Changes in juvenile growth rates of Norway spruce and European beech from Central Europe in link with climate warming
Montane forests of Central Europe are experiencing faster changes in climate than the average trends. It is still unclear how these natural forests, and especially saplings, have responded to recent modifications in growth conditions. We took advantage of the REMOTE forests network to look at the changes in juvenile growth rates of Norway spruce and European beech, two economically important tree species in Europe. This network includes ring-widths series of more than 19,000 spruces and 14,000 beeches located in montane forest patches within the Carpathians and Balkans mountains. We first assessed changes in growth rates since 1750, with a focus on the juvenile life-stage, i.e. the period when trees were below a diameter of 10cm at breast height. Then, we studied the trees’ sensitivity to climatic water balance, vapor pressure deficit, temperature, nitrogen and sulfur depositions. Finally, we assessed the modulating effects of competition, stand structure and species diversity. Juvenile growth of both species accelerated since 1750, the trend in spruce being twice the trend in beech. Spruce saplings responded positively to temperature and this response was higher in species-rich and low competitive environments. Spruces were also sensitive to moisture availability, suggesting that the boost in growth rates could be only transitory. Beech saplings benefited less from higher temperatures and were even negatively affected at low elevation. High levels of acidic depositions buffered the negative effect of a high competition pressure on growth of both species. These results suggest that sylvicultural practices favoring species-diverse and low-density stands could help maintain high growth rates under a warmer climate.