Climate change and the development of coastal plain disjunctions in the central Great Lakes region PDF
Preview Climate change and the development of coastal plain disjunctions in the central Great Lakes region
1 RHODORA, Vol. 99, No. 898, 101-117, 1997 pp. CLIMATE CHANGE AND THE DEVELOPMENT OF COASTAL PLAIN DISJUNCTIONS THE CENTRAL IN GREAT LAKES REGION and Darren Stephen Jackson K. Singer T. Aven Department of Botany, Nelson Building, WY Wyoming, 8207 Laramie, University of Marsh northwestern abstract. Paleoecological studies Portage in In- at diana provide a Holocene record of local populations of four species disjunct Rhynchospora macrostachya, from the Gulf and Atlantic Coastal Plains: R. The populations Fuirena pumila, and Eleocharis equisetoides. scirpoides, were Portage Marsh when water levels dropped owing to re- established at They drying during the mid-Holocene. persisted until the late gional climatic macrostachya occurs the marsh. Populations of Holocene; only in R. still these and other Coastal Plain disjuncts probably were established in the south- mid-Holocene drying and formation of ern Lake Michigan region following extensive shallow wetlands by coastal processes. Seed sources for these pop- We Lakes propose may Great region. that ulations have been in the eastern Great Lakes region development of Coastal Plain disjunctions in the central of westward range-shifts in response to cli- consisted of a sequential series migration along than a single late-glacial mate-induced habitat changes, rather a corridor of suitable habitat. populations, climate Key paleoecology, disjunct Words: biogeography, Lakes change, Great with Gulf and Atlantic species populations of plant Scattered Great Lakes region occur the central in Coastal Plain affinities have These species typically ex- Reznicek 1994). 1922; (Peattie from ranging south- on Coastal Plain, the tensive distributions New and occasionally southern Jersey Texas southern eastern to most North New through of the and absent or rare England, are geography, ecology, review of the American In a recent interior. Reznicek populations, and conservation biology of the disjunct most lakeshores Although concentrations of the disjunct water fluctuating levels. Lakes Great region, central areas in the occur several species in Lake Michigan southern the and diverse most extensive is the Michigan (Rez- and southwestern Indiana region of northwestern have been populations most Indiana Unfortunately, nicek 1994). 101 Rhodora 102 99 [Vol. extirpated owing to extensive wetland destruction during the past (Wilhelm century 1990). The Coastal Plain disjunctions have long intrigued ecologists and biogeographers (Peattie 1922, 1930; McLaughlin 1932; Cain Keddy Keddy and Reznicek Rez- 1944; 1981, 1983; 1982, 1986). nicek (1994) reviewed hypotheses proposed to explain the dis- which from junctions, range long-distance dispersal to late-glacial migration along a corridor of suitable habitat extending along the ancestral Great Lakes and their outlets. These and other expla- nations are difficult to test empirically, however. The of biogeographic explanations based on difficulty testing (Wood Brown past events widely recognized 1972; and Gibson is 1983; Stuckey 1993). Paleoecological studies are a powerful tool ma- for testing disjunction hypotheses, because pollen and plant crofossils provide direct records of past occurrences and distri- butions of species (Whitehead 1972). For example, plant macro- New fossil studies of late-glacial sediments in England and the eastern Great Lakes region have been used explanations to test for disjunct ranges of high-arctic and cordilleran species south- in Canada and Thompson eastern (Miller 1979; Miller 1987, 1989, 1993). We have completed Marsh a paleoecological study of Portage in northwestern Indiana that provides a long-term record of local populations of four Coastal Plain Eleocharis equiseto- disjuncts: ides (Elliott) Ton*., Fuirena pumila (Torr.) Sprengel, Rhynchos- pora macrostachya and Rhynchospora scirpoides (Vahl) Torr., Griseb. (Singer et 1996). That record, combined with recent al. advances in understanding of the Holocene climatic history of the Webb Great Lakes region (Baker 1992, 1996; R.S. et al. et al. 1993; Singer et 1996), provides a framework for evaluating al. We the origin of the disjunction. describe here our paleoecolog- summarize ical results, the postglacial geological, paleoecologi- and cal, paleoclimatic history of the Great Lakes region, and dis- cuss mechanisms potential responsible for the Coastal Plain dis- junctions. MATERIALS AND METHODS Portage Marsh, a shallow, 18 hectare marsh in the towns of Portage and Garyton in northwestern Indiana (Figure among is 1), the few remaining undrained or marshes northwestern unfilled in — Jackson and Singer Coastal Plain Disjuncts 103 1997] America. North Marsh eastern in Location of Portage site Figure 1 . The on Lakeshore. Dunes National site is Indiana Indiana outside C and 13,000 l4 years between 12,200 deposited sediments shore Glenwood phase of ancestral B.P) during the II Before Present (yr The marsh Thompson 1992). and (Chrzastowski Lake Michigan century. human during the past by activity has been disturbed 1950s before the marsh urban; The surrounding the is landscape Marsh vegetation was predominantly agricultural. landscape the Rhodora 104 99 [Vol. dominated today by Carex lasiocarpa Ehrh. and Calamagrostis is Typha canadensis (Michx.) Beauv., with scattered patches of P. Dulichium arundinaceum and Cephal- (L.) Britton, latifolia L., m Open deep anthus occidentalis L. water patches (ca. 0.6 in summer 1994) are occupied by Proserpinaca palustris L., Pota- A mogeton gramineus and Utricularia vulgaris L. complete L., observed marsh provided Singer of plants in the is in et al. list Plant nomenclature follows Gleason and Cronquist (1996). Of Rhynchospora ma- (1991). the Coastal Plain species, only crostachya has been observed in the modern flora and seed bank of the marsh. We analyzed pollen and plant macrofossils from two sediment was cores (Singer et 1996). Core obtained using a land- al. 1 cm based vibracorer diameter) and included a complete se- (7.6 quence of Holocene and sediments. Core which late-glacial 2, was included only the top meter of sediment, collected using a cm Both modified Livingstone piston-corer (10.2 diameter). cores were taken approximately 75 meters from northern edge of the the basin, where sediments are deepest. Sediment age estimates were based on radiocarbon dating of both cores. cm Sediment samples (50 3 each) were dispersed, sieved (710 X and 355 jim mesh), and scanned 6.5 magnification using a at stereomicroscope for macrofossil analysis. All well-preserved microspo- macrofossils (including seeds, conifer needles, fruits, rangia, and oospores) were identified by comparison with her- barium-documented specimens were reference specimens. All as- sumed Lake modern to be constituents of the flora of the southern Deam Michigan region (Peattie 1930; 1940; Voss 1972, 1985; Swink and Wilhelm Notes on morphological used 1979). criteria Appendix macrofossil provided an Sing- in identification are in in er et (1996). al. Sediment samples for pollen analysis were prepared using stan- dard dispersion and digestion procedures, suspended silicone in 400 X and scanned (Singer 1996). Pollen percentages oil, at et al. sum were on for terrestrial taxa calculated based a of arboreal, all shrub, and upland herb types. Cyperaceae pollen was not included in that sum. Pollen percentages for wetland and aquatic plants, sum including Cyperaceae, were calculated from a of terres- all and wetland, aquatic types. trial, — Jackson and 1997] Singer Coastal Plain Disjuncts 105 RESULTS and Pollen plant macrofossils from sediment cores provide a record of regional upland and local aquatic/wetland vegetation for the past 11,000 years (Singer The et al. 1996). pollen per- centages for tree taxa, Ambrosia, and Poaceae (Figure primar- 2) ily represent vegetation on the surrounding regional uplands. Some Poaceae and most Cyperaceae pollen probably derives from wetland Marsh The pollen record during (> the late-glacial interval 10,000 yr B.P) indicates regional vegetation consisting of open dom- forest by inated Picea spp. (Figure together with Abies, Larix, Pop- 2), ulus, and Pinus banksiana Lambert (Singer 1996; Jackson et al. Webb et al. 1986; et 1983). During this period, the basin was al. occupied by an open, hard water Plant macrofossils lake. are rare sediments 0,000 and of mixed Que Fraxinus and Carya The L., nigra Marshall, (Figure reus, 2). was basin occupied by submersed a lake containing plants [Najas & flexilis (Willd.) Rostk. Schmidt, Chara Figure Emergent sp.; 31. plants were scarce in the basin, and sediments consisted of fine- mud textured algal lake (gyttja). Between 6000 and 5500 yr B.P., the mesic forests of the region were replaced by Quercus savanna (Figure At the same time, 2). water Marsh dropped levels in the Portage basin rapidly, leading to development of a shallow, peat-forming marsh with a diverse Many assemblage of emergent plants (Figure of the emergents 3). are annuals or perennials characteristic of exposed shorelines and drawdown The mudflats during temporary occurrence of events. submersed Potamogeton and (Najas Chara, spp.) floating- flexilis, drawdown leaved emergents plants (Brasenia) together with the in the assemblages regime of interannual water level indicates a fluctuations. Figure Pollen percentage diagram from Portage Marsh Core Only 2. 1. taxa am Figure 3. Core 1. Only selected taxa are included. Modified from Singer et al. (1996). Rhodora 99 106 [Vol. o dC5 E o> a <u in o -M H M -H cd in en IT) Oio ro o CD C\J in CO < — Jackson and 1997] Singer Coastal Plain Disjuncts 107 O % ^ ro ^ CVJ I i i SK ^ Li % C9 L^ %> £ «i\X £ xl I i „] X CVJ ^> o "O ll-l.llll S>> II o o \n o o o CO ir<* f * f « < * » « t « * i f • « * f[44 *4 i 4 «f — t 4 t 4 t { 4 < « 4 r^ * * i i 4 < 4 4 D E 400' 300 200 o 100 "-"***-'-"**'*' * a CD 0> IT) CDo ro o CD into CVJ CO CO 2 Rhodora 99 108 [Vol. o •H o 5 o CVJ — Jackson and Singer Coastal Plain Disjuncts 109 1997] The {Rhynchospora four Coastal Plain species scirpoides, R. macrostachya, Fuirena pumila, and Eleocharis equisetoides) col- Of onized the marsh between 5700 and 4500 yr B.P (Figure 3). shallow-water submersed while these, E. equisetoides a plant, is and pumila emergents of R. scirpoides, R. macrostachya, F. are Rhynchospora and wet scirpoides mudflats other exposed, sites. and pumila F. are annuals. Quercus savanna on uplands of northwestern Indiana persisted American 1980), al- and Fagus developed though mesic of Acer, Betula, lo- forests Holocene cally on with fine-textured soils during the late sites Futyma 3000 yr B.P; Singer 1996; 1985; Bailey 1972). (ca. et al. mixed assembla Marsh underwent some changes emergent turbance, although the flora 2800 and Eleocharis equisetoides dis- after yr B.P. (Figures 3 4). Rhynchospora scirpoides and Fui- appeared 2800 B.P, but yr ca. American may marsh Euro- rena pumila have persisted in the until disturbance (Figures 3 and The exact timing of their extirpa- 4). spanning owing a depositional hiatus at least tion uncertain, to is Rhynchos- the 19th and early 20th centuries (Singer et al. 1996). have persisted in the pora macrostachya populations evidently and 4500 years (Figures 3 4). basin throughout the past DISCUSSION The Holocene climate and water lev- wetland vegetation, changes in regional vegetation, Marsh 10,000 yr pattern Holocene (10,000 warm, moist climate during the early 5700 mid J Holocene (2800 yr B.P. to cooler, moister conditions in the late Marsh from Portage diagram concentration Figure macrofossil 4. Plant spanning the past meter of sediments, Core Core 2 includes only the top 2. from Modified Sing- included. 2800 Only taxa are radiocarbon selected years. er et (1996). al. 110 Rhodora [Vol.99 documented present). This a regional climate pattern in east- fits ern Iowa, northern southern Wisconsin, northern Indiana, Illinois, (Webb and Michigan Baker southern 1983; 1992, et et al. al. 1996; Singer 1996). et al. The Holocene period of highest effective moisture pre- (i.e., minus Lake Mich- cipitation evapo-transpiration) in the southern 6000 igan region, 10,000 to yr B.P., corresponds to the Milan- summer maximum kovitch insolation following withdrawal of the Webb Laurentide ice sheet (Kutzbach and 1991). Wright (1992) summer proposed monsoonal that higher insolation amplified summer flow into the southern Great Lakes region, increasing Reduction monsoonal flow summer rainfall. in as insolation de- 6000 B.R creased after yr led to drier conditions. Continuing late Holocene summer reduction in insolation led to cooling and in- creased effective moisture after 3000 yr B.P. (Wright 1992; T. Webb et 1993). al. The eastern Great Lakes, Lawrence and Hudson- valley, St. Mohawk lowlands experienced sequence of Holocene a different climatic changes. These regions also were cool and moist during the late-glacial period (> 10,000 yr B.R). However, they were warmest and Holocene 6000 driest during the early (9000 to yr Webb Webb Webb B.R; Gaudreau and 1986; 1990; R. S. et al. Webb summer High were 1993; temperatures T. et 1993). re- al. summer maximum. lated to the insolation Regional lake levels were low (Webb and upland was 1990), vegetation relatively xer- Webb Summer may have been ic (R. S. et 1993). precipitation al. low summer because storm tracks were shifted north (relative to today) along the steep thermal gradient the edge of the Lau- at (Webb much rentide ice sheet 1990), which occupied of eastern Canada from 9000 to 7000 yr B.P. (Dyke and Prest 1987). Ef- Webb fective moisture increased after 7000 yr B.R (R. S. et al. and 1993), the region has undergone a cooling trend for the past Webb Webb 5000 years (Gaudreau and 1986; 1993). T. et al. Climate change and the origin of coastal plain disjunc- Hypotheses proposed tions. to explain the Coastal Plain dis- junctions in the Great Lakes region into two general cate- fall Holocene gories: (1) long-distance dispersal from the Coastal and Plain populations, (2) late-glacial migration along the ances- Great Lakes and Lawrence and Hud- their outlets in the tral St. son-Mohawk and up valleys the Mississippi and River Illinois
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