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Variable-density thinning promotes differential recruitment and development of shade tolerant conifer species after 17 years

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Abstract

Promoting patchy recruitment of shade tolerant tree species into the midstory is an important step in developing structural diversity in second-growth stands. Variable-density thinning (VDT) has been proposed as a strategy for accelerating structural diversity, as its combination of within-stand treatments (harvest gaps, thinning, and non-harvested skips) should create variable overstory and understory conditions. Here we report on western hemlock (Tsuga heterophylla (Raf.) Sarg.) seedling and sapling densities in five mixed-conifer stands and Sitka spruce (Picea sitchensis (Bong.) Carr.) seedling and sapling densities in two stands in western Washington at 3,7, 10, and 16–17 years after VDT. Additionally, we report on western hemlock advance regeneration growth and survival in two stands over 14 years. Western hemlock seedling density was highest in the thinned treatment but only significantly so in Year 10. In contrast, the gaps contained significantly more western hemlock saplings in Years 7 and 10 and significantly greater growth of western hemlock advance regeneration through Year 10. Skips embedded within the VDT did not differ significantly from unharvested reserves in terms of seedling or sapling densities of either species. Sitka spruce seedling density was highest in the gap and thinned treatments, but saplings were uncommon in all treatments. Collectively, these results indicate that our variant of VDT promoted patchy, midstory recruitment of western hemlock but failed to recruit Sitka spruce saplings in either stand where it established. Consequently, more intensive variants of VDT may be required to promote midstory recruitment of species less tolerant of shade than western hemlock.

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Acknowledgements

We thank the USDA Forest Service Olympic National Forest for implementing the operational treatments, the USDA Forest Service Pacific Northwest Region and Pacific Northwest Research Station for funding, and our many coworkers for their efforts in plot installation and measurement. We would also like to thank Nancy Koerth for her statistical advice.

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Authors and Affiliations

  1. USDA Forest Service, Southern Research Station, G.W. Andrews Forestry Sciences Laboratory, 521 Devall Drive, Auburn, AL, 36849, USA

    John L. Willis

  2. USDA Forest Service, Pacific Northwest Research Station, Olympia Forestry Laboratory, 3625 93rd Avenue SW, Olympia, WA, 98512-9193, USA

    Constance A. Harrington & Leslie C. Brodie

  3. Department of Forestry, Mississippi State University, 361 Thompson Hall, 775 Stone Blvd., Mississippi State, MS, 39762, USA

    Scott D. Roberts

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  1. John L. Willis
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Willis, J.L., Harrington, C.A., Brodie, L.C. et al. Variable-density thinning promotes differential recruitment and development of shade tolerant conifer species after 17 years. New Forests 52, 329–348 (2021). https://doi.org/10.1007/s11056-020-09797-z

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