Response of bog and fen plant communities to warming and water-table manipulations

Large-scale changes in climate may have unexpected effects on ecosystems, g iven the importance of climate as a control over almost all ecosystem attri butes and internal feedbacks. Changes in plant community productivity or co mposition, for example, may alter ecosystem resource dynamics, trophic stru ctures, or disturbance regimes, with subsequent positive or negative feedba cks on the plant community. At northern latitudes, where increases in tempe rature are expected to be greatest but where plant species diversity is rel atively low, climatically mediated changes in species composition or abunda nce will likely have large ecosystem effects. In this study, we investigate d effects of infrared loading and manipulations of water-table elevation 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 m(2) surface area, 0.5-0.7 m depth ) each from a bog and a fen in northern Minnesota to construct a large meso cosm 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 treatme nts, and two ecosystem types (bogs and fens), with three replicates of all treatment combinations. Overhead infrared lamps caused mean monthly soil te mperatures to increase by 1.6-4.1 degreesC at 15-cm depth during the growin g season (May-October). In 1996, depths to water table averaged -11, -19, a nd -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, gramin oid, and shrub life-forms was determined for the dominant species in the me socosm plots based on species specific canopy/biomass relationships. Belowg round net primary production (BNPP) was estimated using root in-growth core s. Bog and fen communities differed in their response to infrared loading and water-table treatments because of the differential response of life-forms a nd 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 f en community, production of graminoids and forbs increased. Along a gradien t of in creasing infrared loading in the bog, shrub production increased wh ereas graminoid production decreased. In the fen, graminoids were most prod uctive at high infrared loading, and forbs were most productive at medium i nfrared loading. In the bog and fen, BNPP:ANPP ratios increased with warmin g 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 dif fer between water-table treatments because BNPP was greatest in the dry tre atment whereas ANPP was greatest in the wet treatment. The differential responses of species, life-forms, and above- and belowgrou nd biomass production to the treatments suggest that bog and fen plant comm unities 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 ene rgy, carbon, and nutrient budgets through differential responses of the pla nt communities. Thus, predictions of the response of peatlands to changes i n climate should consider differences in plant community structure, as well as biogeochemistry and hydrology, that characterize and differentiate thes e two ecosystems.