Effects of residual trees on growth of young to mature Douglas-fir and western hemlock in the western central Oregon Cascades

The purpose of this study was to assess how growth of young to mature Douglas-fir(Pseudotsuga menziesii (Mirb.) Franco) and western hemlock (Tsuga heterophylla (Raf.) Sar.)in mixed stands was influenced by the presence of residual trees. Fourteen paired plots withand without residual trees were examined in a retrospective study on the Willamette NationalForest, Oregon. Growth was analyzed at the stand level, and at the individual tree level,contrasting growth behavior of Douglas-fir and western hemlock. Stands were generally highlystocked with relative densities between 0.38 and 1.05 and age ranged between 55 and 121years.
Results indicate that both understory and residual tree densities had a major influenceon average tree size and growth and yield of the young cohort. At the stand level, residualtrees and high understory densities reduced volume, basal area, and the mean squareddiameter of the young cohort, while understory mortality increased. The influence of residualtree density on total understory and Douglas-fir volume and basal area was best fit by anegative logarithmic function. After accounting for understory density effects, the decrease ofunderstory volume and basal area per individual residual tree decreased with increasingresidual tree density. With 5 to 50 residual trees/ha, total understory volume reduction was22 and 45%, respectively, averaging 2.4 and 1.5% per residual tree, respectively. In mixed
stands, Douglas-fir volume and basal area declined more rapidly than the volume and basalarea of the entire young cohort, when residual tree density exceeded 15 trees/ha. This wasprobably due to the relative shade-intolerance of Douglas-fir. Douglas-fir volume and basalarea on southerly aspects was more than double the values on northerly aspects.
Examination of quadratic mean diameters and radial growth rates by crown classrevealed that the average size and growth rates of dominant Douglas-fir were not reduced byresidual trees. However, the number and basal area of understory trees, particularlydominant and codominant Douglas-fir, declined with increasing residual tree densities.Understory volume was highest in stands that had lowest understory densities. Highunderstory stocking levels were associated with reduced growth and high mortality rates ofthe young cohort, suggesting stands were undergoing self-thinning.
At the individual tree level, basal area growth and diameter growth of trees increasedwith increasing size and dominance. The marginal effect of residual trees declined withincreasing numbers of residual trees per hectare. No residual tree effects on heights ofdominant Douglas-fir was found, perhaps because the few dominant Douglas-firs may havebeen located sufficiently far from residual trees to minimize interaction.
Due to high stocking levels in the young cohort, results from this study only apply tostands with relative densities above 0.38. In addition, leave-trees in managed stands may beyounger, smaller, and may have different growth rates from residual trees in this study, thusaffecting the young cohort differently. Finally, due to the observational nature of this study,cause-and-effect relationships cannot be established. However, since understory volume washighest in stands with low understory densities, understory density management may reducegrowth losses from self-thinning of the young cohort and competition from leave-trees.