Temporal ecology and biogeochemistry across millennia: A paleoecological approach
The emerging field of temporal ecology has identified non-stationarity and temporal scaling as two important features requiring further study. Biogeochemical processes exemplify these features, and additionally, they are often slow and difficult to observe on the timescale of one human lifetime. Paleorecords can be used to access these processes. For example, insight into controls on nitrogen cycling and patterns of availability has been recently provided by century- to millennial-scale reconstructions of biogeochemistry from lacustrine sediment records. However, there are a large number of unanswered questions about the non-stationarity and temporal scaling of nutrient dynamics as seen in paleorecords. The main objective of this study was to investigate the biogeochemical consequences of primary and secondary ecosystem succession of forested areas in North America as reconstructed from lacustrine sedimentary records. Secondary objectives were: (1) to evaluate coherence of biogeochemical records from local to regional and global spatial scales, (2) to examine features of temporal data that allow detection of a biogeochemical response to a disturbance, and (3) to identify key ecosystem characteristics that confer biogeochemical resilience to disturbance events.
Elemental concentrations and stable nitrogen isotopes in lacustrine paleorecords demonstrate a wide variety of biogeochemical trajectories along primary successional pathways. Some of these trajectories correspond with theories developed from chronosequences, but some do not. A high degree of spatial variability among records indicates that regional-scale factors may be influencing these trajectories. There is less information about biogeochemical responses during secondary succession, but generally there is evidence for both initial responses and century-scale alterations to the nitrogen cycle if the disturbance event signal is strong enough. Finally, I introduce the concept of the partitioning ratio (the ratio of nitrogen stocks in the soil to aboveground biomass) as a key ecosystem characteristic that may confer biogeochemical resilience to disturbance events. Evaluation of this concept with data from forested systems in North America indicates that the partitioning ratio is at least partially supported, but further work needs to be done to test the stability of this ratio and mechanisms of biomass loss during disturbance events.