Interspecific covariation between stomatal density and other functional leaf traits in a local flora

Despite the importance of stomata in leaf functioning, and despite the recent interest in interspecific leaf trait covariation in functional ecology, little is known about how stomatal density relates to other leaf traits in a broad interspecific context. This is especially important because stomatal density has been widely used to deduce temporal variation in atmospheric CO2 concentrations [CO2atm] from fossilized or herbarium leaves. We therefore measured stomatal density, specific leaf area (SLA) and its components, leaf thickness, and leaf chlorophyll content in both sun and shade leaves of 169 individuals from 52 angiosperm species in southwestern Quebec. Using mixed models, we show that stomatal density decreases allometrically with increasing SLA and chlorophyll content, and increases allometrically with increasing lamina thickness. The sun–shade contrast changes the intercepts, but not the slopes, of these relationships. It is important to take into consideration these relations when correlating stomatal density with [CO2], to avoid spurious interpretations.