Sensitivity of north temperate lakes to past climatic variability and terrestrial disturbances over the past ca. 200 years
Understanding how freshwater ecosystems respond to changes in climatic drivers and interactions of climate with terrestrial disturbances over time are critical challenges in contemporary ecology. Paleoecological and paleolimnological research in conjunction with historical archival research, regional drought records, published climatic reconstructions, and data from the North Temperate Lakes LTER program were used to reveal changes in historical fire regimes and interactions of fire and land clearance disturbance with climatic variability over the past ca. 200 years. Charcoal and pollen records from lake sediment cores were used to generate local disturbance histories around six northern Wisconsin lakes and event analyses were used to evaluate synchronicity of local fire disturbance with regional drought and global ENSO activity. Lake ecosystem responses to past climatic events and local disturbance events of fire and logging were investigated using paleolimnological analyses of sediment carbon and algal pigment concentrations.
Fire episodes were synchronous with drought events over a time period prior to historical clear-cut logging (1810-1889 C.E.) and were not synchronous with drought over subsequent time periods. Fire episodes occurred closely in time (within < five years of one another) at all five sites only twice (ca. 1890 - 1894 C.E. and ca. 1914 - 1916 C.E.) over the 200-year record. This turn of the century time period coincided with intensive regional land clearance and years of very strong to extreme La Niña, suggesting that the clearcut logging of this period favored more widespread fire that may have been more responsive to very strong La Niña conditions. Change-point analyses of sediment carbon burial rates reveal significant change-points in several lakes over this same time period while multivariate analyses of algal pigment concentrations show significant shifts in community structure (MANOVA, p < 0.001) in lakes of low landscape positions over this same period of intensive land clearance and very strong ENSO activity. Results suggest that lakes occupying lower landscape positions may be more responsive to landscape disturbance than lakes in higher landscape positions but that lakes in both high and low positions show sensitivity to the combined effects of land clearance, strong ENSO activity, and multiyear droughts.