Plant species diversity and composition along an experimental gradient of northern hardwood abundance in Picea mariana plantations

Maintenance of biodiversity in managed forest ecosystem is an increasing concern, particularly in plantation forests. To test the two hypotheses that (i) past vegetation treatments modify species diversity and that (ii) a trade-off exists between plantation productivity and diversity, we measured plant species diversity in the upper, intermediate and lower strata of 16–17-year-old Picea mariana plantations along a gradient of hardwood abundance. The gradient was obtained with various intensities of non-chemical release and thinning treatments. P. mariana productivity linearly decreased as the proportion of hardwoods within the canopy increased. As the proportion of P. mariana increased, total species richness and plant diversity (Shannon index [Hʹ]) first increased up to an intermediate level, and then decreased. Proportion of non-crop to crop tree species explained 66% of the variation in species diversity of the upper strata, 0% of the intermediate strata, and 20% of the lower strata. Intensity of past vegetation treatments slightly affected species composition, as evaluated by the S?rensen’s index of similarity. Maintaining an almost exclusive cover of P. mariana had marginal effects on species richness and understory diversity. But, a large increase in hardwoods proportion occurred at the expense of species richness and diversity of the understory stratum. Differences in canopy light transmission between covers could not explain this result. Variation in species richness and diversity along the gradient of hardwood abundance had a hump shape, a characteristic of the classic disturbance-diversity hypothesis. But, the intensity of past vegetation control had a weak predictive capability for plant species richness and diversity. Thus, vegetation control to maximise crop tree productivity in P. mariana plantations is not likely to affect understory plant diversity or composition. It may however contribute to an increase in stand structural diversity.