Permian and Triassic Tree Rings as High Latitude Paleoclimate Indicators

Fossil wood from the Late Permian and early Middle Triassic of the central Transantarctic Mountains of Antarctica provides information on a growth environment with no modern analog. This period in Earth's history represents a time of global warming with vegetation growing at high latitudes. Gymnospermous wood found at polar latitudes (75-85º S) in the Permian and Triassic is similar to that seen in extant conifer woods. Dendrochronology and wood anatomical techniques utilized on extant wood to determine growth conditions and environmental stresses were used on the fossil wood. Ring widths for both periods ranged from 1.69-2.3 mm with some ring widths reaching nearly 10 mm. The large amount of earlywood and extremely small amount of latewood is not seen in any extant plants, illustrating a unique environment not present on the Earth today. With knowledge of growth habitat from extant tree rings, the resulting environment for high latitudes during the Permian and Triassic would be that of a long growing season with a rapid transition to dormancy. The rare presence of frost or false rings is an indicator that the seasons in which the rings were formed were not often subject to temperature extremes. The large diameter of the tracheids in the earlywood (45-97 μm) would also suggest that water availability was not a factor in the formation of ring boundaries in this fossil wood. The factor most likely influencing the formation of rings in these fossil woods is light availability. At polar paleolatitudes the sun was already at a low angle and, as it dropped lower in the fall, the plants quickly became dormant and wood production ceased. Fossil tree ring structure from the Permian and Triassic of Antarctica is not similar to any extant wood, but may be similar to growth conditions in a future greenhouse environment.