THE RESPONSES OF 3 RIPARIAN COTTONWOOD SPECIES TO WATER-TABLE DECLINE

The influence of rate of water table decline was studied with three No rth American cottonwood (poplar) species: the prairie cottonwood, Popu lus deltoides; the narrowleaf cottonwood, P. angustifolia; and the bal sam poplar, P. balasamifera. Shoot cuttings were rooted and transplant ed into rhizopods, experimental devices that permit the controlled man ipulation of water table depth. Three rates of water table decline wer e applied, 0, 4 and 10 cm day(-1), and growth and transpiration were s tudied. Two clones of each species performed relatively similarly; the P. balsamifera clones grew fastest under all three treatments, follow ed by P. deltoides under 0 and 4 cm day(-1) conditions. Under the 10 c m day(-1) treatment, the P. deltoides grew as slowly as P. angustifoli a. In all genotypes, shoot growth and apparent transpiration were prog ressively reduced with increasing rate of water table decline. Convers ely, root growth was promoted by water table decline and root elongati on was most rapid under the gradual 4 cm day(-1) treatment; root elong ation was insufficient for the abrupt 10 cm day(-1) decline and some P . angustifolia and P. deltoides saplings died under that treatment. Th e present study demonstrates that tolerance to water table decline var ies across cottonwood genotypes and that P. balsamifera saplings were the most vigorous. This is relevant to the natural distribution in whi ch P. balsamifera occurs in mountain regions where stream stages and r iparian water table depths often change abruptly. The vigor of P. bals amifera is also consistent with the reproductive mechanism of 'branch propagation', a process of clonal recruitment in which browsed or brok en branch fragments root along stream edges, enabling dispersive propa gation, particularly of P. balsamifera and P. trichocarpa. (C) 1998 El sevier Science B.V.