Abstract:
Summary
Introduction
Fens are minerotrophic peatlands with the water table located very close to the
ground level. The water flows through the peatland by internal seepage and occasional
regions of surface overflow (Rydin and Jeglum, 2006). The composition of plant communities
found in fens depends on a complex interplay of pH (Sjörs and Gunnarsson, 2002, Wheeler
and Proctor, 2000, Økland et al., 2001), nutrient availability (Pauli et al., 2002, van der Hoek
et al., 2004), light availability (Kotowski et al., 2001, Kotowski and van Diggelen, 2004) and
water table depth (Kotowski et al., 2001, Mälson et al., 2008).
This interplay makes fen ecosystems vulnerable to changes in any of the above
mentioned factors, and often a modification of one variable alters another variable. An
example of this is drainage of fens, which lowers the water-table, which in turn aerates the
previously anoxic peat, resulting in mineralization of the peat, with release of nutrients
(Turner and Haygarth, 2001). This causes increased primary productivity (Joyce, 2001), which
in turn lowers the light availability (Kotowski and van Diggelen, 2004), which has a
detrimental effect on characteristic fen species (Kotowski and van Diggelen, 2004, Sundberg,
2012, Vermeer and Berendse, 1983). The lowered water-table increases the influence of
rain-water. This results in acidification (van Diggelen et al., 2006), usually followed by an
increase in peat mosses (Sphagnum spp.) which causes a positive feedback of further
acidification, due to ion-exchange between Sphagnum spp. and its local surrounding (Rydin
and Jeglum, 2006).
Degradation and restoration
During the 20th-century large areas of peatlands in Europe were drained for
agricultural production, forestry purposes or peat extraction (Vasander et al., 2003). In 1997
it was reported that 62 % of Europe’s mires (peat accumulating peatlands) have been lost,
and only 5 % is protected (Joosten, 1997). Attempts to counteract this development include
conservation and restoration, although even the protected areas include peatlands that are
in strong need of restoration (Sundberg, 2006).
Ecological restoration as defined by the Society for Ecological Restoration is the
process of assisting the recovery of an ecosystem that has been degraded damaged or
destroyed (Society for Ecological Restoration International Science & Policy Working Group,
2004). Ecological restoration of fens aims at restoring both the abiotic factors (hydrology,
light-availability and nutrient-concentration) and the biotic factors (species assemblage).
Expressed in terms of community assembly rules, the restoration aims to alter the
environmental filters (abiotic and biotic) that limits the restoration site from developing
species communities similar to those found in analogous undisturbed ecosystems (further on
called ‘reference ecosystems’ or ‘reference sites’). In moderately degraded fens, i.e. cases of
moderate drainage for forestry purposes, a common restoration method is the combination
of rewetting by blocking drainage ditches and tree-cutting to restore the light-conditions
(Haapalehto et al., 2011, Hedberg et al., 2012, Laine et al., 2011, Lanta et al., 2006, Mälson
et al., 2008, Mälson et al., 2010). In heavily degraded fens, were the peat has mineralized,
resulting in excess nutrient-concentration, an increasingly applied restoration method is topsoil
removal. This restoration method has the capacity to increase the ground-water level,
remove the unwanted seed-bank, increase light-conditions and remove excess nutrients
(Klimkowska et al., 2010, Patzelt et al., 2001, Rasran et al., 2007, Tallowin and Smith, 2001).
In cases where the original species community has vanished, and dispersal limitation makes
natural colonization into the restoration-site improbable, species have to be introduced in
order to restore the species-community. Among methods used to introduce communities of
species, direct seeding (Fraser and Madson, 2008, Tallowin and Smith, 2001) and haytransfer
from reference-meadows (Hölzel and Otte, 2003, Klimkowska et al., 2010, Patzelt et
al., 2001, Rasran et al., 2007) are commonly used.
Due to the mentioned multitude of factors influencing the species community in a fen
(e.g. hydrology, light, pH and nutrient concentration), ecological restoration is difficult, and
restoration sites rarely reach the conditions of the reference sites (Moreno-Mateos et al.,
2012). The relative success of ecological restoration has mostly been measured in terms of
numbers of target species present, indices that combine species identity and relative
abundance of each species (Shannon, 1948, Simpson, 1949) or ordination methods based on
species identity and abundance. These methods can provide information regarding how the
species composition at the restoration site compares to that of the reference site, but
provide no ecological explanation for any similarity or difference that is detected. Secondly,
an evaluation based on species identities loses its usefulness outside the geographical
distribution-range of the studied species.
The advent of functional diversity
A solution to the limits of the species-identity focused analysis is to turn the attention
to the functional characteristics the species possess. In functional ecology functional
diversity measures have been put forward as a measure that takes into account the
functions of species (Diaz and Cabido, 2001, Díaz et al., 2007, Garnier et al., 2004, Garnier et
al., 2007, Laliberté and Legendre, 2010, Mason et al., 2005, Villéger et al., 2008). Although
functional diversity appears to have the potential to highlight if and how a restoration site
differs in functional composition from the reference site, as well as describing the
environmental filters that influence the outcome of the restoration project, the methods
have not been applied to restoration ecology. Considering that species composition in fens is
influenced by several ecological filters, i.e. anoxia (a strong abiotic filter) (Kotowski et al.,
2010) and competition for light (a biotic filter) (Kotowski and van Diggelen, 2004), the
application of a functional analysis for traits corresponding to these filters may be useful for
ecologists when analysing if the species community in the restoration site is functionally
similar to the species community in the reference site, and how specific traits are filtered by
the restoration measures. With this tool ecologists would have the potential to, based on
functional analyses of similar restoration projects, determine which target species are likely
to establish, and adapt restoration measures in order to increase the chance of successful
establishment of species carrying certain traits.
My research focused on the potential value of a functional diversity analysis in the
analysis of fen restoration. The value lies in the potential in obtaining information on how
restoration actions modify the environmental filters that only species with certain traitvalues
can pass, and how the species community in the restoration site differs in functional
composition from the species community in the reference site. A difference in the functional
composition between the species communities should indicate a difference in the
environmental filters, and thereby pinpoint factors that constrain the restoration site from
reaching the state of the reference-site.
A variety of functional diversity indices exists today. I have limited my research to 4
indices that, based on their structure, should have the potential to provide information on
environmental filters in fen restoration. These four indices are the three indices presented
by Mason et al. (2005) (functional richness, functional evenness and functional divergence),
as well as functional dispersion (Laliberté and Legendre, 2010). Apart from these, the
Community Weighted Means of the functional traits are analysed, as well as the functional
group abundance.
The three indices put forward by Mason et al. (2005) and placed into a multivariate
context by Villéger et al. (2008) (functional richness, functional evenness and functional
divergence) measure the size of the filled niche space, the evenness of biomass distribution
within a niche space and the abundance distribution within a niche space respectively. These
three indices all describe different facets of functional diversity. However, Laliberté and
Legendre pointed out that the connection to the convex-hull makes functional richness
sensitive to outliers, and that functional richness does not take into consideration the
relative abundance of species. Functional evenness and functional divergence on the other
hand include the relative abundance of species, but lack information on the distribution of
species in the trait space. Their solution to this was the index functional dispersion, which is
a measure of the dispersion of trait values in a trait space, defined as the average distance of
all species to the centroid of all species in a trait space (Laliberté and Legendre, 2010).
Description of the research conducted for this thesis
This thesis consists of four individual publications, in which various aspects of the restoration
of fens have been explored.
The first publication is a meta-analysis of the diversity and relative success of species
introduction methods in restoration of fens and semi natural grasslands, carried out by
conducting an extensive search for studies that used introduction of species as a part of
ecological restoration. The introduction-methods used were described and quantified, and
the success of each method was evaluated based on how many of the introduced species
that established in the restoration site. When available, the authors written comments
about their view of the restoration were included as a variable in the evaluation.
The analysis showed that in many cases species introduction through the transfer of
hay from donor-meadows is a successful method for restoring species communities. This is
not to say that the method is superior to any other species introduction method, since
external factors may considerably change which method is most suitable. An example of this
is that direct seeding or hay spread may not be recommended if the species are rare, have
low germination rate or low seedling survival. Further the application of hay-transfer in the
restoration of Calowanie fen showed that hay transfer without ground-disturbance (such as
top-soil removal) has very little impact on the species community, and if the degradation is
moderate, as in the Swedish sites, then complete removal of the vegetation may not be
recommended.
Following this a Swedish fen restoration project covering three fens that were drained
for forestry purposes in the 1950s and restored in 2002 by cutting planted trees and blocking
drainage ditches was analysed both through a functional group-focused analysis (publication
2) and through a functional trait diversity analysis (publication 3). The vegetation changes in
the three Swedish restoration sites were monitored between 2002-2010 and compared to a
reference-site that was monitored between 1978-1979 prior to it was drained. The
application of a functional group-focused analysis in the Swedish fen restoration projects
(publication 2) clearly showed that both rewetting and tree cutting increased the cover of
Sphagnum, wetland bryophytes and sedges. Tree cutting increased the cover of grasses,
wetland vascular plants and juvenile trees. Both treatments resulted in an increase in species
richness, and the combination of the two treatments resulted in the highest speciesrichness.
However, the rich-fen specialists that were the target of the restoration did not
recover, most likely due to dispersal limitation. By applying a functional trait analysis
(publication 3) we could detect that the restoration sites had a higher functional richness, a
higher functional dispersion and a higher canopy height than the reference-sites, which
indicates a too relaxed filter. Tree cutting resulted in an increase in functional richness and
functional dispersion, which indicates a lowered habitat filter after the removal of the
shading canopy.
As a contrast to the Swedish restoration project, I analysed the outcome of an
ecological restoration of Calowanie fen located 30 km south-east of Warsaw that previously
had been drained for agricultural production (publication 4). Fen restoration in Calowanie
was conducted in 2008 by removing the degraded top-soil, and introducing target species
from donor meadows via hay-transfer. The top-soil removal site was divided up into belts,
and hay was dispersed on every second belt, providing the possibility to analyse the effect of
top-soil removal with and without hay-dispersal. As a control, hay from the donor meadows
were also dispersed on control plots where no top-soil removal was conducted. I monitored
permanent plots located in all treatment-types (top-soil removal with and without haytransfer
and control-plots with and without hay-transfer), as well as in donor meadows.
Within the top-soil removal sites, plots were distributed in such a way that the gradient of
relative water table depth was covered so that each plot’s distance to the ground-water
level could be included as an explanatory variable in the community analysis. The outcome
of the restoration was analysed both through a classical species-identity focused analysis
and through a functional trait analysis. A multivariate RDA analysis of the restoration in
Calowanie fen (publication 4) showed clearly that the top-soil removal site and the
reference-site were separated from the control site along the first ordination axis, which
followed a wetness gradient. Hay-dispersal had very little influence on the species
community compared to the effect of top-soil removal. We could also through the classical
analysis detect that characteristic species responded significantly in abundance to the
ground-water level. By applying a functional trait analysis we could detect that the groundwater
level imposes a strong habitat filter revealed by a significant decrease in functional
richness and functional dispersion with increasing ground-water level. At high ground-water
level this habitat filter selects for capacity for clonal lateral spread, high Ellenberg moisture
values, capacity for hydrochorous dispersal and low specific leaf area and lack of capacity for
autochorous dispersal. Further, we could detect that the restoration site differs in trait
composition from the reference site by having a significantly lower proportion of species
that disperse through autochory, which was probably caused already at the harvesting of the
hay from the donor meadows, as indicated by the functional trait analysis of species present
as seeds in the harvested hay. Among the species groups that are disfavoured by this are
sedges, which were a target species group of the restoration.
Conclusions
My research has taken place in two countries in sites located up to 960 km
from each other. The research sites differ in geological history, land-use history, degree of
degradation, climatic conditions and length and start of vegetation season. As a
consequence, there are also differences in species pools as well. Whereas the Swedish sites
had a large bryophyte cover, including an abundance of Sphagnum species in two out of the
three Swedish sites, the Calowanie fen was dominated by grasses, sedges and herbs
spanning from species typical for heavily degraded fens that were found in the unrestored
control sites, to species typical for wet meadows and fens found in restored sites. Limnic
species could be found in the deepest parts of the restored sites.
Large differences in species-composition between geographic areas can cause
considerable challenges in drawing site-independent conclusions regarding which species
are likely to benefit from a specific restoration action. By applying a functional approach we
switch the focus from which species will benefit from the restoration measures to which
trait values and trait composition will species favoured by the restoration measures have.
This switch of focus enables general site-independent conclusions to be drawn despite
differences in species pools. Having data on how species possessing different trait-values are
filtered by specific restoration actions, allows us to better predict how specific species are
likely to respond to a specific restoration action, as long as trait-data for these species are
available.
With the functional-group based analysis we could in the Swedish fenrestorations
conclude that certain groups of species benefitted from one or both of the
restoration actions - tree cutting and rewetting. We could also detect that the species
richness was higher in the restored sites than in the reference sites. We could however not
find any ecological explanation to why this was the case. With the functional-trait analysis
we could detect that the habitat filter in the restoration site was relaxed when the shading
by canopy was removed. This weaker habitat filter allowed species of previously disfavoured
trait-composition to flourish. Rewetting can influence productivity both by decreasing
productivity by imposing anoxia, or increasing productivity by the release of phosphorous
following rewetting of degraded peat. The higher canopy in the restoration site compared to
the reference site indicates that productivity has increased due to release of phosphorous
following rewetting of areas where previous drainage have caused the breakdown of peat,
making nitrogen and phosphorous readily available. Similarly the functional-trait analysis of
the restoration of Calowanie fen allowed us to get an ecological explanation to the results
provided by the species based analysis. Together, the Swedish and Polish studies provide
examples of how the functional trait analysis can be used in analysing the effect of
restoration measures. Rather than being a replacement for the traditional analysis, it is a
complement that provides an ecological explanation to why a specific species is likely to be
favoured or disfavoured by a specific restoration action. With the combination of the
traditional species-identity focused analysis and the functional-trait analysis we have the
possibility to conduct restoration analyses that provides results that are both detailed in
terms of effect on species in the study, but also relevant outside the distribution range of the
present species, thus bringing added value to the science of ecological restoration.