Late 20th century growth acceleration in greek firs (Abies cephalonica) from Cephalonia Island, Greece: A CO2 fertilization effect?

Abies cephalonica or greek fir is indigenous to the island of Cephalonia in the Ionian Sea, which gives the species its name. Here I report ring-width variations from eight trees growing between 1300 and 1600 m elevation on the southern slopes of Mt. Ainos on this island, indicating strong acceleration of growth over the second half of the 20th century. The sustained increase in growth since 1990 in particular is unprecedented over the full length of the dataset (1840–2005 AD). These trends bear no relationship to regional temperature or precipitation variations and therefore are unlikely to be climatically induced. Disturbance effects from human activities are also unlikely as the study site lies in a remote forest area with difficult access. A plausible alternative hypothesis is that the enhanced growth reflects a fertilization effect due to rising CO2 in the global atmosphere. Based on the timing of individual tree responses, it is further hypothesized that the crossing of a CO2 threshold was responsible for the enhanced growth, and that this threshold may be age dependent (decreasing with age). A more systematic sampling of the Mt. Ainos firs is needed to further test this hypothesis. If interpreted correctly the results imply that CO2 fertilization may already be an important factor in tree growth at this location.