The cold and the drought: effects of extreme weather events on Stem Carbon dynamic in a Mediterranean beech forest

The effects of short-term extreme events on tree functioning are still rather elusive. Considering beech one of the most sensible species to late frost and water shortage, we investigate the intra-annual C dynamics in stems under such conditions. Wood formation and stem CO2 efflux were monitored in a Mediterranean beech forest for three years (2015-2017) which were characterized by a late frost (2016) and a summer drought (2017). The late frost event reduced radial growth and consequently the amount of carbon fixed in the stem biomass by 80%. Stem carbon efflux in 2016 was reduced by 25%, which can be attributed to reduced effluxes due to growth respiration. Consequently, the amount of carbon emitted was higher than the carbon fixed in stems, causing an net emission of 0.468 Mg C ha-1 yr-1. Counter to our expectations, we found no effects of the 2017 summer drought on radial growth and stem carbon efflux. Even if forests are potentially susceptible to all weather extremes, which are becoming more frequent in Mediterranean basin, the study evidenced the negative effect of spring late frost but the high resilience of beech that resumed the growth in 2017, and became again C sink.