Asymptotic allometry and transition to the canopy in Abies balsamea

There is a lack of consensus in theoretical and empirical literature on whether height-diameter (H:D) relationships of canopy trees are asymptotic. To investigate H:D allometry, particularly in the transition to the canopy, we focus on shade-tolerant Abies balsamea, across steep physical gradients associated with elevation, and correlated biotically-generated gradients of stem density, canopy height and canopy species composition. We addressed these questions: A. What is the relation between H:D allometric form and emergence into the canopy? B. Is H:D allometry asymptotic? C. What aspects of the H:D relationship are most sensitive to environmental conditions? Using maximum likelihood, we compared the performance of alternative H:D forms (linear, power function, non-sigmoid-asymptotic and sigmoid-asymptotic), incorporating elevation and local canopy height as covariates. A. balsamea H:D allometric form was clearly sigmoid-asymptotic, by a wide AIC margin, across all gradients, to an asym...