Influence of species and abiotic factors on extreme tree ring modulation: Picea abies and Abies alba in Tarentaise and Maurienne (French Alps)

The influence of biotic (tree species) and abiotic factors (regional climat e, altitude and stand aspect) on extreme tree-ring widths is dendroecologic ally investigated. Negative pointer years are observed when abrupt radial g rowth reductions (exceeding 40%) occurred synchronously in a given tree pop ulation. Pointer year intensity is expected to follow several theoretical m odels when trees are sampled along ecological gradients in various site con ditions. Hypotheses are made about expected patterns of pointer year spatia l distribution. They are compared with observed results obtained with Norwa y spruce [Picea abies (L.) Karst.] and silver fir (Abies alba Mill.) sample d in two French Alpine valleys (Tarentaise and Maurienne) with different re gional climates, on two aspects and three altitude levels. Results reveal t hat extreme years follow expected patterns well. Thus, clear site modulatio ns are observed in 1956 and 1986 (only trees located in Maurienne reacted). Moreover, species differences are observed, with typical cases of single s pecies sensitivity (for spruce, occurring in 1962 and 1992). Abiotic factor s such as altitude and aspect also involve clear pointer year patterns, suc h as narrow rings in 1976 due to summer drought that occurred especially at lower altitudes. However, some observed extreme years sometimes appear to be a combination of two (or three) of these one-factor theoretical models ( e.g. 1921 with both lower altitude level and only observed in Tarentaise; 1 948 involving both higher altitude and northern slopes). These results demo nstrate the consistency of various tree-ring and climate relationships alon g ecological gradients.