Reconciling terrestrial and lacustrine paleoecosystem records in North America

Background/Question/Methods Evaluating the direction and magnitude of recent anthropogenic change to ecosystems requires long-term (decadal to millennial scale) records of past ecosystem function. Paleoecologists are beginning to develop tools to reconstruct ecosystem properties from sediment cores, tree cores, and museum specimens. The interpretation of stable nitrogen isotopes along with other proxy records can provide a useful record of nitrogen availability in the past. The objectives of this study were: 1) to determine century- to millennial-scale trajectories for nutrient cycling, 2) to investigate the influence of disturbance by wind and fire on nitrogen cycling, and 3) to evaluate evidence for ecosystem response to increased resources (either nitrogen saturation or progressive nitrogen limitation) during the 20th century. To accomplish these objectives, I reconstructed terrestrial nitrogen cycling using stable nitrogen isotopes from lacustrine, foliar, and dendrochronological records at four sites in North America. These complementary approaches yielded records from four different locations including coniferous forest, deciduous forest, and grassland systems: 1) Mirror Lake, New Hampshire, 2) Deming Lake, Minnesota, 3) Indiana, USA, and 4) Kansas, USA. The records also have different durations: 10, 100, 1000, and 10000 years. Results/Conclusions At each site, trajectories of nitrogen availability prior to the onset of Euro-American land use were dynamic, not static. Wind disturbance (Mirror Lake, NH) appeared to have minimal affect on nitrogen cycling, while fire disturbance (Deming Lake, MN) produced significant cyclicity in nitrogen availability associated with forest succession. Our results indicate decreases in terrestrial nitrogen availability during the 20th century at several sites. Mechanisms for this decrease may be different at each site, but taken together these pervasive declines in nitrogen availability are consistent with the progressive nitrogen limitation hypothesis. I interpret these paleorecords of terrestrial ecosystem change within the context of ecosystem development theory generated from classic chronosequence studies.