Climatic drivers of tree growth at tree line in Southwest Yukon change over time and vary between landscapes

Growth of trees at their altitudinal and latitudinal range limits is expected to increase as climate warms, but trees often exhibit unexplained spatial and temporal variation in climate-growth responses, particularly in alpine regions. Until this variability is explained, predictions of future tree growth are unlikely to be accurate. We sampled Picea glauca (white spruce) growing at forest and tree line on north and south aspects in two mountain ranges of southwest Yukon to determine how and why ring-width patterns vary between topographic settings, and over time. We used multivariate statistical analysis to characterize variation in ring-width patterns between topographic factors and time periods, and calculated correlations between ring-width indices and climate variables to explain this variation. Ring-width patterns varied more between mountain ranges than elevations or aspects, particularly in recent decades when ring-widths increased in one mountain range but not the other. Growth responses to summer temperature were notably weaker during warmer time periods, but growth was not positively correlated to summer precipitation, suggesting trees may not be suffering from temperature-induced drought stress. Rather, ring-width indices began responding positively to spring snow depth after 1976. We conclude that tree growth is unlikely to increase in synchrony with rising air temperatures across subarctic tree lines in southwest Yukon. Instead, they may decline in areas that are prone to thin snowpacks or rapid spring runoff due to the negative influence warming springs will have on snow depth and, consequently, early growing season soil moisture.