Variations of construction cost associated to leaf area renewal in saplings of two co-occurring temperate tree species (Acer platanoides L. and Fraxinus excelsior L.) along a light gradient

The yearly renewal of leaves and their holding organs ( the leafy shoot) represent an energetic cost for saplings. The contribution of both biochemical (tissue construction cost, CC) and morphological traits (biomass partitioning and leaf mass per unit area, LMA) to the cost associated with leaf area renewal (construction cost of the leafy shoot per unit leaf area, shoot CCA) was studied in saplings of Acer platanoides L. and Fraxinus excelsior L. growing in a natural light gradient below forest canopy. Decrease in LMA with shade and change in biomass partitioning from stems towards leaves and petioles accounted for the strong decrease in mass per unit area of the leafy shoot (SMA) with shade and for most of the plasticity of shoot CCA in both species. In addition, a decrease in leaf CC in A. platanoides also contributed to the overall decrease of shoot CCA with shade in this species. Leaf CC was positively correlated with LMA in F. excelsior, positively correlated with both LMA and epidermal absorbance of UV (A(UV)) in A. platanoides. Leaf CC was negatively correlated with ash content in both species. The strong negative correlation between A(UV) and nitrogen content might have damped variations in leaf CC in F. excelsior.