Master's Thesis Abstract by Summer Kemp-Jennings (2017)

Climate-growth relationships in treeline forests are particularly informative because they represent the upper limit of a species range where growth is often especially sensitive to climatic variation. Radial growth response to climatic variables typically ranges from energy limited to water limited. In the Pacific Northwest (PNW) region of North America, treeline forests are typically energy limited. Results from a recent study (Marcinkowski et al. 2015) indicate a change in climate-growth relationships over time in treeline mountain hemlock (Tsuga mertensiana) in northern Washington State. Here, we investigate whether these same relationships hold across 700 km of the range of treeline mountain hemlock in Oregon and Washington. Using cores from trees >200 years old, we examined temporal and spatial variabililty of climate-growth relationships since the early 20th Century. Results indicate (1) a potential weakening of energy limitation in forest growth at treeline, (2) a nonstationary growth response to climate variables through time, and (3) different patterns of growth response to climate from north to south. Growth-climate relationships were generally similar between different aspects in the same geographic location, with some exceptions. Correlations between radial growth and climate vary from significantly positively correlated to significantly negatively correlated for some variables. These results helped identify where changes in growth-limiting variables may be occurring, and suggest that the effects of a warmer climate on growth at treeline will be more complex temporally and spatially than has been suggested by previous analyses.