The role of species mixtures in plantation forestry

Nearly all forest plantations are established as monocultures, but research has shown that there are potential advantages to be gained by using carefully designed species mixtures in place of monocultures. This paper reviews recent studies that compare stand development and productivity of mixed and pure plantations. Higher stand-level productivity in mixtures has been found with two kinds of species interactions: (1) complementary resource use between species that arises from development of a stratified canopy (and possibly root stratification); (2) facilitative improvement in nutrition of a valuable timber species growing in mixture with a nitrogen-fixing species (but only if combined with complementary resource use as well). These mixtures can also improve economic return through greater individual-tree growth rates and provision of multiple commercial or subsistence products. More complex plantation mixtures of 5–70 species have been used for ecological restoration of degraded lands; these large numbers of species of different successional stages are combined to reduce the need for a series of sequential plantings. Future research needs to examine many more tree species across a wider range of sites. Innovative planting designs have been developed to reduce the land area needed for mixed-species plantation experiments, by focusing on individual-tree analysis rather than plot-level analysis