Cenozoic paleotemperatures and leaf physiognomy — A European perspective
For 21 European leaf-floras (with a focus on Central Europe), which span a stratigraphic range from the Late Eocene to the Pliocene, paleoclimate estimates have been calculated using five different quantitative techniques: (a) leaf margin analysis (LMA1), using a regression model based on data from East Asia, (b) the multivariate Climate Leaf Analysis Multivariate Programm (CLAMP) technique, based on data from East Asia and N.-America, (c) a recently developed multivariate technique based on modern European vegetation (ELPA), (d) a provisional LMA regression model based on the vegetation of several wet localities from N.-America (LMA2), and e) the Coexistence Approach (CA), a technique based on comparisons with the nearest living relatives (NLRs) of fossil taxa. According to our results there seems to be certain discrepancies where the different techniques are compared for paleotemperatures estimates, depending mainly on the stratigraphic age of the floras. For Paleogene floras, both multivariate leaf physiognomic techniques are in rather good agreement with CA, although both techniques may differ considerably from each other. In contrast, for the Neogene, CLAMP shows a tendency to produce estimates that are considerably colder than CA, whereas ELPA, provides generally warmer estimates, and is in better agreement with CA and other independent evidence. Our results and interpretations add some caveats to temperature reconstructions based on leaf physiognomy, especially when applied to European floras from older periods (i.e. Paleogene, Cretaceous). Possible changes of the relationship between climate and leaf physiognomy over time should be taken into account as a possible source of error whenever such techniques are used. There is the possibility that the actual correlation between climate and leaf form may be modified by long-time evolutionary responses or floral changes, leading to erroneous paleoclimate estimates, if a calibration data-set is used, which is not suited for the region and time-interval in question. However, further research will be needed to test whether such changes in the relationship between climate and leaf physiognomy over time can also be detected on other continents, or whether this is a problem restricted to Europe.