Leaf structure and anatomy as related to leaf mass per area variation in seedlings of a wide range of woody plant species and types

The structural causes of variation in leaf mass per area, and of variations in leaf structure accounted for by leaf habit and life form, were explored in a set of laboratory-grown seedlings of 52 European woody species. The l eaf traits analysed included density, thickness, saturated mass/dry mass, a nd leaf nitrogen per mass and per area. Other traits described the anatomy of leaves, most of them relating: to the lamina (proportions of palisade an d spongy parenchymata, epidermis, air space and sclerified tissues, express ed as volume per leaf area, and per-cell transversal areas of epidermis and parenchymata), and another referring to the mid rib (transversal section o f sclerified tissues). Across the whole set of species leaf mass per area w as correlated with leaf density but not with thickness. and this was confir med by taxonomic relatedness tests. Denser leaves corresponded with greater proportion of sclerified tissues in the lamina, smaller cells and lower wa ter and N contents, but no relation was found with the proportion of air sp ace in the lamina. Taxonomic relatedness analysis statistically supported t he negative association of leaf density with saturated to dry leaf mass rat io. Thicker leaves also exhibited greater volume per leaf area and greater individual cell area in each of the tissues, particularly parenchyma. Mean leaf mass per area and leaf thickness were lower in deciduous than in everg reen species, but no significant differences in leaf density, proportion of sclerified tissues in the lamina or cell area were found between the two g roups. Leaf mass per area was higher in trees and subshrubs than in shrubs and climbers-plus-scramblers, this rank being equal for leaf density and pr oportion of sclerified tissues in the lamina, and reversed for cell area. G iven the standardised environment and ontogenetic phase of the seedlings, w e conclude that variation in leaf structure and anatomy among species and s pecies groups has a strong genetic basis, and is already expressed early in the development of woody plants. From an ecological viewpoint, we can inte rpret greater leaf mass per area across this species set as greater allocat ion to support and defence functions, as shown predominantly by species fro m resource-poor environments.