Relic or Recruit? Newly Discovered Aronia arbutifolia (Ell.) Pers. (Red Chokeberry) in Kent County Raises Questions Regarding Past and Future DistributionsSkip other details (including permanent urls, DOI, citation information)
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RELIC OR RECRUIT? NEWLY DISCOVERED ARONIA ARBUTIFOLIA (ELL.) PERS. (RED CHOKEBERRY) IN KENT COUNTY RAISES QUESTIONS REGARDING PAST AND FUTURE DISTRIBUTIONS
Michael P. Ryskamp and David P. Warners Department of Biology Calvin College Grand Rapids, MI 49546
The nomenclatural history of the genus Aronia is replete with name changes, misidentifications, and debates regarding classification. In Michigan, collections of Aronia arbutifolia (Ell.) Pers. (Red chokeberry) from the early 20th century are thought to be misidentifications of Aronia prunifolia (Marsh.) Rehder (Purple chokeberry). A recent discovery of A. arbutifolia in West Michigan could represent a rare species that has naturally occurred in Michigan in the past or an adventive individ- ual that has only recently arrived. We provide documentation of the new state record and discuss its discovery in the context of earlier ambiguous records of Red chokeberry in the Great Lakes region. We also consider whether this species is exhibiting a coastal plain disjunct distribution, and we ex- amine the possibility that Red chokeberry is expanding its range in response to climate change. Fu- ture genetic research and molecular analysis could help answer some of the questions raised by this interesting discovery in Michigan. Keywords: Red chokeberry; Aronia arbutifolia, coastal plain disjunct, climate change
In 1933, at the annual meeting of the Torrey Botanical Society, renowned botanist and landscape architect Henry Teuscher (1933) gave an address on the question “what is a species?” In his short talk, he used the Aronia (chokeberry) complex to illustrate some of the inherent problems of using morphological cri- teria to define species of plants. Four decades later, Professor James Hardin (1973) revisited “the enigmatic chokeberries” in a paper that outlined the taxo- nomic and nomenclatural confusion surrounding the shrubs since the early 20th century. Today, the taxonomy of the two (or three) polymorphic chokeberry species in the Aronia genus (or sub-genus) is still debated, its history likely clouded by interbreeding (Hardin 1973; Voss 1985; Voss and Reznicek 2012), different reproductive strategies (Campbell et al. 1991; Hovmolm 2004), and inter-population variability (Hardin 1973; Uttal 1984; Brand 2010). Over the years, several different genus names have been assigned to this small group, in- cluding Mespillus L., Pyrus L. f., Crataegus Lam., Adenorachis Nieuwl., and Photinia K.R. Robertson & J.B. Phipps (MacMillin 1892; Nieuwland 1915; Brit- ton and Brown 1970) [In this paper, we follow the nomenclature outlined by Voss (1985)]. More recently, studies involving nucleotide sequencing have es- tablished molecular phylogenies of Pyrinae (Rosaceae) that distinguish Aronia
from Photinia (Campbell et al. 2007; Potter et al. 2007; Guo 2011). Campbell et al. (2007) recommend maintaining Photinia and Aronia as separate genera, as their results yielded no molecular evidence for placing them together, “or even near one another,” on a phylogenetic tree. The state of Michigan is no stranger to the botanical confusion associated with these shrubs. Early plant surveys in the state, circa 1900, reported Aronia arbutifolia (Ell.) Pers. (Red chokeberry) in St Clair (Dodge 1900), Berrien (Beal 1904; Hebert 1934), Ionia (Beal 1904), Kent (Cole 1901), and other counties. Botanical literature from that period, e.g., W. J. Beal’s Michigan Flora (1904), lists two species of Aronia, A. arbutifolia and A. melanocarpa (Michx.) Elliot. (Black chokeberry) [the latter actually listed as A. nigra in Beal (1904)]. How- ever, these early records of Red chokeberry in Michigan are universally inter- preted as either reports or misidentifications of A. prunifolia (Marsh.) Rehder (Purple chokeberry) (which sometimes appeared in older literature as a variety of A. arbutifolia). This taxon (A. prunifolia) is typically understood to have orig- inated as a hybrid between A. melanocarpa and A. arbutifolia (Farwell 1918; Hardin 1973; Voss 1985). Our recent finding of A. arbutifolia in Kent County clarifies the occurrence of Aronia in Michigan by providing the first confirmed report of this taxon in the state. In this paper, we describe the A. arbutifolia dis- covery and then investigate some possible scenarios to explain its presence in West Michigan.
MATERIALS AND METHODS
On November 4, 2011, a group of eight potentially clonal Red chokeberries was discovered dur- ing a field trip to Townsend Park, Kent County, MI. The shrubs were found in a moist bottomland east of Bear Creek, growing in a backwater corridor that ran along the bottom of a west-facing hill- side. A small grove of large Thuja occidentalis (Northern White cedar) (10–18???dbh) filled the canopy around the chokeberries, affording a relatively open understory characterized by groundwa- ter upwelling and several waste deep pockets of muck. To the west, a shrub zone of Cornus spp. (Dogwood), Betula pumila (Bog birch), Zanthoxylum americanum (Prickly ash), and Physocarpus opulifolius (Ninebark) transitioned into an open wet meadow dominated by Carex stricta (Tussock sedge), Onoclea sensibilis (Sensitive fern), and Cirsium muticum (Swamp thistle). A shrubby ripar- ian zone marked the western edge of the meadow, the eastern banks of Bear Creek. At the time of the discovery in early November, dozens of bright-red cymose infructescences populated the distal ends of the shrubs’ branches. Tomentose beneath, the scarlet-colored leaves were elliptical and had short acuminate tips (Figure 1). The tallest shrub reached six meters in height and was supported by a cedar trunk. All the shrubs were elongate, their lower reaches mostly un- branched. Collections of flowering material were made on May 8 2012.
VOUCHER SPECIMENS: Michigan, Kent Co., Cannon Twp. Townsend Park, c. 200 m SE of Ramsdell Dr. and Six Mile Rd., 43° 03?N, 85° 27??W, 4 Nov. 2011, M.P. Ryskamp 1 (CALVIN, MICH); M.P. Ryskamp 2 (CALVIN, NY); 8 May 2012, M.P. Ryskamp 5 (CALVIN, MICH).
RESULTS AND DISCUSSION
The presence of Red chokeberry in Kent County raises a variety of questions with regard to the origin, establishment, and mechanism of dispersal to West
FIGURE 1. Photograph taken November 4, 2011 at Townsend Park in Kent County, Michigan showing the tomentose leaves and hairy stems of Aronia arbutifolia, with canescent berries still at- tached.
Michigan. Below, we explore several different possibilities that might account for this surprising discovery, we discuss some of the implications of these possi- bilities, and we propose some future research initiatives. Given the history of misidentifications of Red chokeberry in Michigan (and in other states), one possible explanation is that what we have identified as Aro- nia arbutifolia is actually an introgressant of the hybrid ‘A. prunifolia complex’ (Uttal 1984). This possibility, however, can be eliminated due to several distinct morphological features of the Townsend Park chokeberries. Firstly, the red- colored fruits observed on the shrubs at the time of discovery are unique to A. ar- butifolia, the fruits of A. prunifolia having a much darker purple to purple-black hue when ripe (Gleason and Cronquist 1963; Hardin 1973; Voss 1985). Uttal (1984) calls fruit color “the only reliable basis for separating chokeberry species.” The fact that both leaves and berries remained on the shrub in Novem- ber is also indicative of A. arbutifolia; both Purple and Black chokeberry pro- duce fruit that ripen and drop earlier in the season (Hardin 1973; Voss 1985; Brand 2010). Additionally, the size of the pomes, which we measured to range 4–7 mm in diameter, is congruent with descriptions of A. arbutifolia—the pomes of A. prunifolia, in contrast, range from 8 to 10 mm (Gleason 1958; Gleason and Cronquist 1963). Lastly, Robinson and Fernald (1908) describe Red chokeberry as having cymes that are “more numerously fruited (9–18)” than those of A. prunifolia, which have 3–10 fruits. Even as late as November, the Townsend Park shrubs had some cymes with 10 or more pomes still attached (Figure 2), and during a field trip in May 2012, with the shrubs in full bloom, we counted 10–16 florets per cyme (Figure 3). In light of all these traits [and with confirma-
FIGURE 2. Another photograph taken on November 4, 2011 showing the bright red fall foliage and pomes of Aronia arbutifolia.
tion from A.A. Reznicek (personal observation)], we are confident that the shrubs in question are indeed A. arbutifolia. A second possible explanation for the presence of these shrubs is that they represent an escaped horticultural form. Again, we think this possibility is un- likely in light of how closely the shrubs match morphological descriptions of wildtype Aronia arbutifolia (as described in the above paragraphs). The most commonly sold cultivar (and perhaps the only one), A. arbutifolia ‘Brilliantis- sima,’ has a more compact form, more lustrous leaves, and has larger, glossier fruits than the wildtype species (Flint 1997; Dirr 2011). This description signifi- cantly contrasts with the small canescent berries and the tall elongate growth habit of the Townsend Park shrubs. Additionally, the habitat immediately sur- rounding the Townsend Park chokeberries was replete with native species that indicate relatively pristine conditions, which differs from the typical disturbed habitat in which most non-native escapees are found (Herman et al. 2001). Though we are confident the Townsend Park shrubs are wildtype Aronia ar- butifolia, there still remain several questions regarding how, when, and from where this species arrived. Distribution patterns of coastal plain species may pro- vide important insight to these questions. There are several communities of dis- junct flora around the Great Lakes region that occur outside their primary range of the Atlantic and Gulf coastal plains (Reznicek 1994). These concentrations
FIGURE 3. Aronia arbutifolia in full bloom, Townsend Park, Kent County, Michigan, May 8, 2012.
occur in three main areas: 1) a post-glacial drainage occupying southwest Michi- gan and northwest Indiana; 2) the region in Ontario east of the Georgian Bay; and 3) the sand deposits of central and northern Wisconsin. Smaller representa- tions are scattered around the Great Lakes region, in northern Michigan and in a few different locations along the shores of Lake Erie and Lake Ontario. One of the most widespread of the disjunct species, Rhexia virginica (Meadow Beauty) occurs in all three main regions and in some of the minor areas from the western shore of Lake Erie eastward toward the St. Lawrence River (Figure 4a). The range of A. arbutifolia is similar to Rhexia (Figure 4b); it centers on the south- eastern coastal plain, extending west along the Gulf coast into eastern Texas (Hardin 1973) and northward into the coastal areas of New England and into the southern portion of Nova Scotia (Eaton et al. 2010; Haines 2011). With A. arbu- tifolia identified in Kent County, it seems reasonable to ask if the same (or some of the same) edaphic and dispersal factors responsible for the presence of dis- junct species around the Great Lakes are also responsible for bringing A. arbuti- folia to Michigan—however recent or long ago that may have been. Purported records of Aronia arbutifolia in Ontario (Accessed through GBIF Data Portal, data.gbif.org, 2012-01-21) would seem to support a coastal plain
FIGURE 4. Distributions of Rhexia virginica (4a) and Aronia arbutifolia (4b). The original distribu- tion map of Rhexia appears in Reznicek (1994); images used with permission from the author. The distribution of A. arbutifolia (4b) was based on Hardin (1973), Gardner et al. (2005), Haines (2011), and on selected records from the New York State Museum (available upon request).
disjunct distribution pattern, but these records have never been confirmed, and neither Shrubs of Ontario (Soper and Hiemburger 1982) nor the Ontario Plant List (Newmaster et al. 1998) recognizes A. arbutifolia as a component of On- tario’s flora. Voucher specimens (collected in late summer and early fall) refer- enced in The New York Flora Atlas do show populations of Red chokeberries in the western region of the state, near Lake Ontario and Lake Erie, isolated from the main population along the Atlantic Coast. Further from the Great Lakes, in Nova Scotia, Red chokeberry is considered a member of the community of dis- junct coastal flora that occurs in the southern region of that province (Eaton et al.
2010), where Rhexia virginica and several other disjunct species occur. Although some records remain dubious, it does appear that A. arbutifolia has a tendency to initiate disjunct populations outside its coastal plain range. Reznicek (1994) hypothesizes that birds or possibly other dispersal agents played a role in shaping and sustaining populations of disjunct species. He adds that migrations of flora from the coastal plains into post-glacial drainages near the Great Lakes could have provided a seed source for dispersal agents. This may describe the causative events behind the establishment of Aronia arbutifo- lia in Kent County—dispersed by migrating birds that ingested Red chokeberry fruits in New York, Pennsylvania, Ontario, or elsewhere. If this scenario does de- scribe the origin of A. arbutifolia in West Michigan, then it could provide sup- port for Reznicek’s hypothesis regarding long-range dispersal of coastal plain flora. A more significant task than describing how the shrub came to Michigan, however, might be discerning when this species arrived. Past records of Aronia arbutifolia in Michigan, which date back to over a century ago, appear to be er- roneous. Many of these identifications had been made before the hybrid was de- scribed, when floral keys relied on leaf pubescence to distinguish the tomentose- leaved A. arbutifolia from the glabrous A. melanocarpa. When Emma Cole included Red chokeberry in Grand Rapids Flora, for example, she was almost certainly using degree of leaf pubescence as a determining trait during her spring-time field surveys, when fruit formation was still months away (Cole 1901). But it is difficult—if not impossible—to say with certainty that Red chokeberry was never native to Michigan. However, if Red chokeberry has been a longstanding component of Michigan’s flora, given the clear morphological differences between A. arbutifolia and both A. melanocarpa and A. prunifolia, we would expect it to have been documented at some point in the past, unless it was very rare and local. Another possibility, and perhaps the most plausible one, is that the Townsend park shrubs are recent arrivals, and that Aronia arbutifolia is simply expanding its range, perhaps in response to climate change. If climate change is in fact in- fluencing the northern and northwestern range expansion of A. arbutifolia, then we would expect to see increasing frequencies of the shrub in Kentucky and other places where it is considered infrequent or absent. A 2003 discovery of Red chokeberry in southwestern Ohio, a state record collected from Clermont County (Gardner et al. 2005), might fit the pattern of northward range expansion. Inter- estingly, two other state records were found in association with the Clermont County discovery, Bartonia paniculata (Twining screwstem) and Woodwardia areolata (Netted chain fern) (Gardner et al. 2005), both of which are listed in Reznicek’s (1994) paper as disjunct coastal plain species that occur in south- western Michigan. The presence of a disjunct community of flora at the site in Clermont County may indicate a more complex and potentially integrated inter- action between climate change and the distribution of coastal plain species, es- pecially if the populations in southwestern Ohio have only recently arrived to the area. If climate change is expanding the ranges of coastal plain flora, then we would expect to see increasing frequencies of A. arbutifolia and other coastal
plain species occurring in the ‘hotspots’ defined by Reznicek (1994), or, con- versely, in new areas outside the classically recognized regions of disjuncts. Further insight into the origin of this shrub could be gained through genetic and molecular analyses. If the molecular composition of our Kent County Aro- nia arbutifolia is found to be similar to representative individuals from Ohio or from other regions close to Michigan, a recent dispersal event from that particu- lar population would be the most likely explanation. But if the Kent County shrubs are notably distinct, such a finding would support the long-term presence of an indigenous Michigan population. A more thorough examination of herbar- ium specimens from the Great Lakes region and elsewhere could also be valu- able in determining whether or not A. arbutifolia ever occurred in the state prior to its recent discovery in West Michigan. Genetic comparisons between local Aronia prunifolia, A. melanocarpa, and A. arbutifolia could also add clarity to the debate over the relative uniformity or variability of the Aronia genus in this region. Both diploid and polyploid popu- lations of A. arbutifolia are known to exist along the Atlantic Coast (Holvmam 2004; Brand 2010), and knowing the ploidy level of the Kent County shrubs as well as other molecular traits may provide information as to their potential in- volvement in local genetic introgression within this enigmatic Aronia complex.
Trying to understand the presence of a recently discovered stand of Aronia arbutifolia in Michigan raises several possible explanations, each with its own set of implications. Its presence in an area known to be a ‘hotspot’ for coastal plain disjuncts may support the possibility that an indigenous population of A. arbutifolia has existed in Michigan for some time. However, in the present con- text of climate change, and given that the discovery of the species in Ohio is also recent, we think it is more likely that this species is a recent arrival from the southern or eastern United States. If this is indeed the case, then A. arbutifolia could either be expanding its range opportunistically or, more complexly, doing so in a manner that follows the patterns of the disjunct coastal plain species. It remains to be seen whether this shrub represents a vestige of an undisturbed Michigan ecosystem or a harbinger of a warming climate—molecular analyses could help illuminate this and some of the many other unknowns surrounding Aronia. If history is any indication, future investigations into this enigmatic genus will elicit more questions than answers.
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