Linking Functional Leaf Traits to Ecophysiology in South African Pelargoniums

Plant responses to drought impact the distribution and persistence of species yet are usually assessed over time frames that may not reflect dynamic stomatal responses, i.e. isohydry and anisohydry. Iso-/anisohydry is a well-known continuum across species from many lineages and communities, but few studies associate iso-/anisohydry with functional trait variation or evolutionary history. We investigated phylogenetic effects on iso-/anisohydry in species from the two major clades with the specific intent of determining whether functional leaf traits link to differences in stomatal behavior. We compared the extent of isohydry/anisohydry in a total of 23 Pelargonium species at two different sites across a seasonal dry-down in the field and a total of 12 species in a dry-down experiment in a greenhouse by measuring functional leaf traits, photosynthetic rates, stomatal conductance and predawn water potentials during the dry-down period on the same individuals. In the greenhouse and the field, species range in expression of iso-/anisohydry but clades differ significantly; we show that linkages between functional leaf traits and stomatal responses are largely clade-specific. We propose that differences in stomatal behavior reflect adaptations to contrasting climates during clade diversification and/or current species distributions across a climatically diverse region. While interactions between functional leaf traits reflect contrasting strategies of resource acquisition across diverse plant lineages, seasonal variation in traits among co-occurring congeners and the extent to which traits predict changes in physiology during a single growing season is relatively unexplored. In field species of Pelargonium, we investigate seasonal shifts in functional leaf traits and photosynthesis, and ask if photosynthetic shifts are predicted by leaf longevity, functional traits and climate. Amazingly, we find that variation in leaf longevity for 23 Pelargonium species parallels the global range of leaf longevities for deciduous species, and variation in functional traits and their interactions are strongly affected by season of measurement. Compared to global datasets in which leaf longevity links strongly to climate, we find no association, and show that seasonal shifts in photosynthetic rates are also not strongly dictated by climate but instead predicted by individual species’ leaf longevities and traits reflecting access to resources (light and water)