Water supply and tree growth. Part I Water deficits
Forestry Abstracts 43(2): 57-95
Water supply is the most important environmental factor determining distribution, species composition, and growth of forests. Net annual primary production of forests varies from as much as 3000 g/m2 in wet regions to negligible amounts in dry regions. The water balance of trees has been characterized by visible wilting, tissue moisture content, relative water content, saturation deficit, and water potential. Problems in use of these measures are discussed. Water deficits develop readily in forest trees, even in trees growing in wet soil, because of excess transpiration over absorption of water. Water deficits adversely affect seed germination and cause shrinkage of leaves, stems, roots, fruits, and cones. Shrinkage of tissues influences stomatal aperture, photosynthesis, movement of gases, growth measurement, dissemination of pollen and seeds, flow of latex and oleoresins, and absorption of water and ions. Some of the decrease in photosynthesis during drought is the result of increased resistance to diffusion of CO2 to chloroplasts and some to decrease in photosynthetic capacity. Water deficits inhibit shoot growth, wood production, and root growth. Yield of fruits and seeds can be inhibited at various stages of reproductive growth such as flower bud initiation, anthesis, pollination, fertilization, embryo growth, and fruit and seed enlargement. Water deficits may also induce leaf scorching and abscission, dieback of twigs and branches, and drought cracks. Severe water deficits often kill trees. Drought tolerance of trees may reflect desiccation avoidance or desiccation tolerance, with the former much more important. Desiccation avoidance is the result of one or more adaptations in leaves, stems, or roots. Among the most important of these are reduction in number and size of leaves; small, few, and sunken stomata; rapid stomatal closure; abundant leaf waxes; leaf shedding during droughts; extensive root development; capacity for twig and stem photosynthesis; living wood fibres; and strong development of palisade mesophyll. In some species osmotic adjustments result in maintenance of leaf turgor during drought periods.