Geoflora concept – does it apply to the ‘Baltic amber forest’?

Based on the highly diverse Baltic amber flora with presumed tropical and temperate elements, Szwedo and Sontag (2009) rejected the theories of Wheeler (1910, 1915) and Archibald and Farrell (2003) (see chapter 5.3). As an explanation for the peculiar mixture of (sub)tropical and temperate taxa, Szwedo and Sontag (2009) followed Kohlman-Adamska (2001), who argued that the Baltic amber plant diversity not only indicated an altitudinal stratification of the source area, but mainly showed the convergence of a temperate and a ‘paratropical’ climatic zone, the latter being equivalent to a ‘subtropical’ climate (Szwedo and Sontag 2009). Thus, the ‘Baltic amber forest’ comprised two geofloras: the Arcto-Tertiary geoflora which is associated with temperate climate and the Palaeotropical geoflora which is indicated by thermophilous plant taxa (Szwedo and Sontag 2009).

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The concept of geofloras dates back to Engler (1882) who studied Tertiary fossil floras from the Arctic. He observed that fossil plant taxa from Arctic assemblages are affiliated with extant floras of northern hemispheric temperate regions (North America, Europe and extratropical East Asia). For addressing this group of plant taxa, Engler (1882) introduced the term ‘Arcto-Tertiary element’.

Plant taxa which are confined to extant Old World tropics were termed

‘Palaeotropical elements’. This concept was elaborated by Chaney (1959) who changed these terms to ‘Arcto-Tertiary/Palaeotropical geofloras’, stating that they all

“must […] have had a common area of origin”, meaning that for instance, all Arcto-Tertiary geofloras originated in northern latitudes, but alternated in their distribution according to environmental changes. Furthermore, geofloras “maintained [themselves] with only minor changes in composition for several epochs or periods of earth history…” (Chaney 1959). In the following years, the terms Arcto-Tertiary and Palaetropical geoflora were widely used for taxa which were on the one hand deciduous and from temperate vegetation zones (Arcto-Tertiary geoflora) or on the other hand wintergreen and parts of (sub)tropical vegetation zones (Palaeotropical geoflora) (pers. comm. L. Kunzmann, Dresden).

This rather undifferentiated usage of the geoflora concept subsequently led to confusion and criticism. For instance, Wolfe (1972) and Graham (1972) rejected the geoflora concept, since to them, it was unlikely that a flora maintained stability for geological epochs. Such a concept would ignore physiological and genetic changes of plant lineages which can alter their ecological preferences through time (Wolfe 1972). Also, both authors criticized that the geoflora concept was too simple to reflect the rather complex history of Cenozoic floras (Graham 1972, Wolfe 1972). In contrast, Mai (1991) supports Engler’s (1882) and Chaney’s (1959) geoflora concept, stating that a classification of palaeofloristic units in geological time was possible.

A further revision of this concept was supplied by Kvaček (1994) who distinguished between ‘ancient’ and ‘modern’ Arcto-Tertiary elements. The first category contained extinct and extant taxa of angiosperms and gymnosperms which were mainly summergreen (e.g. Metasequoia, Platanus schimperi, Corylites, Fagopsis); the second category encompassed genera that migrated to Europe from eastern and north-eastern regions after the closure of the Turgai Strait (late Eocene-Oligocene). These genera either remained in Europe until recent time or became extinct after the Pliocene. Examples of these ‘modern’ Arcto-Tertiary elements are Pseudolarix, Alnus and Acer. Kvaček (1994) concluded that Arcto-Tertiary geofloras are heterogeneous, not only in terms of taxonomic diversity but also in their evolutionary and palaeoecological history.

A recent publication of Grímsson et al. (2015) raised doubts that the geoflora concept as such is reasonable. These authors studied fagaceous pollen floras of the middle Eocene of Western Greenland which should belong, according to the definitions presented above, to an Arcto-Tertiary geoflora. Grímsson et al. (2015) discovered a very diverse Fagaceae flora which, however, does not represent the oldest record so far. For instance, Fagus fossils were already found in early Eocene sediments of Washington and thus, predated the Fagus pollen of Western Greenland.

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The same holds true for other genera of the Fagaceae which were found in the Western Greenland pollen record of the middle Eocene. This shows that the lineages of the Fagaceae were already diversified during the Eocene before they inhabited Greenland. Therefore, Grímsson et al. (2015) rejected the concept of Chaney (1959) and Mai (1991), since presumed Arcto-Tertiary elements, as the Fagaceae migrated into the Arctic, but certainly did not originate there.

Considering all these different perceptions of the geoflora concept, the question arises whether it is really suitable to apply to the Baltic amber flora. If applied in its strictest sense [sensu Engler (1882) and Chaney (1959)], do these geofloras really occur in the Baltic amber source area as it was suggested by Kohlman-Adamska (2001) and Szwedo and Sontag (2009)? First of all, it is not clear which kind of geoflora interpretation these authors used. Assuming that Kohlman-Adamska (2001) and Swzedo and Sontag (2009) applied the original concept of Engler (1882) and Chaney (1959), it initially seems that the Baltic amber flora indeed contained Arcto-Tertiary elements. For instance, Sadowski et al. (2017a [5]) highlighted that the ‘Baltic amber forest’ conifer diversity was comparable to temperate fossil floras of Spitzbergen. However, to be clearly Arcto-Tertiary (following Chaney’s definition), the Baltic amber conifers should be of an Arctic origin as well. This is for example not the case for Pseudolarix whose first fossil record was reported from the Early Cretaceous of southeastern Russia (Bureya Basin) and northeastern China (Fuxin Basins; LePage and Basinger 1995). If Kohlman-Adamska (2001) applied Kvaček’s (1994) revision of the Arcto-Tertiary element, then Pseudolarix would be assigned to ‘modern’ Arcto-Tertiary elements.

‘Ancient’ Arcto-Tertiary elements sensu Kvaček (i.e. Metasequoia), however, were not observed in the Baltic amber flora. Regarding palaeotropical elements, it should be taken into account that (as discussed in chapters 4.2.2 and 5.2) the validity of presumed tropical indicator plant taxa is doubtful. Thus, the presence of a Baltic amber palaetropical geoflora lacks confirmation and should not be applied tothe

‘Baltic amber forest’.

As outlined above, the geoflora concept is not generally accepted among scientists, as different interpretations and definitions of this concept exist. Also, as explained in the previous chapters (5.1, 5.3), the diverse flora from Baltic amber can be justified by its habitat heterogeneity. Thus, it does not appear reasonable to apply the rather uncertain geoflora concept for explaining the composition of the Baltic amber flora.

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