Tree-ring detection of CO2 fertilization in Mediterranean firs

from Cephalonia, Greece


Greek fir (Abies cephalonica) is indigenous to the island of Cephalonia in the Ionian Sea west of mainland Greece. Tree ring data from firs growing on this island reveal surprising increases in growth over the second half of the 20th century. As in most of the Mediterranean Sea, the main climate trend on the island has been toward aridification. Climate response analysis of the tree-ring data indicates that growth is favored by cool and wet conditions during growth, particularly in the month of June. Climate trends since the mid-20th century are marked by increasing June temperature and aridity, which predicts a decline in growth. Despite this,overall growth has accelerated, especially since 1990 (click here  for a figure of the data).

We are investigating the possibility that the fir trees are experiencing a fertilization effect from increasing atmospheric CO2. Photosynthetic stimulation due to elevated CO2 has been demonstrated in Free Air CO2 Enrichment (FACE) experiments, but its impact on natural vegetation under ambient conditions is more ambiguous and controversial. Because of the lack of confounding climatic factors contributing to growth stimulation, this site may offer an advantage for detecting a CO2 fertilization effect. This effect can improve water-use efficiency by lowering stomatal apertures thus limiting water loss via transpiration. As such, it should be easier to detect in arid and semi-arid regions like the Mediterranean where the growing season is dry. Our results suggest that such a growth boost is already over-compensating for growth declines anticipated from aridification. A secondary research objective is to assess the potential for a tree-ring record of paleoseismicity in this earthquake prone area. Some samples show abrupt changes in growth corresponding to historic earthquakes, for example the 1953 quakes that devastated the island.



  • Koutavas, A. (2012), CO2 fertilization and enhanced drought resistance in Greek firs from Cephalonia Island, Greece, Global Change Biology, doi: 10.1111/gcb/12053.
  • Koutavas, A. (2008), Late 20th century growth acceleration in greek firs (Abies cephalonica) from Cephalonia Island, Greece: a CO2 fertlization effect? Dendrochronologia, Vol. 26, Issue 1, 13-19,  doi:10.1016/j.dendro.2007.06.001. (PDF)


  • Koutavas, A., Detection of CO2 fertilization in Abies cephalonica tree rings from Cephalonia Island, Greece, First American Dendrochronology conference (AmeriDendro 2008), University of British Columbia, Vancouver, Canada, Jun 23-27, 2008. (PDF)

Images: (top left) view toward the west from 1300 m elevation on the island; (top right) a mature fir in the Ainos forest; (lower left) suppression of ring growth most likely as a result of the 1909 Magnitude 5.7 earthquake in nearby Helia, Peloponeese.