Cottonwoodcottonwood Establishment, Survival, and Stand Characteristics

$1.50 CottonwoodCottonwood Establishment, Survival, and Stand Characteristics

EM 8800 March 2002 OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO Contents Site conditions ...... 3 Methods of reproduction ...... 4 Seedling establishment...... 5 Survival ...... 6 Stand characteristics ...... 7 Summary of life history traits ...... 8 Concluding remarks ...... 9 For more information ...... 10

Michael Borman, associate professor and Extension rangeland resources specialist, and Larry Larson, professor of rangeland resources; Oregon State University.

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ottonwood provides structural and hybridization within the section is common functional benefits to riparian areas (Farmer 1996). Black cottonwood is the C where it is adapted. Cottonwood is largest of the American poplars, with excep- adapted to specific growing conditions. If tional individuals exceeding heights of 200 you want to promote cottonwood establish- feet on the best sites (DeBell 1990). ment, it is important to understand its Much of the discussion below relates to ecology, including site characteristics and cottonwoods throughout North America conditions required for establishment and because research relating to establishment, survival. This publication is not intended to survival, and stand characteristics has been be a “how-to” guide. It is intended to conducted on a variety of cottonwood spe- describe site characteristics and the condi- cies throughout the continent. Various tions necessary for cottonwood to establish species seem to share the characteristics and sustain itself naturally. Artificially estab- discussed below. If the discussion is specific lishing cottonwood is possible and will be to black cottonwood, it is noted as such. addressed in a subsequent publication. Cottonwood belongs to the genus Site conditions Populus, which also includes aspens and Black cottonwood grows primarily on other poplars. Black cottonwood is the moist sites, the most productive of which are species native to Oregon. In most refer- bottomlands of major streams and rivers ences, the scientific name is Populus (DeBell 1990). It can grow on a variety of trichocarpa Torr. & Gray (DeBell 1990). On soils and sites, ranging from moist silts, the U.S. Department of Agriculture Natural sands, and gravels on islands and new river Resources Conservation Service Web site bars to rich humus soils, loams, and occa- (http://plants.usda.gov/), the scientific name sionally clay soils on upland sites. Optimum is Populus balsamifera L. ssp. trichocarpa (Torr. growth requires abundant moisture, nutri- & Gray ex Hook.) Brayshaw. ents, oxygen, and nearly neutral soil reaction The range of black cottonwood encom- (pH 6.0 to 7.0) (Everitt 1968, DeBell 1990). passes much of the interior and coastal Growth is best at low elevations on deep, Northwest, including Oregon, except most moist, alluvial soils (those made up of mate- of the southeast quarter of the state. The rial deposited by running water). best growth occurs in the humid coastal Black cottonwood is a drought- and forests (DeBell 1990). In the Blue Moun- shade-intolerant pioneer species (DeBell tains, black cottonwood populations are 1990). In drier areas such as eastern Oregon, found up to approximately 3,500 to 4,000 cottonwood usually is limited to protected feet elevation (Crowe and Clausnitzer 1997). valleys and canyon bottoms, along In the Wallowa Mountains, populations are streambanks, and edges of ponds and mead- found at elevations higher than 4,000 feet, ows. West of the Cascades, Farmer (1996) perhaps as a result of the more moderate cited studies that found decreased climate. branchiness and increased growth and leaf Black cottonwood, a member of the size at higher latitudes (corresponding to Tacamahaca section within the Populus genus increasing water availability from south to (Eckenwalder 1996), expresses a high degree north). Reduced height, decreased leaf size, of genetic variation, and natural and increased branchiness are associated

Cottonwood Establishment, Survival, and Stand Characteristics • 3 OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO with the drier conditions of interior river root system often is more rapid than seed- systems east of the Cascades. ling growth (Rood and Mahoney 1990). Winter wind and snow can break off Methods of reproduction branches, which may fall at the water’s edge. Cottonwoods reproduce by both seed These tree parts represent potential sources and vegetative means, including root and for new tree establishment (Braatne et al. shoot sprouting. 1996). High water may bury the branches, Reproduction by seed is a primary means either onsite or after transporting them of cottonwood establishment (Hines 1999). downstream. As the high water recedes, the Cottonwoods are dioecious; male and female branches may sprout, forming new plants. flowers (catkins) are borne on separate trees Black cottonwood also can shed (DeBell 1990). Large, mature trees yield branchlets (twigs and small branches) millions of seeds, which normally are throughout the winter and early spring as released during spring flooding. Seeds are part of a natural pruning process called extremely light and can be carried great cladoptosis (Galloway and Worrall 1979). distances by wind or water (DeBell 1990). Cladoptosis is the physiological abscission Read (1958) reported high seed viability (removal) of lateral branchlets brought about (near 100 percent) for plains cottonwood by the formation of a corky layer of young during the first 5 days following dispersal, cells at the base of a branchlet. The but viability dropped rapidly if seeds were branchlet eventually breaks off at this “ball not kept moist. Braatne et al. (1996) and socket” abscission zone. Shed branchlets described seed viability for cottonwoods as usually consist of short shoots (5 to 10 cm or short, generally lasting only 1 to 2 weeks 2 to 4 inches long) 3 or 4 years old. Gallo- under natural conditions. Once a seed way and Worrall (1979) found that black becomes wet, viability is lost in 2 to 3 days if cottonwood twigs that had been shed by a favorable microsite is not encountered. cladoptosis could form new plants, and that The ideal environment for germination such twigs could remain viable during water typically is found on moist silt, sand, or transport for great distances downstream. gravel in full sunlight along river and stream In research conducted at a variety of floodplains (DeBell 1990). sites, Galloway and Worrall found that most Vegetative reproduction (sprouting from regeneration from areas already forested roots, stumps, or branches) occurs both in with black cottonwood and understory low, moist environments and on higher conifers was from root suckers. Most repro- terraces where moisture is more limiting duction on new gravel and sand bars was (Wilson 1970). It often is associated with from seedlings. Of the rest, most originated disturbance stress caused by fire, beaver, ice from twigs and branches that had been scouring, animal browsing, or the burial or removed mechanically from the parent plant toppling of saplings during floods (Rood and (e.g., by ice, heavy snow, or wind breakage). Mahoney 1990). Sprouting ability is excel- During each of their searches, they found at lent from both roots and stumps of young least one 1- to 2-year old plant that arose trees, but declines as trees grow older. from twigs shed by cladoptosis. Regrowth from stumps with an established

4 • Cottonwood Establishment, Survival, and Stand Characteristics PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP Seedling establishment environment free of competition, a mineral Cottonwood flowering and pollination soil in which roots can maintain contact with generally coincides in the spring with rising a zone of moist substrate as waters slowly water in riparian systems. Seed development recede, and an environment that is not and dispersal follow as water levels recede subject to additional erosion, deposition, or (DeBell 1990). The timing of these events is prolonged flooding during the first growing critical to cottonwood seedling establish- season. ment. Seed germination and establishment These conditions tend to recur on a require a specific environment and have a 5-year return (or longer) interval. A 5-year narrow window of opportunity. return interval implies that a given discharge Black cottonwood germination is favored occurs in only 20 percent of the years based by conditions at the water’s edge. However, on the historical record. Actual occurrence seedlings there are threatened by flooding, can be quite variable, and one should not burial, and scouring. Seedlings at higher expect a regular 5-year spacing between point bar locations are less likely to be events. disturbed, but are more likely to succumb to From a stream classification perspective the rapid drainage of a temporarily elevated (Rosgen 1996), these conditions generally water table (Hines 1999). are seen on C and low-gradient B channels. As high flows recede, freshly deposited These streams provide colonization oppor- mineral substrate (fine sand or a fine sand/ tunities through point bar formation and the gravel mix) on active point bars provides deposition of substrate in remnant channels moist, bare conditions ideal for seed germi- that also carry floodwater. The stream nation. Moss (1938) and Noble (1979) noted gradient in this scenario likely is less than that cottonwood seed germination typically 2 percent, allowing fine sands or a sand/ occurs within 8 to 24 hours on moist sur- gravel mix to form the surface layer of faces. The seedling crop will fail if the exposed point bars. Skeletal layers of mixed surface dries during the first several days and coarse material are found beneath, after germination. Moisture must be avail- having been deposited during periods of able in the upper layer of soil for 1 to higher stream velocity. The low gradient 3 weeks because seedling root growth is suggests that floodwaters tend to pond slow. Seedling water absorption depends within such stream reaches and then recede largely on a brush of delicate hairs (collet- more slowly than on steeper gradient hairs) located near the soil surface. The streams. This sequence of events might collet-hairs also anchor the plant by attach- occur only once in 5 to 10 years or longer ing to sand particles, making seedling (Bradley and Smith 1986, Hines 1999). displacement difficult. As a river or stream migrates, it erodes The best conditions for establishment sediments from the concave side of mean- are associated with sustained high flows ders and deposits them downstream on the during the establishment period (Bradley convex side, creating point bars (Leopold and Smith 1986). Noble (1979) suggested 1994). Point bars are potential cottonwood that a halt in the drop of water enhances habitat, as are migrating edges of seedling survival by keeping seedlings moist. midchannel islands in larger rivers (Barnes This combination of conditions yields an 1985, Everitt 1968).

Cottonwood Establishment, Survival, and Stand Characteristics • 5 OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO Survival sapling against all but extreme floods. As Cottonwoods are susceptible to both point bars expand laterally and build up extended drought and flooding. Young vertically, the saplings will be in less danger plants are especially susceptible to drought of extreme flooding (Bradley and Smith when the water table drops below their 1986). rooting zone. This is a major cause of seed- Because seedlings on point bars are ling death on point bars that have built up highly vulnerable to flooding and scouring, too high and on steep stream gradients Hines (1999) postulated that the majority of where water levels can drop faster than roots colonization on point bars results from can grow. Most seedling mortality occurs in vegetative reproduction as opposed to seed. late summer as sites begin to dry and stream Barnes (1985) suggested that colonization levels and water tables drop (Rood and depends on sprouting and resprouting of Mahoney 1990, Stromberg and Patten seedlings rather than the number of success- 1996). ful instances of seedling establishment. There exists a critical elevation zone on Vegetative propagation allows immediate point bars where seedbeds are high enough local colonization of cottonwood following to avoid drowning, but low enough to avoid the successful establishment of a few drying out. By the end of the first growing seedlings. season, cottonwood seedlings lower on the Cottonwood seedlings remain vulnerable point bar have a better chance of surviving to drought in the year following establish- than those higher on the point bar if the ment because of their small root systems. higher positioned seedlings’ roots have not Growth accelerates during the second year kept pace with the lowering water table. and, after 2 years, roots may be almost However, rapidly rising floodwaters can 3 meters long (Ware and Penfound 1949). scour and uproot young cottonwoods; The longer roots improve the sapling’s therefore, seedlings on point bar surfaces ability to tolerate flooding and drought high enough to escape mortality from flood- stress (Pezeshki and Hinckley 1988). ing during the first few years following Growth rates tend to slow after this recruitment have a better chance of surviv- initial growth period and then remain rela- ing than those at lower elevations. tively constant for several decades. Flower- Hosner (1958) reported that plains ing occurs after about 7 years (Rood and cottonwood seedlings survived a period of Mahoney 1990). 8 days of inundation, but most died after Juvenile and mature trees, while less 16 days. Black cottonwood has been susceptible to drought, can show signs of observed to persist under longer periods of pruning, leaf drop, and yellowing due to inundation (David Hibbs, professor, Forest cavitation (formation of air bubbles in water- Science Department, Oregon State Univer- transporting tissue). Extended periods of sity, personal communication). drought will result in stunted growth and/or If cottonwood seedlings escape mortality death in both juvenile and mature trees from desiccation or flooding during the first (Albertson and Weaver 1945, Rood and few years of growth, their chances of survival Mahoney 1990). probably are very good. Roots will have Cottonwood has several adaptations that grown enough to ensure a stable water allow it to survive floods, but it is not as well supply during dry periods and to anchor the adapted to prolonged flooding as some

6 • Cottonwood Establishment, Survival, and Stand Characteristics PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP riparian species. Mature cottonwoods typi- flooded roots than through lenticels. Conse- cally show signs of stress when flood condi- quently, species utilizing aerenchyma tend to tions last more than a few weeks (Neuman et dominate on sites that are flooded for al. 1996). Cottonwood often is associated extended periods of time and on flooded with soils that contain a layer of coarse soils composed primarily of silt and clay- substrate (Crowe and Clausnitzer 1997). sized particles. These soils drain more quickly than fine- Cottonwoods are susceptible to a num- textured soils, reducing the length of time a ber of other damaging agents. Seedlings and root system must survive in a flooded envi- young saplings can be injured or killed by ronment (Richardson and Vepraskas 2001). unseasonably early or late frosts (Crowe and The roots on mature trees survive Clausnitzer 1997, DeBell 1990). A variety of flooded conditions by utilizing anaerobic insects and fungi can cause problems for respiration (respiration without oxygen) to black cottonwoods, but most damage has continue essential metabolic functions. been observed in plantations rather than in However, anaerobic respiration cannot be natural stands, especially if the trees are continued indefinitely. It is roughly vigorous (DeBell 1990). Excessive browsing 20 percent as efficient as aerobic (oxygen- by wildlife and livestock can suppress both based) respiration, and it produces toxic seedlings and sprouts (Crowe and by-products that accumulate within plant Clausnitzer 1997). Seedlings and saplings are tissue. Reliance upon anaerobic respiration highly susceptible to intense fire because of requires a slowing or stoppage of plant their thin bark and shallow root systems growth and is limited by the amount of (Crowe and Clausnitzer 1997). After 10 to carbohydrate reserves stored within the 20 years, thicker bark may provide some roots and by the subsequent accumulation of protection against low-intensity fires, but toxic compounds. fire wounds can allow heartwood decay to A second way that cottonwood over- begin (Crowe and Clausnitzer 1997). comes the lack of oxygen in flooded soils is Sprouting from stumps can follow top kill by through the presence of shallow adventitious fire. roots and lenticels along the stem and root crown area (Nilsen and Orcutt 1996). Len- Stand characteristics ticels are small cracks or pores in the bark. Maintenance of a riparian cottonwood Oxygen entering the tree through these forest depends on establishment of new trees pores migrates toward areas of low oxygen to compensate for those that die. Forests concentration. In most cases, oxygen sup- rely principally on seedling establishment plied to adventitious roots enters via len- for expansion to keep up with river and ticels or is obtained directly from shallow, stream migration. As discussed above, suc- oxygenated layers within the soil. All of cessful seedling establishment does not these adaptations can take place within the occur regularly, as it requires complemen- plant at the same time but in different por- tary seed production and hydrologic events. tions of the root system. Thus, we often find arc-shaped bands of Aerenchyma, a special type of oxygen- even-aged trees parallel to the river channel. conducting tissue found in herbaceous Each band contains trees similar in height species such as sedges and rushes, is a much and size because a large number of seedlings more efficient means of delivering oxygen to

Cottonwood Establishment, Survival, and Stand Characteristics • 7 OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO tend to become established at the same time In a study of vegetation dynamics of and then thin as the band of trees matures. woody plants (including cottonwood) on an Everitt (1968) described bands of even- island in Wisconsin, the height of woody aged trees on point bars that could be associ- species seemed to be correlated with island ated with specific hydrologic events. He elevation (Barnes 1985). Height seemed to noted that tree age increased with distance be a result of time elapsed since the last from the channel. Each band began as a disturbance rather than of the growth rate. thicket of seedlings on a bar at the channel’s Height generally indicated the average edge and was “transported” inland as the length of time since the last resprouting channel migrated and more trees were from a damaged clone. Island elevation was established on newer sandbars. related to the frequency of disturbance, at Within these bands, asexual (vegetative) least by flooding, and plants that were at reproduction through suckering and stump higher elevation were less likely to sustain regrowth may contribute to forest mainte- damage. nance (Rood and Mahoney 1990). If vegeta- Riparian cottonwoods often have a tive reproduction does not occur, old cotton- ragged shape because of the death of wood stands eventually die out and are branches during dry periods. This may be an replaced by other species. Observations adaptation to deal with occasional droughts. indicate that this is the more common The plant apparently drops branches during outcome (Hibbs, personal communication). periods of limited water supply to reduce the As discussed above, cottonwood estab- shoot mass and leaf area supported by the lishment is determined largely by reach type root system (Rood and Mahoney 1990). and stream flow. Differences in geology, Longevity of mature cottonwoods is elevation, climate, and tributary influence influenced by environmental factors, espe- (Frissell et al. 1986) produce differences in cially drought stress. During the drought of cottonwood distribution along various the 1930s, many older cottonwoods died reaches of the same stream. Thus, high flow across the western prairies (Albertson and along one reach might promote cottonwood Weaver 1945). The largest and oldest trees establishment, while an equivalent flow generally were found on the most favorable along a second reach might scour away sites, where drought stress was avoided. established cottonwood (Everitt 1968, Bradley and Smith 1986). Summary of life history traits Conditions for cottonwood establish- P Black cottonwood can reproduce either ment and survival also vary throughout a by seed or vegetative means. basin as flood frequency, timing, and magni- P Reproduction by seed is the primary tude interplay with the structure of stream means of initial establishment. Trees channels (Asplund and Gooch 1988, Scott et produce millions of seeds, which are very al. 1996, 1997). Conditions vary across time light and can be carried long distances by as well, occurring at irregular intervals of wind or water. Seed viability is high for about 5 to 10 years (Bradley and Smith 1 to 2 weeks, then drops off rapidly if 1986, Braatne et al. 1996). As a result, a seeds are not kept moist. Once a seed shifting mosaic of cottonwood patches in the becomes wet, viability is lost in 2 or riparian landscape reflects the pattern of 3 days if a favorable microsite is not stream disturbance cycles.

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encountered. Reproduction as seedlings P Cottonwoods can tolerate substantial tends to occur on moist sand, gravel bars, flooding, but not as well as many herba- or scoured streambanks. ceous riparian species. Cottonwood trees P Vegetative regeneration typically is survive flooded environments through associated with disturbance stress caused anaerobic respiration, the production of by fire, beaver, ice scouring, animal shallow adventitious roots, and the browsing, or burial or toppling of sap- presence of lenticels along the stem and lings during floods. It also can be a root crown. Cottonwood often is associ- means of colonizing bare surfaces when ated with soils that contain a layer of conditions do not permit establishment coarse substrate, which drains more by seed. As distance from the stream quickly than fine-textured soils, provid- increases, shoot suckers tend to be more ing a quick return to aerobic conditions. prevalent than seedlings, probably due to P Cottonwood stands tend to form as the increased presence of established separate, arc-shaped bands of even-aged trees farther from the water’s edge. trees on point bars. The age of each band P Black cottonwood is a drought- and increases with distance from the channel shade-intolerant pioneer species. because of channel migration. Older, larger trees tend to be farther from the P Long-term survival generally is associ- water’s edge than younger, smaller trees ated with high flows during the period of and seedlings. establishment. These discharges tend to P recur on a 5- to 10-year (or longer) Cottonwood establishment is determined return interval. As these flows recede, the largely by reach type and stream flow. large amounts of freshly deposited min- Differences in geology, elevation, cli- eral substrate (fine sand or a fine sand/ mate, and tributary influence can exist gravel mix) on active point bars provide between two reaches of the same stream. moist conditions ideal for seed germina- Thus, cottonwood distribution also tion. The environment must be free of varies among different reaches of the competition, consist of a mineral soil in same stream. which roots can maintain contact with moist substrate as waters slowly recede, Concluding remarks and be free of additional erosion, deposi- A riparian corridor is a complex mosaic tion, or prolonged flooding during the of moisture and disturbance patterns. Plants first growing season. that form communities within these corri- P Cottonwood seedlings remain vulnerable dors survive on sites where their basic to drought in the year following estab- requirements for establishment, growth, and lishment because of their small size and reproduction are satisfied. Restoration small root systems. Growth during the efforts in riparian areas require an under- second year is more rapid than during standing of both the environmental mosaic the initial year. After the first 2 years, and the life history/adaptations of riparian cottonwood saplings become increas- species. ingly tolerant of flooding and drought stress as they develop larger root systems.

Cottonwood Establishment, Survival, and Stand Characteristics • 9 OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO References Eckenwalder, J.E. 1996. Systematics and evolution of Populus. In: Biology of Albertson, F.W. and J.E. Weaver. 1945. Populus and Its Implications for Manage- Injury and death or recovery of trees in ment and Conservation. R.F. Stettler, prairie climate. Ecological Monographs G.A. Bradshaw, P.E. Heilman, and 15:395–433. T.M. Hinckley, eds. (NRC Research Asplund, K.K. and M.T. Gooch. 1988. Press, Ottawa, Ontario, Canada). Geomorphology and the distributional Everitt, B.L. 1968. Use of the cottonwood in ecology of Fremont cottonwood (Populus an investigation of the recent history of a fremontii) in a desert riparian canyon. flood plain. American Journal of Science Desert Plants 9:17–27. 266:417–539. Barnes, W.J. 1985. Population dynamics of Farmer, R.E. 1996. The genecology of woody plants on a river island. Canadian Populus. In: Biology of Populus and Its Journal of Botany 63:647–655. Implications for Management and Con- Braatne, J.H., S.B. Rood, and P.E. Heilman. servation. R.F. Stettler, G.A. Bradshaw, 1996. Life history, ecology and conserva- P.E. Heilman, and T.M. Hinckley, eds. tion of riparian cottonwoods in North (NRC Research Press, Ottawa, Ontario, America. In: Biology of Populus and Its Canada). Implications for Management and Con- Frissell, C.A., W.J. Liss, C.E. Warren, and servation. R.F. Stettler, G.A. Bradshaw, M.D. Hurley. 1986. A hierarchical P.E. Heilman, and T.M. Hinckley, eds. framework for stream habitat classifica- (NRC Research Press, Ottawa, Ontario, tion: Viewing streams in a watershed Canada). context. Environmental Management Bradley, C.E. and D.G. Smith. 1986. Plains 10:199–214. cottonwood recruitment and survival on Galloway, G. and J. Worrall. 1979. a prairie meandering river floodplain, Cladoptosis: a reproductive strategy in Milk River, southern Alberta and north- black cottonwood? Canadian Journal of ern Montana. Canadian Journal of Forest Research 9:122–125. Botany 64:1433–1442. Hines, C.A. 1999. Evaluating the Restora- Crowe, E.A. and R.R. Clausnitzer. 1997. tion Potential of Black Cottonwood Mid-Montane Wetland Plant Associa- (Populus trichocarpa) from Multiple Scales tions of the Malheur, Umatilla and of Observation, Grande Ronde River Wallowa-Whitman National Forests, Basin, Oregon, USA (M.S. Thesis, Technical Paper R6-NR-ECOL-TP- Oregon State University, Corvallis, 22-97, pp. 85–95 (U.S. Forest Service, Oregon). Pacific Northwest Region, U.S. Depart- ment of Agriculture). Hosner, J.F. 1958. The effects of complete inundation upon seedlings of six bottom- DeBell, D.S. 1990. Populus trichocarpa Torr. land tree species. Ecology 39:371–373. & Gray.; Black Cottonwood. In: Silvics of North America, volume 2, Hard- Leopold, L.B. 1994. A View of the River woods. R.M. Burns and B.H. Honkaka, (Harvard University Press, Cambridge, eds. pp. 570–576 (U.S. Forest Service, Massachusetts). U.S. Department of Agriculture).

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Moss, E.H. 1938. Longevity of seed and Rood, S.B. and J.M. Mahoney. 1990. Collapse establishment of seedlings in species of of riparian poplar forests downstream Populus. Botanical Gazette 99:529–542. from dams in western prairies: Probable Neuman, D.S., M. Wagner, J.H. Braatne, and causes and prospects for mitigation. J. Howe. 1996. Stress physiology—abiotic. Environmental Management 14:451–464. In: Biology of Populus and Its Implications Rosgen, D. 1996. Applied River Morphology for Management and Conservation. (Wildland Hydrology, Pagosa Springs, R.F. Stettler, G.A. Bradshaw, Colorado). P.E. Heilman, and T.M. Hinckley, eds. Scott, M.L., J.M. Friedman, and G.T. Auble. (NRC Research Press, Ottawa, Ontario, 1996. Fluvial process and the establish- Canada). ment of bottomland trees. Geomorphol- Nilsen, E.T. and D.M. Orcutt. 1996. Physiol- ogy 14:327–339. ogy of Plants Under Stress (John Wiley & Scott, M.L., G.T. Auble, and J.M. Friedman. Sons, Inc., New York). 1997. Flood dependency of cottonwood Noble, M.G. 1979. The origin of Populus establishment along the Missouri River, deltoides and Salix interior zones on point Montana, USA. Ecological Applications bars along the Minnesota River. The 7:677–690. American Midland Naturalist 102:59–67. Stromberg, J.C. and D.T. Patten. 1996. Pezeshki, S.R. and T.M. Hinckley. 1988. Instream flow and cottonwood growth in Water relations characteristics of Alnus the eastern Sierra Nevada of California, rubra and Populus trichocarpa: Responses to USA. Regulated Rivers: Research and field drought. Canadian Journal of Forest Management 12:1–12. Research 18:1159–1166. Ware, G.H. and W.T. Penfound. 1949. The Read, R.A. 1958. Silvical Characteristics of vegetation of the lower levels of the Plains Cottonwood, Station Paper No. 33 floodplain of the South Canadian River in (U.S. Forest Service, Rocky Mountain central Oklahoma. Ecology 30:478–484. Forest and Range Experiment Station, Wilson, R.E. 1970. Succession in stands of U.S. Department of Agriculture). Populus deltoides along the Missouri River Richardson, J.L. and M.J. Vepraskas. 2001. in southeastern South Dakota. The Wetland Soils (Lewis Publishers, Boca American Midland Naturalist 83:330–342. Raton, Florida).

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Published March 2002.