Seedlings and snow: The impacts of climate change on Pacific Northwestern Forests

Background/Question/Methods

Coniferous forests in the Pacific Northwest sequester large amounts of carbon and are home to many plants and animals of conservation interest. Thus, understanding how global climate change will affect this biome is critical; yet efforts are hampered by our limited understanding of how component tree species differ in their sensitivity to the climate variables most likely to change in the future. Especially lacking are data on the very youngest trees (seedlings), the life history stages critical for driving range expansion rates. Here, we monitor microclimate, seed germination, seedling survival and seedling growth in fifteen permanent vegetation plots located across the large climatic gradients existing on Mount Rainier, Washington (USA). We use these data to ask 1) how dominant conifers vary in their sensitivity to climate; 2) whether differences match those expected from their altitudinal distributions; and 3) how the warmer temperatures and decreased snowpack in the region will impact recruitment of dominant conifers. 

Results/Conclusions

We found that conifers differed in their sensitivity to key climate variables, especially snowpack, with growth and survival of some species negatively (e.g. Western hemlock), and others positively (e.g. Pacific silver fir) impacted by snowpack. Negative impacts of snowpack appeared to be mediated through shorter growing season lengths, with positive impacts of snowfall resulting from increased soil moisture late in the growing season. The magnitude of climatic effects on seedling growth and survival generally matched species altitudinal distributions, with upper-elevation species more sensitive to late-season drought and lower-elevation species more sensitive to short growing seasons. In all, high mortality and low growth at early life history stages of these conifers imply slow responses to the rapid rates of warming forecast for the Pacific Northwest.