title: Intraspecific variation in stomatal traits, leaf traits and physiology reflects adaptation along aridity gradients in a South African shrub.
abstract: OBJECTIVE
  Trait-environment relationships are commonly interpreted as evidence for local adaptation in plants. However, even when selection analyses support this interpretation, the mechanisms underlying differential benefits are often unknown. This study addresses this gap in knowledge using the broadly distributed South African shrub Protea repens. Specifically, the study examines whether broad-scale patterns of trait variation are consistent with spatial differences in selection and ecophysiology in the wild.
  
  
  METHODS
  In a common garden study of plants sourced from 19 populations, associations were measured between five morphological traits and three axes describing source climates. Trait-trait and trait-environment associations were analysed in a multi-response model. Within two focal populations in the wild, selection and path analyses were used to test associations between traits, fecundity and physiological performance.
  
  
  RESULTS
  Across 19 populations in a common garden, stomatal density increased with the source population's mean annual temperature and decreased with its average amount of rainfall in midsummer. Concordantly, selection analysis in two natural populations revealed positive selection on stomatal density at the hotter, drier site, while failing to detect selection at the cooler, moister site. Dry-site plants with high stomatal density also had higher stomatal conductances, cooler leaf temperatures and higher light-saturated photosynthetic rates than those with low stomatal density, but no such relationships were present among wet-site plants. Leaf area, stomatal pore index and specific leaf area in the garden also co-varied with climate, but within-population differences were not associated with fitness in either wild population.
  
  
  CONCLUSIONS
  The parallel patterns of broad-scale variation, differences in selection and differences in trait-ecophysiology relationships suggest a mechanism for adaptive differentiation in stomatal density. Densely packed stomata may improve performance by increasing transpiration and cooling, but predominately in drier, hotter climates. This study uniquely shows context-dependent benefits of stomatal density--a trait rarely linked to local adaptation in plants.
authors.affiliation: Department of Biological Sciences, Nicholls State University, P.O. Box 2021, Thibodaux, LA 70310, USA and jane.carlson@nicholls.edu.
authors.forename: Jane E
authors.initials: JE
authors.lastname: Carlson
authors.affiliation: Department of Biological Sciences, Nicholls State University, P.O. Box 2021, Thibodaux, LA 70310, USA and.
authors.forename: Christopher A
authors.initials: CA
authors.lastname: Adams
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Connecticut, U-3043, Storrs, CT 06269, USA.
authors.forename: Kent E
authors.initials: KE
authors.lastname: Holsinger
mesh_terms: D000222:Adaptation, Physiological; D002980:Climate; D003889:Desert Climate; D005298:Fertility; D008954:Models, Biological; D054046:Plant Stomata / Q000502:physiology*; D039105:Proteaceae / Q000502:physiology*; D019655:Quantitative Trait, Heritable; D012044:Regression Analysis; D012639:Seeds / Q000502:physiology; D013045:Species Specificity
keywords: Cape Floristic Region; Protea repens; Proteaceae; South Africa; ecophysiology; functional traits; leaf morphology; local adaptation; photosynthesis; selection analysis; stomatal density; sugarbush; trait–environment associations