## Introduction

A decade ago, Wright *et al*. (2004) published a paper that described a ‘leaf economic spectrum’ (LES) of traits. Based on measurements of leaf area, leaf mass, photosynthetic capacity, dark respiration, nitrogen (N), phosphorus (P) and leaf lifespan (LL), collected for a wide range of plant species measured worldwide in their natural habitats, Wright *et al*. (2004) showed that ecophysiological variables form a spectrum of correlated traits. When parameters were expressed per unit dry mass, photosynthetic capacity scaled positively with N and P concentrations, but negatively with leaf mass per area (LMA) and LL. Recently, two groups have cast doubt on the biological significance of these correlations. Osnas *et al*. (2013) and Lloyd *et al*. (2013) independently drew attention to the statistical implications of using leaf biomass as the main factor for normalization. The basis for this concern dates back to the work of Pearson (1897) on ‘spurious’ correlations: if random variables X, Y and Z are uncorrelated with each other, then X/Z will often correlate positively with Y/Z, simply because the denominator Z will be small in some cases, with large values for both ratios as a consequence, and large in others, with small values for both ratios as a result. In terms of the LES: the rate of photosynthesis per unit leaf area may not or just weakly be correlated with the amount of N per unit leaf area, but if both are divided by LMA (or multiplied by specific leaf area (SLA), which is the inverse of LMA), a significant correlation is expected between the photosynthetic capacity per unit mass and the N concentration per unit mass. Both groups conclude that the significance of the correlation coefficient is misleading if the correction for randomly varying and unrelated values is not considered. They further conclude that leaf area is the logical basis for the expression of photosynthesis. In a response, Westoby *et al*. (2013) agree that the strong *r*^{2} between LMA and mass-based leaf traits can correspond with the same leaf traits on an area basis being uncorrelated with LMA, but claim an independent role for mass-based expressions in the comparative analysis of plant traits.

In this letter, we first discuss whether there is indeed a logical basis for expression of the photosynthetic process. We argue that most insight is gained by evaluating several normalizations. Subsequently, we draw attention to the fact that the LES is part of a larger whole-plant economic spectrum, with a biological model that requires transformation of carbon (C) gain per leaf area to a mass basis if we aim to understand the C-economy of the whole plant. Finally, we discuss ways forward to analyse the observed patterns of traits.