Emerging drought sensitivity for large Norway spruce trees at high elevation in the High Tatras, Slovakia

Understanding tree growth and forest dynamics under climate change is paramount to predict changes in carbon cycling, forest development, and ecosystem services. At temperature limited sites, such as alpine treelines, tree growth is often assumed to benefit from rising temperatures, while increased drought may offset potential benefits. Tree size is known to be related to climate sensitivity and drought induced mortality, with large trees generally suffering the most from drought. To assess the relationship of tree size and climate sensitivity for Norway spruce trees at treeline, we collected 158 tree cores at treeline and the adjacent closed canopy forest in the High Tatra Mountains in Slovakia. Size classes were established based on size class isolation of the total sample set, yielding artificial tree ring chronologies with a constant size over time. We ran moving-window correlation analyses to assess the temporal development of climate sensitivity. We found climate-growth correlations to be non-stationary and with similar trends among size classes. In general, trees are temperature limited during the growing season, but correlations have shifted from June to July in recent decades. Additionally, the largest trees show an increased and significant sensitivity to August and September drought conditions in the year preceding growth. These findings emphasize the increasing influence of drought constraints on tree growth, even at supposedly temperature limited treeline sites.