Temporal and Spatial Variation in Peatland Carbon Cycling and Implications for Interpreting Responses of an Ecosystem‐Scale Warming Experiment

Abstract

We are conducting a large-scale, long-term climate change response experiment in an ombrotrophic peat bog in Minnesota to evaluate the effects of warming and elevated CO 2 on ecosystem processes using empirical and modeling approaches. To better frame future assessments of peatland responses to climate change, we characterized and compared spatial vs. temporal variation in measured C cycle processes and their environmental drivers. We also conducted a sensitivity analysis of a peatland C model to identify how variation in ecosystem parameters contributes to model prediction uncertainty. High spatial variability in C cycle processes resulted in the inability to determine if the bog was a C source or sink, as the 95% confidence interval ranged from a source of 50 g C m -2 yr -1 to a sink of 67 g C m -2 yr -1 . Model sensitivity analysis also identified that spatial variation in tree and shrub photosynthesis, allocation characteristics, and maintenance respiration all contributed to large variations in the pretreatment estimates of net C balance. Variation in ecosystem processes can be more thoroughly characterized if more measurements are collected for parameters that are highly variable over space and time, and especially if those measurements encompass environmental gradients that may be driving the spatial and temporal variation (e.g., hummock vs. hollow microtopographies, and wet vs. dry years). Together, the coupled modeling and empirical approaches indicate that variability in C cycle processes and their drivers must be taken into account when interpreting the significance of experimental warming and elevated CO 2 treatments. Core Ideas• We compared spatial vs. temporal variation in C cycle processes and drivers in a bog.• The bog was indistinguishable as a C source or sink because of high spatial variation.• Sensitive C cycle parameters in the model differed under ambient vs. warming scenarios.• Characterizing pretreatment variability is necessary when interpreting warming effects.Abbreviations: ELM_SPRUCE; SPRUCE-specific version of the Energy Exascale Earth System model; ANPP, aboveground net primary production; CI, confidence interval; [CO 2 ], CO 2 concentration; DOC, dissolved organic C; flnr, fraction of leaf N in ribulose-1,5-biphosphate carboxylase/

Constraints on microbial communities, decomposition and methane production in deep peat deposits

“…In August 2016 two cores were taken from each of these three transects with a Russian peat borer (Eijkelkamp, Netherlands) with deep peat deposits (150-200 cm below surface) from each transect serving as separate replicates for the experiments below. Prior characterization of peat from this depth in the S1 bog has shown it to be typically largely hemic, 6000-7000 years old, with ash content of~10% and bulk density of~0.15 g/cm 3 [28][29][30]. Peat from each transect was homogenized in the field, distributed into one liter Nalgene bottles, topped off with porewater taken from the same depth to a zero headspace, then capped and stored at 4˚C in a coldroom until laboratory microcosm construction.…”

“…Sphagnum species vary in with microtopography but Sphagnum magellanicum or S. angustifolium generally dominate hummocks and S. fallax dominate in hollows [25]. The S1-Bog and the SPRUCE experiment has been extensively studied with complete above and belowground characteristics described in previous publications and data products [13,22,[26][27][28]. The SPRUCE experiment includes three raised metal boardwalks extending 88, 112, and 92 m into the S1-Bog.…”

Rapid loss of an ecosystem engineer: Sphagnum decline in an experimentally warmed bog

“…In preliminary measurements (Griffiths et al, 2017), we realized that common methods for measuring Sphagnum growth were not effective at this site. Crank or brush wires (Clymo, 1970 2013), which measure increases in stem length along a vertical wire, could not capture growth of Sphagnum in hollows, which were only rarely vertically oriented, and they could not be used over winter to capture growth immediately after snowmelt in spring.…”

“…As described by Hanson et al (2017) and Griffiths et al (2017), the bog is dominated by Picea mariana and Larix laricina, with an understory of ericaceous shrubs, including Labrador tea (Rhododendron groenlandicum (Oeder) Kron and Judd) and leatherleaf (Chamaedaphne calyculata (L.) Moench), and a limited number of herbaceous plants.…”

Local Spatial Heterogeneity of Holocene Carbon Accumulation throughout the Peat Profile of an Ombrotrophic Northern Minnesota Bog

“…Average carbon concentration was 51.6 ± 2.8% for the entire peat profile, with relatively little variation within or across profiles (Figure 4). Average peat bulk density was also lower in the acrotelm (0.04 ± 0.02 g cm -3 ) than in the catotelm (0.18 ± 0.05 g cm -3 ), although it increased with depth from the peat surface to -50 cm, or approximately 2000 BP, and then declined slightly to -100 cm, or approximately 4000 BP ( Figure 4 in this paper, see also Figure 2 in Griffiths et al (2017) for plots by peat depth). Peat carbon stock, not including carbon stored in raised hummocks, for the S1 Bog averaged 176 ± 40 kg C m -2 to -225 ± 58 cm depth across depth profiles.…”