Stomates on the leaf epidermis are fundamental mediators of water status and carbon assimilation in plants. The conductive potential of a leaf, which is largely determined by the pore size and density of stomates, presumably places an upper boundary on the maximum rate of carbon assimilation among C3 plants. These stomatal traits would therefore be expected to covary with environmental factors characteristic of the habitats in which a plant species is typically found, such as soil water availability and vapor pressure deficit, and to be related to a species’ water-use efficiency (WUE). To quantify these relationships, we measured stomatal densities and pore sizes of 28 tree species in a Bornean rain forest. Twenty-four species are congeneric pairs representing specialists of either a well-drained, nutrient-poor soil (sandy loam) or a moister, more-fertile soil (clay); the remaining four species are habitat generalists sampled on both soils. We compared inter- and intraspecific variation in stomatal traits with instantaneous and time-integrated estimates of water-use efficiency and maximum rates of stomatal conductance and carbon assimilation. We expected that conductive potential, measured by the stomatal pore index (SPI=density*guard cell length2), would be larger for species and individuals growing on the clay, relative to the sandy loam, soil.
For all but two species, stomates were located only on the abaxial leaf epidermis. There was a strong negative correlation between stomatal density and guard cell length among species. As a result, genera varied as to whether or not density or guard cell length was larger for species growing on the clay soil. Four of 12 congeneric comparisons were consistent with our prediction that SPI should be larger on the moister, more fertile soil type, although one comparison showed statistically significant variation opposite to our prediction, and seven comparisons showed no significant differences. For generalists, there was no significant variation among individuals attributable to soil type. SPI was not significantly correlated with any estimate of water-use efficiency or any gas exchange parameter. Among species, stomatal guard cell length was positively correlated with the ratio of the leaf-internal:ambient partial pressures of CO2 (pi/pa), but negatively correlated with instantaneous WUE. Density also showed significant relationships with pi/pa and instantaneous WUE, but in the opposite direction. To the extent that the SPI is a good correlate of the maximum conductive potential of a leaf, we found little evidence of variation in conductive potential between species and individuals on soils of varying water-availability.