The impact of subcanopy light environment on the hydraulic vulnerability of Rhododendron maximum to freeze‐thaw cycles and drought

ABSTRACT The vulnerability of xylem to embolism development in Rhododendron maximum L., an evergreen diffuse‐porous shrub, was investigated in relation to the frequency of winter freeze–thaw cycles in high and low light sites of the Eastern US. Though the frequency of freeze–thaw cycles during the winter was lower in North Carolina than in Virginia, the hydraulic conductivity of 3‐year‐old branches was reduced by up to 60% by winter embolism development in North Carolina compared to less than 30% in Virginia. Generally, small vessel diameters and volumes were associated with a significant resistance to embolism formation resulting from repeated freeze–thaws of xylem sap. In stems grown in high light sites (gaps), larger vessel volumes, and greater diameter growth of stems were associated with a significantly higher degree of freeze–thaw embolism development than in those grown in the low light sites. Thus, the growth patterns of R. maximum stems, under conditions of higher light availability, rendered them more susceptible to freeze–thaw‐induced embolisms. Vulnerability to drought‐induced embolism in stems was not affected by light environment. Rhododendron maximum was relatively sensitive to drought‐induced embolism because 50% loss of hydraulic conductivity occurred at a water potential of ‐2.2 MPa. The distribution and gas exchange of R. maximum are constrained by the dual effects of freeze‐thaw cycles and drought on vascular function.