Reprint from: R. E. Ricklefs and D. Schluter (editors). 1993. Species Diversity in Ecological Communities: Historical and Geographical Perspectives. University of Chicago Press.
Continental Comparisons of Temperate-Zone Tree Species Diversity
Roger Earl Latham and Robert E. Ricklefs
Biogeographers and ecologists have sought for nearly two beyond 40° to 45° north latitude over or around the east centuries to make sense of the striking differences in tree west-trending mountain ranges and the Mediterranean species diversity among the major forest regions of the basin (e.g., Gray 1878). The effects were less severe in earth. Alexander von Humboldt and Aime Bonpland may eastern North America, where the barriers-the Gulf of have been the first to comment on these differences (1807) Mexico and the Mexican highlands-lay south of 30° when they remarked on the far greater species diversity north latitude. For 1,200 km inland across continental among tropical tree species than in the temperate zone. eastern Asia from the eastern coast, no physical barriers With some hyperbole, they also claimed that North would have impeded southward migration by plants well America had three times as many species of oaks alone as beyond the Tropic of Cancer. Europe had tree species. Moist tropical forests have since Ecologists generally have invoked geographical varia been found to have at least ten times as many tree species tion in the outcome of small-scale deterministic processes as moist forests of the northern and southern temperate to explain global-scale diversity patterns (Connell and zones, at several spatial scales. Biologists have proposed Orias 1964; MacArthur 1972; Ricklefs 1977, 1987; Hus various explanations for the temperate-tropical disparity ton 1979). These hypotheses describe a world in equilib in tree species diversity (Pianka 1966, 1989b; Hubbell rium. Even when so-called nonequilibrial mechanisms and Foster 1986; Stevens 1989). No one mechanism has such as natural disturbance are called into play (Huston found general acceptance, and several may contribute to 1979), it is assumed that disturbance and interactions be the pattern. Within temperate latitudes, the mesic forests tween populations, such as competitive exclusion and col of eastern Asia have three times more tree species than onization of disturbed patches, would show balance forests in eastern North America and six times more than within a landscape if it were sampled at large enough spa those in Europe. Although this pattern has beell recog tial and temporal scales (Petraitis, Latham, and Niesen nized for more than a century (Gray 1878), it has at baum 1989). tracted far less attention from theorists than the A major problem with invoking competition and other temperate-tropical disparity. interactions to explain global diversity patterns is that the Suggesting an explanation for regional disparities in scale of the patterns is grossly mismatched to the scale of temperate tree species diversity, Gray (1878) and other the putative causal process. It is a large step from local early authors pointed to probable differences in extinc scale processes to predictions of regional-scale species tion rates due to differences between the continents in the richness. Such predictions would require plausible severity of climatic cooling during the Quaternary Ice hypotheses linking the expression of local processes to re Ages. Continental ice sheets covered much of Europe's gional variation in the physical environment (Ricklefs present-day temperate forest region and advanced deeply 1987). Furthermore, the fossil record gives no reason to into eastern North America, but they never reached the expect equilibrium in regional species richness; The global mid-latitudes of eastern Asia (fig. 26.1). Fossil floras show number of vascular plant species apparently has risen that forests in Europe were far more diverse in the Ter steadily throughout the Phanerozoic, most sharply since tiary than at present (Reid and Chandler 1933; Mai 1960; the radiation of angiosperms in the late Cretaceous Kilpper 1969; van der Hammen, Wijmstra, and Zagwijn (Crepet 1984; Niklas, Tiffney, and Knoll 1985), while the 1971; Mai 1971a, 1971b; Takhtajan 1974; tancucka area of land in terrestrial planr communities has perhaps Srodoniowa 1975; Collinson and Crane 1978; Friis 1979; decreased, at least since the Oligocene, due to the growth Mai 1980, 1981; Gregor 1982; Friis 1985; Kvacek and of continental ice sheets. Local processes could determine Walther 1987; Mai 1987a, 1987b; Mai and Walther regional species richness if the region were merely a col 1988; Sauer 1988; Kvacek, Walther, and Buzek 1989). lection of habitats and localities within which diversity Most, if not all, of the genera lost from Europe in the late were regulated. However, differences in diversity between Tertiary continue to inhabit forests of eastern Asia or regions with similar climate suggest that local and re North America. Plant distributions uriderwent severe con gional processes contribute separately to regional species traction and confinement in Europe, according to some richness. views, because most plants failed to migrate southward In this chapter we attempt to understand geographical
294 COMPARISONS OF TEMPERATE-ZONE TREE SPECIES DIVERSITY 295
Figure 26.1 Maximum Quaternary advance of continental ice North America (excluding Greenland), Europe (including western sheets (white areas) superimposed 01) present-day coastlines (after Siberia), and eastern Asia at approximately 16, 9.4, and 1.2 X 106 Nilsson 1983). Maximum areal coverage has been estimated for km" respectively (Nilsson 1983). patterns in the contemporary diversity of regional tree ents the criteria used in compiling the tree floras and an floras in the temperate Northern Hemisphere by examin abridged version of the floras themselves, with numbers ing both historical and ongoing processes. We begin by of species tabulated by genus and region, information comparing diversity in contemporary regional floras at about the contemporary distributions of the families and several taxonomic levels. Next, we compare regional fos genera, and Tertiary fossil data on the genera. Table 26.1 sil floras beginning in the early Tertiary, when forests of summarizes the total flora by the number of species, gen modern aspect first appeared, with present-day regional era, families, orders, and subclasses occurring in each re floras, seeking patterns in the survival and extirpation (re gion. A total of 1,166 species make up the characteristic gional extinction) of genera. We then review tests of equi north temperate tree flora. Species are distributed among librial hypotheses developed to explain local diversity. Fi Europe (including the Caucasus), eastern Asia, North nally, we suggest a scenario for the establishment of global America's Pacific slope, and eastern North America, re patterns in tree species diversity. We infer that contempo spectively, approximately in the ratio 2:12:1:4. rary patterns of tree species diversity owe much to histori cal and evolutionary contingency, in contrast with some Adjoining Subtropical Forest other authors who have interpreted the patterns as arising Of the four moist temperate forest regions, only the one from, and maintained in equilibrium by, ecological inter in eastern Asia shares a long, common border with moist actions. We propose ancient roots for contemporary di subtropical forest. Thus, it is reasonable to conjecture that versity anomalies, considerably older than the Pliocene the high diversity of temperate eastern Asia's tree flora and Quaternary climate cooling and resulting widespread maybe due to the incursion of subtropical elements. extirpations. We conclude by discussing testable predic Many tree species occurring in the eastern Asian temper tions suggested by our interpretation. ate zone that have mainly subtropical ranges were already excluded from our list. We retallied the list also excluding those species that have mainly temperate ranges but that, CONTEMPORARY GEOGRAPHICAL PATTERNS IN nonetheless, belong to genera with predominantly tropi TEMPERATE TREE SPECIES DIVERSITY cal distributions (table 26.1). This did change the eastern Asian term in the ratio of species numbers among regions, Species Diversity in the Four Regions but a large disparity between regions remained. When In order to compare taxonomic diversities among the ma predominantly tropical genera were excluded, the ratio jor moist temperate forest regions of the Northern Hemi became approximately 2:9:1:4. sphere, we defined the areas covered by moist temperate forest and compiled lists of all of the characteristic tree Area species. The regions are the four warm-temperate humid The disparities in tree diversity among the four moist tem and temperate-nemoral climate biomes of Walter (1979). perate forest regions cannot be explained as an area effect, These mid-latitude regions extend varying distances to because three of the regions (excluding the much smaller ward mid-continent from the east and west coasts of Eu temper:lte forest zone of North America's Pacific slope) rasi:l and North America (fig. 26.2). Appendix 26.1 pres- cover similar areas. The areas of the three larger regions 296 ROGER EARL LATHAM AND ROBERT E. RICKLEFS
Biome 1 Biome 2 Biome 3 .. Tropical everwet D Tropical seasonal mill Subtropical desert Biome4 Biome 5 Biome 6 Winterwet ~, ... ~ Warm temperate, wet Cool temperate Biome 7 Biome8 Biomes 9 and 10 •[Sill Interior desert D Cold temperate D Tundra and Glacial Figure 26.2 Contemporary biomes. Areas in black are major highlands. The study focuses on biomes 5 and 6. (After Wa lter 1979.)
Table 26.1. Summary by Taxonomic Level and Region of Moist Temperate Forest Trees in the Northern Hemisphere Number of tree taxa characteristic of moist temperate forests in: Northern, central, & Pacific Northern eastern East-central slope of Eastern Hemisphere Taxonomic leve l Europe Asia North America North America (total) Subclasses 5 9 6 9 10 Orders 16 37 14 26 39 Families 21 67 19 46 74 Genera 43 177 37 90 213 Species 124 729 68 253 1,166 Families excluding those of predominantly tropical distribution 18 37 18 29 41 (% of total) (86%) (55%) (95%) (63%) (55%) Genera excluding those of predominantly tropical distribution 41 121 35 77 149 (% of total) (95 %) (68%) (95%) (86 %) (70%) Species exclusive of predominantly tropical genera 122 570 66 236 987 (% of total) (98%) (78%) (97%) (93%) (85%)
were estimated by transferring Walter's (1979) biome de cultural vegetation) in only one region. For example, lineations to 1: 12,OOO,OOO-scale Miller oblated srereo Wolfe's (1979) map of eastern Asian mid-latitude forests graphic projection maps (Rand McNally 1969) and plan shows approximately 1.8 X 106 km! in forest types domi imetering. In Europe, eastern Asia, and eastern North nated by broad-leaved deciduous trees. Braun's (1950) America, moist temperate forest biomes were estimated map of deciduous forest associations in eastern North by this method to cover approximately 1.2, 1.2, and 1.8 America indicates approximately 2.4 X 106 km! in the en X 106 km2 respectively. These estimates are generally tire temperate forest region. Despite the discrepancies in lower than those from sources that present more detailed magnitude, the ratio of these estimates-3:4-is similar surveys of potential natural vegetation (roughly, preagri- to the ratio of the areas defined by Walter- 2:3. (Potential COMPARISONS OF TEMPERATE-ZONE TREE SPECIES DIVERSITY 297
0 east-central ASi a Figure 26.3 Numbers of tree taxa of the four o eastern North America 0 , ' moist temperate fores t regions in the N orthern ac tual 0 Europe (including the Caucasus) o} predicted by rarefaction of Hemisphere a nd res ults of simulated rarefacti on 1o Pacific slope of North America 0 east-central ASian tree flora of the east-central Asian tree flora to the species 1000 numbers of the other tree floras (see table 26, 1), D o x .2 100
Q) .Q E :::l Z
+ + + + + + SUBCL, ORD,FAM,GEN, Spp, SUBCL, ORD,FAM,GEN, spp, SpecieS of predominately All species included tropical genera excluded natural vegetation maps are common for countries or re gio~s ' tree floras by simulating the rarefaction (Simberloff gions within Europe but are virtually nonexistent, [except 1979) of the most diverse floras to match the species num for Walter 1979] for the entire temperate forest region of bers in other, less diverse floras. We performed the rar Europe including the Caucasus, even in the most geo efaction tests by computer, picking randomly from the graphically comprehensive treatments [Rubner and Rein species pool of one temperate tree flora until the number hold 1953; Mayer 1984; Jahn 1991].) By area alone, one present in a less diverse flora was reached. For example, would expect North America's temperate forest zone to in simulating the rarefaction of the 729-species eastern have the highest tree species diversity. Asian temperate tree flora to 124-the number of species that belong to the European temperate tree flora-the Taxonomic Diversity Patterns program randomly picked 124 species from the eastern There are many other means of detecting patterns in taxo Asian list and tallied the genera, families, orders, and sub nomic diversity among regions with similar climate and classes represented by those species. Each simulation was vegetation besides simply comparing the numbers of spe repeated 1,000 times. Rarefaction was simulated initially cies. We explored several, including comparing numbers using the entire tree list and again using only species that of higher taxa (table 26.1; fig. 26.3), comparing numbers do not belong to genera with predominanrly tropical dis of species per genus and other ratios of lower to higher tributions. taxa, comparing numbers of genera and families con The actual numbers of genera, families, and orders in sisting of only one or two species, and examining patterns the temperate tree floras of Europe and extreme western in the overlap of taxa among regions. It is implicit that North America differ substantially from the mean num higher taxa are older than lower taxa. Thus, contempo bers obtained by rarefaction of the eastern Asian temper rary distributions of tree genera, families, and orders ate tree flora (tables 26.2 and 26.3; figs. 26.3 and 26.4). among regions may offer clues about the historical rela Statistical analysis of the rarefaction results shows that tionships among the regions' tree floras. the European temperate tree flora is consistently poorer in genera, families, and orders than the eastern Asian tem Simulated Rarefaction. Whether or not regions differ in perate tree flora, whether or not the predominantly tropi their distributions of species among higher taxa could cal genera are omitted. Rarefaction of the eastern North have broad implications for interpreting historical rela American tree flora to the numbers of species in Europe tionships among regional floras and possible causes of re yielded similar differences, which are also highly signifi gional diversity differences. For example, suites of physio cant. Rarefaction revealed almost no significant differ logical or anatomical traits associated with plant families ences in hierarchical patterns of tree diwrsity between may have been crucial to species' regional survival or ex east-central Asia and eastern North America or between tinction during episodes of climate change that affected Europe and the Pacific slope of North America. the regions differenrly. In this case, we would expect re We assume, for the moment, that frost tolerance is a gions to differ significantly in taxonomic structure; that characteristic of higher taxa (we will return to this as is, the flora of one region should diverge significanrly from sumption later) and that frost tolerance was the key to a a random subset, containing the same number of species, taxon's regional survival through Quaternary cooling. It of a more diverse flora in another region, in the num follows that regional extinction would haye been nonran ber of families represented and in the frequency distribu dom among higher taxa. Based on these assumptions, the ti on of species per family. rarefaction results support the hypothesized link between We compared the taxonomic structures of the four re- extirpation and low tree species richness in Europe and 298 ROGER EARL LATHAM AND ROBERT E. RICKLEFS 300 western North America, since the distributions of species among higher taxa in these regions differ sharply from random subsets of the east-central Asian and eastern 200 North American forests. However, given the same as sumptions, the 'rarefaction results suggest that something other than differences in regional extinction rates may be 100 responsible for the lower tree species richness in eastern North America relative to east-central Asia. To interpret these results, we seek a factor that is unbiased toward or 20 25 against particular genera, families, or orders, in contrast to regional extinction based on frost tolerance or intoler 200 Orders ance, which we assume to be phylogenetically selective.
Species per Higher Taxon. The temperate tree floras of Europe and the Pacific slope of North America clearly are 100 depauperate at the higher taxonomic levels relative to those of eastern Asia and eastern North America. Further more, their higher taxa, on average, have fewer species, even though many genera and families are represented in 0 + the eastern Asian temperate forest region by only one or 20 25 30 35 40 a few species (fig. 26.5A). While numbers of genera repre Families sented by one or a few species are far higher in eastern 100 Asia than in Europe, eastern Asia and Europe have almost identically low proportions of these regionally low diversity genera, lower than the corresponding propor tions in North America (fig. 26.5B). In genera common to 50 each pair of regions, Asian temperate forests are two to five times more speciose, on average, than forests in the other regions (table 26.4 columns A and B) .
o .l.!...... + =----lsrlTc.J....J....L" .L.l....LL· c.l...LLLLL.L.L.l....LLTh..LLJ...ITL 45 50 55 60 65 Globally Depauperate Genera. We examined the distribu Genera tion of globally mono typic or ditypic (comprising only one or two species) tree genera among the regions (table Figure 26.4 Frequency distributions of higher taxa resulring from 26.5). We were interested in the biogeography of these simulated rarefaction of the east-central Asian moist temperate tree flora to the species number of the European moist temperate tree very low diversity genera because presumably some are flora, with genera of predominantly tropical distribution omitted relicts of formerly diverse lineages, and some are autoch (see table 26.3). Numbers of taxa actually present in the European thonous and perhaps new taxa that have not diversified. moist temperate tree flora are marked by asterisks ('). In either case they probably are more likely to go extinct
Table 26.2. Results of Simulated Rarefactions of Contemporary Tree Species Taxa present Mean taxa in 1,000 floras
Regions compared Taxon in region 1 drawn randomly from region 2 t1df _ 999) Significance 1 Europe Order 16 25.2 -4.80 2 East-central Asia Family 21 38.5 -6.22 Genus 43 70 .2 -6.46 1 Pacific slope of North America Order 14 20.7 -3.11 2 East-central Asia Family 19 29.1 -3.65 Genus 37 47.0 -3.16 1 Eastern North America Order 26 30.4 -2.42 (NS) 2 East-central Asia Family 46 50.4 -1.63 (NS) Genus 90 105.5 -3.34 1 Europe Order 16 22.4 -4.58 2 Eastern North America Family 21 36.5 -7.13 Genus 43 58.8 -4.73 1 Pacific slope of North America Order 14 18.8 -2.99 2 Eastern North America Family 19 28.1 -3.80 Genus 37 39.8 -0.90 (NS) 1 Pacific slope of North America Order 14 13.5 0.46 (NS) 2 Europe Family 19 16.8 1.51 (NS) Genus 37 32.6 2.01 (NS)
Note: We used t-tests to compare the actual numbers of taxa present with the means from 1,000 randomly generated floras in which the moist temperate tree flora of region 2 is rarified to the number of species in the moist temperate tree flora of region I. The Type [ error rate was adjusted using the Dunn-Sid:ik method (Sakal and Rohlf 1981): fat ex = ,05, ex' = .0085 and for ex = .01, ex' = .0017 (OP < ,05, 'op < .01). COMPARISONS OF TEMPERATE-ZONE TREE SPECIES DIVERSITY 299
Table 26.3. Results of Simulated Rarefactions of Contemporary Tree Species Taxa present Mean taxa in 1,000 floras
Regions compared Taxon in region 1 drawn randomly from region 2 t'df - 999) Significance 1 Europe Order 15 20.6 -3.30 ** 2 East-central Asia Family 20 32.0 - 5.07 Genus 41 57.5 -4.30 1 Pacific slope of North America Order 14 17.3 -1.95 (NS) 2 East-central Asia Family 19 24.9 -2.45 (NS) Genus 35 40.0 -1.56 (NS) 1 Eastern North America Order 25 23.8 0.75 (NS) 2 East-central Asia Family 41 39.0 0.97 (NS) Genus 77 80.6 -0.92 (NS) 1 Europe Order 15 21.3 -4.49 2 Eastern North America Family 20 32.9 -6.41 ** Genus 41 53.2 -4.01 1 Pacific slope of North America Order 14 17.8 -2.42 (NS) 2 Eastern North America Family 19 25.2 - 2.78 Genus 35 36.4 - 0.47 (NS) 1 Pacific slope of North America Order 14 12.9 0.97 (NS) 2 Europe Family 19 16.1 2.00 (NS) Genus 35 31.2 1.83 (NS) Note: Simulations were performed and tested for significance as described in table 26.2.
200 Figure 26.5 Frequency distributions of species >- per genus in the four regions. (A) Cumulative fre 0 quency; (B) cumulative proportion of total fre c quency. (]) ::J 0- (]) '- 'I- (]) 100 :;::::> 0 ::J E ::J U -- 0 10 100 c 1,0 0 t B 0 Q. Q 0,8 Q. Q) > Europe +- 0 0,6 east-central Asia ::J Pacific slope of North America E eastern North America ::J U 0,4 " 1 10 100 Species per genus 300 ROGER EARL LATHAM AND ROBERT E. RICKLEFS
Table 26.4. Comparison of Species Diversity of Tree Genera among Reg ions (A) Mean species! Significance (B) M ea n species! Significance (C) Mean species! genus in region 1 of Wilcoxon genus in region 2 of Mann- genus in region 2 of genera common to Ts of ge nera common to Whitney U of genera extirpated Regions compared both regions (Ho: YA = YB) both regions (Ho: YB = yel from region 1 1 Europe 3.03 P < .000001 10.1 P < .000001 2.69 2 East-central Asia n = 39 n = 39 n = 74 1 Pacific slope of North America 2.00 P < .00001 10.0 P < .0005 3.54 2 East-central Asia n = 31 n = 31 n = 35 1 Eastern North America 3.57 P < .000005 7.59 P < .01 2.00 2 East-central Asia n = 61 n = 61 n=8 1 Europe 3.43 (NS) 5.20 P < .0005 1.82 2 Eastern North America n = 30 n = 30 n = 34 1 Pacific slope of North America 2.08 P < .005 5.19 (NS) 2.67 2 Eastern North America n = 26 n = 26 n = 21 1 Pacific slope of North America 2.27 (NS) 3.64 (NS) 2.50 2 Europe n = 22 n = 22 n=8 Note: We compared species diversities of tree genera currently inhabiting regions of higher overall diversity (B) with the same genera persisting in regions of lower overall diversity (A), and with' genera extirpated from regions of lower overall diversity during the mid- to late Tertiary and Quaternary (C). Nonparametric methods were used to test the statistical significance of differences between groups in species numb~rs per genus: the Wilcoxon signed-rank test for paired samples and the Mann-Whitney test for unpaired samples. Adjusting the Type I error rate by the Dunn-Sid:ik method (Sokal and Rohlf 1981), for ex = .05, ex' ;= .0085 and for ex = .01, ex' = .0017.
Table 26.5. Geographical Distribution of Globally Mono- and Ditypic Genera of Moist Temperate Forest Trees in the Northern Hemisphere. Entire Northern, central, & Pacific slope of Eastern Northern Hemisphere eastern Europe East-central Asia North America North America (total) Genera of > 2 species worldwide 41 148 34 78 168 Genera of :5 2 species worldwide 2 29 3 12 45 (% of total) (4.7%) (16%) (8.1%) (13%) (21%) Note: One ditypic tree genus, Liriodendron, lives in two of the regions,
than are multispecies genera. Their distribution may re include Ginkgo, 7 conifers (including Glyptostrobus, flect differences among regions in rates of extirpation or which ranges in eastern Asia's temperate zone but occurs production of new taxa. primarily southward), and 11 hamamelids. The known There are 45 globally monotypic or ditypic tree genera relicts thus belong disproportionately to the older classes native to the north temperate forest regions. Six make up and subclasses. Ginkgo and conifers belong to the oldest globally mono- or ditypic families: four in eastern Asia surviving lineages of temperate trees, and hamamelids in (Ginkgoaceae, Eucommiaceae, Cercidiphyllaceae, Tetra clude the oldest known angiosperm temperate trees, mem centraceae) and two in eastern North America (Leitneria bers of the formerly diverse and now depauperate Plat ceae, Cyrillaceae). The globally mono- and ditypic genera anaceae (Schwarzwalder 1986). are distributed among Europe, east-central Asia, North Of the 21 globally mono- and ditypic genera that ap America's Pacific slope, and eastern North America, re pear to be endemic to a single region, 13 (62 %) are absent spectively, at a ratio of approximately 1:12:1:5, compared from the fossil record in any of the four regions. They may with the ratio of total numbers of genera of approxi have occurred sparsely in Tertiary forests, their Tertiary mately 1:4:1:2. We used the G-test to compare the distri ranges may have been small, they may have first appeared bution among regions of genera consisting of only one or only recently, or they may be cryptic in the fossil record two species worldwide with the distribution of genera owing to low pollen output, non-wind-dispersed pollen, \Vith more than two species (table 26.5) . The test showed or restriction to habitats not conducive to fossilization of :he two distributions to be marginally significantly djffer leaves, flowers, fruits, or seeds. Most are probably within ~nt (G = 6.7, df = 3, .05 < P < .1). Thus, globally mono region relicts or lineages that never were diverse or md ditypic genera may be overrepresented in eastern Asia abundant. md eastern North America relative to Europe and North '\merica's Pacific slope. Global Distribution of Genera. We tallied genera that oc The fossil record shows 25 globally mono- and ditypic cur in either two or three of the regions, that is, those that :emperate tree genera to have relict distributions; that is, are neither endemic nor cosmopolitan (fig. 26.6). Of the :hey formerly ranged across at least one more of the four 63 genera so distributed, 59 (94%) are present in eastern 'egions than they do currently. These genera (table 26.6) Asia. The largest tally-20 genera common only to east- COMPARISONS OF TEMPERATE-ZONE TREE SPEC I ES DIVERSITY 301
Table 26.6. Taxonomic Distribution of Globally Mono- and Ditypic Tree Genera with Relict Distributions Mono- and ditypic genera Fossils in No fossils in Class Subclass Total genera other regions other regions Ginkgoopsida 1 1 0 Pinopsida 27 7 5 Magnoliopsida Magnoliidae 18 ] 1 Hamamelidae 45 11 2 Dilleniidae 32 2 5 Rosidae 67 1 7 Asteridae 18 1 1 Liliopsida Arecidae 3 1 0 Commelinidae 3 0 0 Liliidae 1 0 0
ern Asia, and Crataegus, the hawthorns, with approxi mately 18 tree-sized species (:2: 8 m maximum height) in Europe eastern North America, 2 each in eastern Asia and Eu rope, and 1 on North America's Pacific slope. Most Cra taegus species are shrubs, but the trend in total species distribution parallels that of the few tree-sized members of the genus: approximately 220 of the global total of 306 species in a recent reexamination of Crataegus taxonomy (Phipps et a1. 1990) are centered in the eastern North American moist temperate zone. Another genus that strik ingly defies the trend is Quercus, the oaks, with tree-sized species numbering 32 in eastern North America, 21 in eastern Asia, 11 in Europe, and 5 in Pacific North America. Juglandaceae, the family to which Carya belongs, is among the very few angiosperm tree lineages distributed widely across the north temperate zone for which there is fossil evidence that early diversification took place outside of eastern Asia, in this case in eastern North America and Figure 26.6 Numbers of tree genera native to either two or three of Europe (Manchester 1989). The tribe Querceae of the Fa the four north temperate forest regions. Straight lines and the middle curved lines (bowing out) indicate genera that occur in two regions. gaceae, including Quercus, may also have originated in The inner curved lines (bowing in) indicate genera that occur in three eastern North America and Europe (Crepet and Nixon regions including east-central Asia. The outer ellipse indicates genera 1989). Crataegus, on the other hand, may have originated that occur in the three regions excluding east-central Asia. in southern China in the early Tertiary (Phipps 1983) de spite its current locus of highest diversity in eastern North America. ern Asia and eastern North America- reflects the well known range disjunction displayed by many moist tem perate forest plants that inhabit both regions (Li 1952; CLUES FROM THE FOSSIL RECORD Graham 1972; Boufford and Spongberg 1983). Three tal In examining fossil distributions, we focused on the Pa lies nearly tie for second rank: Europe and eastern Asia; leogene, over 40 million years of warm, relatively stable Europe, eastern Asia, and Pacific North America; and Eu climate during which forests spanned most of the present rope, eastern Asia, and eastern North America. Despite day Arctic and covered nearly the entire breadths of Eu their proximity, the temperate forests at the two ends of rasia and North America, but were interrupted intermit North America share no genera uniquely, and they share tently on both continents by north-south trending shal the fewest genera as members of three-region groups. low seas in mid-continent (Figure 26.7). We used two Eastern Asia emerges strikingly and overwhelmingly as different estimates of the fossil tree floras for the four con the core area of Northern Hemisphere temperate tree ge temporary north temperate forest regions: (a) genera ac nus distributions. tually represented in the fossil record of each region (Reid and Chandler 1933; Hu and Chaney 1940; Traverse Exceptional Genera. A few genera run counter to the gen 1955; Mai 1960; Kilpper 1969; \"an der Hammen, Wijm eral trend of greatest diversity in eastern Asia followed, in stra, and Zagwijn 1971; Mai 1971a, 1971b; Tanai 1972; sequence, by eastern North America, Europe, and Pacific Takhtajan 1974; Lancucka-Srodoniowa 1975; Rachele North America. Most notable are C1rya, the hickories, 1976; Collinson and Crane 1975; Friis 1979; Mai 1980; with 13 species in eastern North America and one in east- Potter and Dilcher 1980; Mai 1981; Gregor 1982; Freder- 302 ROGER EARL LATHAM AND ROBERT E. RICKLEFS
Biome 1 IIfIIlIIlI1lI1l Biome2 Biome 3 Tropical everwet UllIllI1II1IllU Tropical seasonal Subtropical desert Biome4 Biome5 Biome 6 • Winterwet Warm temperate, wet Cool temperate Biome 7 • Interior desert
Figure 26.7 Maestrichtian (late Cretaceous) biomes. Areas in black are major highland regions. (Redrawn from Horrell 1991.) iksen 1984a, 1984b; Wing and Hickey 1984; Friis 1985; Europe and east-central Asia, but its occurrences in the Kvacek and Walther 1987; Mai 1987a, 1987b; Mai and temperate zone of eastern North America are peripheral Walther 1988; Sauer 1988; Kvacek, Walther, and Buzek or disjunct from the main North American range of the 1989; Guo 1990; McCartan et al. 1990; Manchester, un genus, which is boreal. For another example, Platanus published data; Friis, personal communication) and (2) an species belong to the moist temperate tree floras of eastern expanded list of genera also including those that are ab and Pacific slope North America, but in Eurasia, native sent from the Tertiary fossil record of the region but pres stands of the genus are confined mainly to seasonally arid ent in its contemporary flora and in the Tertiary fossil re (eastern Mediterranean), montane (the Himalayas), and cord of at least one other region. The latter we term the tropical (Southeast Asia) regions. Such genera may have "inferred" Paleogene tree flora of each region. We ac contributed to moist temperate forest fossil assemblages knowledge that these are imperfect estimates of the actual both in regions where they were characteristic of the regional Paleogene tree floras for many reasons, including moist temperate flora and in those where their presence the infrequency and nonrandom distribution of fossiliza was infrequent, signaling spillover from a neighboring tion events, the scarcity of Tertiary sedimentary deposits, biome. and the rarity of paleontologists with the skills, specific Our compiled fossil data show a tally of 107 tree gen curiosity, and time to analyze Paleogene tree floras. era that have disappeared since the Oligocene from some, In comparing fossil and contemporary floras, we in but not all, of the present-day north temperate regions (ta cluded some present-day occurrences of genera not in ble 26.7). We sought patterns in the division of tree genera cluded in considerations of contemporary floras alone. All in each region into three categories: present as Paleogene such occurrences (marked (P) in Appendix 26.1) fall into fossils but extirpated, present as fossils and persisting in one of two categories: certain trees at the edges of their the contemporary flora, and known only from the con ranges, and certain shrubs. We included a genus in a re temporary flora (see Appendix 26.1). gion's contemporary flora even if represented there only by shrubby species if the genus also includes tree-sized Comparability of Fossil Data among Regions species in another region, because trees may not be distin First, we compared total extant genera in each region with guishable from non-tree congeners in fossils. For such extant genera that are also represented in the fossil record comparisons we also included a genus in a region'S con for that region. This comparison provides a rough esti temporary flora even if its range is peripheral to the region mate of the consistency among regions of the fossil data if it is also a characteristic ill'ember of a contemporary reliability (table 26.8), assuming that most genera oc moist temperate tree flora in another region. For example, curring in each of the four regions now also occurred Larix is a member of the moist temperate tree floras of somewhere within that regIOn during the Tertiary. The COMPARISONS OF TEMPERATE-ZON E TREE SPECIES DIVERSITY 303
Table 26.7. Genera Extirpated Regionally during the Late Tertiary and Quaternary Northern & Western Eastern Northern & Western Eastern east-central North North east-central North North Genera Europe Asia America America Genera Europe Asia America America Acanthopanax x • LinderaE x • X (P) AilanthusE X • X LiquidambarN X • X • AlangiumE X • X X LiriodendronN X • X • AlbiziaE • X LithocarpusE X • • Aphananthe X • X • Litsea E X • (P) Aralia X • • Lyonia X (P) • AsiminaN X • Magnolia X • X • Broussonetia X 0 Mallotusl' X • X(?) BumeliaE x • Manglietia E X • Calocedrus X (P) • MeliosmaE X • N Q CamelliaI' X " Metasequoia X X CarpinusN o 0 X c Michelia E X CJ CaryaN X 0 X o Neolitsea E X • CastaneaN o 0 X () Nyssa X X E • • Castanopsis X 0 @ OsmanthusE X ~ • Catalpa X 0 X OstryaN Q o X • Cedrela E X 0 X Paulownia X G X Cephalanthus X Persea E X X N • Cephalotaxus X I) Phellodendron X • CercidiphyllumN X • X PhoebeE X e ChamaecyparisN X • • • PlaneraN X • ChionanthusN X • • PlatycaryaN X • X X Cinnamomum E X • PoliothyrsisN X • ClerodendrumE XU) • PseudolarixN X • Clethra x • (P) PseudotsugaN X • • Cunninghamia" X • PterocaryaN • • X X CyciobalanopsisN X • PteroceltisN X • CyciocaryaN X • Rhus (P) • XI') • CyrillaN X • Robinia X X • DendropanaxE • X Sa balE X X • DiospyrosE x • x • SapindusE X • X • Disanthus" x • SapiumE X • Distylium" x • SassafrasN X • X • EmmenopterysN • X Scheff/eraE X • EngelhardtiaE X (P) X SciadopitysN X • X EnkianthusN X(?) • SequoiaN X X • EucommiaN X • x X( ?) SerenoaN X • EupteleaN X • Sino wilsoniaN • X EvodiaE X • StaphyleaN X • • FagusN • • X • StewartiaN X • • FortuneriaN X • SymplocosE X • X • GinkgoN X • X TapisciaN X • X GlyptostrobusE X (P) X X TaxodiumN X X X • GordoniaE X (P) • TernstroemiaE X • Halesia N X • TetracentronN X(?) • HamamelisN X • • ThujaN x • • • HemipteleaN X • Tilia N • • X • Hydrangea X • (P) TorreyaN x • • • IIIiciumE X • TSlIga N x • • • KalmiaN X • TllrpiniaN X • KeteleeriaE X • x Ulmus" • • X • Koelreuteria X • Z'lI1thoxylum E X • • LagerstroemiaE X • ZelkovaN • • X X(?) LeitneriaN X X • Note: I X) indicates extirpation from a region, Ie) indicates contemporary tree srecies existing in a region. Contemporary genera are marked (P) if they do not attain tree height or if they rarely occur in the region's fl o ra but do inhabit an adjoining biome and are characteristic members of a moist temperate tree flora in another region. Fossil genera are marked (') if identified onlv tentatively b,' recem authoriries (cited in this chapter). Present distributions of genera are indicated by superscripts: E. predominantly tropical; N, predominantly tempe"lte or extending into tropical latitudes mainly at high elevations. (See Appendix 26.1 for sources of fossil and distributional dara.) completeness of the fossil record for extant genera does ble 26.9). The four regions differ significantly in tree ge not differ significantly among the regions (G = 6.21, df = nus survival rate (G = 53.9, df = 3, P « 0.001). 3, P > .1). Extirpation rates of tree genera were radically unequal among the four regions. Europe was especially hard hit. Persistence/Extirpation Rates among Regions Next, we compared total fossil genera in each region with Extirpated Genera fossil genera that are also extant in that region, as an esti Next, we looked at contemporary floras for differences mate of the relative survival of genera among regions (ta- bdween the genera that died out regionally and those that 304 ROGER EARL LATHAM AND ROBERT E. RICKLEFS
Table 26.8. Relative Index of Completeness of Tertiary Fossil Record, by Region, of Moist Temperate Forest Tree Genera in the Northern Hemisphere Northern & Western North Eastern North Europe east-central Asia America America Total extant genera" 53 185 42 99 Represented in fossil record" 38 117 35 49 (72%) (6 3%) (83%) (49%)
"Includes extant genera that do not attain tree height or that rarely occur in the region's flora but do inhabit adjoining biomes and are characteristic members of a moist temperate tree flora in another region. "Includes a few fossil genera identified only tentatively by recent authorities (c ited in this chapter).
Table 26.9. Survival since Mid-Tertiary, by Region, of Moist Temperate Forest Tree Genera in the N orthern Hemisphere Northern & Western North Eastern North Europe east-central Asia America America Total fossil generaa 130 122 75 60 Survivingb 38 117 35 49 (29%) (96%) (47%) (82% ) aInciudes a few fossil genera identified only tentatively by recent authorities (cited in this chapter). bIncludes extant genera that do not attain tree height or that rarely occur in the region's flora but do inhabit adj oining biomes and are characteristic members of a moist temperate tree flora in another region.
survived. The mean numbers of species per genus in the - extant eastern Asian temperate tree flora are significantly greater - extirpated in the genera that persist in the other regions than in the genera that were extirpated from them. A similar relation (5 2a. 1. 0~ ship holds between the eastern North American temperate o tree flora and that of Europe (table 26.4, columns Band a.OJ 05 East-central I j :00 00',,"' > . Asian genera L//1 ~io n g e nera extant and C; fig. 26.8). In other words, genera that are currently '5 extant and ' extirpated in more speciose in eastern Asia (or eastern North America) "5 extirpated in Pa c ific sl ope of E j Europe are more likely to have survived in other regions than gen North America <3 0.0 +-1 ______l era that are currently less speciose. For random extinction to have produced this effect, c o :;::: species diversity would have to be a distinctive property (5 a. of individual genera that endures across many millions of o years and among continents. The data suggest that this is a. East-centra l Eastern Nort h OJ Asian genera Ameri can unlikely. We compared numbers of species in genera com > extant and gene ra mon to each pair of the six possible pairs of continental '5 extirpated in ext ant a nd "5 eastern No rth exti rpa ted in areas using the G-test (adjusting Type I error rate as in E America Europe ::J table 26.2), testing genus-by-genus whether the species U 0.0 +-, ------_ I 10 100 I 10 100 numbers in the less diverse region of the pair differ sig SpeCies per genus Spec ies per genus nificantly from expected values generated by proportion ally reducing the species numbers in the more diverse re Figu~e 26.8 Cumulative frequency distributions (as proportions of gion. Two of the pairs show significant differences: roral frequency) of species per genus in high-diversiry regional tree eastern North America and eastern Asia (G = 255, df = floras for genera that are also extant in less diverse regioml tree fl o ras versus genera extirpated from less dive rse regions. 60, P < .01) and eastern North America and Europe (G = 61.1, df = 29, P < .01); eastern North America and the Pacific slope of North America differ marginally signifi than three-fourths of the conifer genera persisted on the cantly (G = 49.4, df = 26, .05 < P < .1). Pacific slope of North America, the other region hit hard Furthermore, extirpated genera are nonrandomly dis by extirpation. There, also, as in Europe, the magnoliids tributed among higher taxa (table 26.10). For example, in experienced the greatest attrition. Europe all tree genera of the now mostly tropical sub classes Magnoliidae and Arecidae died out. In the sub Global Distribution of Paleogene Genera classes Hamamelidae and Rosidae, which have radiated We tallied fossil genera that occurred in either two or widely in temperate, boreal, and high-altitude habitats, three of the regions defined earlier for contemporary flo nearly half of the tree genera persisted. Counter to the ras (fig. 26.9). In parallel with the contemporary pattern, trend of species dropping out that belong to subclasses of 88 of the 95 genera in the inferred Tertiary floras that mainly tropical distribution, two-thirds of the genera of were so distributed (78 of 81 genera in the fossil-only flo conifers, which inhabit mostly temperate and boreal re ras) were present in eastern Asia. Of the 170 genera in our gions, disappeared. However, over half of these genera inferred Tertiary flora, 19 (11 %) have been found in only now are monotypic and most have very small global one region, 95 (56%) are represented in two or three re ranges, indicating their relictual status. In contrast, more gions, and 56 (33%) are represented in all four regions. COMPARISONS OF TEMPERATE-ZONE TREE SPECIES DIVERSITY 305
Table 26.10. Taxonomic and Regional Distributions of Extant and Extirpated Genera of North Temperate Forest Trees Belonging to the Conifer and Dicot Classes Northern & Western Eastern Europe east-central Asia North America North America Class Subclass Extant Extirpated Extant Extirpated Extant Extirpated Extant Extirpated Pinopsida 7 15 24 2 14 4 11 2 Magnoliopsida Magnoliidae 0 14 15 0 1 5 7 0 Hamamelidae 16 21 42 2 9 13 20 6 Dilleniidae 7 13 23 0 6 4 20 0 Rosidae 19 16 61 10 8 28 2 Asteridae 4 6 15 0 2 3 9 0 Note: Includes a few fossil genera identified only tentatively by recent authorities (cited in this chapter) and extant genera that do not attain tree height or that rarely occur in the region's flora but do inhabit adjoining biomes and are characteristic members of a moist temperate tree flora in another region.
Of the 213 genera in our extant flora, 122 (57%) are en demic to one region, 63 (30%) are present in two or three Europe regions, and 28 (13%) are in all four regions. The Tertiary and extant distributions differ highly significantly (G = 240, df = 2, P « .001). The low numbers of endemic fossil genera may be due in part to the lower likelihood of discovering fossil genera that were present in only one region. However, the pattern of decline in cosmopolitan distributions is striking. The well-known concurrence be tween the temperate floras of eastern Asia and eastern North America (Li 1952: Graham 1972; Boufford and Spongberg 1983) appears to be merely a vestige of the for merly even stronger affinity among the floras of these two regions and that of Europe. Most of the changes from figure 26.9 to figure 26.6 are due to extirpations in Europe. Rarefaction of Paleogene Genera We compared the higher taxonomic structures of the in Figure 26.9 Numbers of tree genera occurring in either two or ferred Paleogene tree floras (table 26.11) by simulating three of the four contemporary north temperate forest regions during the rarefaction of the eastern Asian flora to match the the Paleogene (genera represented by fossil remains plus those in numbers of genera in the other regions' floras (method ferred as present during the Tertiary because they are present in the given above). Unlike the simulated rarefaction of contem region's contemporary flora and in the Tertiary fossil record of at least one other region). See figure 26.6 for an explanation of the porary floras, the analysis of Paleogene floras dc~s not lines. compare regions in numbers of tree species among genera, families, and orders because species number cannot be re liably estimated from fossil remains. Rarefaction of the Wright 1983; Turner, Lennon, and Lawrenson 1988). Pat ~astern Asian fossil tree genera indicated that the distribu terns of this type have been demonstrated for trees in both tion of fossil genera among families and orders in Europe, temperate (Currie and Paquin 1987; Adams and Wood the Pacific slope of North America, and eastern North ward 1989) and tropical (Gentry 1988a) regions, using America did not differ from random samples of the Asian sampling areas ranging from grid blocks as large as fossil genera (table 26.12). We also simulated rarefaction 100,000 km2 to small plots of 0.1 to 1 hectare. Further Jf the contemporary tree genera of temperate east-central more, several authors have cited the consistency of these Asia to contemporary numbers of tree genera in the other relationships between regions (e.g., Adams and Wood three regions, for comparison with the rarefactions of fos ward 1989) as evidence of convergence and determination ;il genera and of contemporary species (table 26.13). Rar of species richness by local physical factors. ~fying contemporary floras by genera paralleled the re In general, tree species richness increases in direct rela mits of rarefying contemporary floras by species (see tion to precipitation and AET, suggesting that a positive table 26.2). relationship exists between diversity and productivity of the habitat (but see Tilman and Pacala, chap. 2, and Rosenzweig and Abramsky, chap. 5 for evidence that di CLIMATE AND TREE SPECIES DIVERSITY versity declines at high habitat productivity). This rela ;everal comparative studies have revealed a direct rela tionship forms the basis of "species-energy theory" or :ionship between species diversity and various climate energy-diversity theory (Wright, Currie, and Maurer, {ariables, particularly precipitation or estimates of actual chap. 6), which relates diversity to energy flux by means ~vapotranspiration (AET) (Richerson and Lum 1980; of several possible mechanisms. In general, high produc- 306 ROGER EARL LATHAM AND ROBERT E. RICKLEFS
Table 26.11. Summary by Taxonomic Level and Region of Forest Trees in "Inferred" Tertiary Floras Number of tree taxa in inferred Tertiary floras of: Northern & Western Eastern Taxonomic level Europe east-central Asia North America North America Total Subclasses 8 7 8 7 8 Orders 33 33 24 26 34 Families 61 63 43 49 67 Genera 140 156 81 98 170
Table 26.12. Results of Simulated Rarefactions of Tertiary Tree Genera Taxa in early Tertiary Mean taxa in 1,000 fl oras
Region Taxon tree flora of region drawn randomly by genus tldf ::: 999) Significance Europe Order 33 31.2 2.25 (NS) Family 61 59.4 1.04 (NS) Western North America Order 24 26.5 - 1.45 (NS) Family 43 44.1 -0.39 (NS) Eastern North America Order 26 28.2 - 1.50 (NS) Family 49 49.0 0.02 (NS)
Note: We used t-tests to compare the numbers of tree taxa inferred to have inhabited high-latitude forest regions during the Tertiary with the mea ns from 1,000 randomly generated floras in which the Tertiary tree flora of northern and east-central Asia is rarified to the number of genera in the Tertiary tree flora of each of the other regions (see text for method of inferring paleofloras). The Type I error rate was adjusted using the Dunn-Sidak method (Sokal and Rohlf 1981): for a = .05, a' = .017 and for a = .01, a' = .0033 (* P < .05, .. P < .01).
Table 26.13. Results of Simulated Rarefactions of Contemporary Tree Genera Taxa in present-day Mean taxa in 1,000 floras Region Taxon tree flora of region drawn randomly by genus Significance Europe Order 16 21.6 - 2.78 Family 21 30.5 - 4.14 Pacific slope of North America Order 14 19.9 - 3.00 Family 19 27.3 -4.00 Eastern North America Order 26 30.0 -2.16 (NS) Family 46 48.2 -0.82 (NS)
Note: We used t-tests to compare the actual numbers of taxa present with the means from 1,000 randomly generated floras in which the moist temperate tree flora of contemporary east-central Asia is rarified to the number of genera in the moist temperate tree flora of the other regions. Simulations were performed and tested for significance as described in table 26.12. tivity maintains larger numbers of individuals per species dence for a parallel diversity anomaly at the local level (I and thus reduces the probability of stochastic extinction. to lO-ha plots) is weak due to inadequate sampling in High production in habitats with little stress may also in Asia, but appears to be consistent with the regional trend crease the total variety of microhabitats and permit between Europe and eastern North America (Latham and greater microhabitat specialization. For certain types of Ricklefs 1993). Lacking contrary evidence, we accept the organisms, notably trees, high precipitation and tempera possibility that eastern Asian temperate forests contain ture may be associated with the occupation of a greater markedly more species at the local level than do temperate variety of habitats, thereby increasing sample diversity forests elsewhere. through increased habitat heterogeneity on a regional Both Currie and Paquin (1987) and Adams and Wood scale and through spillover or mass effects on a habitat ward (1989) claimed general similarity among eastern scale. Regardless of the mechanism, the correlation be Asia, Europe, eastern North America, and temperate re tween diversity and physical factors suggests that the out gions of the Southern Hemisphere in species richness of come of species interactions depends on the physical con temperate forests. We have reviewed and criticized these ditions of the environment. conclusions in detail elsewhere (Latham and Ricklefs Alternative viewpoints must be entertained when one 1993). Briefly, in these comparisons Asian data cited by identifies diversity anomalies, in which habitats with simi Adams and Woodward (1989) were restricted to boreal lar physical conditions are occupied by different numbers and island localities. Moist temperate continental Asia, of species in different regions (Schluter and Ricklefs, chap. where regional tree species diversity is higher by far than 21). Such is the case in both local and regional compari anywhere else in the earth's temperate zones, was not sons of mangrove species between the depauperate Carib sampled. Although diversity in Europe was claimed to be bean and species-rich Indo-Pacific regions (Ricklefs and comparable to that in eastern North America, seyen out Latham, chap. 20). It is also certainly the case in the re of eight European sampling areas fell below the North gional comparisons of species richness in north temperate American regression of species richness on AET. In these deciduous broad-leaved forests presented here. The evi- studies, species richness was tabulated for large grid COMPARISONS OF TEMPERATE-ZONE TREE SPECIES DIVERSITY 307 blocks (51,000 to 100,000 km2), which introduces un omes, belong to families that are characteristic of north specified contributions of habitat heterogeneity to species temperate regions, primarily Betulaceae, Fagaceae, Ha diversity. In arid regions, for example, in which average mamelidaceae, Juglandaceae, Salicaceae, Cornaceae, Ro climate conditions do not support forest vegetation, tree saceae, and Aceraceae. Toward the southern parts of the species were recorded only from riparian habitats. Fur biomes, representatives of more tropical families appear, thermore, both Currie and Paquin (1987) and Adams and but generally not in large numbers. Woodward (1989) mixed broad-leaved and needle-leaved Second, the difference in diversity between tropical and (including boreal) forests, perhaps making direct compar temperate floras, and between temperate floras in differ isons inappropriate. Latham and Ricklefs (1993) found ent regions, resides at high taxonomic levels. There are that tree species richness in 0.5- to 10-ha samples of tem roughly 11 species per family in temperate eastern Asia perate broad-leaved forests was unrelated to AET. Be and half that number in temperate eastern North cause the outcome of any ecological interaction that may America, which has about a third the total number of spe restrict local coexistence of species is determined at the cies (see table 26.1). Rarefaction of the Asian species indi local scale, we feel that our analysis more directly tests cates that the distribution of North American species the relationship between species richness and energy flux among higher taxa does not differ from that in a random within the temperate broad-leaved deciduous forest bi sample of the Asian taxa. On O.l-ha plots, temperate flo ome of eastern North America. ras exhibit about 1.4 to 2.3 species per family, while low The increase in tree species richness from temperate to land tropical floras , having up to 10 times as many spe tropical latitudes is generally thought to reflect parallel cies, exhibit specieslfamily ratios of between 2 and 4, with gradients of physical conditions (Ricklefs 1977; Gentry as many as 58 families represented at a single site (Gentry 1988a), with temperature and moisture generally higher 1988a). Thus, patterns of diversity are expressed at a high toward the equator at low elevation. While species rich taxonomic level (Ricklefs 1989b). This suggests that con ness of trees, large shrubs, and lianas on O.l-ha plots in temporary patterns were established long ago by coloni the tropics appears to increase with annual precipitation zation and cladogenesis, which played roles at least as im up to 300 to 500 cm (Gentry 1988a), Latham and portant as that of extinction. Ricklefs (1993) failed to find a relationship between spe Third, temperate flora diversity and the proportion of cies number of trees alone and AET on 1-ha plots in tropi species belonging to predominantly tropical families and cal forests. A latitudinal gap of at least 15° separates genera are both highest in eastern Asia, where there is and broad-leaved forests in the Neotropics and in temperate perhaps has been since before the Tertiary a continuous North America. As noted by Latham and Ricklefs (1993), corridor of mesic forest connecting tropical and temperate a corresponding discontinuity exists in tree species rich latitudes. Colonization of temperate biomes in Asia from ness. In an analysis of covariance of tree species richness the tropics over long time periods has probably played and AET on 0.5- to 10-ha plots, temperate and tropical an important role in the development of temperate forest plots differed significantly between each other (by an or communities there. der of magnitude) but independently of AET, even though Fourth, although both European and western North the ranges of temperate and tropical AET values over American temperate floras suffered extinctions during the lapped. Thus, the latitudinal difference in tree species di mid- to late Tertiary, the primary temperate diversity versity is not a direct consequence of a latitudinal differ anomaly, that distinguishing eastern Asia from other tem ence in physical conditions, because tree species diversity perate regions, is old and probably was established pri is statistically unrelated to AET within latitudinal belts. marily by regional differences in colonization and autoch In general, we conclude that regional effects influence thonous production of new taxa. tree species richness independently of, and in addition to, We propose that the differences in diversity of temper local effects of climate. Diversity-climate correlations ate tree .floras among continents reflect the history of colo among large sampling blocks may reflect increased variety nization of temperate biomes, which appears to have oc of habitats suitable for trees as productivity increases (in curred more frequently in Asia, and the subsequent creased beta diversity). Temperate-tropical differences in production of new autochthonous taxa and their geo diversity in the Americas and Europe/Africa may repre graphical spread within temperate biomes. Furthermore, sent discontinuities in diversity along continuous environ we propose, as have Farrell, Mitter, and Futuyma (1 992) mental gradients. Greater sampling, particularly of tem for insects, that differences between temperate and tropi perate and subtropical forests in eastern Asia, will be cal floras reflect a physiological barrier to colonization of required to clarify these relationships. But for the present, temperate zones that can be crossed only by the evolution we feel that a simple, continuous relationship between di of freezing tolerance mechanisms. Thus, the relatively low versity and local climate does not provide an adequate de diversity of angiosperm trees in temperate areas arises be scription of contemporary patterns of tree species di cause of the difficulty of colonizing temperate regions, ,'ersity. rather than, or in addition to, any intrinsic limits either on species production or on coexistence of species within temperate areas. Accordingly, explanations for latitudinal THE HISTORICAL DEVELOPMENT OF TEMPERATE gradients (actually disjunctions) in diversity can be traced TREE FLORAS to historical and evolutionary factors rather than to con North temperate broad-leaved deciduous tree floras pres temporary ecological interactions. We discuss these ideas ent the following patterns. First, a large proportion of in more detail below. It is not our purpose here to provide these floras, particularly in the northerly parts of the bi- a balanced evaluation of alternative models. Rather, we 308 ROGER EARL LATHAM AND ROBERT E. RICKLEFS advocate a particular model that must be properly evalu 6 Families with predominantly temperate distributions ated in the future. 4 Most angiosperm families arose during the late Creta ceous and Paleogene. During this time, frost-free climates en 2 .~ covered much of the world's land surface (see fig. 26.7). E 0 The oldest fossils of contemporary moist temperate zone 014 Other families tree families in the Northern Hemisphere date from over '+- 100 mya to less than 15 mya, with most falling within the 0 12 range 30 to 90 my a (fig. 26.10). During the early part of Q) 10 this period, eastern Asia was the only region in the North .Q ern Hemisphere where a more or less continuous zone of E 8 :::J forest vegetation might have existed between the tropics Z 6 and high latitudes (see fig. 26.7). Fossil data from the late 4 Cretaceous to the mid-Tertiary indicate an arid zone cov ering most inland fossil collection sites between subtropi 2 cal moist forests in northeastern Asia and tropical moist 0 forests in southeastern Asia (see figure 26.7; Song, Li, and c He 1983; Horrell 1991). However, moist conditions are 0 OJ E c ~
Number of tree species Northern, Pacific central, & East slope of Eastern Class eastern central North North /subclass Order Family Genus Europe Asia America America I Ginkgoopsida Ginkgoales Ginkgoaceae*N Ginkgo*N I t t 1 I t I Pinopsida Pinales CephalotaxaceaeN CephalotaxusN t 3 CupressaceaeN Calocedrus t (P) t 1 ChamaecyparisN I t 2 t 2 1 Cupressus 1 2 t 3 Juniperus 3 t 4 t 1 3 Platycladlls*N 1 I Thuja" I t t 3 t 1 I 1 Thujopsis*N I t 1 N ! Pinaceae Abies" t 4 14 t 6 I t 2 Keteleeria E t 1 t i Larix" t 1 t 5 I t (P) PiceaN t 2 t 9 t 2 t 1 Pinus t 6 t 14 t 7 t 12 Pseudolarix"N t t 1 Pseudotsllga" t t 3 t 1 Tsuga" t t 7 t 2 , t 2 TaxaceaeN TaxlIs" t 1 3 t 1 (P) Torreya" t 4 t 1 1 TaxodiaceaeN Cryptomeria"N t 1 Cunninghamia"N t t 1 G lyptostroblls" E t t (P) t ; t Metaseqlloia"N t t 1 t Codes: E (Equatorial!. family or genus is predominantly tropical in distribution; N (Nemor"l), family or genus is predominantly temper,lIe in distribution or extends into tropical latitudes mainly at high elevations; ., globally monotypic or ditypic family or genus; t, genus is represented in the Tertiary lossil record of tl", region; (P), geogr" phical range of the genus is peripheral to the region andlor the maximum height of the largest species in the genus occurring in the region is iess than 8 m (gil'l;n only for those genera lor which Tertiary fossil information is inc/uded); (?), fossil genera identified only tentatively by recent authorities (cited above). (continued) 312 ROGER E AR L LAT H AM AN D ROBER T E . R I CKLEFS
Number of tree species Northern, Pacific central, & East- slope of Eastern Class eastern central North North Isubclass Order Family Genus Europe Asia America America Sciadopitys*N t 1 t Sequoia"' N t t t 1 Sequoia dendron "N t 1 Taiwania*N t 1 Taxodium'oN t t t t 2 Magnoliopsida Illiciales Ill iciaceae Illicium E t 3 IMagnoliidae Laurales LauraceaeE ActinodaphneE t 3 CinnamomumE t t 11 LinderaE t t 8 t (P) LitseaE t t 9 (P) Machi/usE t 10 NeolitseaE t t 5 NothaphoebeE 1 PerseaE t t 1 PhoebeE t t 11 SassafrasN t t 1 t 1 Umbeliularia" N t 1 Magnoliales AnnonaceaeN AsiminaN t 1 Magnoliaceae Liriodendron"'N t t 1 t t 1 Magnolia t t 14 t 7 ManglietiaE t 4 MicheliaE t 3 Ranunculales SabiaceaeE MeliosmaE t 5 Magnoliopsida Daphniphyllales Daphniphyllaceae DaphniphyliumE 2 lHamamelidae Eucommiales Eucommiaceae ". N Eucommia" N t t 1 t t(? ) Fagales BetulaceaeN AlnusN t 3 t 12 t 1 t 2 BetulaN t 3 t 14 t (P) t 5 CarpinusN t 1 t 14 t t 1 CorylusN t 3 t 3 t (P) OstryaN t 1 t 4 t t 1 FagaceaeN CastaneaN t 1 t 4 t t 2 CastanopsisE t t 7 t 1 CyclobalanopsisE t t 4 FagusN t 3 t 6 t t 1 LithocarpusE t t 5 t 1 QuercusN t 11 t 21 t 5 t 32 Hamamelidales Cercidiphyllaceae '·N Cercidiphyllum"N t t 1 t EupteleaceaeN EupteleaN t 2 HamamelidaceaeN AltingiaE 1 Disanthus*" t t 1 DistyliumN t t 1 Fortuneria" " t t 1 Hamamelis" t t 2 t 1 Liquidambar" t t 2 t t 1 Loropetalum"N 1 Sinowilsonia" N 1 t Platanaceae PlatanusN t (P) t (P) t 1 t 1 Juglandales JuglandaceaeN CaryaN t t 1 t t 13 CyclocaryaN t t 1 EngelhardtiaE t t (P) t }uglans t 1 t 6 t 1 t 2 Platycarya"" t t 1 t t PterocaryaN t 1 t 6 t t Leitneriales Leitneriaceae"N Leitneria" N t t 1 Mvricales Myricaceae Myrica t (P) 1 1 t 1 Tr~chode nd ra l es Tetracentraceae ,. N Tetracentroll"N t(?) t 1 Urticales MoraceaeE Brollssonetia t t 1 Clidrania E 1 Maclura" N 1 Moms t (P) 7 t I Ulmaceae Aphananthe t 1 t CeltisN t 3 t 8 t (P) t 4 Hemiptele,j"" t I Plallera"N t t 1 Pteroceltis"" t t 1 UlmlisN t 6 t 8 t t 6 ZelkovaN t 2 t 2 t t(?) UrticaceaeE Villebntne,jE 1
(continued ) COMPARISONS OF TEMPERATE-ZONE TREE SPECIES DIVERS I TY 313
Number of tree species Northern, Pacific central, & East- slope of Eastern C lass eastern centra l North North Isubclass Order Family Genus Europe Asia America America Magnoliopsida Ebenales EbenaceaeE DiospyrosE t t 3 t 1 IDili eniidae SapotaceaeE Bume/ia E t(?) 3 Styracaceae HalesiaN t 3 Pterostyrax N 2 Styrax E t (P) t 7 t (P) 1 SymplocaceaeE Symp/ocosE t t 2 t t 1 Ericales ClethraceaeE C/ethra t t 1 t (P) CyrillaceaeE C/iftonia*N 1 Cyrilla*N t t 1 Ericaceae Arbutus 1 t 1 ElIiottia"' N 1 EnkianthusN t(?) 1 Ka/miaN t 1 Lyonia t t (P) t 1 Oxydendrum"'N t 1 Rhododendron" t (P) t 6 t 1 t 2 Vaccinium" (P) (P) (p) t 1 Malvales ElaeocarpaceaeE SloaneaE 1 SterculiaceaeE Firmiana E I t 1 TiliaceaeE Tilia N 5 14 3 t 1 t t t Primulales MyrsinaceaeE A rdisia E 1 MyrsineE 1 Salicales SalicaceaeN PopulusN t 2 t 14 t 1 t 3 SalixN t 4 t 23 t 8 t 8 Theales TheaceaeE Camellia E t t 2 Franklinia ,. N 1 GordoniaE t t (P) t 1 StewartiaN t t 4 1 TernstroemiaE t t 1 Violales FlacourtiaceaeE Idesia"N t 1 Poliothyrsis * N t 1 Xylosma E t 1 M agnoliopsida Apiales Araliaceae Acanthopanax t 2 IRosidae Aralia t t 2 1 DendropanaxE 2 t Evodiopanax':'N 1 Kalopanax"' N t 1 Schefflera E 1 1 t I Celastrales Aquifoliaceae I1ex t 1 t 14 I t 5 CelastraceaeE Euonymus 1 t 6 t 1 Cornales AlangiaceaeE AlangiumE t t 2 t t CornaceaeN At/cubaN 1 ComusN t 1 t 7 t 1 t 3 Macrocarpillm N 2 Torricellia 1 l\"yssaceaeN Davidia"'N 1 Nyssa t t 1 t t 3 Euphorbiales Buxaceae Buxlts t 2 t (P) EuphorbiaceaeE Mallotus E t t 3 W ) Sapilim E t t 1 Fabales CaesalpiniaceaeE CercisN 1 t 3 1 Gleditsia t 5 t 2 G lymnocladllsN t 1 1 Fabaceae Cladrastis" t 5 1 Dalbergia" 1 Erythrilla l' 1 LaiJllrl11ll'1l':' N 2 MaackiaN t 2 Ormosial' 1 Robinia t t 2 Sophora t 1 t (P) :-'!imosaceael' AI/Jizia' t 3 t Myrtales LlThraceael' Lagerstroelllia' t 2 :-'!ntaceae' SZYZygilllll E t I Rhamnales Rhamnaceae Ho!'enia N t 2 Rhamnll s N (P) 2 t 2 t (P) Zi:;iphlls' ! t 1 Rosales HydrangeaceaeN Hydrangea t t I (P) konrifltled) 314 ROGER EARL LATHAM AND ROBERT E_ RICKLEFS
N um ber of tree species Northern, Pa cific central, & East- slope of Eastern Class eastern central North North Isubclass Order Family Genus Europe Asia America America RosaceaeN AmelanchierN 1 1 1 2 ChaenomelesN 1 CrataegusN t 2 t 2 t 1 18 EriobotryaE t 1 MalusN 3 t 15 1 5 Mespilus· N 1 Photinia 3 PrunusN t 6 t 32 t 2 t 10 PyrusN 4 10 Sorb USN t 8 t 15 1 2 Santalales OlacaceaeE Schoepfia 1 Sapindales Aceraceae" Acer" t 12 t 58 t 3 t 9 Dipteronia'-N 1 AnacardiaceaeE Choerospondias"N 1 CotinusN 1 PistaciaE 1 t 1 Rhus t (P) t 4 t(?) t 3 ToxicodendronN 2 t 1 Hi ppocastanaceae AesculusN t 1 t 3 t 1 4 MeliaceaeE Cedrela E t t 1 t RutaceaeE EvodiaE t t 5 Phellodendron t t 5 Ptelea (P) t 1 Zanthoxylum E t t 3 2 SapindaceaeE Koelreuteria t t 2 SapindusE t t 1 t 2- SimaroubaceaeE Ailanthus" t t 3 t PicrasmaE 1 Staphylaceae StaphyleaN t 1 1 Tapiscia" N t 1 t Turpinia E t 1 Magnoliopsida Dipsacales CaprifoliaceaeN Sambllcus t 1 2 1 1 IAsteridae Viburnum t (P) t 2 4 Lamiales Boraginaceae EhretiaE t 3 VerbenaceaeE Clerodendrum E t (?) 1 Premna E 1 Rubiales RubiaceaeE AdinaE 1 Cephalanthus t 1 Emmenopterys*N 1 t Pinckneya'" 1 RandiaE 1 Scrophulariales BignoniaceaeE Catalpa t t 3 t 2 Palliownia t 4 t Oleaceae ChionanthllsN t t 1 1 Forestiera E 1 FraxinusN t 6 t 14 t 1 t 7 Ligllstrum (P) t 1 OsmanthusE t t 4 1 SyringaN 1 Liliopsida Arecales ArecaceaeE Sab,liE t t 1 IArecidae Serenoa" N t I 1 Tr.lChycarplIsE I 2 Liliopsida Cyperales Poaceae Anmdinaria 3 1 ICommelinidae PhyIlostachysN 12 Semiarundinaria 1 I Liliopsida Liliales Agavaceae Yucca E 1 ILiliidae I COMPARISONS OF TEMPERATE-ZONE TREE SPECIES DIVERSITY Re f. 1
ACKOWLEDGMENTS paleontology of the outcropping Tertiary beds in the Pamunkey River region, central Virginia Coastal Plain Special thanks are due Else Marie Friis for comments, - guidebook for Atlantic Coastal Plain Geological references and data; Mark Horrell for contributing Association 1984 field trip. Atlantic Coastal Plain digitized global biome maps; Paul Chung for reading and Geological Association. abstracting Zheng (1983, 1985); Steve Demos for Frederiksen, N. o. 1984b. Stratigraphic, paleoclimatic, programming computer simulations; and Florence and paleobiogeographic significance of Tertiary Raynal for translating selections from Humboldt and sporomorphs from Massachusetts. u.s. Geological Bonpland (1807). We also thank Brenda Casper, Steve Survey Professional Paper 1308: 1-25. Heard, Kirk Johnson, Rachel Merz, Dolph Schluter, Jake Friis, E. M. 1979. The Damgaard flora: a new middle Weiner, Tim Williams, Scott Wing and Fred Ziegler for Miocene flora from Denmark. 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