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Cottonwoods are poplar trees that are well adapted to dynamic riparian, or streamside, zones throughout the Northern Hemisphere. Here we assess the influences of three prominent physical disturbances, floods, fire, and ice, on cottonwood population ecology. We emphasize cottonwoods along rivers from the “Crown of the Continent”, the central Rocky Mountain zone around the Canada – United States border, where five Populus species overlap and four hybridize. Moderate to major floods scour banks and deposit bars, creating barren and moist colonization sites that are essential for cottonwood seedling recruitment. Floods also scarify shallow roots, thus promoting clonal suckering, especially for the section Tacamahaca species: narrowleaf cottonwood ( Populus angustifolia James), balsam poplar ( Populus balsamifera L.), and black cottonwood ( Populus trichocarpa Torr. & A. Gray). Fire would naturally be less frequent in some riparian zones because of the moist conditions and firebreaks provided by the streams, but with human use, floodplain forest fires have probably increased. Following fire, regrowth through clonal root and shoot suckers can be prolific for the Tacamahaca species, but is limited for the section Aigeiros , prairie cottonwood ( Populus deltoides Bartr.). River ice, and especially ice drives that accompany winter or spring break-up, provide powerful riparian disturbances that have often been neglected. Ice drives generate barren sites for seedling colonization, shear shoots, and scarify roots promoting shoot and root suckering, and sever branches, enabling dispersive clonal branch propagation. Following studies along many regional rivers, we conclude that: (i) riparian cottonwoods are tolerant of, and dependent upon, occasional physical disturbance for population rejuvenation; (ii) differing disturbance responses contribute to niche differentiation across the Populus species; (iii) different disturbances enable varied spatial and temporal patterns of cottonwood establishment, including fringe, general, and patch recruitment; and (iv) natural disturbance regimes probably favor native cottonwoods and disfavor some invasive, woody plants. River damming and flow regulation often attempt to attenuate flood and ice disturbance, a management objective that may hinder the perpetuation of native floodplain forests. We recommend that river resource managers seek to allow flood and ice disturbance, and additionally, fire may provide a managed disturbance that could rejuvenate overmature cottonwood groves along some regulated rivers.

Cottonwoods occur in riparian areas where water table depth generally varies with the elevation of the adjacent river. Plant adaptation to the riparian zone requires the coordination of root elongation to maintain contact with the water table during the summer decline. We investigated the effects of rate of water decline on growth allocation and concentrations of endogenous gibberellins (GAs) in black cottonwood (Populus trichocarpa Torr. & A. Gray ex Hook.) saplings. Rhizopods were used to achieve water decline rates of 0, 2 and 4 cm day −1 . Root elongation approximately doubled in response to the 2 cm day −1 treatment, whereas leaf area was reduced. A water decline rate of 4 cm day −1 led to water stress, as evidenced by reduced growth, increased leaf diffusive resistance, decreased water potential, and leaf senescence and abscission. Endogenous GAs were extracted, purified and analyzed by gas chromatography-selected ion monitoring with internal [ 2 H 2 ]GA standards. Across the sampled plant organs, GAs were generally highest in shoot tips and sequentially lower in basal stems, root tips, leaves and upper roots; GAs were thus abundant in rapidly growing tissues. Of the GAs measured, GA 1 tended to predominate, followed sequentially by GA 3 , GA 8 , GA 19 , GA 20 , GA 29 and GA 4 . There was little relationship between GA concentration and growth allocation across the water table decline treatments, although GA 8 was consistently reduced in plants experiencing water table decline. Because GA 8 is the final gibberellin in the metabolic sequence, it might be useful for assessing historic patterns of GAs and growth rate. This study demonstrated changes in growth allocation in response to water table decline, but provided little evidence that endogenous GAs play a primary role in the regulation of root elongation in response 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 .

Completed in 1951, the St. Mary Dam enables water storage and diversion for irrigation; river flows downstream are consequently dramatically reduced during summer months. To assess historical changes in the abundance of riparian cottonwoods (Populus balsamifera, Populus angustifolia, and a few Populus deltoides), airphoto analyses were conducted for 40-km river reaches upstream and downstream from the dam and along adjacent dammed and undammed rivers. Cottonwoods along the lower St. Mary River are confined by steep-walled canyons to narrow bands and consequently analyses of the lineal river distance associated with cottonwoods were conducted. These revealed a 68% decline from 1951 to 1985. The decline was progressive, with 28.9, 27.6, 15.1, and 7.6% of the reach associated with cottonwoods in 1951, 1961, 1981, and 1985, respectively. Ground surveys from 1985 to 1994 indicated further decline after 1985 and an absence of cottonwood seedlings and saplings. Cottonwood stands upstream from the St. Mary Dam and along adjacent rivers are more extensive and analyses of the areal extent of stands were consequently appropriate. These indicated minor change along the upper St. Mary (−0.5%), the upper (+1.9%) and lower Waterton (+3.5%), and the upper Belly (−9.1%) rivers, and an increase in forest abundance along the lower Belly River (+52.2%), between 1951 and 1985. Thus, the decline of cottonwoods along the lower St. Mary River was not symptomatic of a general pattern of decline in the region. Analyses of historical stream flows indicated that the cottonwood mortality was drought induced as a result of insufficient flows during the hot, dry summer periods and abrupt flow reductions following the high-flow period in the late spring. The riparian water table was determined to be closely coordinated with river stage, as changes in river elevation were followed by quantitatively similar changes in water table depth. Along the St. Mary River, reduced sedimentation downstream from the dam was not considered to be responsible for the cottonwood decline. The historically sparse cottonwood abundance along the lower St. Mary River may have reflected environmental conditions that were naturally only marginally suitable, and those groves may have been particularly vulnerable to the impacts of river flow regulation. Key words: Populus, cottonwoods, instream flows, mortality, riparian vegetation.

From 1990 to 1993, 690 small cottonwood saplings were excavated along four rivers in the Oldman River Basin to determine their method of origin. Overall, 52% began as seedlings while 48% originated through clonal mechanisms, of which 30% (of the total) were root suckers, i.e., adventitious shoots from preexisting roots, and 18% were shoot suckers, i.e., new shoots originating from buried shoots. The shoot suckers arose primarily through (i) flood training, i.e., new shoot emergence from young shoots that had been toppled and buried during flood events, and (ii) shoot regrowth following decapitation by ice scouring or animal browsing. Only two saplings originated through cladoptosis, propagation from excised shoot fragments. The proportion of seedlings versus clonally established shoots varied somewhat across years and sites. Within a site, saplings near the river's edge were more commonly seedlings, whereas saplings further from the river were more commonly root suckers. Three cottonwood species occurred in the forests studied and their reproductive strategies varied somewhat. Unlike the balsam poplar (Populus balsamifera), narrowleaf cottonwood (Populus angustifolia), and interspecific hybrids, the prairie cottonwood (Populus deltoides) did not propagate through root suckers; all species produced seedlings and shoot suckers. This study demonstrates that clonal reproduction has the potential to contribute to the regeneration of riparian cottonwood forests in southern Alberta. The contribution through clonal propagation complicates the forest population structure and should be considered in programs to conserve the remaining riparian cottonwood forests in southern Alberta and elsewhere in North America. Key words: Populus, cottonwoods, hybrid poplar, reproduction, clones, suckering, seedlings.

The effects of substrate texture and rate of water table decline on the growth of a Populus balsamifera x Populus deltoides hybrid were studied using rhizopods. Rooted shoot cuttings were transplanted into growth tubes filled with gravel, sand, or a 1:1 (v/v) mixture of sand and were treated with water level declines of 0, 2, 5 or 10 cm day−1. Water drained fastest from the gravel-filled tubes, intermediate in the mixture and slowest from the sand-filled tubes. The effects of rapid water table decline were most severe on plants grown in the gravel and least severe on those grown in sand. Transpiration, height, leaf number, leaf area and plant health decreased with increasing rates of water table decline and increasing gravel content in the substrate. The reduction in transpiration and plant growth indicates that rapid water table decline caused drought stress of the poplars. Root elongation was promoted in all substrates by water table decline. The results indicate that alterations to river flow that cause abrupt drops in riparian water table will retard the transpiration and growth of riparian poplars. These effects will be more severe along flood-plains with coarse substrates.