Populus alba L. var. pyramidalis (Bunge)

More info for the terms: fire management, formation, introgression, invasive species, natural, prescribed fire, restoration, top-kill

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.

Integrated management: No information is available on this topic.

More info for the terms: cover, introgression, layering, phenology, tree

Vegetative regeneration and reproduction are vital to white poplar growth, spread, persistence, and recovery from injury [91,105]. Clonal growth and spread most commonly occur by root sprouting [60,119,122] but may also occur through fragmentation and layering [94]. Fragmentation describes the process by which new clones can develop from twig or root pieces that become partially buried in sand or silt ([94], review by [17]). Observations in South Africa led researchers to conclude that white poplar and P. alba × P. tremula were dispersed through the movement of vegetative parts in water [46].

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].

More info for the term: breeding system

White poplar predominantly regenerates vegetatively. Because only female trees are generally present at a given location [24], sexual reproduction is usually limited to white poplar hybrids [101].

• Vegetative regeneration
• Seed production
• Seed dispersal
• Seed banking
• Germination
• Seedling establishment and plant growth
• Pollination and breeding system