Intra-annual tree-ring parameters indicating differences in drought stress of Pinus sylvestris forests within the Erico-Pinion in the Valais (Switzerland)

The effects of drought on radial growth of Pinus sylvestris were investigated by comparing sites along hydric gradients. The gradients were located in Valais, an inner Alpine dry valley in Switzerland, with each consisting of two site types, an extreme dry, xeric site and a less dry, moderate site. The two site types were assigned to phytosociological associations within the Erico-Pinion. The investigation covered the responses of tree growth to climate and particularly concentrated on intra-annual features of tree-rings such as earlywood/latewood ratio, intra-annual density fluctuations (IADFs) and traumatic tissues (TTs) as well as the sapwood/heartwood ratio. Radial growth differed according to the site types, with trees on dry sites generally showing more missing rings, lower mean ring widths, lower autocorrelation, higher mean sensitivities, reduced latewood proportions and lower sapwood areas than trees on moderate sites. The relationships between climate and tree-ring width, studied using response function analysis, varied strongly between the site types within the Erico-Pinion: Tree growth on dry sites was positively influenced by precipitation at the end of the winter and the beginning of the growing season and negatively influenced by temperature in June. Winter precipitation was positively correlated with radial growth, demonstrating its importance for the successful root and shoot growth of the plants in spring on dry sites. On moderate sites, tree growth was less controlled by climate than by prior growth. The intra-annual density fluctuations (IADFs) provided a valuable means to differentiate between the site types. In comparison to the moderate sites, the trees on dry sites contained more IADFs, and their frequency was increased. Moist-cool conditions in the middle of the growing season were the triggering factor for IADFs on dry sites, whereas on moderate sites, there must be an additional warm period in early summer in order to initiate IADFs. Most IADFs were found in latewood. We found no relationship between climate and traumatic tissues (TTs). It is unclear whether other abiotic or biotic factors such as wounding by insects or birds are responsible for the development of TTs. The assignment of these differences in tree growth behaviour to phytosociological associations will enable a deeper understanding of the site types and will facilitate the comparison with similar studies. Furthermore, the results can be combined with studies from other scientific disciplines concerning these phytosociological associations. The ecological indicator values of the vegetation was a precise method for the distinction of site types.