Patterns of leaf conductance and water potential of five Himalayan tree species

We studied variations in water relations and drought response in five Himalayan tree species: chilaune ( Schima wallichii ) and dhale katus ( Castanopsis indica ), at an elevation of 1400 m; banjh ( Quercus lanata ) and lali gurans ( Rhododendron arboreum ) at 2020 m; and khasru ( Q. semecarpifolia ) at 2130 m; at Phulchowki Hill, Kathmandu, Nepal. Soil water potential at 15 (Ψ_s15) and 30 cm (Ψ_s30) depths, tree water potential at predawn (Ψ_pd) and midday (Ψ_md), and leaf conductance during the morning ( g _wAM) and afternoon ( g _wPM) were observed from December 1998 to April 2001, except during the monsoon months. There was a significant variation among sites, species and months in Ψ_pd, Ψ_md, g _wAM and g _wPM, and among months for all species for Ψ_s15. Mean Ψ_pd and Ψ_md were lowest in Q. semecarpifolia (- 0.40 and -1.18 MPa, respectively) and highest in S. wallichii (-0.20 and -0.63 MPa, respectively). The minimum Ψ value for all species (-0.70 to -1.79 MPa) was observed in March 1999, after 4 months of unusually low rainfall. Some patterns of Ψ_pd were related to phenology and leaf damage. During leaf development, Ψ_pd often increased. Mean g _wAM and g _wPM were highest in Q. semecarpifolia (172 and 190 mmol m^{- 2} s^-1, respectively) and lowest in C. indica (78 and 74 mmol m^-2 s^-1, respectively). Soil water potential (Ψ) at 15 cm depth correlated with plant Ψ in all species, but rarely with g _wAM and not with g _wPM. Plant Ψ declined with increasing elevation, whereas g _w increased. As Ψ_pd declined, so did maximal g _w, but overall, g _w was correlated with Ψ_pd only for R. arboreum . Schima wallichii maintained high Ψ, with low stomatal conductance, as did C. indica , except that C. indica had low Ψ during dry months. Rhododendron arboreum maintained high Ψ_pd and g _w, despite low soil Ψ. Quercus lanata had low g _w and low Ψ_pd in some months, but showed no correlation between tree Ψ and g _w. Quercus semecarpifolia , which grows at the highest elevation, had low soil and plant Ψ and high g _w.