Stomatal behavior and functional leaf traits reflect phylogenetic history in co-occurring congeners
At a global scale, predictable interactions among functional leaf traits reflect resource acquisition strategies across diverse plant lineages. At the community level, phylogenetic conservatism predicts that co-occurring congeners will share similar strategies and trait values. Here we ask if co-occurring congeners from different lineages differ in their stomatal responses to dry-down and if differences in stomatal responses can be predicted by functional leaf traits. South African Pelargonium species exhibit remarkable morphological variation in growth form and leaf shape and species from each major clade often co-occur in the field. 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 leaf functional traits, photosynthetic rates, stomatal conductance and predawn water potentials during the dry-down period on the same individuals.
Species displayed a continuous range of isohydric and anisohydric behaviors but consistent clade-specific patterns emerged in response to both rapid and gradual soil drying. Clade A species were more anisohydric and exhibited less stomatal control than clade C species, providing evidence that physiology has been conserved at the major clade level. Functional leaf traits predict the extent of isohydric/anisohydric responses, and clades differ in which traits best explain these responses. We propose that differences in functional strategies reflect the contrasting evolutionary histories of the two clades. Clade C presumably began diversification during the early Miocene when South Africa experienced a mesic-subtropical climate. Under warm temperatures and high evaporative demand, isohydry preserves leaf function by closing stomata to maintain plant water status. The more recent diversification of clade A in the late-Miocene occurred during a period of increasing aridity and seasonality in the winter rainfall region. Under cool, wet winter growing conditions, anisohydry in clade A promotes rapid carbon gain during a short growing season followed by dormancy during a hot, dry summer. We conclude that phylogenetic effects on stomatal behavior can provide important insights into leaf functional trait variation at the community level.