Research Assistant and Data Analyst Ohio University, Ohio, United States
Abstract: Climate change has already altered weather patterns worldwide and is projected to continue to increase mean annual temperature, in particular summer temperatures, and change precipitation patterns. In the Pacific Northwest (PNW), less precipitation in the summer coupled with the increase in summer temperatures could produce overall drier, more stressful summer conditions. Water stress has already been shown to have a strong negative effect on seedling survivorship in species like Abies lasiocarpa (subalpine fir), Abies amabilis (Pacific silver fir), Picea engelmannii (Engelmann spruce), and Pinus flexilis (limber pine). In addition, regeneration failure is increasingly being recorded among PNW species along warmer species range boundaries and even inside their core range areas for certain species. Thus, it is imperative to understand the factors that determine seedling survivorship at the individual species level. The goals of this study are to quantify those seedling survival relationships for tree species in the PNW. We used the FIA (Forest Inventory and Analysis) data for seedling and stand information, and PRISM data for climate. We ran an inverse model fitted with maximum likelihood methods, to predict recruitment and annual survival as a function of mean annual temperature, water deficit, total canopy basal area and conspecific basal area. We fit parameters for 22 species, that ranged in FIA subplot presence from 0.4 – 13%. We attempted to fit relationship for 27 additional species, however these species were present in < 1% of FIA plots and the models did not converge. Most species had reduced seedling survival at high water deficits (e.g., Abies amabilis, Abies procera, Tsuga heterophylla), however there were some species that appeared insensitive to water deficits (e.g., Pseudotsuga menziesii, Pinus ponderosa) or had some moderate tolerance to drought (e.g., Abies grandis, Picea engelmannii). Seedling survival responses to temperature were also species-specific, although the most common response was higher survival at cooler temperatures (e.g., Abies amabilis, Abies procera, Alnus rubra, Pinus monticola, Tsuga heterophylla). It is interesting to note that many of these species have distributions that include lower elevations, which could potentially indicate an increase in seedling mortality as temperatures continue to warm. In conclusion, these species-specific seedling survival relationships can be used to parameterize forest models and to better understand risks associated with climate change and recruitment limitations.