Twentieth century treeline ecotone advance in the Sudetes Mountains (Central Europe) was induced by agricultural land abandonment rather than climate change

Abstract

AbstractQuestionsWhat 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?LocationSudetes Mts., Central Europe (50° N, 15‐17° E).MethodsData 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.ResultsWe 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.ConclusionsTreeline 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.