Vascular plant diversity in eastern Asia and North America: historical and ecological explanations

Botanical Journal of the Linnean Society (1998), 128: 123–136. With 2 figures Article ID: bt980186

Vascular plant diversity in eastern Asia and North America: historical and ecological explanations QINFENG GUO1 Department of Biology, University of California at Los Angeles, 900 Veteran Avenue, Los Angeles, CA 90095-1606, U.S.A. ROBERT E. RICKLEFS Department of Biology, University of Missouri, St Louis, St. Louis, MO 63121-4499, U.S.A. MARTIN L. CODY Department of Biology, University of California at Los Angeles, Los Angeles, CA 90095-1606, U.S.A. Received January 1998; accepted for publication April 1998

Taxonomic diversity of vascular plants (ferns, gymnosperms and angiosperms) was compared between eastern Asia and North America. Eastern Asia has significantly higher species richness in all three classes but the difference was greatest in ferns and least in angiosperms. Differences in taxonomic treatments between the two continents are not likely contributors to these patterns.The relationship of regional to global species richness across the three plant classes suggested that diversity patterns were relatively homogeneous at three taxonomic levels. Thus, differences in species richness are established at the family level and are therefore relatively old. The previously noted fact that eastern Asia has a higher proportion of primitive taxa was shown by analyses both among and within plant classes. Diversity patterns across three taxonomic levels (i.e. family, genus and species) of the three classes may reflect the relative historical positions of the two continents (following continental drift) to the centre(s) of their origin, neighbouring land masses, differential speciation/extinction rates, and switches in dominance levels associated with climate change (including glaciation), as well as reproductive/dispersal mechanisms of the three plant classes.  1998 The Linnean Society of London

ADDITIONAL KEY WORDS:—angiosperm – continental drift – fern – gymnosperms – plant classes – species diversity.

1

Correspondence to Dr Q. Guo. Present address: School of Natural Resources, 102 Plant Industry, University of Nebraska, Lincoln, NE 68583-0814, USA. Email: [email protected] 0024–4074/98/100123+14 $30.00/0

123

 1998 The Linnean Society of London

Q. GUO ET AL.

124

CONTENTS

Introduction . . . . . Data sources and analyses Results . . . . . . Discussion . . . . . Phylogeny and history Ecological dispersal . Regional environments Acknowledgements . . References . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

124 125 126 129 130 132 133 134 134

INTRODUCTION

The well-documented floristic affinities between eastern Asia and North America (Li, 1952, 1972; Boufford & Spongberg, 1983; see also Graham, 1972; Raven, 1972; Thorne, 1972; Wood, 1972; Wu, 1983) have recently attracted ecologists and biogeographers to compare species diversity and ecological relationships between the two regions (e.g. White, 1983; Ricklefs & Latham, 1992; Latham & Ricklefs, 1993a; Li & Adair, 1994; Guo, 1998). The similarities in area and latitude between the two regions make such studies among the most prominent of many such intercontinental comparisons. Initial work has demonstrated that, despite the similar latitudes and close floristic relationships of both Northern Hemisphere regions, temperate forests of eastern Asia support significantly more diverse tree floras than forests in climatically similar areas of North America and Europe (Gray, 1878; Latham & Ricklefs, 1993b). Eastern Asian temperate forests possess most of the total global diversity of temperate trees, whether measured by distributions of families (95% of the total Northern Hemisphere temperate tree flora), genera (87%), or species (63%). On a continental-wide basis, moist temperate forests in Asia support about three times as many species of trees as similar forests in North America (Latham & Ricklefs, 1993b). Many explanations for these patterns have been based on intercontinental differences in geological and evolutionary history (Takhtajan, 1969; White, 1983; Ricklefs & Latham, 1992; Latham & Ricklefs, 1993b). Such historical effects include three important considerations. First, eastern Asia has a larger landmass within the tropics and a longer history of overland connection with tropical areas having rich species pools. Tropical south-eastern Asia exhibits great plant taxonomic diversity and is considered by many as the centre of origin and dispersal of vascular plants (Wolfe, 1975; Wu, 1980; White, 1983; Lidgard & Crane, 1990; but see Raven & Axelrod, 1974; Tiffney, 1985a, b). Second, continental drift during the period between the Middle Cretaceous and the Middle Miocene moved North America farther away from the Eurasian land mass and reduced its connection across the North Atlantic Ocean to sources of temperate zone diversity in south-eastern Asia. North America was also separated by water from the tropics of the Western Hemisphere for much of this period (Raven & Axelrod, 1974). Therefore, the interchange of species, especially primitive ones, between Asia and North America was interrupted and sister taxa in the two regions then took separate evolutionary pathways. Third, the eastern Asian continent is geologically and geomorphologically more ancient than North America (e.g. the Rocky Mountains and the deserts were relatively more recently formed, see Fiero, 1986; McLaughlin, 1986) and North America was more strongly influenced by Quaternary glaciation than was Asia (Brown & Gibson, 1983).

PLANT DIVERSITY IN EASTERN ASIA AND NORTH AMERICA

125

Although comparable habitats on the two continents may have similar productivity, or actual evapotranspiration (AET), eastern Asia supports higher species diversity in several taxa (Latham & Ricklefs, 1993b; Li & Adair, 1994). Climatic changes through geological history (including those related to glaciation) have been recognized as having major effects on global biodiversity patterns. Sauer (1988) pointed out that the amount of land at various latitudes and the nature of barriers to movement of populations during worldwide climatic fluctuations differ greatly among the continents and across geological times. These geographical factors could have potentially affected the sizes of forest refugia during glacial maxima and the degree of connection between temperate forests and species-rich tropical forests (Gray, 1878; Butzer, 1974; Prance, 1982; Delcourt & Delcourt, 1987; Sauer, 1988). Comparisons of the genera of plants in fossil and present-day floras have revealed differential effects of late Tertiary climate change and glaciation on plant diversity in Eurasia and North America, largely due to the different orientations of landforms on both continents (Brown & Gibson, 1983; Tiffney, 1985a; Brown, 1993). Another factor that has been frequently neglected is that eastern Asia also has broader connections to the west and northwest. For example, central Asian, western Asian, and Mediterranean floristic elements (genera) make up 16.2% of the flora of China (Wu, 1980). Previous studies compared species diversity in different classes separately (e.g. ferns: Kramer, 1993; Kato, 1993; trees: Latham & Ricklefs, 1993b). Here, we compare species richness at three taxonomic levels across three vascular plant classes (ferns, gymnosperms, and angiosperms) between eastern Asia and North America. Our null hypotheses for this study were: (1) that diversity did not differ between eastern Asia and North America, (2) that differences in diversity between eastern Asia and North America were homogeneous with respect to class of vascular plant and taxonomic level, and (3) that diversity at each taxonomic level was a similar multiple of diversity at the next higher level independently of class or region. Significant deviations from the null hypotheses would suggest differences in the historical diversification of each of the vascular plant classes in each region related to different attributes of the plants or different histories of climate and geographical relationship of the regions. We use analysis of variance to separate the effects of taxon, taxonomic level, and region on diversity, allowing the three taxonomic groups to serve as controls for each other with respect to differences in taxonomic practices and habitat distributions. We pay special attention to the origins of each group in relation to dispersal power and the role of continental drift in each region’s geological history. We assume that the three plant classes might have responded differently to such historical events as continental drift and Quaternary glaciation. We suggest that phylogenetic positions of plants, either across classes or within classes, have largely determined their diversities and distribution in different regions or continents with various geological histories. We discuss the evolutionary and biogeographic implications of the differential diversities among plant classes and between isolated continents. In particular, we are interested in whether explanations for differences in tree diversity between eastern Asia and North America can be applied to the diversity patterns of ferns and gymnosperms. DATA SOURCES AND ANALYSES

Analysis of the diversity of the three plant classes was based on data for eastern Asia and North America compiled by Li & Adair (1994). The particular data set

Q. GUO ET AL.

126

T 1. Comparison of taxon richness of native vascular plants between eastern Asia and North America relative to the world totals (adapted from Li & Adair, 1994)

Ferns Gymnosperms Angiosperms

Eastern Asia

North America (family/genus/species)

World

52/204/2300 10/34/180 238/2637/25000

15/60/341 5/19/118 202/2261/15827

65/443/11820 11/57/670 542/12500/225000

used in this study was drawn from their Table 2, which was based on broad geographic areas. In this data set, eastern Asia refers to the eastern forest region of China, eastern Siberia (excluding the Kamchatka Peninsula and Sakhalin Island), and Korea. North America includes the entire continental region of Canada and United States. Although both represent large regions of temperate vegetation, the two areas differ in several important respects. China extends southwards to about 21°N latitude whereas, except for the Florida Peninsula, little of North America north of Mexico extends southwards of 30°N latitude. The northern extents of the two regions, about 70°N latitude, are similar. Balancing the subtropical area of southern China is the fact that North America is bordered by ocean on two sides, which has resulted in the presence of species-rich Mediterranean-climate habitats along the Pacific coast. The western border of the eastern Asian region is primarily dry and cold north of 30°N latitude. Li & Adair (1994) tabulate the number of families, genera, and species of ferns, gymnosperms, and angiosperms in eastern Asia, North America, and the World. These data are reproduced in Table 1 and shown graphically in Figure 1. Numbers of taxa were log10-transformed for all statistical analyses. We performed two analyses of variance. In the first, the effects were taxon and region with the global diversity as covariate, and including all two-way interaction terms. The main effects test whether diversity differs significantly between the three classes of plant and between the two regions. The interaction terms test whether the slopes of the local-versusglobal diversity regressions differ significantly, i.e. whether numbers of species, genera, and families are similar samples of the world floras for taxonomic group (taxon∗world interaction) or region (region∗world interaction). The third interaction term (taxon∗region) tests whether diversities of the three classes of plant respond in a nonparallel fashion to region. A second analysis of variance included three main effects (taxonomic level, taxon and region) and all three two-way interactions. In this case, the taxon∗ level and region∗ level interactions test whether the numbers of species per genus and genera per family are similar across classes and regions. This is related to the taxon∗world and region∗world interactions described above. The taxon∗region interaction has the same interpretation as in the first analysis of variance. In both cases, we first applied the full model to the data, and then applied a second model in which non-significant effects and interactions were dropped.

RESULTS

Figure 1 portrays numbers of taxa in the three plant classes at three taxonomic levels (family, genus and species) against the world totals (see also Table 1). Data

PLANT DIVERSITY IN EASTERN ASIA AND NORTH AMERICA

127

Figure 1. Comparison in species richness of vascular plants between eastern Asia and North America. Data from Li & Adair (1994). A, ferns; B, gymnosperms; C, angiosperms.

points for each taxon within each region are connected to show by the slope of the line the ratios of species to genera and genera to families. The graphical representation is paralleled by the analyses of variance (ANOVA). In the first ANOVA (Table 2A), neither the taxon∗world (F=2.65, df=2,8, P=0.13) nor the region∗world (F=2.23, df=1,8, P=0.17) interactions were significant, indicating that the local flora of each of the plant classes and in each of the regions samples the world flora in a homogeneous manner. The allometric (log-log) slope of regional with respect to

Q. GUO ET AL.

128

T 2. Analysis of variance in regional diversity with respect to global diversity, taxon, and region (eastern Asia versus North America) Source

df

Type III SS

(A) Model 1: F=537, df=9,8, P<0.0001, R2=0.998 Global diversity (covariate) 1 7.053 Taxon 2 0.017 Region 1 0.014 Global diversity∗taxon 2 0.021 Global diversity∗region 1 0.009 Taxon∗region 2 0.235 Error 8 0.032 (B) Model 2: F=570, df=6,11, P<0.0001, R2=0.997 Global diversity (covariate) 1 7.769 Taxon 2 0.250 Region 1 0.480 Taxon∗region 2 0.228

Mean Square

F

P

7.053 0.009 0.014 0.011 0.009 0.118

1776.5 2.2 3.5 2.7 2.2 29.6

0.0001 0.1765 0.0970 0.1311 0.1738 0.0002

7.769 0.125 0.480 0.114

1386.6 22.3 85.6 20.4

0.0001 0.0001 0.0001 0.0002

T 3. Analysis of variance in regional diversity with respect to taxonomic level, taxon, and region (eastern Asia versus North America) Source

df

Type III SS

Model: F=234, df=13,4, P<0.0001, R2=0.999 Taxonomic level 2 7.550 Taxon 2 10.688 Region 1 0.480 Level∗taxon 4 0.245 Level∗region 4 0.010 Taxon∗region 2 0.228 Error 4 0.025

Mean Square

F

P

3.775 5.344 0.480 0.061 0.005 0.114 0.007

598.4 847.1 76.0 9.7 0.8 18.1

0.0001 0.0001 0.0010 0.0245 0.5171 0.0099

global diversity was 0.71±0.02 SE. The reduced model presented in Table 2B reveals a highly significant taxon∗region interaction (F=20.4, df=2,11, P=0.0002) in addition to significant taxon (F=22.3, df=2,11, P=0.0001) and, especially, region (F=85.6, df=1,11, P=0.0001) effects. The results of the first ANOVA were reinforced by those of the second (Table 3), in which the level∗region interaction (F=0.8, df=2,4, P=0.52) was insignificant and the taxon∗region interaction (F=18.1, df=2,4, P=0.01) was significant. In this case, however, the level∗taxon interaction was marginally significant (F=9.7, df= 4,4, P=0.025), indicating that either the number of species per genus, genera per family, or both, differs among the three plant classes. Further analysis of the model showed that angiosperms were significantly less diverse at the family level than expected from main effects in the model (−0.63 log10 units, a factor of 0.24; t= −5.8, P=0.001) and that ferns were significantly more diverse in Asia than expected from main effects in the model (0.39 log10 units, a factor of 2.5; t=4.4, P=0.005). Among the main effects in these models, angiosperms (log10 species richness= 3.336) were the most diverse class and the ferns (1.473) the least diverse class; gymnosperms (2.146) were intermediate. More importantly, diversity in Asia (2.482) exceeded that in North America (2.155) by 0.33 log10 units (a factor of 2.1), on average, or by 0.25 log10 units (a factor of 1.8) when global diversity or other main

PLANT DIVERSITY IN EASTERN ASIA AND NORTH AMERICA

129

T 4. Eastern Asian and North American vascular floras as percentages (%) of the world total flora

Eastern Asia North America

Fern family genus species

Gymnosperm family genus species

Angiosperm family genus species

80 23

91 45

44 33

46 14

11 3

60 33

27 18

21 18

10 7

effects were considered in the model. This is consistent with the frequently cited conclusion that plant species richness in temperate Asia exceeds that in temperate North America. This pattern is difficult to interpret, however, inasmuch as the proportions of different climate zones in each of the two regions differ significantly. What is more interesting in a comparative context is the significant taxon∗region interaction in both analyses. As pointed out above, what this shows is that ferns are significantly more diverse in Asia than they are in North America, by 0.39 log10 units (a factor of 2.5) relative to the average differences in diversity between classes and regions. In other words, the regional difference in diversity in ferns exceeds that in angiosperms and, especially, gymnosperms. Among the three classes of plants, the smaller the total number of families, genera, and species in a class, the higher proportion of the world’s totals occur within either eastern Asia or North America (Table 4). At the family level, eastern Asia has 10 of the world’s 11 families of gymnosperms (91%) and 52 of the 65 families of ferns in the world (80%), but only 45% of the world’s angiosperm families. As one would expect, these proportions are smaller at the genus and species level, although eastern Asia has 60% of the world’s gymnosperm genera and 27% of the total gymnosperm species. The patterns are the same, but the proportions of the global flora are smaller in North America.

DISCUSSION

Comparisons of plant diversity between eastern Asia and North America raise two general issues. One of these, the origin of the differences in diversity between the two continents, has been discussed at length in a number of papers (e.g. Latham & Ricklefs, 1993a, b; Li & Adair, 1994). The second issue arises from the observation that ferns are significantly more diverse in Asia compared with North America, and angiosperms somewhat less diverse, than one would expect based on comparisons of all three plant classes together. It is possible that this taxon-time-region interaction may shed some light on the differences between continents in general. The slopes and the lengths of the F-G-S (or f-g-s) lines in Figure 1 may be influenced by two kinds of factors. First, they may reflect differential speciation and/or extinction rates on both continents; second, they may reflect differential taxonomic treatments on familiar, generic, and specific levels between eastern Asian and North American taxonomists. Because the F-G-S (or f-g-s) lines are close to parallel (no significant global diversity∗region or global diversity∗taxon interactions), differences in diversity between regions and between taxa reside primarily at the family level and are therefore evolutionarily very old. Furthermore, because few families are endemic to either region, family circumscriptions are presumably widely agreed upon by

Q. GUO ET AL.

130

T 5. History of eastern Asia and North America floras and major geological events∗ Eras and time (Myr)

Geological and biological events

Pleistocene (0.5–3)

periodic glaciation; invasion from Asia across Beringia predominants (Vermeij, 1991) availability of Beringia and Atlantic landbridges mountain glaciers; forming of Beringia and Atlantic landbridges (Tiffney 1985a) collision of Himalaya with Eurasia (Hamilton, 1983) separation of North America from E Asia first angiosperm first gymnosperm first fern

Miocene (25) Eocene (49–58) Late Cretaceous-Cenozoic (63) Middle Cretaceous (100) Jurassic (130–155) Permian (248–286) Devonian (360–408)

∗ Data from Coulter & Dittmer (1964), Niklas et al. (1983), and Signor (1994).

systematic botanists world-wide. In this case, parallel F-G-S lines suggest that differences in diversity at the genus and species levels are not artifacts of different approaches to taxonomic work in the Old World as opposed to the New World (Kramer, 1990). Ferns may be relatively more diverse in eastern Asia because of certain ecological or abiotic factors that favour ferns there relative to gymnosperms and angiosperms, or because of historical factors connected to the different histories of the plant groups within changing continents. That is, the intercontinental difference in the species richness of ferns, gymnosperms, and angiosperms may be caused by the differences in the ages and locations of their origins, their past distributions, their present habitat distributions, and the dispersal capabilities of their plants. Timing of continental drift (plate tectonics) in relation to changes in relative dominance among the three plant classes in history also may be a critical factor in forming the intercontinental diversity patterns Phylogeny and history Ferns and gymnosperms first appeared during the Late Devonian to Early Carboniferous (c. 360 Myr ago), and were abundant and diverse by the Late Carboniferous (c. 300 Mya) (Behrensmeyer et al., 1992). Angiosperms apparently arose in the Late Jurassic-Early Cretaceous (130–155 Mya) (Table 5, Fig. 2; Coulter & Dittmer, 1964; Niklas, Tiffney & Knoll, 1983, 1985; Signor, 1994). The two megaplates (Eurasia and North America) separated shortly after the rise and establishment of angiosperms, i.e., this process began in the middle Cretaceous (c. 95 Mya; Hallam, 1994). The Atlantic Ocean widened until the Miocene when a North Pacific (Bering) land-bridge was formed (Hamilton, 1983). Although the Bering land-bridge has probably been available through much of the Tertiary, access was limited to plants tolerant of winter darkness, presumably deciduous species, throughout this period (Tiffney 1985b; but see Hsu¨, 1983). During this time, ferns and gymnosperms were continuously declining and angiosperms were expanding dramatically both in taxonomic richness and in abundance (largely replacing the other two classes; Fig. 2), especially in North America where new kinds of habitats were becoming available due to dramatic changes in landscapes in the west including the rise of Rocky Mountains and the forming of the southwestern deserts (Bond,

PLANT DIVERSITY IN EASTERN ASIA AND NORTH AMERICA

131

Figure 2. Taxonomic diversity of vascular plants (Northern Hemisphere only) through geological time. Redrawn from Niklas et al. (1983, 1985) and Signor (1994).

1989; Signor, 1994; see also Li & Adair, 1994). At present, these environments tend to support more evolutionarily advanced herbaceous plants. Even though ferns occurred much earlier than angiosperms and had greater dispersibility (Klekowski, 1972; Kramer, 1993; Smith, 1993), it has been hypothesized that the disjunct distribution of ferns was formed later than that of flowering plants (Kato, 1993) because most of the disjunct fern taxa are closely related species or varieties of the same species, that is, of lower rank than disjuncts of flowering plants (Li, 1952; Kato, 1993). Fossil data show that ferns had broader distribution in the geological past (e.g. Parris, 1985; Barrington, 1993; Kato, 1993).Quaternary glaciation in North America could have caused the disappearance of some fern species there. It is not clear, however, how ferns responded to Pleistocene glaciation compared with seed plants. Fossil records from eastern North America indicate relatively few extinctions of tree taxa in eastern North America during the late Tertiary, although the tree floras of western North America were diminished considerably (Latham & Ricklefs, 1993b). However, because the ecological dominance of ferns has declined continually for almost 200 Myr, it is possible that ferns are more restricted ecologically than angiosperms and consequently more vulnerable to extinction. One is tempted to relate regional differences in diversity in declining taxa to differences in extinction rates rather than to differences in the rate of origin of new taxa. Gymnosperms dominated the earth’s vegetation before angiosperms and remain dominants in certain vegetation types in some regions. They are all woody, perennial and wind-pollinated, and have long reproductive cycles (Bond, 1989). Floristically,

132

Q. GUO ET AL.

the rise of angiosperms 130–155 Mya apparently suppressed the earlier dominance of pteridophytes and gymnosperms (Wing et al., 1993). This is evident both on global scales, as shown by the dramatic increase of species diversity of angiosperms and the decrease of pteridophytes and gymnosperms (Signor, 1994; Bond, 1989), and locally, as shown by the limited geographical distribution of gymnosperms to high latitudes and elevations or nutrient-poor soils (Bond, 1989). Bond (1989) proposed that certain functional constraints could restrict gymnosperms to areas where growth of angiosperm competitors is reduced (i.e. cold temperatures and nutrient-poor soils). While this seems like a reasonable explanation for the distribution of gymnosperm taxa within regions, it cannot account for differences in diversity between regions. In particular, in Asia, where angiosperm diversity is comparable to, or slightly higher than, that of North America, gymnosperm diversity is also disproportionately higher. History appears to be the major determinant that controls continental-scale gymnosperm diversity, but the role of the emergence of angiosperms in causing the difference in gymnosperm diversity between eastern Asia and North America is not completely understood. Because diversity is not synonymous with ecological significance or dominance (Stebbins, 1981; Guo & Rundel, 1997), the relative importance of the three vascular plant classes in local communities in eastern Asia and North America still needs to be examined. One must also be particularly careful about the meaning of history in this context. On the one hand, history may refer to a situation or event in the past that can explain contemporary patterns of biological diversity better than present-day ecology. On the other, history may merely describe the change in a pattern of diversity towards a new ecologically driven equilibrium (Ricklefs, 1987). To illustrate the first pattern, the reduced diversity of forest trees in Europe appears to have been caused by conditions present during glacial maxima that are not representative of the current ecological situation. From the standpoint of local ecology, it is probable that European soils and climates today could support and maintain much of the diversity that was lost during the late Tertiary. By way of contrast, the extinction of tree species from western North America appears to have been caused by a gradual change in climate related to mountain building that has persisted to the present. It is likely that many of the species that have disappeared from the area would not re-establish themselves easily in the contemporary ecological setting. Ecological dispersal Ferns may have different distribution patterns from the seed plants because of their unique reproductive and dispersal mechanisms (Li, 1952). For all three classes of vascular plants, dispersal may have been critically important in forming the intercontinental patterns in diversity. One interpretation of these patterns is that relatively more higher taxa originated in Asia (including southeast Asia) but many of these failed to reach North America (“asymmetrical invasion”; Vermeij, 1991; Latham & Ricklefs, 1993a, b; see also Chaney, 1947; Wolfe, 1972). In contrast, Cracraft (1994) recently argued that long-distance dispersal may play a minor role compared to vicariance and differential extinction in structuring continental species assemblages (biotas). More importantly, dispersal may play different roles in the distribution of ferns, gymnosperms, and angiosperms. Vermeij (1991) suggested that the predominantly eastward invasion of temperate floristic elements from Asia to

PLANT DIVERSITY IN EASTERN ASIA AND NORTH AMERICA

133

North America during the Miocene and Pliocene is correlated with higher rates of extinction on the North American side, although the Tertiary fossil record for angiosperms does not indicate significant extinction of woody species in eastern North America (Latham & Ricklefs 1993b). Detailed fossil evidence is critically needed to link phylogenetics, dispersal, and intercontinental diversity patterns (Vermeij, 1991). If dispersal were critical to the establishment of observed diversity patterns, then the greatest differences in diversity should occur in taxa that diversified during periods of low dispersal between regions. This would argue against dispersal being important to the diversity pattern in ferns if fern diversity was established primarily during the Mesozoic period when Eurasia and North America were part of a single land mass. Regional environments The differences in the diversity of three vascular plant groups between eastern Asia and North America might be explained in part by recent geological events and by the contemporary ecological setting. Eastern Asia has a broad land connection to subtropical and tropical regions to the south, which may have served as refugia for plant species and permitted later northerly expansions. Compared with the other two vascular plant groups, ferns may also be relatively more diverse in mesic and tropical habitats (Page, 1979). Not only does eastern Asia contain greater area of such habitat than North America, it has closer access to tropical centres of diversity and may have more readily provided refuge to ferns during periods of climate cooling. The same argument could apply to gymnosperms and angiosperms. To understand the degree to which the contemporary distribution of environments contributes to differences in diversity between eastern Asia and North America would require a more detailed examination of the relative histories, areas, and diversities of plants in the major vegetation zones in each continent. This is not yet possible with the information available to us. However, it is unlikely that eastern Asia contains greater diversity of vegetation types than North America, although it may contain larger areas of subtropical habitat with high plant taxonomic diversity. Comparisons of tree species diversity between eastern Asia and eastern North America (Latham & Ricklefs, 1993b) indicate that for one plant growth form in one habitat type the diversity difference between Asia and North America is quite pronounced. Intercontinental comparison using phylogenetic reconstruction analysis could be a powerful tool in the study of evolution and dispersal of terrestrial organisms in geological history (Cracraft, 1994; Ricklefs, 1996). Most earlier comparisons of this kind were undertaken to detect convergent/divergent evolution in community organization (e.g. Good, 1974; Cody, 1975; Mooney, 1977; Cody & Mooney, 1978; White, 1983; Goldblatt, 1993; Mooney, Fuentes & Kronberg, 1993; Brown, 1993; Westoby, 1993; Cowling et al., 1996), and few of these focused on species diversity (but see Cody, 1983, 1996; Latham & Ricklefs, 1993b; Westoby, 1993; Guo, 1998). While considerable progress has been made in studies of disjunct distributions of either plants or animals, it may be critical to synthesize data for multiple taxonomic levels combining both plants and animals that share the same type of disjunct distribution to improve our knowledge of the coevolution, distribution, and regional/ global biodiversity patterns (Vrba, 1992). The procedure shown here could be

134

Q. GUO ET AL.

extended to all seven of the continental plates that made up Laurasia and Gondwana, and to many more of the groups of organisms that inhabited them.

ACKNOWLEDGEMENTS

We thank G. Stevens and W. Berry for helpful comments. Q. Guo was supported by a fellowship from K. C. Wong Foundation (Hong Kong) and a grant from the Chinese National Science Foundation.

REFERENCES

Barrington DS. 1993. Ecological and historical factors in fern biogeography. Journal of Biogeography 20: 275–280. Behrensmeyer AK, Damuth JD, DiMichele WA, Potts R, Sues H-D, Wing SL (eds.). 1992. Terrestrial ecosystems through time. Evolutionary paleoecology of terrestrial plants and animals. Chicago: University of Chicago Press. Bond WJ. 1989. The tortoise and the hare: ecology of angiosperm dominance and gymnosperm persistence. Biological Journal of the Linnean Society 36: 227–249. Boufford DE, Spongberg SA. 1983. Eastern Asian-North American phytogeographical relationships – A history from the time of Linnaeus to the twentieth century. Annals of the Missouri Botanical Garden 70: 423–439. Brown JH. 1993. Assessing the effects of global change on animals in western North America. In: Mooney HA, Fuentes ER, Kronberg BI, eds. Earth system responses to global change: contrasts between North and South America. San Diego: Academic Press, 267–284. Brown JH, Gibson AC. 1983. Biogeography. St. Louis: Mosby. Butzer K. 1974. Geological and ecological perspectives on the middle Pleistocene. Quaternary Research 4: 136–148. Chaney RW. 1947. Tertiary centers and migration routes. Ecological Monographs 17: 139–148. Cody ML. 1975. Towards a theory of continental species diversities. In: Cody ML, Diamond JM, eds. Ecology and evolution of communities. Cambridge: Harvard University Press, 214–257. Cody ML. 1983. Continental diversity patterns and convergent evolution in bird communities. In: Kruger F, Mitchell DT, Jarves JUM, eds. Mediterranean-type ecosystems. Ecological Studies 43. Berlin: Springer-Verlag, 347–402. Cody M. 1996. Introduction to Neotropical diversity. In: Gibson AR, ed. Neotropical biodiversity and conservation. Occasional publication of the Mildred E. Mathias Botanical Garden 1. Los Angeles: University of California, 1–20. Cody ML, Mooney HA. 1978. Convergence versus nonconvergence in Mediterranean-climate ecosystems. Annual Review of Ecology and Systematics 9: 265–321. Coulter MC, Dittmer HJ. 1964. The story of the plant kingdom. Chicago: University of Chicago Press. Cowling RM, Rundel PW, Lamont BB, Arroyo MK, Arianoutsou M. 1996. Plant diversity in Mediterranean-climate regions. Trends in Ecology and Evolution 11: 362–366. Cracraft J. 1994. Species diversity, biogeography, and the evolution of biotas. American Zoologist 34: 33–47. Delcourt PA, Delcourt HR. 1987. Long-term forest dynamics of the temperate zone. New York: SpringerVerlag. Fiero B. 1986. Geology of the Great Basin. Reno: University of Nevada Press. Goldblatt P (ed). 1993. Biological relationships between Africa and South America. New Haven: Yale University Press. Good R. 1974. The geography of flowering plants. 4th edition. London: Longman. Graham A (ed). 1972. Floristics and paleofloristics of Asia and Eastern North America. New York: Elsevier. Gray A. 1878. Forest geography and archaeology. American Journal of Science 16: 85-94, 183-–196. Guo Q,. 1998. Ecological comparisons between eastern Asia and North America: historical and geographical perspectives. Journal of Biogeography. In press.

PLANT DIVERSITY IN EASTERN ASIA AND NORTH AMERICA

135

Guo Q, Rundel PW. 1997. Measuring dominance-diversity in ecological communities: choosing the right variables. Journal of Vegetation Science 8: 405–408. Hallam A. 1994. An Outline of Phanerozoic Biogeography. Oxford: Oxford University Press. Hamilton W. 1983. Cretaceous and Cenozoic history of the northern continents. Annals of the Missouri Botanical Garden 70: 440–458. Hsu¨ J. 1983. Late Cretaceous and Cenozoic vegetation in China, emphasizing their connections with northern America. Annals of the Missouri Botanical Garden 70: 490–508. Kato M. 1993. Biogeography of ferns: dispersal and vicariance. Journal of Biogeography 20: 265–274. Klekowski EJ. 1972. Genetic features of ferns as contrasted to seed plants. Annals of the Missouri Botanical Garden 59: 138-–151. Kramer KU. 1990. The American paradox in the distribution of fern taxa above the rank of species. Annals of the Missouri Botanical Garden 77: 330–333. Kramer KU. 1993. Distribution patterns in major pteridophyte taxa relative to those of angiosperms. Journal of Biogeography 20: 287–291. Latham RE, Ricklefs RE. 1993a. Global patterns of tree species richness in moist forests: energydiversity theory does not account for variation in species richness. Oikos 67: 325–333. Latham RE, Ricklefs RE. 1993b. Continental comparisons of temperate-zone tree species diversity. In: Ricklefs RE, Schulter D, eds. Species diversity in ecological communities. Chicago: University of Chicago Press, 294–314. Li H-L. 1952. Floristic relationships between eastern Asia and North America. Transactions of the American Philosophical Society 42: 371–429. Li H-L. 1972. Eastern Asia-North America species-pairs in wide-ranging genera. In: Graham A, ed. Floristics and paleofloristics of Asia and North America. New York: Elsevier, 65–78. Li S, Adair KT. 1994. Species pools in eastern Asia and North America. Sida 16: 281–299. Lidgard S, Crane PR. 1990. Angiosperm diversification and Cretaceous floristic trends: a comparison of palynofloras and leaf macrofloras. Paleobiology 116: 77–1593. McLaughlin SP. 1986. Floristic analysis of the southwestern United States. Great Basin Naturalist 46: 46–65. Mooney HA (ed). 1977. Convergent evolution in Chile and California: Mediterranean climate ecosystems. Stroudsburg: Hutchinson & Ross. Mooney HA, Fuentes ER, Kronberg BI (eds). 1993. Earth system responses to global change: contrasts between North and South America. San Diego: Academic Press. Niklas KJ, Tiffney BH, Knoll AH. 1983. Patterns in vascular plant diversification. Nature 303: 614–616. Niklas KJ, Tiffney BH, Knoll AH. 1985. Patterns in vascular plant diversification: An analysis at the species level. In: Valentine JW, ed. Phanerozoic diversity patterns: Patterns in macroevolution. Princeton: Princeton University Press, 97–128. Page CN. 1979. The diversity of ferns: an ecological perspective. In: Dyer AF, ed. The experimental biology of ferns. London: Academic Press, 9–56. Parris BS. 1985. Ecological aspects of distribution and speciation in Old World tropical ferns. In: Dyer AF, Page CN, eds. Biology of pteridophytes. Edinburgh: Royal Society, 341–346. Prance GT (ed). 1982. The biological model of diversification in the tropics. New York: Columbia University Press. Raven PH. 1972. Plant species disjunctions: a summary. Annals of the Missouri Botanical Garden 59: 234–246. Raven PH, Axelrod DI. 1974. Angiosperm biogeography and past continental movements. Annals of the Missouri Botanical Garden 61: 539–673. Ricklefs RE. 1987. Community diversity: relative roles of local and regional processes. Science 235: 167–171. Ricklefs RE. 1996. Application of phylogenetically independent contrasts: a mixed progress report. Oikos 77: 167–172. Ricklefs RE, Latham RE. 1992. Intercontinental correlation of geographical ranges suggests stasis in ecological traits of relict general of temperate perennial herbs. American Naturalist 139: 1305–1321. Sauer JD. 1988. Plant migration. Berkeley: University of California Press. Signor PW. 1994. Biodiversity in geological time. American Zoologist 34: 23–32. Smith AR. 1993. Phytogeographic principles and their use in understanding fern relationships. Journal of Biogeography 20: 255–264. Stebbins GL. 1981. Why are there so many species of flowering plants? BioScience 31: 573–577.

136

Q. GUO ET AL.

Takhtajan A. 1969. Flowering plants: origin and dispersal. Edinburgh: Oliver and Boyd. Thorne RF. 1972. Major disjunctions in the geographical ranges of seed plants. Quarterly Review of Biology 47: 365–411. Tiffney BH. 1985a. The Eocene Atlantic landbridge: its importance in Tertiary and modern phytogeography of the Northern Hemisphere. Journal of the Arnold Arboretum 66: 243–273. Tiffney BH. 1985b. Perspectives on the origin of the floristic similarities between eastern Asia and eastern North America. Journal of the Arnold Arboretum 66: 73–94. Vermeij GJ. 1991. When biotas meet: understanding biotic interchange. Science 253: 1099–1104. Vrba ES. 1992. Mammals as a key to evolutionary theory. Journal of Mammalogy 73: 1–28. Westoby M. 1993. Biodiversity in Australia compared with other continents. In: Ricklefs RE, Schulter D, eds. Species diversity in ecological communities. Chicago: University of Chicago Press, 170–177. White PS. 1983. Eastern Asian-North American floristic relations: the plant community level. Annals of the Missouri Botanical Garden 70: 734–747. Wing SL, Hickey LJ, Swisher CC. 1993. Implications of an exceptional fossil flora for Late Cretaceous vegetation. Nature 363: 342–344. Wolfe JA. 1972. An interpretation of Alaska Tertiary floras. In: Graham A, ed. Floristics and paleofloristics of Asia and North America. New York: Elsevier, 201–233. Wolfe JA. 1975. Some aspects of plant geography of the Northern Hemisphere during the Late Cretaceous and Tertiary. Annals of the Missouri Botanical Garden 62: 264–279. Wood CE Jr. 1972. Morphology and phytogeography: the classical approach to the study of disjunctions. Annals of the Missouri Botanical Garden 59: 107–124. Wu Z. (ed). 1980. The vegetation of China. Beijing: Science Press. Wu Z. 1983. On the significance of Pacific intercontinental discontinuity. Annals of the Missouri Botanical Garden 70: 577–590.