Branch sacrifice: cavitation-associated drought adaptation of riparian cottonwoods

In their native riparian zones (floodplains), Populus deltoides (prairie co ttonwood) and P. fremontii (Fremont cottonwood) commonly experience substan tial branch die-back. These trees occur in semi-arid areas of North America and unexpectedly given the dry regions, they are exceptionally vulnerable to xylem cavitation. drought-induced air embolism of xylem vessels. We prop ose that the vulnerability to cavitation and branch dieback are physiologic ally linked; drought-induced cavitation underlies branch die-back that redu ces transpirational demand enabling the remaining shoot to maintain a favor able water balance. This proposal follows field observation along various w estern North American rivers as precocious branch senescence. the yellowing and death of leaves on particular branches during mid- to late summer, was common for P. deltoides and P. fremontii during hot and dry periods of low stream-flow. Branches displaying precocious senescence were subsequently d ead the following year. The proposed association between cavitation, precoc ious senescence and branch dieback is also supported by experiments involvi ng external pressurization of branches to about 2.5 MPa with a branch colla r or through an adjacent cut-branch. The treatments induced xylem cavitatio n and increased leaf diffusive resistance (stomatal closure) that was follo wed by leaf senescence and branch death of P. deltoides. P. trichocarpa (bl ack cottonwood) appeared to be less affected by the pressurization treatmen t and this species as well P. angustifolia (narrowleaf cottonwood) and P. b alsamifera (balsam poplar) seldom display the patchy summer branch senescen ce typical of P. deltoides and P. fremontii. 'Branch sacrifice' describes t his cavitation-associated senescence and branch die-back that may provide a drought adaptation for the prairie and Fremont cottonwoods.