Diverging growth performance of co-occurring trees (Picea abies) and shrubs (Pinus mugo) at the treeline ecotone of Central European mountain ranges

Abstract

Although there are ample data on growth trends and climate growth relationships of trees from the leading edge of their distribution at treeline, information from the neighbouring trailing edge of the vegetation belt dominated by alpine shrubs is missing. We expected trees at their upper limit to exhibit unambiguous temperature limitation with a clearly positive growth response to recent warming. On the other hand, shrubs at the lower limits of their distribution and because of their low-stature are assumed to be less constrained by temperature, with ambiguous growth trends as compared to trees. We collected tree-ring series from sites with co-occurring Norway spruce (Picea abies) and Mountain pine (Pinus mugo) in four mountain ranges of Central Europe (the High Tatras, the Hrubý Jeseník Mts, the Krkonoše Mts and the Bavarian Alps), assembled a suite of tree-ring chronologies containing either long-term trends or high-frequency variability, and compared climate-growth relationships and growth trends between spruce and pine. Our results show that in all areas under study, growth patterns of spruce statistically differ from those of pine. The growth of spruce is characterized by a tight relationship with June–July temperatures, maximum correlations ranging between 0.5 and 0.6. The climatic signal in tree-rings of pine is also characterized by a significant influence of summer temperature, albeit weaker than that observed in spruce (mostly between 0.3 and 0.4). All sites exhibited increasing growth trends for spruce since the 1980s; trend slopes for pine were either less positive (Hrubý Jeseník and Krkonoše Mts) or negative (High Tatras). To conclude, the growth of spruce at its leading edge clearly resembled temperature-limited growth with corresponding recent growth acceleration, while the response of low-stature mountain pine to warming was weaker because of its tight coupling with microsite conditions and the location at the trailing edge of its distribution.