Changes in environmental controls on the growth of Abies alba Mill. in the Vosges Mountains, north-eastern France, during the 20th century

We analyse the radial growth of silver fir (Abies alba Mill.) in the Vosges mountain in north-eastern France in relation to soil, climate and interspecific competition factors and to check the temporal stability of their effects during the 20th century. Data were collected from 143 plots regularly distributed over three altitude ranges (from 330 to 1100 m), three humus forms (moder to mull) and seven types of stands, including a pure stand of silver fir and six mixed stands containing this species. For each plot, an index of mean radial growth of silver fir was calculated, independent of tree age and calendar year of ring formation. Relationships between radial growth and ecological factors were analysed using linear mixed-effects models. Radial growth of silver fir is low in the presence of Picea abies or of a high density of other dominant trees. Low aluminium concentration and good nitrogen supply, measured by the soil C:N ratio, are correlated with high radial growth. Water reserves, related to available water storage capacity and to lateral run-off, are also correlated with high radial growth. Analysis of environmental effects during the 20th century showed that growth was strongly correlated with nitrogen supply at the start of the century, and with aluminium toxicity and climate at the end of the century. The radial growth of trees located on sites with low nitrogen supply was lower before 1970 than that of trees located on nitrogen-rich soils. At the end of the century, radial growth of silver fir was the same for all levels of nitrogen nutrition. Our results indicate that nitrogen supply was a limiting factor for the mean radial growth of silver fir before the 1970s. Eutrophication during the 20th century may then have eliminated limitation by nitrogen.