Tree growth at polar latitudes based on fossil tree ring analysis

Permineralized trunks and mature wood samples with well-preserved growth rings are described and analyzed from the Upper Permian and Middle Triassic of the central Transantarctic Mountains. This fossil wood is unique in that the plants lived in an environment with no modern analogue and exhibited luxuriant tree growth above 75°S paleolatitude. Ring width averages 1.69–2.3 mm, with maximum width of 6.83–9.9 mm, an order of magnitude larger than ring widths produced at near-polar latitudes today. Tree rings in both the Permian and Triassic woods show similar structure, consisting almost entirely of earlywood (spring wood), with between 0–12% of each ring classified as latewood (summer wood). The small amount of latewood (0–6 cells) indicates a very rapid transition to seasonal dormancy, probably in response to decreasing light levels at these paleolatitudes. In order to accurately delimit the earlywood–latewood boundary, a comparison was done of classical dendroclimatological techniques and alternative techniques utilized primarily by paleobotanists analyzing fossil woods. We found that classical wood anatomy techniques provided a more accurate explanation of wood development and tree growth for these high-latitude samples. The suggested cool-temperate Late Permian Glossopteris flora from this area differs substantially from the warm-temperate Middle Triassic corystosperm flora (leaf type, Dicroidium) and very different paleoclimates have been reconstructed for these two time periods. Ring structure and wood growth from both sites, however, are very similar, indicating that these plants were responding to the environment in very similar ways. The structure of the tree rings, including a large number of earlywood tracheids and a very low number of latewood cells, provides evidence that growth at these polar latitudes was limited by light levels rather than water and temperature as occurs in modern high-latitude forests. These fossil tree rings have important implications for understanding woody shoot growth and cambial function at high latitudes during periods of global warmth.