Trees control leaf temperature, but how much?

2008/07/01 Elhuyar Zientzia Iturria: Elhuyar aldizkaria

• Botanika

An article published in the journal Nature has generated controversy. In this article, a team at the University of Pennsylvania has proposed that trees are able to accurately control leaf temperature. According to them, the trees, with winters or summers and located north or south, manage not to distance the temperature of their leaves from 21ºC. But there are experts who question whether measurements have been done correctly.

Maintaining the temperature of the leaves almost constant can be important for the tree from the point of view of photosynthesis, since it is a very sensitive process to temperature. And the temperature of 21ºC is suitable for photosynthesis. A slight cooling or heating makes photosynthesis less effective. Therefore, it seems logical to control the temperature of the leaves.

However, the chemistry of trees is not that of the largest animal, they cannot control the temperature by chemical reactions. But they use other systems. They can change the angle of the leaves so that they receive more or less heat from the Sun. In addition, fir trees and many northern trees accumulate their leaves together to form layers of air that protect them from the cold. Moreover, leaf stomata can also control open and closed leaf temperature.

The question is the accuracy with which trees control temperature through these systems. According to measurements from the Pennsylvania team, control is very accurate. Other researchers have criticized the measurement system.

The Pennsylvania team has studied the oxygen atoms of the tree trunk, namely the ratio between two oxygen isotopes (16 oxygen and 18 oxygen).

In the air this proportion varies with temperature and humidity. Therefore, with the data of a given moment and place, it can be compared with the proportion of isotopes accumulated by the tree in the trunk. If there are differences it is considered that it is due to the natural temperature of the leaves, from where the temperature can be calculated at that time. So the Pennsylvania team has measured the temperature of its leaves.

However, the results of these measurements do not coincide with those made long ago and with more correct methods. For example, the placement of thermocouples in the leaves has allowed some researchers to measure very variable temperatures in the leaves. Also the German researcher Körner, who measured the influence of the air layers between the leaves, has criticized the method of measurement, since the trees do not grow at all times and therefore do not accumulate oxygen in the trunk.