Responses of wood anatomy and carbon isotope composition of Quercus pubescens saplings subjected to two consecutive years of summer drought

To withstand and to recover from severe summer drought is crucial for trees, as dry periods are predicted to occur more frequently over the coming decades. In order to better understand growth-related tree responses to drought, wood formation, vessel characteristics and stable carbon isotope composition (delta C-13) in tree rings of Quercus pubescens saplings imposed to two consecutive summer droughts were compared with regularly watered control trees. In both years, photosynthetic activity was strongly inhibited during the drought periods of five to seven weeks but quickly restored after re-watering, reinitiating wood formation. Stress caused more than a 20% reduction in ring width, a 0.5 parts per thousand increase in latewood delta C-13 and changes in vessels characteristics in both the current year latewood and the next year earlywood. The latewood displayed up to 90% increased hydraulic conductivity than control trees, likely to compensate for a cavitation-induced reduction of water transport. The earlywood after the first drought year was characterized by more but smaller vessels suggesting the attempt of restoring conductivity while minimizing the risk of hydraulic failure. However, after the second year, the reduction of hydraulic conductivity and the increased delta C-13 values indicate a structural adjustment towards a reduced growth induced by exhaustion of carbon reserves.