Karen Updegraff

RESPONSE OF BOG AND FEN PLANT COMMUNITIES TO WARMING AND WATER-TABLE MANIPULATIONS

Large-scale changes in climate may have unexpected effects on ecosystems, given the importance of climate as a control over almost all ecosystem attributes and internal feedbacks. Changes in plant community productivity or composition, for example, may alter ecosystem resource dynamics, trophic structures, or disturbance regimes, with subsequent positive or neg- ative feedbacks on the plant community. At northern latitudes, where increases in temperature are expected to be greatest but where plant species diversity is relatively low, climatically mediated changes in species composition or abundance will likely have large ecosystem effects. In this study, we investigated effects of infrared loading and manipulations of water-table ele- vation on net primary productivity of plant species in bog and fen wetland mesocosms between 1994 and 1997. We removed 27 intact soil monoliths (2.1 m2 surface area, 0.5-0.7 m depth) each from a bog and a fen in northern Minnesota to construct a large mesocosm facility that allows for direct manipulation of climatic variables in a replicated experimental design. The treatment design was a fully crossed factorial with three infrared-loading treatments, three water-table treatments, and two ecosystem types (bogs and fens), with three replicates of all treatment combinations. Overhead infrared lamps caused mean monthly soil temperatures to increase by 1.6-4.1?C at 15-cm depth during the growing season (May-October). In 1996, depths to water table averaged -11, -19, and -26 cm in the bog plots, and 0, -10, and -19 cm in the fen plots. Annual aboveground net primary production (ANPP) of bryophyte, forb, graminoid, and shrub life-forms was determined for the dominant species in the mesocosm plots based on species- specific canopy/biomass relationships. Belowground net primary production (BNPP) was esti- mated using root in-growth cores. Bog and fen communities differed in their response to infrared loading and water-table treatments because of the differential response of life-forms and species characteristic of each community. Along a gradient of increasing water-table elevation, production of bryophytes increased, and production of shrubs decreased in the bog community. Along a similar gradient in the fen community, production of graminoids and forbs increased. Along a gradient of in- creasing infrared loading in the bog, shrub production increased whereas graminoid production decreased. In the fen, graminoids were most productive at high infrared loading, and forbs were most productive at medium infrared loading. In the bog and fen, BNPP:ANPP ratios increased with warming and drying, indicating shifts in carbon allocation in response to climate change. Further, opposing responses of species and life-forms tended to cancel out the response of production at higher levels of organization, especially in the bog. For example, total net primary productivity in the bog did not differ between water-table treatments because BNPP was greatest in the dry treatment whereas ANPP was greatest in the wet treatment. The differential responses of species, life-forms, and above- and belowground biomass pro- duction to the treatments suggest that bog and fen plant communities will change, in different directions and magnitudes, in response to warming and changes in water-table elevation. Further, results of this and complementary research indicate that these peatlands may mediate their energy, carbon, and nutrient budgets through differential responses of the plant communities. Thus, predictions of the response of peatlands to changes in climate should consider differences in plant community structure, as well as biogeochemistry and hydrology, that characterize and differentiate these two ecosystems.

RESPONSE OF CO2 AND CH4 EMISSIONS FROM PEATLANDS TO WARMING AND WATER TABLE MANIPULATION

Projected changes in climate could shift northern peatlands from their current status as net C sinks toward that of being net C sources by changing soil temperatures and hydrology. We assessed the importance of water table and soil temperature as controls over ecosystem respiration in a bog and sedge fen in northern Minnesota, USA, by means of a manipulative mesocosm experiment. Fifty-four intact monoliths were removed from a bog and a fen and installed in insulated tanks that permitted control of the water table and were heated by overhead infrared heaters. The experimental design was a fully crossed factorial combination of two communities, three water tables, and three heat levels. Ecosystem respiration as indicated by emission of CO2 and CH4, dissolved nutrient fluxes, and productivity were measured and summarized for each growing season from 1995 to 1997. Seasonal ecosystem respiration (ER) as indicated by CO2 emissions responded almost exclusively to soil temperature and did not differ between community types (∼630 g C/m2) or with water table level. These results suggest that community type, within certain limits, will not be an important factor in predicting temperature-driven increases in ER. The response of CH4 flux to soil temperature and water table setting became progressively stronger in each succeeding growing season. Seasonal CH4 emissions were on average three times higher in the bog than in the fen mesocosms (21 vs. 7 g C/m2). Aboveground net primary productivity and dissolved N retention were also higher in the bog mesocosms. There were strong correlations between CH4 flux and N retention, but generally weak correlations between CH4 and plant primary production. The relatively lower CH4 emissions from the fen mesocosms appear to result mainly from higher rates of methanotrophy in the aerated zone, possibly reinforced by the effects of higher porewater N concentrations and lower primary productivity compared to the bogs. The results confirm the existence of strong environmental controls over ER and methanogenesis, which are modulated by complex interactions between plant community and soil nutrient dynamics. The differential responses of these ecosystem functions to climate change may complicate efforts to predict future changes in C dynamics in these important repositories of soil C.

Potential feedbacks of northern wetlands on climate change

Atmospheric concentrations of many greenhouse gases, in particular carbon dioxide and methane, are rapidly increasing from preindustrial levels. This paper explores the potential feedback from northern wetlands, particularly with regard to carbon dioxide and methane flux, to climate change. Although progress has been made in determining current trace gas fluxes from major biomass, the authors believe that a mechanistic approach is necessary to predict the direct and indirect feedbacks between climate and northern wetlands. Relevant areas covered in the article include the following: feedback mechanisms; carbon and nutrient mineralization; the kinetics of carbon and nutrient mineralization; methane oxidation and the role of plants; field dynamics of gases in a meadow and bog in Minnesota; and extrapolating to larger scales. 80 refs., 8 figs., 1 tab.

NITROGEN, PHOSPHORUS, AND CARBON MINERALIZATION IN RESPONSE TO NUTRIENT AND LIME ADDITIONS IN PEATLANDS

This objective of this study was to determine if mineralization of C, N, and P was pH stressed and/or nutrient limited in a bog and fen in northeastern Minnesota. Although soil activity in northern peatlands can be limited by low pH, low temperatures, high C:N and C:P ratios of soil organic matter, and/or anaerobic conditions, different classes of peatlands (bog and fen) vary in the type and degree of stress affecting soil processes. We employed in situ studies, laboratory incubations, and a fertilization and liming experiment to understand peatland mineralization dynamics further. Nitrogen mineralization in the field was higher in the fen than in the bog, but net P mineralization was close to zero in both sites. Soil cores were removed from plots that had been treated for 2 years with NH4+, PO4−2, and/or CaCO3, followed by a 30-week lab incubation. Nutrient additions in the bog increased the labile N pool (N0), and cumulative N and P mineralization over 30 weeks, but lime addition had no effect. Nutrient additions to the fen did not significantly alter nutrient pool sizes (N0, P0), cumulative mineralization totals, or mineralization rates. However, lime additions decreased potentially mineralizable N and cumulative N mineralization in the fen aerobic incubations, but increased cumulative P mineralization in the anaerobic incubations. Although both are peatlands, bog and fen nutrient cycles are controlled by different factors that may explain the differences in mineralization, total soil N and P, and respective labile pools. This information may be especially useful in anticipating changes brought about by anthropogenic nutrient/cation inputs and hydrologic alterations.