Spatial patterns of historical growth changes in Norway spruce across western European mountains and the key effect of climate warming
Résumé
Productivity changes in Norway spruce show important regional and local spatial variations, highlighting their context dependence at different spatial scales. These variations suggest the enhancing role of climate warming, and interplay with local water and nutrient limitations. While forest growth changes have been observed in many places worldwide, their spatial variation and environmental origin remain poorly documented. Analysis of these historical changes in contrasted regional contexts, and their mapping over continuous environmental gradients, may help uncover their environmental causes. The approach was tested on Norway spruce (Picea abies) in three western European mountain contexts (Massif Central, Alps and Jura), using National Forest Inventory (NFI) data. We explored the environmental factors influencing stand basal area increment (BAI) in each context. We then estimated and compared mean regional changes in BAI and related these to the regional environmental limitations evidenced. Within each region, we further mapped local BAI trends using a geographically weighted regression (GWR) approach. In each region, local estimates of BAI changes were finally correlated to environmental indicators. We found an increase in BAI in the three regions over 1980-2005, greater in the Massif Central (+71 %) than in the Alps (+19 %) and the Jura Mountains (+21 %). Inter-regional differences in BAI changes suggested the release of a thermal constraint-found more important in the Massif Central-by the strong temperature increase over the period, and a limitation by water availability in the Jura and the Alps Mountains. Spatial patterns of BAI change revealed significant local variations in the Massif Central and the Alps. From the correlation analysis, these were again found consistent with the hypothesis of an enhancing effect of climate warming in these mountain ranges. They were also related to local soil nutritional status in the two regions, and negatively related to nitrogen deposition level in the Massif Central. As a main outcome, a strong context and spatial scale dependence of productivity changes is emphasized. In addition, the enhancing effect of climate warming on productivity is suggested, with local modulation by climatic and nutritional conditions.