European rusty willow S. atrocinerea in eastern Massachusetts

Alexey Zinovjev and Irina Kadis

Rusty willow (S. atrocinerea Brotero) is an Atlantic West-European species that naturally grows in Britain, France, Belgium, Spain, Portugal, on some Mediterranean islands, and in North Africa. It belongs to a large section Vetrix or Cinerella, whose members are difficult for identification. It has been treated as a subspecies of a continental Eurasiatic species, gray willow (S. cinerea L.), hence in the literature, S. atrocinerea has been often referred as S. cinerea ssp. oleifolia (Smith) Macreight or even just S. cinerea. However, the world authorities on the genus Salix prefer to treat rusty willow and gray willow as two different species (Skvortsov 1968, 1999; Argus 1997, 2005). Indeed there is not very much that these two willows really have in common. Remarkably, phenetic analysis of a Spanish population of S. atrocinerea demonstrated that S. atrocinerea "clusters with S. aurita, S. starkeana and S. bebbiana, not with S. cinerea" (Argus 1997: 76, footnote). Therefore, the treatment of the two as subspecies of a single species seems not enough justified. Unless the opposite is proved, we prefer to treat them as two separate species.

In the US, naturalized S. atrocinerea was first identified by George Argus (1985) as herbarium specimens from North Carolina, the oldest of which dated back to 1929 and 1939. The second finding occurred a decade later in New York (Tucker 1996). Tucker discovered an alien-willow infestation and sent his vouchers to Argus, who identified them with rusty willow previously known only from North Carolina.

However, the alien willow still remained unknown to the majority of field workers. Until recent years its expansion on the North American Atlantic Coast has remained unnoticed, even though it has been very abundant and in some areas even dominant. Rusty willow has been taken for native American willows, mostly Bebb's willow (S. bebbiana Sarg.) or pussy willow (S. discolor Muhl.). This situation changed only when Tom Rawinski (Rawinski 2005) identified the common willow on the New England coast with the European species and demonstrated its invasive character. Rawinski addressed the scientific community as well as general public through a number of newspaper and online publications. Since then, this willow has been discovered and discussed in all of coastal New England. The findings triggered a multitude of reports, notes, and publications (Tucker 2006, 2007; Northeastern Area News Notes February 2006), particularly on the web, including our own, at www.salicicola.com (2006, 2007) and in the Friends of the Blue Hills Newsletter (Kadis 2007). In autumn of 2005 we had a chance to make two short field trips in the Boston area, one with T. Rawinski, the other with G. Argus. Consequently, we could conclude with more confidence that the most common willow around Boston is indeed Salix atrocinerea. Its identity in eastern Massachusetts is now quite obvious.

At the same time, many questions regarding the "invasive willow" remain unresolved. First of all, not only S. atrocinerea but also S. cinerea is present within the alien complex. Yet the latter seems to play a much more modest role. Another problem is that many specimens exhibit characters intermediate between S. atrocinerea and the native American willows. This situation makes one assume that S. atrocinerea has expanded in eastern MA at least partially by way of producing hybrids with the American willows. While in balanced ecosystems there exist mechanisms preventing hybridization of closely related species, these isolation mechanisms may fail when humans promote species from continent to continent. The fact that the expansion of the alien willow and/or its hybrids has remained unnoticed for decades might be explained by taking into consideration another strange phenomenon: the native willow that is supposed to be one of the commonest all across Massachusetts, S. bebbiana, has become impossible to find in the Boston area. We found pure S. bebbiana when we visited Williamstown (western MA) and New London (New Hampshire); previously we had observed S. bebbiana in the upstate New York, Churchill (Manitoba), Schefferville (Quebec), Ottawa (Ontario), Lapland (Finland), Yakutia, Magadan, Amur Obl., Khabarovsk and Maritime Province (Russia). However, we never identified a single typical S. bebbiana in eastern Massachusetts.

It is most probable that in the absence of pure S. bebbiana, S. atrocinerea has been habitually taken for this native willow. The facts suggest that pure, typical Bebb's willow has been largely eliminated around Boston, perhaps partially due to hybridization with rusty willow. Map 1 depicts communities of eastern Massachusetts in which we found rusty willow, gray willow, or their hybrids within the time period 2002-2007. Map 2 shows localities for putative hybrids of Bebb's and rusty willow found during the same time. S. atrocinerea might have produced hybrids not only with S. bebbiana, but also with other native willows, such as S. discolor and S. humilis Marsh. We have found the latter species unexpectedly scarce around Boston. We are going to address the problem of S. atrocinerea hybrids in a different publication. Yet most of the time, S. atrocinerea can be separated from other willows of the section Vetrix, including S. cinerea.

Diagnostic characters of rusty willow

If not damaged, grows as a tree (Photo 1) to 10-15 m tall, but may also be a high shrub (Photo 2). Of New-England Vetrix willows, only S. bebbiana can grow as a tree (Photo 3). Tree growth is also characteristic of S. caprea L. and, as a rare exception, of S. aurita L. (Berg 2000); however, the latter two have hardly ever become naturalized in New England.

Dense and long prominent ridges (striae) are formed on the wood underneath the bark (Photo 4). The wood ridges correspond to longitudinal depressions on the bark surface of the older branches. Bark on mature trunks and limbs is thus characteristically "wavy," smooth, light gray (Photo 5). This character is shared with S. cinerea, but that species does not grow as a tree. In S. aurita, S. caprea, and S. bebbiana, wood ridges are either completely absent or short and sparse; the respective wavy bark is never formed.

S. atrocinerea produces buds of two contrasting types, floriferous buds being much larger than vegetative buds (Photo 6). This is typical of most species of Sect. Vetrix: S. caprea, S. cinerea, S. aurita, and the native S. discolor. This character helps separate S. atrocinerea from the more distantly related S. bebbiana, which has nearly uniform buds, all of a different shape.

Branchlets are rather slender, glabrous, frequently with reddish coloration. In this respect, S. atrocinerea resembles the rarely cultivated S. aurita as well as native S. bebbiana; this should help distinguish it from the native S. discolor and introduced S. cinerea, both of which have much thicker and, at least in case of S. cinerea, entirely gray-colored branchlets and branches.

Aments are short-stalked, densely flowered, rachis is never exposed, bracts black (Photo 7). In S. bebbiana aments are on foliated stalks that elongate with age and become very loose when ripe; rachises clearly exposed; bracts pale, light-brown, rarely black at apex (Photo 8).

Veins (up to the third order) are much prominent on the lower leaf surface (Photo 9) forming reticulum and a corresponding rugose pattern on the upper surface. This character is shared by the majority of Vetrix species including S. bebbiana; however, this makes S. atrocinerea different from S. discolor, which has rather smooth lower leaf surface (Photo 10).

Rusty willow has received its spoken name for the ferruginous (Photo 11) hairs that may be present on its lower leaf surface together with grayish hairs. While the presence of these rusty hairs is diagnostic, their absence does not necessarily exclude a possibility of S. atrocinerea. Ferruginous hairs may be more or less abundant or completely absent. Ferruginous hairs may be sometimes also found in S. discolor, but never in S. bebbiana or S. cinerea.

Upper leaf surface is bright green and shining (Photo 12). S. cinerea leaves (Photo 13) are characteristically dull—hence the name gray willow or ash willow.

As to leaf shape, it is highly variable. Even though the leaves of S. atrocinerea differ from other species, it is difficult to describe the difference concisely. In order to use the leaf shape as a diagnostic character, one must have a trained eye and try to compare the entire variability ranges of certain species rather than solitary leaves.

Having said all that, we have no intention to produce an impression that separating S. atrocinerea from S. cinerea and other Vetrix species or hybrids in nature is an easy task. We cannot confidently attribute every willow that we find to one or the other species. At this stage the most efficient way to consider the alien-willow infestation is to treat it in the entirety, that is, to look at the complex that includes both European willows along with their possible hybrids with native willows — with the overall dominance of rusty willow.

S. atrocinerea might be present in all of eastern Massachusetts counties. Our field observations confirm that it is common at least in Essex, Middlesex, Norfolk, Suffolk, Bristol, Plymouth, and Barnstable counties of eastern Massachusetts, i.e., everywhere where we made observations (see Map 1). Indeed, it probably has become the commonest of all the willows, particularly in Norfolk and Suffolk.

Rusty willow is now known to be present, with some gaps, from eastern Canada to North Carolina. Even though it is a recent discovery, rusty willow most probably was introduced to North America centuries ago. It has been long known in the US trade under the name "florist's gray willow" and perhaps is still sold under various erroneous names.

Part of the problem with the belated recognition of rusty-willow expansion might be explained by the confusion in the taxonomic treatment. Around the world (particularly in Australia and New Zealand) the willow that is notorious as the most significant menace to wetlands and riparian communities has been known as "S. cinerea." Any such listings in the literature are to be treated with caution, as they might actually contain references to S. atrocinerea or both. Cremer (2003) listed both willows as the two subspecies of a single species for rural Australia, hence the presence of rusty willow was explicitly confirmed for Australia.

Even though the situation with S. atrocinerea and S. cinerea appears to be bad enough in Australia, in eastern North America it might turn out to be even worse. In Australia the alien willows at least do not pose a threat for the existence of native willow species, since there are no native willows there. In New England, a few of the native willow species might become extinct by way of producing hybrid specimens with rusty willow. This situation reminds of another case in Europe. Brittle willow (S. fragilis L.) was introduced to Europe from Asia Minor during the Middle Ages or even earlier, produced there a huge secondary range and hybridized with the native European S. alba L., so that in disturbed habitats all across Europe boundaries between the two species have been eroded (Skvortsov 1973, translation 2007; 1999).

Our account is only based on populations in eastern Massachusetts. The variability range of the willow we call here S. atrocinerea may not be the same as in the area of its natural distribution in Atlantic Europe for two reasons. First of all, we do not know to which extent original introductions were representative of natural European populations. Most probably, original plants included cultivars and/or cultivated hybrids. Besides, the willows in question might be products of subsequent hybridization with native willows. Remarkably, other willows capable of becoming invasive (S. alba, S. fragilis, and perhaps also S. babylonica L.) are most probably hybrid clones of all the three species in different combinations rather than pure species. While the hybrid nature of introduced "S. fragilis" is now a well-known fact, the nature of the invasive willow in New England is yet to be studied.

Some herbarium collections pertaining to this note have been forwarded to George Argus, (CAN), duplicates to be deposited at NEBC Herbarium. We are grateful to George Argus for his confirmation of the identity of the alien willow and for a short survey of S. atrocinerea populations that we completed together in the Boston area under pouring October rain. Tom Rawinski's breakthrough played the major part in our own understanding of the alien willow. He was the first person with whom we could share our initial concerns. (Photo 14).

Literature Cited

Argus, G. W. 1985. Computerized catalogue of herbarium specimens of Salix in the Southeastern United States. - Natural Museum of Canada. Ottawa.

Argus, G. W. 1986. The genus Salix (Salicaceae) in the Southeastern United States. - Systematic Botany Monographs. Vol 9. 170 pp.

Argus, G. W. 1997. Infrageneric classification of Salix (Salicaceae) in the New World. - Systematic Botany Monographs. Vol. 52. 121 pp.

Berg, T. 2000. Salix cinerea L., Salix aurita L. — In: Jonsell, B. (ed.), Flora Nordica 1, Stockholm, pp. 153-156

Cremer, K. W. 2003. Introduced willows can become invasive pests in Australia. Biodiversity 4(4): 17-24.

Gould, L. L. 2005. The invasive beat: what's here and where is it? Rhode Island Naturalist. 12(2): 13-14.

Kadis, I. 2007. Unexpected guests become unwelcome owners. Friends of the Blue Hills Newsletter. 32(1): 1, 5.

Rawinski, T. 2005. European Gray Willow (Salix cinerea): A message from Tom Rawinski, USFS. Invasive Plants Newsbriefs 14, New England Invasive Plant Group [NIPGro], 25 August 2005. [cited after: Gould 2005]

Skvortsov, A. K. 1973. [Present distribution and probable primary range of brittle willow (Salix fragilis L.)]. Problemy biogeotsenologii, geobotaniki i botanicheskoy geografii. Leningrad, Nauka, pp. 263-280.

Skvortsov, A. K. 1999 Willows of Russia and adjacent countries. Taxonomical and geographical revision. — University of Joensuu, Faculty of Mathematics and Natural Sciences, Report Series # 39. Joensuu. English translation. 307 pp.

Skvortsov, A. K. 2007. Present distribution and probable primary range of brittle willow (Salix fragilis L.) Problemy biogeotsenologii, geobotaniki i botanicheskoy geografii. Leningrad, Nauka, pp. 263-280. English translation: www.salicicola.com

Tucker, G. C. 1996. Salix atrocinerea — an overlooked willow in New York State. New York Flora Association Newsletter. 7(2): 2

Tucker, G. C. 2006. Additions to the flora of Rhode Island. Rhodora 108: 65-71.

Tucker, G. C. 2007. Additions to the flora of Connecticut. Rhodora 109: 459-463.