Abstract: Plant trait economic spectra are viewed as a result of natural selection favoring different ways to be a plant in a given environment. However, plant trait economic spectra atomize the individual without really having a clear idea of how selection of individuals is occurring given phenotypic differences. Corner’s rules potentially connect leaf-stem trait coordination to understand how selection shapes plant economy. Corner’s rules offer an ideal starting point for understanding how leaves, stems, and perhaps even roots and reproductive structures, evolve as a unit. More important still, robust theory is available explaining Corner’s rules, providing the cornerstone for explaining how and why plant subindividual units should evolve in a coordinated fashion. Corner's rules are a widespread pattern of trait correlations between leaf and stems. At one end of the continuum, species have small leaves and slender terminal twigs that are highly-branched. At the other end of the continuum, species have large leaves and thick terminal twigs that are sparsely-branched. Species with thick twigs tend to have large inflorescences; those with thin twigs, smaller ones. This tendency has mostly been documented across species, but a global effort to test the universality of this pattern is lacking. Here, we covered with new data all the gymnosperm and angiosperm families ( >400) and with more than 1,200 woody species from 5 continents and from several biodiversity hot spots (e.g., Chile, Madagascar, South Africa, New Caledonia, Taiwan), we found support for a tight coordination between twig cross-section area and twig total leaf area with a slope of 1 in a logarithmic scale. Although some biologically significant outliers are present (mostly, thick stems with small leaves, e.g., Oxalis gigantea [Oxalidaceae], common of dry systems), other morpho-space regions were devoid of any species, e.g., narrow stems with large leaves. Other leaf-shoot allometric relationships were distilled, including a positive and significant relationships between wood density and leaf size. Potential causes for the universality of the Corner’s rules include 1) plants need to fix similar amounts of carbon per unit leaf area, 2) the avoidance of self-shading, and 3) the existence of a growth and mechanical tradeoff. With all this, we assert that Corner’s rules become the cornerstone to explain the whole-plant economics spectrum.
