Biodiversity and Conservation 14: 97–117, 2005.
# Springer 2005
Biodiversity of floodplain forests in Europe and eastern North America: a comparative study of the Rhine and Mississippi Valleys ANNIK SCHNITZLER1, BRACK W. HALE2,* and ESTHER ALSUM3 1
Laboratoire Biodiversite´ et Fonctionnement des e´cosyste`mes, Universite´ de Metz, F-57070 Metz, France; Nicholas School of the Environment and Earth Sciences, Duke University, P.O. Box 90328, Durham, NC 27705, USA; 3Department of Botany, University of Wisconsin-Madison, Madison, WI, USA; *Author for correspondence (e-mail:
[email protected]; fax: þ1-919-684-8741) 2
Received 20 May 2003; accepted in revised form 29 September 2003
Key words: Biodiversity, Floodplain forests, Mississippi River, Productivity, Rhine River, Succession, Wisconsin River, Woody species richness Abstract. In this study, we investigate the mechanisms driving biodiversity in floodplain forests with a comparison of the composition and dynamics in the warm-temperate floodplain forests of the lower Mississippi Valley and the cool-temperate floodplain forests of the lower Wisconsin and Rhine River Valleys. We employ data from original research, as well as from the literature. We compare species, genus, and family diversity across regions with respect to species richness, numbers of species per family and genus, and a similarity index. We examine these results within a historical context, as well as with respect to river-floodplain dynamics. We also compare productivity data and successional stages for each region. We find a lower species, genus, and family richness in the cool-temperate forests of the Rhine compared to the cool-temperate forests of the Wisconsin, a probable result of the lack of available refugia for Rhine species in times of glacial expansion. We find the highest richness in the lower Mississippi Valley, likely a result of climatic factors and the availability of refugia in this region. In each of the regions, floodplain forests are more diverse than their upland counterparts, demonstrating the role of river-floodplain dynamics in maintaining species diversity. Each region maintains a high and relatively similar level of productivity in the floodplain forests. They also experience similar stages of succession, although succession becomes more complex in the warm-temperate forests of the Lower Mississippi.
Introduction Bottomland forests of large floodplains in the temperate zone differ from other temperate forest communities because of their luxuriant, dense, and generally more diverse vegetation (Braun 1950; Wendelberger 1970; Robertson et al. 1978; Schnitzler 1994). Such forests have been compared to a temperate jungle (Carbiener 1970). The periodic flood pulses, which drive active exchange processes of matter and information across floodplain landscapes, affect this ecological particularity (Carbiener 1970; Dister 1984; Sparks 1995; Ward et al. 1999). Since floodplain forests in the temperate zone develop under similar conditions (i.e., the flood pulses), we expect relatively similar forest communities to develop across different systems. At the same time, studies at the global level indicate that species diversity varies greatly across continents, with European forests tending to be much less diverse than their North American counterparts (e.g., Latham and Ricklefs 1993).
98 In this paper, we examine floodplain forests in both Europe and North America to analyze the similarities and differences that exist between the two continents, with an emphasis on vegetation diversity. We compare cool-temperate floodplain forests on each of the continents (the upper Rhine Valley and the lower Wisconsin Valley) and compare these forests with the warm-temperate forests of the lower Mississippi Valley. This paper summarizes available information on species composition, forest structure, and successional processes of floodplain forests in these regions. This study follows an earlier comparison between floodplain forests of the QuercoUlmetum type in western Europe (Schnitzler 1994). In the present paper, we emphasize the respective implications of river dynamics, past history and climate to explain the ecological similarities of the floodplain forests of the temperate zone in North America and Europe. Study sites Upper Rhine valley The Upper Rhine Valley is located in northwestern Switzerland, northeastern France, and southwestern Germany. Floodplain forests of the upper Rhine include softwood forests (Salicetum albae, Oberd. 53), and hardwood forests1 (Querco-Ulmetum association, Oberd. 53). The climate is suboceanic with continental tendencies because of its location in a deep rift valley. Mean annual temperature is 10 8C. Mean annual precipitation is 500–600 mm along the Rhine and 700 mm along the Loire. Annual floods typically occur in spring and the beginning of summer for the Rhine. Along the Rhine, the initial successional stage in floodplain forests is composed of various patterns of shrubs (Salix viminalis L., S. triandra L., S. eleagnos Scop., S. purpurea L.) or tree saplings of Populus nigra L. and Salix alba L. or S. fragilis L. These different Salicaceae are selected by flooding severity and soil texture. With time, two trees dominate: Salix spp. in moist, low sites that are regularly flooded; Populus nigra in dry, elevated terraces; and Populus alba and Alnus incana L. in silty and moist sites. These pioneer forest stands are characterized by two main strata: an even-aged canopy (about 20 m high) and the grass layer (up to 2 m high), composed of clones of Urtica dioica L., Impatiens glandulifera Royle, Phalaris arundinacea L. or exotic species such as Solidago gigantean Ait. (Schnitzler 1995b). With time, hardwoods (Quercus, Ulmus, Fraxinus, Acer, Populus alba) enter under Salicaceae and develop till co-dominance, in about 100 years. Woody lianas and shrubs become more abundant during the course of succession. Their active growth by clones may block gap closure for decades (Schnitzler 1995a).
1 The usage of the word ‘hardwood’ differs among countries. In English-speaking countries, ‘hardwoods’ refers to angiosperm species; ‘softwoods’ to coniferous species. In contrast, French and German scientists often use both these terms to refer to deciduous species – hardwoods refer to species such as oaks and ashes; softwoods to willows and poplars. In this paper, we employ both definitions, using the English definition for American forests, and the European definition for European forests.
99 Lower Wisconsin valley The lower Wisconsin River flows through southwestern Wisconsin before joining the Mississippi River at Prairie du Chien, WI. Floodplain forests of the lower Wisconsin are typically classified as Elm-Ash-Cottonwood (Parker and Merritt 1995) or as lowland hardwoods (Finley 1976). The climate is continental, with cold winters and warm summers. Mean annual temperature is 7.8 8C, ranging from 6 8C in the winter to 20 8C in the summer (Lindstrom and Young 2002). Average annual precipitation is 825 mm. Annual floods typically occur in spring, in concert with snowmelt. In these forests on the Lower Wisconsin, succession can be initiated by two different groups of species depending on the environmental conditions; however, both pioneer communities are replaced by an Acer saccharinum-dominated forest in the absence of further disturbance (Ware 1955; Curtis 1959; Barnes 1985). Salix nigra Marsh. and Populus deltoides Bart. Ex Marsh. are the pioneer species that colonize freshly cleared or deposited substrate. Salix nigra appears to be restricted to recently deposited sandbars while Populus deltoides can invade mid-elevation sites and be a minor component of the canopy in mature stands (Evans 1970; Hale and Alsum, unpublished data). The second pioneer community consists primarily of Quercus bicolor and Betula nigra L., which establish in gaps and other open areas on the floodplain that are more elevated than sites colonized by Salix nigra and Populus deltoides (Ware 1955). Historically, such canopy openings were maintained by fire sweeping down from the upland (Curtis 1959). In the absence of major disturbances, both pioneer forest types tend to be replaced by a community dominated by Acer saccharinum, Fraxinus pennsylvanica, and Ulmus americana L. Evans (1970) reports that the replacement of a Salix and Populus dominated community occurs within 50 years. Since the 1950s, U. americana has been significantly reduced in importance in the Wisconsin floodplain due to Dutch Elm disease (Dunn 1986). Thus, A. saccharinum is the overwhelming dominant in mature floodplain forests and tends to be the major colonizer in small canopy gaps and backwater sloughs (Barnes 1997; Knutson and Klaas 1998). As river dynamics typically prevent further improvement of the soil, succession to the typical A. saccharum-dominated mesic forests of southern Wisconsin does not generally occur (Ware 1955; Curtis 1959). Common associates of these later successional forests include Carya cordiformis (Wangenh.) K. Koch, Tilia americana L., and Celtis occidentalis L. On sandier, slightly elevated sites, Q. bicolor can remain a dominate canopy species (Hale and Alsum, unpublished data). Woody lianas can be an important component of these floodplain forests at all successional stages and elevation levels. This is especially true of light generalist lianas (Toxicodendron radicans and Smilax hispida Mull. Ex Torr.), which are found at all elevations in these forests (Bell 1974; Menges and Waller 1983). Despite their importance (Curtis 1959), woody lianas have not yet been shown to play an important role in either inhibiting or facilitating succession.
100 Table 1. Cover types and succession in the lower Mississippi Valley. Cover type
Dominant species
Successional stage
Willow Cottonwood
Salix nigra Populus deltoides
Pioneer Pioneer
Riverfront hardwoods
Platanus occidentalis L. Ulmus americana Acer saccharinum
Early-mid successional
Cypress-Tupelo
Taxodium distichum (L.) L.C. Rich. Nyssa aquatica L. Nyssa sylvatica Marsh.
Mid-successional Sub-climax on sites with substantial flooding
Sugarberry-Elm-Ash
Celtis laevigata Ulmus americana Fraxinus pennsylvanica
Mid–late successional
Overcup oak-Water hickory
Quercus lyrata Walt. Carya aquatica (Michx. f.) Nutt.
Late successional on low-lying sites
Sweetgum-Water oaks
Liquidambar styraciflua Quercus phellos Quercus nigra Fraxinus pennsylvanica
Late-successional on higher elevation sites
Oak-Hickory mix
Quercus michauxii Quercus pagodifolia Carya spp.
Late successional on highest floodplain sites
Adapted from Putnam et al. (1960), Wiseman (1982) and Sharitz and Mitsch (1993).
Lower Mississippi valley The lower Mississippi Valley (LMV) extends from southern Illinois to the mouth of the Mississippi on the Gulf Coast (Louisiana). It is included in the southeastern evergreen forest region (Braun 1950). Despite the large extent of the LMV, the forests are remarkably uniform (Wiseman 1982). A variety of classification schemes have been developed for the floodplain forests of the southeastern United States. These schemes range in number from three general types (Hodges 1995) to 13 types (Eyre 1980). This diversity of schemes reflects the complicated structure and processes that occur in this system and the difficulty in delineating specific community types. The climate of southern Illinois is continental with cool winters and warm summers, whereas the Gulf Coast experiences an almost subtropical climate. Mean annual temperatures range from 14.7 8C in southern Illinois to 21 8C in the southernmost reaches of the Mississippi (Robertson et al. 1978; Wiseman 1982). Maximum average precipitation is 1500 mm per year. The Mississippi and its tributaries reach flood stage during the spring. The first stages of succession in the LMV resemble those of its Rhine and Wisconsin counterparts (see Table 1). Pioneer stages typically occur on fresh substrate (either recently cleared or deposited by floodwaters) and are typically
101 pioneered by stands of S. nigra or P. deltoides, or a mix of the two species (Wiseman 1982; Sharitz and Mitsch 1993). Willow stands start to break up around 35 years; cottonwoods around 45 years (Hodges 1995). Unless the willow stands are in a frequently flooded area, such as a low-lying slough, cottonwood will eventually dominate. These stands are slowly invaded by other hardwoods and eventually shift to a Riverfront hardwoods type and=or the Sugarberry-Elm-Ash. Wiseman (1982) considers the latter type to be the quasi climax community of the batture lands (the lands between the river and the artificial levees) along the lower Mississippi. The willow stands that develop in low-lying areas are replaced by communities of Cypress-Tupelo (Hodges 1995; Kellison et al. 1998). Water elm (Planera aquatica J.F. Gmel.) is a common associate of this type (McKnight et al. 1981; Sharitz and Mitsch 1993). Succession tends to remain at this stage, as long as flooding remains semi-permanent. Should these sites be drained or silt up, the community tends to shift to an Overcup oak-Water hickory type at lower elevations and the Sugarberry type at slightly higher levels of the floodplain. On sites higher than those upon which the Sugarberry type are found, succession often leads to a community dominated by Liquidambar styraciflua L., Q. nigra L. and Q. phellos L. (Sharitz and Mitsch 1993; Hodges 1995). On the highest areas in the floodplain, a mix of Quercus and Carya species dominate the canopy. Wiseman (1982) notes that due to disturbances from periodic flooding, succession in the floodplain forests of the Mississippi rarely continues longer than 200 years. The abundance of woody lianas contributes to the unique environment of the floodplain forest. In general, vines tend to rapidly colonize gaps created in the forests, as well as old fields (Battaglia et al. 2002). Vines tend to be sparse in the lower-lying habitats, primarily occurring on edges (Kellison et al. 1998). Vines are much more common in the Riverfront Hardwoods and Sweetgum types (Wiseman 1982; Sharitz and Mitsch 1993; Kellison et al. 1998). Wiseman (1982) found vines to be most abundant in the Sugarberry type and cites the ‘ubiquitous occurrence of large woody vines (214)’. The diversity of vine species is remarkably high (19þspecies). The most common vines are Toxicodendron radicans, Ampelopsis arborea (L.) Koehne, Campsis radicans (L.) Seem. ex Bureau, and Rubus trivialis Michx. (Wiseman 1982). Kellison et al. (1998) report that two vines, T. radicans and Vitis rotundifolia Michx., can grow high into the canopy. Other vines, such as Smilax L. spp., Berchemia scandans (Hill) K. Koch and C. radicans, can reach heights of up to 3 m.
Methods In the Rhine floodplain, floristic data come from studies along the Rhine floodplain (Schnitzler 1994, 1995b, 1996). One of the best preserved, still flooded hardwood forests of the Rhine is located in the Rhinau Natural Reserve (France). Different methodologies were utilized to survey this area: phytosociology (30 releve´ s of 1000 m2 chosen in different forest phases, including gaps of various sizes), and forest architecture.
102 Data for the Lower Wisconsin River derive from Ware (1955), Curtis (1959), Fulton (1987) and a current project, Hale and Alsum (unpublished). These data were collected using traditional plot methods, as well as the point-quarter method (Cottam and Curtis 1956). Such methods provide data on density, dominance, and frequency of woody species, as well as presence–absence data. Data from the LMV were collected from the literature (Braun 1950; Shelford 1954; Hosner and Minckler 1963; Robertson et al. 1978; Wiseman 1982; Wall and Darwin 1999). The methods used in these studies were similar to those in the Wisconsin studies. The differences in methodology between continental Europe and America create certain challenges in comparing the environments on the two continents. Species biodiversity is well documented in Europe where phytosociological data are plethoric. In America, however, many papers detail tree densities and dendrology; but phytosociological studies similar to those of the Braun–Blanquet school are extremely rare. Studies dealing with relationships between species and habitat gradients are abundant on both continents. Using the above data sources, we assembled lists of woody species, genera, and families for each of the three regions. We calculated the number of species, genera, and families present in each region, as well as the average number of species per genera and species per family. For the latter calculations, we used the Wilcoxon rank test (S-Plus, Insightful Corp., Seattle, WA) to analyze for significant differences among regions (a ¼ 0.05). Using Latham and Ricklefs (1993) and Watson and Dallwitz (2000), we classified families as those which are primarily tropical, those which are primarily temperate, and those which are widespread in both regions. We then compared numbers among regions, in terms of both family richness and a similarity index (SI): SI ¼
2w AþB
where w ¼ families in common; A ¼ families in area A and B ¼ families in area B (adapted from Curtis (1959)). The influence of fluvial dynamics on biodiversity in floodplain forests can be estimated by a comparison between the diversity indices of flooded environments with neighboring upland ecosystems, which are subjected to the same climate and the same forestry practices. For such an analysis, we compare data on the number of woody species in neighboring upland environments, which we obtained from the literature, with those in this study.
Results Species richness The compiled species list with associated families and genera is located in the Appendix. Table 2 provides floristic richness for woody species (trees, shrubs and
103 Table 2. Woody species richness.
Families Genera Species Species per family Species per genus a
LMV
WI River
Rhine River
42 78 146 3.5a 1.9
26 41 67 2.6 1.6
21 35 53 2.5a 1.5
Values significantly different (p < 0.05).
Table 3. Presence of families with primarily temperate distributions. Family
LMV
Wisconsin
Rhine
Aceraceae Annonaceae Berberidaceae Betulaceae Caprifoliaceae Cornaceae Fagaceae Hamamelidaceae Juglandaceae Leitneriaceae Nyssaceae Rosaceae Salicaceae Saxifragaceae
X X
X
X
X X X X X X X X X X X
X X X X
X X X X X
X
X
X X X
X X X
Total
13
9
10
vines). The forest communities in the LMV exhibit considerable woody floristic diversity with 146 species compared with 67 in Wisconsin and 53 along the Rhine; similar trends exist at both the genus and family level. The forests of the LMV have higher numbers of both species per genus and species per family compared to either of the cool-temperate forests. However, only the number of species per family proved to be significantly different, and only between the forests of the LMV and the Rhine. Floodplain forests on the Rhine and Wisconsin present similar numbers of both species per family and species per genus, indicating that the higher species richness on the Wisconsin is mainly a result of the presence of more families (26 vs. 21). This higher number of families along the Wisconsin is due to larger numbers of both tropical and widespread families (Tables 3–5). The three floodplain forest regions include a similar number of families with primarily temperate distributions, in which many genera recur (Table 3). However, we see a common trend of higher numbers of genera and species on the Mississippi, followed by the Wisconsin, and, lastly, the Rhine. The higher number of species for genera in the LMV forests is demonstrated well in the temperate families. One of the most striking examples is the Juglandaceae family, which includes only one genus and one species in the Rhine forests (Juglans regia L.) against two genera in
104 Table 4. Presence of families with widespread distributions. Family
LMV
Wisconsin
Aquifoliaceae Asteraceae Dioscoreaceae Fabaceae Hippocastanaceae Liliaceae Loranthaceae Magnoliaceae Myricaceae Oleaceae Platanaceae Ranunculaceae Rhamnaceae Staphyleaceae Taxodiaceae Thymeleaceae Tiliaceae Ulmaceae Vitaceae
X X
X
Total
X X X
Rhine
X X X X X
X X X X X X
X X X
X
X
X X X
X X X
X X X
X X X X
15
12
9
X
America: Juglans L. (two species in WI and MS) and Carya Nutt. (with two species in the WI forests and nine species in the LMV). In the Fagaceae family, the Quercus L. genus has 16 species that coexist in the LMV, five in the Wisconsin forests, and only one on the Rhine. However, at least one species provides an exception to the general trend: the Salix L. genus (Salicaceae) is richer along the Rhine, with eight species against three in the Wisconsin forests and two in the LMV. This trend is also present in some of the families with more widespread distributions (Table 4). In the Ulmaceae family, LMV forests include Ulmus L., Planera J.F. Gmel and Celtis L., Wisconsin forests include Ulmus and Celtis, while European forests include only Ulmus. In the Oleaceae family, the LMV has two genera, Fraxinus L. (four species) and Forestiera Poir. (one species). The Wisconsin has only the genus Fraxinus with three species. The Rhine also only has the Fraxinus genus with only one species. One last example is the Vitaceae family, which has four genera in the LMV, two on the Wisconsin, and only one on the Rhine (Vitis L.). Vitis species are well represented in number and abundance in the LMV with four species, while both the cool temperate floodplain forests of the Wisconsin and the Rhine only support one species each. The abundance of families with primarily tropical distributions provides further explanation for regional differences in diversity (Table 5). The LMV forests include 14 families with tropical affinities. These include trees (e.g., Diospyros virginiana L.), shrubs (e.g., Cephalanthus occidentalis L.), and vines (e.g., Toxicodendron radicans (L.) Kuntze). In contrast, fewer species of tropical families are present in the WI forests, only five families. All of these are either vines (e.g., Toxicodendron
105 Table 5. Presence of families with primarily tropical distributions. Family
LMV
Wisconsin
Anacardiaceae Araliaceae Bignoniaceae Celastraceae Ebenaceae Lauraceae Meliaceae Menispermaceae Moraceae Rubiaceae Rutaceae Sapotaceae Styracaceae Theaceae
X X X X X X X X X X X X X X
X
Total
14
Rhine X
X
X
X X X
5
2
radicans) or shrubs (e.g., Cephalanthus occidentalis). Along the Rhine, only two families with tropical affinities are represented, one vine (e.g., Hedera helix L.) and one shrub (e.g., Euonymus europea L.). Figure 1 compares the similarity values calculated for the three pairs of regions. In terms of overall similarity, the two connected forests, the LMV and Wisconsin prove the most similar (0.71), followed by the two cool-temperate forests (0.68). Indeed, the LMV-Rhine forests demonstrate the least similarity among all groups. Looking solely at the primarily temperate families, the cool-temperate forests are the most similar (0.95), followed by the LMV-Wisconsin forests (0.82). The widespread and tropical groupings show similar trends, where the LMV-Wisconsin forests are much more similar than either of the other two groupings (0.74 and 0.53, respectively).
River dynamics and biodiversity The forests bordering the Mississippi floodplain forests occupy the loess bluffs of the Mississippi river. Like their lowland counterparts, these forests are known for their high levels of diversity. Johnson and Little (1967) report 99 total woody species in the Bluff Experimental forest in Mississippi, including 66 tree species. Hodges (1995) reports that more than 75 tree species are present in these loess bluff forests. In contrast, our results indicate that the floodplain forests of the LMV contain over 100 tree species. Curtis (1959) reports a similar situation in southern Wisconsin, where the floodplain forests contain more tree species than their upland counterparts. Lowland (floodplain) forests contain 37 tree species, whereas the mesic (Acer saccharum Marsh.-dominated) and xeric (Quercus-dominated) forests of the uplands contain 26 and 29 species, respectively.
106
Figure 1. Similarity indices among study regions for overall, temperate, widespread, and tropical families.
The hardwood floodplain forests of Europe present the same trend in species richness compared to adjacent upland forests. For example, 18 woody species are found in the Galio-Carpinetum and 22 woody species in the Ulmo Acereti pseudoplatani (elm=maple) (Oberdorfer 1992). The canopy of these forests includes only half as many tree species as in the Rhine forests, and is overwhelmingly dominated by Quercus spp. and Fagus sylvatica L.
Productivity Trees of floodplain forests rapidly reach impressive sizes. Schnitzler (unpublished data) found Quercus robur L. as large as 129 cm dbh and 41 m high; Fraxinus excelsior L. 109 cm dbh, Populus alba L. 102 cm dbh; Ulmus minor Mill. em Richens 113 cm dbh; Tilia cordata Mill. 88 cm dbh; in the Rhine floodplain. She also found that small trees and lianas may also achieve exceptional sizes, such as individuals of Malus sylvestris Mill. with a dbh of 30 cm dbh or Hedera helix with a dbh of 16 cm. As seen in Table 6, Schnitzler (1994) found a basal area of 28 m2 along the Rhine. The basal area data for the Wisconsin in Table 6 are slightly higher compared to 1950s data from Curtis (1959) who reported an average basal area of 22.6 m2 per ha. Individual species also achieve large sizes. Hale and Alsum (unpublished) found Acer saccharinum as large as 104 cm dbh, Quercus bicolor Willd. 87 cm dbh,
107 Table 6. Selected data from the three regions. Region
LMV–Illinoisa
LMV– Mississippib
Wisconsinb
Rhinec
Source
Robertson et al. (1978) 491 674 34.4
Wiseman (1982) 294 759 27.3
Hale and Alsum (unpublished) 321 2293 25.4
Schnitzler (1994) 337 2582 29.9
Tree density (stems per ha) Sapling=shrub density (stems per ha) Tree basal area (m2 per ha) a
This study defined sapling=shrubs as individuals with a dbh of 2.54–6.6 cm; trees had a dbh of 6.6þ cm. These studies defined sapling=shrubs as individuals with a dbh of 1–10 cm; trees had a dbh of 10þ cm. c The figures in this column assume that sapling=shrubs have a dbh of less than 10 cm; trees have a dbh of 10þ cm. Further, these figures only include data from active floodplain sites. b
and Fraxinus pennsylvanica Marsh. trees 80 cm dbh. Curtis (1959) reports A. saccharinum L. individuals up to 197 cm dbh. The basal areas in Table 6 for the two LMV sites are not directly comparable, due to the slightly different definitions for adult trees and saplings and shrubs (see table footnote). As such, the numbers from Robertson et al. (1978) are a slight overestimate compared to those of Wiseman (1982). However, both estimates lie within the range of 27–48 m2 per ha that Kellison et al. (1998) report for bottomland forests in the Southeast (US).
Discussion The role of the climate, present and past is important for biodiversity in all systems including those used in this study. The high woody plant diversity in the LMV as compared to the other two sites can be viewed as part of the general trend of increasing within-habitat diversity for woody plants toward warmer and wetter climates. The continuity of a favorable climate during the Quaternary is another explanation for the differences in biodiversity in the three floodplains. Forests of the southeastern United States are close descendants of a mixed Tertiary forest with a complex history of alternating glacial expansions and contractions (Braun 1950; Monk 1967). During expansion times the Mississippi region was able to provide refugia, in particular for families with tropical affinities. In contraction times during the Quaternary, no significant barriers to migration for the vegetation existed between northern areas and the Mississippi region. This helps to explain the greater abundance of species from widespread and, particularly, tropical families that are found in the cool temperate region of the Lower Wisconsin, which, as tributary of the upper Mississippi, has remained connected to the refuge areas in the LMV. In contrast, the Rhine, cut off from areas that could have served as refugia, has fewer tropical elements despite a milder climate than Wisconsin’s. The lack of available refugia in Europe has resulted in a progressive disappearance of Tertiary elements from the forests from late Tertiary to middle Quaternary (Table 7). East–
108 Table 7. Genus extinction in Europe. Genus
Period
Year BP (MA)
Asimina Aesculus Diospyros Liquidambar Nyssa Taxodium Liriodendron Magnolia Carya Juglans Ostrya Celtis Parthenocissus
End of Miocene End of Miocene Pretiglian Pretiglian Tiglian Tiglian Tiglian Tiglian Tiglian Waalian-Menapian Waalian-Menapian Middle Cromerian Middle Cromerian
7.3 7.3 2.3 2.3 2 3 4 5 6 0.9–1 0.9–1 0.4 0.4
Source: Tralau (1963).
west mountain chains, as well as the Mediterranean Sea, presented significant obstacles for species migration during glacial periods. Emergence of the Mediterranean climate with a marked dry season at the end of the Tertiary (3.1 million years ago) also caused dramatic changes in the flora, including a slow disappearance of tropical families (evergreen elements such as Taxodiaceae or Cathaya Chun et Kuang) and an expansion of xerophytic elements (Suc 1989; Pons et al. 1995). Nonetheless, some rare warm temperate genera survive in small pockets around the Mediterranean area. Some Celtis or Zelkova Spach still survive in Crete and Sicily, Laurus nobilis L. in the southwestern part of France, Platanus orientalis L. in alluvial forests of Greece, and Liquidambar L. spp. in alluvial forests of Turkey (Poli et al. 1974; Follieri 1986; Pascale et al. 1992; Fineschi et al. 2002). Alluvial forests of the Rhoˆ ne in the Camargue include one Lauraceae (Laurus nobilis) and one Moraceae (Ficus carica L.) (Grillas and van Wijck, unpublished data). North of the Mediterranean area, including the Rhine River basin, all these species are absent, primarily due to the geographical barriers existing between these regions and central Europe. Analyzing the similarity among the families in the three regions provides further support of the refugia idea, and demonstrates the importance of connection to refugia for long-term biodiversity (Figure 1). The importance of refugia is particularly evident in a comparison of similarity among sites with respect to the presence of tropical families. The LMV and the Wisconsin are considerably more similar than any of the other two site comparisons. The site of origin of a particular species is another important factor influencing current patterns of biodiversity. For example, the reason for higher abundances of Carya and Quercus genera in North American forests compared to the European forests is explained, at least in part, by the notion that both genera originated from North America (Latham and Ricklefs 1993). In addition to climatic factors and species origin, human activities have also played an important and relatively discrete role in biodiversity, at least in species
109 richness. Of the species discussed in this paper, a prime example is the European vine (Vitis vinifera L. ssp. silvestris (Gmelin) Hegi) which survived the last glacial period (Planchais 1972), but is now nearly extinct along the Rhine because of anthropogenic impacts, including introduced pathogens, deforestation, river management, and forestry practices (Arnold et al. 2002). River-floodplain dynamics also influence species diversity and succession in these systems. As the climatic influence naturally extends beyond the floodplain, it contributes to increased diversity throughout the region. However, the increased diversity within floodplain forests as compared to the surrounding area reflects the role of the river-floodplain dynamics (Kozlowski 2002). The periodic flood pulses provide disturbances necessary for the establishment of pioneer species and create a diverse microelevational environment across the floodplain. Slight elevational differences, in turn, create large differences in flooding frequency of individual sites. Thus, there are areas high enough within the floodplain where less-flood tolerant species and typically upland species can persist and compete. All three river valleys studied contain a dynamic, diverse assemblage of species from all successional stages, which is, in part, due to the periodic flood events and microsite elevation differences. The three regions in this study present the same general trend of succession: Salicaceae species form pioneer communities. With time, ‘hardwoods’ enter under Salicaceae and develop until dominance. Differences do exist between regions, however. Successional stages become longer and more complex in the warmer climate because of the greater number of tree species. In addition, individual species presence can vary in space and time in floodplain forests. Forest composition within sites varies greatly in relation to ecological conditions in the floodplain (soil texture, water level, flooding severity), which explains the high number of phytosociological associations described in Europe. All three regions also present relatively similar levels of productivity, particularly for adult trees. In general, these forests present high levels of productivity as compared to adjacent forested systems, as is typical of floodplain forests worldwide. The slightly lower levels of basal area for the Wisconsin forests possibly reflect the colder climate (and shorter growing season). Some differences exist between regions with respect to the number of understory stems. In Wisconsin, high numbers for sapling and shrub density reported by Hale and Alsum reflect the invasion of these floodplain forests by several shrubs, primarily Zanthoxylum americanum Mill. and the non-native Rhamnus cathartica L. Both the Wisconsin and Rhine understories have been heavily impacted by the loss of Ulmus species, which created many openings in the forest canopies. The impacts of Dutch Elm disease in the LMV have been less severe. Further, in the LMV, Kellison et al. (1998) report that the thin understories in these forests result in part from flooding impacts and the lack of understory tolerant tree species. An analysis of the vegetation in floodplain forests of Europe and the United States presents several trends in patterns of biodiversity across regions. These trends can be attributed in part to the role of climate, to the importance of available refugia, and to the complexity of river-floodplain dynamics and the resulting successional processes. However, the biodiversity of floodplains is also greatly
110 influenced by man’s activities in the region. Floodplains are particularly vulnerable systems because of their close dependence on natural flooding regimes. Any anthropogenic alteration of hydrology or influence on forest dynamics can significantly modify diversity levels by both losses and inputs of species. Several anthropogenic factors have greatly influenced biodiversity in all three of the study regions. Landscape and river fragmentation has occurred as rivers have been engineered and floodplains converted to agriculture, industry, or settlements. Forest management and timber harvesting remain active in all of the three regions and have already influenced each of these regions’ forests. Introduced pathogens, such as Dutch Elm disease, have been particularly damaging in the two cooltemperate regions. Finally, exotic species are a growing concern in all of the regions mentioned here. Some examples include Acer negundo, Robinia pseudacacia, Fallopia japonica, and Solidago gigantea on the Rhine; Rhamnus cathartica and Phalaris arundicacea on the Wisconsin; and Ligustrum sinense and Sapium sebiferum in the LMV. All of these species are proliferating and possibly altering forest structure and composition. As the forests face each of these threats, the differences seen in the cool-temperate floodplain forests of North America and Europe teach us the particular importance of maintaining and protecting potential refuges themselves, as well as the connectivity to these refugia, in order to preserve long-term biodiversity. This study should provide an even greater impetus to protect the remaining natural forests, which in the future could provide important refugia for species and may function to prevent future biodiversity crises.
Appendix Species list for study regions. Family
Genera
Mississippi
WI
Rhine
Aceraceae
Acer
barbatum Michx. negundo rubrum L. saccharinum saccharum
negundo rubrum saccharinum saccharum
campestre L. platanoides L. pseudoplantanus L.
Anacardiaceae
Rhus Toxicodendron
copallinum L. glabra L. radicans
Annonaceae
Asimina
triloba (L.) Dunal
Aquifoliaceae
Ilex
decidua Walt. opaca Ait. vomitoria Ait.
Araliaceae
Aralia Hedera
spinosa L.
Asteraceae
Baccharis
halimifolia L.
Berberidaceae
Berberis
radicans
verticillata (L.) Gray helix
vulgaris L.
111 Appendix (continued) Family
Genera
Mississippi
Betulaceae
Alnus
serrulata (Ait.) Willd.
Betula
nigra
Carpinus Corylus
caroliniana Walt. americana Walt. rostrata Ait. virginiana (P. Mill.) K. Koch
Ostrya Bignoniaceae
Bignonia Campsis Catalpa
capreolata L. radicans bignoniodes Walt. speciosa (Warder) Warder ex Engelm.
Caprifoliaceae
Lonicera
dioica L.
Sambucus Viburnum Celastraceae
WI
lutea Michx. f. nigra caroliniana americana
Rhine glutinosa (L.) Gaertn. incana (L.) Moench pendula Roth betulus L. avellana L.
virginiana
canadensis L. prunifolium L. rufidulum Raf.
canadensis Bartr. Ex Marsh. reticulata Raf. canadensis lentago L. acerifolium L.
nigra L. opulus L. lantana L.
Celastrus Euonymus
scandens L. atropurpureus Jacq.
scandens atropurpurea
europeaus L.
Cornaceae
Cornus
drummondii C. A. meyer florida L. racemosa Lam. stolonifera Michx. stricta Lam.
racemosa rugosa Lam. stolonifera
Dioscoreaceae
Dioscorea
Ebenaceae
Diospyros
virginiana
Fabaceae
Albizzia Amorpha Amphicarpa
juiibrissin Durazzini fruticosa L.
Cercis Daubentonia Gleditsia
canadensis L. drummondii Rydb. aquatica Marsh. tricanthos L. dioicus (L.) K. Koch pseudoacacia
Gymnocladus Robinia Fagaceae
Fagus Quercus
xylosteum L.
mas L. sanguinea L.
villosa L.
grandifolia alba L. bicolor
fructicosa bracteata (L.) Fernald
tricanthos dioicus
alba bicolor
robur
112 Appendix (continued) Family
Genera
Mississippi
WI macrocarpa rubra velutina
Hamamelidaceae
Liquidambar
imbricaria Michx. lyrata macrocarpa Michx. michauxii muelhenbergii Engelm. nigra nuttalli Palmer pagodafolia palustris Muenchh. phellos rubra L. shumardii Buckl. stellata Wangenh. velutina Lam. styraciflua
Hippocastanaceae
Aesculus
pavia L.
glabra
Juglandaceae
Carya
aquatica cordiformis glabra (P. Mill.) Sweet illinoensis (Wang.) K. Koch. laciniosa (Michx. f.) G. Don. myristiciformis (Michx. f.) Nutt. ovalis (Wangenh.) Sarg. ovata (Miller) K. Koch. tormentosa (Lam. ex Poir.) Nutt. cinerea L. nigra L.
cordiformis ovata
Juglans Juglans Lauraceae
Lindera Persea Sassafras
benzoin (L.) Blume melissaefolium (Walt.) Blume palustris (Raf.) Sarg albidum (Nutt.) Ness
Leitneriaceae
Leitneria
floridana Chapman
Liliaceae
Smilax
bona-nox L. rotundifolia L. tamnoides L.
Loranthaceae
Loranthus Viscum
Magnoliaceae
Liriodendron Magnolia
tulipifera L. tripetala (L.) L.
Meliaceae
Melia
azedarach L.
cinerea nigra
Rhine
regia
herbacea L.
europaeus L. album L.
113 Appendix (continued) Family
Genera
Mississippi
WI
Menispermaceae
Cocculus Menispermum
carolinus (L.) DC. canadense L.
canadense
Broussonetia Maclura Morus
papyrifera (L.) L’He´ r. ex Vent. pomifera (Raf.) Scheid. rubra L.
Myricaceae
Myrica
cerifera (L.) Small
Nyssaceae
Nyssa
aquatica L. sylvatica Marsh.
Oleaceae
Forestiera Fraxinus
acuminata Poir. americana L. caroliniana P. Mill. pennsylvanica profunda (Bush) Bush
Moraceae
americana nigra Marsh pennsylvanica
Ligustrum Platanus
occidentalis L.
occidentalis
Ranunculaceae
Clematis
virginiana L.
virginiana
Rhamnaceae
Berchemia Frangula Rhamnus
scandens
Crataegus
spp.
serotina
Pyrus
angustifolia Marshall hortulana Bailey mexicana S. Wats. persica (L.) Batsch serotina Ehrh. umbellata Ell. coronaria L.
Rosa
caroliana L. argutus Link trivialis Michx.
vitalba L. alnus P. Mill cathartica L.
lanceolata Pursh. marshallii Egglest. paludosa Sarg. speciosa Sarg. velutina Sarg. viridis L.
Malus Prunus
excelsior L.
vulgare L.
Platanaceae
Rosaceae
Rhine
allegheniensis Porter flagellaris Willd.
strigosus Michx. occidentalis L. pubescens Raf. Rubiaceae
Cephalanthus
occidentalis
occidentalis
Rutaceae
Zanthoxylum
clava-herculis L.
americanum
monogyna Jacq.
sylvestris Mill. avium L. padus L. spinosa L.
pyraster (L.) Burgsd. arvensis Huds. canina L. caesius L. fructicosus L. non Weihe et Nees
114 Appendix (continued) Family
Genera
Mississippi
WI
Rhine
Salicaceae
Populus
deltoides heterophylla L.
Salix
interior Rowlee
deltoides grandidentata Michx. tremuloides Michx. amygdaloides Andersson interior nigra
alba nigra tremula L. alba
americanum P. Mill.
rubrum L.
nigra
Sapotaceae
Bumelia
Saxifragaceae Ribes
cinerea eleagnos fragilis purpurea
lanuginosa (Michx.) Pers. lycioides (L.) Pers. gracile Pursh, non Michx.
cynosbati L. Staphyleaceae Staphylea Styracaceae
Styrax
americana Lam.
Taxodiaceae
Taxodium
distichum
Theaceae
Hydrangea
arborescens L.
trifolia L.
pinnata L.
americana
cordata Mill. platyphyllos Scop.
Thymeleaceae Daphne
mezereum L.
Tiliaceae
Tilia
americana L.
Ulmaceae
Celtis
laevigata occidentalis mississipiensis Bosc. occidentalis aquatica alata Michx. americana americana rubra crassifolia racemosa D. Thomas rubra Muhl.
Planera Ulmus
Vitaceae
Ampelopsis arborea Cissus stans Pers. Parthenocissus quinquefolia L. Planchon Vitis aestivalis Michx. cinerea Engelm. cordifolia Michx. riparia Michx.
glabra Huds. laevis Pallas minor Mill. em Richens
quinquefolia riparia
vinifera ssp. silvestris
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