Site ecological differences to the climatic forcing of spruce pointer years from the Lötschental, Switzerland

Abstract

Summary Variations in ring width and ring coloration of 89 spruce trees from six sites in the Lotschental, Switzerland, are analyzed. Sites are located along an altitudinal transect spanning 1500 to 2000m a.s.l. on a SSE- and a NNW-facing slope. Site ecology is further determined by local differences in micro relief, allowing differentiation into locally wet and dry sites. Growth reactions are classified as “site pointer years” representing extreme years common within a site, and as “valley pointer years” representing extreme years common between sites. These site and valley pointer years are classified and analyzed separately for ring width, light rings and dark rings. In so doing, 44 ring width and 9 light ring pointer years are reported for the 20th century. Dark rings do not crossdate within sites, so that no such pointer year is documented. Comparisons with instrumental data show that May precipitation and temperatures are crucial for both negative and positive ring width pointer years. Climate variability in all other months only modifies the intensity of these pointer years. Light ring pointer years correlate with low temperatures recorded towards the end of the growing season, particularly in September. By further analyzing the climate response patterns of site and valley pointer years, a conceptual classification of six climate/pointer year groups is presented: (1)Moderately cold and moist growing seasons cause wide rings at upper and lower sites. (2)A cold May followed by moderately cool summers cause wide rings at lower sites. (3)Warm (and moist) summers cause wide rings at upper sites. (4)Warm and dry summers cause narrow rings at lower sites. (5)Cold and moist summers cause narrow rings at upper sites. (6)A cold September cause light rings at upper sites.