Detection and effects of climate extremes on Mediterranean forests

Research on occurrence and effects of natural and anthropogenic disturbances on forests is largely widespread around the globe. The Mediterranean basin due to its geographic but also socio-economic features, is one of the regions where climate changes are expected to affect the most forests and ecosystems in general. With this research we have integrated different methodologies to study the effects of climate induced disturbances on different Mediterranean forests. In particular, we focused on the detection and the intensity, of climatic stress on tree growth in some deciduous forests. We conducted analyses at population and individual tree-level using mainly tree-ring series to assess the species and individual sensitivity to spring frost (Central and Southern Apennines, Italy) and to summer drought (and thinnings) (Sub-Pyrenees, Spain), detecting occurrence of extreme events and post-disturbance radial growth recovery and resilience. Within a sub-sample (Central Apennines) we also analysed time series of wood anatomical traits to extract possible alteration signals on tree inner structure. Finally, we applied remote sensing analysis for the spatio-temporal detection of late frost effects on forest cover. We also checked how reliable are short tree-ring series for assessing climate-growth relationship. This report, after an overall introduction, is organized as a compilation of: one published article (Chapter 2), one manuscript under review (Chapter 3), one in preparation (Chapter 4) and one ready for submission (Chapter 5). Each chapter stands alone, containing its own introduction, materials and methods, results, discussion, conclusions (and eventually supplementary materials). A short general conclusion chapter and a general reference list are closing the report. Chapter 2 is a dendrochronological study conducted on naturally regenerating European black pines (Pinus nigra J.F.Arnold) aimed to identify the potential and limitations of young trees in recording climate signals. The results confirm the presence of individualistic growth trends in many studied trees, but also the ability of the population to respond uniformly following particularly favourable or unfavourable growing seasons. Chapter 3 is a study on European beech (Fagus sylvatica L.) sensitivity to spring frosts in the Apennines forests, in collaboration with the University of Padova (UNIPD, Italy), the University of Basilicata (UNIBAS, Italy) and the Pyrenean Institute of Ecology (IPE, Zaragoza, Spain). Here we combined tree-ring and remote-sensing data to assess the vulnerability and the recovery capacity of beech populations to late frosts. Late frost occur in spring and defoliation rates are strictly related to the forest leaf phenology and stand elevation. Mature beech trees revealed to be resilient and to recover rapidly their growth trends, showing no legacy effects. Chapter 4 presents the preliminary results of an ongoing study aimed to search the effects of late frost events on fine wood anatomical traits in beech tree-rings. We conducted the anatomical slide preparation in the laboratories of the Department of Land, Agriculture and Forestry Systems (TESAF) at UNIPD. Frost events do not seem to affect the plasticity of vessel traits, which could be more related to the tree size (especially tree height) and to stand elevation. Chapter 5 is a study in collaboration with IPE Zaragoza, Spain for assessing the radial growth response to thinning and drought of Quercus subpyrenaica. Drought events limit the radial growth of trees, and the effects are clear when the trees are in a release phase, for example after thinning. Trees in thinned plots have higher basal area increment (BAI) even in co-occurrence of drought events, and recovery periods do not differ between thinned and unthinned plots. Trees change their phenotypic traits in response of extreme climate condition, not only in terms of vegetation greenest or leaf area but also in inner structure (e.g. tree rings and xylem anatomical traits). For this reason, only a multiscale and multidisciplinary approaches allow to identify all the impact of the ongoing climate variability. The resulting information can guide sylvicultural strategies to better adapt forests to climate change.