Upland deforestation triggered an ecosystem state-shift in a kettle peatland

Background/Question/Methods

European settlement of eastern North America resulted in dramatic changes to ecosystems, although the dynamics and underlying causes of these changes are not always obvious.  For example, land clearance likely exposed soils to increased wind erosion, potentially impacting downwind ecosystems indirectly through enhancement of dust deposition.  We hypothesized that otherwise undisturbed wetlands were altered through this indirect disturbance mechanism, increasing nutrient availability and initiating a cascade of ecosystem-level changes.  We tested this hypothesis in a floating kettle peatland located in northwestern Pennsylvania, USA using an interdisciplinary approach.  A series of peat cores were collected along a transect oriented parallel to the dominant wind direction and paleoecological techniques were used to identify signatures of upland deforestation and mineral matter deposition within the peat profiles.  Elemental analyses were used to reconstruct historic availability of major macronutrients, plant macrofossils and tree rings (Pinus strobus) were used to reconstruct wetland plant communities and the timing of tree recruitment on the peatland surface, and testate amoebae were used as a proxy for microbial-community dynamics. 

Results/Conclusions

Strong correlations between the concentrations of ragweed (Ambrosia) pollen and fine-grained mineral matter linked upland deforestation to enhanced dust deposition on the peatland surface.  Elemental analyses indicated that nitrogen, phosphorus, and potassium concentrations increased coincident with dust deposition.  Plant communities shifted from Sphagnum-dominance to vascular-plant-dominance coincident with enhanced dust deposition, including increased recruitment of Pinus strobus onto the peatland.  Testate amoeba communities shifted toward those adapted to highly variable micro-environmental conditions, and likely reflect broader changes in microbial communities.  Together, all data suggest that upland deforestation by European settlers triggered a cascade of ecological changes on a nutrient-poor peatland by enhancing dust deposition and nutrient delivery on the surface.  These results demonstrate that indirect, unintended, and often overlooked human disturbances can lead to dramatic structural and functional alterations of carbon-rich wetland ecosystems, highlighting the potential vulnerability of these systems in human-dominated landscapes.