new contenders for the parent trees of Baltic amber
Written by Jacek Szwedo & Ryszard Szadziewski
In spite of investigations carried out for more than 160 years, the mystery of Baltic amber’s parent plant remains unsolved. Many species of trees were suggested as the parent tree, chiefly belonging to two conifer families: the Pinaceae and the Araucariaceae. The traditional name for the parent plant of the Baltic amber resin was coined as early as in the mid-19th century as Pinus succinifera or the amber pine (GÖPPERT, 1836; CONWENTZ). However, the actual systematic affiliation of this taxon remains uncertain. The serious contenders for the title of Baltic amber resin’s parent plant also included the golden larch Pseudolarix (GORDON) from the same family Pinaceae.
Palaeobotanical and physio-chemical research has failed to resolve many doubts or even raised new questions. A review of the issues related to the “mysterious origin of Baltic amber” was presented in a monograph on botanical resins by LANGENHEIM (2003). Infra-red spectroscopy (IRS) tests have shown that Baltic amber has a different spectrum from known resins produced by trees from the family Pinaceae. Baltic amber’s distinguishing feature comes in a part of the spectrum curve between 1250 and 1175 cm-1, known as the “Baltic amber shoulder.” This arm does not appear in the spectra of contemporary tree resins, with the exception of the Pinus lambertiana, a pine from North America (DOUGLAS). However, Baltic amber’s spectrum is similar to that of the kauri Agathis australis (LAMBERT) from the family Araucariaceae.
Other analytical methods, including pyrolysis-mass spectrometry (PyMS), have confirmed the results obtained by spectrum analysis (POINAR & HAVERKAMP, 1985). Baltic amber does not contain abietic acid, which is a chemical feature of resins produced by plants from the family Pinaceae. However, most Baltic amber varieties contain amber acid, which is not found in resins produced by plants from the family Araucariaceae, and is recognised as Baltic amber’s distinguishing chemical ingredient. However, this chemical feature, once thought of as unique to Baltic amber, has also been found in other resins from the Cretaceous and Palaeogene (ANDERSON & LEPAGE 1995 NGUYEN TU et al. 2000, OTTO & SIMONEIT 2001, WOLFE A.P. et al. 2009). The resins of contemporary pines from the genera Keteleeria (CARRIÉRE) and Pseudolarix (GORDON) also contain amber acid (GRIMALDI 1996). The Pseudolarix is especially interesting because a resin with acid amber content was discovered in the fossilised Eocene cones of the Pseudolarix wehri (GOOCH) from Axel Heiberg Island in the Canadian Arctic (ANDERSON & LEPAGE 1995). This amber was found in its primary deposit together with traces of fossilised wood. Today, such trees are represented by the sole species Pseudolarix amabilis (NELSON, REHDER) limited in its natural occurrence exclusively to some mountainous areas in south-eastern China. Such a distribution of the representatives of the Pseudolarix in ancient and contemporary times may suggest that this genus used to reach far to the north, including the area of Fenno-Sarmatia in the Eocene, and that it could have been part of the amber forests.
It has been suggested that amber acid develops as a result of the fermentation of cellulose and resin polysaccharides, the decomposition of lignin, the biodegradation of organic substances with the participation of microorganisms and redox processes with the participation of ferric ions (which is the likely reason for pyrite traces in amber). Therefore, amber acid is most likely a diagenetic product and should not be treated as a chemotaxonomic indicator of Baltic amber.
Recently, a hypothesis was published to suggest that the parent-plants of the resin that became Baltic amber (WOLFE P.A. et al. 2009) are extinct trees from the family Sciadopityaceae, related to the contemporary genus Sciadopitys (SIEBOLD et ZUCCARINI). This hypothesis was confirmed by Fourier-transform infrared spectroscopy (FTIR) and by tests using gas chromatography/mass spectrometry (GC/MS). The spectrum of the resin of the Sciadopitys verticillata Japanese umbrella-pine (THUNBERG, SIEBOLD et ZUCCARINI) is very similar to that of Baltic amber (WOLFE P.A. et al. 2009) (Fig. 1).
In nature today, this plant occurs only in Japan, where it grows to a height of 40 m. In other parts of the world it can only be found in parks and gardens. In Poland, it grows only in the west. (Fig. 2)
The Japanese umbrella-pine belongs to the independent family Sciadoptyaceae (STEFANOVIAC et al. 1998) whose closest relative is the Cupressaceae cypress family. In sediments from the early Palaeocene, Sciadopitys pollens constitute up to 60% of all the pollen in the samples (KRUTZSCH 1971, GOTHAN &WEYLAND 1973), which indicates that the geographical spread of these trees reached middle latitudes, which were located in the subtropical climate zone at the time. Forests dominated by the Sciadopitys were also found in Miocene brown coal sediments in the Rhine River Valley (MOSBRUGGER et al. 1994). The results of pollen analyses suggest that trace populations of these trees could have survived in Europe until the Pliocene (VAN DER HAMMEN et al. 1971). (Fig.3)
The latest hypothesis that it was Sciadopityacae that was the parent-plant of the resin that became Baltic amber is well documented by the results of physio-chemical analyses (IR spectroscopy, gas chromatography/mass spectrometry). Also morphological analyses of the impressions of the wood and bark with a typical “cypress-like” appearance which differs from “pine-like,” the presence of needles and cones among Baltic amber inclusions, as well as the contemporary umbrella-pine’s strong immunological and allelopathic properties (WOLFE P.A et al. 2009) are strong evidence to confirm this hypothesis.
In reference to the umbrella-pine’s antibiotic properties, it is worth emphasising that the resins of trees from this family are utterly non-resistant to rapid degradation due to biological (bacteria, fungi) or chemical agents, so they could not be deposited in an external environment for any extended period of time. The araucaria family of plants produce resins which are resistant to biodegradation (e.g. kauri resin). The problem is that these plants were found in the Northern Hemisphere only until the end of the Mesozoic period, so they could not have formed part of the amber-bearing forests when Baltic amber came into being.
The question remains open, whether the umbrella-pine trees, which had previously not been considered, could have produced such enormous amounts of resin which later turned into Baltic amber?
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Prof. Ryszard Szadziewski: Faculty of Invertebrate Zoology, University of Gdańsk, Al. Piłsudskiego 46, 81-376 Gdynia, Poland; e-mail: email@example.com