WATER AND FOREST PRODUCTIVITY

Water availability is a major factor influencing the distribution and productivity of the earth's vegetation, but details of the mechanisms by which its effects are felt are not well understood. This is due in large part to the interactions between water and vegetation, such as through interception and change in leaf-area, which affect rates of canopy photosynthesis and transpiration. Physiological differences among species are not always directly translated to differences among stands, emphasizing the importance of climate and microclimate as controls. Leaf-area index (L) is a critical integrator of water availability and productivity, and changes in leaf-area, such as occur through thinning and understory control, may have dramatic effects on both. There is increasing evidence that L changes significantly within an annual cycle and from year to year, even in closed-canopy conifer stands. Consequently, the season and year in which a measurement of L is made may explain much of the variability noted before in its response to water availability and effects on productivity. Because carbon, water, and nutrient cycles are so closely coupled, simulation models that represent both direct and indirect relationships are useful tools for understanding and managing forest ecosystems. © 1990.