The enigmatic ‘Baltic amber forest’

The ‘Baltic amber forest’ is a term describing the Baltic amber source vegetation.

However, numerous perceptions about this palaeoecosystem exist, comprising partly contradictory opinions about its floristic composition, habitat types, climate and the topography of the source area (see Tab. 3 for summary and references). Thus, the

‘Baltic amber forest’ is a rather abstract term, representing one (or more) palaeoecosystem(s) which harboured the Baltic amber source plant(s). Therefore, the term ‘Baltic amber forest’ will be written in quotation marks in the following text.

Historic descriptions of the ‘Baltic amber forest’ divide the amber source area into lowlands and mountains in which subtropical to warm-temperate plant taxa inhabited lowland areas while plant taxa with extant analogoues in the northern high latitudes grew in higher elevations (Goeppert 1853, Caspary 1872a, Goeppert and Menge 1883). Following these authors, the ‘Baltic amber forest’ had affinities to extant floras of northern America, East Asia and Europe. Contrarily, Conwentz (1890) highlighted the dominance of pines which formed pure and dense stands. In his opinion, further deciduous tree species were scarce and only located at the margins of pine forests. Also, Conwentz (1890) mentioned that it must have been a humid and warm forest in which, however, snowfall occurred.

Based on insect inclusions, Ander (1942) concluded that the Baltic amber source area harboured many kinds of aquatic habitats. He divided the ‘Baltic amber forest’ into altitudinal zones and based on studies of Baltic amber plant inclusions from the 19^th century, Ander (1942) describes the ‘Baltic amber forest’ as dense and moist, with affinities to extant forests of Central China and North America.

Following his interpretation, the forest was dominated by conifers, but intermingled with oaks. Further broad-leaved deciduous trees and shrubs were only located in open glades or along southern slopes. In Ander’s opinion (1942), the main climate was humid, warm-temperate and only locally subtropical. In a comprehensive review, Czeczott (1961) agreed with Ander’s (1942) interpretations, but highlighted the high proportion of tropical and subtropical plant taxa (e.g. Apocynaceae, Araceae, Lauraceae and Theaceae) which comprised 23 % of all plant taxa from Baltic amber while temperate taxa only constitute 12 %. Due to the presence of temperate taxa along with tropical ones, Czeczott (1961) concluded that the source area of Baltic amber was mountainous with a warm-temperate to subtropical climate.

Further studies on the ‘Baltic amber forest’ by Bachofen-Echt (1949) and Larsson (1978) draw a similar picture and only differ from other interpretations in few details. Bachofen-Echt (1949) emphasized the diversity of the Baltic amber source area, comprising meadows, arid areas, but also stagnant waters and mixed forests. In contrast, Larsson (1978) underlined the diversity of Quercus species, but only saw little evidence for the presence of meadows.

Contrarily to these perceptions of the ‘Baltic amber flora’, Schubert (1953, 1961) and Rüffle and Helms (1970) found indicators for drier climates with affinities to ‘hammocks’ of Florida and mountain steppe forests of Cuba and Honduras.

Following their interpretation, subtropical and sclerophyllous woods with pines and

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palms dominated the lowland of the Baltic amber source areas, while mountainous sides were inhabited by pine-oak forests (Rüffle and Helms 1970). Kohlman-Adamska (2001) differentiated between altitudinal zones with specific vegetation types and climates: swamps habitats close to subtropical lowland river systems, pine-oak-steppe forests in mid-altitudinal warm-temperate zones and pur conifer stands on temperate high altitudes. Contrarily, Weitschat (1997), Wichard et al. (2009) and Weitschat and Wichard (2010) interpreted Baltic amber arthropod inclusions as evidence for tropical rain forests in lowland areas and subtropical to warm-temperate oak-pine forests at higher elevations of Fennoscandia.

The most recent summary on the ‘Baltic amber forest’ was published by Alekseev and Alekseev (2016) who used beetle inclusions from Baltic amber to estimate the habitat types. Along with historic and recent literature on the Baltic amber plants, they concluded that the forest was a climax community, located in a plain landscape. In contrast to the other mentioned interpretations above, they did not see evidence for mountain ranges in the Baltic amber source area, although hills (300 m above sea level) might have been present (Alekseev and Alekseev 2016). Based on their study, the ‘Baltic amber forest’ was thermophilous, moist and layered.

Contrasting with Kohlman-Adamska (2001), Alekseev and Alekseev (2016) denied the presence of any inundated areas such as swamps, but instead underlined the presence of different kinds of stagnant waterbodies. As mentioned by other authors before, they also saw affinities to East and Southeast Asian forests.

In summary, the ‘Baltic amber forest’ is still controversial and strongly disparate theories about its vegetation exist. Most studies about the Baltic amber source area state that it was a humid, warm-temperate to ‘subtropical’ forest.

Following these studies, conifers, especially pines, were dominant, while broad-leaved deciduous plant taxa intermingled (rarely) with pine forests or grew in more open areas. In contrast, other studies found evidence for tropical or dryer climates and vegetation (see Tab. 3 for summary). These conflicting theories also evoked speculative ideas about a Fennoscandian locality and a vast and mountainous distribution area of the ‘Baltic amber forest’, as well as different age estimations for Baltic amber. It is noteworthy that the majority of the mentioned studies are either based on analyses of arthropod inclusions or on interpretations of plant inclusions that were published much earlier by Goeppert and Berendt (1845), Goeppert (1853), Goeppert and Menge (1883), Conwentz (1886b, 1890), and Caspary and Klebs (1907). Since the 19^th century, the majority of these plant inclusions have not been restudied. Besides literature based revisions by Czeczott (1961) and Spahr (1993), there were no new comprehensive studies about plant inclusions from Baltic amber.

This lack of knowledge contributes to the vast number of contrasting theories about the ‘Baltic amber forest’.

Tab. 3: Perceptions of the ‘Baltic amber forest’, taken from references indicated in the first column. Results of this thesis are indicated in the last row.

Reference Particular characteristics Topography Habitats and vegetation Climate Affinities to extant

floras

Age of Baltic amber Dense and close pine forests, broad-leaved deciduous taxa scarce

Conwentz 1890 pines as dominant tree taxa -

amber source trees (pines) formed closed, pure stands; broad-leaved deciduous trees scarce and rarely intermingling with pine forests

humid, warm,

but also snowfall North America, East Asia Eocene Warm-temperate to subtropical forest, humid, mountainous mountains plant taxa with extant analogues in northern high latitudes

Ander 1942 different kinds of aquatic habitats altitudinal

zones - broad-leaved deciduous trees and shrubs only in open glades or along southern slopes

- dense moist forest

- conifers, mixed with Quercus species

warm-temperate, only Czeczott 1961 high proportion of tropical and

subtropical plant taxa mountainous subtropical to

warm-temperate - Eocene

Bachofen-Echt 1949 diverse landscape and biota lowlands mountains

- mixed forests with conifers and broad-leaved deciduous trees - meadows, arid areas

- stagnant water bodies

- ‘Malaya’ early Eocene?

Larsson 1978

- conifers and Quercus species very dominant and diverse

lowlands humid swamp habitats along river valleys subtropical

- early

Palaeogene mountains pine-oak steppe forests in lower mountainous areas warm-temperate

pure conifer forests at higher altitudes temperate Subtropical to cold-temperate forest, ‘hammock’-like, dry, mountainous

Schubert 1953, 1961

Rüffle and Helms 1970 -

lowlands

- pine-palm-woods

- sclerophyllous pine woods along rivers - subtropical vegetation along coast

lowlands tropical rain forest tropical _extremely

warm and mountains subtropical to warm temperate oak-pine forests subtropical

Thermophilous, humid, broad-leaved forest in a plain landscape Alekseev and Alekseev

2016 forest was a climax community plain landscape

to slightly hilly

- thermophilous, moist, broad-leaved forest

- stagnant waterbodies (acidophilous, dystrophic, oligotrophic), streams, no inundated areas

- high conifer diversity plain landscape

- coastal swamps: brackish-water influenced areas and raised bogs

- back swamps and riparian forests: inundated (non-brackish) - mixed-mesophytic conifer-angiosperm forests with meadows

humid,

warm-temperate East Asia, North America late Eocene (Priabonian)

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