mires and the ecological effects of drainage and restoration
Sinikka Jauhiainen
Academic dissertation
To be presented with the permission of the Faculty of Agriculture and Forestry of the University of Helsinki, for public criticism in the Auditorium 2 of the Info Centre, Viikinkaari 11, Helsinki, on August 15th, 2003, at 12 o´clock.
Sinikka Jauhiainen
Helsingin yliopiston metsäekologian laitoksen julkaisuja 29
University of Helsinki Department of Forest Ecology Publications 29 Julkaisija: Helsingin yliopiston metsäekologian laitos
Publisher: University of Helsinki Department of Forest Ecology
Kansikuva: Ilmakuva © Maanmittauslaitos. Lupanumero 49 / MYY / 03
Cover picture: Aerial photograph © National Land Survey of Finland. Permission number 49 / MYY / 03
Taitto / Layout: Sinikka Jauhiainen
Kielentarkastus / Revision of English: Michelle de Chantal
Työn ohjaaja / Supervisor: Docent Harri Vasander, Ph.D., Department of Forest Ecology, University of Helsinki, Finland
Esitarkastajat / Reviewers: Docent Tapio Lindholm, Ph.D., Finnish Environmental Institute, Helsinki, Finland and Docent Heikki Seppä, Ph.D., Department of Earth Sciences, University of Uppsala, Sweden
Kustos / Custos: Professor Juhani Päivänen, Department of Forest Ecology, University of Helsinki, Finland
Vastaväittäjä / Opponent: Professor Atte Korhola, Department of Ecology and Systematics, Division of Hydrology / ECRU, University of Helsinki, Finland
ISBN 952-10-1269-2 (nid) ISBN 952-10-1270-6 (pdf) ISSN 1235-4449
Yliopistopaino, Helsinki Finland 2003
The investigation of drainage and restoration effects on peatland ecosystems has created the need to study their natural development. In order to restore drained peatlands successfully, we need to know what factors have regulated their development and at what stage of succession the peatlands were at the time of management.
Two different mire types, which represent a typical fen and a bog in southern Finland were studied. The chemostratigraphy and past vegetation in relation to climate change were investigated. The influence of drainage on peatland ecosystems was studied using aerial photographs that covered the drainage period. Differences in vegetation structure and moisture conditions were investigated. The potential of the peatlands’
own seedbanks to regenerate the mire vegetation was studied. Restoration was performed by filling the ditches or damming them. The restoration effect was accelerated by clear-cutting trees in order to avoid strong evaporation. The success of restoration was investigated annually using water table level measurements and vegetation surveys.
Both peatlands initiated by paludification soon after the glacial retreat, around 9000
14C BP. Peat accumulated at different rates in the fen (average 0.28 mm yr-1) and in the bog (average 0.47 mm yr-1) reaching peat thicknesses of 230 cm and 430 cm, respectively. They accumulated carbon at the average rates of 11.1 g m-2 yr-1 for the fen and 13.2 g m-2 yr-1 for the bog. The main trend in peat chemistry of both mires was the change from higher to lower nutrient levels with an increased acidity towards the surface peat. The main changes in chemostratigraphy occurred between 4600 and 420014C BP, simultaneously with the spread of Picea abies in the area.
Drainage during the 20th century caused greater changes at the fen site than at the bog site. Two new drained mire site types appeared at the fen site with ground vegetation species common to a forest floor. Continuously high moisture contents and low nutrient levels at the bog site prevented the formation of an economically valuable forest stand on the site. Also, the restoration caused stronger changes to the fen than to the bog. This was obviously due to the removal of the forest cover.
Restoration of both mires started successfully. The water table remained high and peatland vegetation spread on the areas in two to three years. However, it takes a much longer time period to restore a mire, and the restored state may not be the same as the pre-drainage one.
Key words: aerial photographs, bog, drainage, fen, hydrology, mire vegetation, peat chemistry, peatland restoration, peat stratigraphy, plant remains, seed bank
This thesis is based on the following articles, which are referred to in the text by their Roman numerals:
I Jauhiainen, S., Pitkänen, A. and Vasander, H. 2003. Chemostratigraphy and vegetation in two boreal mires during the Holocene. Accepted by The Holocene.
II Jauhiainen, S., Rasinmäki, A. and Holopainen, M. 2003. Drainage-driven changes in peatland ecosystem; Aerial photograph investigation. Submitted manuscript.
III Jauhiainen, S. 1998. Seed and spore banks of two boreal mires. Ann. Bot. Fennici 35:197-201.
IV Jauhiainen, S. 2002. Testacean amoebae in different types of mire following drainage and subsequent restoration. Europ. J. Protistol. 38:59-72.
V Jauhiainen, S., Laiho, R. and Vasander, H. 2002. Ecohydrological and vegetational changes in a restored bog and fen. Ann. Bot. Fennici 39:185-199.
Sinikka Jauhiainen participated in planning the research and setting-up the experimental sites. She has been responsible for field sampling, preparing the samples and making the analyses, except the ICP and plant tissue remain analyses. She has made the vegetation mapping of the fen and bog sites. She has been the main author of all papers. Harri Vasander participated in planning the research and writing Papers I and V; commented on the other manuscripts and thesis summary. Raija Laiho has participated in the writing of Paper V, and has given critical comments on the manuscripts. Markus Holopainen was responsible for digitizing the aerial photographs and participated in the writing of Paper II. Aki Pitkänen has participated in the writing of Paper I. Annukka Rasinmäki made the vegetation mapping of the Lakkasuo mire for Paper II.
Terms and their definitions
Acrotelm= upper layer of peat in which the water table fluctuates during the growing season.
Catotelm = lower layer of peat under the acrotelm, permanently waterlogged.
Chemostratigraphy= classification of sediment units according to variations in their element concentrations.
Diplotelmic = peat that contains acrotelm and catotelm.
Holocene = post-glacial period, covers approximately the last 10 000 years.
Microfossils = microscopic remains of organisms.
Macrofossils = larger remains of organisms.
Minerotrophic mire = mire that receives water which has been in contact with mineral soil.
Ombrotrophic mire = mire that receives water and nutrients from precipitation only.
Palaeoecology = study of past ecology.
Paludification = formation of organic peat soil directly on mineral substrate.
Propagule = any part of a plant capable of growing into a new organism.
Terrestrialization = the process by which a shallow water body is gradually infilled with accumulated debris from organic and inorganic sources.
Testate amoebae = shell-covered amoebas of the class Rhizopoda, Protozoa.
von Post scale = humification classification (scale H1-10).
R% = Razlochenie (Russian) = decay, the percentage of decomposed organic matter.
Stratigraphy = chronological order of sediments.
Abstract Papers included
Terms and their definitions
1 Introduction... 7
1.1 General framework of the study ... 7
1.2 Peatland initiation and peat formation ... 7
1.3 Peatlands as archives... 9
1.4 Human impacts on Finnish peatlands ... 10
1.4.1 Drainage... 10
1.4.2 Restoration ... 10
1.5 Aims of the study ... 11
2 Material and methods... 12
2.1 Study sites ... 12
2.2 Experimental design... 12
2.3 Measurements and analysis... 13
2.4 Investigation of peatland development ... 14
2.5 Investigation of drainage effects ... 14
2.6 Investigation of restoration effects... 14
2.7 Data analysis ... 17
3 Results and discussion ... 18
3.1 Development of peatlands... 18
3.1.1 Initiation and accumulation... 18
3.1.2 Changes in chemostratigraphy and past vegetation ... 20
3.2 Drainage impacts ... 21
3.3 Restoration impacts... 22
Conclusions... 24
Acknowledgements ... 24
References... 25
1 Introduction
1.1 General framework of the study
Peat is organic soil which is composed of annually accumulated, partly decomposed plant remains and populations of decomposers (Clymo 1983). Existing functional peatlands in Finland have developed after the latest glacial period, during about 10 000 years. Peat covers about 1/3 of the land area in Finland, forming various types of peatland ecosystems (Ruuhijärvi 1983). Moist, flat, stonefree areas created an interest in the 20th century to convert peatlands to productive use such as for agriculture and especially for forestry. By 1980, more than 50 % of the peatland area in Finland, including different site types, was drained for forestry use (Paavilainen and Päivänen 1995). The economic goal, increased tree growth, was obtained in about 83 % of the drained areas. The remaining 17 % of drained peatlands turned out to be unsuitable for forestry (Eurola et al. 1991), resulting in ecologically disturbed areas. Values of biodiversity, the desire to maintain functional peatland ecosystems and to form buffer zones for capturing nutrient losses after logging or complimentary ditching created a need to restore some disturbed peatland areas to their predrainage stage. How much drainage has changed the ecosystem of different mire types and how mires are being restored is still poorly known. This thesis examines how a minerotrophic fen and an ombrotrophic bog developed and how their ecosystems changed due to drainage and following restoration.
1.2 Peatland initiation and peat formation
Topography plays an important role in the initiation process of peatlands. With the retrieval of ice after the last glaciation period, the exposed land provided moist depressions where water stayed for long periods of time. These places were favorable for peatland initiation, which often started by paludification. Paludification is a process of colonization rather than infill (Heatwaite et al. 1990). It is an efficient peat forming process, thus the transition zone between the mineral bottom and paludified organic peat soil usually remains thin (Korhola 1992 and Paper I). Also, the lateral expansion of peatlands by paludification may occur quite efficiently. Peatlands may start also by vegetation spreading over the open water, i.e. terrestrialization. This peatland forming process, however, is less extensive than paludification (Sjörs 1983, Charman 2002).
Organic matter starts to accumulate when biomass production exceeds the rate of decay (Moore 1974, Tallis 1991). In cool and humid northern climatic conditions, in which precipitation exceeds evaporation, the decay rate is low. When initiated, the peat forming process is a self-sustainable system and continues until disturbances occur.
Most of the mires in the raised bog region in Finland started as minerotrophic (Korhola and Tolonen 1996). The source of water determines the nutrient level of a peatland and it also affects the vegetation composition of a site, Sphagna being the most important peat forming genus in nutrient poor and acid environments (Pakarinen and Tolonen 1977a, Damman 1978, 1986, 1990, Malmer 1990).
The minerotrophic development continues as long as the connection to minerogenous waters is maintained. The accumulation of peat mass finally causes a diminishing
supply of minerogenous waters from the surroundings and the surface peat layer becomes more dependent on atmospheric supply. With lowered nutrient levels, the mire starts to develop towards ombrotrophy. The date of ombrotrofication can be detected from stratigraphical changes, such as changes in vegetation composition and element concentrations in peat (Mörnsjö 1968, Paper I). Korhola and Tolonen (1996) suggested that the ombrotrophication in Finnish peatlands is a time-transgressive feature from south to north starting about 8000 14C BP in southern Finland. In central Finland, ombrotrophication has been dated most commonly at about 3000 14C BP (Korhola and Tolonen 1996, Paper 1).
As peat accumulation continues, clearly different kind of layers begin to form. At the time when the surface peat rises above the water table level (WT), it differs hydrologically and functionally from the peat below. The diplotelmic structure (Ingram 1978) is formed. The surface layer, acrotelm is also known as ‘active layer’ because most of the living organisms exist in this oxidized layer. The lower limit of the acrotelm is the lowest level of the fluctuating water table (Damman 1978, 1986), which varies according to the type of mire and the precipitation-evaporation balance.
The lower layer, catotelm (Ingram 1978), consists of the rest of the peat. It is constantly water saturated, lacking aerobic microorganisms. However, as increasing methane concentration at greater depths indicates, decay continues in the catotelm (Clymo 1984). The transition zone between acrotelm and catotelm is not clear in most cases (Ingram 1978).
The surface peat structure of drier and wetter places, i.e. hummocks and hollows in bog peat (e.g. Aaby 1976), and ridges and pools in fen peat (Foster et al. 1983), causes heterogeneity in the structure of accumulated peat. The formation of hummocks and hollows is influenced by climatic factors (Aaby 1976), and by the physical and hydrological characteristics of the peat (Karofeld 1998). The area of hummocks increases during warmer and drier periods, whereas colder and wetter periods are favorable for the expansion of hollows. More hollows have been formed in Finnish bogs during the last 3000 years (Aartolahti 1967, Tolonen 1987 and Paper I).
Decomposition of plants and other organic material mostly occur in the acrotelm (Ingram 1978, Clymo 1984). The decomposition rate depends on oxygen and water content, vegetation species composition, population of decomposers and temperature (Damman 1978, 1979, Belyea and Warner 1996, Klarqvist 2001). Wet and acid conditions create a poor environment for decomposers. Furthermore, most of the peat- forming species are resistant to decay, Sphagna being the most resistant ones (Dickinson 1983, Johnson and Damman 1993, Beleya 1996). This results in high peat accumulation rates especially in Sphagnum mires. After being deposited to the anoxic, water-saturated catotelm, material undergoes only minor physicochemical changes.
Some anaerobic decomposition occur in the catotelm, but the rate of decay is very slow (Clymo 1978). Thus peatlands form excellent archives for later studies.
1.3 Peatlands as archives
The macro- and microfossils of plants and animals are well preserved in deposited peat. They can be used as proxy data in reconstructing past ecological conditions at a site (e.g. Warner 1990). Their deposition time in peat is most commonly dated with radiocarbon dating system, which, based on a half-life of 5568 years for 14C, can be used for samples younger than 40 000 years. Accordingly, the conditions of the past habitat can be reconstructed.
The peat accumulation rate of organic matter can be determined according to the dated depths. The accumulation rate depends on the rate of decay, which further depends on the habitat conditions, such as moisture content, air and soil temperature and species present (Tolonen 1973; Damman 1978, Ikonen 1993, Beleya and Warner 1996).
About 50% of the plant material is carbon. Soil macro- and microorganisms obtain their carbon by eating dead plant material. Most of the carbon dioxide (CO2) is returned back to the atmosphere by vegetational and microbial respiration, and decay of organic matter. The rest is annually accumulated as carbon with dead plants and microorganisms. The carbon accumulation rate depends on the species present and the decay rate (Tolonen and Turunen, 1996). It is also influenced by environmental conditions such as ecohydrology, temperature and environmental hazards such as wild fires (Kuhry 1994, Pitkänen et al. 1999, Klarqvist 2001).The amount of accumulated carbon in northern mires may be overestimated (Mäkilä et al. 2001, and Paper I).
The vegetation history since the last glacial period has been investigated in detail in numerous studies using pollen analysis. Although most of the pollen rain is from short distances, long distance transport of pollen also occurs (Moore and Webb 1978). Thus changes in pollen stratigraphy indicate both human activities in the area and large- scale climatic conditions. Plant tissue remains are autochthonous in origin and represent the vegetational development on the peatland in question (Warner 1990).
Their remains preserve well in acid, anaerobic conditions, but need much more delicate sample preparation for microscopical analysis than pollen (Warner 1990 and Paper I). Species composition is influenced by hydrological and chemical environment of the peatland.
Peat chemistry is determined mostly by the source of water (e.g. Damman 1986).
Changes in element concentrations occur in the acrotelm due to growth and decay of flora and fauna. Once deposited, some of the changes still occur, but very slowly (Clymo 1984). Changes in the chemostratigraphy of ombrotrophic peatlands reflect differences in atmospheric deposition (Damman 1990). In minerotrophic peatlands, the geology and mineralogy of the catchment area play a more important role (Damman 1995). The element concentrations in the upper 15 cm of the surface peat differ from those below in both types of mires (e.g. Damman 1978, 1990 and Paper I).
Peat microfauna indicates the environmental conditions of the peatlands, especially the hydrological ones. Testate amoebae (Protozoa) form a major component of peat microfauna, being especially abundant in Sphagnum-mires (Heal 1962, Corbet 1973, Meisterfeld 1977, 1979, Sleigh 1989). Their shells are well preserved in accumulated
peat. Because of their sensitivity to changes in moisture content, peat chemistry, and vegetation, their species composition can be used to reconstruct palaeohydrological conditions (Tolonen 1966, Warner 1987, Ikonen 1993, Mitchell et al. 2000, Paper II).
1.4 Human impacts on Finnish peatlands 1.4.1 Drainage
Drainage has changed Finnish peatland ecosystems drastically during the 20th century.
The management of peatlands was extensive at the beginning of the 1940’s, and during the period between 1960 and 1980. In 1953, about 9.4% of the peatland area in Finland was drained, whereas the drained area in 1994 was 52.5%. The percentages for southern Finland were 17.7 and 74.9%, respectively (Hökkä et al. 2002). Drainage has been primarily concentrated in the forested mire types, although Sphagnum fuscum bogs and short sedge pine fens have been drained to some extent (Eurola et al. 1991).
The growth of pines increases shortly after drainage and continues for about 15 years, at which point growth decreases again; in comparison, spruces show a slower response (Seppälä 1969).
The ditch density and depth used depend on site fertility and moisture conditions. A ditch depth of 70-90 cm and a spacing of 30-50 m, (Paavilainen and Päivänen 1995 and Paper IV) have been commonly used. Also the area nearby is often affected by drainage (Aapala and Lindholm 1999).
Increased oxygen conditions mean increased microbial activity and thus an increased amount of plant available nutrients (Johnson and Damman 1993, Malmer and Wallen 1993). Drainage means also accelerated decomposition in the aerated acrotelm.
However, in some peatland sites drained for forestry, and especially in ombrotrophic mires nutrients have shown to be a limiting factor for tree growth (e.g. Damman 1990).
Better growing conditions exist in mires with catchment areas that have provided minerogenous waters to the mire.
Aerial photographs can be converted to digital form to enable the numerical classification of vegetation in large areas or changes in vegetation over long periods of time (King 1991, 1995, Næsset 1996, Holopainen 1998, Holopainen and Wang 1998a, 1998b, Pellikka 1998, Holopainen and Jauhiainen 1999). As a method, it is more objective and faster than field mapping. This method was used to study drainage- driven changes in vegetation structure (Paper II).
1.4.2 Restoration
Restoration became topical in forest management during the late 20th century. The importance of biodiversity, the protection of key biotopes, the renovation of disturbed peatlands, and those drained peatlands located in nature conservation areas created needs to restore these areas to their predrainage condition (Aapala et al. 1996, Vasander et al. 1998, 2003, Kuuluvainen et al. 2002).
Peatland restoration means that ecological and vegetational changes are induced or accelerated by blocking ditches with dams or filling them to elevate the water table to
the predrainage level. In forested mires, trees are also often cut to diminish evapotranspiration in the area (Heikkilä and Lindholm 1994). How mire vegetation starts to recover and at what rate is not accurately known.
Regeneration of plant communities has been found to occur from seed banks in habitats such as marshes (van der Valk and Davis 1978, Smith and Kadleg 1985, ter Heerdt and Drost 1994), shorelines and flooding areas (Nicholson and Keddy 1983, Schneider and Sharitz 1986, McDonald 1993, Jutila 1994) and cutover peatlands (Salonen 1992, Tuittila et al. 2000). Several species in wet habitats regenerate from other parts of plants also, such as buds and capitulae (Van der Valk and Davis 1978, Sastroumo 1981, Zobel and Antos 1992).
By restoration, runoff of water from peatland is often prevented by damming or infilling ditches to raise the WT. The consequence of successful restoration is a self- sustainable functioning peatland ecosystem (Wheeler 1995). Restoration may well contain a variety of eventual objectives including either the restoration of the site to its original state or to a former state. What is ‘original’? At what stage of the succession was the peatland at the time of the first drainage management? The successional state which occurred on the site before drainage may never be recovered. Even if the exact conditions of surface vegetation ecology and hydrological functioning could be reproduced, at the very least the stratigraphic record would certainly be disturbed.
Restoration may create a habitat that never existed on the site previously.
1.5 Aims of the study
The overall aims were to study 1) the Holocene development of two different mire sites, a fen and a bog in the same geographical area, 2) how drainage has changed their ecosystems and 3) how successfully they can be restored.
In order to achieve these aims, the stratigraphy of both peatlands was studied for peat and carbon accumulation rates, and for changes in physical and chemical properties during the peatlands’ development. Past vegetation composition was investigated according to pollen and plant tissue remains analysis. The factors that have influences on stratigraphical changes were considered (Paper I).
The effects of drainage on ecohydrology and vegetation were studied by using aerial photographs which covered the drainage period (Paper II). The abundance and distribution of ecohydrological indicators, testacean amoebas were studied in the upper 100 cm of peat, a depth that encompasses natural, drained and restored indicators (Paper III).
Restoration was started by studying the potential of seed and spore banks to regenerate peatland vegetation (Paper IV), and continued with a study of changes that occurred in ecohydrology and species cover during the three restoration years and what stage we achieved with restoration (Paper V).
2 Material and methods
2.1 Study sites
The mires studied are located about 60 km NE of Tampere in southern Finland (Figure 1, Papers I-V). The area belongs to the boreal raised bog region (Ruuhijärvi, 1983).
Mean annual precipitation in the area is 709 mm, and mean annual temperature +2.9 ºC (Finnish Meteorological Institute, Juupajoki Station, average 1961-1990).
The fen site (Konilamminsuo mire) forms the northern part of the Hanhisuo mire complex (61º48’N, 24º17’E, 155 m a. s. l.) and is located between the esker Vatiharju and the lake Hanhijärvi. When restoration started in 1994, the site was dominated by a pine (Pinus sylvestris) stand of 104 m3 ha-1, mixed with some spruce (Picea abies) and birches (Betula pubescens and B. pendula). The field layer vegetation consisted of dwarf shrubs, namely Ledum palustre, Vaccinium vitis-idaea, V. uliginosum, V.
myrtillus and B. nana. The moss layer was composed of Sphagnum angustifolium, S.
magellanicum and S. russowii with the forest moss Pleurozium schreberi.
The bog site (Viheriäisenneva mire) is located about 10 km north from the studied fen (61º 51’ N, 24º 14’ E, 160 m a. s. l.). The prerestoration vegetation consisted of seedlings of pines (Pinus sylvestris and P. contorta) with a density of 4400 seedlings per ha. Most of the seedlings were less than 1 m tall, and not taller than 3.1 m. The field layer vegetation consisted of Calluna vulgaris, Empetrum nigrum and V.
uliginosum. The bottom layer was densely covered by Cladonia species and some ombrotrophic Sphagna.
The Lakkasuo mire which vegetation data is used in Paper II, is described in Pitkänen et al. (2001).
Nomenclature follows Hämet-Ahti et al. (1998) for vascular plants, Koponen et al.
(1977) for bryophytes, and Ahti (1993) for lichens.
2.2 Experimental design
Areas of 1.1 ha in the fen site and 10.5 ha in the bog site were chosen for restoration.
Smaller experimental areas, 0.6 ha in the fen and 0.5 ha in the bog site, with control sites were delimited (Figure 1). The closest distance from the experimental area to the mire margin was more than 100 m in the fen site and about 500 m in the bog site. To monitor the changes in vegetation species cover (Paper V) and WT during restoration (Papers IV, V) permanent vegetation plots and ground water wells were set on the areas. Twelve vegetation plots (50x100 cm) were located in restoration sites and 9 in control sites. Ground water wells were located adjacent to each of the vegetation plots.
In the fen site an additional set of ground water wells was located as a gradient line starting from the mineral soil, close to the esker and reaching the middle of the restoration site. The WT was also measured from the wells set in the areas in 50x50 m grids, 29 wells in the fen site and 52 wells in the bog site.
Trees in the experimental areas of 0.6 and 0.5 ha in the fen and bog, respectively, were measured, clear-cut and removed from the areas late in autumn 1994. Ditches were filled or partly blocked in February 1995. In addition, water was directed to the restoration area in the fen site along the newly-constructed feeder ditch.
2.3 Measurements and analysis
Water table level was measured weekly during the growing seasons between 1994 and 1997 (Papers IV, V). The field layer vegetation was investigated annually at the end of July in 1994 and 1997, starting before restoration operations in 1994. The peat cores for the analyses (seed bank, testacean amoebas, radiocarbon dating, pollen, plant tissue remains and peat chemistry) were taken in autumn 1994 and repeated for testacean amoebas and physicochemical analysis 3 summers after restoration, i.e. in autumn 1997 (Table 1 and Paper IV). The cores down to a depth of 100 cm were taken using a volumetric peat sampler (8.3x8.4x100 cm). Deeper peat was cored using a Russian peat sampler (6.5x50 cm). The degree of decomposition was estimated in the field using von Post’s (1922) 10-grade scale (H1-H10).
A
B
200 m 200 m
Helsinki Tampere
x Study site
A B
200 m 200 m
Figure 1. Location of study sites in southern Finland.
A) Konilamminsuo mire (fen site) B)Viheriäisenneva mire (bog site)
2.4 Investigation of peatland development
In order to reconstruct the initiation and development of the mires, both mires were studied using palaeoecological methods (Paper I). Radiocarbon dating for the peat profiles was made systematically at 50 cm intervals, as well as for the basal peat, at 230 cm at the fen site and 430 cm at the bog site. Pollen and plant tissue remain analyses were done from the same cores. Both conventional and calibrated ages were determined. Pollen analysis was made for peat profiles at 5 and 10 cm intervals for the fen and the bog, respectively, following the method of Faegri and Iverssen (1964).
Plant tissue remains were analyzed at 25 cm intervals for both mires according to the method of Korotkina (1939). Physical (bulk density, organic matter content) and chemical properties (pH, Al, Fe, Ca, K, Mg, Mn, Na, P, C and N) were analyzed before and after restoration at 10 cm intervals between the depth of 0-100 cm and at 12.5 cm intervals for the rest of the peat profile (Paper I). Peat and carbon accumulation rates were examined according to dated peat profiles (Paper I).
2.5 Investigation of drainage effects
Drainage-driven changes in moisture conditions and vegetation composition were studied using aerial photographs (Figures 2 and 3, and Paper II) which covered the time period between 1946–1995. Color-infrared (1995) and panchromatic (1946) aerial photographs (Paper II, Table 1) were used for the final analysis of vegetation change.
The infrared aerial photograph of year 1995 was digitized, and the vegetation was classified according to tone and texture values of the photo windows (Paper II). The digitized data (tone and texture values of the aerial photo) were analyzed together with the corresponding field data of vegetation survey. The vegetation in the 1946 photograph was reconstructed according to the analysis of the 1995 photograph and the study of Holopainen and Jauhiainen (1999). The mire site types of predrainage (1946) aerial photograph were reconstructed according to the analysis of the 1995 vegetation data and to the known vegetation classes of the 1946 mire site types.
Drainage effects were also studied by stratigraphical methods (Paper I). What physicochemical changes have occurred in the surface peat? The abundance and distribution of the testaceans were studied from peat profiles of both mires before and after restoration at 10 cm intervals to the depth of 100 cm (IV). This section was supposed to include the different stages of peatland: natural, drained and restored.
Shells of testaceans at different depths were identified using a high power microscope.
Changes in the abundance and distribution of testaceans were studied in relation to changes in WT and changes in peat chemistry. Also the relationship between testaceans and vegetation was studied according to contemporaneous occurrence of testacean cells and plant tissue remains in peat (Paper IV).
2.6 Investigation of restoration effects
The potential of peatland species to recover from the seedbank was studied by germinating peat samples in the greenhouse (Table 1 and Paper III). Nine cores of both mires were taken at a depth of 50 cm. The upper 20 cm was divided into slices of 5 cm,
Figure 2. Aerial photographs of years 1946, 1966, 1984 (pancromatic) and 1995 (colour infrared) for the fen site.
1946 1966
1984 1995
100 m
1946 1966
1984 1995
Figure 3. Aerial photographs of years 1946, 1966, 1984 (pancromatic) and 1995 (colour infrared) for the bog site.
100 m
and the rest into 10 cm slices. Peat-quartz sand mixtures of samples were placed on trays which were arranged in the greenhouse. The number of seedlings germinated were recorded weekly during twelve weeks.
The abundance and distribution of testacean amoebas (Paper IV) were investigated in the surface peat profile before and after restoration.
Restoration-driven changes in species cover of vegetation during the period of 1994- 1997 were studied by determining the percentage cover of each species in permanent vegetation plots each year (Paper V). The main comparison in species cover was made between the years of prerestoration (1994) and post-restoration (1997). Restoration driven changes in moisture content were estimated with water table level measurements. Differences in element concentrations in the uppermost 20 cm of peat before and after the restoration were studied (Paper V).
Table 1. Summary of all the samples taken for the different studies: 1= the number of samples taken in restoration sites, 2=control sites, 3= sampling depth, 4= analysis to use, 5=paper described in, and sampling years.
1 2 3 4 5 1994 1995 1996 1997
Peat samples
12 9 0-50 cm Chemical analysis (I,IV,V) x x 12 9 0-50 cm Seed and spore bank (III) x
1 0-basal peat Phys.+chem. analysis (I,IV,V) x x 1 0-basal peat Radiocarbon dating (I,IV) x
1 0-basal peat Pollen analysis (I) x 1 0-basal peat Plant tissue remains (I,IV) x 1 0-100 cm Testacean amoebas (IV) x Vegetation
12 9 50x100 cm Vegetation survey (V) x x x x
Water table
12 9 and Grids WT measurements (III,IV,V) x x x x 2.7 Data analysis
Stratigraphical data for testaceans (Paper IV), plant tissue remains and pollen (Paper I) are presented using the TILIA program (Grimm 1992). The diagrams describe the relative percentage of each species occurring in each sample studied. The relationship between the species studied (testacean amoebas, vegetation) and environmental variables (peat chemistry, tone and texture values of digitized IR aerial photographs) (Paper II, IV, V) were analyzed with CCA (canonical correspondence analysis) of the CANOCO program (ter Braak and Šmilauer 1998). Annual changes (1994-1997) in vegetation composition at the sample plot level were further studied using DCA (Detrended Correspondence Analysis) of the CANOCO program (Paper V). Cluster analysis (Legendre and Vaudor 1991) was used to study possible patterns in time- depth scale in testacean species occurrence. A distance tree was constructed using
UPGMA (Unweighted Pair Group Method with Arithmetic Mean) classification with Steinhaus index (C=2w/(a+b) as a distance measure (Paper IV, Figure 4). Element concentrations in peat before and after restoration were compared using Paired t-test (Paper IV) and two-sample Kolmogorov-Smirnov (K-S) test (Paper V).
3 Results and discussion
3.1 Development of peatlands 3.1.1 Initiation and accumulation
The peatlands studied initiated soon after the retrieval of the latest glacier, when the land was revealed after the Ancylus regression. The radiocarbon dates (uncalibrated years) for the basal peat was 8130 ± 160 14C BP for the studied fen and 9110±120 14C BP for the bog (Figure 4 and Paper I). It is supposed, however, that the fen site in the study area initiated before 9000 14C BP, but the oldest part of the mire is located in the southern part of the large Hanhisuo mire. The mire has grown laterally towards north and reached the surroundings of the Konilampi area about 1000 years later. The lateral expansion of mires is an effective peat forming process, as reported by Korhola (1992).
An estimate of mire initiation time in the area to be before 9000 14C BP is supported by Tolonen et al. (1979) who dated the nearby Siikaneva mire initiation at 9700 14C BP.
In some more southern parts of Finland, favorable mire initiation period was dated later: 7200-6500 14C BP (8000-7300 cal BP) (Korhola (1995).
The initiation of both mires occurred by paludification under the influence of minerogenous waters, as happens commonly for mires in the raised bog region (Korhola and Tolonen, 1996). The transition zone between the mineral bottom and the organic peat soil remained thin, i.e. less than 10 cm. The fen started to accumulate at a rate of 0.18 mm yr-1, whereas the bog site started at a rate of 0.48 mm yr –1. The mean accumulation rates for the fen and the bog were 0.28 mm yr-1 (range 0.16-0.58 mm yr-1) and 0.47 mm yr-1 (range 0.25-1.19 mm yr-1), respectively (Figure 4 and Paper I).
Peatlands in Finland have been measured to accumulate peat at a rate of 0.2-4.0 mm yr-1 (Korhola and Tolonen 1996).
The peat accumulation rate varied during the mire development mostly according to the vegetation composition and decomposition rates, which is species-specific. The lowest peat accumulation rates in the fen were for the bottommost (0.18 mm yr–1) and for the uppermost peat (0.16 mm yr–1). Both of these were well-decomposed peat, with a von Post’s scale of H6-7. Both sections were also rich in woody remains (Paper I, Figure 4). Betula remains were abundant in the bottommost peat, whereas Pinus and Betula remains were abundant in the surface peat.
The basal peat developed by paludification under the influence of minerogenous waters provided by the esker nearby. The decomposition rate was high due to the nutrient rich, oxic conditions. Accelerated decomposition in the surface peat occurred due to drainage, which caused increased oxygen content and microbial activity.
Figure 4. Relationship between peat depth and age of the mires. Filled circles with error bars represent uncalibrated years BP, and open circles with error bars represent calibrated years BP. The values represent the peat accumulation rates (mm yr-1) according to uncalibrated years during the development of mires.
The peat between 175-50 cm depth (5600-3200 PB) was composed mostly of Carex remains (Paper I, Figure 4). It was less humified peat and accumulated at a higher rate (about 0.5 mm yr-1) than woody peat.
The bog site had a more variable peat accumulation rate than the fen site. The Carex peat of the early state followed by the Eriophorum-dominated peat accumulated almost at the same rate, i.e. 0.41 and 0.35 mm yr-1, respectively (Paper I, Figure 4). S. fuscum was dominant between 4600 and 4200 14C BP, and reached its maximum of 80% of species composition at about 4400 14C BP. During this period, the poorly humified peat accumulated at its highest rate (1.19 mm yr –1).Sphagnum(especiallyS. fuscum) is the most resistant species to decay (Johnson and Damman 1991, 1993, Malmer and Wallen 1993, Beleya 1996). Since Sphagna contain phenolic compounds and uronic acids, they act also as acidifying agents (Verhoeven and Liefveld 1997) which further restrain the decomposition rate (Johnson and Damman 1993, Charman, 2002). The high accumulation rate might have been influenced partly by climate also. After the warm and moist Atlantic period, the climate became cool and humid, which favored peat accumulation. Accelerated peat accumulation rates during this time period have been found also by Korhola (1995) and Korhola and Tolonen (1996).
Carbon accumulated at an average rate of 11.1 g m-2 yr-1 at the fen site and 13.2 g m-2 yr-1 at the bog site (Paper I). These were lower than the averages reported for fens (15.1 g m-2 yr-1) and bogs (24.0 g m-2 yr-1) in Finland by Tolonen and Turunen (1996).
However, there is a large variation in carbon accumulation rates according to mire types, age of the mire and decomposition rates. The long-term rate of carbon accumulation (LORCA) has been found to be lower for old than young mires, and lower for drained than undrained mires (Turunen et al. 2002).
Fen 0
50
100
150
200
250
0 2000 4000 6000 8000 10000 A ge (years BP)
Depth, cm
Bog 0
100
200
300
400
0 2000 4000 6000 8000 10000 A ge (years BP)
Depth, cm
0.16
0.53
0.58
0.36 0.18
0.74 0.63 0.25
0.68 1.19
0.35 0.41
0.41 0.48
Clearly different stages in peat and carbon accumulation occurred during peat development. Carbon accumulation rate varied from 6.6 to 15.3 g m-2 yr-1 at the fen site and from 6.1 to 21.1 g m-2 yr-1 at the bog site (Paper I, Table 2). Klarqvist (2001) showed also high variation in C accumulation rate in Swedish mires. Factors such as species composition, decomposition rate (Tolonen and Turunen, 1996), climate change, and mire fires (Kuhry, 1994, Pitkänen et al., 1999) have an influence on carbon accumulation rates during different periods of mire development.
3.1.2 Changes in chemostratigraphy and past vegetation
Both mires initiated under the influence of minerogenous waters. Minerotrophic development continued as far as minerogenous waters were provided to the root zone.
The fen site initiated as a nutrient-rich reed thicket with abundant Phragmites australis and Betula spp. The initiation, however, occurred in relatively acid conditions, as shown by a pH of 3.6 for the basal peat. Between the time period of 8000 and 4800 14C BP, the fen gradually developed towards a poorer Carex-dominated fen. The mire remained minerotrophic during all its development, but developed as an extremely poor fen between the time period of 4800-4200 14C BP (between 150 and 100 cm).
Probably the input water from the esker did not reach the surface peat anymore. Thus, the input waters became diluted by nutrient poor precipitated water. The element concentrations decreased to the levels of the ombrotrophic bog (Paper I, Figure 2). The vegetation composition, however, was still Carex-dominated, but the Ca/Mg ratio decreased below 10 which indicates ombrotrophication (Chapman 1964, Mörnsjö, 1968).
The extremely poor Carex-dominated fen continued to develop without remarkable changes in the peat stratigraphy until the physicochemical boundary at 50 cm depth, which represent the radiocarbon age of 3220 14C BP.
The upper 50 cm of peat was compacted due to drainage. The BD (bulk density) of the compacted surface peat was 52% higher than that of the peat below (Paper I, Figure 2).
Silins and Rothwell (1998) found in Alberta an even higher increase in BD (63%) over a shorter period of drainage. If the fen site had continued to accumulate at the mean rate of 0.28 mm yr–1instead of 0.16 mm yr–1since 3220 14C BP, the thickness of the upper section would have been 90 cm instead of 50 cm. Minkkinen et al. (1999) reported that the surface peat in minerotrophic mires subsides 20-28 cm during a 30- year-drainage period.
The element concentrations are normally higher for any soil surface. The differences in element concentrations between the surface peat and the peat below were greater at the fen site than at the bog site. Low nutrient levels at the bog site and small changes during the last decades in the concentrations of precipitated water do not allow for large chemical changes in the surface peat. Greater changes may occur in ombrotrophic mires located in coastal areas (Damman 1995). Analysis at smaller than 10 cm intervals in the compacted surface peat would have given more detailed information about the development of that section and also about the effects of drainage. A large amount of information remains unnoticeable when the decay rate is high (Aaby 1976).
The bog site started also under the influence of minerogenous waters. The pH of the basal peat, which was composed of PhragmitesandCarex was 3.2. The bog site turned ombrotrophic around 7200 14C BP after going through a minerotrophic Carex state during 2000 years. At that time, the peat had reached a thickness of 80-90 cm, when theCarex-dominated mire started to develop towards a Sphagnum-mire. This, together with a strongly decreased Ca/Mg ratio indicates that the mire lost contact with minerogenous waters and developed towards an ombrotrophic bog (Chapman 1964, Mörnsjö 1968, Seppä 1991, Korhola 1992, Heikkilä et al. 2001). Otherwise, changes in the element concentrations were gradual between 9100 and 4800 14C BP.
Large-scale changes occurred in peat chemistry between the period of 4800 and 4200
14C BP (250-200 cm). The element concentrations oscillated strongly during those 600 years and remained at extremely low levels for the rest of the mire development (Paper I, Figure 3). Large scale climatic changes from the warm Atlantic chronozone to the cooler and relatively humid Subboreal chronozone at about 5000 14C BP (Donner et al.1978) preceded changes in peat chemistry and were the most probable factor influencing chemostratigraphical changes in peat. Simultaneous to the major changes in peat chemistry of both mires, the surrounding deciduous forests changed to coniferous ones (Paper I, Figures 6 and 7).
Since this climatic boundary, the climatic conditions of the last 5000 years have been cooler and more unstable (Eronen and Zetterberg, 1996). Variations in climate and moisture conditions affected the formation of hummocks and hollows in the upper 200 cm of bog peat. The formation of hummocks indicates drier periods whereas increased precipitation favors the formation and expansion of hollows (Aaby 1976, Karofeld 1998). Karofeld (1998) found that more hollows developed between 3500-1000 14C BP in Estonian mires. Species such as E. vaginatum, S. balticum and S. papillosum were abundant in the upper 125 cm of the bog peat. The composition of which indicate more moist conditions (Lindholm and Markkula 1984). The hummock-hollow pattern also influenced the oscillating element concentrations in the peat profile. Similar findings have been reported by Pakarinen (1978) and Damman (1978).
Drainage-driven changes were hardly visible in the bog stratigraphy (Paper I, Figure 3). Low nutrient levels do not allow great concentration or vegetation composition changes (Vasander et al. 1996).
3.2 Drainage impacts
An immediate change occurred in the WT after the ditches were opened (Papers IV and V). The changes depend on the source of water and the density and depth of the ditches (Paavilainen and Päivänen 1995, Lundin 1999). In drained conditions, the WT in forested peatland fluctuated during the growing season from 20 cm in early summer to 65 cm in early August (Figure 5 and Paper V). The evapotranspiration of the tree stand was likely the main factor behind the large variation in WT.
Drainage caused more changes in the vegetation structure at the fen site than at the bog site. Pine-dominated areas increased (Figure 2), and two drained mire site types:
drained, pine-dominated Vaccinium type (Ptkg) and drained, pine-dominated dwarf shrub type (Vatkg) were formed.
Figure 5. The depth of the WT in restoration areas before (1994) and after (1997) restoration.
The stand volumes, 104 m3 ha-1 for the restoration site and 96 m3 ha-1 for the control site in 1994 were slightly lower than the average for similar drained mires in the region, but still within the normal variation (cf. Laiho and Laine 1994). Field layer vegetation changed from peatland vegetation to dwarf shrub vegetation. V. vitis idaea, V. myrtillus and L. palustre were common at the fen site.
The ombrotrophic bog site lacked strongly evaporative vegetation (Figure 3). As such the WT fluctuation was smaller (Figure 5) and more dependent on changes in precipitation (Lindholm and Markkula 1984, Reinikainen et al. 1984). At the bog site, tree growth did not reach an economically valuable level during the drainage period.
Seedlings at the site were not taller than 3.1 meter, and about 78% of pine seedlings in the restoration site and 50% in the control site were under 1 m tall. Density was 4400 seedlings ha-1 in both sites. Drainage made the uppermost 10 cm of peat dry and crispy, and the surface became densely covered by lichens, Cladonia and Cladina species (Paper V).
3.3 Restoration impacts
The study of the seed and spore banks of the sites to be restored showed that their role in regeneration of peatland vegetation was minute. Seed germination from the seed bank was relatively poor in these study sites (Paper III). Seeds of V. myrtillus andV.
uliginosum germinated relatively well, but V. uliginosum and especially E. vaginatum and E. nigrum germinated vigorously from propagules. Vegetative growth started as soon as the samples were placed in the greenhouse, which seems to be the main reproduction strategy in mire ecosystems, as found also by Van der Valk and Davis (1978), who suggested that a more appropriate term for wetland seed banks would be
“propagule banks”. No conifers germinated in this study, although the fen site is surrounded by a pine-spruce stand. The abundant C. vulgaris at the bog site did not germinate from seeds. The low pH of the substrate and the simultaneous occurrence of Cladonia species in the bog surface have been found to reduce germination of Calluna (Helsper and Klerken 1984, Hobbs 1985).
Fen site
0 10 20 30 40 50 60 70
23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 June I July I A ugust I S eptem ber
1994 1997
W eek
B og site
-10 0 10 20 30 40 50 60
23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 June I July I A ugust I S eptem ber
1994 1997
W eek
The seed banks, however, have been found to play an important role in the regeneration of plant communities in other wetlands, such as marshes (van der Valk and Davis 1978, Smith and Kadleg 1985, ter Heerdt and Drost 1994), shorelines and flooding areas (Nicholson and Keddy 1983, Schneider and Sharitz 1986, McDonald 1993, Jutila 1994).
The tree stand clear-cut, ditch filling, and the constructed feeder ditch caused an immediate change in WT at the beginning of restoration. Because of strongly decreased evapotranspiration at the fen site, the WT fluctuation decreased, and the mean WT remained to a level typical of pristine mires (Lindholm and Markkula 1984, Reinikainen et al. 1984).
Smaller changes in WT occurred at the bog site because it lacked an evaporative tree stand at the prerestoration stage. Damming the ditches at the bog site elevated the WT to a level of a natural bog. The WT changes in ombrotrophic mires are more sensitive to the amount of precipitation (Lindholm and Markkula 1984, Reinikainen et al.
1984).
Vegetation changes were greater at the fen site, where tree removal changed light conditions and the ecohydrological shift was larger. After clear-cutting, the increased light and moisture conditions favored germination of vegetative propagules, especially of E. vaginatum (Paper III). Being efficient in using the increased nutrients (Komulainen et al. 1999), Eriophorum started to grow vigorously. Because Eriophorum is an opportunistic pioneer species, it can rapidly take over the habitat created by restoration of different kinds of peatlands (Grosvernier et al. 1995, Pfadenhauer and Klötzli 1996, Robert et al. 1999, Tuittila et al. 2000). The Eriophorum stage may, however, be a transitional stage towards Carex- and Sphagnum-dominated mire vegetation (Grosvernier et al. 1995, Tuittila et al. 2000). In 1997 true fen species, such as Carex rostrata, Calla palustris, and Potentilla palustris, as well as several species of Sphagna were already present at the fen site. Due to clear- cutting at the restoration sites, the increased light conditions increased vegetation growth in the area nearby (Paper V). Moore and Bellamy (1974) found also that land use changes affect the areas nearby as well, not only the area treated.
At the bog site, dry lichen-covered surface was overtaken by moist Sphagna vegetation and the growth of E. nigrum in hummocks increased considerably, because it was released from epiphytic lichens. Although the Sphagnum cover of lawns and hollows already increased during the first three years, a major change can only be expected after several years (Heikkilä and Lindholm 1994).
Raised WT caused changes in element concentrations in the surface peat. The surrounding watershed, including an esker, served as the main source of mineral nutrients at the fen site. The strongly decreased K concentration may be due to leaching from peat (Damman 1986, 1990, 1995), or it may have been used by the vigorously growing Eriophorum (Malmer 1958, Damman 1986). Restoration-driven changes in element concentrations occurred mostly in the upper 20 cm of peat (Paper V). Because of the low concentrations of mineral elements in the ombrotrophic peat and in precipitation (Mörnsjö 1968, Damman 1986, 1990) the postrestoration changes
in the surface peat nutrient regime of the bog remained small and the change in ecohydrology was less marked than at the fen site.
The abundance and distribution of testacean amoebas, which have been used as ecological indicators (Tolonen 1966, 1986), changed during the restoration years. At the fen site, Nebela militaris almost disappeared during the restoration and was replaced by Cryptodifflugia oviformis. C. oviformis, however, decreased at the surface peat of the bog. This may suggest that conditions in the surface layer at the two sites changed in different ways, because of differences in vegetation cover and the quality and quantity of input water. Increased moisture content at the bog surface deteriorated the living conditions for Amphitrema flavum, which decreased in numbers during restoration, whereas Amphitrema wrightianum and different species of Hyalosphenia benefited from the increased moisture content.
Conclusions
The two studied peatland types - minerotrophic and ombrotrophic - had some similarities during their development. The main trend in the development of both peatlands was from higher to lower nutrient levels. Large-scale climatic changes had an influence on both mires, causing similar types of changes in the chemostratigraphy.
However, the development is strongly influenced by the source of water and changes in the catchment area as well as in quality and quantity of precipitation. The accumulation rates are species-specific and dependent also on decomposition rates.
Drainage changed the ecosystems in both mires, although more for the fen than for the bog. The fen site developed a functional forested site, with two drained, forested mire site types, whereas the bog remained economically unproductive. However, renovation of ditches in the fen site would have been necessary periodically.
When restored, the peatland vegetation regenerated mostly vegetatively, but also from modern seeds and spores which spread into the restored areas from the surroundings.
The potential of seed and spore banks to regenerate the peatland vegetation was rather poor, but the initiation occurred from propagules. Peatland vegetation in both mires spread relatively fast. This means successful restoration. Forest shrubs declined at the fen site and E. vaginatum became dominant. At the bog site, the dense lichen cover almost disappeared and Sphagna increased in size and abundance. The water table level remained high and the peatland ecosystems of the fen and the bog became self- sustainable.
Acknowledgements
This work was carried out at the Department of Forest Ecology, Division of Peatland Ecology and Forestry, University of Helsinki. I am grateful to my supervisor Harri Vasander and to Docent Jukka Laine for introducing me to this field of reseach.
Without them this work would not have started. I want to thank warmly Harri Vasander and Raija Laiho for their encouragement, comments and valuable suggestions on the manuscripts and the thesis. I want to thank Aki Pitkänen with whom I had enjoyable paleoecological discussions. Special thanks also to Markus Holopainen with whom I discovered the interesting field of aerial photography. Silja Aho always
offered her friendly help in the laboratory. I greatly acknowledge the head of the Division, Prof. Juhani Päivänen, for providing me with working facilities. Docents Tapio Lindholm and Heikki Seppä have done a great and laborious job with the pre- examination of my thesis. Many thanks to Michelle de Chantal who revised the English language. My warm thanks go to my friends Leila, Pasi, Paula, Seppo, Timo, Tuula among others for their support during this long process.
This work was financed by the Graduate School of Forest Ecology, the Jenny and Antti Wihuri Foundation, the Finnish Cultural Foundation and the Niemi Foundation. I am grateful to these institutions for making this study financially possible.
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