Growth trends and temperature responses of treeline Norway spruce in the Czech-Polish Sudetes Mountains

The Earth's climate system has recently experienced substantial warming, and associated effects are often most pronounced at species-specific distribution limits. Treeline ecotones may therefore be particularly useful to assess the complex interplay of biotic and abiotic factors in relation to environmental change. Here, we present site-dependent growth trends and climate responses of 22 Norway spruce Picea abies (L.) H. Karst. tree-ring chronologies from the treeline ecotone in the Sudetes Mountains (Mts.) along the Czech-Polish border. Annually resolved and absolutely dated ring-width measurements from 2 regions (Giant Mts. and Hruby Jesenik Mts.), separated by aspect and altitude, resulted in robust chronologies for the 20th century. All sites reveal a close relationship between ring widths and growing season temperatures. The main site-dependent differences in growth trends and temperature responses were attributed to altitude. Trees located at the upper boundary of the treeline ecotone reacted positively to preceding autumn temperatures, and their temperature response was slightly weaker than that of trees from the upper forest limit. Temporal trends in growth-climate responses consisted mainly of a gradual decrease in the influence of preceding October temperatures and an increasing effect of May temperatures towards the present. The proportion of younger trees generally increases upwards within the ecotone, and the most recent growth rates appear unprecedented in a century-long context.