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. 2018 Mar 14;121(4):681-689.
doi: 10.1093/aob/mcx199.

Under Pressure? Epicormic Shoots and Traumatic Growth Zones in High-Latitude Triassic Trees From East Antarctica

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Under Pressure? Epicormic Shoots and Traumatic Growth Zones in High-Latitude Triassic Trees From East Antarctica

Anne-Laure Decombeix et al. Ann Bot. .
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Abstract

Background and aims: Investigating the biology of trees that were growing at high latitudes during warmer geological periods is key to understanding the functioning of both past and future forest ecosystems. The aim of this study is to report the first co-occurrence of epicormic shoots and traumatic growth zones in fossil trees from the Triassic of Antarctica and to discuss their biological and environmental implications.

Methods: Permineralized woods bearing scars of epicormic shoots were collected from the Triassic Fremouw Formation in Gordon Valley, Central Transantarctic Mountains, Antarctica in 2010. Samples from different portions of three specimens were prepared using standard thin section and hydrofluoric (HF) acid peel techniques, and anatomical details were studied in transmitted light.

Key results: The fossil woods represent the outer part of trunks, with at least 40 growth rings that are 0.2-4.8 mm in width. Anatomical comparisons suggest that they represent a new tree taxon for the Triassic of Antarctica. Numerous small epicormic shoots can be seen crossing the wood almost horizontally and are locally branched. Each specimen also contains several occurrences of traumatic growth zones located in the early wood, in the cells produced either at the very start of the growing season or slightly later.

Conclusions: This is the first report of epicormic shoots and traumatic growth zones in the wood of a Triassic tree from Antarctica. Their co-occurrence indicates that these trees from Gordon Valley were subjected to environmental stresses not seen in Triassic trees previously described from this region. This suggests that they had a different biology and/or were growing in a different habitat, which offers a new glimpse into the diversity of high-latitude trees in the Triassic greenhouse climate.

Figures

Fig. 1.
Fig. 1.
(A) Map of present-day Antarctica showing the location of Fremouw Peak and Gordon Valley in the Central Transantarctic Mountains. (B) Paleogeographical reconstruction for the Triassic with the position of the Transantarctic localities indicated.
Fig. 2.
Fig. 2.
Triassic trees with epicormic shoots from Gordon Valley: general aspect. (A) General view of the three specimens. (B) Enlargement showing conspicuous shoot scars (S) within the wood. Scale bar = 1 cm. (C–E) Peels of selected transverse sections in the three specimens showing the distinct growth rings and several shoots (S) crossing the wood. (C) 18,292 Bbot1; (D) 18,291 Dtop3; (E) 18,294 E3. Scale bar = 1 cm.
Fig. 3.
Fig. 3.
Triassic trees with epicormic shoots from Gordon valley: wood anatomy. (A) Transverse section of wood showing growth rings of various thicknesses. Arrows indicate ring boundaries. 18,292 Cbot1 (slide 30,291). (B) Detail of a wide growth ring (R: ring boundary). 18,292 Cbot1 (slide 30,291). (C) Detail of a smaller ring (R: ring boundary). 18,291 Ctop1 (slide 30,290). (D) Frost ring (Fr) occurring early in growth. 18,291 Ctop1 (slide 30,290). (E) Detail of cell distortion in a frost ring (Fr). 18,294 Btop1 (slide 30,293). (F) Frost ring (Fr) occurring later during the growth season. 18,291 Ctop1 (slide 30,290). (G) Disturbed radial arrangement of tracheid files. 18,294 Btop1 (slide 30,293). (H) General view in tangential longitudinal section showing low uniseriate rays. 18,294 A1 (slide 30,282). (I) Detail of rays in tangential longitudinal section. 18,294 A1 (slide 30,282). (J) Radial longitudinal section showing low parenchymatous rays, pitting on the radial wall of tracheids and a ring boundary. 18,294 A1 (slide 30,282). (K–N) Radial pitting. 18,294 A1 (slide 30,282). (K) Vertically spaced rounded pits. (L) Partly biseriate pitting with opposite pits. (M) Uniseriate crowded pits. (N) Detail of numerous, small crowded cross-field pits. Scale bar in (A) 1 mm, (B) 250 µm, (C), (D), (F), (H), (J) 100 µm, (E), (G), (I) 50 µm, (K), (L), (M), (N) 25 µm.
Fig. 4.
Fig. 4.
Triassic trees with epicormic shoots from Gordon Valley: variations of tracheid lumen radial diameter (black line) vs. wall thickness (grey line) along a typical growth ring. Arrows indicate the location of latewood (sensu Mork, 1928; Denne 1989, formula 2, i.e. where the radial diameter of the tracheid lumen becomes smaller than the thickness of the two succeeding cell walls).
Fig. 5.
Fig. 5.
Triassic trees with epicormic shoots from Gordon Valley: variations of growth ring size (mm) along a transverse section in each specimen. (A) 18,291. (B) 18,292. (C) 18,294.
Fig. 6.
Fig. 6.
Triassic trees with epicormic shoots from Gordon Valley: shoot anatomy. (A) View of a cross-section of wood showing several epicormic traces (S) with an oblique to horizontal trajectory. The shoot on the right is branching (arrow). Note regularly spaced sclerotic nests in the pith of the shoots (Sn). 18,292 Dbot1 (slide 30,292). (B) Cross-section of a shoot at a level with no sclerotic nest in the pith. 18,294 Btop1. (C) Oblique view of a shoot showing some sclerotic nests in the pith. 18,292 Cbot1 (slide 30,291). (D) Oblique view of a shoot for which only the secondary xylem is visible. Note that there are no conspicuous frost rings associated with the trace. 18,291 Cbot1 (slide 30,291). (E) Detail of a shoot seen in longitudinal section, with an alternation of parenchyma and sclerotic nests in the pith. 18,292 Cbot1 (slide 30,291). (F) Detail of the pith on (E) 18,292 Cbot1 (slide 30,291). (G) Shoot producing a trace to a lateral organ (lt). 18,294 A6 (slide 30,286). (H) Transverse-oblique section of a shoot showing a large sclerotic nest (Sn) in the pith. 18,294 B3 (slide 30,289). (I) Longitudinal section of a shoot showing the pith and secondary xylem with uniseriate pits on the tracheid walls. 18,294 A6 (slide 30,286). Scale bars: (A) 1 cm; (B–D) 500 µm; (E, G) 250 µm; (H) 100 µm; (F) 50 µm.

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