Reduced stomatal frequency with rising elevation for Kobresia royleana on the Tibetan Plateau

Knowledge about within-species variation in stomatal frequency with varying elevation at very high elevations is rare, which is crucial for us to understand how alpine plants are adapted to the extreme environment. Here, we focus on the variation in stomatal frequency in Kobresia royleana (Nees) Boeckeler (Cyperaceae, Cyperales) along two altitudinal transects (elevation ranges from 3723 m to 5081 m) in the center of the Tibetan Plateau. The result shows the stomatal density (SD) varied from 303 ± 55.6 mm−2 to 542 ± 81.8 mm−2, and stomatal index (SI) ranged from 21.0% to 29.6%. In contrast with most cases, an unexpected negative response of stomatal frequency to rising elevation was observed. Among abiotic factors, the growing season mean temperature and CO2 partial pressure significantly declined with increasing elevation, while the growing season precipitation did not vary. Therefore, the decreasing SD and SI were mainly due to the declining temperature rather than the decreasing CO2 partial pressure. Further, SD and SI were negatively related to leaf functional traits of specific leaf area (SLA), leaf nitrogen concentration (N) and stable carbon isotope ratios (δ13C), and all these morphological and physiological traits tended to covary with rising elevation and declining temperature. Meanwhile, the increasing δ13C, N and SLA with elevation seem to be strategies for alpine plants to cope with the low-temperature environments. Therefore, the observed covariance between stomatal frequency and leaf functional traits also suggests that the low temperature rather than low CO2 partial pressure mainly leads to the elevational pattern of stomatal frequency for this alpine species. •Stomatal density and index declined with rising elevation.•Leaf functional traits increased with increasing elevation.•Stomatal frequency was negatively related to leaf functional traits.•Low temperature mainly led to the variation of stomatal frequency and related functional traits.