Growth patterns of sugar maple seedlings and mature trees in healthy and in declining hardwood stands

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Abstract
Openings created in the forest canopy as a result of the decline of sugar maple (Acersaccharum Marsh.) may increase microsite heterogeneity and favor the growth of tree seedlings on the forest floor and possibly neighboring healthy trees because of resource release. To corroborate these hypotheses, I studied the growth of sugar maple seedlings and mature trees, and some microsite characteristics, in healthy and in declining hardwood stands. Sampling was carried out in 400-m2 quadrats in four stands of similar composition. In two of the stands, the trees showed no apparent symptoms of decline (healthy stands), but in the other two (declining stands), dieback had caused tree cover to be reduced by ≈25 to 30%. Photosynthetically active radiation below the canopy was significantly lower and less variable in the healthy than in the declining stands, under both cloudy and sunny conditions. In one of the declining stands, soil pH was higher and soil organic matter content was lower than in both healthy stands. Stem elongation of sugar maple seedlings did not differ among the stands prior to 1984, but following that date it was significantly higher and more variable among seedlings in the declining stands. Ring width of apparently healthy trees decreased markedly in the early 1980s and increased somewhat during the 1985–1987 period on the declining sites. Microsite heterogeneity and growth of tree seedlings on the forest floor were thus greater in the declining than in the healthy stands. Neighboring healthy trees did not necessarily respond to the opening of the canopy (as a result of dieback) by increased ring width; this possibly resulted from the hierarchical position within the canopy, the differential time of reaction, and the age and (or) the health status of each individual.