Models of sapling mortality as a function of growth to characterize interspecific variation in shade tolerance of eight tree species of northwestern British Columbia

Abstract

We have developed models of sapling mortality for the eight dominant tree species of northwestern British Columbia in order to better understand forest community dynamics and succession in this important forest region. The species-specific models characterize an individualis probability of mortality as a function of recent growth (a surrogate for whole-plant carbon balance). Interspecific comparisons of survival under low growth rates (i.e., suppression) provide a quantitative measure of the effective shade tolerance of these tree species. In particular, the more shade tolerant species exhibited more rapid decays in mortality probability with increased radial growth. The broad and continuous range in shade tolerance ranking was as follows: Thuja plicata Donn ex D. Don > Tsuga heterophylla (Raf.) Sarg. = Abies lasiocarpa (Hook.) Nutt. > Picea glauca (Moench) Voss × Picea sitchensis (Bong.) Carrire > Pinus contorta Dougl. ex Loud. var. latifolia Engelm. > Populus tremuloides Michx. > Populus balsamifera ssp. trichocarpa (Torr. & A. Gray) Brayshaw = Betula papyrifera Marsh. At low growth rates, mortality varied between T. plicata and B. papyrifera by more than an order of magnitude. For some species, the three replicate sites exhibited significant variation, suggesting that shade tolerance may vary with site conditions (presumably soil moisture in our study sites). The mortality models are consistent with previous qualitative categorizations into shade tolerance classes and parallel the dominance of different species in postdisturbance succession. Our results suggest that species differences in noncatastrophic mortality are critical to understanding and predicting forest dynamics.