Unique floral composition

The extinct polar forests resembled extant cool-temperate rainforest in the Southern Hemisphere to the extent that the most commonly preserved macro- and micro-fossils belong to the Antarctic Beech Nothofagus or one or more of the conifers Araucariaceae, Cupressaceae and Podocarpaceae (see references in Enright & Hill Reference Enright and Hill1995, Veblen et al. Reference Veblen, Hill and Read1996, Hill & Brodribb Reference Hill and Brodribb1999, Hill et al. Reference Hill, Lewis, Carpenter and Whang2008, Reference Hill, Carpenter, Carpenter and Paull2019, Jordan et al. Reference Jordan, Carpenter, Bannister, Lee, Mildenhall and Hill2011, Cantrill & Poole Reference Cantrill and Poole2012). The similarity, however, becomes much less pronounced at the subgenus and species level as many of the nearest living relatives (NLRs) of the conifers and Nothofagus forming temperate rainforest in the Late Cretaceous and Palaeogene now have highly restricted distributions (and different ecological tolerances at least in terms of photoperiod) across the Southern Hemisphere or are extinct (see Dettmann Reference Dettmann and Hill1994, Macphail et al. Reference Macphail, Alley, Truswell, Sluiter and Hill1994, Enright & Hill Reference Enright and Hill1995, Veblen et al. Reference Veblen, Hill and Read1996, Poole & Cantrill Reference Poole and Cantrill2006, Cantrill & Poole Reference Cantrill and Poole2012, Pross et al. Reference Pross, Contreras, Bijl, Greenwood, Boharty and Schouten2012, Bowman et al. Reference Bowman, Francis, Askin, Riding and Swindles2014, Barreda et al. Reference Barreda, Palazzesi and Olivero2019). Read & Francis (Reference Read and Francis1992) and Osbourne et al. (Reference Osborne, Royer and Beerling2004) note that species habits such as winter deciduousness are highly suited to the survival of trees during warm, dark polar winters, an observation that is supported by the apparent higher diversity and possibly greater abundance of deciduous species of Nothofagus during the Late Cretaceous and Palaeogene (Hill & Scriven Reference Hill and Scriven1995). An example is New Zealand, where deciduous Nothofagus species persisted into the Miocene but are now extinct (Pole Reference Pole1994, McGlone et al. Reference McGlone, Mildenhall, Pole, Veblen, Hill and Read1996).

Canopy ‘openness'

Interpretations of community structure from the fossil record are less certain given that fossil leaves, small wood fragments and pollen and spores do not usually provide direct evidence of community structure (Greenwood Reference Greenwood and Hill1994, Macphail et al. Reference Macphail, Alley, Truswell, Sluiter and Hill1994). Moreover, many of the NLRs can thrive as trees or shrubs (see references in Enright & Hill Reference Enright and Hill1995, Veblen et al. Reference Veblen, Hill and Read1996) and, just as now, tree spacing appears to be closely related to stand age and environment. For example, several sites on the Antarctica Peninsula and other landmasses within or bordering the Antarctic Circle preserve closely spaced in situ stumps or transported wood on sites that were influenced by fluvial, volcanic and other disturbances (e.g. Pole Reference Pole1994, Francis Reference Francis, Stillwell and Feldmann2000, Thorn Reference Thorn2005, Olivero Reference Olivero2012, Iglesias Reference Iglesias2016, Kvaček & Vodrážka Reference Kvaček and Vodrážka2018). Similarly, interpretations of canopy shape based on NLRs of the palaeoconifers is conjectural. For example, the emergent species Araucaria hunsteinii (New Guinea) has few or no branches within the broadleaf angiosperm canopy stratum (and apparently avoids competition with other tree species), whilst other New Guinean populations of the closely related species Araucaria cunninghamii tend to branch within, as well as above, the canopy layer (Enright Reference Enright1982).

Despite these caveats, we argue that the following generalizations are uncontested: 1) modern communities classified as temperate rainforest mostly occur between ~25° and 50° latitude in both hemispheres, 2) temperate rainforests in the Southern Hemisphere almost always have closed canopies (closed forests sensu Specht Reference Specht, Gillison and Anderson1982) dominated by evergreen or, rarely found outside southern South America, deciduous species of Nothofagus, in which evergreen conifers typically form an overstory (Busby & Brown Reference Busby, Brown and Groves1994, Enright & Hill Reference Enright and Hill1995, Veblen et al. Reference Veblen, Hill and Read1996) and 3) extensive or pure stands of conifers are rare at high latitudes in the Southern Hemisphere except in areas subject to past or ongoing disturbances or that are at high elevations, such as Huon Pine (Lagarostrobos franklinii) gallery rainforest along rivers in western Tasmania and piñonero (Araucaria araucana) stands between ~900 m and the treeline on the Andes in Chile (Veblen et al. Reference Veblen, Burns, Kitzberger, Lara, Villabra, Veblen, Hill and Read1995, Read Reference Read, Reid, Hill, Brown and Hovenden1999).

Notwithstanding these caveats, the spacing, degree of canopy ‘openness’ and, in some instances, tree heights in the ancient polar forests have been indirectly estimated from affinity, physiognomy and ecophysiological responses of NLRs and other plants to conditions analogous to the polar regimes, wood anatomy (including growth rings) and the architecture and isotopic analysis of the fossil foliage, as well as from in situ fossil stumps (see Jefferson Reference Jefferson1982, Read & Francis Reference Read and Francis1992, Specht et al. Reference Specht, Dettmann, Thompson and Davies1992, Brodribb & Hill Reference Brodribb and Hill1997, Reference Brodribb and Hill2003, Hill & Brodribb Reference Hill and Brodribb2003, Poole & Cantrill Reference Poole and Cantrill2006, Taylor & Rhyberg Reference Taylor and Rhyberg2007, Takahashi & Murata Reference Takahashi and Murata2008, Truswell & Macphail Reference Truswell and Macphail2009, Read et al. Reference Read, Hill and Hope2010, Biffin et al. Reference Biffin, Brodribb, Hill, Thomas and Lowe2011, Pross et al. Reference Pross, Contreras, Bijl, Greenwood, Boharty and Schouten2012, Graham et al. Reference Graham, Herrera, Jaramillo, Wing and Freeman2019). However, assuming mean summer temperatures exceeded 10°C, we regard the most compelling evidence for wide spacing of mature trees, especially conifers, and a woodland-like structure in the ancient polar forests is the astronomical constraints of alternating periods of near to total darkness in winter and continuous light (photoinhibition) in summer and the inferred diversity of an understory dominated by shrubs such as Proteaceae (see Specht et al. Reference Specht, Dettmann, Thompson and Davies1992). For example, on the West Antarctic Peninsula, the structure of conifer forests in the Late Albian (c. 105–99.6 Ma) appears to have been similar to modern open woodland. These had transitioned into Podocarpaceae and Nothofagus open-canopy ‘forests’ by the Maastrichtian (c. 70 Ma), with conifers continuing to dominate the canopy stratum up to the mid- to late Eocene (c. 40–34 Ma) despite the presence of Nothofagus (see Askin Reference Askin1992, Dettmann Reference Dettmann and Hill1994, Falcon-Lang & Cantrill Reference Falcon-Lang and Cantrill2001a, Poole & Cantrill Reference Poole and Cantrill2006, Pujana et al. Reference Pujana, Inglesias, Raffi and Olivero2018). Whether or not the canopy or overstory trees had conical-shaped crowns as proposed by Specht et al. (Reference Specht, Dettmann, Thompson and Davies1992) or more laterally distributed foliage is unresolved (cf. Pole Reference Pole1994, Brodribb & Hill Reference Brodribb and Hill2003). Nevertheless, it is improbable that the diverse shrub or fern understories (e.g. Cantrill & Poole Reference Cantrill and Poole2012: 260) could exist at very high latitudes under a closed-canopy forest, even if more densely spaced canopy or emergent trees had sparsely distributed or closely adhering branches (Dettmann Reference Dettmann and Hill1994: 156). The impacts of infrequent wildfires and large browsing animals such as sauropod dinosaurs on community ‘openness’ or on creating open spaces within the polar forests are uncertain (di Pasquo & Martin Reference Di Pasquo and Martin2013, Gnaedinger et al. Reference Gnaedinger, Coria, Koppelhus, Cassadío, Tunik and Currie2017, Mays et al. Reference Mays, Cantrill and Blevitt2017).

Circumpolar distribution

Unlike the Northern Hemisphere, contiguous landmasses no longer occur around the South Pole. This was not the case during the Mesozoic and early Palaeogene, where the circumpolar distribution of fossil Araucariaceae, Podocarpaceae and Nothofagaceae allows the palaeofloras to be grouped into large-scale phytogeographical provinces, such as the ‘Austral Microcachrys phytogeoprovince’ in the Jurassic–Early Cretaceous (c. 200–100 Ma) and the ‘Nothofagus phytogeoprovince’ in the Late Cretaceous–Early Palaeogene (see Dettmann Reference Dettmann and Crame1989, Reference Dettmann and Hill1994, Horrell Reference Horrell1991, Dettmann et al. Reference Dettmann, Molnar, Douglas, Burger, Fielding and Clifford1992, Srivastava Reference Srivastava1994, Quattrocchio et al. Reference Quattrocchio, Volkheimer, Marquillas and Salfity2005, Macphail et al. Reference Macphail, Hill, Partridge, Jordan, Corbett K.D. and & Calver2014). Significantly, the palaeofloras comprising these ‘provinces’ have remained disjunct from those at low palaeolatitudes despite minor exchanges of taxa with the African-Indian, Asian and Boreal phytogeoprovinces (e.g. extinct members of the Northern Hemisphere Triprojectacites (Aquilapollenites) Group and a Droseraceae related to the North American Venus fly-trap Dionaea in the Palaeogene; Morley Reference Morley2000, Truswell & Macphail Reference Truswell and Macphail2004). We note that if geographical extent rather than structural criteria is used, then the polar forests were forests in the sense in which this term is used in north-west Europe and North America.