Effects of drought conditions on the carbon dioxide dynamics in a temperate peatland

The study site, Fäjemyr, is a temperate, ombrotrophic peatland (raised bog) located in southern Sweden (56° 15'N, 13° 33'E). The long-term (1961–90) mean annual temperature is 6.2 °C with July being warmest month (15.1 °C) and January coldest (−2.4 °C). Long-term mean annual precipitation is 700 mm. The water table fluctuates 0–20 cm below the surface, and the topographical pattern is dominated by hummocks, lawns and carpets, while hollows and open pools are scarce. The vegetation is dominated by dwarf shrubs (Calluna vulgaris, Erica tetralix), sedges (Eriophorum vaginatum) and Sphagnum mosses (S. magellanicum, S. rubellum). The depth of accumulated peat is approximately 5 m. More detailed site descriptions can be found in Lund et al (2007, 2009).

Measurements of the land–atmosphere net ecosystem exchange (NEE) of CO₂ were performed using an eddy covariance system, consisting of a closed-path infrared gas analyser (Li-Cor 6262, Li-Cor Inc., USA) and a three-dimensional sonic anemometer (Gill R3, Gill Instruments, UK). The anemometer was mounted on a mast at a height of 3.4 m above ground. In all directions surrounding the mast, there were at least 290 m of homogeneous surface properties. According to previous footprint modelling, more than 90% of the flux emanated from this area for atmospheric conditions ranging from weakly stable to unstable (Lund et al 2007). Additional environmental variables such as air (EMS 33, EMS Brno, Czech Republic) and soil temperatures (Type T thermocouples), precipitation (ARG100, Skye Instruments, UK), water table depth (WTD; SKPS 1830, Skye Instruments, UK) and photosynthetic photon flux density (PPFD; JYP 1000, SDEC, France) were monitored continuously at the site. Water table depth was measured in a lawn community type, i.e. the zero datum was set at an intermediate topographical level.

Normalized difference vegetation index (NDVI) data was acquired from the Terra/MODIS vegetation index product MOD13Q1 (16 day temporal resolution, 250 m spatial resolution). The selected single grid cell subset included the eddy covariance mast and covered only peatland area, excluding surrounding forest. Due to the prevailing south-westerly winds at Fäjemyr, a cell where the eddy covariance tower was situated in the eastern part was found to largely overlap the flux footprint (Schubert et al 2010).

Raw data from the eddy covariance system was collected at a frequency of 20 Hz and processed using the Ecoflux software, producing half-hourly fluxes according to Fluxnet methodology (Aubinet et al 1999). The software corrects for sensor separation by covariance optimization, air density fluctuations and frequency response losses. The storage term was calculated based on the single-point CO₂ concentration measurements and added to the flux. Data was screened for periods of low atmospheric mixing (defined as friction velocity <0.1 m s⁻¹). The system setup and data processing are described in more detail in Lund et al (2007).

Gaps in CO₂ flux data were filled using the following approaches. Firstly, small gaps (≤2 h) were filled using linear interpolation. Secondly, the Misterlich function (Falge et al 2001) was parameterized for daytime periods (PPFD > 10 μmol m⁻² s⁻¹) using an 8 day moving window (time step 1 day) with PPFD as independent variable;

$$\begin{eqnarray} &&\mathrm{NEE}=-({F}_{\mathrm{csat}}+{R}_{\mathrm{d}})\left (1-{\mathrm{e}}^{\frac{-\alpha (\mathrm{PPFD})}{{F}_{\mathrm{csat}}+{R}_{\mathrm{d}}}}\right )+{R}_{\mathrm{d}} \end{eqnarray} \tag{ 1 }$$

where F_csat is CO₂ exchange at light saturation (mg CO₂ m⁻² s⁻¹), R_d is dark respiration (mg CO₂ m⁻² s⁻¹) and α is initial slope of the light response curve (mg CO₂ μmol⁻¹ PPFD). The parameterization of the light response curve was only considered significant when all parameters (F_csat,R_d,α) were significantly different from zero (p < 0.05). Gaps in PPFD were filled using data from STRÅNG (http://strang.smhi.se/), an operational mesoscale radiation model from SMHI (Swedish Meteorological and Hydrological Institute) that produces radiation data covering Scandinavia at a resolution of about 11 km. The obtained light response curve parameters and PPFD data were subsequently used to fill gaps in CO₂ flux data. Remaining CO₂ flux gaps, smaller than 7 days, were filled with mean diurnal variation using an 8 day window (Falge et al 2001). Remaining longer gaps (DOY 208–255 and 300–327 in 2006; DOY 97–108 in 2007; DOY 225–235 in 2008; DOY 161–169, 221–231 and 233–240 in 2009) were filled using artificial neural networks (Papale and Valentini 2003).

The total uncertainty in annual CO₂ sums was estimated based on the procedure described in Elbers et al (2011) and Lund et al (2012). In addition, the effect of the long gaps (>7 days) was evaluated by using alternative gap-filling methods. The parameters of the light response curve (F_csat,R_d,α) were estimated based on available PPFD and NEE data 4 and 8 days prior to and following the gap, respectively. In case the parameterization of the light response curve was not significant, the gap was filled using average NEE 4 and 8 days prior to and following the gap, respectively. The uncertainty related to long data gap-filling was determined from the standard deviation of the three resulting NEE sums (calculated from artificial neural network and alternative gap-filling method based on 4 and 8 days, respectively).

Gross primary production (GPP) was subsequently modelled using the light response curve (equation (1)) by subtracting R_d (Lindroth et al 2007). Daytime ecosystem respiration (R_eco) was calculated as the difference between measured and gap-filled NEE and modelled GPP; while night-time R_eco equalled measured and gap-filled NEE.

To extract seasonality and reduce random noise, time series of original MODIS NDVI were smoothed using local Savitzky–Golay curve fits in the TIMESAT software (Jönsson and Eklundh 2004). Based on the quality information of the MOD13Q1 data, higher weights were assigned to data points with higher quality and lower weights to those with lower quality (Schubert et al 2010).

The annual temperature during the study period was on average 1.6 °C above the long-term (1961–90) average (figure 1). Most of the excess heating occurred during the winter months; the period DJF was on average 2.1 °C warmer while MAM, JJA and SON ranged from 1.0 to 1.5 °C warmer. The annual precipitation was on average slightly above the long-term average (+88 mm); however, there were large variations within and between years. For example, in 2006 the months of June and July only received 31% of normal (1961–90) precipitation, while in 2007 the same months received 272% of normal precipitation.

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The integrated effects of temperature and precipitation on the Fäjemyr peatland hydrology can be studied through measurements of water table depth (WTD; figure 2). In 2006, the water table was close to the surface during early growing season while it fell below 10 cm below ground on DOY 164 and reached −24.9 cm on DOY 208. Heavy rainfall in early August 2006 rapidly raised the water table above 10 cm on DOY 213. In 2007 and 2009 the water table generally varied between 0 and −10 cm, while in 2008 the water table was below −10 cm from DOY 126 until DOY 217 (WTD data is missing from DOY 211 but there was low precipitation until DOY 217). Outside of the time period shown in figure 2, water table was above −10 cm in all years. Using WTD =− 10 cm as a threshold for dry soil conditions in Fäjemyr, two long time periods with dry conditions can be identified; DOY 164–213 in 2006 and DOY 126–217 in 2008. These two periods in 2006 and 2008 will therefore be referred to as drought periods. The mean WTD (±1 standard deviation) for DOY 100–300 was −3.0 ± 7.3, −0.2 ± 3.9, −7.3 ± 6.7 and −5.0 ± 3.1 cm for 2006, 2007, 2008 and 2009, respectively.

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The inter-annual variation in net ecosystem exchange (NEE) was large with a range of −108 to 87 g CO₂ m⁻² yr⁻¹ (figure 3). Most of this variability was caused by differences in NEE during the growing season between late May (DOY 150) and early September (DOY 250). In 2008, the summer-time CO₂ net uptake rate was lower compared with other years, which coincided with the drought period (figure 2). The summer-time net CO₂ uptake during 2008 barely balanced the emissions during previous winter and spring, and after taking the net CO₂ emissions during late part of the year into account the peatland acted as an annual source of 86.4 g CO₂ m⁻², with an associated uncertainty of ±29.1 g CO₂ m⁻². In 2006, the net uptake rates during early summer were similar to the wetter years (2007 and 2009); however, around DOY 201 the peatland no longer acted as a sink of CO₂ on a daily basis. Again, this event corresponded to drought conditions with low water table (figure 2). In 2006, the peatland was an annual source of 52.4 ± 26.0 g CO₂ m⁻² while for 2007 and 2009, when water tables were close to the surface throughout the growing season, the annual CO₂ balances were −107.7 ± 28.1 and −106.1 ± 16.3 g CO₂ m⁻², respectively.

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The four year CO₂ balance of this temperate bog (−18.8 ± 102.7 g CO₂ m⁻² or −5.1 ± 28.0 g C m⁻²) was close to neutral, i.e., zero net CO₂ exchange. Other studies on comparable peatland sites have generally reported stronger CO₂ sinks: in a boreal poor fen in Sweden, the annual CO₂–C balance ranged between −48 and −61 g C m⁻² (Sagerfors et al 2008) based on three years of measurements. Roulet et al (2007) reported six years of annual CO₂–C balances for a temperate Canadian bog ranging −2 to −112 g C m⁻². The annual CO₂–C balance of an Irish blanket bog based on six years of measurements ranged −13 to −84 g C m⁻² (Koehler et al 2011). Although being multi-year averages, these budgets may still not reflect the true contemporary CO₂ exchange in northern peatlands, as extreme events such as droughts may occur on timescales longer than 3–6 yr. Similarly, the CO₂ balance at Fäjemyr may be biased by the fact that summer-time droughts occurred during two out of the four years in this study.

In 2008, ecosystem respiration (R_eco) rates were similar compared with other years (figure 4), while gross primary production (GPP) rates were generally lower (i.e. less negative GPP rates) during the drought period (DOY 126–217). In 2006, when the drought period began DOY 164, GPP rates were comparable to or even higher than the wet years of 2007 and 2009; however, R_eco rates during the drought period of 2006 were also higher compared with other years.

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Daytime NEE and GPP are largely regulated by the amount of incoming photosynthetic photon flux density (PPFD). To be able to assess how the drought periods affected the Fäjemyr peatland ecosystem independent from variations in PPFD, daily GPP rates at PPFD > 1000 μmol m⁻² s⁻¹ were calculated, which represent GPP at or close to light saturation (figure 5). Such GPP rates were not lower (i.e. less negative) during the drought period in 2006, while in 2008 GPP rates were consistently lower during DOY 140–220.

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3.4.1. Short-term effects (days to years).

The effects of drought and low water tables at Fäjemyr on the land–atmosphere exchange of CO₂ and its components were regulated by the timing of the drought event. In 2008, when water tables were low during early growing season (figure 2), the vascular vegetation of the peatland was likely affected by water stress during its developing phase. It was also observed in the field that Sphagnum mosses suffered from desiccation during this period. This resulted in decreased biomass build-up (figure 6) and lowered photosynthetic rates throughout large parts of the growing season (figure 5), and the peatland acted as a source of CO₂ on an annual basis. In 2006, the drought period occurred later when vascular plants were already established. Therefore, GPP at light saturation was not affected (figure 5); however, the unusually low water table exposed a thicker layer of peat for aerobic decomposition, and the ecosystem turned into a source of C at an earlier moment in time compared with the wet years. This argument is further supported by TIMESAT-smoothed NDVI data (figure 6). Summer-time NDVI were similar in 2006, 2007 and 2009, with average NDVI values during summer (DOY 152–243) being 0.68, 0.71 and 0.69, respectively. In 2008, the average NDVI was 0.63 during the same period, indicating less dense vegetation and greenness during this year. During a prolonged drought litterfall may decline due to reduced GPP (van der Molen et al 2011); such carry-over effect from the drought period in 2008 may be an explanation for the low CO₂ emissions during early 2009 (figure 3), caused by the reduced supply of labile C.

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Previous studies have suggested that droughts in peatlands primarily affect GPP (Shurpali et al 1995, Arneth et al 2002, Lafleur et al 2003), while others have argued that R_eco is more affected (Alm et al 1999, Aurela et al 2007). The results in this study indicates that the timing, severity and duration of the drought periods regulate the effects on GPP, R_eco and NEE in a temperate bog. A short but severe drought period during the peak growing season may not significantly affect GPP, but can increase R_eco due to increased heterotrophic respiration. On the other hand, a long drought period during the vegetation developing phase may significantly decrease GPP and to a lesser extent also R_eco. Furthermore, it is likely that various peatland types will show dissimilar responses to dry conditions (Lafleur 2009, Sulman et al 2010). Aurela et al (2004) argue that peatlands characterized by hydrological buffers, such as fens (minerotrophic peatlands) with a dynamic connection to the catchment scale water system, will increase their C uptake in a warmer climate. Bogs (ombrotrophic peatlands) on the other hand, which are decoupled from the groundwater of surrounding watershed, are dependent on precipitation inputs to maintain their water balance. A possible generalization may thus be that bogs are more sensitive to dry conditions in terms of their CO₂ balance as compared with fens, since the former are rainwater-fed while the latter is fed by rain and groundwater. The relative contribution of vascular plants and mosses will act to regulate peatland ecosystem resilience to drought, due to differences in water uptake and evapotranspiration processes (Sulman et al 2010).

3.4.2. Long-term effects (years to centuries).