Co-Occurring Stress - Drought : Biologic Markers of Air-Pollution Stress and Damage in Forests. Workshop Papers

1989

S. 357-363

32 Lit. Ang.

Unselbständiges Werk

200069357

Drought stress always restricts the growth of trees. The immediate factor most influencing growth response to drought is turgor pressure which is the force causing plastic enlargement of cells, leaves and stems. Reduced shoot and leaf growth in one dry season can reduce the vigor and growth potential of trees for several subsequent years. Net assimilation of carbohydrates is also reduced by drought its effect on stomatal closure, increased diffusional resistance to CO2 transport in mesophyll cells, reduction of electron flow in photosystem I and II and disruption of enzyme activity, thereby reducing the dark reactions of photosynthesis (1). Reduction in carbohydrate reserves can cause loss of frost hardiness and subsequent stem die-back (2). Winter dehydration can also be a very important cause of stem die-back, a symptomatology normally associated with forest decline. Winter dehydration and consequent stem embolism can reduce the capacity of small stems to conduct water by more than 80% (3). Models indicate that during a growth season a loss of water conductivity greater than 20% can cause mid-day stomatal closure or catastrophic xylem dysfunction (4). Although mechanisms are normally in place to reverse embolism prior to bud break, anthropogenic stresses could interfere with these mechanisms, thus leading to crown die-back.