Allometry and size structure of trees in two ancient snow forests in coastal British Columbia

Abstract

Shade-tolerant trees that start in the understory but ultimately reach the canopy persist through different microhabitat conditions during their ontogeny. We evaluate how the height to diameter ratio and the crown ratio (crown length/tree height) change during development and differ among four coniferous species ( Abies amabilis (Dougl. ex Loud.) Dougl. ex J. Forbes, Chamaecyparis nootkatensis (D. Don) Spach, Tsuga mertensiana (Bong.) Carrière, and Tsuga heterophylla (Ref.) Sarg.) in ancient forests. At two sites, we measured diameter, height, and height to the base of live crown for trees ≥4.0 cm diameter at breast height in four 0.25 ha plots. For each species, we constructed models of height based on diameter and crown length using part of the data for model development and part as the test data set. Models predicting tree height were effective for all species, and adding crown length considerably improved the prediction over diameter alone. The height to diameter ratio increased until the subcanopy and then decreased for two shade-tolerant species (A. amabilis and T. heterophylla) but decreased linearly throughout ontogeny for C. nootkatensis. Crown ratio increased as trees became larger except for C. nootkatensis, where the reverse occurred. Differences in allometric relationship among species reflect patterns of crown development and also suggest different approaches to a common structural constraint in these forests: heavy snow loads.