A lost generation of trees: Investigating the causes of low tree recruitment in urban coniferous forests

Background/Question/Methods

Why are urban forests not replacing themselves? For example, recent surveys demonstrate that adult trees are dying faster than new saplings are recruiting in Seattle (Washington), implying that the health of local coniferous forests is in decline. Extensive resources are spent planting trees to combat this problem, but the underlying causes of this “lost generation” of trees remains unknown.  We investigate factors limiting seedling establishment of native conifers (Thuja plicata, Tsuga heterophylla) in Seattle, and use results to suggest strategies resource managers can use to address the ‘lost generation’ problem. We investigate the role of invasive plants (which can out-compete native species for water, nutrients, and light) and the lack of coarse woody debris or rotting wood (which benefits seedling survival; second growth forests typically have little downed deadwood).  Specifically we examine the effect of removing non-native English Ivy (Hedera helix) and adding mulch to understand what is limiting conifer regeneration, and to explore possible management techniques for improving growth and survival of transplanted seedlings.  We measured survival, general health (visually assessed), and height as response variables to the two treatments. As of yet, no studies have empirically tested both of these factors.

Results/Conclusions

Mulch addition had the most consistent overall effect on seedling health and survival.  We observed the highest transplant survival rates in areas where mulch was added, for both focal species. Surviving transplants in the mulch treatment were also healthier than the transplants with no mulch added.  Ivy presence did not affect health or survival of either species. Treatments affected growth of the two species differently.  Growth of Thuja plicata was not affected by either treatment.  For Tsuga heterophylla, however, growth was highest in the ivy-removed treatment, but was not significantly affected by the presence of mulch.  Our results suggest that seedling survival is limited by a lack of mulch and that competition with invasive understory plants may limit growth for some species.  Mulch addition may be beneficial due to higher moisture retention, by providing seedlings with additional nutrients, or by aiding mycorrhizal associations.  Our recommendations for park management are therefore that restoration transplants should be paired with mulch addition, as well as Hedera helix removal for Tsuga heterophylla, to maximize survival and growth.