Plant pathogens may promote or destabilize diversity through a number of mechanisms, depending on pathogen host range, and host abundance, susceptibility, and tolerance of pathogens. Early plant life stages are thought to be especially vulnerable to plant pathogens that may otherwise be tolerated by larger individuals. In 2016, we established a network of 117 seed traps and seedling monitoring plots in the Wind River Forest Dynamics Plot (WRFDP) in southwestern Washington to quantify the dynamics and diversity of natural regeneration processes in this old growth conifer forest and the potential role of oomycete pathogens in driving those dynamics. We also established 30 experimental sites, each with one seed trap and four seedlings plots, outside the perimeter of the WRFDP, where we applied a broad-spectrum fungicide and oomyceticide in a factorial design in late summer 2017. Beginning in 2016, we have quantified the number of conifer germinants, marked and measured all surviving woody seedlings and new seedling recruits, and assessed herbaceous cover in each plot. Seed trap contents have been collected annually since 2017, with contents dried, sorted, identified and counted.
Results/Conclusions
Western hemlock (Tsuga heterophylla) and Douglas Fir (Pseudotsuga menziesii) dominate the seed rain and are the most common seedling recruits, but we found considerable year-to-year variation in seed fall, germinant number, and overall estimated survival rate from seed-to-seedling. There were significantly higher germination rates in the 2017 mast year than in 2018. The composition of the seedling community also responds to the biotic neighborhood, including herbaceous cover and soil pathogens and symbionts. For example, Vanilla leaf (Achlys triphylla) appears to exclude conifer seedlings, and has newly been shown to host oomycete pathogens, but there was little response in abundance of this rhizomatous herb to the oomyceticide treatment in our experiment. Year-to-year variation in seed production is a large driver of early regeneration dynamics in this old-growth forest, and survival of established seedlings responds to abiotic gradients as well as conditions of the biotic neighborhood.