Ecophysiology of Long-Distance Water Transport in Trees

Abstract

We give a short overview over some basics of tree water relations and the likely impact of future climate changes on the functionality of the soil-plant-atmosphere continuum. We start with a short account of the biophysics of water transport and explain some methods for determining relevant parameters in the field and in the laboratory. Important results are described next: the variable values of total water potential in crowns, the use of pressure-volume curves established in the lab for a detailed analysis of leaf water relations in the field, parameters indicating water stress, and the stability of water columns in the xylem. A look at the water relations of small plants shows that the numerical values of key parameters in seedlings or herbs are not much different from those in tall trees; this enigma may have to do with evolutionary selection for a cautious use of soil water reserves. There are still a number of weak spots in our understanding of water transport in trees. We have only a very general knowledge of root distribution over the soil layers and of fine root turnover. Also, cavitation in the different size classes of roots has not been investigated sufficiently. The most important unknown however is probably the distribution of resistances over the whole length of the xylem, which depends on xylem structure at different points in the plant. Finally we speculate about possible effects of future climate changes. There are a number of relevant climate factors which can influence plant water relations in both positive and negative ways; they are described in some detail. Changes of these factors will not be uniform, not even on small scales, let alone over continents or the whole globe. It depends on their combinations whether tree water stress will increase or decrease at a given site. It seems however safe to conclude that the existence of trees on our planet is far from being jeopardized: trees have existed uninterruptedly since the late Devonian, including long periods with climates far more extreme than the ones predicted for the next 100 years. This positive outlook does however not pertain to the fate of single tree species, which may well succumb to increased competition by better adapted ones.