Polar forests on the edge of extinction: what does the fossil spore and pollen evidence from East Antarctica say?

摘要

Diverse pollen and spore assemblages, spanning the Late Eocene preglacial-glacial transition, have been recovered from Ocean Drilling Program cores from Prydz Bay, East Antarctica. These microfloras are mostly in situ and provide an unparalleled record of terrestrial plant communities growing in Antarctica during the earliest stages of ice-cap formation. The evidence provides a basis for assessing the phytogeographic relationships of the Antarctic floras with other high-latitude floras in the southern hemisphere, including possible migration routes for some taxa. Preliminary studies (Macphail and Truswell 2004a) suggested the Late Eocene vegetation at Prydz Bay was floristically impoverished rainforest scrub, similar to Nothofagus-gymnosperm communities found near the climatic treeline in Patagonia and Tasmania. Re-evaluation of the microfloras indicates the diversity of shrubs, especially Proteaceae, was underestimated and the Late Eocene vegetation was a mosaic of dwarfed (krumholtz) trees, scleromorphic shrubs and wetland herbs, analogous to the taiga found in the transition zone between the boreal conifer forest and tundra biomes across the Arctic Circle. Microfloras similar to although much less diverse than the Prydz Bay assemblages occur in coreholes from the Ross Sea region on the opposite side of Antarctica. Interpretation of the latter is complicated by reworking and low yields but the combined evidence points to the collapse of taller woody ecosystems during the Eocene-Oligocene transition and their replacement by tundra-like or fell-field vegetation during the Oligocene and Neogene. This temperature-forced regression seems to have been broadly synchronous across the continent. The high-palaeolatitude location (similar to 70 degrees S) means that the Prydz Bay flora was adapted to several months of winter darkness and short-summer growing seasons. The nearest living relatives of identifiable woody taxa suggest year-round high humidity, with an annual precipitation between similar to 1200 and 1500 mm. Palaeotemperatures are more difficult to quantify although the inferred humid microtherm climate is consistent with mean annual temperatures less than 12 degrees C and freezing winters.