Differentiated impact of climate and soil on primary and secondary growth of Abies alba Mill.
Résumé
The objective of this work was to determine environmental drivers of primary and secondary growth of silver fir (Abies alba Mill.) at the decade scale. We aimed to evaluate the respective importance of nutritional and water resources, climate and inter-specific competition on these two dimensions of the growth. For this purpose, 143 plots were carried out in the Vosges Mountains (northeastern France), using a stratified sampling design according to stand composition, elevation and nutritional status of soils. All of the stands selected were mature, even-aged, pure and mixed silver fir stands, with Fagus sylvatica, Picea abies, Pinus sylvestris, Quercus petraea and Acer pseudoplatanus as co-existing tree species. Dendrometric, climatic and soil variables were measured for each plot, and three trees of silver fir were core sampled. Cores were then analyzed using standard dendrochronological methods. For each plot, a site index (SI) and a radial index (RI) covering 100 years were calculated, independent of tree age, providing dominant fir height and mean radial growth data, respectively. Relationships between SI and RI and ecological factors and stand composition descriptors were analyzed using multiple - regression analyses and linear mixed - effects models, respectively. Our study highlighted the fact that the relationship between environmental factors and height growth is different from the relationship between environmental factors and radial growth. Height growth is strongly correlated with climatic variables (elevation, aspect, winter temperature) and nutritional factors (soil acidity, aluminum toxicity, nitrogen).
Moreover, spring water balance and water supply (related to topography conditions) showed a positive effect on the site index at low elevations. On the other hand, climatic effects on radial growth were weak or not significant, although the effect of water storage capacity and topographic position were revealed to be important. Moreover, there are significant and differentiated effects on silver fir growth depending on tree species composition: for example, there was an enhancing effect of high silver fir growth as a result of conifer stocking, whereas radial growth decreased with Picea abies presence. This result suggests important competition for light, resulting in silver fir changes in carbohydrates allocation from secondary to primary growth. These differentiated ecological patterns between height and radial growth produce a change in tree shapes with environmental conditions, which invalidate Eichhorn’s rule (1904) that states that homothetic changes between primary and secondary growth occur with the change in site fertility. This invalidation should lead to the replacement of the use of site index parameter with that of direct environmental variables in stand growth models.