The interpretation of stem diameter-height allometry in trees: biomechanical constraints, neighbour effects, or biased regressions?

Static diameter-height allometry data have been used by many ecologists to demonstrate that diameter should increase at a faster rate than height during tree growth, as predicted by biomechanical models. We review the available evidence and examine the potential problems that arise in the interpretation of this relationship. In particular, we reveal how few studies investigating patterns of diameter-height allometry in trees have adequately controlled for neighbour effects. We also demonstrate how the interpretation of diameter-height allometry has suffered from a lack of uniformity in the selection of regression models, and how the use of least squares regression to estimate allometric scaling exponents can be biased. We conclude that most of the published data on static diameter-height relationships in trees tell us virtually nothing about either age (developmental) effects or neighbour effects; they are completely confounded. Further studies are required to analyse the long-term dynamic growth trajectories of individual trees in relation to local neighbour effects, and greater effort must be made to establish the consistent use of unbiased statistical methods between studies.