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Corner’s rules hold up across the growing season

The relationship between twig cross-sectional area and leaf surface area varied little over the course of the season.

In 1949, E.J.H. Corner published a set of rules, later to be known as ‘Corner’s rules,’ describing the relationship between the cross-sectional area of a twig and the surface area of its leaves; namely, that they are isometrically correlated across species. This implies that a larger twig will carry a greater area of leaf surface, the rationale being that a) more foliage will require a proportionally greater vascular supply, and b) a stem’s nutritional needs will be proportional to the energy provided by the leaves it bears. Studies comparing different species have found support for Corner’s rules, though little work has been done in determining whether they hold true over the course of a growing season in deciduous trees, whether the presence of flowers on a branch changes the relationship, or whether plant height has an effect.

In a new article published in Annals of Botany, lead author Alex Fajardo and colleagues tested the effect of phenology, flowering, and height on the relationship described by Corner’s rules. The researchers studied six woody Patagonian species comprising trees, shrubs, and treelets, sampling them weekly over the course of a 20 week growing season.

Branches, twigs and inflorescences of Embothrium coccineum (Proteaceae) and a
scheme of the different twigs that were considered in our sampling: V stands for a completely
vegetative twig, while R stands for a floral twig and everything it bears; VR stands for the special
case where a vegetative twig was borne by a floral twig. Source: Fajardo et al. 2020.

Phenology did not have an effect on the relationships between leaf area and twig cross-sectional area or between leaf dry mass and twig dry mass. However, there was significant variation in the slopes and intercepts of the leaf area/twig cross-section plots, suggesting adaptive significance. Of the six species studied, two diverged significantly from an isometric relationship, showing that departure from the general pattern is possible and likely has functional consequences.

“This finding certainly gives a new dimension to the Corner’s rules, as the production of new tissues during the growing season resembles the spectrum found elsewhere when several species are compared, going from slender shoots with small leaf area at the start of the season to thicker twigs and larger leaf area at the peak of the season,” write the authors.

Leaf-shoot scaling relationships do vary across species of different heights. The authors found that for the same cross-sectional area, twigs from high up in a tree will have less foliage area on average than twigs collected from shrubs and treelets. A plant’s foliar habit also affected the relationship, with a deciduous species appearing to need more twig cross-sectional area to support a given leaf area than does an evergreen species, even for nearly equal leaf sizes.

Finally, the presence of an inflorescence on a branch did not affect the leaf area/twig cross-section scaling relationship, but flowering branches produced a lower slope on the plot than did vegetative branches. This means that there is less leaf mass on a flowering shoot than a non-flowering shoot for a given shoot mass.

Still, relatively few species have been examined to determine how the above factors affect Corner’s rules. “The inclusion of more species belonging to most of the orders across the flowering plants appears essential to map the universality of these scaling relationships,” write the authors.

Erin Zimmerman

Erin Zimmerman is a botanist turned science writer and sometimes botanical illustrator. She did her PhD at the University of Montréal and worked as a post-doctoral fellow with the Canadian Ministry of Agriculture. She was a plant morphologist, but when no one wanted to pay her to do that anymore, she started writing about them instead. Her other plant articles (and occasional essays) appear in Smithsonian Magazine, Undark, New York Magazine, Narratively, and elsewhere. Read her stuff at www.DrErinZimmerman.com.
Erin can also be found talking about plants and being snarky on Twitter @DoctorZedd.

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