On the Distributional History of the Arctic-Alpine Moss Cyrtomnium hymenophylloides (Mniaceae) in North America

Herbarium specimens of Cyrtomnium hymenophylloides document that this calcicole moss is widespread at lowland stations across the North American Arctic, largely north of treeline. It is found southward in the Rocky Mountains of western Canada, often in high elevation vegetation, and in New York-New England-eastern Canada in mountainous areas, but at lower altitudes beneath deciduous or conifer-deciduous forest. Its southeasternmost disjunct stations, which occur north of the glacial boundary, are interpreted as relicts on the basis of a latePleistocene fossil recovered from sediment of glacial Lake Hitchcock, southeastern Vermont. Extant populations occur near where the fossil was found. Spore production is unknown in North America in this dioicous moss, but its fragile stems may allow local dispersal at edaphically favorable sites and over longer distances in treeless vegetation. The southeasternmost disjunct populations may be clones, the persistence of which is favored by vegetative spread through the proliferation of new leafy branches from portions of the plants growing at or below the surface of the soil. The presence of disjunct arctic-alpine mosses in the conifer and conifer-deciduous forest regions of the northeastern United States is well documented. Some of the disjuncts are restricted to the small areas of alpine tundra in these regions, while others occur on cliffs, gorge walls, and edaphically favorable sites at lower altitudes and in forests of differing composition. These two patterns may have had different histories. Disjuncts at lower elevation sites represent possible examples of populations persisting as relicts in favorable microhabitats, which have remained similar to those that existed at the end of the Pleistocene, whereas the mountain-top disjuncts perhaps arose mainly by post-glacial dispersal southward (or northward) from regions of more continuous distribution. Although it may be impossible to decide which of these explanations (or some other one) fits a specific example of disjunction, studies into the fossil history and reproductive biology of such disjunct species allow an improved understanding of factors that led to the often fragmented ranges of arctic-alpine mosses. A noteworthy arctic-alpine moss in eastern North America is Cyrtomnium hymenophylloides (Hiib.) T. Kop., a rare species in New York-New England, that was first discovered in North America at Trenton Falls near Utica, New York. At the southern range limit of this moss in North America, in New York-New England, C. hymenophylloides typically grows well below the alpine zone in small patches on ledges and crevices of bedrock that either is calcareous or weathers to release calcium. I recently discovered a small fragment of a plant of C. hymenophylloides with other moss fossils and the remains of vascular plants in an exposure of glacial Lake Hitchcock sediment in Dummerston, Vermont. This late-Pleistocene record indicates that populations at the southern range limit of the species in eastern North America are likely to be relicts rather than recent immigrants. I describe and illustrate the fossil and document and evaluate the distribution of C. hymenophylloides in North America, using recently discovered aspects of its reproductive (dispersal) biology to explain how the disjunct southern populations may have originated and are maintained. GEOLOGICAL AND PALEOBOTANICAL SETTING Glacial Lake Hitchcock was a large body of fresh water that existed in the Connecticut River Valley between a dam at Rocky Hill, Connecticut, and the receding margin of the Laurentide ice sheet. Sediments of glacial Lake Hitchcock occur over a distance of 350 km northward to Burke, Vermont. Glacial Lake Hitchcock appears to have drained at 12,300-12,400 radiocarbon years before the present, i.e., 1950 (yr B.P.) (Ridge & Larsen 1990), and the lake began prior to 15,000-16,000 yr B.P., an age inferred by Koteff et al. (1988), or at about 15,600 yr B.P., the inception of the Antevs New England varve chronology, according to Ridge and Larsen (1990). Sediment carried into the lake was deposited as deltas and as varves consisting of coarse-grained summer layers and fine-grained winter ones. Recently discovered plant material 0007-2745/96/187-192$0.00/0 This content downloaded from 40.77.167.32 on Sun, 31 Jul 2016 04:49:43 UTC All use subject to http://about.jstor.org/terms 188 THE BRYOLOGIST [VOL. 99 from outcrops of these deposits, studied paleobotanically and radiocarbon dated (Miller 1995; Ridge & Larsen 1990; Stone & Ashley 1992), shows that throughout the lake's history, vegetation consisting mainly of tundra plants existed in the nearby uplands. Studies of pollen and macrofossils (fruits, seeds, flower parts, and leaves), from samples of fresh sediment I collected at the Canoe Brook site, Dummerston, Vermont, supplement information about late-Pleistocene vegetation described from studies at other sites in western New England. The Canoe Brook pollen assemblage (Miller 1993) is similar to those extracted from basin-bottom sediments in small lakes of the region and consists of a high percentage of herb pollen (59%: sedge and grass 41%, various herbs 18%) and a smaller amount (41%) of pollen of various trees and shrubs (pine 22%, spruce 11%, other trees 4%). Vascular plant macrofossils included the leaves of Dryas integrifolia, Salix herbacea, and S. reticulata and seeds or fruits of Oxyria digyna, Sibbaldia procumbens, and Silene acaulis. The relatively small amount of spruce and pine pollen and the presence of seeds and other remains of these and additional arcticalpine plants indicate that the late-Pleistocene landscape and vegetation of the region were tundra-like in aspect. An assemblage of fossil mosses from the Canoe Brook site, described elsewhere (Miller 1993), supports this conclusion. The plant assemblage is securely radiocarbon dated at 12,350 _ 90 yr B.P. (Beta-53633, CAMS-2667) by accelerator mass spectrometry of a twig of willow (Miller 1993).