Changes in radial tree growth for Picea abies, Larix decidua, Pinus cembra and Pinus uncinata near the alpine timberline since 1750

Abstract

Changes in radial growth of the four conifer- ous species growing in the French Alps near the upper treeline are investigated. Thirty-seven populations of Norway spruce [Picea abies (L.) Karst.], European larch (Larix decidua Mill.), Swiss stone pine (Pinus cembra L.) and mountain pine (Pinus uncinata Mill. ex Mirb.) were sampled by taking 1320 cores and analysing tree- ring widths. Sites were chosen in various climatic condi- tions (macroclimate and aspect) and on two kinds of bedrock in order to take into account the ecological be- haviour of these species. Belledonne, Moyenne-Tarent- aise, Haute-Maurienne and Briançonnais areas were sampled along increasing gradients of summer aridity and winter continentality. The calculation of time series after removing the age trend brings strong evidence for an increase in radial growth during the two last centuries, but with different stages and fluctuations for each spe- cies. This growth trend is significantly enhanced since 1860 for the spruce, and since 1920 for the two pine spe- cies. Furthermore, it also appears on Larix decidua with the same pattern despite periodical growth reduction due to attacks of the larch bud moth (Zeiraphera diniana Gn.). The analysis of ring-widths at a given cambial age reveals that this enhanced phenomenon is observed espe- cially during the tree’s early years (25–75 years). The analysis of four regional climatic series, and three longer series of temperature (in farther single sites) reveals syn- chronous decadal fluctuations and an evident secular in- crease in minimum temperatures (especially in January and from July to October), that may be involved in tree- growth enhancement. Thermic amplitudes are signifi- cantly reduced during the whole growing period, what is more pronounced in Belledonne, the most oceanic re- gion. Long term growth changes are well described by stepwise regression models, especially for the pine spe- cies. These models involved both a linear trend (CO2 concentration or N-deposition) and low frequency of Tu- rin monthly temperatures. However, they show different patterns than those observed from response functions at a yearly scale.