- Uneven-aged management has been suggested as a method for balancing biodiversity conservation and wood production goals from managed forests in a variety of regions. In coastal Douglas-fir (Pseudotsuga menziesii) forests of the Pacific Northwest, implementation of uneven-aged management is hindered by a lack of experience with uneven-aged silvicultural systems, including approaches for converting even-aged Douglas-fir stands into uneven-aged structures. Twenty years ago, researchers developed the Uneven-Aged Management Project (UAMP) on the H.J. Andrews Experimental Forest to examine the effects of different uneven-aged management strategies on wood production and ecological goals. UAMP treatments included three different thinning treatments to begin the process of converting monoculture, even-aged Douglas-fir stands into mixed-species, uneven-aged structures. These three thinning treatments included: (1) a Light Overstory Removal where 100% of the stand was thinned from below to a relative density of 4.3, (2) a Group Selection where 90% of the stand was thinned from below to a relative density of 4.3 and the remaining 10% was cut in circular gaps of 24.38 m in diameter, and (3) a Heavy Overstory Removal where 100% of stand was thinned from below to a relative density of 2.8, as well as (4) an unharvested Control treatment. In the Summer of 2017, we re-measured all overstory trees > 5cm DBH, and tree seedlings and saplings < 5 cm DBH on a series of long-term plots to compare overstory growth and yield, regeneration establishment, tree size variability, and species composition of the overstory and understory tree communities 17 years after application of these initial treatments. Overall, the harvest treatments had no significant effects on stand scale volume increment, basal area increment, and overstory species diversity when compared to controls, but harvest treatments reduced mortality, increased individual tree growth rates, and showed a trend towards greater tree size variability than controls due to increased representation of small-diameter trees. There were no significant differences among the four treatments in any of the metrics of stand-scale growth, individual tree growth, mortality, tree size variability, or species diversity.
These harvest treatments had a significant effect on the density of Douglas-fir and western redcedar (Thuja plicata) seedlings, species richness of seedlings, and total seedlings in the stand. Douglas-fir seedling densities were generally greatest in the heavy removal and gap treatments, depending on seedling size, and lowest in controls. Western redcedar densities were generally higher in the light removal treatment than in controls. There were no significant differences in among the three harvest treatments for western hemlock (Tsuga heterophylla), Pacific yew (Taxus brevifolia), or hardwood seedling densities. The results suggest that wood production levels may be comparable across a range of even-aged to uneven-aged conversion treatments, and that overstory thinning and underplanting may accelerate the development of tree size variability relative to passive management. Although significant levels of tree regeneration occurred in all treatments, heavy overstory removals or larger gaps are likely needed to promote Douglas-fir regeneration establishment and recruitment, while western redcedar regeneration establishment and recruitment appear to benefit from light overstory removals. Collectively, these results suggest that uneven-aged conversion treatments that create a variety of residual overstory densities or incorporate gaps are likely to promote greater diversity in the regenerating tree community, while producing wood volume at rates similar to unthinned plantations.