Scott D. Bridgham

Manipulative Climate Change Experiment on Peatlands in Minnesota

“Carbon and Energy Flow and Plant Community Response to Climate Change in Peatlands” (National Science Foundation DEB9707426), $1,200,000, Aug. 1, 1997–July 31, 2003. (Renewal of DEB9496305, $800,000, July 1993 - June 1997).

Principal Investigators:  Scott D. Bridgham (University of Oregon), John Pastor (University of Minnesota, Duluth), Jiquan Chen (University of Toledo)

Collaborators:  Jake Weltzin (University of Tennessee), Jeffrey White (Indiana University), Robert Shannon (University of Pennsylvania)

Graduate Students:  Carmen Chapin (USDA-ARS), Jason Keller (Notre Dame)

Technicians:  Karen Updegraff (South Dakota State University), Cal Harth, Brad Dewey (University of Minnesota, Duluth), Jason Keller (Notre Dame), Mark Schmisek, Kathleen Lysyshyn

   From 1994-2002, we examined the effects of climate change on the whole-ecosystem response of peatlands by controlling infrared energy inputs and water-table level in a bog and fen in northern Minnesota.  The goal of this research was to identify and quantify the relative importance of internal and external processes and feedbacks that govern peatlands in a changing global environment. 

   In 1993-94, we constructed 54 mesocosms (2.1-m2 surface area, ~0.6-m depth) in insulated plastic tanks that were buried in the ground at a field station in northeastern Minnesota.  Each tank was filled with a single frozen, minimally disturbed peat monolith with intact vegetation from a bog or fen.  The mesocosms were heated with overhead infrared lamps at ambient, ~ +40 W m-2, and ~ +90 W m-2 above ambient. Soil temperature increased in the heated plots approximately 1.6 to 4.1 ºC above ambient during the growing season, well within the range of temperature increases predicted by global climate models.  Heating treatments and energy flux measurements continued on a year-round basis.  Additionally, we had three hydrology treatments with water tables set weekly at ~ -3, -16, and -25 cm below the mean initial peat surface.  Water tables were controlled by a simple manostat system.  Each combination of heating and water tables was crossed in a full factorial design with three replicates.

   Given our focus on feedbacks and interactions that control the response of peatlands to climate change, we have measured a broad variety of variables in each plot, including changes in total carbon accumulation, methane fluxes, ecosystem respiration (ER), net ecosystem production (NEP), net and gross primary production (NPP, GPP), dissolved organic carbon (DOC) budgets, cover, production, and photosynthetic response of individual plant species, canopy light attenuation and geometry, changes in surface microtopography, nutrient availability with exchange resins, nutrient concentrations in pore water and vegetation, nutrient budgets, and detailed energy budgets.  To derive annual carbon fluxes from instantaneous measurements, we have also developed phenomenological models of ER, CH4 flux, NEP, and GPP as functions of soil and air temperature, water table, canopy phenology, and diurnal photosynthetic response.  Thus, we have accurate estimates of changes in carbon storage and the individual components of the carbon budget (GPP, ER, NEP, CH4, NPP, and DOC flux) as a function of nine different climate scenarios in a bog and fen. 

   The most important findings of this experiment are: (1) Bogs and fens have different, intrinsic water-table equilibria, and they very rapidly gain or lose carbon if they are put into disequilibrium with this water-table level.  (2) Correspondingly, the carbon balances of bogs and fens responded in opposite directions to heating and changes in water tables.  (3) These treatments caused large changes in plant community composition, with warmer, drier conditions favoring shrub dominance at the expense of other plant life forms, particularly mosses.  (4) However, compensatory responses of plant species and above- and belowground productivity to warming and water-table manipulations result in small, or no, changes in overall productivity.  (5) Methane emissions were strongly correlated with nitrogen availability, plant productivity, soil temperature, and water table height, whereas respiratory carbon dioxide fluxes were controlled almost entirely by soil temperature. (6) Net radiative fluxes and soil temperatures differ between bogs and fens at the same infrared loading, and these differences are a result of biotic feedbacks.

   See curriculum vitae button on home page for publications resulting from this research.

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