Woody plants — evolution and distribution since the Tertiary

The last decades have witnessed great advances in paleogeography (reconstructing the former distribution and topography of land and sea) and paleoclimatology (inquiring into the climatic changes of the past). Paleobotany has enormously expanded the documentation of fossil plant groups, floras and vegetation types, supporting its conclusions by technically much improved analyses of microfossils (pollen) and anatomical details. An increasing quantity and quality of all these informations from the geosciences is available when we follow the history of the biosphere up to the present. Simultaneously, research from the biosciences on the morphology, ecology, distribution, systematics and evolution of extant vascular plants, and on the ecogeographical differentiation of the vegetation cover of our planet, has made enormous progress. Thus, a synthetic geo-and bioscientific approach becomes more and more feasible and urgent for further advances in the many problems of common concern. Nevertheless, considerable obstacles for an exchange of relevant informations and subsequent cooperative efforts result from the fact, that paleo- and neobotanists usually work in separate institutions, use methods of their own, and publish in different journals.

After presenting the methodological bases of historical phytogeography (paleochorology; Fig. 1), a new paleographic map series is introduced (Fig. 2). They serve for ten examples of paleochorological analyses, in which the major phases of the development of a group in space and time are documented. In addition to purely tropical taxa (Nypa, Ctenolophon),others which originated in the warm and humid N. Tethyan belt (Bombacaceae p.p., Olacaceae p.p., Symplocos, Alangium, Nepenthes) or which presently occur as eastern N. American/E. Asian disjuncts (Sarcococca and Pachysandra) are discussed (Figs. 3 – 14).Finally, analyses of two very old angiosperm groups with complicated paleochorological patterns (Restionaceae and Ascarina, Chloranthaceae p.p.; Figs. 15 – 16) are presented.

The evolution of the Australian flora through the Tertiary has not been reviewed by a megafossil palaeobotanist for almost a century. Based on material available and published studies, the Australian Tertiary can be considered in three units: Eocene, OligoMiocene, and Pliocene. Key taxa in the Eocene include Casuarinaceae, Proteaceae, Fagaceae, Podocarpaceae, and Lauraceae. Many known Eocene deposits are interpreted as warm, humid rainforests.

Oligo-Miocene floras reflect a climatic deterioration, with greater occurrence of sclerophylly and xerophylly. A reduction of tropical taxa is seen with an increase of Myrtaceae and earliest records of Mimosaceae, Chenopodiaceae, and Poaceae. These trends continue into the Pliocene, which is not well represented in Australia.

The Tertiary vegetation of Europe evolved from paratropical to warm-temperate and temperate forms in response to a progressive, non-linear, climatic cooling. Its vegetational forms are composed mainly of two separate ecological units: the evergreen, laurophyll “paleotropical geoflora” and the deciduous, broad-leaved “Arctotertiary geoflora”. The development of the Tertiary climate and its interaction with the vegetation are convincingly indicated by the geoflora’s migration; the changes in its composition; and the development of the Tertiary forest, swamp, and aquatic plant communities. The “paleo-tropical geoflora” is characterized in the upper Cretaceous to the upper Miocene by para-tropical rain forest, subtropical rain and laurel forests, temperate laurel forests and edaphically-mediated formation of laurel-conifer forests. The “Arctotertiary geoflora” advanced into Europe in waves since the Paleocene and formed the basis for the Tertiary mixed mesophytic forests. These can be divided into warm-temperate rain forests, oak-hornbeamchestnut or mixed beech-oak-hornbeam forests, and edaphic formations such as bottomland and swamp forests. Beginning in the lower Cretaceous, the hydrophytic vegetation developed independently of the forest vegetation and formed very diverse herbaceous fresh water, swamp, salt water, and coastal formations. Considerable differences in composition allow to separate floral regions and provinces in Eurosiberia. Instead of three ill-defined floral regions in the Paleocene, there are four well-defined floral regions in the Pliocene. A Mediterranean region cannot be recognized, although Mediterranean (eumesogeic) floral elements appear in the Eocene/Oligocene and thereafter. The Mediterranean sclerophyll forests probably arose after the destruction of the laurophyll forests during the Pleistocene.

The evolution of important woody plant groups, ancestral to the modern Mediterranean dendroflora, is surveyed. Altogether, the history and phytogeography of 86 fossil species or species-groups is considered. The major part of the Paleomediterranean woody plants appears in Miocene mixed mesophyllous and mesoxerophyllous, evergreen and deciduous forests. The initial formation of basic Mediterranean sclerophyllous woody vegetation types is referred to periods from the Late Sarmatian to the Late Pontian, in geographic areas between 37° and 45° N latitude.

From the vascular plant genera of the tropical Andean montane forests about 65% have a tropical-neotropical, 15% tropical-Andean, 5% amphi-pacific (Malayo-American), 5% Austral-antarctic and 10% Holarctic distribution. The explosive evolution of Andean centred taxa probably started in the lower Tertiary from tropical Gondwana stock, when in the area of the present day Andes, locally there may have been hills up to 1 000 m. With the final upheaval between the Late Miocene and Late Pliocene, extensive areas with a montane climate were created and populated from the low mountain flora by evolutionary radiation. Taxa of Austral-antarctic origin migrated along the Andean chain to the tropics. At the same time the formation of the Panama isthmus connected N. and S. America, and enabled many plants to enter the tropical Andes from the north. First to cross were tropical to subtropical and warm-temperate species from the Tertiary Laurasian flora that had migrated southward during the period of Miocene cooling, taking refuge in the montane tropics. Because the same happened in SE. Asia there is a good number of amphi-pacific taxa known as fossils from Tertiary Laurasia but largely extinct in the present-day Holarctic (e.g., Trigonobalanus). Later to cross were cool-temperate to cold elements that still exist in the Holarctic (e.g., Alnus and Quercus).

To understand the history of the montane forests of the northern (tropical) Andes, it is necessary to know the general historical background of the South American continent, and of the flora and vegetation of tropical S. America, in relation to temperate S. America and temperate N. America. For that reason we will first give some background information on the earlier history, then treat shortly the vegetation and phytogeography of the Andes, followed by an overview of the Late Tertiary and Quaternary history of the area, and finally general conclusions.

Results of pollenanalytical investigations in the Highlands of Mexico (area of Puebla — Tlaxcala) are presented together with a survey of the last 35 000 years of vegetation history. After 3 pine periods from 35 000 to about 7 000 – 8 000 B. P., according to high and late Pleistocene condition, the Holocene vegetation history becomes more diversified: periods of alder, pine, fir and mixed oak forests alternate and end in a period of cultivated landscape.

The assembly of Mediterranean pines in the sense of MIROV is inhomogeneous in respect to morphological, geographical and evolutionary affinities. Considering new or neglected characters (vegetative and particularly reproductive, cone scales, apophyses, mucros, seeds: Figs. 1 – 3) in extant populations and fossils, three groups are recognized. The group of coast and island pines extends from the Canary islands to the Himalaya region and is closely related to Caribbean and C. American taxa. This complex evidently has originated from haploxyl ancestors of sect. Parrya during the Mesozoic (Upper Jurassic/ Lower Cretaceous) in the NW. Tethys area (Fig. 13). P. rzedowskii can be regarded as an extant survivor of this first phase of differentiation. The extremely variable P. canariensis (together with P. roxburghii in sect. Sula) marks a transitional phase towards the more advanced diploxyl species of sect. Pinea (with P. pinea, P. halepensis and P. brutia, and subsect. Oocarpae) and sect. Pinaster (with P. pinaster, etc.) (Figs. 4–12). — The second group consists of diploxyl mountain pines from the areas surrounding the Mediterranean. They are classified as members of the Eurasiatic sect. Pinus subsect. Sylvestres, have differentiated along the northern Parathetys area, and exhibit close links with E. Asiatic taxa. The third group includes the haploxyl mountain pines P. cembra and P. peuce which can be regarded as western outposts of the circumpacific centred sect. Strobus with a pre-Tertiary origin. — As an appendix, an improved classification scheme is presented for the pine groups discussed.

Paleobotanical studies indicate that several isolated and systematically depauperate groups of extant woody dicotyledons originated in the Mid Cretaceous. The Chloranthaceae had probably differentiated into insect-pollinated (Chloranthus and Sarcandra) and wind-pollinated (Ascarina and Hedyosmum) forms by the end of the Albian, and leaves referable to the Trochodendrales are known from the Albian and Cenomanian. In the latest Cretaceous and Early Tertiary, extinct representatives of the Trochodendrales included Nordenskioldia and the Joffrea-Nyssidium complex. The Platanaceae also differentiated before the end of the Albian and initially had insect-pollinated, unisexual flowers with five carpels or stamens. Some of these features persisted in the platanoid lineage until the Early Tertiary, and during the Paleocene and Eocene the Platanaceae included forms with elliptical, palmate and pinnate foliage. The history of the Platanaceae suggests that several features of the reproductive morphology of extant taxa may have arisen in association with a trend toward wind pollination. In the Mid Cretaceous, platanoid foliage partially inter-grades with pinnate Sapindopsis and pedate Debeya-Dewalquea leaves suggesting a close relationship between Platanaceae and Rosidae and Fagaceae respectively. The Chloranthaceae, Trochodendrales, and Platanaceae all occupy a somewhat intermediate position between the Magnoliidae and Hamamelidae and are of considerable interest with respect to their role in the initial radiation of nonmagnoliid (“higher”) dicotyledons.

New investigations on the flower and fruit structure of extant Hamamelidaceae and other Lower Hamamelididae together with new finds of fossil flowers and seeds from the Upper and Lower Cretaceous provide the outline of an increasingly more differentiated picture of the early evolution of the subclass. Three patterns of valvate anther dehiscence are recognized in the subfamily Hamamelidoideae (and the subclass Hamamelididae). The basic (plesiomorphic) type within the Hamamelididae has 2 valves per theca. The type with 1 valve but 2 pollen sacs per theca is both consistent and exclusive for the 5 southern genera of the Hamamelidaceae. They seem to be the remnants of a homogeneous group that originated before the Upper Cretaceous. This is supported by fossil hamamelidaceous flowers from the Upper Cretaceous that have thecae with 1 valve. Since several-seeded Hamamelidaceae predate one-seeded forms in the fossil seed record (in Europe) and the systematic structure of the one-seeded group is relatively more homogeneous, several-seeded groups are considered to be more ancient. Several parallel evolutionary trends are recognized within the Hamamelidaceae as well as within the Lower Hamamelididae: anther dehiscence with 2 valves per theca → 1 slit or 1 valve; pollen sacs per theca 2 → 1; pollen tricolpate → polyforate; exine coarsely reticulate → finely reticulate; loss of perianth (tepals or petals and sepals) and concomitant loss of fixed number of floral organs; differentiation of exposed nectaries.

A systematic reassessment of megafossil records of Fagaceae in Central Europe has been undertaken on the basis of leaf cuticular characters. The oldest representatives date back to the Eocene: Quercus subhercynica spec. nova, Dryophyllum furcinerve (Rossm.) Schmalh., Trigonobalanopsis rhamnoides (Rossm.) gen. & comb. nov. In the Oligocene other members of extant genera appear: Quercus rhenana (Weyl. & Kilpp.) Knobloch & Kvaček, Fagus attenuata Goepp., Lithocarpus saxonicus spec. nova. In the Neogene these ancient taxa (except in Fagus lineage), are gradually replaced by deciduous species of Quercus and Castanea. Trigonobalanus and Castanopsis are recorded by fruits (or wood) only.

The major radiation of the Juglandaceae occurred during the early Tertiary as recorded by the proliferation of juglandaceous pollen and the appearance of fruits representing extinct and extant genera of the family. Juglandaceous pollen types of the Paleocene were predominantly triporate and exhibited a greater diversity in patterns of exinous thinning than occurs in the family today. Analyses of in situ pollen from early Tertiary juglandaceous inflorescences confirms the taxonomic value of certain patterns of exinous thinning. Data from co-occurring fruits and pollen indicate that relatively unspecialized, isopolar triporate pollen of the type presently confined to the tribe Engelhardieae also occurred in other tribes of the family during the Paleocene. Pollination has been mostly anemophilous throughout the Tertiary. Both wind and animal fruit-dispersal syndromes were established early in the radiation of the family but a greater diversity of wind-dispersed genera has prevailed.

A comparative analysis of the seed morphology and anatomy of fossil and extant Rutaceae (mainly Zanthoxyleae and Toddalioideae) is presented. This allows to place the most important fossil taxa in a time-table and on paleogeographical maps. A phylogenetic scheme demonstrates the postulated historical relationships of Evodia, Zanthoxylum, Fagara, Rutaspermum, Acronychia, Toddalia, Fagaropsis, and Phellodendron.

Due to the considerable annual fluctuations of water level of the Amazonian rivers, their river banks are fringed with periodically flooded forests of vast extension. The biota of these communities are adapted to annual inundations that can last for more than half a year. Water chemistry is most important for the floristic differentiation of these flooded forests. White water rivers, which carry a rich load of suspended material originating from the erosion of the Andes, have a floristic composition related to that of the noninundatable Amazonian forest. Clear water and black water rivers, which originate in the Amazon Basin or its adjacent crystalline shields, are nutrient-poor and more or less acidic; their flora is related to that of peculiar woodland and savannah vegetation on oligotrophic white sand. The distribution patterns of floodplain species of nutrient-poor waters point to a centre of diversity in the Upper Rio Negro region, and another one in the Guayana lowland. These coincide with diversity centres for species of non-flooded habitats. Hence it seems unlikely that species diversity is directly influenced by pluviosity. The flooded forests have developed biotic interactions with the fish fauna of the Amazon Basin, which are vital for their continued existence. It is assumed that the origin of these habitats, their biota and their interactions dates back long into the Tertiary.

The Euxinian and the Hyrcanian floristic province are analysed in respect to their trees and shrubs. These provinces mark the southern limit of the Euro-Siberian Region in SW. Asia. Mesophyllic forests dominate; they are deciduous, but there is a substantial component of evergreen shrubs in the understorey. Characteristics species frequently have a relic nature. Species lists and some exemplary distribution maps are presented for the Euxinian and the Hyrcanian element, both with endemic and more wide-spread species, including the characteristic Euxine-Hyrcanian group.

The deciduous wood flora of southern Europe is characterized as a special “nemoral-Submediterranean element”. The distribution patterns of some representatives (total ranges and local areas in Roumania) are described and explained by recent climatic conditions. The northern limits of Submediterranean and Submediterranean-middle European deciduous forest taxa exhibit a continuous gradation, but only a few species with a wide ecological amplitude extend into the temperate zone.