The effects of drought in xylem anatomical traits of Austrian pine (Pinus nigra) in Venosta valley

In recent decades, drought events have increased in frequency and intensity all over the world, inducing more and more cases of forest decline. Forest mortality episodes are expected to increase in the near future, in parallel with the increasing frequency of drought events. All this represents a challenge for trees, which can withstand occasional and mild episodes of water stress, but could hardly survive in face of their substantial increase in frequency and intensity. An example is Pinus nigra, a species that generally shows a high tolerance to harsh environmental conditions, but whose geographical distribution is expected to shrink in the future. Indeed, increasingly drier climate, could likely negatively influence its xylem anatomical features and the consequent physiology of the species. In this thesis I investigated the effects of drought in wood anatomical traits of Austrian pine (P. nigra) in Venosta valley (Italy). In particular, through a dendro-anatomical approach, I retrospectively identified the differences between non-declining (ND) and declining (D) trees in terms of radial growth (ring width), cell size (lumen area), cell wall dimension (cell wall thickness) and number of cells per ring (cell number), considering a time-span of 75 years. Data collection and analysis was carried out with ROXAS, R and Excel programs. The series of anatomical traits for the D-ND trees were correlated with climatic variables (temperature, precipitation and the SPEI-3 drought index). Despite the similar trend for both groups analysed (D-ND), the results obtained showed a greater climatic sensitivity on the declining trees, and the differentiation of certain anatomical traits only in some years such as 1996 and 2007. The sudden mortality of certain trees, therefore, may probably be due to some other factors such as one or more subsequent extreme events and/or a pest outbreak, rather than the persistent drought of the area. This thesis underlines the need to obtain an accurate understanding of the forest response to this type of events, leaving room for further future investigations on this issue.