Why should we investigate the morphological disparity of plant clades?

Background
Disparity refers to the morphological variation in a sample of taxa, and is distinct from diversity or taxonomic richness. Diversity and disparity are fundamentally decoupled; many groups attain high levels of disparity early in their evolution, while diversity is still comparatively low. Diversity may subsequently increase even in the face of static or declining disparity by increasingly fine subdivision of morphological ‘design’ space (morphospace). Many animal clades reached high levels of disparity early in their evolution, but here have been few comparable studies of plant clades, despite their profound ecological and evolutionary importance. We offer a prospective and some preliminary macroevolutonary analyses.
Methods
Classical morphometric methods are most suitable when there is reasonable conservation of form, but lose traction where morphological differences become greater (e.g., in comparisons across higher taxa). Discrete character matrices offer one means to compare a greater diversity of forms. We explore morphospaces derived from eight discrete data sets for major plant clades, and discuss their macroevolutionary implications.
Key Results
Most of the plant clades in our study show initial, high levels of disparity that approach or attain the maximum levels reached subsequently. These plant clades are characterised by an initial phase of evolution during which most regions of their empirical morphospaces are colonised. Angiosperms, palms, pines and ferns show remarkably little variation in disparity through time. Conifers furnish the most marked exception, appearing at relatively low disparity in the latest Carboniferous, before expanding incrementally with the radiation of successive, tightly clustered constituent subclades.

Conclusions
Many cladistic datasets can be repurposed for investigating the morphological disparity of plant clades through time, and offer insights that are complimentary to more focused morphometric studies. The unique structural and ecological features of plants make them ideally suited to investigating intrinsic and extrinsic constraints on disparity.