Effects of light availability and sapling size on the growth and crown morphology of understory Douglas-fir and lodgepole pine

Information on the dynamics of sapling growth of Douglas-fir (Pseudotsuga m enziesii var, glauca (Beissn.) France) and lodgepole pine (Pinus contorta D ougl. ex Loud. var. latifolia Engelm), two dominant species in the interior of British Columbia, Canada, is incomplete and thus the objective of this study was to understand how light availability and sapling size interact to influence their growth and crown morphology. in an undisturbed forest, 360 saplings were randomly selected in three light classes 0-15, 15-30, and >3 0% PPFD (photosynthetic photon flux density). A number of morphological and growth parameters were measured, including height and lateral branch growt h. Douglas-fir had a more plastic crown morphology than lodgepole pine with its leader to lateral branch growth ratio, live crown depth, and number of branches increasing with increasing light class. Sapling size had little e ffect on morphological characteristics, but larger saplings of both species had greater absolute height growth and lateral branch growth than did smal ler saplings. Both Douglas-fir and lodgepole pine were able to survive up t o 50 years and attain a height of 3 m at less than 5% PPFD. These results f urther suggest that shade tolerance is greater on drier sites, although the mechanisms for such increases in tolerance are unknown. The ecological imp lications of these findings are discussed in a forestry context.