Douglas-fir dominates when grown in dual species mixtures with three western conifers

Planting a mixture of species during reforestation is of increasing interest, underlying a need to quantify the long-term effects of mixtures on stand development for a wide array of species. Here we report on ∼40 year response of three dual species mixture trials at a moderate quality site in Washington, USA where coast Douglas-fir (DF, Pseudotsuga menziesii var. menziesii) was grown at 3 m spacing in a 50:50 mix with either western hemlock (WH, Tsuga heterophylla), noble fir (NF, Abies procera) or western white pine (WWP, Pinus monticola). Stand metrics were compared for each trial between replicated plots of the mixture and pure monocultures of each component species. Across all trials, DF grown in a mixture had greater height and diameter growth than any of the companion species (in either mixture or monocultures) and also outperformed DF grown in a monoculture. Height to diameter ratios for DF were lower in mixtures for all trials compared to DF monocultures, indicating lower interspecific competition for light when grown in mixture. In the DF-NF and DF-WWP trials, mixtures had lower total volume than the DF monoculture, but higher total volume than the NF and WWP monocultures. In contrast, there were no differences in volume between the mixture and monocultures in the DF-WH trial, likely because of complementary shade tolerances between species; overyielding in the mixture may occur with further stand development. These findings can inform decisions on the utility of species mixtures to achieve various management objectives related to wood production.