Regenerating montane conifers with variable retention systems in a coastal British Columbia forest: 10-Year results

As a component of the Montane Alternative Silviculture Systems (MASS) project, this study investigates limits on the growth of montane conifers resulting from silvicultural systems with varying amounts of overstory retention. In spring, 1994, Abies amabilis (amabilis fir) and Tsuga heterophylla (western hemlock) seedlings were planted in replicated blocks of clearcut and alternative systems with retention aggregated in patches (patch cut: PC) and dispersed at high density (shelterwood: SW) and low density (green tree: GT). In addition, fertilization (F) and vegetation control (V) treatments were applied alone and in combination (FV) in each silviculture system to test the extent to which growth limitations are related to nutrient availability and competing vegetation. While total tree height and volume in the CC, GT and PC were similar after 10 years, recent growth of both species tended to be greatest in the PC, which had more shallow-soil growing degree days than the CC, possibly due to earlier depletion of the spring snow pack. The growth response in the PC (in comparison to the CC) was most pronounced in the vegetation control treatments, but significant only for western hemlock. In the reduced light environment of the SW, height and volume after 10 years was markedly lower in both species compared to the more open CC, GT and PC systems. Both western hemlock and amabilis fir showed the greatest growth response (20–80% increase in height) to vegetation control alone and in combination with fertilization in all silvicultural systems, indicating the presence of below-ground (nutrient) resource limitations. Foliar nitrogen concentrations in both species were at deficient levels in all silvicultural systems, indicating reduced availability of nitrogen on the study site, and may be a precursor to growth check, a phenomenon of decreasing or variable annual height increment in conifer regeneration that has been observed following clearcutting on nitrogen-poor, mid- and high-elevation coastal sites in British Columbia.