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Over thousands of years, peatlands around the world have accumulated carbon (C) stocks of global importance. Drainage for agriculture, forestry and peat extraction has transformed many peatlands from long-term sinks into strong sources of carbon dioxide (CO2). Peat extraction is worldwide responsible for about ten percent of drained peatlands and is mainly carried out in northern countries and Eastern Europe. In Belarus, 0.3 Mha of peatlands are drained for peat extraction, which is twelve percent of the country's peatland area. From 2006 to 2013, 21,333 ha of this area have been rewetted to protect these peatlands from fire and further degradation, reduce their greenhouse gas (GHG) emissions, turn them back into C sinks and promote biodiversity. A further 260,000 ha are no longer used for peat extraction and their rewetting would be a great benefit for nature conservation and climate protection.
Rewetting of abandoned peat extraction areas usually leads to inundation of large areas where not adapted plants die and new species establish, depending on water level and nutrient conditions. Beavers, of which there are many in Belarus, also play an important role in the rewetting of peatlands. They dam up ditches in drained and rewetted peatlands, thus contributing to water level increases and vegetation changes. The aim of this PhD thesis was to investigate the impact of inundation on vegetation and GHG emissions in formerly extracted fens in Belarus, to determine the role of water level in this process, and to study whether such fens develop back into C sinks with an almost neutral GHG balance within one or two decades after rewetting (Papers II and III). Also the potential of beaver activities for peatland restoration was assessed (Paper III).
Two very different fens, rewetted after peat extraction, were chosen as study areas. The first one, Giel'cykaŭ Kašyl, is a former flood mire and was rewetted with water from the Jasiel'da River in 1985. During the study period 2010–2012 this site was a shallow lake (~ 1 m deep) dominated by very productive, tall reed. Shallower areas along the edges had a partly floating vegetation cover of cattail (Typha latifolia, T. angustifolia) and sedges (Carex elata, C. vesicaria). The second fen, Barcianicha, is fed by groundwater. Rewetting from 1995 onwards resulted in water levels at or slightly above surface and a lower nutrient availability compared to Giel'cykaŭ Kašyl'. This was reflected in the establishment of mesotrophic communities of Eriophorum angustifolium and Carex rostrata. Phragmites australis stands, which were also dominant here, were shorter and less productive than in Giel'cykaŭ Kašyl'. The southern area of Barcianicha was not used for peat extraction and has not been rewetted. Until 2009 vegetation of this part was characterized by forbs (Urtica dioica) and wet meadows (Agrostis stolonifera). From autumn 2009, a beaver dam in the main drainage ditch caused flooding of these areas and led to diverging vegetation development depending on water levels.
Within the framework of this doctoral thesis annual fluxes of CO2, methane (CH4) and nitrous oxide (N2O) and the development of water levels and vegetation were monitored for two years at nine sites and evaluated (Papers II and III). Three of the sites, respectively, were located (a) in Giel’cykaŭ Kašyl’, flooded in 1985, (b) in the central area of Barcianicha, which was rewetted in 1995, and (c) in the southern part of Barcianicha, which was flooded by beavers end of 2009. GHG measurements were carried out with manual chambers from August 2010 to September 2012. Annual net CO2 exchange rates (NEE) were modeled based on light response curves of gross primary production (GPP) and on temperature response curves of ecosystem respiration (Reco), which were determined every third to fourth week by alternating measurements with transparent (cooled) and opaque chambers (both with fan) along the daily amplitude of photosynthetically active radiation (PAR) and temperature. Annual CH4 emissions were calculated mainly based on the temperature response of CH4 fluxes over the course of the year, based on biweekly (in summer) to monthly (in winter) repeated single measurements with opaque chambers (without fan). This was done, although all longer rewetted sites were dominated by aerenchymatic plants whose gas transport during the vegetation period may change over the course of the day and can be influenced by shading. This might apply to the six longer rewetted sites, two of which were dominated by Phragmites australis, and the others by Typha latifolia, Carex elata, Carex rostrata or Eriophorum angustifolium. For these six sites therefore studies on the daily course of CH4 release and the influence of chamber shade were conducted, covering 8–24 hours and lasting at least from sunrise to afternoon. Also the extent to which flux rates were affected by a lack of chamber headspace mixing by fans was investigated in the mentioned studies (Papers I and II).
The daytime course of CH4 emissions showed a pronounced dynamic for Phragmites australis in both fens, with minimum release during the night and maximum during the day (Paper I). The other sites in contrast did not show a significant diurnal CH4 flux dynamic (Paper II). Lack of headspace mixing by fans as compared to chambers with fan resulted in a slight underestimation of CH4 emissions at very high chambers (220 and 250 cm), as used for Phragmites australis in Giel'cykaŭ Kašyl', while there was no difference at lower chambers (≤185 cm), as used for the other sites. Opaque chambers resulted for sites dominated by Typha latifolia and Carex elata in significantly (1.2 times and 1.1 times, respectively) lower CH4 fluxes compared to transparent chambers. For the other sites, opaque chambers did not significantly reduce CH4 emissions. This result was unexpected, especially for Phragmites australis, as PAR out of all parameters tested had the strongest influence on CH4 emissions from both reed sites, and clouds directly led to reduction of their emissions. Presumably the gas flow in the reed shoots located within opaque chambers was maintained by shoots outside the chamber that were connected to the enclosed shoots by rhizomes (Paper I). The investigations showed that single measurements between 9 a.m. and 6 p.m. with opaque chambers without fan, as performed for the determination of annual CH4 fluxes, resulted for Carex rostrata and Eriophorum angustifolium in estimates similar to the daily mean, but for Phragmites australis in estimates that were rather above the daily mean. Annual CH4 fluxes from Phragmites australis could therefore be slightly overestimated. CH4 fluxes from Typha latifolia and Carex elata during the vegetation period were corrected by a factor of 1.2, although darkness inside of opaque chambers matters only at day, not at night. Daily and annual CH4 fluxes from these sites have been therefore most likely slightly overestimated, too.
Water saturation and the establishment of adapted vegetation were the most important conditions for the restoration of C sinks (gaseous CO2 and CH4 fluxes) in the investigated peatlands. The only site with falling water levels in summer and thus temporarily aerated peat was the beaver flooded forbs (Urtica dioica) site at Barcianicha. This site was a very strong CO2 emitter and the only significant N2O source of the entire study (Paper III). All other sites were permanently wet, had much lower CO2 emissions or were even net C sinks (Papers II and III). Establishment of adapted vegetation depended on inundation depth and time since rewetting. For example, within one year the meadow site in Barcianicha shallowly flooded by beaver was colonized by Carex rostrata and other adapted helophytes and developed into a CO2 sink, while the deeper flooded site at the same meadow initially attracted only Chara and some individuals of Alisma plantago aquatica and remained a moderate CO2 source. However, the results of the longer rewetted sites show, that also deeply (~ 1 m) flooded fen areas can become densely populated with mire plants in the course of 25 years and develop into net C sinks. Highest annual C uptake in both fens was achieved by the reed sites. Eriophorum angustifolium and Carex rostrata in mesotrophic Barcianicha were smaller C sinks. Typha latifolia and Carex elata in the eutrophic Giel'cykaŭ Kašyl', on the other hand, released CO2, presumably because the high and fluctuating water levels imposed stress to the plants, and because the large supply of nutrients and dead plant material allowed for strong heterotrophic respiration (Paper II). The simultaneously high CH4 emissions made Typha latifolia and Carex elata major sources of GHG. CH4 emissions from Phragmites australis in Giel'cykaŭ Kašyl' were even higher, but due to extremely high CO2 uptake the site was only a small net GHG source. CH4 emissions in Barcianicha were much lower and comparable to undisturbed sedge fens. The difference between Giel'cykaŭ Kašyl' and Barcianicha was mainly due to the different nutrient supply and the related productivity of the plants. Important conclusions are that stable inundation is an appropriate measure for restoration of the C sink of formerly extracted fens, but nutrient input with water needs to be stopped or reduced in order to decrease CH4 production. If this is not possible, establishment of Phragmites australis and other strong C sinks could help to compensate for the climate impact of high CH4 emissions from eutrophic sites.
The effect of the beaver dam on the development of the southern part of Barcianicha depended not only on the initial situation but mainly on the water level. Under optimal conditions, it led to the rapid establishment of adapted mire plants, the restoration of a C sink and a significant reduction of GHG emissions. However, this situation in the shallowly flooded meadow was achieved by chance. In comparison to planned rewetting measures, which aim to raise the water level evenly over the entire peatland, beavers dam ditches in order to improve their immediate habitat, thus influencing water levels only up to a certain distance, but rarely over the entire peatland. Nevertheless, beaver activity is of high value both for mire conservation projects, where existing dams are supplemented by beaver dams, and for abandoned, drained peatlands, like former peat extraction areas in Belarus, many of which at least partially have been rewetted by beavers.
Late Quaternary evolution and carbon cycling of tropical peatlands in equatorial Southeast Asia
(2014)
Peatlands are an important component in the global carbon cycle as they act as both long-term sinks for carbon dioxide and significant sources for methane. Over the Holocene period (the past 11,700 years) continuous CO2 uptake by peat accumulation exceeded methane emissions in northern peatlands and resulted in a net-radiative cooling effect on the global climate.Although 11% of the global peatland area is located in the tropics, the role of tropical peatlands in the global carbon cycle and in influencing the Earth’s radiative budget has not been resolved. Climate-carbon cycle models have thus far not included tropical peatlands because reliable data on their past rates of carbon uptake and release are not available. In this thesis this problem has been approached by reconstructing peatland expansion and rates of carbon storage and release over the Late Quaternary (Latest Pleistocene and Holocene) for the largest tropical peatland area, which is located in equatorial SoutheastAsia (i.e. Sumatra, Borneo, Peninsular Malaysia). Peat accumulation in the tropics remains an enigmatic phenomenon, because the constantly high temperatures of 26-27°C should theoretically drive rapid soil carbon turnover and thus not enable the accumulation of peat. Therefore this thesis also explores the mechanisms that cause peat formation in the SoutheastAsian tropics as well as the drivers behind changing rates of carbon accumulation. Carbon dynamics were analyzed at the regional scale (103–105 km2) of SoutheastAsia over millennial timescales (paper, I, II) and at the local scale (101–102 m2) of a peatland site on annual to centennial timescales (paper III, IV). Paper I presents the first systematic classification of the nearly 160,000 km2 SoutheastAsian lowland peatlands (below 70 m a.s.l.) into geographic peatland types. The peatlands were divided into 1) coastal peatlands of PeninsularMalaysia, Sumatra, and Borneo (~130,000 km2) and into inland peatlands (~30,000 km2) of 2) Central Kalimantan (southern Borneo), 3) the Kutai basin (eastern Borneo), and 4) the Upper Kapuas basin (western Borneo). Coastal peatlands formed by primary mire formation directly on freshly exposed marine or mangrove soils with the lowering of the sea level during the Late Holocene. In contrast, inland peatlands formed via paludification on either terrestrial sand soils (Central Kalimantan) or by both paludification and terrestrialization (Kutai basin, Upper Kapuas basin). The sequence of peatland initiation was established by applying the common cumulative basal date frequency approach (paper I). This method revealed clear differences in the timing of peatland initiation: 1) the Upper Kapuas peatlands are the oldest postglacial peat formations and date from 20,000-13,000 cal BP (calendar years before present), 2) inland Central Kalimantan peatlands date from 14,500-9000 cal BP, 3) the Kutai peatlands date from 8300-4900 cal BP, and 4) and the coastal peatlands date from 7700-200 cal BP. Coastal peatlands have a Holocene average carbon accumulation rate of 77 g C m-2 yr-1, being recognized as the globally most effective terrestrial ecosystems in terms of long-term carbon sequestration. Except for the Kutai peatlands, the Holocene average carbon accumulation rates of inland peatlands are significantly lower (20-30 g C m-2 yr-1) and very similar to the average long-term rates of northern peatlands. Fluctuations in past rates of carbon accumulation of SoutheastAsian peatlands could for the first time be linked to paleoclimatic changes, primarily variations in moisture availability (paper I, II). Hydroclimatic influences on carbon accumulation rates were related to shifts in the mean position of the Intertropical Convergence Zone, changes in the intensity of theAustral-Asian monsoon system, and variations in the frequency of the El Niño- Southern Oscillation. In contrast, peatland initiation and expansion was driven by sea-level change (paper I, II). The deglacial rise in sea-level is identified as the primary driver for inland peatland formation in Borneo, because the rising sea-level 1) lowered the hydrological gradients in the SoutheastAsian island archipelago inducing rising ground and surface water levels on these islands, and 2) led to higher atmospheric moisture availability due to the associated expansion of marine water masses on the shelf floor. Paper II shows that inland peatland initiation and expansion was most extensive during deglacial meltwater pulses, when the rate of sea-level rise exceeded 10 mm yr-1. Only when the rate of sea-level rise had slowed down to a threshold of 2.4 mm yr-1 by ~7000 cal BP could peat accumulation along the coasts keep up with the sea-level rise and coastal peatlands could form. Hydro-isostatic adjustment of the Sunda Shelf led to a sea-level lowering by ca. 5 m over the past 4500 years. Falling sea levels exposed extensive marine areas that were rapidly colonized by peat swamp forests.Anewly 140 developed method for the reconstruction of past peatland area based on transfer functions (paper II) reveals that 70%of the peatlands of Sumatra and Kalimantan only formed during the past 4000 years.Moreover, this new transfer function approach shows that the common basal dates approach overestimated the extent of peatlands in the past. This method, in general, leads to higher rates of reconstructed cumulative peat carbon uptake for the past. By combining reconstructed peatland areas and mean rates of carbon accumulation over millennial timescales from each peatland type the carbon uptake of all peatlands from Sumatra and Kalimantan could be quantified for the past 15,000 years (paper II). Carbon uptake remained below 1 Teragram (Tg) C yr-1 from 15,000-5000 cal BP because the total area of peatlands was less than 30,000 km2. Rapid peatland expansion driven by the lowering of sea-level over the past 5000 years increased carbon uptake on Sumatra and Kalimantan to over 7 Tg C yr-1 and resulted in an exponential growth of the regional peat-carbon reservoir to a size of over 20 Pg C. SoutheastAsian peatlands therefore had no significant role in the Late Pleistocene and Early Holocene global carbon cycle. However, because of their rapid expansion after 5000 cal BP by over 100,000 km2 the peatlands of SoutheastAsia became a globally important carbon sink during the Late Holocene and likely caused an atmospheric CO2 drawdown of 1-2 ppm (paper II). This previously unrecognized biospheric carbon sink partly compensated for contemporaneous terrestrial carbon losses associated with the desertification of Northern Africa. The mechanisms that enable high rates of carbon accumulation of coastal peatlands were explored in a peat core study presented in paper III. Here the use of a new coring technique for the tropics and the application of noninvasive geophysical measurements were employed to derive a high-resolution record of carbon accumulation rates. This study provides the first description of peatland pools for SoutheastAsia, which form as tip-up pools from falling trees such as Shorea albida. Based on a pollen and macrofossil record a fossil tip-up pool could be identified in the core and an associated carbon accumulation rate of 100 to over 900 g C m-2 yr-1 determined. Thus tip-up pools function as local hot spots for carbon accumulation, fundamentally different from northern hemisphere peatland pools, which act as net-carbon sources. From a time-series of aerial photographs the rate of tree fall and thus pool formation was determined at 0.4 tree ha-1 yr-1 (paper III).Asimulation model indicates that up to 60%of the peat deposited in peat domes of Borneo is derived from filled up fossil pools – changing the paradigm that Southeast Asian peatlands mainly form from belowground biomass and providing an explanation for the rapid carbon accumulation of these ecosystems. The climate impact of peatlands is, however, not only related to their capacity to rapidly store carbon, because peatlands also release the strong greenhouse gas methane – a by-product of anaerobic decomposition.Ametaanalysis of methane emission data from SoutheastAsian peatlands (paper IV) shows that their average annual methane release of 3 g CH4 m-2 yr-1 is lower than the average annual release of ~9 g CH4 m-2 yr-1 from northern peatlands, although the higher tropical soil temperatures should lead to significantly higher emissions. The limited degree of anaerobic decay is explained by the recalcitrance of the deposited biomass, which contains high amounts of lignin and tannin, providing another explanation for rapid carbon accumulation. Low anaerobic decomposition together with high rates of carbon accumulation imply that limits to vertical peat bog growth in SoutheastAsia are not set by cumulative anaerobic decay as in northern raised bogs. Instead peat bog growth is limited by aerobic decomposition related to water-table lowering as shown by a derived linear relationship between the amount of released CO2 from aerobic peat decomposition and the mean annual depth of the peatland water-table (paper IV). The climatic effect of Southeast Asian peatlands was determined by the global warming potential (GWP) method, which compares carbon uptake with methane emissions in terms of CO2-equivalents. The low methane emissions and high carbon accumulation rates of coastal peatlands result in a net annual uptake of 1340 kg CO2- equiv. ha-1 yr-1 over a 100 year GWP time-horizon. Under natural conditions coastal Southeast peatlands exert a significant net cooling effect on the global climate in contrast to northern peatlands, which have a warming effect or act climatic neutral on this time frame. It can be concluded that the tropical peatlands of SoutheastAsia are the strongest carbon sinks among all peatlands globally with a notable influence on the Earth’s radiative budget. However, today an estimated 90,000 km2 of peatlands in SoutheastAsia is drained for agriculture (e.g. oil palm plantations) and deforestation. These drained peatlands release annually over 140 Tg C yr-1 from aerobic peat 141 decomposition. Drainage also facilitates the regular spread of peat fires in this region, which on average release around 75 Tg C yr-1. Ongoing total carbon losses (~220 Tg C yr-1) exceed the natural carbon uptake by a factor of 25 and demonstrate that the entire SoutheastAsian peatland region has recently switched from a globally important carbon sink to a globally significant source of atmospheric CO2 (paper II, IV).