Evidence that soil fertility controls the mixing of jack pine with trembling aspen

Abstract

Jack pine and trembling aspen are boreal tree species that are found growing either in naturally regenerated mono-specific stands, or in mixed-wood stands. We conducted a field survey and a manipulative field study to test the productivity–diversity hypothesis, which predicts that mixed-wood stands are more likely to occur on fertile soils, or following fertilization. We surveyed 44 mixed-wood stands and found 43 of these occurring on fertile clay deposits, and only one occurring on a nutrient poor till deposit. By contrast, the area surveyed comprised 45% clay and 55% till deposits. In a second study, we conducted a five year fertilization and brushing trial in a recently burned area dominated by jack pine saplings with patches of regenerating trembling aspen. Fertilization without brushing improved the growth and recruitment of aspen stems, but had no effect on jack pine growth and recruitment. Fertilization + brushing increased the growth of jack pine. Brushing the aspen, with or without fertilization, resulted in higher recruitment of jack pine. We conclude that soil fertility controls the mixing of jack pine with trembling aspen, that fertilization increases the likelihood of encroachment of aspen into areas formerly dominated by jack pine, and that brushing along with fertilization is necessary to promote jack pine growth.