Are mixed-species stands more productive than single-species stands: an empirical test of three forest types in British Columbia and Alberta

Stand volumes were determined for naturally established, even-aged, single- and mixed-species stands involving three combinations of shade-tolerant and shade-intolerant conifers on similar sites: (i) western redcedar – western hemlock (Thuja plicata Donn ex D. Don – Tsuga heterophylla (Raf.) Sarg.), (ii) lodgepole pine – western larch (Pinus contorta Dougl. ex Loud. – Larix occidentalis Nutt.), and (iii) lodgepole pine – black spruce (Pinus contorta – Picea mariana (Mill.) BSP). Stand volume was significantly increased with site index and breast-height age in all three studies. Stand volume was also related to relative stand density in the lodgepole pine – black spruce study. When both species were shade tolerant (hemlock–redcedar), stand volume increased linearly with the proportion of hemlock; the mixed-species stands had intermediate volume compared with single-species stands. The combination of two shade-intolerant species (pine–larch) had lower stand volume than that anticipated from single-species stands, implying that one species may inhibiting the growth of the other. Mixtures of shade-intolerant and shade-tolerant species with different growth patterns (spruce–pine) may be more productive than single-species stands in specific ecological contexts and developmental stages. The effect of one species on the productivity of another species is tree-species and site specific: at maturity, even-aged, mixed-species coniferous stands are not necessarily more productive than single-species stands.