Effects of light and intraspecific competition on growth and crown morphology of two size classes of understory balsam fir saplings

This paper characterizes the growth and crown morphology of young balsam fir saplings naturally regenerated under a gradient of understory light environments and intraspecific competition densities for two size classes (50–100 cm and 100–200 cm). Most growth and crown morphological parameters investigated were strongly related to the natural light gradient investigated (3–83% full sunlight), but the relationship tended to plateau at around 25% full sunlight. The relationships were generally better for the larger size class. Intraspecific competition did not significantly affect growth and crown morphology of saplings receiving less than 25% full sunlight, but it affected relative height growth, relative radial growth and the apical dominance ratio for those receiving more than 25% full sunlight (R²=0.506; p<0.001; R²=0.403; p<0.002; R²=0.348; p<0.001, respectively). These results suggest that live crown ratio, apical dominance ratio and the number of internodal branches can provide, alone or in combination, useful indicators of vigour for understory fir. Such a study provides the basic data inputs required for the development of empirically-derived mechanistic models that can predict understory tree growth and survival.