The fossil plant record and global climatic change

Abstract

The generally sedentary character of terrestrial plants gives them a special dependence on their adaptation to the climate under which they live. As a consequence, plants normally show structural adaptations which are characteristic of their habitat, and fossil plants constitute particularly sensitive palaeoenvironmental indicators. In Quaternary pollen analysis the assumption is generally made that the species recognised as pollen had the same climatic constraints as their present-day representatives. As we go back through Tertiary time, and extant species become progressively rarer, we seek the nearest living relatives of the plant fossils as a basis for palaeoclimatic interpretation. This approach relies on the accuracy of the taxonomic assignment of the fossil material. Various ‘non-taxonomic’ features of Tertiary fossils have also been used in attempts to read a ‘palaeoclimatic signal’, independent of the correctness of the taxonomic assignment. These include most notably leaf physiognomy, and growth responses to seasonality such as growth rings in fossil wood. When we look to Palaeozoic plants, even leaf physiognomic features are of limited value, but fossil plants of this age can still give us significant information about their palaeoenvironment. The presence of charcoal (fusain) produced by wildfire puts a constraint on the level of oxygen in the palaeoatmosphere. Stomatal density and index may be used to give a proxy measure of palaeo-CO 2 levels. The realisation of the link between the carbon-dioxide greenhouse phenomenon and climate makes the use of stomatal data from fossil plants of particular relevance to palaeoclimatic interpretation. Our results from a study of stomatal index in plants from the devonian to Permian interval are consistent with evidence from physical sources of major changes in global CO 2 levels through that period.