No abstract

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 Hrubý Jeseník 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 sitedependent 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.

No abstract

Questions What was the main trigger of treeline ecotone advance – rising temperature or agricultural land abandonment? Were the triggering factors of tree expansion homogeneous or did they differ between upper and lower parts of the treeline ecotone? Location Sudetes Mts., Central Europe (50° N, 15‐17° E). Methods Data were gathered from the treeline ecotone formed by Picea abies at elevations ranging from 1250 to 1490 m. The study area experienced a 1 °C temperature increase over the last 100 yr and termination of cattle grazing and grass mowing in the first half of the 20th century. At 38 plots situated at lower (‘timberline’) and middle or upper (‘treeline’ and ‘outpost treeline’) parts of the treeline ecotone, the age structure of all seed origin P. abies was determined. Changes in tree cover and number of trees over the last 60–70 yr were assessed from aerial imagery. The history of agricultural land use for each plot was compiled. Finally, changes in tree establishment were modelled using climatic variables and land‐use intensity. Results We found that tree establishment at treeline had occurred with a 30–40‐yr lag after the main establishment peak at timberline. Whereas all treeline plots showed gradual increases in tree cover, timberline tree cover first increased, with some plots then undergoing thinning. Enhanced tree establishment was dependent mainly upon agricultural land abandonment. The effect of land‐use changes was more important in the lower than in the upper part of the treeline ecotone. Increasing summer temperatures had a negative influence on seedling establishment in the last few decades. Conclusions Treeline ecotone densification was attributable to agricultural land abandonment across the entire treeline ecotone with the most important effect at timberline. More recently, seedling establishment has been limited by the effects of drought and/or absence of suitable microsites. We documented that one or two establishment pulses over 120 yr, together with enhanced growth since the 1980s, were able to trigger treeline ecotone advance.

Tree rings and documentary evidence are the most important palaeoclimatic archives with annual resolution that continuously span several centuries. Despite this benefit, local to regional-scale temperature reconstructions and their spatial signatures tend to be irregularly distributed, and the appropriate extent of low-frequency variability captured in these proxy records remains uncertain. Here, the first summer temperature reconstruction from the Czech Sudetes Mountains that extends to 1700 AD was introduced. An ensemble reconstruction approach using 251 new high-elevation spruce ring width samples suggests particularly cold June-July temperatures at the beginning of the 18th century, in the 1740s and around 1820. Markedly warm conditions occurred in the 1790s and during the most recent decades. The reconstructed decadal summer temperature amplitude from 'Little Ice Age Cooling' to 'Recent Anthropogenic Warming' ranges from −3.5 ∘ C between 1700 and 1710 to 1.3 ∘ C in 1999-2009, with respect to the 1961-1990 mean climatology. Comparison of our new reconstruction with existing tree-ring chronologies from the Alps reveals a significant level of coherency that is much higher than the agreement with geographically closer documentary evidence from Central Europe. Our study confirms the importance of independent regional climate reconstructions, which capture the full range of past variability and also fill spatial gaps in large-scale networks.