Geographically Structured Growth decline of Rear-Edge Iberian Fagus sylvatica Forests After the 1980s Shift Toward a Warmer Climate

Warming-related growth decrease on southern Fagus sylvatica forests has been observed in different regions; however, whether it is a generalized fact or not remains unclear. Here we investigate the geographical pattern on growth response of the southwestern European beech forests to the warming climate shift which started in the 1980s. We sampled 15 beech forests (215 trees) across four climatically contrasting regions (Mediterranean, Pyrenean, low- and high-elevation Atlantic areas) near the southern distribution limit of the species in the Iberian Peninsula. Dendrochronological analyses were carried out to evaluate the growth of European beech since the 1950s. Growth responses quantified as pointer years, abrupt growth changes and long-term growth trends were compared between periods (before and after the 1980s climate shift), geographical regions and tree sizes. Analyses of the studied variables indicated a growth decrease in basal area increment after the climate shift in three of the four studied regions. Pyrenean stands were not negatively influenced by the climate shift, although an increase in the frequency of negative abrupt growth changes was also found there. Growth after the climate shift presented divergent patterns depending on the geographical region. Although Mediterranean and Atlantic stands presented different indicators of constrained growth, Pyrenean stands showed rising long-term growth trends. Such results suggest that regional characteristics differentially determine the growth response of the southern European beech forests to recent warming periods. Iberian beech forests located at the Pyrenees would benefit from forecasted warming conditions, whereas Atlantic and Mediterranean forests would be more prone to suffer warming-related growth decline.