Growth response of ponderosa pine (Pinus ponderosa) to climate in the eastern Cascade Mountains, Washington, USA:: Implications for climatic change

Climatic change is expected to cause dramatic shifts in low-elevation treeline in mountainous environments. Ponderosa pine (Pinus ponderosa) were sampled across an elevation gradient adjacent to the Methow Valley of the Okanogan National Forest in the eastern Cascades, Washington to examine the potential response of ponderosa pine to climatic change. Response function analyses were used to compare climate-growth relationships among 12 sites, four elevations on three different mountains. Response function analysis attributes 42-55% of the inter-annual variation in growth to climate. Growth is positively correlated with November precipitation prior to the growing season on all 12 sites, suggesting that November precipitation is critical for increased root growth, increased nutrient availability through decomposition, building snowpack, or non-growing season photosynthesis and carbon storage. Growth is positively correlated with previous October, January, June, and July precipitation at more than one site. Temperature is not correlated with growth on any sites. Climate models predict that the Pacific Northwest will experience warmer and wetter winters and drier summers in the future. Growth-climate correlations suggest that the short-term growth response of ponderosa pine is most sensitive to non-growing season precipitation. Therefore, predicting ponderosa pine's response to projected climatic change is problematic, with wetter falls increasing growth and drier summers decreasing growth. Our results indicate that ponderosa pine is much more sensitive to precipitation than temperature and that any predictions of this and species' response to climatic change are difficult, due to uncertainty in predicting future precipitation patterns.