The recent compilation of large trait datasets has facilitated global analyses of trait covariation that have revealed fundamental evolutionary plant trade-offs between ‘resource acquisitive’ and ‘resource conservative’ life history strategies. For instance, the Leaf Economic Spectrum (LES) has highlighted a global spectrum between plants with short-lived, inexpensive leaves that have high assimilation capacity vs. plants with long-lived, expensive leaves with low assimilation capacity. This global spectrum implies fundamental evolutionary trade-offs that suggest that a plant can’t have it all (i.e. long lived, high leaf mass per area (LMA) leaves with high physiological capacity). However, at certain scales (e.g. between individuals of the same species subjected to different environmental stresses), many of these fundamental trait tradeoffs break down, and some actually show opposite relationships from those documented across many species at a global scale. This begs the question: at what taxonomic scale does the global leaf fast-slow evolutionary trade-off emerge? We combine a large dataset of within-species geographic variation in leaf traits with a global leaf trait database to test how the trait correlations that make up the Leaf Economics Spectrum vary across taxonomic scales from within-species to between families.
We find that the trait correlations between leaf nitrogen content (per unit leaf mass), leaf mass per area (LMA) and leaf lifespan become progressively weaker at smaller taxonomic scales. In the case of LMA vs. leaf lifespan, the sign of the relationship actually switches from strongly positive across families to negative within species. These three leaf traits differ in the taxonomic level that contains the most trait variation, highlighting how differential phylogenetic trait conservatism can alter the strength of ‘universal’ trait syndromes. Our findings suggest that the evolutionary trade-offs that create strong trait correlations across the tree of life do not always operate at the level at which we understand speciation to occur, the level of individuals within a population. Thus, although these tradeoffs likely represent fundamental physiological constraints, they probably do not drive sympatric speciation. The potential contrast between macro- and microevolutionary selection pressures has implications for trait based studies of community ecology and ecosystem function.