Effects of summer drought and winter freezing on stem hydraulic conductivity of Rhododendron species from contrasting climates

We studied the limits to maximum water transport in three diffuse-porous evergreen shrubs exposed to frequent winter freeze-thaw events (Rhododendron maximum L. and R. catawbiense Michaux from the Appalachian Mountains) and to a severe summer drought (R. macrophyllum G. Don. from the Oregon Cascades). Percent loss of hydraulic conductivity (PLC), vulnerability curves to xylem embolism and freezing point temperatures of stems were measured over 2 years. Controlled freeze-thaw experiments were also conducted to determine the effect of thaw rate on PLC. During both years, native PLC was significantly higher in winter than in summer for R. macrophyllum. Seasonal changes in PLC were variable in both R. catawbiense and R. maximum. Only R. maximum plants growing in gaps or clearings showed higher PLC than understory plants. A rapid (2-4 day) natural recovery of high native PLC during the winter was observed in both R. maximum and R. macrophyllum. Compared with the bench-dehydration method, vulnerability curves based on the air-injection method consistently had less negative slopes and greater variation. Fifty percent PLC (PLC50) obtained from vulnerability curves based on the dehydration method occurred at -1.75, -2.42 and -2.96 MPa for R. catawbiense, R. maximum and R. macrophyllum, respectively. Among the study species, R. macrophyllum, which commonly experiences a summer drought, had the most negative water potential at PLC50. In all species, stem freezing point temperatures were not consistently lower in winter than in summer. A single fast freeze thaw event significantly increased PLC, and R. catawbiense had the highest PLC in response to freezing treatments. Recovery to control PLC values occurred if a low positive hydraulic pressure was maintained during thawing. Rhododendron macrophyllum plants, which commonly experience few freeze thaw events, had large stem diameters, whereas plants of R. catawbiense, which had small stem diameters, suffered high embolism in response to a single freeze-thaw event. Both drought-induced and winter-induced embolism caused a significant reduction in hydraulic conductivity in all species during periods when drought or freeze-thaw events occurred in their native habitats. However, rapid recovery of PLC following freezing or drought maintained the species above their relatively low margins of safety for complete xylem dysfunction.