Long-term effect of temperature and precipitation on radial growth in a threatened thermo-Mediterranean tree population

The combined effect of climate change and habitat destruction and fragmentation threatens many plant populations and even entire communities in Mediterranean ecosystems. The Iberian pear, Pyrus bourgaeana Decne, a characteristic species of Mediterranean ecosystems, is threatened by both habitat and climate changes. We ask whether and how the growth of mature P. bourgaeana in the thermo-Mediterranean zone (i.e., altitude <700 m) has been affected by long-term climate changes during the last century in a fragmented landscape. Dendrochronological methods were used to find growth-climate relationships. We made the first dendroclimatological analyses and constructed a first 103-year tree-ring chronology (1905-2007) of this species. The tree-ring series revealed large growth variability. We found a clear, strong relationship between tree growth and climate, with annual precipitation being the most important climate factor enhancing radial growth. Our results also showed that warm autumns and winters positively affect growth. There was no temporal stability in the relationship between tree growth and climate. The most general trend was in the relationship between annual precipitation and tree growth: the decrease of rainfall in the last decades of the twentieth century was associated with a constant increase of the correlation coefficient. Water accumulated in the soil in autumn and winter proved to be a key factor augmenting tree growth in the following vegetation period. The climate-growth relationship in P. bourgaeana has strengthened in recent decades apparently due to decreased precipitation levels.