CRETACEOUS POLAR CONIFER FORESTS: ASSESSING LEAF HABIT FOR CLIMATE MODELLING

Climate models used to simulate past climates have commonly generated ice-covered polar regions. However, abundant fossil wood indicates that during the Cretaceous the polar regions were covered with dark dense forests. These forests would have significantly modified the climate on both regional and global scales due to their low albedo and their effect on the land-surface heat budget, the hydrological and carbon cycles. The deciduous or evergreen habit of the conifers and the length of time they held their leaves would have played an important part in this feedback. In the past it has been assumed that these forests would have been deciduous due to the long periods of winter darkness they would have endured.

A new method has been developed to determine the evergreen or deciduous habit of fossil polar conifers by analysing the cell patterns within growth rings in fossil wood to determine how long the trees kept their leaves. Analysis of Cretaceous fossil wood from the Arctic (Canadian Arctic Archipelago and Svalbard) and the southern polar regions (Antarctica and Australia) indicates that the forests were composed of a mixture of both deciduous and evergreen conifers. These results have been compared to outputs of the Sheffield University Conifer Model, which models the growth of forests at high latitudes. The model predicts evergreen and deciduousness values similar to those seen within the fossil record. These results have important implications for climate modelling, allowing us to include accurate representations of polar vegetation. The presence of a high proportion of evergreen trees indicates that the seasonal effects produced by these forests would not have been as large as if only deciduous trees were present. The Conifer Model will now be coupled with regional and global climate models to examine the effect of polar forest vegetation on global climate during past greenhouse climates.