Foliar responses of understorey Abies lasiocarpa to different degrees of release cutting in a Betula papyrifera and conifer mixed species stand

Foliar responses of subalpine fir [Abies lasiocarpa (Hook.) Nutt.] to thinn ing were studied in a 35-yr-old mixed stand of paper birch (Betula papyrife ra Marsh.) and conifers. The stand regenerated naturally after a wildfire w ith a canopy dominated by paper birch (average height 9.8 m) and an underst orey dominated by subalpine fir (average height 1.6 m). The stand was thinn ed to four densities of birch: 0, 600 and 1200 stems ha(-1) and control (un thinned at 2300-6400 stems ha(-1)) in the autumn of 1995. The understorey c onifers, mainly subalpine fir, were thinned to 1200 stems ha(-1). The study used a completely randomized split-plot design. Three sample trees were sy stematically selected from each treatment replicate and each tree stratum ( upper, intermediate and lower understorey). One-year-old and older age clas s needles were collected from one south-facing branch within the fifth whor l from the tree top. Thinning of paper birch significantly (p < 0.001) incr eased leaf area and dry weight per 100 needles for intermediate and short t rees except in the 0 birch treatment. Understorey subalpine fir trees in 60 0 stems ha(-1) birch (T3) had the largest leaf area and leaf dry weight per 100 l-yr-old needles. Specific leaf area (SLA) decreased from unthinned (T 1) to 0 birch (T4). Lower understorey trees had the largest SLA. One-year-o ld needles had significantly higher N, P and K concentrations in all the th inning treatments. These responses are consistent with the shade tolerance of subalpine fir. The results suggest that when managing a paper birch-coni fers mixed-wood forest it may be of benefit to understorey conifers to leav e a birch canopy as a nursing crop.