Antiquity and turnover of North American terrestrial ecosystems: Multivariate and ROC analyses of North American pollen data

Background/Question/Methods

We assessed i) antiquity existing ecosystems, and ii) patterns of appearance, persistence, and disappearance of ecosystems since the Last Glacial Maximum. These questions are particularly important for understanding environmental controls of ecosystem properties and the persistence of terrestrial plant ecosystems under future global change scenarios. Pollen records in the Neotoma Paleoecology Database, which document vegetational history in North America over the past 21,000 years were used to address these questions. We undertook two complementary approaches, multivariate analysis and receiver operating characteristic (ROC) analysis, to objectively identify plant ecosystem types from fossil pollen assemblages. The transitions between ecosystem types were identified for pollen sequences at various sites in the database. We mapped the origination and termination of ecosystem types at individual sites to investigate the spatial and temporal pattern of ecosystem turnover.

Results/Conclusions

Our results show that: i) Most modern ecosystems originated within the past several thousand years, and all ecosystems are sensitive to environmental change of the magnitude experienced since the last glacial period. ii) At time scales of decades to millennia, changes in ecosystems generally correlate with changes in climate. The ecosystem turnover rate averaged over North America through time showed the same pattern as the magnitude of climate change through the glacial-interglacial period, including abrupt climatic events such as the Younger Dryas at 11-12k BP and the gigadrought at 4.2k BP. As a result of this correlation between environment and ecosystem, the timing of origination and duration of different ecosystems at different places shows some systematic pattern. iii) Ecosystems in some areas persist longer than in other areas. These findings from paleoecological records help us assess the historical range of environmental conditions under which ecosystems are maintained, and identify critical environmental conditions beyond which modern ecosystems may not be sustainable.