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The water-cohesion-tension insufficiency syndrome of forest decline


Journal of theoretical biology, 156(2): 235-267
The water-cohesion-tension insufficiency syndrome of forest decline
The water-cohesion-tension insufficiency hypothesis, which suggests the primary reason for forest decline produced by air pollutants to be an increase of cavitational rupture and embolism of negative pressure water within the water carrying tracheids and vessels of the tree xylem, has been examined in detail. A thermodynamic model of water under tension is presented which is based on the van der Waals equation in which the interaction between water molecules subject to negative pressure has been elaborated and tested by computer simulation. The hypothesis explains typical reactions of affected trees (e.g. formation of "stork nests", "anxiety" sprouts, reduction of leaf and needle sizes, shedding of needles) as adaptations to decreased tensile strength of water in tree xylem water conduits. In support of the proposed mechanism a comparative study of tree diseases has been made in which rupture of sap flow (cavitation) is caused by fungi (e.g. Dutch elm disease). It is shown that symptoms occur comparable to those trees damaged by air pollution. The presented hypothesis predicts that the forest decline is basically proportional to the product of dry deposition of pollutants (particulate matter, ozone, hydrogen peroxide and acidic gas) and the water stress experienced by trees. A comparative study of forest decline in Europe and Japan, as well as an evaluation of preliminary results of large scale experimental research on forest decline in Germany suggests that the observed patterns can qualitatively be explained. Possible mechanisms for pollution induced cavitation and embolism in xylem water conduits are discussed and necessary experimental research for verification of the hypothesis outlined.

Accession: 002530994

DOI: 10.1016/s0022-5193(05)80675-7

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