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Dr Alan Channing

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Fossil conifers from Macaronesia
Poster presented at EPPC 2010 in Budapest here:
Alba Zamuner, Alan Channing and Cajsa Lisa Anderson 2010

Recently we published a diverse record of fossil plants from Gran Canaria based on the presence of compressions, casts and moulds, charcoal and permineralised leaves, wood and other plant organs. These reports appear to certify the existence of laurisilva and Pinus dominated ecosystems in the Canary Islands at various points in the Miocene-Pliocene (Anderson et al. 2009). We have extended our searches for Canarian fossils geographically and stratigraphically to another island of the archipelago, Tenerife, where plant moulds and less common charcoal is present in Pleistocene ignimbrites and ash/pumice fall deposits of the Bandas Del Sur Group. Three different genera have so far been determined: Pinus (Pinaceae), Tetraclinis and Juniperus (Cupressaceae) - the former two on Gran Canaria and the latter on Tenerife. Generally the conifer wood is very well preserved either as charcoalified wood chips or as permineralised branches or trunks. Fossils are studied using light and scanning microscopy as is comparative material from the most likely closely related extant representatives according to phylogenetic evidence.

Tetraclinis sp. (Cupressaceae)
The conifer genus Tetraclinis was an element of the widespread European subtropical forests during the Palaeogene, but became almost extinct during the Neogene climate change. Today one single species, T. articulata, exists in the Atlas mountains of northwest Africa, with a few relict populations in Malta and south-east Spain. This is the first evidence that this genus had a distribution that included Macaronesia. It also appears to be the first direct palaeobotanical evidence of extinction on Gran Canaria (Anderson et al., 2009).
wood description
Permineralised coniferous wood with distinct annual rings and a gradual transition from early to latewood. Earlywood tracheids, 24-37 um wide (28-29 on average) with almost circular lumen, thin walls 3.6-5.1 um up to 10.71 um. Latewood tracheids with elliptical lumen and thick walls. Radial walls mainly with abietoid uniseriate bordered pitting, frequently separate, sometimes contiguous, rarely with a pair of opposite pits. Pits are round or somewhat elliptic 13.19 um x11.12 um to 21.39 um x15.28 um. Inner aperture commonly round 3.57 um to 5.95 um, sometimes weakly elliptic 5.91 x 3.94 um. Callitroide thickenings absent. Rays are homogeneous, parenchymatic and quite frequent; generally 1-14 cells in height and apparently uniseriate. Inner cells 15.77 um in high, marginal ones 30.83 um; cells apparently circular in cross section. Crossfields cupressoid, commonly with 1-4 (1-2 in inner cells; 4 in marginal cells) semiareolate pits that are variable in size from 7.76 um to 8.58 um and organized in 1-2 rows. Inner aperture circular or elliptical inclined. Horizontal and vertical walls of cells smooth and unpitted, never nodular. Horizontal wall thickness 3.78 um whilst vertical walls are 6.74 um. Pits in a horizontal row or as diagonal pairs are present. Sometimes in the marginal fields they are more numerous. We have not observed any indentures. Axial parenchyma present, sparce with minute cupressoid pits (diameter 22.22 um) in 2 alternate rows; possible weak nodules occur in the thin and smooth horizontal walls. Probable globular resin contents occur in axial parenchyma and tracheids.

Fossil Tetraclinis wood has been described under several names, but based on leaf- and cone characters there were probably two main species during the Miocene; T. salicornioides and T. brachyodon (Kvaček 2007 and refs. therein). Tetraclinis salicornioides is considered a thermophilous element of humid mixed evergreen and deciduous forests, while T. brachyodon was more likely a Mediterranean-climate xeromorphic element. The extant species T. articulata and T. brachyodon have ecological preferences in common, but T. salicornioides has been proposed to be the more closely related to T. articulata. The phylogeny ofTetraclinis must however be said to be unresolved, and we are not ready to speculate about the affinities of the Gran Canarian fossil.

Pinus canariensis

Pinus canariensis is the dominant species in the high altitude forests (up to 2400m) of Gran Canaria, Tenerife, La Gomera, El Hierro and La Palma. The ecosystem, that also includes Cistus, Bystropogon, Erica, Sonchus and other shrubs and woody herbs, is highly adapted to regular burning.

wood description

Growth rings clearly defined, with a relatively gradual change from earlywood to latewood; latewood well developed. Axial tracheids with polygonal cross section, lacking spiral thickenings. The axial tracheids of the earlywood are of irregular shape, tracheids of latewood are thick walled with narrow lumen. Resin ducts scattered but mainly in the transition with latewood, with preserved epithelial cells. Tracheids of earlywood mainly with radial uniseriate separate bordered pits, occasionally with 1-2 pairs of opposite pits, uniseriate contiguous in the whole wall or forming patches; sometimes totally biseriate, opposite in large elements. Pits are sometimes preserved with an intact torus. In latewood scarce uniseriate small bordered pits are typical. Crossfields pinoid, with 3-4 simple elliptical pits in earlywood, 1-2 in latewood. Uniseriate rays 1-4 cells high. Probable axial parenchyma that, in contact with rays, show a very thin wall and circular hollows that could be primary pit fields. In latewood, rays uniseriate between 7 – 18 cells high.

The fossil Pinus charcoal was found at high altitude in the Pajonales area (ca 1400 m), within transported Roque Nublo breccia (see map).

A number of characters that would confirm the diagnosis of the fossil material as Pinus canariensis are either lacking or have not been observed. Important features we have failed to identify include - dentate ray tracheids; biseriate and opposite bordered pits with crassulae on the radial wall of the axial tracheids; axial resin canals with associated subsidiary cells and pinoid cross-field pits. Also, axial resin canals appear in the transition wood or latewood and very rarely in the earlywood. The lack of these features precludes us from providing a specific diagnosis though the anatomy suggests placement within Section Pinus Subsection Pinaster (which is a mainly Mediterranean group, with outlying species in Macaronesia and the Himalayas), see phylogeny.

Juniperus sp.
Juniperus cedrus (Macaronesian) and J. oxycedrus (a mainly Mediterranean species with several subspecies) might form a species complex rather than separate species (see molecular phylogeny).

wood description

Juniperus aff. J. cedrus Webb & Berthelot
Gymnospermous homoxylic secondary xylem, with scarce and disperse axial parenchyma but without resin canals. Annual rings fairly sharp, up to 35 tracheids wide (app. 250-1100 um in thickness). Earlywood distinct from latewood in diameter and wall thickness of vascular elements. In latewood tracheids with narrow lumen and circular to oval section. In earlywood, polygonal and thin-walled. Apparent thin false rings with approximately 7 files. Radial intervascular pitting bordered, loose, uniseriate, separate to more or less contiguous, with a pit aperture round or included lenticular inclinated in both, early and latewood; tangential pitting not apparent. Rays uniseriate low 1-8 cells height, thin-walled, crossfields commonly with 1 (rarely 2) cupressoid pits in crossfield; Horizontal and vertical walls of ray parenchyma are smooth. Axial parenchyma near the latewood with horizontal walls generally smooth and pits similar to that of crossfields. Indentures not apparent.

Typically, J. oxycedrus wood has distinct growth rings, tracheids always with square section, bordered pits in tangential walls, axial parenchyma is abundant and terminal. A few less closely related species to J. cedrus - oxycedrus have similarities in their anatomy compared to the fossil charcoal: J. procera Hochst.ex Endl. is a tree living in NE, E and S of Africa and the Arabian Peninsula. It shares many important anatomical features with the fossil charcoal. J. procera differs in having tracheids with commonly 2-3 (up to 5) cupressoid pits per crossfield, ray cells with pitted horizontal walls and vertical walls which are sometimes beaded, axial parenchyma horizontal walls are also sometimes beaded and uniseriate tangential pitting (Esteban et al. 2002). J. phoenicea L. is a Mediterranean tree, also present in the Canaries and Madeira. It is similar in anatomy with the above species but with higher rays (1-13 cells) and vertical walls of ray parenchyma cells always beaded (Esteban et al. 2002). J. brevifolia occurs in the Azores Islands, generally the habit of the plant does not exceed 2 m in height. Annual ring boundaries in this species are hardly discernible; there is an abundant axial parenchyma regularly distributed along the annual ring, crossfields may include up to 6 pits, commonly 1-4. Ray cells with thick and perforate horizontal walls, vertical walls beaded.

Whilst our study has not as yet investigated ecological information from samples, all three taxa show very well marked growth rings, with distinct, wide latewood, that suggest the presence of important limiting factors for annual growth, most likely the existence of a well defined dry season.