Climate factors and oak decline based on tree-ring analysis. A case study of peri-urban forest in the Mediterranean area

The main objective of this research was to investigate climate-growth relationships and if climate is a possible cause of oak decline in a very representative peri-urban forest in the Mediterranean area. The dendrochronological behaviours of healthy trees and declining trees were compared according to compromised vitality as assessed by crown defoliation. The analysed parameters were ring and latewood widths. The results showed that ring widths and latewood depended directly on spring precipitation (April-May-June) and, inversely, on spring-summer temperature (April-May-June-July). The growth histories of healthy and declining deciduous oaks in Castelporziano, as evaluated by their crown vitality, were very similar. The onset of decline dates back to the middle of the 1940s, and a further irreversible growth depression was based on the increase in temperature in the 1980s. This climatic change is reflected roughly 13 years later in declining trees, 17 years later in the healthy ones, in a strong growth depression that appears very exacerbated by the recurrent warm extreme events after the 2000s. Latewood in our study did not add more to the total ring-width measurements, potentially because of a high state of tree suffering. Declining trees were in general older than healthy ones, but in this study, the difference in age was not different from a statistical point of view.