Abstract: Signs of dieback and declining tree growth trends have recently been observed in montane conifers such as Fir or Spruce in South Europe. Given that Europe is among one of the temperate zones that have experienced the greatest warming, this decline could be interpreted as an early signal of regression processes at the rear edge of tree species’ distribution. When changes have been examined over altitude ranges, the growth responses of trees remained more ambiguous. The present study aims to compare historical and recent growth trends of Abies alba Mill., one of the most important mountainous tree species of Europe, under the influence of climate change.
The study was conducted in the Vosges Mountains through a forest inventory of Abies alba over 30 pure and mixed plots. Trees were cored and a dendrochronology analysis was carried out to obtain series of tree-ring width. A radial growth model dependent on climate and stand variables was also constructed to explore what variables have determined A. alba growth from 1900 to 2022, focusing on the last decade, which contains 7 of the 10 warmest years ever measured.
Results show an increasing trend during the XXth century followed by a significant decreasing trend in the two last decades. Growth model highlighted that temperature were significantly correlated to growth, with recent highest temperatures correlated to declining growth over the 1999-2022period. Growth trends over time can be explained by a shift of limiting factors constraining tree growth from cold temperatures during the 20th century to low water supply for the last decades. Study also revealed a natural decreasing growth trend with increasing altitude which have occurred during the first part of the XXth century. A progressive decay of this decreasing trend occurred during the last decade leading to a current lack of fir radial growth change over the studied elevation gradient ranging from 600 to 1200 m.
Overall, results complement existing observations of the downturns in growth carried out in South European forests, and contribute to highlight the ecological drivers explaining this shift. The issues at stake on the evolution of resources and services provided to forest users of Fir forests such as wood production, carbon sequestration or biodiversity conservation. If growth trend reversal can be confirmed over a large spatial scale, it will have consequences in terms of adapting forest management and the forestry sector.
