Xylogenesis responses to drought stress in Quercus ilex L. at Mediterranean site

Droughts are becoming more frequent in the Mediterranean forests due to warmer climate and longer periods without precipitation. In the Mediterranean region, cambial activity is mostly determined by water availability and it occurs during favorite seasons. However, a few studies have focused on the seasonal patterns of wood formation (xylogenesis) response to ongoing climatic conditions in pure stand oak forests. To investigate tree growth response under predicted climate change scenarios, xylogenesis was assessed in Quercus ilex L. trees growing at a site located in the Vesuvio National Park, southern Italy. Sample were taken every two weeks for two consecutive years, and quantitative wood anatomy was used to study the progressive production of cambium, wood and phloem cells. The primary xylogenesis phases (cambium cell formation, occurrence of post-cambial cells, cell-wall thickening, and maturity) were linked to the climate data over 5 to 15 days’ time periods. In helm oak, cambial activity onset in early April, with the development of wood and secondary phloem simultaneously and stopped in early August. Early June was a favorable season for both late wood and late phloem production. Cell wall thickening increased in June to July and August to September as the temperature rose, and vapor pressure decreased in these months. In conclusion, warm climatic conditions and high evaporation rate during spring and summer enhanced plant growth. Herein, earlier growth peaks in helm oak and maximum responsiveness to spring-summer season allowed mediterranean species to mitigate the impact of summer drought.