Crown dynamics and wood production of Douglas-fir trees in an old-growth forest

Abstract

Large trees are the most prominent structural features of old-growth forests, which are considered to be globally important carbon sinks. Because of their large size, estimates of biomass and growth of large trees are often based on ground-level measurements (e.g., diameter at breast height, DBH) and little is known about growth dynamics within the crown. As trees increase in size, growth of the crown may not be reflected in DBH measurements contributing to inaccuracy of aboveground forest productivity. Here we present data from a 10-yr re-measurement of crown structure and branch/trunk growth of 400-year-old Pseudotsuga menziesii trees in an old-growth forest in western Washington, USA. In six study trees, 40–60% of branches occurred in the upper third of the live crown. Branch mortality over 10years was highest in the lower third of the crown. Of live-branch mass (including leaf mass), 41–78% occurred in the middle third of the live crown. During the study period, live-branch mass increased in the upper half of the crown and there was little loss due to fragmentation or death. In contrast, increment of live-branch mass was negligible and live-branch mass decreased in the lower half of the crown. On average, 70–99% of the increment of live-branch mass per tree occurred in the upper half of the crown. Core samples taken from various heights indicated that trunks became less tapered with increasing age as a result of greater increments of upper-trunk radius during the most recent 10years. Increment of live-branch mass contributed 42 and 66% of the whole-tree and upper-crown increments of mass, respectively, and its vertical distribution corresponded to that of leaf mass density. Increments of trunk mass contributed 88% of the lower-crown increment. Growth increments of the crown were not reflected in core samples taken at lower heights. Our estimates of trunk, branch, and leaf mass were consistently smaller than those calculated using empirical allometric equations based on tape-wrap measurements of DBH. Moreover, leaf mass decreased in four trees, whereas allometric equations predicted increases. Our results indicate that large P. menziesii trees can sustain wood mass production, especially in trunk and branches of the upper crown, while leaf mass change can be more dynamic, and that such growth dynamics of the crown are difficult to detect via DBH-based measurements.