Examining post-drought recovery in three sub-Mediterranean species: Some trees not affected while some never recover

Abstract

The study aimed to evaluate and compare growth resilience to hotter droughts of three common sub-Mediterranean species: black pine (Pinus nigra Arnold), manna ash (Fraxinus ornus L.) and pubescent oak (Quercus pubescens Willd). Over 200 trees were sampled across eight different sites in two countries, Slovenia and Italy. Our primary objective was to evaluate and compare diversity in response to drought within individual species. Increment cores were extracted from trees from each site, which were then processed and their tree-ring width measured. Potential droughts were identified using a Summer-Heat moisture index in the following years 1983, 1985, 1992, 2000 and 2003. Radial growth resilience was analyzed using two indices, post-drought recovery period and average relative growth reduction. Climate-growth relationships were also examined. The individual droughts affected the majority of studied trees. Of the three examined species, F. ornus appears to be the least sensitive to drought stress. Across sites, species or individual droughts, a small proportion of trees always remained unaffected by the droughts. Some of the affected trees never recovered their radial growth within the studied period, indicating that their recovery period exceeded 20 years after individual drought events. The proportion of both varied between species, site and drought. These facts would indicate that more emphasis should be given in the future to studying “winner” trees, since they may hold the answer to species’ better adaptability to the occurrence of hotter droughts.