Biotic responses to rapid climatic changes

Qualitative and quantitative changes in fossil flora and fauna have been used in many studies to infer climatic change. Here we ask a different question: how do flora and fauna respond to climatic changes such as rapid warming or cooling? As an independent proxy for paleotemperature we take the ratio of oxygen isotopes in biogenically precipitated lake marl and in ostracod shells. This introductory paper describes the project design and the five sites on an altitudinal transect from 600 m to about 2300 m asl in the western Swiss Alps. As cases of climatic cooling and warming we use the beginning and end of the Younger Dryas as major changes, and the Gerzensee and Preboreal oscillations as minor changes. At the two sites of Gerzensee and Leysin these changes are recorded in stable-isotope ratios, and there the time scales can be derived by correlations to the GRIP ice core (Schwander et al., 2000 and von Grafenstein et al., 2000). Biotic responses to climate changes are treated in individual papers using pollen (Wick, 2000), plant macrofossils (Tobolski and Ammann, 2000), and remains of chironomids (Brooks, 2000), beetles and other insects (Lemdahl, 2000), and chydorid Cladocera (Hofmann, 2000). They are followed by a synthesis focusing on quantification of biotic responses (Ammann et al., 2000). In addition, a reconstruction of summer temperatures for the Allerød and the Younger Dryas at Gerzensee is provided by Lotter et al. (2000).

Quantification of biotic responses to rapid climatic changes around the Younger Dryas — a synthesis

To assess the presence or absence of lags in biotic responses to rapid climatic changes, we: (1) assume that the δ18O in biogenically precipitated carbonates record global or hemispheric climatic change at the beginning and at the end of the Younger Dryas without any lag at our two study sites of Gerzensee and Leysin, Switzerland; (2) derive a time scale by correlating the δ18O record from these two sites with the δ18O record of the GRIP ice core; (3) measure δ18O records in ostracods and molluscs to check the record in the bulk samples and to detect possible hydrological changes; (4) analyse at Gerzensee and Leysin as well as at two additional sites (that lack carbonates and hence a δ18O record) pollen, plant macrofossils, chironomids, beetles and other insects, and Cladocera; (5) estimate our sampling resolution using the GRIP time scale for the isotope stratigraphies and the biostratigraphies; and (6) summarise the major patterns of compositional change in the biostratigraphies by principal component analysis or correspondence analysis. We conclude that, at the major climatic shifts at the beginning and end of the Younger Dryas, hardly any biotic lags occur (within the sampling resolution of 8–30 years) and that upland vegetation responded as fast as aquatic invertebrates. We suggest that the minor climatic changes associated with the Gerzensee and Preboreal oscillations were weakly recorded in the biostratigraphies at the lowland site, but were more distinct at higher altitudes. Individualistic responses of plant and animal species to climatic change may reflect processes in individuals (e.g. productivity and phenology), in populations (e.g. population dynamics), in spatial distributions (e.g. migrations), and in ecosystems (e.g. trophic state). We suggest that biotic responses may be telescoped together into relatively short periods (50 to 150 years), perhaps disrupting functional interactions among species and thus destabilising ecosystems.

Macrofossils as records of plant responses to rapid Late Glacial climatic changes at three sites in the Swiss Alps

Plant macrofossils from the end of the Younger Dryas were analysed at three sites, Gerzensee (603 m asl), Leysin (1230 m asl), and Zeneggen (1510 m asl). For the first two sites an oxygen-isotope record is also available that was used to develop a time scale (Schwander et al., this volume); dates refer therefore to calibrated years according to the GRIP time scale. Around Gerzensee a pine forest with some tree birches grew during the Younger Dryas. With the onset of the isotopic shift initiating the rapid warming (about 11,535 cal. years before 1950), the pine forest became more productive and denser. At Leysin no trees except some juniper scrub grew during the Younger Dryas. Tree birches, pine, and poplar immigrated from lower altitudes and arrived after the end of the isotopic shift (about 11,487 B.P.), i.e., at the beginning of the Preboreal (at about 11,420 B.P.). Zeneggen is situated somewhat higher than Leysin, but single tree birches and pines survived the Younger Dryas at the site. At the beginning of the Preboreal their productivity and population densities increased. Simultaneously shifts from Nitella to Chara and from silt to gyttja are recorded, all indicating rapidly warming conditions and higher nutrient levels of the lake water (and probably of the soils in the catchment). At Gerzensee the beginning of the Younger Dryas was also analysed: the beginning of the isotopic shift correlates within one sample (about 15 years) to rapid decreases of macrofossils of pines and tree birches.