An evolutionary perspective on leaf economics : phylogenetics of leaf mass per area in vascular plants

In plant leaves, resource use follows a universal trade-off between rapid capture and conservative storage. This “worldwide leaf economics spectrum" consists of a suite of inter-correlated leaf traits, among which leaf mass per area, LMA, is one of the most fundamental as it indicates the cost of leaf construction and light-interception borne by plants. Despite sound knowledge of the physiological and ecological correlates of LMA, how this key trait has evolved to give rise to present-day variation remains largely unknown. Vegetative traits such as LMA may be evolutionary labile and vary independently from phylogeny across species or, by contrast, show phylogenetic patterns structured by drift and selection. Here, we show that the phylogenetic signal in LMA displays low but significant conservatism, that is leaf economics in relatives tended to be more similar than expected by chance alone, and that LMA evolved under weak stabilizing selection. We conducted a broad scale analysis of the evolutionary history of LMA across a large dataset of 5401 vascular plants species. Models of continuous trait evolution revealed that different optimal phenotypes existed across large clades in which extremes tended to be selected against. Meanwhile conservatism in LMA was strongly patterned by growth form, as were selection intensity and phenotypic evolutionary rates: woody plants show higher conservatism in relation to stronger stabilizing selection and lower rates compared to herbaceous taxa. Our findings demonstrate the adaptive value of LMA as an indicator of resource use strategies. Optimal phenotypes evolved in response to different stabilising selective regimes across clades. These evolutionary trajectories involved coordination and trade-offs between leaf traits and plant traits related to growth form. Consequently, ecological processes within communities are likely restricted by fundamental limits on organisms structure and physiology.