The responses of three riparian cottonwood species to water table decline

The influence of rate of water table decline was studied with three North American cottonwood (poplar) species: the prairie cottonwood, Populus deltoides; the narrowleaf cottonwood, P. angustifolia; and the balsam poplar, P. balsamifera. Shoot cuttings were rooted and transplanted into rhizopods, experimental devices that permit the controlled manipulation of water table depth. Three rates of water table decline were applied, 0, 4 and 10 cm day −1, and growth and transpiration were studied. Two clones of each species performed relatively similarly; the P. balsamifera clones grew fastest under all three treatments, followed by P. deltoides under 0 and 4 cm day −1 conditions. Under the 10 cm day −1 treatment, the P. deltoides grew as slowly as P. angustifolia. In all genotypes, shoot growth and apparent transpiration were progressively reduced with increasing rate of water table decline. Conversely, root growth was promoted by water table decline and root elongation was most rapid under the gradual 4 cm day −1 treatment; root elongation was insufficient for the abrupt 10 cm day −1 decline and some P. angustifolia and P. deltoides saplings died under that treatment. The present study demonstrates that tolerance to water table decline varies across cottonwood genotypes and that P. balsamifera saplings were the most vigorous. This is relevant to the natural distribution in which P. balsamifera occurs in mountain regions where stream stages and riparian water table depths often change abruptly. The vigor of P. balsamifera is also consistent with the reproductive mechanism of `branch propagation', a process of clonal recruitment in which browsed or broken branch fragments root along stream edges, enabling dispersive propagation, particularly of P. balsamifera and P. trichocarpa.