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.