Global meta-analysis shows that relationships of leaf mass per area with species shade tolerance depend on leaf habit and ontogeny.

It was predicted that relationships of leaf mass per area (LMA) with juvenile shade tolerance will depend on leaf habit, and on whether species are compared at a common age as young seedlings, or at a common size as saplings. A meta-analysis of 47 comparative studies (372 species) was used to test predictions, and the effect of light environment on this relationship. The LMA of evergreens was positively correlated with shade tolerance, irrespective of ontogeny or light environment. The LMA of young seedlings (<or=1 yr) of deciduous species in low light was also positively correlated with shade tolerance; more weakly so in high light. By contrast, size-specific comparisons of deciduous saplings gave negative correlations of LMA with shade tolerance. Independent contrasts and cross-species analyses yielded broadly similar results. We conclude that ontogeny strongly influences the relationship of LMA with shade tolerance of deciduous trees, but has little impact on that of evergreens. Size-specific comparisons reveal opposing trends in deciduous and evergreen taxa: the negative relationship of LMA with shade tolerance of deciduous species is probably dominated by interspecific differences in palisade thickness, whereas patterns in evergreens are probably shaped more by the degree of structural reinforcement, linked to wide variation in leaf lifespan.