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Vegetation Changes on Swedish Mires
Effects of Raised Temperature and Increased Nitrogen and Sulphur Influx
BY
URBAN GUNNARSSON
Dissertation for the Degree of Doctor of Philosophy in Ecological Botany presented at Uppsala University in 2000
ABSTRACT
Gunnarsson, U. 2000. Vegetation changes on Swedish mires. Effects of raised temperature and increased nitrogen and sulphur influx. Acta Universitatis Upsaliensis.
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 561. 25 pp. Uppsala. ISBN 91-554-4792-9.
Since the start of the industrialisation, the deposition of nitrogen and sulphur and the atmospheric concentrations of greenhouse gases have increased. The main objectives of this study were to find how these changes in climate and deposition can change the vegetation of mire ecosystems and the growth of Sphagnum species. Two main approaches were applied: re-investigated of two mires previously investigated 40-50 years ago and experimental manipulations.
The plant species diversity had decreased on one of the two re-investigated mires (Skattlösbergs Stormosse; central Sweden), but the total number of species was unchanged on the other (Åkhultmyren; southern Sweden). On Skattlösbergs Stormosse, an acidification was found in the high pH areas, coinciding with a reduction in rich fen species. At Åkhultmyren, there was a similar reduction in pH, but the changes in the plant composition also indicated increased nutrient levels and a drier mire surface. There were large changes in species composition on Åkhultmyren.
For instance Scheuchzeria palustris had disappeared from the investigated area.
Further, the cover of Scots pine (Pinus sylvestris) had increased, which can be explained by a changed ground-water table regime. Monitoring of pines growing on a bog over a ten-year period showed that pines growing higher above the ground-water table had higher survival than lower-growing pines.
Experimental addition of nitrogen during 3-4 years reduced Sphagnum growth in bogs and poor fens repressenting a wide range of ambient nitrogen deposition. A changed interspecific competitive relation was found between S. lindbergii and S.
balticum when increasing nitrogen influx, but the competitive relations between two hummock-growing species pairs did not change in a three-year nitrogen fertilization study. Sulphur addition did not affect the production or length increment in S.
balticum. An increased temperature reduced Sphagnum growth, but there were no indications of altered competitive relationships between hummock and hollow inhabiting Sphagnum species in a four-year experiment.
Key words: Acidification, growth, long-term studies, mires, nitrogen, species richness, Sphagnum, succession, sulphur, temperature.
Urban Gunnarsson, Uppsala University, Evolutionary Biology Centre, Department of Plant Ecology, Villavägen 14, SE-752 36 Uppsala, Sweden
Urban Gunnarsson 2000 ISSN 1104-232X
ISBN 91-554-4792-9
Printed in Sweden by Tryck & Medier, Uppsala 2000
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This thesis is based on the following five papers, which will be referred to in the summary by their Roman numerals (I - V).
I. Gunnarsson U, Rydin H. 1998. Demography and recruitment of Scots pine on raised bogs in eastern Sweden and relationships to microhabitat differentiation.
Wetlands 18: 133-141.
II. Gunnarsson U, Rydin H, Sjörs H. 2000. Diversity and pH changes after 50 years on the boreal mire Skattlösbergs Stormosse, Central Sweden. Journal of
Vegetation Science 11: 277-286.
III. Gunnarsson U, Malmer N, Rydin H. Changes in diversity and species distributions after 40 years on the mire Åkhultmyren, Southern Sweden.
Manuscript.
IV. Gunnarsson U, Rydin H. 2000. Nitrogen fertilization reduces Sphagnum production in bog communities. New Phytologist 147, in press.
V. Gunnarsson U, Granberg G, Nilsson M. Growth and interspecific
competition in Sphagnum after temperature, nitrogen and sulphur treatments on a boreal mire. Manuscript.
Papers I, II and IV have been reproduced with kind permission from the publishers.
In all five papers the planning of the experiments have been done together with the co- authors, while I had the major responsibility for the field and laboratory work, the data analyses and the draft writing. In paper I, II and III the first field investigations were made by H. Rydin, H. Sjörs and N. Malmer, respectively, and I did the recent field investigations.
INTRODUCTION 7
Mire types, gradients and succession 7
Recent environmental changes 8
Potential effects on mire ecosystems 9
AIMS 11
STUDY SITES 12
THE OCCURRENCE OF SCOTS PINE ON MIRES 12
DIVERSITY AND pH CHANGES ON TWO RE-INVESTIGATED
MIRES 15
SPHAGNUM RESPONSES TO EXPERIMENTAL MANIPULATIONS 16
Effects of raised nitrogen influx 17
Effects of raised sulphur influx 19
Effects of raised temperature 19
CONCLUSIONS 19
SAMMANFATTNING 19
TACK 20
INTRODUCTION
Mires are defined and characterised as ecosystems that accumulate peat. The main plant constituents of the peat and the living biomass in northern peatlands are species of the genus Sphagnum. There may be more carbon accumulated in Sphagnum, alive or as peat, than in any other plant genus (Clymo & Hayward 1982). Therefore, factors affecting the accumulation of Sphagnum peat are of importance for the global carbon cycle.
Nitrogen is a limiting essential element in most boreal ecosystems (Tamm 1991), including mire ecosystems. Increased levels of nitrogen deposition may be a threat for plant species in the most nitrogen poor ecosystems, which are adapted to low nitrogen levels (Ellenberg 1985). Instead, more nitrogen demanding, taller species would benefit if the nutrient level increases (Bobbink 1998). In areas with high levels of nitrogen deposition, many of the species typical for bog and poor fens are decreasing in abundance or are even endangered (Mennema et al. 1980-1989, Greven 1992, Benkert et al. 1996, Tyler & Olsson 1997). A reduced growth of Sphagnum species might even affect peat accumulation and the carbon cycle.
Most plants (especially bryophytes) growing on mires have narrow ecological niches regarding pH and the height above the ground-water table (Rydin 1986, 1993, Gignac 1993), and hence changes in pH and in the ground-water table will probably affect the species composition. Several studies have shown a derease in the
occurences of many rich fen bryophyte species in connection with an acidification (Kooijman 1992, Hedenäs & Kooijman 1996, Thygesen 1997). Many Swedish mires have been drained for agriculture, silviculture or peat extraction, mainly in central and southern Sweden (Rydin et al. 1999) with detrimental effects on the mire flora. The still undrained mires (in particularly fens) often suffer from changed regional ground- water conditions, which affect the flora on apparently untouched mires.
This thesis describes how mire ecosystems react to changed environmental
conditions. Two approaches have been applied: first, re-investigations of the floristic composition of mire vegetation to quantify changes during the last 40-50 years, and second, experimental manipulations of temperature, and of the nitrogen and sulphur influx to atudy the effects on mire vegetation.
Mire types, gradients and succession
In Sweden, the area covered by natural or near-natural mires comprise about 4.9 million ha (i.e., 11 % of the total land area, Rydin et al. 1999). The mires are not evenly spread over the country; peatlands are less abundant in eastern Sweden than in western Sweden (Lundqvist & Lundqvist 1955).
Mires are divided into two types, bogs and fens. Bog vegetation, has no supply of minerotrophic water (ground-water) which means that the only supply of water and nutrients to the vegetation is through precipitation (ombrotrophic water). Fens are at least to some degree affected by minerotrophic water. Most mire plant species can not survive under ombrotrophic conditions and are thus only found in fens. Such species are used as fen indicators (listed in Rydin et al. 1999). Some of these indicator species (for example Erica tetralix, Eriophorum angustifolium and Sphagnum papillosum) have in areas with oceanic influences, e.g., in south-western Sweden, been found growing in ombrotrophic sites, but the reasons for the ombrotrophic occurrences are unclear (∅kland 1990).
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Three main vegetation gradients have been identified in northern mires: the poor - rich gradient, the mire margin - mire expanse gradient and the microtopographical gradient (Sjörs 1948, Malmer 1962a, ∅kland 1989).
The gradient from bogs to extreme rich fens has been described in several studies (e.g. Sjörs 1948, Du Rietz 1949). It comprises mainly a gradient in mineral richness and pH (Rydin et al. 1999) from very mineral poor acid bogs (pH below 4) to extremely rich fens with high pH (sometimes as high as 8.5). Even if this gradient usually is reflected in the pH, it is strictly defined by the species composition (Sjörs 1948, Du Rietz 1949). It is not either strictly linked to nutrient conditions - also rich fens are often nutrient poor.
Mire margins are quite different from the mire expanses in their vegetation. Raised bogs are surrounded by a lagg fen, and usually have a marginal forest of pine trees gradually thinning out towards the open bog plain. The difference between the mire margin and the expanse is caused by several biotic and abiotic conditions. The tree and dwarf-shrub cover in the mire margin creates a different microclimate for the understory vegetation and a larger litter production compared to the mire expanse (Rydin et al. 1999). Usually species in the mire margin have supply of minerotrophic water from the surroundings.
The microtopographical gradient can be seen on aerial photographs as patterns of hummock banks perpendicularly oriented against the direction of the slope, and between them elongated hollows. Hummocks are dominated by some hummock forming Sphagnum species (mainly S. fuscum and S. rubellum). The dwarf shrubs Calluna vulgaris and Empetrum nigrum have been used as indicators of the lower limit of the hummocks (Du Rietz 1949). Occasionally small Pinus sylvestris trees can be found on the hummocks, but rarely in hollows. Hollows are dominated by other Sphagnum species (mainly species of section Cuspidata) and graminoids (e.g.
Eriophorum vaginatum, Rhynchospora alba and Scheuchzeria palustris). Hummocks have a higher proportion of aerated peat than hollows (Zobel 1986) and usually a lower yearly Sphagnum length growth (Rydin 1993). However, the peat production is about the same (Belyea & Clymo 1999), since the decomposition in hollows exceeds that in hummocks (Johnson & Damman 1991).
The development of Sphagnum bogs has been described by e.g. Rydin et al. (1999).
In a classic hydrosere, the succession starts with the infilling of shallow lakes by organic sediments leading to the development of marsh communities of aquatic plants, followed by a fen community with Sphagnum, and finally developing into an open bog. Nowadays open bogs are considered as a climax community in boreal areas (e.g.
Colinvaux 1986, van Breemen 1995, Klinger 1996). Invasions of trees have rarely been found on open bogs (Klinger 1996) and there are no indications that further succession will lead to the development from bogs into another vegetation type (Klinger 1996, Rydin et al. 1999).
Recent environmental changes
Peat cores from mires have been used as archives of postglacial climatic changes, reflected by the occurrences of macrofossil and pollen deposited in the peat cores (e.g.
Winkler 1988, Kuhry et al 1993).
Since the start of the industrialisation and the accompanying increase in human populations, dramatic and rapid changes in the environment have occurred.
Atmospheric levels of several greenhouse gases (for example CO2 and CH4) have increased together with atmospheric levels of nitrogen (N) and sulphur (S; Lee 1998,
Wuebbles et al. 1999). These changes have led to or will lead to global warming, acidification and eutrophication (Vitousek 1994, Lee 1998).
The atmospheric levels of the greenhouse gases have had a steady post-
industrialisation increase of 0.4 % year-1 for CO2 and 0. 6 % year-1 for CH4 (IPCC 1996). Considering these increases, regional climate models over Sweden predict a 2- 4°C increase in temperature in the next 100 years, with increased precipitation in northern Sweden (SWECLIM 1999). However, according to these models southern Sweden will have about the same level of precipitation as today (SWECLIM 1999).
Much effort has been put into detecting the main sinks and sources for the
greenhouse gases. It has been estimated that boreal mires store 300-455 Gt of carbon, equalling 20-30 % of the global carbon in the soil (Sjörs 1981, Gorham 1991).
However, mires can be important carbon sources as well, contributing about 4-11 % of the global CH4 emission (IPCC 1996). Thus, the intricate relations between the carbon accumulation and CH4 emission of boreal mires are important for the global carbon cycle and temperature.
Air-borne deposition of nitrogen is large in central and western Europe with average levels of 3-4 g m-2 year-1 and levels above 8 g m-2 year-1 at high deposition regions (Houdijk & Roelofs 1991). In Sweden the wet deposition of nitrogen in 1997 ranged from 0.1 g m-2 year-1 in northern Sweden to about 2.5 g m-2 year-1 in southern Sweden (data from Swedish Environmental Research Institute, home page:
http://www.ivl.se). The nitrogen deposition levels over central Sweden are still high, but signs of reduced deposition have been observed since the beginning of the 1990s (Lennart Granat, Stockholm University).
The level of air-borne sulphur deposition has steadily increased since the
industrialisation until the highest record in the 1970s, and has thereafter been halved (Mylona 1993). In 1997, the deposition levels ranged from 0.2 g m-2 year-1 in northern Sweden to 1.6 g m-2 year-1 in the south (data from Swedish Environmental Research Institute).
Potential effects on mire ecosystems
It has been suggested that one of the most important environmental changes caused by global warming will be lowered ground-water levels (Manabe & Wetherald 1986, Laine et al. 1996), but it is also possible that increased precipitation would counteract this. It is probable that the increased temperature will result in increased ground-water table fluctuations. Raised temperature and lowered water table would increase the risk of capitulum desiccation in Sphagnum species and thus reduce their productivity (Schipperges & Rydin 1998). A lowered ground-water table will also increase decomposition (Malmer & Wallén 1999) and increased temperature can increase the rate of mineralization (Nadelhoffer et al. 1991, Robinson et al. 1995). If the raised temperature gives lower ground-water tables, dominance in the mire vegetation might change from hollow species to hummock species.
Nitrogen is one of the main growth limiting elements on boreal mires. If the level increases above the limiting demands of the plants, they will become limited by another element. It was shown that Sphagnum growth in a mire in southern Sweden is now limited by phosphorus, whereas growth in a north Swedish mire is nitrogen limited (Malmer 1990, Aerts et al. 1992). A few nitrogen fertilization studies on mire plants have been performed, both in the laboratory (Jauhiainen et al. 1994, 1999) and in the field (Rochefort et al. 1990, Li & Vitt 1997). A major problem, however, is that most experiments have not lasted longer than a few years.
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Most nitrogen added to Sphagnum is immediately sequestered into the moss layer (Li & Vitt 1997). Sphagnum species are adapted to low nutrient levels, and increasing the influx may give toxic effects (Rudolph & Voigt 1986). It has been suggested that these species lack a down-regulating mechanism in the uptake of nitrogen (Jauhiainen et al. 1998).
Sulphur has not been shown to limit the growth of mire plants (Malmer 1993), but it may have an effect on the mire pH (acidification). It can also have direct toxic effects (Ferguson & Lee 1980), which is the probable explanation for the
disappearance of Sphagnum from the highly polluted southern Pennines, UK (Tallis 1964). Bogs have a low pH (usually below 4) and would hardly be acidified by acid precipitation (Westling 1990), but fens (with higher pH) would show reduced pH- values and hence, rich fen species may decrease in occurrence (Kooijman 1992, Hedenäs & Kooijman 1996).
The above scenarios are for one factor acting at a time, in reality they work together and interactions are expected. In studies of vegetation changes in reference areas it is therefore difficult to separate changes caused by one factor from those caused by another. However, if changes in abundance of individual species with known
environmental requirements are quantified, it is possible to disentangle the effects of different environmental factors. In addition, the results from experiments that are specifically designed to test the effect of individual factors can help us interpret the vegetation changes in the reference areas.
AIMS
The main task of this study was to find out how mire vegetation will react, and has reacted to changed large scale environmental conditions in areas with different levels of air-borne deposition.
The specific objectives of this thesis were to:
1. examine the relation between the water table position and the occurrences and establishment of Pinus sylvestris on different parts of raised bogs (paper I), and to find evidence for the assumed increase of pines on south Swedish mires (paper III);
2. investigate changes in species diversity and pH during the last 40-50 years in two mires with different levels of air-borne deposition of nitrogen and sulphur:
Åkhultmyren (higher deposition, paper III) and Skattlösbergs Stormosse (lower deposition, paper II);
3. investigate how experimentally increased nitrogen and sulphur influxes and
increased temperature affect Sphagnum production, and how increased nitrogen influx affects the growth of Sphagnum on mires with different loads of air-borne deposition (papers IV and V);
4. to test if experimentally increased nitrogen and sulphur influxes affects the interspecific competition between Sphagnum species, and notably if increased temperature is beneficial for hummock species (papers IV and V).
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Fig. 1. Map of Sweden showing the positions of the investigated mire sites.
STUDY SITES
The investigations have been carried out on mires representing a deposition gradient from the high deposition sites, at Åkhultmyren in Småland, to the low deposition site Degerö Stormyr in Västerbotten (Fig. 1, Table 1).
Ryggmossen and Åkerlänna Römosse (3 km south from Ryggmossen) are typical raised bogs of the east Swedish type (Sjörs 1948) and Åkhultmyren is an eccentric, faintly sloping bog with a wide lagg fen (Malmer 1962a). Skattlösbergs Stormosse (Sjörs 1948), Degerö Stormyr (Malmström 1923) and Luttumyren are mixed mires.
The experimental sites are: on Åkerlänna Römosse bog hollows and hummocks, on Åkhultmyren and Luttumyren poor fen or bog hummocks and on Degerö Stormyr a poor fen lawn community (sensu Sjörs 1948).
THE OCCURRENCE OF SCOTS PINES ON MIRES
To examine the relationships between the microhabitat (the height above the ground- water table) and germination and seedling establishment of Scots pine (Pinus
sylvestris), we performed a sowing experiment on Åkerlänna Römosse. Ten 50 x 50 cm plots were selectively placed on high hummocks and ten plots in hollows. In each
Table 1. Location of the studied mires and precipitation, nitrogen and sulphur wet deposition at the closest recording station. The name and distance (km) to the recording station are given in parenthesis.
Wet deposition and precipitation data from Swedish Environmental Research Institute (IVL, home page: http://www.ivl.se), except the data from Svartberget, which were collected by Svartberget Research Station (Swedish University of Agricultural Sciences, Vindeln). The precipitation and wet deposition values are yearly mean values (± s.d.) covering the period 1983-1998 at Aneboda and Forshult, 1987-1998 at Tandövala, 1997-1999 at Högskogen and 1990-1992 at Svartberget.
Site (Station, distance in km) Lat./Long. Province Altitude (m)
Precipitation (mm)
Nitrogen dep.
(g m-2 year-1)
Sulphur dep.
(g m-2 year-1) Degerö Stormyr (Svartberget, 25) 64°11'N, 19°33'E. Västerbotten 270 5231 0.20 0.302 Luttumyren (Tandövala, 25) 61°02'N, 13°22'E. Dalarna 500 814 0.42 ± 0.09 0.40 ± 0.15 Skattlösberg Stormosse (Forshult,70) 60°10'N, 14°42'E. Dalarna 265-285 7502 0.55 ± 0.14 0.51 ± 0.15 Ryggmossen (Högskogen, 7) 60°01'N, 17°22'E. Uppland 60 6003 0.64 ± 0.18 0.43 ± 0.64 Åkhultmyren (Aneboda, 2) 57°06'N, 14°33'E. Småland 225 789 0.72 ± 0.12 0.65 ± 0.15
1Measured at Kulbäcksliden (64°12'N, 19°34'E, 1 km from the site at Degerö Stormyr)
2 Precipitation on Skattlösbergs Stormosse, data from Sjörs (1948).
3 Precipitation data for Ryggmossen from the Swedish Meterological and Hydrological Institute’s (SMHI) station in Uppsala (25 km south-east of Ryggmossen).
plot 36 seeds were sown in June 1993, and germination and mortality were followed until August 1995 (I).
In the sowing experiment, the germination was not found to differ significantly between the hummocks (76 %) and hollows (66 %). However, there was a habitat effect on seedling mortality: both the total mortality level and the rate of mortality was highest in hollows (Fig. 2). Differences in mortality can explain why pines usually occur on high hummocks (safe sites, sensu Harper 1977) and not in wet hollows. The high germination frequency and the low seedling survival in the hollows show a conflicting pattern of germination and survival, which has been found to be a rather common pattern (Schupp 1995).
On Ryggmossen, pines growing in three permanent plots (10 x 10 m) have been mapped and followed for more than ten years (I). The plots were situated on the open bog plain, in the marginal pine forest and in a transition zone between those. The height above the ground-water table was measured and related to the survival of each pine. To test if the field and the bottom layers affect the pines, we mapped the layers, measured the area of each type and counted the numbers of seedlings (ó 1 mm in diam.), juveniles (1 < diam. < 10 mm) and established pines (ò 10 mm) in each type.
The seedlings in the permanent plots were found to be randomly distributed among the field and bottom layers, probably because the selective mortality in hollows had still not started to act upon the seedlings. Meanwhile, the juveniles and established pines were more often found on Sphagnum fuscum hummocks or in Pleurozium schreberi-carpets, where the conditions for survival were better.
To show the importance of the height above the ground-water table for survival, a logistic regression analysis was performed. Survival of the pines increased with increasing height above the ground-water table (Table 2), and there was also a difference in survival between plots (highest in the transition zone and lowest on the open bog). Additionally, a temporal variation in mortality was found, in 1981 a 61 % mortality of the established pines in the marginal pine forest plots was observed. This high mortality coincided with a constantly high ground-water table over the growing period that year.
The study of pines growing on Ryggmossen was initiated mainly because of the general impression that the pine cover has increased on mires in southern and central
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Table 2. Logistic regression analysis of pine survival with plot (marginal pine forest, transition zone and open bog) and the log-transformed elevation above ground-water table as explanatory variables.
Terms were included into the model in the order shown in the table.
Source df mean
deviance
F- value Regression coefficient
Plot 2 12.43 9.80***
Elevation 1 5.35 4.22* 1.35
Residual 329 1.27
* = P < 0.05; *** = P < 0.001.
Sweden (Åberg 1992, Ihse et al. 1992). Comparisons with old photographs over Ryggmossen and Skattlösbergs Stormosse and the documented increase in cover of pines on Åkhultmyren (III) confirm the increase of mire growing pines. However, the results from the permanent plots on Ryggmossen included a period with a strong reversal of the general trend, which shows that these marginal populations are sensitive to changes in the ground-water table. In a successional perspective, the increased cover of pines indicates that changes in allogenic factors (e.g., a changed climate or a changed deposition) can change open bogs into more tree covered ones (c.f. Klinger 1996). The position of, and the variation in, the ground-water table are key factors for the occurrences of trees on bogs.
Fig. 2. Cumulative mortality for seedlings in the experiment on Åkerlänna Römosse for hummocks (open bars) and hollows (filled bars). Error bars indicate one standard error.
DIVERSITY AND pH CHANGES ON TWO RE-INVESTIGATED MIRES To find out how undrained mires have changed under “natural” conditions (i.e., under ambient deposition regimes) it was necessary to identify objects that had been
described in detail and areas that could be re-positioned with high certainty. From these criteria two areas were selected, Skattlösbergs Stormosse (low deposition, studied area ca. 10 ha) and Åkhultmyren (high deposition, studied area ca. 17 ha). The first inventories on Skattlösbergs Stormosse were made in 1944-1945 (Sjörs 1948) and it was re-investigated in 1995 (ca 50 years after the first inventory). At
Åkhultmyren the first inventories were made in 1953-55 (Malmer 1962a, b) and the site was re-investigated in 1997 (more than 40 years after the first inventory).
On Skattlösbergs Stormosse, we mapped the different species (most of the fen species) with the help of a hydrotopographical map prepared by Sjörs (1948; see II:
Fig. 1). Later on, all species maps were transferred into a 10 x 10 m grid of plots, enabling comparison between years. pH measurements were made at 251 localities, directly in the mire water, in 1945 and in 1995 (II).
At Åkhultmyren, plots (20 x 20 m) were established at the same positions in both investigations (maximum deviation between plot corners was 3 m between the surveys). All species present in the plot (except six common species) were noted. pH was measured, directly in water, over the mire in 1954 and in 1997 (III).
Skattlösbergs Stormosse showed a reduction in total number of species and in number of species per plot for both vascular plants and bryophytes (Table 3, II). On Åkhultmyren a reduction in the number of vascular plant species per plot was found, while bryophyte species richness had increased (Table 3). Of the 80 species found in 1954, eight species were not re-found in 1997, while 10 new species were recorded (III).
On Skattlösbergs Stormosse, most rich fen bryophytes decreased in plot frequency and most Sphagnum species increased (II). In contrast, at Åkhultmyren most bryophytes (except Sphagnum) increased (III), most notably hummock
inhabiting species, but also a few poor fen species. On the whole Sphagnum species had not
Table 3. Number of mapped species (vascular plants and bryophytes) per plot in 1945 and 1995 (Skattlösbergs Stormosse, 10 x 10 m plots) and 1954 and 1997 (Åkhultmyren, 20 x 20 m plots).
Mean number of species per plot (s.d.)
1945/1954 1995/1997 Difference Paired-t Skattlösbergs Stormosse (n=813)
Total 6.3 (7.5) 5.9 (6.7) -0.4 (2.5) -4.67***
Vascular plants 4.9 (5.7) 4.6 (5.2) -0.3 (1.9) -4.23***
Bryophytes 1.4 (2.4) 1.3 (2.1) -0.1 (1.2) -2.84**
Åkhultmyren (n=245)
Total 18.3 (0.4) 18.3 (0.4) 0.0 (0.3) 0.12n.s.
Vascular plants 10.3 (0.3) 8.4 (0.2) -1.9 (0.2) -8.12***
Bryophytes 7.9 (0.2) 9.9 (0.2) 2.0 (0.2) 9.66***
n.s. = P ≥ 0.05; ** = P < 0.01; *** = P < 0.001.
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changed much in plot frequency (III). On both mires many of the herb and the graminoid species decreased, while trees and shrubs tended to increase.
pH values had declined on both mires, particularly in the high pH areas (II and III). However, in the ombrotrophic parts of the mires pH was unchanged. Large amounts of acid deposition would be needed to reduce the already low pH in ombrotrophic bogs (Westling 1990).
The rich fen vegetation areas on Skattlösbergs Stormosse had lost species, coinciding with the decreasing pH, and the rich fen vegetation was in 1995 found in smaller areas than in 1945 (II). Also at Åkhultmyren the rich fen species had
decreased in occurrence (III). Similar patterns of reduced species numbers have been found in other Swedish rich fens (Hedenäs & Kooijman 1996, Thygesen 1997). On Skattlösbergs Stormosse, species with the highest pH-demands in 1945 had decreased in plot frequency in 1997, in contrast to the increase among species that grew in more acid conditions in 1945.
Skattlösberg Stormosse showed signs of acidification, probably as a combined result of autogenic succession and anthropogenic deposition. It is known that
Sphagnum species can act as acidifiers (Clymo 1964), and this process can be rapid, particulary when Sphagnum establishes in the brown moss dominated fens in the pH range 5-6 (Gorham 1956, Vitt & Kuhry 1992, Kuhry et al. 1993).
In paper II, it was suggested that the expansion of the bog vegetation was a possible explanation for the reduction in of lagg fen areas and soaks. A similar
reduction in the lagg fen area was found on Ryggmossen during the period 1947-1993 (Thygesen 1997). In contrast, at Åkhultmyren, which has a large fen between the bog and the mineral soil, the fen limit had moved over the former bog area (III), as reflected in the more bogward occurences of the fen species Narthecium ossifragum and Eriophorum angustifolium, and from stratigraphical studies. This shows that the fen limit can be unstable. The differences between the mires may be caused by differences in climate and/or deposition levels. Åkhultmyren showed signs of increased surface dryness (possibly caused by a changed hydrology), increased nutrient status and acidification, or combinations of those (III). At Åkhultmyren, the effects of the acidification were of subordinate importance. In contrast, on
Skattlösbergs Stormosse acidification was more important and there was no clear evidence of increased nutrient levels or surface dryness could be observed, and the ombrotrophic part was very similar in species composition in 1945 and in 1995.
SPHAGNUM RESPONSES TO EXPERIMENTAL MANIPULATIONS To find out whether the same Sphagnum species reacts differently on additional nitrogen in areas with high (Åkhultmyren) and low (Luttumyren) levels of deposition, the following experiment was arranged. In the middle of the summer of 1996,
20 x 20 cm plots were established on hummocks with natural species mixtures of either S. fuscum + S. magellanicum or S. rubellum + S. magellanicum. Five rates of nitrogen additions (as NH4NO3) were used: 0, 1, 3, 5 and 10 g m-2 year-1, with five replicates of each treatment on both mires (IV). The length growth and the area covered by the two species in the mixtures were measured twice a year until
September 1998. Production was calculated by multiplying the length growth with the bulk density of the biomass produced and correcting for the change in capitulum mass (IV).
In a factorial experiment on Degerö Stormyr, the effects of increased nitrogen and sulphur influx and increased temperatures on Sphagnum growth and on interspecific competition between S. lindbergii and S. balticum were studied (V). Two levels of each factor were used, with duplicate experimental combinations. Additionally four midpoint treatments for nitrogen and sulphur were applied. Twenty-one plots (2 x 2 m) were established in 1994 and the experimental treatments started in 1995. The amount of nitrogen and sulphur applied were 2.8 g m-2 year-1 and 1.7 g m-2 year-1, respectively, and the temperature treatment increased the mean daily air temperature by 3.6° C during the growing period. Midpoint plots were treated with 1.3 g m-2 year-1 nitrogen and 0.7 g m-2 year-1 sulphur (Granberg et al. 2000). Length growth was measured twice a year until the harvest in 1998 and converted to production as described above (V).
To measure the interspecific competition between a hummock inhabiting species and the lawn species naturally occurring in the plot (S. balticum), transplants (8 cm in diam.) of hummock growing S. papillosum were put into the plots. In addition, the competitive relations between two naturally occurring lawn species (S. lindbergii and S. balticum) were followed. The areas covered by S. lindbergii and the transplanted S.
papillosum were mapped twice a year.
Effects of raised nitrogen influx
Nitrogen additions had a negative effect on Sphagnum biomass production for all investigated mires (Figs 3, 4). However, in the first growing season on Luttumyren, we found an increased length growth in the intermediate nitrogen treatments (i.e. 1 and 3 g m-1 year-1), which together with the low N/P quotient in the mosses indicated that growth was nitrogen limited at this site (IV). The positive effect on the length growth disappeared in the second year, which shows the danger of drawing
conclusions from one-year experiments. No indication of nitrogen limited growth was found on the other mires (IV, V). Nitrogen additions probably caused Sphagnum growth to become limited by other nutrients, probably phosphorus (Aerts et al. 1992, van der Heijden et al. 2000). A nutrient imbalance or even a toxic effect might occur in the nitrogen treated Sphagnum, reflected for instance by increased accumulation of amino acids (Baxter et al. 1992, A. Nordin & U. Gunnarsson, manuscript), or
decreased photosynthesis and soluble sugar content (van der Heijden et al. 2000).
In a fertilization study, Lütke Tweenhövden (1992) found that the minerotrophic S.
fallax could outcompete the more ombrotrophic S. magellanicum when treated with increased nitrogen levels. At Degerö Stormyr, S. lindbergii increased in area in the
high nitrogen treatment, at the expense of S. balticum (V). This shows that the differences between species, regarding nitrogen tolerance or utilisation, are important
for the competitive outcome when deposition increases. However, the nitrogen treatments on neither Åkhultmyren nor Luttumyren changed the competitive relations between S. fuscum and S. magellanicum or between S. rubellum and S. magellanicum (IV). The differences in nitrogen tolerance or utilisation between S. magellanicum, S.
fuscum or S. rubellum were probably to small too result in changed competitive
Urban Gunnarsson
18
Fig. 3. Mean annual dry mass production (+ one standard error) of different Sphagnum mixtures at two mires after treatment with different N supplements. The mixtures were on Åkhultmyren and Luttumyren hummocks with either S. fuscum + S.
magellanicum (S. fuscum mix) or S. rubellum + S. magellanicum (S. rubellum mix).
-3 0 -2 5 -2 0 -1 5 -1 0 -5 0 5 1 0 1 5 2 0
H e ig h t N T S N x T T x S N x S
M o d e l v a r ia b le s Model coefficients for production (g m-2 yr-1 )
Fig. 4. Scaled and centered coefficients from a multiple regression model of
Sphagnum balticum production during the period 1995-1998 at Degerö Stormyr. Error bars indicate 95 % confidence intervals. For significant coefficients, at the 5 % level, the bars do not include zero. The variables are: height above the ground-water table (height), nitrogen (N), temperature (T) and sulphur (S) together with the interactions.
relation, while the differences between S. lindbergii and S. balticum were sufficient.
In a three year nitrogen fertilization study (3 g m-2 year-1) S. balticum was found to decrease in occurrence, while S. papillosum increased (Saarnio 1999). Such a difference between S. balticum and S. papillosum was not found on Degerö Stormyr (V). However, it might be that S. balticum is sensitive to high nitrogen levels.
Effects of raised sulphur influx
The studies on Degerö Stormyr showed no main effect of the sulphur treatments on production or length increment (Fig. 4, V). Probably the availability of sulphur is of minor importance in regulating Sphagnum production on most Swedish mires, but high levels might have toxic effects (Ferguson et al. 1978). The area occupied by S.
lindbergii increased and S. balticum decreased as an effect of the sulphur addition.
Probably the species also have different tolerances to high sulphur levels, which have been shown for other Sphagnum species (Ferguson et al. 1978, Austin & Wieder 1987).
Effects of raised temperature
The mean daily air temperature 0.3 m above the moss surface was increased by 3.6°C in the temperature treated plots, leading to increased evapotranspiration and dryness of the Sphagnum shoots. The water content has been found to be one of the main factors regulating growth and photosynthesis in Sphagnum shoots (Silvola & Aaltonen 1984, Rydin & McDonald 1985). Extensive periods with dry capitula (top cm) will reduce Sphagnum production (Schipperges & Rydin 1998), and this was also found in the temperature treated plots on Degerö Stormyr (Fig. 4, V).
CONCLUSIONS
My studies show that the composition of the mire vegetation will change if the temperature and nitrogen deposition increase. The production of bog and poor fen Sphagnum species will be reduced, which probably will diminish peat production. On a larger scale this can affect the global carbon balance. It was further confirmed that abundance of Scots pine growing on mires has increased in southern Sweden, probably as an effect of changed ground-water table regimes or increased nutrient deposition. This indicates that open bogs can become more tree-covered if the climate or deposition levels change. At Åkhultmyren in southern Sweden, large changes in the vegetation and pH were found, showing that mire ecosystems, even bogs, may change rapidly. The development of the bogs and poor fens in southern Sweden might lead to more eutrophic and dry ecosystems, and therefore anthropogenic deposition of
nitrogen and changed hydrology is a threat to these ecosystems.
SAMMANFATTNING
Myrar är torvproducerande våtmarker och idag upptar de ungefär 11 % av Sveriges landareal. De flesta myrar är extremt näringsfattiga (i regel är tillväxten
kvävebegränsad). Detta gäller framförallt mossar, eftersom de endast får vatten och näring i form av nederbörd. Myrväxterna är därför anpassade till låga näringshalter,
Urban Gunnarsson
20
bl.a. genom att kunna återanvända näringsämnen, lagra tillfälliga näringsöverskott, och en del växter får extra näring genom att fånga insekter.
Ungefär 35 % av Sveriges myrar har dikats ut, främst i södra delarna, för att öka jordbruks- och skogsbruksarealerna men även inför torvbrytning. De myrar som inte är utdikade påverkas även de av mänsklig aktivitet framförallt genom kväve- och svavelnedfall. I Sverige är det de sydvästra delarna som har den högsta kväve-
belastningen. Den har varit så hög att många myrar i södra Sverige idag inte längre har en kvävebegränsad tillväxt, utan tillväxten har i stället blivit begränsad av fosfor.
I mina studier har jag intresserat mig för hur försurningen och den ökade depositionen av kväve och svavel påverkar myrväxter och ekosystem. Jag har dels gjort experimentella manipulationer, ökat kväveinflödet och temperaturen, och dels gjort två återinventeringar av vegetation och pH i två väldokumenterade myrområden.
De experimentella studierna med ökat kväveinflöde visar att vitmossarterna (Sphagnum) inte klarar av att växa mer då kvävetillgången ökar. En svag initial ökning av tillväxten kunde observeras den första växtsäsongen för mellandoserna på den undersökta myren i västra Dalarna (Luttumyren), vilket visar att tillväxten på myren var kvävebegränsad. Tillväxten minskade dock även på Luttumyren efter första året för alla kvävebehandlingar. Fattigkärrs- och mosselevande vitmossor har svårt att tillgodogöra sig en ökad kvävetillgång. De kvävenivåer som idag deponeras på myrar i sydsverige ligger troligen i närheten av den giva som är optimal för tillväxten. En ökning av temperaturen visade sig även den att vara negativ för vitmossornas tillväxt.
Den naturliga utvecklingen har på de två återinventerade myrarna pågått under 40- 50 år utan att några större ingrepp har gjorts på myrarna eller i deras direkta
omgivningar. På Skattlösberg Stormosse, i Bergslagen, hade antalert arter minskat, framför allt rikkärrsarter, vilket sammanföll väl med försurning främst i de områden som tidigare (1945) hade höga pH-värden. Fattigkärrs- och mossedelarna (med låga pH-värden) var dock relativt oförändrade både i växtsammansättning och pH.
På en myr i södra Småland, Åkhultmyren, fann vi större förändringar i
vegetationen. Tall och björk hade ökat sina utbredningsområden under perioden 1954- 1997 och fanns 1997 på stora delen av den tidigare öppna myren. Med det ökade trädtäcket följde även en ökning av flera skogsväxter. Högvuxna kärlväxter ökade i utbredning, medan lågvuxna kärlväxter typiska för mossar och fattigkärr minskade.
pH minskade i kärrområdena men var oförändrat på mossedelen. Förändringarna på Åkhultmyren har huvudsakligen orsakats av ökad näringstillgång, torrare mosseplan och ett något förändrat vattenflöde.
På sikt kan den höga depositionen av näringsämnen och det varmare klimatet vara ett hot för mosse- och kärrväxterna, eftersom de har svårt att utnyttja förhöjd
näringstillgång och eftersom de är känsliga för förändringar i grundvattennivån.
TACK
Inledningsvis skulle jag vilja tacka min handledare Håkan Rydin för att ha lotsat mig igenom forskarstudierna, från första ansökan tills den sista revisionen av
avhandlingen. Håkan introducerade mig i myrekologi, initierade de flesta studierna, och tragglade tålmodigt igenom manuskript av varierande kvalitet. Tusen tack Håkan!
Jag skulle också vilja tacka Ingvar Backéus för att bidragit med sitt breda
myrkunnande och gett goda råd inför bl.a. inventeringen på Skattlösbergs Stormosse.
Hugo Sjörs och Nils Malmer har låtit mig gå igenom sina gamla anteckningsböcker och kartor - det har varit ett nöje att få samarbeta med er, och få ta del av er stora kunskap om svenska myrar. Jag vill även passa på att tacka Mats Nilsson, Gunnar Granberg och Annika Nordin för ett inspirerande och givande samarbete och för er gästvänlighet. Christer Albinsson, Bertil Gunnarsson, Anders Jacobsson och Christina Munkert har hjälp till med fältarbete på olika myrar.
Jag vill tacka alla som är eller har varit knutna till Växtekologiska avdelningen,
“Växtbio”, under min tid som doktorand. Ett speciellt tack riktas till Ulla Johansson, Willy Jungskär, Stefan Björklund och Folke Hellström för att hjälpt till med allt från fotografering till lönebesked. Eddy van der Maarel, Jon Ågren, Pauli Snoijs, Staffan Karlsson, Håkan Rydin, Ingvar Backéus och Brita Svensson har varit viktiga för att skapa en trivsam och internationell forskarmiljö. Diskussionerna med mina
rumskamrater, Sebastian Sundberg, Henrik Berg, Martin Weih, Tesfaye Bekele och Antonella Soro har varit otaliga och till stor hjälp under arbetet. Zebbe och Henrik har varit med från början till slut och de har hjälpt till att hålla de floristiska och
faunistiska kunskaperna på topp. Segern i fågelracet 97 / 98 tillhör en av de absoluta höjdpunkterna under min doktorandtid. Mikael Lönn och Ulf Grandin har alltid hjälpt till med diskutioner om diverse statistiska tester och mycket annat.
Henrik Weibull, Tomas Hallingbäck och Hugo Sjörs har hjälpt till att bestämma mosskollekt. Ingvar Backéus, Håkan Rydin, Charlotte Sweeney och Jon Ågren har läst och givit kommentarer på sammanfattningen.
Mina föräldrar (Runa och Bertil) och bröder (Anders och Mårten) har hjälpt till, varit intresserade och tålmodigt väntat på att jag skall bil klar med studierna. Min far har varit en stor inspirationskälla för mig och han väckte mitt intresse för natur, jakt och fiske. I jaktsammanhang måste jag även tacka Olle Eriksson för att han tog med mig ut i de Uppländska skogarna och delat med sig av sitt stora kunnande och vilja av att vara ute i markerna. Rune och Margareta Selén har bidragit med bostad, trädgård och vartit ett trevligt sällskap bl.a. under min föräldraledighet. Odd och Gunilla Maad har hjälpt till med barnpassning och datorer.
Slutligen vill jag tacka Johanne för allt du hjälpt till med, fältarbete, SAS-program, manusläsning, matlagning och mycket annat, och det skulle nog ha varit betydligt lungnare hemma om inte Agnes och Hjalmar stökade omkring, men nog hade det varit långsamt utan er.
Så var det det där med minnet, ta därför inte illa upp om jag glömt att näma någon till namn här ovan, därför passar jag på att tacka alla som hjälpt till med studier, fältarbete, diskussioner och undervisning, på ett eller annat sätt. TACK!
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