Temporal and spatial variability in climate-growth response of mountain hemlock at treeline

Abstract

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 changes occured 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 variability of climate-growth relationships since the early 20th Century. Results indicate (1) a change in the climate variables influencing energy limitation in forest growth at treeline, (2) growth response to climate variables changes through time, (3) patterns of growth response to climate are similar between chronologies from similar latitudes, (4) and variable patterns of growth response to climate between chronologies from different topographic aspects. 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.