Factors Affecting Understory Composition and Height Growth 13 Years Following Variable-Density Thinning of a Conifer Forest

Variable-density thinning (VDT) is a management option aimed at accelerating the development of late-successional structure in second-growth conifer stands. The response of the understory is paramount to the success of the treatment. This study examined the species composition, density, and height growth of post-VDT regeneration 13 years following thinning in 50- to 70-year old conifer stands at Ft. Lewis Military Reservation near Tacoma, WA. Thinning intensity had a significant effect on the density of post-VDT regeneration, but that effect varied in intensity throughout stem-mapped plots. Height growth from 2002 to 2005 was assessed for 2,535 Douglas-fir, 43 grand fir, and 39 western white pine in the post- variable-density thinning cohort of second-growth Douglas-fir stands. Thinning intensity was not significantly related to height growth for grand fir or for western white pine. Thinning intensity was significantly related to height growth for Douglas-fir, but the effect varied with initial height. Factors correlated with Douglas-fir height growth in gap-thinned subplots were measures of crown size, measures of intra-cohort crowding, strata and type of overtopping vegetation, and overstory crown coverage. Smaller regeneration (less than 1.3m tall) was also examined. Stocking of smaller regeneration was significantly greater in the light thinning intensity than in the heavier thinning. The results of this study indicate that variable-density thinning can induce variation in the abundance and growth of Douglas-fir in post-VDT cohorts.