Conifer regeneration in Seattle’s urban forests: Influences of seed availability, microsite conditions, and ivy competition on germination

Background/Question/Methods

Urban conifer forests offer extensive ecological benefits, such as storm water retention, carbon sequestration, erosion prevention, and improvements to air quality. In urban areas like the Puget Sound region, these conifer forests are at risk because the natural regeneration of coniferous species in urban forests is low. We examined three possible causes for low regeneration levels: seed and dispersal limitation, microsite limitation, and competition from invasive species.  Seed availability and dispersal limitation are particularly important to consider in urban parks, as small, fragmented forest patches are commonly seed-limited. Because Seattle’s urban forest patches are nearly all second-growth, with low amounts of coarse woody debris in the form of downed wood, another possibility for low regeneration is that microsite conditions are not favorable to native conifer germination, which occurs at higher rates on downed wood (i.e. nurse logs). Introduced invasive species are a third possible cause for low germination rates. English ivy creates dense mats on the forest floor and can outcompete native plants. To address these issues, we therefore examined the influences of seed addition, addition of woody debris, and ivy removal on the germination rates of three native climax conifers (Abies grandis, Tsuga heterophylla, and Thuja plicata). 

Results/Conclusions

In all, we found that seed and dispersal limitation limited recruitment of our three focal species most, with complex effects of wood chip addition and ivy removal. Specifically, we generally found the highest germination in seed addition plots for all species, although total germination was low for all species. Abies grandis and Tsuga heterophylla germinated at higher overall rates than Thuja plicata, for which we observed one germinant. Abies grandis seedlings showed the highest total germination in areas without ivy and without added woody debris, although at the end of the growing season, germinant survival was highest in areas where wood was added and ivy was removed. Tsuga heterophylla germination was highest in control areas (those with ivy and without woodchips). However, similar to Abies grandis, the surviving Tsuga heterophylla germinants only occurred in plots with wood added by the end of the growing season. In summary, Seattle’s urban forests are likely both seed- and microsite-limited. While ivy may reduce germination in some cases, our results clearly show that the addition of seeds and woody debris could help urban conifer forests regenerate, and should be used to improve park management and future planting efforts.