GROWTH OF PSEUDOTSUGA-MENZIESII AND TSUGA-HETEROPHYLLA SEEDLINGS ALONG A LIGHT GRADIENT - RESOURCE-ALLOCATION AND MORPHOLOGICAL ACCLIMATION

Silvicultural alternatives that differ in the degree of overstory remo val may create shady environments that will be problematic for the reg eneration of Douglas-fir (Pseudotsuga menziesii (Mirb.) France). Gradi ents of light in the field were used to compare mortality, growth, and leaf morphological acclimation of two conifer species of contrasting shade tolerances: Douglas-fir and western hemlock (Tsuga heterophylla (Raf.) Sarg.). Results after two growing seasons indicated that Dougla s-fir mortality occurred mainly at relative light intensity (RLT) belo w 20%, while western hemlock mortality was evenly distributed along th e light gradient. Height, diameter, and biomass of the planted seedlin gs increased with increasing light for both species but at different r ates, and maximum biomass accumulation always occurred in the open. Do uglas-fir allocated more resources to stem biomass than western hemloc k, which accumulated more foliage biomass. Increases in specific leaf area for Douglas-fir seedlings occurred at RLI less than or equal to 0 .4 and red I far red (R/FR) ratio less than or equal to 0.6, which app ear to be the minimal optimum light levels for growth, Conversely, wes tern hemlock seedlings adjusted their leaf morphology in a more regula r pattern, and changes were less pronounced at low light levels. These results, along with early mortality results for Douglas-fir, suggest that the most successful way to artificially regenerate this species m ay be by allowing at least 20% of RLI for ensuring survival and at lea st 40% RLI for optimum growth.