Plant biologists have long had a choice of the dimensions in which to express their studied traits and/or processes. For example, a plant physiologist working at the leaf level might typically measure and report photosynthetic rates as a flux per unit leaf area (e.g. μmol CO_{2} m^{−2} s^{−1}) whereas a biochemist might typically express the same process per unit chlorophyll. An agronomist or forester would usually be more interested in dry matter accumulation rate per unit ground area (Mg ha^{−1} yr^{−1}) or sometimes even as a relative growth rate (g DW g^{−1} DW d^{−1}). Although expressing leaf-level photosynthesis on an area basis seems intuitive and was for decades the standard practice, starting with Field & Mooney (1986) and then Reich & Walters (1994), there has been an increasing tendency to express the photosynthetic characteristics of leaves on a dry-weight basis (typically nmol CO_{2} g^{−1} s^{−1}). This trend has been due, at least in part, to stronger correlations for mass-based photosynthetic rates with foliar properties thought to be important in their modulation (Reich *et al*., 1998). Weaker associations between foliar properties when expressed on an area-basis have also provided one rationale for the inclusion of mass-based measures of photosynthesis, nitrogen and phosphorus into a so-called ‘leaf economics spectrum’ (Wright *et al*., 2004) and with mass-based measures of photosynthetic carbon exchange subsequently underlying further analyses (Shipley *et al*., 2006). Some modelling studies investigating the relative importance of nitrogen vs phosphorus as modulators of leaf photosynthetic capacity have likewise been parameterized on a mass rather than an area-basis because of the apparently superior model fit of the former (Domingues *et al*., 2010).

Area- and mass-based measures of any foliar trait are, of course, readily inter-convertible through the simple relationship

where Θ_{m} is the value of trait Θ expressed on a per unit mass basis, Θ_{a} is its equivalent value on an area-basis and *M*_{a} is the leaf mass per unit leaf area (typically in units of g m^{−2}). This means that any investigation of the nature of the variation in Θ_{m} in relation to *M*_{a} is also an investigation into how Θ_{a}/*M*_{a} varies in relation to *M*_{a}; and from which, as first pointed out by Pearson (1897), some interesting statistical properties emerge. Such ‘ratio correlations’ have been studied in some detail (Chayes, 1971) and, as we show later, the relevant theory can simply be applied to help understand the apparently different relationships between plant traits expressed on area vs mass bases. It turns out that when the coefficients of variation of any two traits are of a similar magnitude, then the area- vs mass-based correlations must, in all practical situations, be different. This inevitable difference can apply to both the sign and magnitude of the regression coefficient (*r*) and one must be very careful when interpreting the difference between the two metrics. In many cases this has nothing to do with plant function or with one measure being fundamentally better than the other. But rather to do with some less well known statistical properties of ‘ratio correlations’. Although the mathematical statistics involved are not overly complicated, we start with a simple simulation to help illustrate our points, then explaining observed results through the presentation of established statistical theory. This first analysis is presented for the simplest case: a single area-based trait not correlated with *M*_{a}. The effects of area-to-mass transformations for a single trait significantly correlated with *M*_{a} on an area basis is then examined: the results from which help to understand the final case investigated; viz., How do slopes and correlation coefficients change when two area-based traits are transformed and subsequently regressed on a mass basis? We then consider the implications of our results; especially in terms of whether species-level variations in leaf photosynthetic properties are, indeed, closely associated with intraspecific variability in other leaf traits such as *M*_{a} and leaf longevity (*τ*) as is now generally assumed to be the case (Westoby & Wright, 2006).