2000 in spite of these effects we have found that

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Unformatted text preview: ar was actually positively correlated with abundance of broadleaf neighbors (Simard et al. 2004b), suggesting a prolonged facilitative effect of paper birch, possibly because turnover of nutrient-rich litter helped to meet the redcedar’s high nutrient requirements (Klinka et al. 2000). In spite of these effects, we have found that light level is a poor predictor of diameter growth rates of conifers in young ICH stands, where the vertical and horizontal distribution of broadleaf trees is highly variable, with most conifers growing between paper birch clumps and interspecific interactions thus confounded by variable degrees of crowding as well as root disease. Light availability under 10-yearold paper birch canopies accounted for only 19%–38% of the variation in diameter of same-age Douglas-fir (Simard et al. 2004b), contrasting with more predictable effects of high overstory on understory saplings in older northern subboreal and temperate-zone forests (Wright et al. 1998; Comeau et al. 2003). Stem-exclusion stage With canopy closure at 15–25 years, prior to the onset of significant self-thinning, the importance and intensity of inter-tree competition reach an asymptote in these forests (Simard et al. 2004b). At this time, individual tree growth becomes relatively more affected by soil-resource (water and nutrient) competition from shorter neighbors than by light competition from taller neighbors (Simard and Sachs 2004). Because of its high water and nutrient demands, and more suppressed position in the canopy, western redcedar is more strongly affected by competition from neighbors in these older stands than are Douglas-fir and western larch (Simard and Sachs 2004). Broadleaf trees are still important competitors of conifers at 25 years, but their declining relative height growth rates place them at a comparative height disadvantage with respect to conifers by 50 years (Wang and Kimmins 2002). By 50 years, broadleaf trees have thinned considerably from the stands, and competition from other conifers for soil resources is of greater importance than competition from broadleaf trees (Simard et al. 2004b). Most broadleaf trees have fallen out of the forests by 70 years because of overtopping by the longer lived conifers Can. J. For. Res. Vol. 36, 2006 (Wang and Kimmins 2002). At this time, broadleaf trees persist only where gaps created by root disease, insect mortality, or windthrow are large enough to meet their light requirements (Simard and Vyse 1994). Long-term productivity Ecosystem modeling suggests that nitrogen inputs by paper birch are important for maintaining the productivity of mixed conifer–broadleaf forests over several rotations. Sachs (1996) calibrated the FORECAST ecosystem model (Kimmins et al. 1990) using growth and nutrient information collected in mixed paper birch – Douglas-fir stands in ICH ecosystems (e.g., Wang et al. 1996). Mixedwood-management scenarios over four 100-year rotations (400 years) were simulated using a replacement series design with a constant density of 1600 stems·ha–1 (Harper 1977). Sachs (1996) predicted that Douglas-fir productivity was lower than that of paper birch, declining steadily and significantly over 400 years when Douglas-fir was grown in pure stands. However, Douglas-fir yield increased and then stabilized over eight rotations with inclusion of at least 400 stems·ha–1 paper birch in mixture. The model predicted that total yield would be higher in mixed stands of 400 stem·ha–1 paper birch and 1200 stems·ha–1 Douglas-fir than in pure stands of either species. This study suggests that paper birch, because of its high tissue nitrogen content and associative nitrogen fixation by rhizosphere bacteria, is important in maintaining long-term productivity of ICH forests (Blenner-Hassett 1996; Sachs 1996). Weeding effects on conifer growth and survival Competition thresholds High densities of paper birch can negatively affect growth of conifers in young stands, but birch density is spatially variable, with high densities occurring in small patches and most mesic microsites supporting low to moderate birch densities that have small and variable effects on conifer growth (Simard et al. 2004b). In 10-year-old Douglas-fir plantations, Simard (1990) examined conifer growth across a range of neighbor birch densities and found that 340– 2100 stems·ha–1 of 3 m tall paper birch had little effect on conifer diameter growth. In a later study, Simard et al. (2001) found that diameter growth of 10- to 15-year-old Douglas-fir was highly variable at low densities of 2–3 m tall paper birch, and was consistently suppressed only at densities above 4400 total birch stems·ha–1 or 2500 taller birch stems·ha–1. In these studies, high densities of paper birch were much less common than low densities. Current free-growing regulations in British Columbia allow lower retention of broadleaf trees than was suggested by Simard (1990) and Simard et al. (2001). In the following section we...
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