Phylogeny and variation in light capture area deployed per unit investment in leaves: Designs for selecting study species with a view to generalising

In recent years a procedure that can be called 'phylogenetic correction' has become widespread. According to advocates of this procedure, trait correlations must be approached through examining correlated divergence from radiations on the phylogenetic tree. Actually, it is incorrect to believe that correlated-divergence methods have superceded correlations across the set of present-day species. Rather, the two procedures address separate questions and are complementary. Correlated divergence tests, like cross-species correlations, need to be interpreted with caution as evidence for a functional relationship between traits in the present day, for the well-known reason that any correlation might be mediated through some third variable.Phylogeny need not be thought of as an 'effect', a source of causation through common ancestry that potentially confounds ecological functionality. Phylogeny is a source of historical information that potentially can be used to generalise functional relationships more efficiently across species. Generally, the problem of representative sampling of species for study has been much underestimated, compared to the rigorous randomisation expected when sampling quadrats in vegetation, or individuals within species. Species might be selected according to habitat, or according to ecological strategy (a scheme is proposed), or according to phylogeny. We outline some alternative question-forms that might be asked, with phylogenetic species-selection designs that address those questions, and we comment on general issues that arise in designing the selection of study species along phylogenetic lines. These questions are illustrated from our recent work on specific leaf area (SLA), one of the key strategic attributes of plants.SLA shifted downwards both in species from lower rainfall, and in species from soils with lower total P. The overall shift consisted both of shifts in representation among different clades, and of shifts across most phylogenetically contrasted pairs of species. SLA downshift consisted both of increasing leaf lamina depth and of increasing tissue density. The contribution from these two effects varied between different phylogenetic contrasts, but there was no evidence for their relative importance being different at low rainfall compared to low soil P. There were some differences in the anatomy of SLA decrease, however. At low rainfall, tissue density increased mainly via the reinforcing ribs running along the leaf contributing a larger proportion of the tissue. At low soil P, some contrasts showed a similar increase in sclerification, but about half actually showed decreased sclerification, together with stronger epidermal casing. Thus there were some differences in the exact anatomical changes along rainfall compared to nutrient gradients, but they had the common effect of decreasing SLA, and this must flow through to other consequences, unifying the two syndromes of adaptation to some extent.