Size-Related Changes in Tree Proportions and Their Potential Influence on the Course of Height Growth

Abstract

As trees grow larger and older they show characteristic changes in their proportions and growth rates. This paper explores the hypothesis that mechanical support costs of growing trees contributes to tree height limitation and general growth decline in aging trees. The chapter reviews scaling hypotheses, including geometric similarity, elastic similarity, stress similarity, and the pipe model, as well as the patterns in tree allometry that have been reported. It then uses the remarkable published record of diameter, volume and height growth, deriving from stem analysis, of one 437-year-old, 83-m-tall noble fir (Abies procera) to evaluate the degree to which different scaling models apply at different stages in that tree's life. The analysis suggests that the overall pattern of height growth could be related to tree allometry and wood production rates, but that the tree showed substantial decadal variation around the general patterns. The chapter suggests that many more studies are needed to characterize individual trees, either with known environments or after manipulations that alter allometry, and in different ecosystems beyond the Pacific Northwest and California. Such data will enable scientists to analyze the degree to which these allometric models pertain in different plant types and at different life stages, and will give insight into the role of mechanical and hydraulic support in the limitation of tree growth and productivity.