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.

促进耐荫树种的斑块募集进入中层，是发展次生林分结构多样性的重要一步。可变密度稀疏（VDT）已被提议作为加速结构多样性的一种策略，因为其机架内处理（收获间隙，稀疏和未收获的跳跃）的组合应会产生可变的上层和下层条件。在这里，我们报告了五个混合针叶林林分和锡特卡云杉（云杉云杉）中的铁杉（Tsuga heterophylla（Raf。Sarg。）西部铁杉幼苗和幼树密度。VDT发生后的3、7、10和16-17年，华盛顿西部的两个林分中有（密密麻麻的）Carr。）此外，我们报道了西部铁杉在14年内在两个林分中的提前再生生长和存活情况。在稀疏处理中，西部铁杉幼苗的密度最高，但在第10年才如此。相反，在第7年和第10年，这些缺口包含明显更多的西部铁杉幼树，而到第10年，西部铁杉的幼苗生长明显加快。 VDT与未采伐的储藏物在两种树种的幼苗或幼树密度方面均无显着差异。Sitka云杉幼苗密度在间隙和稀疏处理中最高，但在所有处理中幼树都不常见。总的来说，这些结果表明，我们的VDT变种促进了西部铁杉的斑块状，中层募集，但未能在其建立的任何林分中募集Sitka云杉树苗。因此，可能需要更强的VDT变体来促进对中型比西方铁杉耐荫性的物种的中层更新。