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

Abstract

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