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 Pinussylvestris were investigated by comparing sites along hydricgradients. The gradients were located in Valais, an inner Alpine dry valley inSwitzerland, with each consisting of two site types, an extreme dry, xeric siteand a less dry, moderate site. The two site types were assigned tophytosociological associations within the Erico-Pinion. The investigationcovered the responses of tree growth to climate and particularly concentratedonintra-annual features of tree-rings such as earlywood/latewood ratio,intra-annual density fluctuations (IADFs) and traumatic tissues (TTs) as wellasthe sapwood/heartwood ratio. Radial growth differed according to the sitetypes,with trees on dry sites generally showing more missing rings, lower mean ringwidths, lower autocorrelation, higher mean sensitivities, reduced latewoodproportions and lower sapwood areas than trees on moderate sites. Therelationships between climate and tree-ring width, studied using responsefunction analysis, varied strongly between the site types within theErico-Pinion: Tree growth on dry sites was positively influenced byprecipitation at the end of the winter and the beginning of the growing seasonand negatively influenced by temperature in June. Winter precipitation waspositively correlated with radial growth, demonstrating its importance for thesuccessful root and shoot growth of the plants in spring on dry sites. Onmoderate sites, tree growth was less controlled by climate than by priorgrowth.The intra-annual density fluctuations (IADFs) provided a valuable means todifferentiate between the site types. In comparison to the moderate sites, thetrees on dry sites contained more IADFs, and their frequency was increased.Moist-cool conditions in the middle of the growing season were the triggeringfactor for IADFs on dry sites, whereas on moderate sites, there must be anadditional warm period in early summer in order to initiate IADFs. Most IADFswere found in latewood. We found no relationship between climate and traumatictissues (TTs). It is unclear whether other abiotic or biotic factors such aswounding by insects or birds are responsible for the development of TTs. Theassignment of these differences in tree growth behaviour to phytosociologicalassociations will enable a deeper understanding of the site types and willfacilitate the comparison with similar studies. Furthermore, the results can becombined with studies from other scientific disciplines concerning thesephytosociological associations. The ecological indicator values of thevegetation was a precise method for the distinction of site types.