Long-term effects of tree spacing during reforestation on survival and growth of three important tree species in the Pacific Northwest, USA

Tree spacing during reforestation is a critical decision that influences the trajectory and rate of stand development over time, but there is limited quantification of responses at meaningful time scales for management. Here we report on ∼40 year response of coast Douglas-fir (DF, Pseudotsuga menziesii var. menziesii), noble fir (NF, Abies procera) and western white pine (WWP, Pinus monticola) to five or six spacing treatments that ranged from 1 to 6 m in the western Cascades of Washington, USA. Spacing effects were tested separately for each species in completely randomized designs applied to 0.4 ha plots with 4–6 replications per treatment. There was no effect of spacing on relative survival in WWP and NF; for DF, relative survival decreased with decreasing spacing, with the effect becoming more pronounced with time. Quadratic mean diameter generally increased with increasing spacing for WWP and DF, but the effect attenuated at 4-m spacing and higher. Total stand volume was inversely related to spacing for all species for much of the study period, but there was no difference in volume among spacings for DF in the most recent measurement because of the density-dependent mortality in narrow spacings as Stand Density Index increased above 55 % of maximum. Our findings indicate that wider spacings (5–6-m) promote the development of larger trees with more complex stand structure (including that arising from ingrowth), while spacings of 3–4-m will maximize wood production. The implications of these findings are discussed in the context of management objectives, silvicultural resource constraints, and species-specific considerations.