Fire Effects Information System (FEIS)

FEIS Home Page

Populus alba and hybrids

• INTRODUCTORY
• DISTRIBUTION AND OCCURRENCE
• BOTANICAL AND ECOLOGICAL CHARACTERISTICS
• FIRE EFFECTS AND MANAGEMENT
• MANAGEMENT CONSIDERATIONS
• APPENDIX: FIRE REGIME TABLE
• REFERENCES

INTRODUCTORY

AUTHORSHIP AND CITATION FEIS ABBREVIATION NRCS PLANT CODE COMMON NAMES TAXONOMY SYNONYMS LIFE FORM
	©Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Hybrids:
Populus × canescens (Aiton) Sm. (gray poplar), a cross between P. alba × P. tremula (European aspen)
Populus × heimburgeri B. Boivin (Heimburger's poplar), a cross between P. alba × P. tremuloides (quaking aspen)
Populus × rouleauiana B. Boivin (Roulwau's poplar), a cross between P. alba × P. grandidentata (bigtooth aspen) ([39], Boivin 1966 cited in [102])
Populus tomentosa Carrière (Chinese white poplar), a cross between P. alba × P. adenopoda (Chinese aspen) [30]

White poplar hybrids with European aspen, quaking aspen, and bigtooth aspen occur primarily east of the Great Plains region of North America [39]. However, hybrids are possible in any area where the parent species' distributions overlap. Hybridization has the potential to affect white poplar invasiveness. This is discussed later in the seed production, dispersal, vegetative spread, and potential impacts sections. Populus alba × P. adenopoda hybrids have been planted in the southeastern United States, although rarely [30].

White poplar hybrids are identified using scientific parent names in this review.

DISTRIBUTION AND OCCURRENCE

• GENERAL DISTRIBUTION
• HABITAT TYPES AND PLANT COMMUNITIES

In North America, white poplar hybrids are known only east of the Great Plains. Populus alba × P. tremula is the most widely distributed hybrid; it occurs in nearly all states and provinces east of the north-south area from western Ontario to Louisiana [114]. Hybrids with native aspens (quaking aspen and bigtooth aspen) occur primarily in the Great Lakes region [101], although P. alba × P. grandidentata has been reported in West Virginia [114]. Populus alba × P. tremuloides is less widely distributed and less common than P. alba × P. grandidentata [100,114,119]. Plants Database provides distributional maps of white poplar and its hybrids.

White poplar was reported in New England by 1785 (Cutler 1785 cited in [102]), and after its introduction, planting was widespread. By the late 1800s, it was noted in many US floras. White poplar occurred in Michigan by 1876 (Almendinger 1876 cited in [102]), in southwestern Texas by 1879 (Watson 1883 cited in [102]), and on Block Island, Rhode Island, by 1892 [4]. In North Dakota and Montana, it was planted by ranchers and farmers for windbreaks [111].

Because white poplar spread is generally limited to clonal growth in the absence of hybridization (see Seed production), the distribution and spread of white poplar is directly related to human plantings [100]. Widespread planting has given white poplar a wider distribution than native aspens in the United States (Barnes personal communication cited in [102]). In northern Cape Breton, Nova Scotia, white poplar is largely restricted to areas where it was planted as an ornamental. It persists through vegetative sprouting [103]. In Farmington, western Maine, the spread of escaped white poplars was easily traced back to areas where white poplar was planted [6]. Throughout the Georgia Piedmont, the distribution of white poplar was directly correlated with the relative number of residences [23]. The composition of the deciduous Black Rock Forest in southeastern New York was compared from 1930 to 2006. White poplar was first reported in the study area in the 1990s. Although researchers indicated that white poplar "invaded naturally", they did not speculate on the establishment method and failed to report the location of the nearest white poplar population [90].

Native habitats: In western and central Europe, white poplar occurs in riparian, forest-steppe, and coastal communities. Gravel bars and hardwood floodplain woodlands are common white poplar habitats [27,63,66,94]. In Hungary, white poplar occurs in forest-steppe vegetation characterized by sandy grasslands with patches of white poplar, Lombardy poplar (P. nigra), and common juniper (Juniperus communis) [79]. White poplar sometimes forms thickets on coastal cliffs in southeastern England [26].

Additional information about white poplar's nonnative and native habitats is available in Site Characteristics and Successional Status.

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

GENERAL BOTANICAL CHARACTERISTICS SEASONAL DEVELOPMENT REGENERATION PROCESSES SITE CHARACTERISTICS SUCCESSIONAL STATUS
	Photo © Richard Old, XID Services, Inc., Bugwood.org		Photo © Paul Wray, Iowa State University, Bugwood.org

White poplar is typically a small tree but may reach 130 feet (40 m) tall and 7 feet (2 m) in diameter [24,47,107]. The crown is open, wide, and rounded [18,47,105,122]. Low branches are reportedly very persistent [102]. Sometimes the white poplar trunk is divided at the base [39], and often it is crooked [102]. Bark is smooth on young trunks [39] but becomes rough on the lower portion of mature trunks [51]. White poplar is short-lived [13,18,117], and its wood is weak and prone to storm breakage [13].

White poplar leaves are alternate, glossy blue-green above, and white with dense hairs below [18,24,47,91,105]. Early leaves are nearly oval and have margins with rounded teeth [28]; mature leaves have 3 to 7 blunt lobes [35,91] with toothed to wavy margins [39,105]. Leaves measure 1 to 5 inches (3-12 cm) long and are longer than they are wide [71,122], but leaf morphology can be variable [117]. White poplar hybrids produced smaller leaves on dry sandy sites than on mesic or wet sites [102].

White poplar is dioecious. Most white poplars introduced to North America are female. Catkins are dense and support about 50 individual flowers [26]. Pistillate catkins measure 1.5 to 3 inches (4-7 cm) long; staminate catkins measure 2 to 4 inches (5-10 cm) long [105]. Because male white poplar trees are rare and generally not available to fertilize female trees, fruit and seed production are typically restricted to white poplar hybrids in North America [101]. White poplar fruits, although not typically produced outside of hybridization, are 3- to 5-mm capsules that typically contain 2 seeds [39,71]. White poplar seeds are tiny (up to 1.5 mm long) with long, fine hairs at the base [26,105].

Hybrids: White poplar hybrids are difficult to distinguish from parents in the field. Molecular markers are the best way to avoid misclassification of white poplar hybrids [31]. Descriptions of P. alba × P. grandidentata and P. alba × P. tremuloides hybrids are available in the following references: [5,30,102]. Populus alba × P. adenopoda is described in the Flora of North America [30].

		Belowground description: Researchers excavated and described the root system of white poplars growing on Rhinau Island in eastern France. Root density was greatest at 12- to 24-inch (30-60 cm) depths. At these depths, some roots were greater than 2 inches (5 cm) in diameter. Root abundance in the top 4 inches (10 cm) of soil increased with increasing distance from the trunk. Three feet (1 m) away from the trunk, white poplar roots reached the gravel layer, which was 43 inches (110 cm) deep. Excavation did not occur beyond the gravel layer [89]. White poplar is capable of prolific root sprouting, which allows for long-term persistence [13,18].
Photo © Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

White poplar root sprouts have been described as prolific [102], "vigorous" [73], and "objectionable" [68]. Dense colonies or thickets from root sprouts [60,119,122] can cover large areas [105]. When white poplar seedlings were grown and evaluated as shelterbelt trees in the northern Great Plains, root suckers were frequently reported long distances from the parent tree [33]. According to a review by Spies [102], vegetative sprouts can occur up to 160 feet (50 m) from the parent. In southeastern Michigan, the average size of individual P. alba × P. grandidentata or P. alba × P. tremuloides clones was 0.02 to 0.5 acre (0.01-0.2 ha). The average number of stems per clone ranged from 1 to 404. Populus alba × P. grandidentata sprouted more "vigorously" than bigtooth aspen or quaking aspen [102]. Surveys of white poplar stands on the Mediterranean island of Sardinia showed that most sites supported single-sex ramets from a single parent that in some cases, formed linear riparian stands extending several kilometers. Four monoclonal stands ranged from 38.6 mile² (100 km²) to over 1,500 mile² (4,000 km²). Fertile seed occurred in just 1 of 80 sampling sites, and no seedlings were observed at any site [10]. In lowland floodplain forests along the Danube in Austria and Slovakia, P. alba × P. tremula and white poplar clones extended over distances of 610 feet (186 m) and 430 feet (132 m), respectively. Researchers indicated that these were likely conservative clone size estimates, since trees were not exhaustively sampled [116].

Vegetative regeneration from root sprouts is important to white poplar persistence and recovery from injury. White poplar produces shoots from surviving roots long after the original parent tree has died [47,102,105]. In 1888, white poplar cuttings were planted at the Grayling Agricultural Experiment Station in Crawford County, Michigan. During visits to this planting site between 1998 and 2000, root sprouts from the original plantings were still present [62]. In Iowa, 2 studies from the 1950s reported that P. alba × P. grandidentata clones persisted and spread vegetatively from 2 or more seedlings that established in the early 1900s [67,72]. Sprouting is rapid and prolific following the death of white poplar trees or stems [73,84,105]. This regenerative potential, however, may decrease with tree age, according to a review of the Populus genus by Dickmann [17]. These studies and findings suggest that although white poplar is short-lived, once planted this species can permanently occupy its original planting site or a much larger area.

Studies indicate that white poplar may disperse and establish new populations from vegetative fragments [17,46], but most studies testing the regenerative capacity of white poplar involve experiments with somewhat artificial conditions. In a controlled study involving plant material from southeastern Canada, the vegetative regeneration potential of white poplar and P. alba × P. grandidentata appeared to be much greater than that of bigtooth aspen. About 90% of white poplar cuttings rooted and 65% to 98% of hybrid cuttings rooted, but only about 5% of bigtooth aspen cuttings rooted [56]. From P. alba × P. grandidentata plant material collected in southeastern Iowa, researchers found that root segments greater than 0.5 inch (1.3 cm) in diameter and 2 inches (5 cm) long often produced more than 1 sprout/root segment. When root segments were planted in unseasonably warm, dry weather, very few sprouted. However, some of the root segments sprouted the following growing season [42].

Pollination and breeding system: White poplar flowers are wind pollinated [18,116]. However, because male white poplar trees are rare, successful pollination of white poplar generally requires hybridization with European aspen, bigtooth aspen, or quaking aspen ([28,101], Schlenker 1953 cited in [29]). Hybridization and backcrossing between white poplar and native aspens could cause nonnative gene introgression into the native gene pool [102,103]. Studies in southeastern Michigan, however, did not find widespread backcrossing or gene flow between white poplar and bigtooth aspen [100].

Hybridization between white poplar and native and nonnative aspens may be affected by the phenology and location of parent populations. In southeastern Michigan, P. alba × P. grandidentata is much more common than P. alba × P. tremuloides, and nearly all hybrids occur to the east of white poplar clones. The easterly distribution of hybrids was expected because of prevailing western winds, but based on the distribution and abundance of parent species, researchers expected a 1:1 ratio of P. alba × P. grandidentata and P. alba × P. tremuloides hybrids. A single season of observations did not reveal a phenological barrier to hybridization between white poplar and quaking aspen. However, researchers noted that female receptivity is difficult to observe and suggested that the degree of phenological differences and overlap may have been missed in just one season of observations [101,102].

In its native habitats, P. alba × P. tremula backcrossed with white poplar more often than with European aspen. Along the Ticino River in northern Italy, P. alba × P. tremula backcrossed with white poplar but not with European aspen. Researchers suspected their findings were related to the distance to European aspen trees or other preferential backcrossing, although these factors were not investigated [31]. Similar findings were reported in the Danube Valley near Vienna, Austria. Most hybridization occurred between white poplar females and European aspen males, and most backcrossing occurred with white poplar. Because European aspen produced male flowers several weeks earlier than white poplar, European aspen may have fertilized white poplar before white poplar pollen was shed [66].

Seed production: Several floras report that white poplar fails to produce seed [13,18,25]. In North America, most planted white poplar were female [101]; however, in a survey of eastern herbia, 5 of 8 had male white poplar branches in their collection [102]. In North America, most seed production by white poplar occurs through hybridization with bigtooth aspen, quaking aspen, or European aspen ([101], Schlenker 1953 cited in [29]). In southeastern Michigan, all white poplar clones surveyed were female. White poplar hybrids, however, were male, female, or hermaphroditic with separate staminate and pistillate catkins (review by [101], Schlenker 1953 cited in [29]).

Although white poplars generally become sexually mature at 5 to 7 years old (Braatne and others 1996 cited in [75]), one genotype grown from seed collected along the Italian peninsula flowered at 1 year old. Subsequent clones regenerated from this genotype failed to flower in their first year without at least 6 months of root chilling treatments [75].

White poplar hybrids can produce an abundance of viable seed. When white poplar and P. alba × P. grandidentata clones were pollinated openly in a greenhouse, white poplar averaged 23.9 seeds/shoot, and P. alba × P. grandidentata averaged 415.9 seeds/shoot [102]. All P. alba × P. grandidentata, P. alba × P. alba × P. grandidentata, P. alba × P. grandidentata × P. tremula, and P. alba × P. grandidentata × P. alba × P. tremula hybrids that were experimentally produced in Ottawa produced viable seed. Seed set was best from crosses between white poplar and bigtooth aspen [57].

Seed dispersal: White poplar and its hybrids produce light-weight, plumose, wind-dispersed seeds ([116], Graham and others 1963 cited in [102]). In the greenhouse, the average weight of seeds produced by 9 open-pollinated white poplar clones was 0.177 mg/seed, and the average weight of seeds produced by P. alba × P. grandidentata clones was 0.199 mg [102]. Although cottonwood (Populus spp.) seeds have been reported to disperse 19 miles (30 km) or more (van der Pijl 1972 cited in [101]), surveys in southeastern Michigan revealed that most (91%) white poplar hybrid seedlings occurred within 1 mile (1.6 km) of white poplar clones [101].

Seed banking: Although field experiments are lacking, white poplar and hybrid seeds are reportedly very short-lived (England Forestry Commission Booklet [26]). A review reports that seed bank longevity is low for Salicaceae [58].

Germination: The optimal conditions for germination of white poplar and hybrid seeds were not reported. A review reports that within Salicaceae, germination is rapid and often occurs within 24 hours of seed shed. Germination percentages are drastically reduced in dry conditions, but germination occurs in moist conditions, at warm temperatures (59-81 °F (15-27° C)), and in dark environments [58]. Soaking reduced the germination of white poplar seeds in the laboratory. Just 23.4% of dry seeds failed to germinate; up to 34.2% of soaked seeds failed to germinate. Nearly 65% of dry white poplar seeds germinated without abnormalities (poor substrate attachment and imperfect geotropism). After soaking 1 to 60 minutes, just 24% to 27% of germinated seedlings lacked abnormalities. Duration of soaking did not greatly affect germination [82]. The survival of abnormal seedlings was not reported, but it could be expected that survival of abnormal seedlings was less than that of normal seedlings.

Germination of white poplar hybrid seeds can be high. When white poplar and P. alba × P. grandidentata were openly pollinated in a greenhouse, germination of seeds collected from white poplar averaged 34.7%, and germination of seeds collected from P. alba × P. grandidentata averaged 81.8% [102]. All P. alba × P. grandidentata, P. alba × P. alba × P. grandidentata, P. alba × P. grandidentata × P. tremula, and P. alba × P. grandidentata × P. alba × P. tremula hybrids that were experimentally produced in Ottawa produced viable seed. Germination was best (61%) for seed produced by crosses between white poplar and bigtooth aspen. Germination of seed produced by the other crosses ranged from 21% to 40% [57].

Seedling establishment and plant growth: In the field, seedling recruitment is generally limited to white poplar hybrids. Information on nonhybrid white poplar seedling establishment is generally limited to studies conducted on trial plantings or in plantations. Establishment of white poplar and hybrid seedlings is likely best on sites with exposed mineral soil that lack other established vegetation.

Although nonhybridized white poplar seedlings are considered unlikely, researchers reported that white poplar "seedlings have been observed to freely colonize neighboring ruderal sands" on western Fire Island in Suffolk County, New York. Bigtooth aspen and quaking aspen were also reported on the island [22], suggesting that seedling recruitment may have been the result of hybridization between white poplar and native aspens. White poplar hybrids can produce seedlings. Many hybrids and backcrosses were experimentally created between white poplar, bigtooth aspen, and quaking aspen. All hybrids produced seed, and seedling survival was at least 29% [57]. When 10-week-old, greenhouse-grown white poplar and P. alba × P. grandidentata seedlings were transplanted outdoors, about 70% of white poplar and 93% of hybrid seedlings survived to the end of the growing season [102].

Excessively dry conditions, harsh winters, and established vegetation may limit survival and growth of white poplar and hybrid seedlings. When white poplar seedlings from Xinjiang, China, were planted at the Northern Great Plains Field Station in Mandan, North Dakota, most failed to survive longer than 10 years. Only 1 seedling survived more than 10 years, and it did not survive 36 years. Dry conditions and winter injury were the most common causes of mortality [33].

In southeastern Michigan, open sites with exposed mineral soil were best for the establishment of white poplar hybrid seedlings. In the Walsh Lake study area in Washtenaw County, P. alba × P. grandidentata established during a 9-year period following agricultural abandonment. In Livingston County, P. alba × P. grandidentata and P. alba × P. tremuloides established at the edge of lakes, ponds, and swamps and on dry, disturbed sites. Both kinds of sites had experienced disturbances that exposed mineral soil. In both counties, hybrid seedling establishment decreased as old-field and floodplain succession progressed [101].

Plant growth: Rapid growth is characteristic of white poplar and its hybrids [70,102]. While hybrids may grow faster than parent species, differences in growth rate may vary by genotype, site, and/or clone age [55,56,86]. Discoveries of rapid tree growth and biomass production by white poplar hybrids in the Great Lakes contributed to increased recommendations for planting hybrids on plantations and in wildlands [40,43,43,72].

Rapid growth of white poplar and its hybrids has been recorded in their native and nonnative ranges. Along the Henares River floodplain in Madrid, Spain, white poplar clones almost doubled in size in 5 years. In 4-year-old plantations, white poplar averaged 2.8 inches (7.2 cm) in DBH and 16 feet (4.9 m) tall. In 9-year-old plantations, white poplar averaged 6.5 inches (16.5 cm) in DBH and 30 feet (9.2 m) tall [70]. In southeastern Michigan, P. alba × P. grandidentata clones that were 28 to 53 years old had annual height increases of 1.1 to 2.3 feet (0.3-0.7 m) and annual DBH increases of 0.2 to 0.4 inch (0.5-1 cm) [102].

In North America, white poplar hybrids are often larger than their parent species of the same age. However, this was not the case in Hungary, where white poplar and P. alba × P. grandidentata grown in a common area were nearly the same size at 3, 7, and 10 years old. Often the maximum DBH reported for white poplar clones exceeded that of hybrids [86]. In a common area in southeastern Canada, P. alba × P. grandidentata was larger than both parent species of the same age. At 5 years old, average stem height was 14 feet (4.3 m) for white poplar clones, 11.7 feet (3.6 m) for bigtooth aspen clones, and 17.6 feet (5.4 m) for P. alba × P. grandidentata clones [56]. The clonal growth capacity of white poplar hybrids and parent species are compared in the vegetative regeneration discussion above. In Quebec, P. alba × P. grandidentata and P. alba × P. tremuloides were typically larger than the parent species when trees were 13 to 19 years old. In young age classes (6-7 years old), hybrid size was much more variable than parent species' sizes, and the maximum height and DBH were generally largest for hybrid clones [55].

In southeastern Michigan, most white poplar hybrids were found around current or former farms. No hybrids occurred in forests considered relatively undisturbed by humans. Nearly half of the hybrids occurred on sparsely vegetated, dry, sandy sites. The other half occurred on mesic sites, often at the edges of lakes, ponds, and swamps [101].

Climate: White poplar occurs in areas as far north as USDA hardiness zone 3, where average annual minimum temperatures can reach -40 °F (4 °C) [19,28,68]. However, some report that white poplar may be killed or injured by extremely low winter temperatures [19,33], suggesting that white poplar may be restricted to protected sites in its northernmost habitats. In a review, Spies [102] reports that white poplar is most common at low elevations where temperatures are moderate and moisture is favorable. Although white poplar commonly occupies moist habitats, it can also occupy upland, somewhat droughty habitats [17]. In Hungary, white polar occurs in semiarid forest-steppe vegetation that grows in a temperate continental climate with annual precipitation averaging 20 to 22 inches (500-550 mm) [79].

In growth chamber experiments, researchers found P. alba × P. grandidentata growth was best when soil moisture was 16% to 30% and temperatures were 77 to 95 °F (25-35 °C) during the day and 59 to 77 °F (15-25 °C) at night. The parameters necessary for interpreting soil moisture values in this study include a level of 8.6%—which was slightly above the permanent wilting point—and a field capacity level of 35.6%. Height and stem diameter growth peaked at 23% average soil moisture. Height and stem size were also high at 30% and 32% soil moisture but were low at 9% soil moisture [20].

Soils: Although growth may be best in moist, deep loams [19], white poplar and its hybrids grow in a variety of soil types and textures [105,117]. In the Southwest, white poplar occurs on dry, well drained sites [117], and in Michigan, white poplar thickets are common in sandy soils [119]. In southeastern Michigan, most white polar and hybrid stands occurred on loamy sands, but soil textures ranged from pure sands to nearly pure loams. Soil pH was mostly neutral but ranged from 5 to 8 [102]. White poplar persisted for more than 100 years at the Grayling Agricultural Experiment Station in Crawford County, Michigan, where soil pH, nutrients, and water-holding capacity were low [62]. In its native forest-steppe habitats in Hungary, white poplar occurred on soils with a sand content of greater than 96% and silt and clay content of less than 2% [79]. In the Danube-Tisza region of Hungary, more than 70% of white poplar stands occurred on calcareous soils [86].

Soil moisture: Floodplain and upland sites provide habitat for white poplar [17,45], and although white poplar may survive episodes of both flooding and low precipitation, experiments suggest that growth is best in moist conditions. In his review, Dickmann [17] reports that on dry soils, P. alba × P. tremula grows better than white poplar.

Studies suggest that white poplar and hybrid growth is best in moist but not saturated soils. In a controlled study, P. alba × P. grandidentata stem height increased with increasing soil moisture levels up to 34%; stem height was lower at a 41% moisture level. A lack of root aeration may have affected hybrid growth at 41% soil moisture. In this study, permanent wilting of the hybrids occurred at 9% to 11%, and soil was near field capacity at 41%. [96]. However, when ranchers and farmers in North Dakota and Montana were surveyed, those who planted white poplar on sites with a shallow water table ranked its windbreak performance lower than those who planted it on sites with deeper water tables [111].

In the few studies available, white poplar exhibited greater flood tolerance in its native than nonnative habitats. Along the Upper Rhine in France, white poplar occurs in a hardwood floodplain forest that is flooded almost every summer (June-August) [94]. When white poplar trees were planted around a reservoir in Davis, California, nearly 80% of white poplar trees survived 61 to 69 days of flooding in 2 consecutive years. Survival was much lower, 20%, after 100 days of flooding in 2 consecutive years. The age of white poplar trees at the time of flooding was not reported [45], but provided photos suggest the trees were less than 10 years old.

Salinity: White poplar tolerates salt spray [19] and grows in saline soils [17]. Some indicate that white poplar can establish and grow in soil with salinity levels of up to 4,000 mg/L (Wong and others 1985 as cited in [52]). In the Camargue in southern France, white poplar occurred in habitats where the soil salinity at 12 inches (30 cm) deep was up to 3,200 mg/L [76]. In a greenhouse experiment, the survival of 1-year-old white poplar seedlings was compared after adding various amounts of sodium to the soil. Growth of white poplar was significantly lower in high-salt than low-salt treatments (P=0.0183). After a year, all seedlings had survived the low-salt treatment, and mortality in the high-salt treatment was only 20% [52]. Populus alba × P. tremula is considered more tolerant of saline soils than white poplar [17], but comparisons between white poplar and its hybrids with aspens native to North America were not found.

Studies suggest that shading does not substantially affect white poplar establishment and survival. Ordination analysis of hardwood floodplain forests along the Upper Rhine River in France showed that white poplar was associated with some of the most shaded plots [94]. During field studies in southeastern Iowa, researchers found that low survival and growth of P. alba × P. grandidentata cuttings were not related to light intensity when cuttings received 33% or more of full sun [32].

FIRE EFFECTS AND MANAGEMENT

• FIRE EFFECTS
• FUELS AND FIRE REGIMES
• FIRE MANAGEMENT CONSIDERATIONS

FIRE EFFECTS:
Immediate fire effect on plant: Even the most severe fires probably result only in top-kill of white poplar ([79], Johnson 2010 personal communication [54]).

Postfire regeneration strategy [106]:
Tree with adventitious buds, a sprouting root crown, sobols, and/or root suckers

Fire adaptations and plant response to fire: As of 2010, very few studies had examined the effects of fire on white poplar. Observations indicate that white poplar sprouted after a fire in forest-steppe vegetation in Hungary [79]. However, details regarding fire season, fire severity, postfire sprout abundance, and postfire sprout survival were lacking. North American studies of white poplar and fire were lacking as of the writing of this review (2010). The supervisory forester for Great Smoky Mountains National Park, Kristine Johnson, suggests that white poplar's prolific and persistent root system would survive and produce sprouts after even severe fire. Based on experiences from prior control attempts, Johnson predicts that white poplar sprouts would be abundant following fire and that repeated fire at short intervals would be necessary to kill white poplar clones (2010 personal communication [54]).

Seedling establishment on burned sites would likely be limited to white poplar hybrids. Thus, seedling establishment is probably limited to areas supporting white poplar together with bigtooth aspen, quaking aspen, and/or European aspen. In southeastern Michigan, sites with exposed mineral soil and limited vegetation cover were colonized by P. alba × P. grandidentata and P. alba × P. tremuloides seedlings [101], suggesting that burned sites may be favorable for white poplar hybrid seedling establishment. For more information on white poplar hybridization as it relates to Seed production, Seed dispersal, and Seedling establishment, follow the links to these earlier sections.

Without additional fire studies, it is impossible to address the long-term survival of white poplar postfire sprouts. However, a postdisturbance study following a major windstorm in a northern pin oak (Q. ellipsoidalis)-dominated woodland on Minnesota's Anoka Sand Plain suggests delayed mortality is possible following top-kill of white poplar. Just months after the storm, the frequency of white poplar was 28%. In the next 2 years, white poplar frequency was relatively unchanged. However, white poplar did not occur on study plots 7, 10, and 14 years after the windstorm [81]. Although this study did not involve fire, the environment after the windstorm is in some ways comparable to a postfire environment.

Fire regimes: The prevailing fire regime in white poplar's native habitats was not described in the available literature (2010). Fire regimes in white poplar's nonnative habitats are difficult to characterize. Widespread planting of white poplar in North America has made many vegetation types potential habitat for white poplar. While dense white poplar and white poplar hybrid stands could alter fire frequency or fire behavior in invaded habitats, their impact on natural fire regimes had not been studied as of 2010. See the Fire Regime Table for more information on the fire regimes in vegetation communities that may support white poplar or its hybrids. Find further fire regime information for the plant communities in which these entities may occur by entering their common or scientific names in the FEIS home page under "Find Fire Regimes".

Preventing postfire establishment and spread: Preventing invasive plants from establishing in weed-free burned areas is the most effective and least costly management method. This may be accomplished through early detection and eradication, careful monitoring and follow-up, and limiting dispersal of invasive plant seed into burned areas. General recommendations for preventing postfire establishment and spread of invasive plants include:

• Incorporate cost of weed prevention and management into fire rehabilitation plans
• Acquire restoration funding
• Include weed prevention education in fire training
• Minimize soil disturbance and vegetation removal during fire suppression and rehabilitation activities
• Minimize the use of retardants that may alter soil nutrient availability, such as those containing nitrogen and phosphorus
• Avoid areas dominated by high priority invasive plants when locating firelines, monitoring camps, staging areas, and helibases
• Clean equipment and vehicles prior to entering burned areas
• Regulate or prevent human and livestock entry into burned areas until desirable site vegetation has recovered sufficiently to resist invasion by undesirable vegetation
• Monitor burned areas and areas of significant disturbance or traffic from management activity
• Detect weeds early and eradicate before vegetative spread and/or seed dispersal
• Eradicate small patches and contain or control large infestations within or adjacent to the burned area
• Reestablish vegetation on bare ground as soon as possible
• Avoid use of fertilizers in postfire rehabilitation and restoration
• Use only certified weed-free seed mixes when revegetation is necessary

Use of prescribed fire as a control agent: Although detailed management guidelines for the use of fire to control white poplar and its hybrids are lacking, some weed control documents suggest that annual repeated fire may reduce sprout abundance and limit spread [16,97]. Solecki [97] reports that cutting may be necessary in dense white poplar stands to encourage enough herbaceous fine fuel growth to carry a fire. For "very large" clones, this process may require several years of cutting and burning-in from the stand edges before the clone center can be burned [97].

MANAGEMENT CONSIDERATIONS

• FEDERAL LEGAL STATUS
• OTHER STATUS
• IMPORTANCE TO WILDLIFE AND LIVESTOCK
• OTHER USES
• IMPACTS AND CONTROL

Palatability and/or nutritional value: No information is available on this topic.

Cover value: It is likely that white poplar and hybrid stands provide shade and cover for wildlife and livestock. White poplar has been recommended to improve wildlife habitat [42]; however, its cover value was not evaluated in the literature as of 2010.

		Impacts: Although details and documentation of white poplar's impacts on native vegetation and drainage structures are lacking, these impacts are commonly noted in floras, weed control handbooks, and landscape manuals. Concern about the impacts of white poplar in wildlands varies, as do recommendations for prioritizing its control. White poplar is listed as a "significant threat" by many eastern weed organizations [61,98,109,118]. In a survey of Wisconsin's authorities on local flora, white poplar ranked 35th out of 66 nonnative invasive plants evaluated for their negative impacts on native plant communities [87]. It was ranked 36th in a list of 81 nonnative, invasive species impacting natural habitats of Canada [14]. Based on models using climatic tolerances, biological traits, and invasiveness in other wildlands, researchers predicted that white poplar was a very high threat for establishing and proliferating in Manitoba's Riding Mountain National Park [80]. White poplar was assigned high priority for removal from Point Pelee National Park, Ontario (Dunster 1990 cited in [123]).
Photo © Richard Old, XID Services, Inc., Bugwood.org

White poplar was not considered a problematic species or a control priority in several other cases. It probably has less potential impact and receives lower priority for control in areas where it has not been widely planted and does not have the potential to hybridize with native aspens. In a survey answered by 35 Canadian botanists, most respondents indicated that white poplar was not a "problem species" and was invasive only locally. The survey was sent to botanists across Canada, but the regional distribution of respondents was not reported [123]. White poplar was relatively rare in Farmington, western Maine, and spread of clones was easily tracked back to areas where white poplar was planted [6]. Surveys of the flora in New London County, Connecticut, revealed that white poplar populations were uncommon and generally restricted to disturbed sites. Population sizes were stable [48].

Impacts from underground growth: The extensive white poplar root system has caused problems near houses or other urban developments. Several sources anecdotally report that white poplar roots can clog drains, sewers, and water channels [19,34,68]. In his manual of woody plants, Dirr [19] indicates that white poplar "becomes a nuisance and liability after a time". Dirr suggests that homeowners "avoid this pest". A pamphlet produced by England's Forestry Commission reports that white poplar can remove soil moisture rapidly during dry, hot days. In low-rainfall areas such as London and Essex, white poplar has caused rapid drying and shrinkage of clay soils, which can upset dwelling foundations [26].

Impacts to associated vegetation: Impacts on associated vegetation may change as white poplar stands expand and age. Through prolific root sprouting, white poplar can develop dense stands, which can crowd and shade native vegetation and reduce species diversity [16,97,120]. As stands age, the breakage of brittle white poplar wood can damage nearby vegetation [73]. In the central Transvaal area of South Africa, where white poplar is nonnative, there is "marked correlation between the occurrence of naturalized and planted white poplar", but white poplar no longer occurs as isolated stands; instead, it occupies whole river reaches and has spread from the water's edge to far outside the riparian zone. White poplar has "out-compete(d)" and suppressed existing vegetation in its formation of "absolutely pure stands" [121].

Hybridization: White poplar hybridizes with native aspens. Researchers fear that this hybridization could change the native aspen gene pool or produce "vigorous" hybrids that could replace native aspens. Because of its hybridization ability, white poplar was rated as a high priority for control in Cape Breton Highlands National Park, Nova Scotia. Although current hybridization was not evaluated, white poplar populations occurred near quaking aspen and bigtooth aspen populations. Managers feared that hybridization could lead to the introgression of white poplar genes into the native aspen gene pool [103]. In southwestern Michigan, white poplar, native aspens, and their hybrids were evaluated. The impact of white poplar on the native aspen gene pools was considered low. As of the early 1980s, white poplar hybrid populations were clustered; recent hybridization, backcrossing, and extensive gene flow were not detected [101]. Hybrids were relatively disease and insect free [102].

Control: Because white poplar regenerates easily after top-kill (see Vegetative regeneration), it is difficult to control once established [13,88].

In all cases where invasive species are targeted for control, no matter what method is employed, the potential for other invasive species to fill their void must be considered [9]. Control of biotic invasions is most effective when it employs a long-term, ecosystem-wide strategy rather than a tactical approach focused on battling individual invaders [69].

Fire: For information on the use of prescribed fire to control this species, see Fire Management Considerations.

Prevention: Although there has been widespread planting of white poplar in North America, eliminating future plantings could improve future control efforts and reduce the potential for contamination of the gene pool for native aspens. Restricting sale of white poplar could limit future use. Informing land owners of white poplar's objectionable traits, as was done by Appleton and others [2], may help to limit future plantings.

It is commonly argued that the most cost-efficient and effective method of managing invasive species is to prevent their establishment and spread by maintaining "healthy" natural communities [69,93] and by monitoring several times each year [53]. Managing to maintain the integrity of the native plant community and mitigate the factors enhancing ecosystem invasibility is likely to be more effective than managing solely to control the invader [50].

Weed prevention and control can be incorporated into many types of management plans, including those for logging and site preparation, grazing allotments, recreation management, research projects, road building and maintenance, and fire management [113]. See the Guide to noxious weed prevention practices [113] for specific guidelines in preventing the spread of weed seeds and propagules under different management conditions.

Cultural control: No information is available on this topic.

Physical or mechanical control: Vegetative regeneration and spread can be encouraged by cutting white poplar stems [73]. To be a viable control option, cutting will likely need to be frequent, repeated, and/or paired with another control method. Weed handbooks suggest controlling white poplar by repeated and frequent cutting [34,120]. In the tallgrass restoration handbook, white poplar spread is said to be controlled by girdling large trees and repeatedly cutting sprouts [97]. In a review, Czarapata [16] reports that white poplar stems with less than a 2-inch (5 cm) DBH may be controlled by cutting followed by herbicide treatments. Sprouting of girdled stems larger than 2 inches (5 cm) in DBH may be limited by applying an herbicide to the wound [16].

Biological control: Biological control of invasive species has a long history that indicates many factors must be considered before using biological controls. Refer to these sources: [115,124] and the Weed control methods handbook [110] for background information and important considerations for developing and implementing biological control programs. For information on pests and diseases known to infect white poplar in the United States, see Spaulding [99].

Chemical control: Herbicides may be useful to control white poplar [34], but effectiveness may be improved if used in conjunction with other control methods [97].

Herbicides are effective in gaining initial control of a new invasion or a severe infestation, but they are rarely a complete or long-term solution to weed management [11]. See the Weed control methods handbook [110] for considerations on the use of herbicides in natural areas and detailed information on specific chemicals.

APPENDIX: FIRE REGIME TABLE

REFERENCES

1. Appleton, Bonnie Lee; Frenzel, Cindy L.; Hillegass, Julie B.; Lyons, Robert E.; Steward, Larry G. 2009. Virginia firescapes: Firewise landscaping for woodland homes. Virginia Cooperative Extension Publication 430-300. Blacksburg, VA: Virginia Polytechnic Institute and State University, Virginia Cooperative Extension; Virginia Firewise Landscaping Task Force. 9 p. Available online: http://pubs.ext.vt.edu/430/430-300/430-300.pdf [2009, October 6]. [76014]

2. Appleton, Bonnie; Huff, Roger R.; French, Susan C. 1999. Evaluating trees for saltwater spray tolerance for oceanfront sites. Journal of Arboriculture. 25(4): 205-210. [37147]

3. Asher, Jerry; Dewey, Steven; Olivarez, Jim; Johnson, Curt. 1998. Minimizing weed spread following wildland fires. Proceedings, Western Society of Weed Science. 51: 49. Abstract. [40409]

4. Bailey, W. W.; Collins, J. F. 1893. A list of plants found on Block Island, R.I., in July and August. Bulletin of the Torrey Botanical Club. 20(6): 231-239. [73907]

5. Barnes, Burton V. 1961. Hybrid aspens in the Lower Peninsula of Michigan. Rhodora. 63: 311-324. [79397]

6. Barton, Andrew M.; Brewster, Lauri B.; Cox, Anne N.; Prentiss, Nancy K. 2004. Non-indigenous woody invasive plants in a rural New England town. Biological Invasions. 6: 205-211. [47715]

7. Braun, E. Lucy. 1989. The woody plants of Ohio. Columbus, OH: Ohio State University Press. 362 p. [12914]

8. Brooks, Matthew L. 2008. Effects of fire suppression and postfire management activities on plant invasions. In: Zouhar, Kristin; Smith, Jane Kapler; Sutherland, Steve; Brooks, Matthew L., eds. Wildland fire in ecosystems: Fire and nonnative invasive plants. Gen. Tech. Rep. RMRS-GTR-42-vol. 6. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 269-280. [70909]

9. Brooks, Matthew L.; Pyke, David A. 2001. Invasive plants and fire in the deserts of North America. In: Galley, Krista E. M.; Wilson, Tyrone P., eds. Proceedings of the invasive species workshop: The role of fire in the control and spread of invasive species; Fire conference 2000: 1st national congress on fire ecology, prevention, and management; 2000 November 27 - December 1; San Diego, CA. Misc. Publ. No. 11. Tallahassee, FL: Tall Timbers Research Station: 1-14. [40491]

10. Brundu, Giuseppe; Lupi, Renato; Zapelli, Ilaria; Fossati, Tiziana; Patrignani, Giuseppe; Camarda, Ignazio; Sala, Francesco; Castiglione, Stefano. 2008. The origin of clonal diversity and structure of Populus alba in Sardinia: evidence from nuclear and plastid microsatellite markers. Annals of Botany. 102(6): 997-1006. [77736]

11. Bussan, Alvin J.; Dyer, William E. 1999. Herbicides and rangeland. In: Sheley, Roger L.; Petroff, Janet K., eds. Biology and management of noxious rangeland weeds. Corvallis, OR: Oregon State University Press: 116-132. [35716]

12. Carpenter, Jackie S.; Chester, Edward W. 1987. Vascular flora of the Bear Creek Natural Area, Stewart County, Tennessee. Castanea. 52(2): 112-128. [75372]

13. Carter, Jack L. 1997. Trees and shrubs of New Mexico. Boulder, CO: Johnson Books. 534 p. [72647]

14. Catling, Paul; Mitrow, Gisele. 2005. A prioritized list of the invasive alien plants of natural habitats in Canada. Canadian Botanical Association Bulletin. 38(4): 55-57. [71460]

15. Curtis, John T. 1959. Beach, dune, and cliff communities. In: The vegetation of Wisconsin. Madison, WI: The University of Wisconsin Press: 402-411. [60532]

16. Czarapata, Elizabeth J. 2005. Invasive plants of the Upper Midwest: An illustrated guide to their identification and control. Madison, WI: The University of Wisconsin Press. 215 p. [71442]

17. Dickmann, Donald I. 2001. An overview of the genus Populus. In: Dickman, Donald I.; Isebrands, J. G.; Eckenwalder, James E.; Richardson, Jim, eds. Poplar culture in North America. Ottawa, ON: National Research Council of Canada, Research Press: 1-42. [79277]

18. Diggs, George M., Jr.; Lipscomb, Barney L.; O'Kennon, Robert J. 1999. Illustrated flora of north-central Texas. Sida Botanical Miscellany, No. 16. Fort Worth, TX: Botanical Research Institute of Texas. 1626 p. [35698]

19. Dirr, Michael A. 1998. Manual of woody landscape plants: Their identification, ornamental characteristics, culture, propagation and uses. 5th ed. Champaign, IL: Stipes Publishing. 1187 p. [74836]

20. Domingo, Ireneo L.; Gordon, John C. 1974. Physiological responses of an aspen-poplar hybrid to air temperature and soil moisture. Botanical Gazette. 135(3): 184-192. [77806]

21. Donahue, Raymon A.; David, Tim D.; Michler, Charles H.; Riemenschneider, Don E.; Carter, Doug R.; Marquardt, Paula E.; Sankhla, Narendra; Sankhla, Daksha; Haissig, Bruce E.; Isebrands, J. G. 1994. Growth, photosynthesis, and herbicide tolerance of genetically modified hybrid poplar. Canadian Journal of Forest Research. 24(12): 2377-2383. [77720]

22. Dowhan, Joseph J.; Rozsa, Ron. 1989. Flora of Fire Island, Suffolk County, New York. Bulletin of the Torrey Botanical Club. 116(3): 265-282. [22041]

23. Duncan, Wilbur H. 1950. Preliminary reports on the flora of Georgia. 2. Distribution of 87 trees. The American Midland Naturalist. 43(3): 742-761. [75535]

24. Duncan, Wilbur H.; Duncan, Marion B. 1987. The Smithsonian guide to seaside plants of the Gulf and Atlantic coasts from Louisiana to Massachusetts, exclusive of lower peninsular Florida. Washington, DC: Smithsonian Institution Press. 409 p. [12906]

25. Duncan, Wilbur H.; Duncan, Marion B. 1988. Trees of the southeastern United States. Athens, GA: The University of Georgia Press. 322 p. [12764]

26. Edlin, Herbert L. 1968. Know your broadleaves. Forestry Commission Booklet No. 20. London: Her Majesty's Stationery Office. 142 p. [20459]

27. Ellenberg, Heinz. 1988. Vegetation ecology of central Europe. 4th edition. Cambridge, UK: Cambridge University Press. 731 p. [English translation by G. K. Strutt]. [73365]

28. Farrar, John Laird. 1995. Trees of the northern United States and Canada. Ames, IA: Blackwell Publishing. 502 p. [60614]

29. Fechner, Gilbert H.; Barrows, Jack S. 1976. Aspen stands as wildfire fuel breaks. Eisenhower Consortium Bulletin 4. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 26 p. In cooperation with: Eisenhower Consortium for Western Environmental Forestry Research. [7611]

30. Flora of North America Association. 2010. Flora of North America: The flora, [Online]. Flora of North America Association (Producer). Available: http://www.fna.org/FNA. [36990]

31. Fossati, T.; Patrignani, G.; Zapelli, I.; Sabatti, M.; Sala, F.; Castiglione, S. 2004. Development of molecular markers to assess the level of introgression of Populus tremula into P. alba natural populations. Plant Breeding. 123(4): 382-385. [77745]

32. Gatherum, G. E.; Gordon, J. C.; Broerman, B. F. S. 1967. Effects of clone and light intensity on photosynthesis, respiration and growth of aspen-poplar hybrids. Silvae Genetica. 16: 128-132. [79613]

33. George, Ernest J. 1953. Tree and shrub species for the Northern Great Plains. Circular No. 912. Washington, DC: U.S. Department of Agriculture. 46 p. [4566]

34. Glass, William. 1996. Populus alba--white poplar. In: Randall, John M.; Marinelli, Janet, eds. Invasive plants: Weeds of the global garden. Handbook #149. Brooklyn, NY: Brooklyn Botanic Garden: 39. [72853]

35. Gleason, Henry A.; Cronquist, Arthur. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. 2nd ed. New York: New York Botanical Garden. 910 p. [20329]

36. Goerndt, Michael E.; Mize, Carl. 2008. Short-rotation woody biomass as a crop on marginal lands in Iowa. Northern Journal of Applied Forestry. 25(2): 82-86. [77746]

37. Goodwin, Kim; Sheley, Roger; Clark, Janet. 2002. Integrated noxious weed management after wildfires. EB-160. Bozeman, MT: Montana State University, Extension Service. 46 p. Available online: http://www.montana.edu/wwwpb/pubs/eb160.html [2003, October 1]. [45303]

38. Graber, Jean W.; Graber, Richard R.; Kirk, Ethelyn L. 1977. Illinois birds: Picidae. Biological Notes No. 102. Urbana, IL: State of Illinois, Department of Registration and Education; Natural History Survey Division, Natural History Survey. 73 p. [64983]

39. Great Plains Flora Association. 1986. Flora of the Great Plains. Lawrence, KS: University Press of Kansas. 1392 p. [1603]

40. Green, Alan W.; Green, LeeRoy. 1959. Fast-starting species best in some Iowa plantations. Station Note No. 133. Columbus, OH: U.S. Department of Agriculture, Forest Service, Central States Forest Experiment Station. 2 p. [79623]

41. Grubbs, Jeffrey T.; Fuller, Marian J. 1991. Vascular flora of Hickman County, Kentucky. Castanea. 56(3): 193-214. [75356]

42. Hall, R. B.; Colletti, J. P.; Schultz, R. C.; Faltonson, R. R.; Kolison, S. H., Jr.; Hanna, R. D.; Hillson, T. D.; Morrison, J. W. 1990. Commercial-scale vegetative propagation of aspens. In: Adams, Roy D., ed. Aspen symposium '89: Proceedings; 1989 July 25-27; Duluth, MN. Gen. Tech. Rep. NC-140. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station: 211-219. [12432]

43. Hall, R. B.; Hilton, G. D.; Maynard, C. A. 1982. Construction lumber from hybrid aspen plantations in the Central States. Journal of Forestry. 80: 291-294. [79616]

44. Hann, Wendel; Havlina, Doug; Shlisky, Ayn; [and others]. 2008. Interagency fire regime condition class guidebook. Version 1.3, [Online]. In: Interagency fire regime condition class website. U.S. Department of Agriculture, Forest Service; U.S. Department of the Interior; The Nature Conservancy; Systems for Environmental Management (Producer). 119 p. Available: http://frames.nbii.gov/frcc/documents/FRCC_Guidebook_2008.07.10.pdf [2010, 3 May]. [70966]

45. Harris, Richard W.; Leiser, Andrew T.; Fissell, Robert E. 1975. Plant tolerance to flooding. Summary report -- 1971-1975. Grant Contract No. A 5fs-16565; RWH-200-7/1/75. Davis, CA: University of California, Department of Environmental Horticulture. 30 p. In cooperation with: U.S. Army Corps of Engineers, U.S. Forest Service. [76584]

46. Henderson, L. 2007. Invasive, naturalized and casual alien plants in southern Africa: a summary based on the Southern African Plant Invaders Atlas (SAPIA). Bothalia. 37(2): 215-248. [77748]

47. Hickman, James C., ed. 1993. The Jepson manual: Higher plants of California. Berkeley, CA: University of California Press. 1400 p. [21992]

48. Hill, Steven R. 1996. The flora of Latimer Point and vicinity, New London County, Connecticut. Rhodora. 98(894): 180-216. [44935]

49. Hitchcock, C. Leo; Cronquist, Arthur. 1973. Flora of the Pacific Northwest. Seattle, WA: University of Washington Press. 730 p. [1168]

50. Hobbs, Richard J.; Humphries, Stella E. 1995. An integrated approach to the ecology and management of plant invasions. Conservation Biology. 9(4): 761-770. [44463]

51. Holmgren, Noel H.; Holmgren, Patricia K.; Cronquist, Arthur. 2005. Intermountain flora: Vascular plants of the Intermountain West, U.S.A. Vol. 2, Part B: Subclass Dilleniidae. New York: The New York Botanical Garden. 488 p. [63251]

52. Imada, S.; Yamanaka, N.; Tamai, S. 2009. Effects of salinity on the growth, Na partitioning, and Na dynamics of a salt-tolerant tree, Populus alba L. Journal of Arid Environments. 73(6-7): 245-251. [77749]

53. Johnson, Douglas E. 1999. Surveying, mapping, and monitoring noxious weeds on rangelands. In: Sheley, Roger L.; Petroff, Janet K., eds. Biology and management of noxious rangeland weeds. Corvallis, OR: Oregon State University Press: 19-36. [35707]

54. Johnson, Kristine. 2010. [Email to Corey Gucker]. April 7. Regarding Populus alba. Gatlinburg, TN: Great Smoky Mountains National Park. On file with: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Missoula, MT; FEIS files. [79621]

55. Johnson, L. P. V. 1942. Studies on the relation of growth rate to wood quality in Populus hybrids. Canadian Journal of Research. 20: 28-40. [79400]

56. Johnson, L. P. V. 1946. A note on inheritance in F1 and F2 hybrids of Populus alba L. X P. grandidentata Michx. Canadian Journal of Research. 24: 313-317. [79401]

57. Johnson, L. P. V.; Heimburger, C. 1946. Preliminary report on interspecific hybridization in forest trees. Canadian Journal of Research. 24: 308-312. [79398]

58. Karrenberg, S.; Edwards, P. J.; Kollmann, J. 2002. The life history of Salicaceae living in the active zone of floodplains. Freshwater Biology. 47: 733-748. [79298]

59. Kartesz, John T. 1999. A synonymized checklist and atlas with biological attributes for the vascular flora of the United States, Canada, and Greenland. 1st ed. In: Kartesz, John T.; Meacham, Christopher A. Synthesis of the North American flora (Windows Version 1.0), [CD-ROM]. Chapel Hill, NC: North Carolina Botanical Garden (Producer). In cooperation with: The Nature Conservancy; U.S. Department of Agriculture, Natural Resources Conservation Service; U.S. Department of the Interior, Fish and Wildlife Service. [36715]

60. Kartesz, John Thomas. 1988. A flora of Nevada. Reno, NV: University of Nevada. 1729 p. [In 2 volumes]. Dissertation. [42426]

61. Kentucky Exotic Pest Plant Council. 2008. Invasive exotic plant list, [Online]. Southeast Exotic Pest Plant Council (Producer). Available: http://www.se-eppc.org/ky/list.htm [2009, January 5]. [72785]

62. Kilgore, Jason S.; Telewski, Frank W. 2004. Reforesting the jack pine barrens: a long-term common garden experiment. Forest Ecology and Management. 189(1-3): 171-187. [47461]

63. Kondolf, G. Mathias; Piegay, Herve; Landon, Norbert. 2007. Changes in the riparian zone of the lower Eygues River, France, since 1830. Landscape Ecology. 22(3): 367-384. [77722]

64. LANDFIRE Rapid Assessment. 2005. Reference condition modeling manual (Version 2.1), [Online]. In: LANDFIRE. Cooperative Agreement 04-CA-11132543-189. Boulder, CO: The Nature Conservancy; U.S. Department of Agriculture, Forest Service; U.S. Department of the Interior (Producers). 72 p. Available: http://www.landfire.gov/downloadfile.php?file=RA_Modeling_Manual_v2_1.pdf [2007, May 24]. [66741]

65. LANDFIRE Rapid Assessment. 2007. Rapid assessment reference condition models, [Online]. In: LANDFIRE. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Lab; U.S. Geological Survey; The Nature Conservancy (Producers). Available: http://www.landfire.gov/models_EW.php [2008, April 18] [66533]

66. Lexer, C.; Fay, M. F.; Joseph, J. A.; Nica, M.-S.; Heinze, B. 2005. Barrier to gene flow between two ecologically divergent Populus species, P. alba (white poplar) and P. tremula (European aspen): the role of ecology and life history in gene introgression. Molecular Ecology. 14(4): 1045-1057. [77755]

67. Little, Elbert L., Jr.; Brinkman, Kenneth A.; McComb, A. L. 1957. Two natural Iowa hybrid poplars. Forestry Science. 3(3): 253-262. [79394]

68. Little, Elbert L. 1961. Sixty trees from foreign lands. Agricultural Handbook No. 212. Washington, DC: U.S. Department of Agriculture. 30 p. [53217]

69. Mack, Richard N.; Simberloff, Daniel; Lonsdale, W. Mark; Evans, Harry; Clout, Michael; Bazzaz, Fakhri A. 2000. Biotic invasions: causes, epidemiology, global consequences, and control. Ecological Applications. 10(3): 689-710. [48324]

70. Manzanenra, Jose A.; Martinez-Chacon, Maria F. 2007. Ecophysiological competence of Populus alba L., Fraxinus angustifolia Vahl., and Crataegus monogyna Jacq. used in plantations for the recovery of riparian vegetation. Environmental Management. 40(6): 902-912. [77725]

71. Martin, William C.; Hutchins, Charles R. 1981. A flora of New Mexico. Volume 2. Germany: J. Cramer. 2589 p. [37176]

72. McComb, A. L.; Hansen, Norman J. 1954. A naturally occurring aspen popular hybrid. Journal of Forestry. 52: 528-529. [79622]

73. Mehrhoff, L. J.; Silander, J. A., Jr.; Leicht, S. A.; Mosher, E. S.; Tabak, N. M. 2003. IPANE: Invasive Plant Atlas of New England, [Online]. Storrs, CT: University of Connecticut, Department of Ecology and Evolutionary Biology (Producer). Available: http://nbii-nin.ciesin.columbia.edu/ipane/ [2008, May 28]. [70356]

74. Meilan, R.; Han, K.-H.; Ma, C.; DiFazio, S. P.; Eaton, J. A.; Hoien, E. A.; Stanton, B. J.; Crockett, R. P.; Tayolor, M. L.; James, R. R.; Skinner, J. S.; Jouanin, L.; Pilate, G.; Strauss, S. H. 2002. The CP4 transgene provides high levels of tolerance to Roundup herbicide in field-grown hybrid poplars. Canadian Journal of Forest Research. 32(6): 967-976. [77726]

75. Meilan, Richard; Sabatti, Maurizio; Ma, Caiping; Kuzminksy, Elena. 2004. An early-flowering genotype of Populus. Journal of Plant Biology. 47(1): 52-56. [77757]

76. Mesleard, F.; Grillas, P.; Lepart, J. 1991. Plant community succession in a coastal wetland after abandonment of cultivation: the example of the Rhone Delta. Vegetatio. 94(1): 35-45. [77758]

77. Mohlenbrock, Robert H. 1986. [Revised edition]. Guide to the vascular flora of Illinois. Carbondale, IL: Southern Illinois University Press. 507 p. [17383]

78. Munz, Philip A. 1974. A flora of southern California. Berkeley, CA: University of California Press. 1086 p. [4924]

79. Onodi, Gabor; Kertesz, Miklos; Botta-Dukat, Zoltan; Altbacker, Vilmos. 2008. Grazing effects on vegetation composition and on the spread of fire on open sand grasslands. Arid Land Research and Management. 22(4): 273-285. [72697]

80. Otfinowski, R.; Kenkel, N. C.; Dixon, P.; Wilmshurst, J. F. 2008. Integrating climate and trait models to predict the invasiveness of exotic plants in Canada's Riding Mountain National Park. Canadian Journal of Plant Science. 87(5): 1001-1012. [70500]

81. Palmer, Michael W.; McAlister, Suzanne D.; Arevalo, Jose Ramon; DeCoster, James K. 2000. Changes in the understory during 14 years following catastrophic windthrow in two Minnesota forests. Journal of Vegetation Science. 11(6): 841-854. [42541]

82. Polya, L. 1961. Injury by soaking of Populus alba seeds. Nature. 189(4759): 159-160. [77759]

83. Poston, Muriel E.; Middendorf, George A., III. 1988. Maturation characteristics of Rubus pennsylvanicus fruit: are black and red the same? Oecologia. 77(1): 69-72. [13541]

84. Radford, Albert E.; Ahles, Harry E.; Bell, C. Ritchie. 1968. Manual of the vascular flora of the Carolinas. Chapel Hill, NC: The University of North Carolina Press. 1183 p. [7606]

85. Raunkiaer, C. 1934. The life forms of plants and statistical plant geography. Oxford: Clarendon Press. 632 p. [2843]

86. Redei, K. 2000. Early performance of promising white poplar (Populus alba) clones in sandy ridges between the rivers Danube and Tsiza in Hungary. Forestry. 73(4): 407-413. [77760]

87. Reinartz, James A. 2003. IPAW working list of the invasive plants of Wisconsin--March 2003: a call for comments and information, [Online]. In: Plants out of place: The newsletter of the Invasive Plants Association of Wisconsin. Issue 4. Madison, WI: Invasive Plants Association of Wisconsin (Producer). Available: http://www.ipaw.org/newsletters/issue4.pdf [2009, June 26]. [74814]

88. Roland, A. E.; Smith, E. C. 1969. The flora of Nova Scotia. Halifax, NS: Nova Scotia Museum. 746 p. [13158]

89. Sanchez-Perez, Jose Miguel; Lucot, Eric; Bariac, Thierry; Tremolieres, Michele. 2008. Water uptake by trees in a riparian hardwood forest (Rhine floodplain, France). Hydrological Processes. 22(3): 366-375. [77729]

90. Schuster, W. S. F.; Griffin, K. L.; Roth, H.; Turnbull, M. H.; Whitehead, D.; Tissue, D. T. 2008. Changes in composition, structure and aboveground biomass over seventy-six years (1930-2006) in the Black Rock Forest, Hudson Highlands, southeastern New York State. Tree Physiology. 28(12): 537-549. [73136]

91. Scoggan, H. J. 1978. The flora of Canada. Part 3: Dicotyledoneae (Saururaceae to Violaceae). National Museum of Natural Sciences: Publications in Botany, No. 7(3). Ottawa: National Museums of Canada. 1115 p. [75493]

92. Seymour, Frank Conkling. 1982. The flora of New England. 2nd ed. Phytologia Memoirs 5. Plainfield, NJ: Harold N. Moldenke and Alma L. Moldenke. 611 p. [7604]

93. Sheley, Roger; Manoukian, Mark; Marks, Gerald. 1999. Preventing noxious weed invasion. In: Sheley, Roger L.; Petroff, Janet K., eds. Biology and management of noxious rangeland weeds. Corvallis, OR: Oregon State University Press: 69-72. [35711]

94. Siebel, Henk N.; Bouwma, Irene M. 1998. The occurrence of herbs and woody juveniles in a hardwood floodplain forest in relation to flooding and light. Journal of Vegetation Science. 9(5): 623-630. [73525]

95. Simpfendorfer, K. J. 1989. Trees, farms and fires. Land and Forests Bulletin No. 30. Victoria, Australia: Department of Conservation, Forests and Lands. 55 p. [10649]

96. Smith, David W.; Gatherum, Gordon E. 1974. Effects of moisture and clone on photosynthesis and growth of an aspen-poplar hybrid. Botanical Gazette. 135(4): 293-296. [77803]

97. Solecki, Mary Kay. 1997. Controlling invasive plants. In: Packard, Stephen; Mutel, Cornelia F., eds. The tallgrass restoration handbook: For prairies, savannas, and woodlands. Washington, DC: Island Press: 251-278. [43127]

98. South Carolina Exotic Pest Plant Council. 2008. Invasive plant list, [Online]. Southeast Exotic Pest Plant Council (Producer). Available: http://www.se-eppc.org/southcarolina/SCEPPC_LIST_offical_2008.xls [2009, January 5]. [72717]

99. Spaulding, Perley. 1958. Diseases of foreign forest trees growing in the United States: An annotated list. Agriculture Handbook No. 139. Washington, DC: U.S. Department of Agriculture. 118 p. [9945]

100. Spies, T. A.; Barnes, B. V. 1981. A morphological analysis of Populus alba, Populus grandidentata and their natural hybrids in southeastern Michigan. Silvae Genetica. 30(2-3): 102-106. [77765]

101. Spies, Thomas A.; Barnes, Burton V. 1982. Natural hybridization between Populus alba L. and the native aspens in southeastern Michigan. Canadian Journal of Forest Research. 12(3): 653-660. [77764]

102. Spies, Thomas Allen. 1978. The occurrence, morphology, and reproductive biology of natural hybrids of Populus alba in southeastern Michigan. Ann Arbor, MI: University of Michigan, School of Natural Resources. 125 p. Thesis. [77815]

103. Stapleton, C. A.; McCorquodale, D. B.; Sneddon, C.; Williams, M.; Bridgland, J. 1998. The distribution and potential for invasiveness of some non-native vascular plants in northern Cape Breton. Technical Report in Ecosystem Science No. 015. Ottawa: Parks Canada, Canadian Heritage, Atlantic Region. 68 p. [77812]

104. Steenhof, Karen; Berlinger, Stephen S.; Fredrickson, Leigh H. 1980. Habitat use by wintering bald eagles in South Dakota. The Journal of Wildlife Management. 44(4): 798-805. [75199]

105. Stephens, H. A. 1973. Woody plants of the north Central Plains. Lawrence, KS: The University Press of Kansas. 530 p. [3804]

106. Stickney, Peter F. 1989. Seral origin of species comprising secondary plant succession in Northern Rocky Mountain forests. FEIS workshop: Postfire regeneration. Unpublished draft on file at: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Missoula, MT. 10 p. [20090]

107. Strausbaugh, P. D.; Core, Earl L. 1977. Flora of West Virginia. 2nd ed. Morgantown, WV: Seneca Books, Inc. 1079 p. [23213]

108. Sweetman, Harvey L. 1949. Further studies of the winter feeding habits of cottontail rabbits. Ecology. 30(3): 371-376. [72521]

109. Tennessee Exotic Pest Plant Council. 2001. Tennessee invasive exotic plant list, [Online]. In: Invasive exotic plants in Tennessee. Fairview, TN: Tennessee Exotic Pest Plant Council (Producer) Available: http://www.tneppc.org/Invasive_Exotic_Plant_List/The_List.htm [2009, June 12]. [74677]

110. Tu, Mandy; Hurd, Callie; Randall, John M., eds. 2001. Weed control methods handbook: tools and techniques for use in natural areas. Davis, CA: The Nature Conservancy. 194 p. [37787]

111. Tuskan, Gerald A.; Laughlin, Kevin. 1991. Windbreak species performance and management practices as reported by Montana and North Dakota landowners. Journal of Soil and Water Conservation. 46(3): 225-228. [15084]

112. Tyser, Robin W.; Worley, Christopher A. 1992. Alien flora in grasslands adjacent to road and trail corridors in Glacier National Park, Montana (U.S.A.). Conservation Biology. 6(2): 253-262. [19435]

113. U.S. Department of Agriculture, Forest Service. 2001. Guide to noxious weed prevention practices. Washington, DC: U.S. Department of Agriculture, Forest Service. 25 p. Available online: https://www.fs.usda.gov /invasivespecies/documents/FS_WeedBMP_2001.pdf [2009, November 19]. [37889]

114. U.S. Department of Agriculture, Natural Resources Conservation Service. 2010. PLANTS Database, [Online]. Available: https://plants.usda.gov /. [34262]

115. Van Driesche, Roy; Lyon, Suzanne; Blossey, Bernd; Hoddle, Mark; Reardon, Richard, tech. coords. 2002. Biological control of invasive plants in the eastern United States. Publication FHTET-2002-04. Morgantown, WV: U.S. Department of Agriculture, Forest Service, Forest Health Technology Enterprise Team. 413 p. Available online: http://www.invasive.org/eastern/biocontrol/index.html [2009, November 19]. [54194]

116. van Loo, Marcela; Joseph, Jeffrey A.; Heinze, Berthold; Fay, Mike F.; Lexer, Christian. 2008. Clonality and spatial genetic structure in Populus x canescens and its sympatric backcross parent P. alba in a Central European hybrid zone. New Phytologist. 177(2): 506-516. [77767]

117. Vines, Robert A. 1960. Trees, shrubs, and woody vines of the Southwest. Austin, TX: University of Texas Press. 1104 p. [7707]

118. Virginia Department of Conservation and Recreation, Division of Natural Heritage. 2003. Invasive alien plant species of Virginia, [Online]. In: Natural Heritage Program--Invasive plants list. Richmond, VA: Virginia Department of Conservation and Recreation, Division of Natural Heritage; Virginia Native Plant Society (Producers). Available: http://www.dcr.virginia.gov/natural_heritage/documents/invlist.pdf [2009, March 23]. [44942]

119. Voss, Edward G. 1985. Michigan flora. Part II. Dicots (Saururaceae--Cornaceae). Bulletin 59. Bloomfield Hills, MI: Cranbrook Institute of Science; Ann Arbor, MI: University of Michigan Herbarium. 724 p. [11472]

120. Weber, Ewald. 2003. Invasive plant species of the world: a reference guide to environmental weeds. Cambridge, MA: CABI Publishing. 548 p. [71904]

121. Wells, M. J.; Duggan, K.; Hendersen, L. 1980. Woody plant invaders of the central Transvaal. In: Neser, S.; Cairns, A. L. P., eds. Proceedings, 3rd national weeds conference of South Africa; 1979 August; Pretoria, South Africa. Cape Town, South Africa: A. A. Balkema: 11-23. [47112]

122. Welsh, Stanley L.; Atwood, N. Duane; Goodrich, Sherel; Higgins, Larry C., eds. 1987. A Utah flora. The Great Basin Naturalist Memoir No. 9. Provo, UT: Brigham Young University. 894 p. [2944]

123. White, David J.; Haber, Erich; Keddy, Cathy. 1993. Invasive plants of natural habitats in Canada: An integrated review of wetland and upland species and legislation governing their control. Ottawa, ON: Canadian Wildlife Service. 121 p. [71462]

124. Wilson, Linda M.; McCaffrey, Joseph P. 1999. Biological control of noxious rangeland weeds. In: Sheley, Roger L.; Petroff, Janet K., eds. Biology and management of noxious rangeland weeds. Corvallis, OR: Oregon State University Press: 97-115. [35715]

125. Wu, Z. Y.; Raven, P. H.; Hong, D. Y., eds. 2010. Flora of China, [Online]. Volumes 1-25. Beijing: Science Press; St. Louis, MO: Missouri Botanical Garden Press. In: eFloras. St. Louis, MO: Missouri Botanical Garden; Cambridge, MA: Harvard University Herbaria (Producers). Available: http://www.efloras.org/flora_page.aspx?flora_id=2 and http://flora.huh.harvard.edu/china. [72954]