Understanding the role of resource use efficiency in determining the growth of trees and forests

Abstract

In the twentieth century, silviculturists commonly thought about the growth of trees and stands in terms of growing space. Trees and stands grew faster when they obtained more growing space. Unfortunately, growing space is intangible and not quantifiable, limiting the opportunities for quantification and hypothesis testing. Patterns of tree and stand growth can be evaluated quantitatively with a production-ecology perspective, testing hypotheses about factors that influence growth. The growth of trees and forests depends on the acquisition of resources (light, water, nutrients), on the efficiency of using these resources for photosynthesis, and on the partitioning of photosynthate to wood growth. Trees and stands with high rates of resource use might be expected to show lower efficiency of resource use as a result of some sort of declining marginal return; however, empirical patterns show that increasing resource use is generally accompanied by sustained or increased efficiency of use. For example, in fast-growing Eucalyptus plantations, large trees may intercept twice as much light as smaller trees, and use the light twice as efficiently to provide a fourfold greater rate of stem growth than smaller trees. At the stand level, increases in water supply (across geographic gradients or from irrigation) often show 50% increases in water uptake by trees, and constant or increasing efficiency of water use leads to large increases in stem growth. These insights are valuable for forest management, including understanding why subordinate trees contribute so little to stand growth, why uniform stands grow better than stands with greater variety of tree sizes, and why some species mixtures grow better than others. The production-ecology approach offers a powerful framework for how to think about the growth of trees and forests.