Resilience to late frost and drought of mixed forests with Turkey oak and silver fir in southern Italy

Abstract

Mixed species can increase the stability of forest ecosystems against disturbances. However, the benefits of mixing cannot be generalized for all forest types and tree species combinations. To investigate the role of tree species composition in the responses of tree growth to late frost and drought, we constructed tree-ring series from forest stands with varying abundances of silver fir (Abies alba) and Turkey oak (Quercus cerris) at two sites in southern Italy. We studied (i) intra- and interspecific responses to late frost and drought stress, (ii) impact of tree species composition on these responses, and (iii) relationships between tree growth and climatic and structural variables. In the drier region, both species were more resistant to drought than to late frost. Moreover, Turkey oak was less resistant than silver fir to late frost in drier site conditions but showed greater resistance in wetter ones. The impact of tree species composition on plot-level responses was observed for drought resilience components at the drier site. Additionally, plot diameter heterogeneity positively influenced growth resistance to both stressors, while basal area negatively affected drought resistance. Water balance played a central role in the growth of both species, but silver fir growing in drier conditions was less influenced than silver fir growing in wetter ones. Silver fir can adapt growth strategies to deal with different environmental settings. This study shows that lower stand density (basal area: 41–83 m²/ha in this study) and greater structural heterogeneity (Gini index: 0.1–0.4) contribute to greater stand-level resilience in Mediterranean forests.