Species mixture effects in northern red oak–eastern white pine stands in Maine, USA

Growth and yield studies of mixed-species stands lack generality, though mixture effects appear to be most likely in stands of species with contrasting traits and/or with vertical stratification. The northern red oak (Quercus rubra L.) – eastern white pine (Pinus strobus L.) forest type of the US Northeast is dominated by species of intermediate shade tolerance, often with a spatially heterogeneous lower canopy of shade-tolerant red maple (Acer rubrum L.) and eastern hemlock (Tsuga canadensis (L.) Carr.). We examined neighborhood scale plots (0.02 ha) with variable mixtures of these species and a range of age structure (44–140 years mean age) in southern Maine. Linear mixed effects regression was used to model canopy surface area, stem biomass growth, and stem biomass yield across gradients of composition. With stocking, age, and site quality held constant, more species-diverse plots had denser canopies and faster growth, but lower yield than compositionally simpler plots. For a variety of age structures, mixtures of oak and pine produced more canopy surface area and more stem biomass growth than would be expected if interspecific competition were equal to intraspecific, and more than would be expected on plots dominated by one species or the other. Lower canopy red maple and hemlock contributed to higher plot-level values of all variables in mixed plots, but there was no evidence to suggest that any species contributed to stocking without displacing other species to some degree. This study demonstrates that white pine and red oak, along with other species, can grow in fine-scale mixtures in a variety of even- and two-aged structures without sacrificing productivity, and in some conditions enhancing it.

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► One hundred and twenty one plots in 6 stands dominated by red oak, white pine, red maple, eastern hemlock.
► Modeled canopy, stem biomass, growth by composition, with stocking, site quality, age.
► Species-diverse plots: more canopy, faster growth, but lower yield than simpler.
► Oak–pine mixed plots had more canopy, growth than expected from linear combination.
► Each species’ proportion negatively correlated with stocking of others: none additive.