Carbon dioxide flux and net primary production of a boreal treed bog: Responses to warming and water-table-lowering simulations of climate change

Abstract

Abstract. Midlatitude treed bogs represent significant carbon (C) stocks and are highly sensitive to global climate change. In a dry continental treed bog, we compared three sites: control, recent (1-3 years; experimental) and older drained (10-13 years), with water levels at 38, 74 and 120 cm below the surface, respectively. At each site we measured carbon dioxide (CO 2 ) fluxes and estimated tree root respiration (R r ; across hummock-hollow microtopography of the forest floor) and net primary production (NPP) of trees during the growing seasons (May to October) of 2011-2013. The CO 2 -C balance was calculated by adding the net CO 2 exchange of the forest floor (NE ff − R r ) to the NPP of the trees.From cooler and wetter 2011 to the driest and the warmest 2013, the control site was a CO 2 -C sink of 92, 70 and 76 g m −2 , the experimental site was a CO 2 -C source of 14, 57 and 135 g m −2 , and the drained site was a progressively smaller source of 26, 23 and 13 g CO 2 -C m −2 . The short-term drainage at the experimental site resulted in small changes in vegetation coverage and large net CO 2 emissions at the microforms. In contrast, the longer-term drainage and deeper water level at the drained site resulted in the replacement of mosses with vascular plants (shrubs) on the hummocks and lichen in the hollows leading to the highest CO 2 uptake at the drained hummocks and significant losses in the hollows. The tree NPP (including above-and belowground growth and litter fall) in 2011 and 2012 was significantly higher at the drained site (92 and 83 g C m −2 ) than at the experimental (58 and 55 g C m −2 ) and control (52 and 46 g C m −2 ) sites.We also quantified the impact of climatic warming at all water table treatments by equipping additional plots with open-top chambers (OTCs) that caused a passive warming on average of ∼ 1 • C and differential air warming of ∼ 6 • C at midday full sun over the study years. Warming significantly enhanced shrub growth and the CO 2 sink function of the drained hummocks (exceeding the cumulative respiration losses in hollows induced by the lowered water level × warming). There was an interaction of water level with warming across hummocks that resulted in the largest net CO 2 uptake at the warmed drained hummocks. Thus in 2013, the warming treatment enhanced the sink function of the control site by 13 g m −2 , reduced the source function of the experimental by 10 g m −2 and significantly enhanced the sink function of the drained site by 73 g m −2 . Therefore, drying and warming in continental bogs is expected to initially accelerate CO 2 -C losses via ecosystem respiration, but persistent drought and warming is expected to restore the peatland's original CO 2 -C sink function as a result of the shifts in vegetation composition and productivity between the microforms and increased NPP of trees over time.

Partitioning Forest‐Floor Respiration into Source Based Emissions in a Boreal Forested Bog: Responses to Experimental Drought

“…was the most abundant type of tree constituting >99% of the tree stand, with 25,766 stems•ha −1 consisting of 37% taller trees (>137 cm height) up to 769 cm high [44]. The black spruce stand had an average canopy height of 168 cm, projection coverage of 42%, and basal area of 73.5 m 2 •ha −1 [17]. Trees were generally evenly distributed across the study plots (i.e., not clustered).…”

“…Immediately after the CO 2 concentration measurements at a plot, soil temperature at a depth of 5 cm (T 5 ) was measured using a thermocouple thermometer, and the WT level relative to moss surface was manually measured from a permanently installed water well adjacent to the plot. The CO 2 flux was calculated from the linear change in CO 2 concentration in chamber headspace over time [17], as a function of air temperature, pressure, and volume within the chamber headspace, following the ideal gas law.…”

“…To estimate R A_T_bg at hummock or hollow microform at each site, we used a trenching method [17,52]. One half of the instrumented hummock or hollow plots (60 cm × 60 cm) were chosen at each site for the trenching procedure, whereas the other half of the plots were left untrenched.…”

“…Therefore, any increase in carbon dioxide (CO 2 ) emissions in response to the expected changes in climate has the potential to provide a positive feedback to global warming [4,[7][8][9]. In general, many northern peatlands are expected to be drier under future climates [10,11], and while the response of peatland ecosystem respiration to water-table drawdown has been extensively studied [12][13][14][15][16][17], controlled field experimentation for partitioning ecosystem respiration into its source-based major components remains largely unexplored [18]. Research is needed to investigate source-based respiration fluxes in relation to potential changes in environmental conditions to improve our understanding of changes in ecosystem C storage or emissions to the atmosphere under climate change scenarios [19,20].…”

Responses of peat carbon at different depths to simulated warming and oxidizing

“…On one hand, in a warmer and O 2 richer environment, SOM becomes more available for microbes that accelerate Rs (Guntiñas et al, 2013). On the other hand, warming may dry up peatlands, which in turn accelerates SOM decomposition (Dorrepaal et al, 2009;Fenner and Freeman, 2011;Munir et al, 2015).…”