Effects of initial spacing and species mix on stand developmental patterns in two field trials in central Oregon

Productivity, stand dynamics, and ecosystem processes characterizing mixed-species forests have been inferred from a mix of inventory data, observational studies, designed experiments. Designed experiments are expensive to implement maintain for sufficiently long time in most temperate species, but associated treatment replication randomization lower the risk confounding spatial correlations among species composition, environmental conditions, productivity. Results replicated spacing trials 45-yr-old Pinus ponderosa/Abies grandis plots 53-yr-old P. ponderosa/P. contorta demonstrated main interaction effects composition on growth structure, as well change these over time. Initial ranged 1.8 m 5.5 square grid. Mixtures ponderosa A. at wide spacings offered opportunity achieve cumulative height nearly equal thereby form uniform mixed canopy stem volume growth. The same initial mixture closer resulted increasingly stratified mixtures with dominating overstory due largely its more rapid juvenile wider crown, relegating shade tolerant understory. A similar pattern emerged contorta, subtle gradients vertical structure attributable less extreme differences tolerance. By end 2019 (plantation age 53), no significant overyielding or underyielding occurred contorta. In contrast, by 2018 ponderosa/A. 45 years) achieved some that was marginally (net production) very (gross only narrowest (1.8-m) spacing. timing intensity mortality shifted mixing periodic annual increment slightly Evidence consistent weak total plantation 53 years, respectively. However, statistical power detecting limited relatively small number replications, size differentiation continues emerge successively plots.