[1] Net ecosystem CO 2 exchange (NEE) was measured from June 2000 through October 2001 by 10 automatic chambers at a peatland in southeastern New Hampshire. The high temporal frequency of this sampling method permitted detailed examination of NEE as it varied daily and seasonally. Summer of 2001 was significantly drier than the 30-year average, while summer of 2000 was wetter than normal. Although NEE varied spatially across the peatland with differences in plant species composition and biomass, maximum CO 2 uptake was 30-40% larger in the drier summer in evergreen and deciduous shrub communities but the same or lower in sedge sites. Ecosystem respiration rates were 13-84% larger in the drier summer depending on plant growth form with water table and temperature as strong predictors. Ecosystem respiration was also correlated with maximum ecosystem productivity and foliar biomass suggesting that plant processes, water table, and temperature are tightly linked in their control of respiratory losses. The ratio between maximum productivity and respiration declined for evergreen shrub and sedge sites between the wet and dry summer, but increased in deciduous shrub sites. A drier climate may reduce the CO 2 sink function of peatlands for some growth forms and increase it for others, suggesting that ecosystem carbon and climate models should account for differences in growth form responses to climate change. It also implies that plant functional types respond on short timescales to changes in moisture, and that the transition from sedges to shrubs could occur rapidly in peatlands under a drier and warmer climate.
Abstract:Net ecosystem exchange of CO 2 was measured at a temperate peatland in southeastern New Hampshire. Classified as a mineral-poor fen owing to deep, water-logged peats that are influenced to a limited extent by groundwater, the ecosystem is dominated by plants such as sedges (Carex spp.) and evergreen shrubs. Ten automatic chambers measured fluxes every 3 h by sampling changes in headspace concentration of CO 2 from November 2000 through March 2001. The fen was covered in snow for most of this period and CO 2 was emitted from the snow pack throughout the winter. The largest fluxes were associated with ground temperatures of 0°C and with declining atmospheric pressure. CO 2 effluxes up to 3 µmol m 2 s 1 were recorded when the ground temperature reached the thaw point. Fluxes were lower when the ground temperature rose above 0°C, however, suggesting that the large fluxes were associated with a build up and release of stored CO 2 degassing as soon as the ground thawed, or by enhanced microbial activity associated with freeze-thaw dynamics. The large number of thaw events coupled with frequent short-term releases of CO 2 suggest that degassing occurred on a regular basis with changes in atmospheric pressure and/or microbial decomposition occurred beneath the snowpack. The extent of soil freezing prior to thaw was also an important factor, with colder soils yielding smaller CO 2 emissions upon thaw. Although most of the observed CO 2 flux was efflux from the ecosystem, occasional CO 2 uptake by the ecosystem of up to 1 µmol m 2 s 1 was also observed, indicating small rates of photosynthesis even during winter. Photosynthesis occurred only when the ground temperature was >0°C. The implications for a warmer climate are unclear. If warmer winter temperatures yield less snow in the temperate region, then soils could freeze more deeply and result in lower CO 2 emissions. However, if less snow results in a higher frequency of freeze-thaw events, then winter CO 2 emissions could be larger with a warmer climate.
We used five analytical approaches to compare net ecosystem exchange (NEE) of carbon dioxide (CO 2 ) from automated and manual static chambers in a peatland, and found the methods comparable. Once per week we sampled manually from 10 collars with a closed chamber system using a LiCor 6200 portable photosynthesis system, and simulated four photosynthetically active radiation (PAR) levels using shrouds. Ten automated chambers sampled CO 2 flux every 3 h with a LiCor 6252 infrared gas analyzer. Results of the five comparisons showed (1) NEE measurements made from May to August, 2001 by the manual and automated chambers had similar ranges: À10.8 to 12.7 lmol CO 2 m À2 s À1 and À17.2 to 13.1 lmol CO 2 m À2 s À1 , respectively. (2) When sorted into four PAR regimes and adjusted for temperature (respiration was measured under different temperature regimes), mean NEE did not differ significantly between the chambers (p < 0.05). (3) Chambers were not significantly different in regression of ln( À respiration) on temperature. (4) But differences were found in the PAR vs. NEE relationship with manual chambers providing higher maximum gross photosynthesis estimates (GP max ), and slower uptake of CO 2 at low PAR (a) even after temperature adjustment. (5) Due to the high variability in chamber characteristics, we developed an equation that includes foliar biomass, water table, temperature, and PAR, to more directly compare automated and manual NEE. Comparing fitted parameters did not identify new differences between the chambers. These complementary chamber techniques offer a unique opportunity to assess the variability and uncertainty in CO 2 flux measurements.
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