Permian and Triassic tree ring analysis provides some of the first data on plant response to the environment across a global transition from icehouse to greenhouse.  Extensive growth is observed from both periods in an environment for which there is no modern analog. Warm temperatures and the presence of adequate water suggest that ring formation and wood growth is mediated by a parameter not observed in modern tree ring formation. Ring structure, especially the proportion of earlywood and latewood in each ring, suggests that light availability was most likely controlling wood production and tree ring formation in Late Permian and Middle Triassic forests in the central Transantarctic Mountains. The extreme seasonal light and dark cycles in this polar environment were no doubt also a major factor contributing to the deciduous nature of both Permian glossopterid and Triassic corystosperm seed ferns in these forests.