Introduction

Nothofagus cunninghamii cool temperate forest in Tasmania. Similar forests exist in New South Wales (Australia), New Zealand and South America. Tropical species of Nothofagus exist in New Caledonia and New Guinea

In the Paleozoic era some 200 million years ago, the southern Hemisphere continents and islands like South America, Antarctica, Australia, Africa, New Zealand, New Caledonia and others were all part of one gigantic land mass known as Gondwana.

In the early Jurassic (circa. 184 million years ago), Gondwana started to break up, leaving a large number of plants and animal lineages on each continent that represent a “Gondwanan” legacy. Indeed, the fact that the same animal and plant genera and sometimes same species is currently shared between many of these continents was taken as a line of evidence to support the now well established idea that these landmasses were once joined. When a plant genus is shared between geographical separate landmasses, the genus is said to have a disjunct distribution and the many instances of disjunctions of the Gondwanic flora has been and continue to evoke immense scientific interest and research.

Nothofagus glauca

Ever since the early writings of the botanist J. D. Hooker (1853), several explanations have been proposed to explain the patterns of disjunctions in the gondwanic flora: geological or climatic vicariance (Hooker, 1853; Croizat, 1952; Seberg, 1991; Weston & Crisp, 1994); short-distance dispersal across much narrower ocean basins (Raven and Axelrod, 1974), and; southward migration from ancient, continuous dispersal from the Northern Hemisphere landmasses (Laurasia). Southern migration might have occurred en route land bridges (van Steenis, 1963), long-distance dispersal events (Darlington, 1965; Pole, 1993, 1994; Hill, 1992) or rafting on drifting continents (Brundin, 1965).

Although no single genus can be used to explain all the intricacies of Gonwanan plant biogeography, the genus Nothofagus has been touted as one of the key angiosperm genera for elucidating the evolutionary and migratory patterns of the South hemisphere biota (Darlington, 1960; van Steenis, 1971; Hill & Dettmann, 1996; Hill, 2001). Nothofagus (Nothofagaceae) are commonly known as southern beeches and consists of 36 species of evergreen or deciduous trees and shrubs which are in appearance somewhat reminiscent for oaks and beeches (Fagus spp.)of the Northern hemisphere.

Nothofagus pumilio

Three reasons for the significance of Nothofagus to biogeography: (1) Nothofagus has an extensive and spatially widespread fossil record of distinctive pollen (Dettmann et al., 1990) and macrofossils dating from the Late Cretaceous till the present (2) the genus consist of distinctive trees that often dominate areas of vegetation in temperate South Australia, New Zealand and South America (Read & Hope, 1996) and high altitude areas of tropical New Guinea and New Caledonia (Poole, 1987), and (3) Nothofagus fruits are not adapted for long-distance dispersal (Rodway, 1914; Preest, 1963), suggesting that land-based dispersal is the only route by which the genus is dispersed.

Investigations into Nothofagus have yielded many advances in the present understanding of the taxonomy, phylogeny and biogeography of the genus (Hill, 2001). Yet after more than thirty years of intensive research, Nothofagus still challenge investigators in many ways (Hill, 2001). This short review aims to discuss some of the problems and solutions gleaned from Nothofagus research.

Phylogeny and classification

The genus Nothofagus was initially erected in 1850 by Carl Ludwig Blume (Poole, 1987) and extant members of the genus are represented by 35 species (Hill & Dettmann, 1996; Hill, 2001) subdivided into four subgenera Brassospora, Fucospora, Nothofagus and Lophozonia (Hill & Jordan, 1993; Manos, 1997; Jordan & Hill, 1999). Previously believed to be related to true Beeches (Fagaceae), Rodway (1914) maintained that Nothofagus differed from Fagus mainly by “having smaller flowers and very narrow medullary rays.”

Only relatively recently was this taxonomic assignment scrutinized (Hill, 2001) and Nothofagus is now recognized to belong to a monogeneric family Nothofagaceae (Manos, 1997). In fact, it is suggested that Nothofagus is more related to birches (Betulaceae) than to beeches (Fagaceae) (Dixon, 1989 cited by Hill, 1992). The initial interpretation of Nothofagus as being allied to the Fagaceae caused monumental biogeographical misinterpretations (Hill & Dettmann, 1996) as we shall later see.

Fossil pollen regarded unquestionably of Nothofagus identity is assigned the genus Nothofagidites Erdtman ex Potonié, 1960 (Dettmann et al., 1990). The persistent usage of the title Nothofagidites is curious (Hill, 2001) considering that Nothofagus is the parental source of Nothofagidites (Dettmann et al., 1990; Hill, 2004). For many years, Nothofagus and Nothofagidites pollen were recognized to belong to three groupings, designated morphologically as the brassii-, fusca- and menziesii-type pollen (Dettmann et al., 1990). In addition, Nothofagidites was recorded to have some ancestral pollen groups not seen in extant Nothofagus (Dettmann et al., 1990). Palynologists used these groupings as an informal taxonomic system for Nothofagus (Hill & Read, 1991) although congruence with extant taxa was incomplete.

A breakthrough in Nothofagus taxomony was achieved in the early 1990’s when Dettman et al. (1990) erected a forth grouping by splitting the fusca-type pollen group into fusca-type (a) and fusca-type (b) and when Hill & Read (1991), doing a cladistic study of extant Nothofagus, proposed four subgenera for the genus, namely: (1) subgenus Brassospora, (2) subgenus Fucospora, (3) subgenus Nothofagus, (4) subgenus Lophozonia. These subgeneric delineations are well supported by molecular evidence (Martin & Dowd, 1993; Manos, 1997; Jordan & Hill, 1999), and provide a stable framework for the classification of fossils (Jordan & Hill 1999). Hill (2001) proposed that Nothofagus pollen types be referred to by their subgeneric names (brassii-type = subgenus Brassospora, fusca-type (a) = subgenus Fucospora, fusca-type (b) = subgenus Nothofagus, menziesii-type = subgenus Lophozonia).

Present and paleo-distributions, origins, and biogeography

From as early as 1851, J. D. Hooker recognized Nothofagus elements in south-east Australia, New Zealand and southern South America (Hooker, 1853) and eminent botanists like Rodway (1914) expounded on the significance of the genus in Gondwanan biogeography. Van Steenis (1953, 1971) later extended the range of the genus to include New Caledonia and New Guinea. The ancient distribution of Nothofagus, as indicated by the distribution of macrofossils and Nothofagidites pollen appears to be even more extensive (Jordan & Hill, 1999). In addition, formally described species of Nothofagidites appear to have a wider range of morphologies than extant Nothofagus pollen (Dettmann et al., 1990), suggesting that Nothofagus may have been more speciose than it is today. The pollen record of Nothofagus is so well characterized that its absence can be taken as strong support for the hypothesis that Nothofagus was never present in Africa and India (Hill, 2001). Why Nothofagus should be absent from these two prominent Southern Hemisphere continents remain a mystery.

Nothofagus nitida

One major preoccupation of Nothofagus biogeographers has been to determine the center of origin for the genus. The discovery of Nothogfagus in New Caledonia and New Guinea had major implications on ideas of the center of origin for the genus. Due to the prevailing idea that Nothofagus was closely related to Fagus at that time, Van Steenis (1971) believed that the center of origin of Nothofagus was somewhere in the region stretching from Yunnan to Queensland where Nothofagus co-existed with other genera of the Fagaceae. This is understandable as the origin and migration of Nothofagus may be expected to have overlapped with at least some related taxa in the Fagaceae (Hill & Dettman, 1996). With a better resolution of Nothofagus phylogeny (Martin & Dowd, 1993) and fossil reinterpretations (Pole, 1992), the consensus is that the southern South America-Antarctic Peninsular region is the center of origin of the genus (Hill & Dettmann, 1996).

Nothofagus cunninghamii, the Myrtle Beech of southern temperate Australia. Note the capsular fruits that resemble those of a beech tree. Those fruits are called cupules and split open at maturity.

Another preoccupation of Nothofagus biogeographers has been to find explanations for the distribution patterns of the genus. Although Alfred Wegener’s theory of “Kontinentaldrift” would have helped to explain some of the distributional patterns of Gondwanan plants, plant geographers like Burbridge (1960) and van Steenis (1962) were skeptical and favored explanations like land-bridges or long distance dispersal. As Wegener’s theory gained wider acceptance, ideas about Nothofagus biogeography started to change as well.

The breakup sequence of Gondwana follows that New Zealand was separated from Australia at around 80 mya and South America and Australia stayed connected via Antarctica until about 35 mya (McLoughlin, 2001). If the breaking up of the continents was the only explanation of current distributions (ie vicariance), it is likely then that the relationships of Nothofagus species between the different Gondwanan fragments should reflect the break-up sequence (Linder & Crisp 1995).

Nothofagus codonandra, a New Caledonian species.

One such prediction follows that Southern Australian Nothofagus would be more closely related to South American Nothofagus than to New Zealand Nothofagus since New Zealand was separated from Australia for a much longer period of time. Australian Nothofagus however, is more related to New Zealand Nothofagus than to South American Nothofagus (Hill, 2001; Knapp et al., 2005). To try to use vicariance to explain the present day distribution of Nothofagus would require investigators to accept the view that Nothofagus evolved in situ on the different southern continents after the Cretaceous break up of Gondwana (Knapp, 2005) and that the extinction of South Australian/and or closely related South American species has occurred. The view is an attractive one and is largely founded on the firmly implanted belief that Nothofagus fruits have a poor dispersal capability (Preest, 1963) and cannot survive prolonged immersion in seawater.

Nothofagus betuloides

Others investigators favor the view that multiple transoceanic dispersals had occurred between Australia and New Zealand (Hill & Dettman, 1996). In light of recent evidence, not only did Nothofagus fruits float well, seeds were viable after prolonged immersion in sea water (M. Sharpe as quoted by Hill & Orchard, 1999). Recent molecular research also indicates that the relationships between Australian and New Zealand Nothofagus species are too young to be explained by continental drift and favors long term dispersal of Nothofagus (Cook & Crisp, 2004; Knapp et al., 2005). Ultimately, it appears that a hybrid theory is needed to explain modern distribution patterns of Nothofagus where some cases may be explained by best by vicariance whereas others can only be explained by transoceanic dispersal (Hill & Dettmann, 1996).

As yet, the mechanism of long-distance dispersal of Nothofagus is seculative (Hill & Dettman, 1996; Hill, 2001) and much basic research on this aspect of Nothofagus biology is needed.

The Future of Nothofagus biogeography

If Nothofagus, the ‘key to plant geography’ (van Steenis, 1971) is to open up the treasure chest of Gondwanan biogeography, a multi-disciplined approach is required. Biogeographers often emphasize the importance of considering all biotic groups while searching for generalizations in biogeography (Pirozynski, 1983). However, such an approach has not been applied extensively in the study of Nothofagus biogeography with the exception of a few investigators (Linder & Crisp, 1995). The fungal component in Nothofagus biogeography, for example, was noted earlier (van Steenis, 1971) but still remains largely ignored by mainstream phytogeographers (Pirozynski, 1983). An interesting fungal genus Cyttaria is an obligate parasite of Nothofagus and known only to attack only South American and Australian Nothofagus. No Cyttaria has ever been reported from New Guinean Nothofagus of the subgenus Brassospora suggesting that Cyttaria had evolved only after Nothofagus subgenus Brassospora had evolved (Korf, 1983).

Korf (1983) proposed an evolutionary history that could account for the distribution of Cyttaria species and their Nothofagus hosts. Other investigators are skeptical but admit that further molecular data might help to resolve relationships (Peterson et al., 1999). Other fungal groups that form mycorrhizal associations with Nothofagus may also provide useful insights (van Steenis, 1971). The insect fauna associated with Nothofagus are another group of biota that has received little attention, the most recent review being that of MaQuillan (1993) nearly two decades ago.

With the increasing use of molecular techniques in scientific research, we are already seeing many exciting and informative studies dealing with the evolutionary ecology and historical phylogeography of Nothofagus in Australia (Worth et al., 2009) and South America (Acosta & Premoli, 2010; Mathiasen & Premoli, 2010). After 150 years of scientific thought and research, Nothofagus continues to be the icon tree genus of Gondwanan biogeography.

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