Are growth-climate relationships at treeline changing? Energy limitation and mountain hemlock in a warming climate
In forest ecosystems, radial growth response to climatic variables typically ranges from energy limited to water limited. Although tree growth can be limited by one or the other or both, plants need both energy and water to survive and grow. In the Pacific Northwest (PNW) region of North America, forests at or near altitudinal treeline are typically energy limited (Albright & Peterson, 2013). Climate-growth relationships in treeline forests can be particularly informative, because they represent the upper limit of a species range where growth is especially sensitive to climatic variation (Brubaker, 1986). Summer’s current research seeks to examine the extent of change in biologically meaningful limiting variables and the effect of these changes on radial growth of treeline mountain hemlock (Tsuga mertensiana (Bong.) Carrière) in a large portion of the Pacific Northwest.
The results of this study may suggest the existence of a climate-change signal, namely a weakening of energy limitation in forest growth at treeline. This would be a significant change in a basic physiological relationship that has likely existed for millennia, affecting forest productivity and carbon storage. Regardless of the specific outcome, the results of this study will improve our understanding of the ecology of subalpine forests, and can be used to develop better growth functions in ecosystem models