DENDROECOLOGICAL ANALYSIS OF FIR (ABIES-A LBA MILL) AND SPRUCE (PICEA-ABIES (L) KARST) AT DIFFERENT STAGES OF DEFOLIATION IN THE FOREST OF VIGODI-DI-TON (TRENTO, ITALY)

Variations in radial growth of 2 Abies alba populations and one Picea abies population from the Trentino (fig1) have been studied in relatio n to the present state of tree crowns (table I) Calculation of statist ical models (fig 2-4) taking into account age trends (ARMA modelling) and tree ring-climate relationships (response function) over 2 success ive periods makes the quantification of growth rate changes possible. Similar growth trends (fig 5,6) were identified for the 3 populations until the 1960s, with differences occurring thereafter. After 2 critic al years, 1960 and 1970, the now dying trees (defoliation classes 2 an d 3) underwent a loss of vitality, whereas the now healthy trees (defo liation classes 0 and 1) rapidly resumed their growth potential. Respo nse functions established for each population and each defoliation cla ss over 4 successive periods at 10 year intervals reveal a gradual dec rease in tree sensitivity to climate from the years 1960 and 1970 onwa rds (figs 7-9). In parallel, the theoretical growth reconstructed for the period 1960-1987, on the basis of the ARMA model calculated for th e previous period 1927-1959 and compared with the actual growth, shows that growth deficiency is directly correlated with the present degree of defoliation (fig 10). The present degree of defoliation appears to be the consequence of a loss of vitality that has been initiated by i nimical climatic conditions. This sensitivity of trees to climate vari es from one tree to another, depending on genetic inheritance (Abies i s more affected than Picea) and site conditions