Diurnal patterns of water potential in the evergreen cloud forests of the Kaimai Ranges, North Island, New Zealand

Abstract

Abstract Water potentials were measured on six species of evergreen trees growing at a range of altitudes, including the cloud zone, on two ridges in the Kaimai Ranges. Diurnal sampling at intervals throughout the summer showed little evidence of plant water stress. Dawn potentials were about -0.1 MPa and minimum potentials were down to -1.2 MPa except in Quintinia acutifolia, a seral shrub. This species yeilded low dawn potentials (-0.6 MPa) at a 600 m a.s.l. site which was below the cloud zone. Plants of unhealthy appearance growing on poor sites had lower potentials than healthy plants on adjacent good sites. Fog occurrence produced higher dawn potentials but had no effect on daily minimum potentials. The results contrast with leaf conductance studies in the same area which showed severe stress with rapid afternoon stomatal closure. It is suggested that the fog promotes soil waterlogging leading to water stress which is reflected in stomatal sensitivity but not shoot water potential.