A QUANTITATIVE DICOT LEAF MORPHOSPACE

Because dicot angiosperm leaves differ demonstrably in shape and size with variation in climate, ecology, and phylogenetic history there is a general interest in describing leaf morphological variation. Traditional descriptive systems have been primarily qualitative or semiquantitative, although some studies have examined the relationship between climate and a limited set of quantitative measurements of leaf form. Here I propose a fully quantitative morphospace that provides a reasonably complete description of dicot leaves, allowing local floras to be characterized without the morphospace being distorted by variation in scoring practices.

The primary dimensions of dicot leaf morphospace are blade area, blade perimeter, apex angle, base angle, base symmetry, whole-blade symmetry, length:width ratio, distance along midrib to maximum blade width, tooth importance, and blade lobation. These dimensions reflect aspects of leaf form typically characterized in qualitative analyses, and leaf outline analyses indicate that these dimensions are fundamental features of leaf shape variation. The morphospace has been tested against eight modern New World floras varying latitudinally from Hubbard Brook Experimental Forest, New Hampshire to Barro Colorado Island, Panama (sites HB, ANF, HP, York, DF, BH, PR, and BCI in Royer et al 2005). Tropical and temperate floras appear to occupy semi-distinct areas of the morphospace; although there is considerable overlap, centroids are significantly distinct between BCI and HB, ANF, HP, and York . Tropical floras also exhibit a narrower range of shape variation (morphological disparity) than do temperate floras, despite having higher species richness. When a set of leaves with more highly unusual leaf forms (including many herbaceous species) is included in the morphospace, most species in both tropical and temperate floras appear to occupy a limited volume of the available morphospace, despite being moderately distinct from each other.

As a broader global range of dicot leaf floras are examined, a more comprehensive view of the volume and filling of dicot leaf morphospace will emerge, providing a modern context for understanding evolutionary-scale changes in angiosperm leaf form.