Stomatal responses and water relations of Eucalyptus pauciflora in summer along an elevational gradient

The spatial and temporal variation of leaf conductance (g) in Eucalyptus pauciflora was analysed with respect to photon flux area density (I), temperature (T), water vapour concentration deficit (Δw), and leaf water potential (ψ) at four different sites between 940 m and 2,040 m altitude in the Snowy Mountains of south-eastern Australia. Along this altitudinal gradient the precipitation/evaporation ratio increases from 1 to 4. The results show that gas diffusion in this tree species is primarily controlled by I and Δw at all sites, independently of the specific soil moisture regime. Even under dry midsummer conditions with predawn leaf water potentials of -1 MPa at the lowest altitude, ψ had no striking effect on g. The humidity threshold for the onset of stomatal closure does not vary greatly between the study sites (12.2 ± 1.3 Pa$\text{kPa}^{-1}$The highest and lowest values observed for ψ, the osmotic potential at water saturation (from pressure/volume curves), the mean and maximum g and stomatal density, all increase with elevation. The highest (least negative) osmotic potentials were obtained at all sites in midsummer. It therefore appears that there is no osmotic adjustment to drought in the seasonal course. The maximum difference between osmotic potentials obtained at the lowest and highest sites is 0.46 MPa. In general osmotic potential varies less than has been reported for other plant species exposed to varying water regimes. This may be the consequence of the pronounced feed-forward response of the stomata to evaporative demand, which led to only moderate tissue desiccation, never exceeding the turgor loss point. E. pauciflora is a tree species with a very conservative utilisation of soil water, which adjusts to drought via stomatal control of water loss, rather than via osmotic properties. These results explain previous reports of the comparatively high susceptibility of E. pauciflora to severe drought and its positive influence on the hydrological balance of mountain ecosystems in the Australian Alps.