Definitive Identification of Larix (Pinaceae) Wood

IAWA Journal, Vol. 22 (1), 2001: 73–83

DEFINITIVE IDENTIFICATION OF LARIX (PINACEAE) WOOD

BASED ON ANATOMY FROM THE MIDDLE EOCENE,

AXEL HEIBERG ISLAND, CANADIAN HIGH ARCTIC* by Richard Jagels1, Ben A. LePage2 & Mei Jiang1

SUMMARY This paper provides the first definitive identification of fossil Larix Miller wood using the characteristic features of ray-tracheid bordered pits. The wood was recovered from the middle Eocene (Lutetian/Uintan; 41.3–47.5 Ma) Buchanan Lake Formation on eastern Axel Heiberg Is- land in the Canadian High Arctic and extends the fossil record of Larix wood further back in geologic time. A new and rapid embedding method is described which provides a firm and non-destructive matrix for thin- sectioning and examining the well-preserved details of the wood. The wood is associated with Larix altoborealis LePage & Basinger, a short- bracted species, which was previously described from this locality as the earliest known species of Larix. Key words: Larix altoborealis, fossil, wood, Arctic, Cenozoic, Eocene, anatomy, ray-tracheid pitting.

INTRODUCTION The fossil record of Larix Miller is well documented and demonstrates that the genus was widely distributed throughout the Northern Hemisphere during the Tertiary (LePage & Basinger 1991a, 1991b, 1995; Miller & Ping 1994; Schorn 1994). The bulk of the Larix fossils described in the literature consist of isolated seeds, cones, leaves and twigs bearing brachioblasts, with reports of wood being rare. While it is difficult to provide a compelling reason as to why Larix wood should be rare in the fossil record, a partial explanation may be the difficulties associated with discrimi- nating between extant Larix and Picea A. Dietrich wood. Bartholin (1979) recognized this problem and provided a set of features that facilitated separation of the wood of European species of these two genera. Recently, Anagnost et al. (1994) tested the accuracy of Bartholinʼs method and concluded that the proportion of one of two types of ray-tracheid pitting was the only anatomical feature that correctly distin- guished between the wood of Larix and Picea in every case.

1) Department of Forest Ecosystem Science, University of Maine, 5755 Nutting Hall, Orono, ME 04469-5755, U.S.A. [[email protected]]. 2) Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA 19104-6316, U.S.A. [[email protected]]. *) Maine Experimental Station Report No. 2420. Polar Continental Shelf Project Contribution No. 00100.

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The middle Eocene (Lutetian/Uintan; 41.3–47.5 Ma) Buchanan Lake Formation of Axel Heiberg Island, Arctic Canada contains a number of exquisitely-preserved in situ fossil forests and associated forest-floor litters that represent the remains of Taxodiaceae-dominated swamp-forest communities. Included among these lowland forest constituents are some of the earliest remains of Larix, Picea and Tsuga. Fertile and vegetative remains of Picea and Larix co-occur in a number of these forest-floor litters and without using the proportion of ray-tracheid bordered-pit types reliable identification of putativeLarix wood was not possible. In this paper we employed the analytical technique outlined by Anagnost et al. (1994) to definitively identify fossil wood that had tentatively been identified as Larix and emend the diagnosis of Larix altoborealis LePage & Basinger to include a detailed description of the wood.

MATERIALS AND METHODS

The fossils that formed the basis of this study were recovered from sediments of the Buchanan Lake Formation, Eureka Sound Group, on Axel Heiberg Island, Canadian Arctic Archipelago (Fig. 1; 79° 55' N, 89° 02' W; Geological Survey of Canada, Map 1301A, Strand Fiord, District of Franklin, 1 : 250,000). The Buchanan Lake Forma- tion, as described by Ricketts (1986, 1991, 1994), consists of four lithologically dis- tinct and mappable members. Larix wood occurs in the sandy facies near the top of Rickettsʼ Upper Coal Member, which consists of interbedded sandstone, siltstone and lignite arranged in fining upwards sequences. In general, the wood collected from the sand layers showed less compression than wood from the stratigraphically lower silty layers.

Fig. 1. Map of Canada showing the location of Axel Heiberg Island (arrow) in the Canadian High Arctic.

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Based on vertebrate remains, structural, petrographic, stratigraphic and palynologi- cal features, the age of these fossiliferous sediments has been determined to be middle Eocene (Lutetian/Uintan; 41.3–47.5 Ma) (Ricketts 1986, 1987, 1994; Ricketts & McIntyre 1986; McIntyre 1991; Eberle & Storer 1999; Harrison et al. 1999). Samples were wrapped in plastic, transported to the University of Maine, Orono and stored in a freezer prior to analysis. A number of main stem logs that appeared to have little compression based on their external morphology were screened for preservation and taxonomic features by examining free-hand thin sections under the microscope. The best-preserved log chosen for this study was approximately 28 cm in diameter and not eccentric. Free-hand sections showed anatomical features that con- formed to wood of extant Picea and Larix. Small cubes (1–1.5 cm2) were cut from the fragile, recently defrosted wood using a hand-held, rotary micro saw (Dremel, variable-speed multi-pro model 3955; Racine, WI, USA). The blocks were either slowly dried at 4°C for 8 months or dehydrated gradually for 2 days through an alcohol series, and infiltrated under vacuum with a proprietary, low-viscosity, epoxy resin (Git-Rot; Life Industries, N. Charleston, SC, USA). A number of other resins were tested, including celloidin (Jagels 1968), but the Git-Rot resin yielded the best results. No differences were observed in the sections from the blocks that were slowly or more rapidly dehydrated. One ideal feature of Git-Rot resin is its compatibility with slightly damp wood (personal communication with resin developer). Resin penetration of the wood was nearly complete and the optical properties are compatible with high-resolution light microscopy (Fig. 14–17). Thin sections of 15–20 μm were prepared using a sliding microtome (A.O. Spencer, model 860) with a steel knife. Sections were mounted without staining in mounting medium on microscope slides with cover slips. Photographs were taken with 35 mm Kodak T-max 100 film using a Leitz Labrolux 12 microscope. All specimens have been deposited in the Paleobotanical Collection at the Canadian Museum of Nature, Ottawa, Ontario, Canada.

SYSTEMATIC DESCRIPTION Larix altoborealis LePage & Basinger emend. Jagels, LePage & Jiang (Fig. 2–20). Larix altoborealis LePage & Basinger (LePage & Basinger 1991a: 89–111, fig. 4–17, 18n, 19, 20, 23–26, 29, 30, holotype fig. 4). Emended species diagnosis Wood consists of longitudinal tracheids up to 44 μm in tangential diameter with one to two rows of bordered pits on the radial surfaces; piceoid crossfield pits, 3–7 pits per ray crossfield; marginal longitudinal parenchyma present, but scarce; transition between earlywood and latewood more or less abrupt; longitudinal and radial resin canals present, with up to 13 thick-walled epithelial cells surrounding the canals in the fusiform rays; fusiform rays with two resin canals rare; uniseriate and partly biseriate rays present; ray tracheids present mostly on the margins of the rays and containing bordered pits that conform to the Larix type; ray parenchyma with sparsely pitted horizontal walls and nodular end walls with indentures. Hypotype: Canadian Museum of Nature Paleobotanical Collection # CMN 51268.

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Fig. 2–9. Wood of Larix altoborealis. – 2 & 3: Transverse sections of the best preserved wood. Note that the crushed cells are confined to the late earlywood zone. – 4: Transverse section of poorly preserved wood showing considerable distortion and some cell separation (arrow). – 5 & 6: Radial longitudinal section showing paired bordered pits in longitudinal tracheids. – 7: Radial longitudinal section and tangential longitudinal section caused by folding distortion as seen in Figure 3. – 8 & 9: Tangential longitudinal sections of uniseriate and fusiform rays. — Scale bars = 100 μm. All slides were prepared from Canadian Museum of Nature specimen # CMN 51268.

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Fig. 10–13. Wood of Larix altoborealis. Radial longitudinal sections showing piceoid pitting, mostly smooth horizontal walls and nodular end-walls, frequently with indentures, in the ray parenchyma (Fig. 13, arrow). — Scale bars = 70 μm.

RESULTS AND DISCUSSION

The fertile and vegetative remains of Larix altoborealis are the earliest known for the genus and indicate that the morphological variability among the various organs examined is as great as that seen among all of the living representatives of the genus, and that affinity with any of the living species could not be determined (LePage & Basinger 1991a). The original species diagnosis of L. altoborealis was based exclu- sively on anatomical and morphological features of the seed cones, leaves and twigs bearing brachioblasts (LePage & Basinger 1991a). The lack of suitably preserved wood assignable to Larix when L. altoborealis was first described precluded the inclusion of wood as part of the original diagnosis. The recent discovery of well-preserved Larix wood from this site provides an opportunity to describe and illustrate the wood

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Fig. 14–20. Wood of Larix altoborealis. – 14–17: Radial longitudinal sections showing the types of bordered-pit pairs in ray tracheids; 14: Larix-type; 15: intermediate type; 16: Picea- type 1; 17: Picea-type 2. – 18–20: Tangential longitudinal sections; 18: nodular end-walls in the longitudinal parenchyma; 19: dark staining deposits in the longitudinal parenchyma; 20: epithelial cells with side-wall pits (left) and longitudinal parenchyma (right). — Scale bars = 50 μm. and emend the diagnosis of L. altoborealis. Given that only one species of Larix was identified and described from the entire suite of sediments assigned to the Buchanan Lake Formation on Axel Heiberg Island, it is not unreasonable to conclude that the wood collected from this site belongs to Larix altoborealis.

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Samples extracted from the best preserved log yielded cross sections as is seen in Figures 2 and 3. An example of more typical preservation and compression is seen in Figure 4. Where compression damage was small (Fig. 2, 3), it occurred in the middle of the earlywood and usually displayed a tangential slip-plane in one direction (Fig. 3). In Figure 7 the last-formed latewood of one year is seen on the left, followed by a few cells of the first-formed earlywood of the next year in radial view, then crushed earlywood in a mostly tangential view, and finally first-formed latewood on the right. Because of the considerable amount of crushing, earlywood features were difficult to find and not possible to quantify. Despite these difficulties, the wood is identified as being that ofLarix based on the following features:

1) Longitudinal and radial resin canals with thick-walled epithelial cells (Fig. 4, 8, 9). 2) Three to seven piceoid pits per ray crossfield between the ray parenchyma and longitudinal tracheids (Fig. 10, 11). Eroded pits are seen in Figure 12. 3) Ray parenchyma with sparsely-pitted horizontal walls and nodular end walls with indentures (Fig. 10, 13). 4) Earlywood tracheids up to 44 μm in diameter in tangential view, with bordered pits in one to two rows on the radial surfaces; paired pitting can extend the full length of the tracheid (Fig. 5, 6). 5) A more or less abrupt transition between the early- and latewood (Fig. 2). 6) Two types of rays: uniseriate and fusiform (Fig. 8, 9). 7) Ray tracheids; examination of 23 bordered pits in sectional view yielded 13 Larix- type (Fig. 14), 8 intermediate (Fig. 15), 1 Picea-type 1 (Fig. 16) and 1 Picea-type 2 (Fig. 17) as described by Bartholin (1979) and Anagnost et al. (1994). Both Picea- types were seen in the last formed latewood. 8) Scarce, marginal longitudinal parenchyma, with up to 8 nodules on the end walls in tangential view (Fig. 18, 19). Some longitudinal parenchyma contained dark-staining contents (Fig. 19, left); and some epithelial cells had side-wall pits (Fig. 20, left).

The presence of mostly Larix or intermediate-type bordered pits in the ray tracheids and marginal longitudinal parenchyma provide conclusive evidence that this wood is that of Larix and not Picea (Anagnost et al. 1994; Panshin & de Zeeuw 1980).

Secondary features Other anatomical features that have been used to separate Larix from Picea from restricted geographical areas, and which we have termed “secondary” features, were examined in the wood of Larix altoborealis. The abrupt transition between early- and latewood has been used to distinguish Larix from Picea for North American species (Panshin & de Zeeuw 1980). Larix altoborealis possessed a transition zone that we interpret as being more or less abrupt (Fig. 2). Anagnost et al. (1994) point out that 72% of the Larix samples they examined had some abrupt transition rings, while 84% of the Picea samples were gradual.

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The number of epithelial cells surrounding the horizontal resin canals in the fusiform rays, particularly the maximum value, has been considered to be a stable and consistent character that has been used as a diagnostic feature to separate Larix from Picea (Budkevich 1961; Sudo 1968; Jane et al. 1970; Barefoot & Hankins 1982). In general, Larix species have 9 or more epithelial cells surrounding the resin canals, whereas Picea species generally have 9 or fewer. Counting the number of epithelial cells in samples of L. altoborealis was difficult because of the amount of wood compression, but 34 canals were measured and the number of epithelial cells ranged from 9–13, with one canal having possibly 8. However, Anagnost et al. (1994) pointed out that the use of this feature alone enabled correct identification of only 64% of their samples, with 28% of the Picea and 56% of Larix samples being mis-identified. Paired pitting in the earlywood longitudinal tracheids is seen in many species of Larix and is less common in Picea (Panshin & de Zeeuw 1980). Paired pitting in L. altoborealis was commonly seen in the earlywood tracheids that were not crushed (Fig. 5, 6). Spiral thickening in the longitudinal or ray tracheids was not seen in L. altoborealis. For extant North American species, the latewood tracheids of Larix some- times possess spiral thickenings (Panshin & de Zeeuw 1980). Jane et al. (1970) and Anagnost et al. (1994) have noted the general unreliability of this feature, but Anagnost et al. (1994) confirm Phillipsʼs (1948) observation that spiral thickening is more common in Picea than in Larix. The presence of marginal parenchyma has been suggested as being a distinguishing feature of Larix (Jane 1934; Sudo 1968; Panshin & de Zeeuw 1980). Anagnost et al. (1994) did not consider this feature in their comprehensive study. However, Noshiro and Fujii (1994) examined extant Asian species of Larix and Picea and found marginal parenchyma in all samples of Larix and none in Picea, with a possible ex- ception in one Picea sample. The wood of L. altoborealis contained marginal longitudinal parenchyma (Fig. 18–20), providing the strongest secondary evidence that the wood is correctly identified as being that ofLarix . The secondary features just discussed provide confirmatory evidence for an identification of Larix, but the definitive evidence is provided by the proportion of Larix versus Picea-type ray-tracheid bordered pits (Anagnost et al. 1994). Therefore, the fossil wood identified here is the first unequivocal evidence ofLarix wood from fossil remains and extends the wood record of this genus to the middle Eocene.

Other fossil Larix wood In a number of other studies of fossil wood, the samples were characterized as being that of Larix/Picea or questionable Larix (Beals & Melhorn 1961; Matthews et al. 1986; Wheeler & Arnette 1994). In the absence of ray-tracheid pitting analysis, these were appropriate conclusions. Alternatively, putative fossil Larix wood has been commonly identified and assigned to new species of Larix or Laricioxylon Greguss (a morphotaxon for wood that closely resembles extant Larix wood) based on features that do not separate it from Picea (Schröter 1880; Penhallow 1892; Roy & Hills 1972; Blokhina 1976, 1979, 1983, 1984, 1985a, 1985b, 1989, 1996). Although the number of fossil wood specimens currently identified as being that of either Larix or Picea is small (< 5 reports each), it is quite likely that some of the specimens have

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been mis-identified given that discrimination between these two genera is not possible without the use of ray-tracheid bordered-pit types and that Larix and Picea commonly occur as part of the same paleoflora. In addition, others have assigned fossil wood to Larix without the benefit of anatomical examination (Matthews et al. 1986; Bennike 1990). Bennike (1990) erected a new species, L. groenlandii Bennike, for cones, seeds, leaves, twigs and wood (including tree trunks) from Pliocene age deposits in Greenland. Although the cones, seeds and leaves were properly described and illustrated, an anatomical description for the wood was not provided, despite inclusion of the wood under this species concept. Moreover, assignment of putative fossil Larix wood to an extant species of Larix is problematic (Kuc 1974, Matthews et al. 1986), for it is not possible to distinguish between extant species of Larix using anatomical features of the wood alone (Budkevich 1955; Anagnost et al. 1994). Kuc (1974) reported that the forests from the interglacial deposits at Worth Point, western Banks Island, Canadian Arctic Archipelago were composed mainly of L. laricina. However, descriptions or illustrations documenting the validity of this identification are lacking in reports of Kuc (1974) and Matthews et al. (1986). To minimize the nomenclatural and taxonomic clutter caused by mis-identifications and the unnecessary creation of new names, we propose that fossil wood samples resembling Larix that have not been identified or cannot be identified using ray-tracheid bordered-pit types be assigned to Laricioxylon. The necessity for a genus for fossil wood showing affinity toLarix , but not equivocally identified as being that ofLarix is recognized and assignment of such specimens to Laricioxylon is appropriate.

CONCLUSIONS

The current study provides the first definitive identification of Larix wood from the fossil record, and extends the putative record of Larix wood to the middle Eocene. Features commonly used to discriminate between Larix and Picea wood have included the earlywood-latewood transition, spiral thickenings, and pit seriation in the longitudinal tracheids, and number of epithelial cells, but as Anagnost et al. (1994) have noted, these criteria are not consistently reliable. They reported that only 71% of the Picea samples and 59% of the Larix samples that they examined were correctly identified at the generic level using secondary anatomical features, whereas all of the samples were correctly identified using the proportion of ray-tracheid bordered-pit types. None of the research prior to 1994 in which fossil wood was assigned to Larix, Laricioxylon or Picea employed ray-tracheid bordered-pit morphology, and to our knowledge none have since. Consequently, the reliability of these assignments is doubt- ful and these specimens should be re-examined using ray-tracheid bordered-pit morphology in order to be definitively assigned generic designations.

ACKNOWLEDGEMENTS

Financial support for this work was provided by the Andrew W. Mellon Foundation and McIntire- Stennis Funds, University of Maine. We would also like to extend our thanks to the Polar Continen- tal Shelf Project of Energy, Mines, and Resources Canada for field and logistical support and to George Visscher for assistance with slide preparation of extant Larix.

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