Inter- and intraspecific variation in stomatal pore area index along elevational gradients and its relation to leaf functional traits

Stomata are mediators of gas exchange and thus important for photosynthesis and plant performance. The aim of this study was to analyze the ecological explanatory power of the stomatal pore area index (SPI) calculated via stomatal size and density. We studied the SPI on sun leaves of 22 herbaceous species on 22 study sites being distributed along two elevational gradients in the northern Alps ranging from 700 to 1800 m a.s.l.. We analyzed its correlation with other functional traits related to plant performance namely specific leaf area (SLA), area-based leaf nitrogen and carbon (Narea and Carea, respectively) as well as carbon discrimination Δ13C within as well as between species. On a subset of four species we also measured light-saturated net photosynthetic rate at ambient CO2 concentration (Asat) and stomatal conductance on all sites. We found that SPI was positively correlated with Asat, yet the relation was weaker than expected. The reaction of SPI along the elevational gradients was highly species-specific and related to variations in other investigated leaf traits. The relationship with functional traits, however, differed between the inter- and intraspecific level in strength and direction. SPI was positively related to Narea and Carea and negatively with SLA and Δ13C for most species. However, we found no significant relation considering species mean values for Δ13C and Narea. The relation of SPI to SLA was the most consistent displaying no difference when comparing the relation between and within species. This research shows that different processes may act on different organizational levels leading to the detected differences in trait–trait correlations on the inter- and intraspecific levels. It may have important consequences also for macroecological and modelling studies.