 

In this process, called photosynthesis (literally: making things with light), the oxygen we breathe is released as a "waste product".

The amount of water needed depends very much on the weather conditions (more water on sunny and hot days) and on the type of tree. Conifers are quite thrifty and consume much less water on summer days (spruce about 10 l) than deciduous trees (beech 30 l, oak 40 l and birch up to over 100 l per day).

Water rises at different rates in different tree species (1-50 m per hour), but water does not flow fast enough from the roots to the leaves to always cover the demand through transpiration. Therefore, trees must take up water from storage tissues in leaves and needles, but especially in the trunk. The depletion of these water reservoirs therefore causes the trunk to shrink during the day, when the tree needs a lot of water. At night, when evaporation is low, the stores are replenished and the trunk expands. The trees shrink and grow again within a day. This cannot be seen with the naked eye, but precise measuring instruments called dendrometers can record these changes, which occur in the order of tenths of a millimetre.

MARTIN BADER, RESEARCHER AT LINNÉ UNIVERSITY

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