Understanding ecological variation across species: area-based vs mass-based expression of leaf traits.

This article discusses the paper by Lloyd et al. (2013, in this issue of New Phytologist, pp. 311–321), a paper that sets out to criticize the leaf economic spectrum (LES) described by Wright et al. (2004). Table 1 identifies the two key arguments from Lloyd et al. (2013) and responds to them. The LES is a dimension of ecological variation across plant species. It concerns construction and maintenance costs of leaves, and duration of photosynthetic returns from those investments (Reich et al., 1997; Wright et al., 2004). The LES is underpinned by a positive relationship between leaf mass per area (LMA, reflecting investment costs) and leaf lifespan (LL, reflecting duration of the revenue stream arising). Variation among species in LMA reflects underlying variation in quantitative anatomy such as thickness of cuticle, mesophyll and other cell layers, and the degree of structural reinforcement (Niinemets, 1999). Photosynthetic rate on a leaf-area basis (Aarea) does not vary nearly sowidely across species as does leafmass per area LMA (c. 40fold vs 100-fold, in the glopnet dataset provided by Wright et al., 2004 and re-analysed by Lloyd et al., 2013). Also, photosynthetic rate on a mass basis (Amass; 140-fold variation) decreases with increasing LMAacross species. These two statements are equivalent to each other because Aarea divided by LMA is the same as Amass. Hence, if Aarea graphed against LMA is a horizontal relationship or nearly so, then a relationship betweenAmass and LMAwill be sloped downwards. The two statements are inter-convertible. Lloyd et al. (2013) are concerned with the equivalence between these two statements. They carry out several statistical manoeuvres, but they all boil down to the fact that mass-based and area-based data contain the same evidence and can be inter-converted. They seem especially concerned that high r between LMA and massbased assimilation rate or nitrogen (N) content can equally be