Leaf traits and photosynthetic parameters of saplings and adult trees of co-existing species in a temperate broad-leaved forest

In Central European forestry the establishment of broad-leaved mixed forests is attaining increasing importance, but little information exists about gas exchange characteristics of some of the tree species involved, which are less abundant today. In an old-growth forest in Central Germany (Hainich, Thuringia), (i) I compared morphological and chemical leaf traits that are indicative of leaf gas exchange characteristics among eight co-existing species, and (ii) analysed photosynthetic parameters of saplings and adult trees (lower and upper canopy level) in four of these species (Acer pseudoplatanus L., Carpinus betulus L., Fraxinus excelsior L. and Tilia platyphyllos Scop.). Leaves from the upper canopy in the eight species studied varied significantly in their specific leaf area (12.9-19.4 M-2 kg(-1)), stomata] density (125-313 stomata mm(-2)), leaf nitrogen concentration (95-157 mmol N m(-2)) and delta(13)C content (-27.81 to -25.85parts per thousand). E excelsior and C. betulus were largely contrasting species, which Suggests that the species, which were studied in more detail, include the widest difference in leaf gas exchange among the co-existing species. The saplings of the four selected species exhibited shade acclimated leaves with net photosynthesis rates at saturating irradiance (A.,,,.) between 5.0 and 6.4 mumol m(-2) s(-1). In adult trees A(max), of fully sunlit leaves was more variable and ranged from 10.5 (C. betulus) to 16.3 mumol m(-2) s(-1) (F. excelsior). However, less negative delta(13)C values in E excelsior sun leaves point to a strong limitation in gas exchange. In the lower canopy of adult trees A(max) of F. excelsior (12.0 mumol m(-2) s(-1)) was also greater than that of A. pseudoplatanus, C. betulus and T platyphyllos (5.0-5.6 mumol m(-2) s(-1)). This can be explained by the small leaf area and the absence of shade leaves in mature E excelsior trees. Thus, a considerable variation in leaf traits and gas exchange was found among the co-existing tree species. The results suggest that species-specific characteristics increase the spatial heterogeneity of canopy gas exchange and should be taken into account in the interpretation and prediction of gas flux from mixed stands.