VULNERABILITY TO DROUGHT-INDUCED CAVITATION OF RIPARIAN COTTONWOODS IN ALBERTA - A POSSIBLE FACTOR IN THE DECLINE OF THE ECOSYSTEM

Vulnerability of xylem to loss of hydraulic conductivity caused by dro ught-induced cavitation was determined for three riparian cottonwood s pecies in Lethbridge, Alberta: Populus deltoides Bartr., P. balsamifer a L., and P. angustifolia James. These species suffered 50% loss of hy draulic conductivity in one-year-old stem segments when xylem pressure potential fell to -0.7 MPa for P. deltoides and -1.7 MPa for P. balsa mifera and P. angustifolia, making them the three most vulnerable tree species reported so far in North America. The possible contribution o f drought-induced xylem dysfunction to the decline of riparian ecosyst ems in dammed rivers is discussed.