Chronosequence and paleoecological studies are complementary for elucidating the patterns and drivers of ecosystem processes occurring at time scales beyond human observation. They also offer indispensible baseline information against which recent and ongoing changes in biogeochemical cycles can be evaluated. This presentation highlights some of the key findings regarding biogeochemical processes derived from chronosequence and lake-sediment records.
Autogenic processes, climate change, and disturbances interact to control biogeochemical cycles at centennial to millennial time scales. These drivers of long-term ecosystem change have different histories, leading to divergent trajectories and varying tempos of different biogeochemical cycles. Chronosequence studies reveal that as a result of autogenic processes, retrogressive forest succession during the late stages of interglacial periods results in a decline in biomass, an increase in P limitation relative to N, and a reduction in the rates of litter decomposition and soil respiration. Paleolimnological studies offer additional details on the role of autogenic changes during long-term ecosystem development. For example, lake alkalinity declined gradually as dissolved organic carbon rose, N experienced a transient increase, and P did not change directionally over the course of the past 10,000 years in Glacial Bay, Alaska. These autogenic changes operate within the boundary conditions set by climate and disturbance regimes. A large dataset of basal 14C ages from northern peatlands indicates that the initiation of carbon storage occurred primarily in response to climatic warming and ice-sheet retreats after the termination of the last Ice Age. In contrast, the most prominent increase in nitrogen cycling occurred much later in certain regions as a result of the pervasive invasion of N-fixing alder in response to increased effective moisture (precipitation – evaporation). Large-scale climate forcings, such as solar output and the Pacific Decadal Oscillation, may synchronize regional-scale variations in soil decomposition, nutrient fluxes, and aquatic productivity. Superimposed on climate-induced changes, anthropogenic and natural disturbances alter patterns of terrestrial biogeochemical cycling and lake evolution. Recent increases in fire activity in northern high latitudes deserve special attention. They may represent a novel expression of ongoing climatic change and foreshadow more dramatic impacts to occur if the warming trend continues. In particular, widespread occurrence of tundra fires has potential to trigger sudden releases of tundra soil carbon, leading to wholesale changes in ecosystem biogeochemical cycles.