An Analysis of Vegetation Restoration on Opencast
Oil Shale Mines in Estonia
Margus Pensa,1,2,5 Arne Sellin,3 Aarne Luud,1 and Ingo Valgma4
We compared four types of 30-year-old forest stands
growing on spoil of opencast oil shale mines in Estonia.
The stand types were: (1) natural stands formed by
spontaneous succession, and plantations of (2) Pinus
sylvestris (Scots pine), (3) Betula pendula (silver birch),
and (4) Alnus glutinosa (European black alder). In all
stands we measured properties of the tree layer (species
richness, stand density, and volume of growing stock),
understory (density and species richness of shrubs and tree
saplings), and ground vegetation (aboveground biomass,
species richness, and species diversity). The tree layer was
most diverse though sparse in the natural stands. Understory species richness per 100-m2 plot was highest in the
natural stand, but total stand richness was equal in the
The destruction of ecosystem through mining for minerals and
other activities to meet industry demands has been an intrinsic
part of modern development. Further need for mineral
resources will accelerate degradation of natural habitats,
which will result in reduced biodiversity (Singh et al. 2002).
In the second half of the twentieth century scientists and
engineers were presented with many challenges to achieve
restoration, but the less difficult goal of reclamation was
more often practiced on human-disturbed areas around the
world (definitions as in www.ser.org). As the utilization of
natural resources continues and opportunities to restore
ecosystems damaged by human activities become more common, restoration is playing an increasingly important role
in environmental protection (Prach et al. 2001). The public
responds emotionally to lands degraded by mining activities
and associates mining with land that has been left devoid
of all topsoil, all vegetation, and any hope of regeneration in
the short to mid time scale.
The Convention on Biological Diversity (1992) signed
by most states of the world in Rio de Janeiro calls for
Institute of Ecology, Tallinn Pedagogical University, 15 Pargi Street, 41537
Rovaniemi Research Station, Finnish Forest Research Institute, P.O. Box 16,
96301 Rovaniemi, Finland.
Department of Botany and Ecology, University of Tartu, 40 Lai Street, 51005
Department of Mining, Tallinn Technical University, 82 Kopli Street, 10412
Address correspondence to M. Pensa, email firstname.lastname@example.org
Ó 2004 Society for Ecological Restoration International
natural and alder stands, which were higher than the birch
and pine stands. The understory sapling density was lower
than 50 saplings/100 m2 in the plantations, while it varied
between 50 and 180 saplings/100 m2 in the natural stands.
Growing stock volume was the least in natural stands and
greatest in birch stands. The aboveground biomass of
ground vegetation was highest in alder stands and lowest in
the pine stands. We can conclude that spontaneous
succession promotes establishment of diverse vegetation.
In plantations the establishment of diverse ground vegetation depends on planted tree species.
Key words: Alnus glutinosa (L) Gaertn., Betula pendula
Roth, forest plantation, opencast mine, Pinus sylvestris L.,
restoration, spontaneous succession.
ecologically sound restoration of degraded ecosystems as
measures to promote the recovery of local biodiversity.
Governments have therefore frequently given resources to
reestablish vegetation on degraded lands, in anticipation
that this will lead to restoration of the preexisting ecological
state and may add economic value to the degraded lands
(Hunter et al. 1998). The traditional approach to reclamation has been to sow grass and legumes and plant trees to
minimize financial and human resource expenditures. Landscape engineers and foresters often establish a low-diversity
plant cover or use monospecific plantations of exotic species
(Hunter et al. 1998; Rebele & Lehmann 2002). Although
plantations can play a key role in restoring forest ecosystems and achieving short-term socioeconomic goals
by protecting the soil surface from erosion, catalyzing
development of native forests, and accelerating the recovery
of genetic diversity (Singh et al. 2002), spontaneous vegetation succession, or natural recovery, as an alternative
approach to restoration or reclamation has gained increasˇ
ing attention (Prach & Pysek 1994, 2001; Prach 1994; Prach
et al. 2001). It has been claimed that spontaneous succession
can be more efficient than human efforts at returning
degraded lands to their original state and reestablishing
the self-regularity of ecosystems (Prach et al. 2001).
Depending on soil conditions, the required time period for
establishment of woody species on degraded mining sites in
Central Europe has been, on average, 20 years (Prach
1994). The first individuals of woody species may be present
at the beginning of succession (Rebele 1992; Prach 1994;
Prach & Pysek 2001; Rebele & Lehmann 2002), and the
Restoration Ecology Vol. 12 No. 2, pp. 200206
Vegetation Restoration on Opencast Oil Shale Mines
probability of their early establishment is higher in moderately wet, nutrient-poor sites (Prach 1994) or in sandy soils
(Rebele 1992). In general the establishment of woody species on degraded lands is highly variable due to the many
stochastic factors that affect vegetation succession.
Promoting spontaneous succession for ecological restoration therefore requires careful consideration of the target
ecosystem and time scale in which the goals of restoration
are to be achieved (Prach et al. 2001). If the goal is to restore
a certain type of plant community, an engineered restoration
process is preferred and succession should be directed
toward intended objectives. When selecting suitable tree
species for plantations, variable results rendered by different
species under the same environmental conditions must be
borne in mind (Singh et al. 2002). For example, Montagnini
et al. (1995) have shown that among 20 indigenous tree
species used for reclaiming forests on degraded lands in
Brazil, four species tended to have more positive effects on
soil properties than others. On abandoned agricultural lands
in India restored with different bamboo species, the diversity
of ground vegetation was greatest under the canopy of a
specific species (Arunachalam & Arunachalam 2002).
In this study we selected three tree species, Betula
pendula (silver birch), Alnus glutinosa (European black
alder), and Pinus sylvestris (Scots pine), growing in
30-year-old plantations established on calcareous spoils of
opencast oil shale mines in Estonia, and evaluated their
impact on understory and ground vegetation. Reclamation
of the Estonian oil shale opencast mining areas is regulated by the Ministry of Environment. It has been determined that the goals of reclamation should encompass
aspects of both economic (forestry) and ecological importance. However, studies carried out on the reclaimed oil
shale opencast mining areas have mainly focused on questions related to forest management (Kaar et al. 1971; Kaar
2002). The outcome of reclamation has been measured in
the volume increment of growing stock in stands. Except
for pedogenesis (Reintam & Kaar 1999; Reintam 2001;
Reintam et al. 2002), other ecological aspects of reclamation have been scarcely studied on the oil shale opencast
mining areas. Laasimer (1973) and Reintam and Kaar
(2002) studied spontaneous succession in the oil shale
opencasts, but they also emphasized the economic outcome, suggesting establishment of plantations as the only
way to achieve rapid recovery of vegetation on mined
areas. If spontaneous succession is permitted, however,
valuable ecosystems can develop over decades, which
may serve as refuges for rare species and communities
(Kirmer & Mahn 2001). This proposition is supported by
the results of Holl (2002), who demonstrated that spontaneous succession can create diverse plant communities on
an abandoned coal mine in the United States. The need for
comparisons between engineered reclamation and spontaneous succession has also been emphasized by Prach et al.
(2001) and Pysek et al. (2001). Therefore, we additionally
sampled mine spoils where spontaneous succession has
occurred since 1950.
Three specific questions are addressed by this study: (1)
are there differences in characteristics of the tree layer
between plantations and spontaneous stands; (2) does biomass and diversity of ground vegetation in plantations differ
from that of spontaneously developed stands; and (3) have
tree species used to establish plantations had different
effects on ground vegetation biomass and diversity.
The study was conducted at Kuttejou (59 200 N, 26 590 E) and
Narva (59 180 N, 27 450 E) oil shale opencast mining areas in
northeastern Estonia. Mean annual temperature is 4.5 C in
both areas; annual precipitation is 600 mm in Kuttejou and
660 mm in Narva. Oil shale is the main mineral resource of
Estonia, and it is mostly used for energy production. Mining
for oil shale began at the beginning of the twentieth century,
but its extraction and consumption was most intensive in the
1980s, declining sharply after the collapse of the Soviet Union
in 1991. A reclamation program for opencast oil shale mines
was initiated in the late 1960s when large areas around Narva
were opened to mining activities. As of 2003, about 9000 ha of
oil shale opencast mines have been reclaimed by leveling the
mine spoils and planting a variety of tree species on them
(Reintam et al. 2001). The most frequently planted tree species
have been Pinus sylvestris (covering approximately 8590% of
the reclaimed area) and Betula pendula. In total 54 species
have been used for reclamation, but most of them have only
been planted experimentally on small plots (Kaar et al. 1971).
The reclamation program did not set targets for restoring the
opencast spoils in Kuttejou, where mining activities had ceased
by the 1950s. Since then, the Kuttejou mining area (200 ha) has
been revegetated by means of spontaneous succession to a
sparse forest with an average stand age of 30 years.
Background of the Mining Areas
Kuttejou opencast mine operated on former agricultural
lands from 1925 to 1947. Oil shale layers were mined by
hand, and the overburden was stripped by a 3-m3 steam
excavator equipped with a spoiler. The width of the mining
pit was 20 m, forming spoils with maximum slopes of 20 .
The thickness of the oil shale layer was 2.6 m, and the
overburden thickness varied from 5 to 12 m, with an average stripping factor of 1.9 m3/t. Consequently, the height of
the ground in the mined out area is 02 m below the
original ground surface. As a spoiler and mechanical shovel
were used for stripping, the estimated swell factor of
the material was 1.3. Maximum size of the material mined
was 1.4 m due to bucket size, while the average size
was 0.15 m. Due to the lack of reclamation requirements
and poor mobility of the spoiler, the spoil depth varies
from 0.5 to 2 m. The organic matter content of the spoil,
which originates from kukersite oil shale, is on average
46% and is evenly distributed in the spoil.
Vegetation Restoration on Opencast Oil Shale Mines
Mining activities at Narva opencast mine began in 1963,
and the mine is still operating today. The area was previously covered with swamp forests and bogs. The oil shale
layers are mined by blasting; the overburden is blasted and
stripped by 15 m3 and 90 mlong boom draglines. The
widths of the mining pits are 50 m, forming spoils with
maximum slopes of 3 . The thickness of the oil shale
layer is 2.0 m, and the overburden thickness varies from
11 to 17 m, with an average stripping factor of 4.1 m3/t.
Consequently, the average height of the ground in the
mined out area is 24 m above the original ground surface.
The higher surface level and water pumping, which is
required for opencast operations, results in lower water
availability and spoil seepage than at the Kuttejou mine.
The Narva mining area has been a test site for a variety of
new stripping methods. As a result, in some areas, the peat
layer is located at the bottom of the spoil and in other
areas, on the top. The organic component of the remaining
oil shale layers is mostly at the bottom of the spoil. Dragline, and in some cases a mechanical shovel, was used for
stripping the blasted overburden rock, and therefore the
swell factor of the material is 1.4. Maximum size of the
material mined was 3.1 m, while the average size was 0.3 m.
Spoil depth is uneven and ranges from 0.5 to 1.5 m, but can
be as much as 6 m in test sections. The organic matter
content of the spoil, which also originates from kukersite
oil shale, is on average 2% (4% at the bottom and 1% at
the surface of the spoil). Due to the stripping method used,
the material is unevenly distributed in the spoil and the
estimated swell factor is up to 1.5 at the bottom. This is an
additional factor that causes water runoff from the spoil.
The spoil chemistry at the Narva mining area was analyzed by Vaus (1970) before reclamation (i.e., before tree
growth had influenced pedogenesis). Similar data were not
available for the Kuttejou mine; therefore we took spoil
samples in the most recently abandoned parts of the opencast. The samples were analyzed for pH, total nitrogen,
and P2O5, and compared with those taken at Narva
(Table 1). It must, however, be recognized that the spoil
chemistry at Kuttejou has been affected by vegetation
establishment over decades and the results do not
represent the pre-successional conditions in this area.
plantation of Pinus sylvestris (referred to as pine stand),
(3) a plantation of Betula pendula (birch stand), and (4) a
plantation of Alnus glutinosa (alder stand). In June 2002
we established five 10 3 10 m randomly located sample
plots in each stand type (20 plots total). According to
forest inventory data the stands were on average 30 years
old, which was confirmed by taking tree cores with an
increment borer from within each plot.
Within the plots we counted all trees with diameter at
breast height (dbh) greater than 10 cm and estimated density of understory (number of shrubs and tree saplings/
100 m2). For analyzing ground vegetation we marked five
small (0.4 3 0.5 m) randomly located quadrats within each
plot. Thus, ground cover in each stand type was represented by 25 quadrats (100 quadrats total). We harvested
all vascular plants from within the quadrats and dried them
at 60 C for 48 hr. We determined total aboveground biomass and biomass by species for each stand type (g/m2).
We also calculated mean number of species, mean species
diversity (H) expressed by ShannonWiener’s index
weighted with biomass and volume stock of trees. Shannon
Wiener’s index of diversity was calculated as follows:
where n is the number of species and pi is the proportion of
the ith species in total biomass.
To compare the effects of restoration technique (spontaneous succession vs. plantations) and tree species on characteristics of the understory and ground vegetation and
tree layer, we applied univariate ANOVA using stand
type as a fixed factor. Post hoc comparisons were carried
out using the LSD test. The assumptions of normality and
homogeneity of variances were checked using the KolmogorovSmirnov D-statistic and the BrownForsythe tests,
respectively. All statistical computations were made with
the aid of the computer package Statistica’98 (StatSoft
Inc., Tulsa, OK, U.S.A.) at the significance level a 5 0.05
Four different types of forest stands were sampled: (1) a
natural stand formed by spontaneous succession, (2) a
Table 1. Characteristics
(mean ± SD).
Total N (%)
7.1 ± 0.2
0.1 ± 0.05
16.6 ± 10.7
7.4 ± 0.1
0.2 ± 0.06
20.7 ± 8.1
Data on Narva were published by Vaus (1970).
The density of trees with dbh greater than 10 cm was
significantly lower in the natural stands than in the plantations, whereas species composition of the natural stands
was more diverse compared with the plantations (Tables 2
3). There was one dominant tree species (abundance
.90%) in all three plantations and a few co-dominant
species with abundance above 5% (Table 3). Unlike the
plantations, no one species dominated in the tree layer of
the natural stands. Although nearly half of the trees were
birches, which is the most common species in secondary
Vegetation Restoration on Opencast Oil Shale Mines
Table 2. Comparison of the vegetative characteristics among the studied stand types (mean ± SD).
Number of tree species/100 m2
Number of trees/100 m2
Volume stock of trees (m3/100 m2)
Number of understory species/100 m2
Understory density (saplings/100 m2)
Biomass of ground vegetation (g/m2)
Number of species in ground vegetation/m2
Diversity of ground vegetation, H
3 ± 1*
9 ± 4*
1.0 ± 0.6*
9 ± 3*
98 ± 54
52.9 ± 20.8*
12.4 ± 6.9*†
1.73 ± 0.44*
1 ± 0.4†
25 ± 11†
1.9 ± 1.1*‡
5 ± 2†
38 ± 17
32.4 ± 13.7†
7.0 ± 2.5*
1.52 ± 0.32*
2 ± 1†
28 ± 2†
3.4 ± 0.8†
5 ± 1†
85 ± 120
59.2 ± 23.6*
7.0 ± 1.9*
0.86 ± 0.47†
1 ± 0.5†
26 ± 7†
2.8 ± 0.5†‡
3 ± 2†
16 ± 19
104.0 ± 41.3‡
12.2 ± 2.8†
1.56 ± 0.46*
Values within rows with different symbols are significantly different at a 5 0.05. Due to high degree of variation, the significance of differences was not estimated for
density of understory.
succession in oldfields in Estonia, other species were also
frequently represented (Table 3). The sparse tree layer in
natural stands had a positive effect on the density of the
understory (Table 2). In other stand types there were fewer
than 50 saplings/100 m2, the only exception being a plot in
a birch stand where Caragana arborescens had been
densely planted (300 saplings/100 m2) under the tree layer.
This is why mean understory density was high in the birch
stands (Table 2); however, the birch stands also had the
highest degree of variation, with densities ranging from 20
to 300 saplings/100 m2. Species richness of the understory
followed the same pattern as that of the tree layer with the
natural stands having significantly more species per plot
than the plantations (Table 2). Although trees were more
sparsely distributed in the natural stands, the difference in
volume stock between the plantations and natural stands
was not as clear. When compared with pine stands, natural
stands would provide similar timber production, but when
compared with birch and alder stands, the potential for
timber production was much less (Table 2).
In alder stands, the biomass of ground vegetation was
significantly greater than that in the other two plantations
as well as in the natural stands (Table 2). Species richness
was significantly higher in the alder stands than that in the
pine and birch stands, where it averaged 7 species/m2
(Table 2). In natural stands, species richness was highly
variable and did not differ clearly from that of any other
stand type (Table 2). ShannonWiener’s diversity index
was smallest in the birch stands, while among other stand
types its variation was not significant (Table 2). Calamagrostis arundinacea (red grass) alone accounted for more
than half the cumulative biomass in birch stands, while five
species in natural, four in alder, and two in pine stands
accounted for more than half the cumulative biomass
The studied sites had different historical backgrounds that
might have influenced the measured characteristics of
vegetation. Mining activity at the Kuttejou mine ended
20 years earlier than at Narva; therefore Kuttejou was
exposed longer to species immigration. A longer immigration period was found to enhance biodiversity in studies of
island colonization (Begon et al. 1996). Differences in
vegetation between Narva and Kuttejou opencast mines,
however, cannot be explained merely by species immigration. First, although less exposed to immigration, the
ground vegetation in alder stands had the same average
species richness as that of natural stands. Second, although
the Kuttejou opencast mine was older than Narva, the
average age of the tree layer was 30 years in both areas.
Thus, there was a time lag of about 20 years in the devel¨
opment of woody stands at the Kuttejou mine. A similar
time lag was characteristic of the formation of a tree layer
by spontaneous succession in lignite opencast mines in
Central Europe (Prach Pysek 1994, 2001; Prach 1994).
The second aspect to be considered when comparing
study areas is their different locations; the Kuttejou mine
is situated at about 50 km west of the Narva mine.
Although this distance does not result in climatic variation,
it may affect propagule availability due to differences in
surrounding plant communities. The Kuttejou mine is
located in a diverse agricultural landscape (fields interspersed with small areas of forests and gardens), while
the Narva mine is surrounded by natural ecosystems
(swampy deciduous forests interspersed with pine bog forests). A more diverse landscape is probably the main
reason why ground vegetation at Kuttejou opencast has
the greater total number of plant species, but spontaneous
succession seems to be a necessary condition for maintaining that diversity. Although Betula pendula was the most
abundant tree species in the natural stands, its dominance
did not exceeded 50% and groups of other woody species
were interspersed within gaps. The stochastic nature of
spontaneous succession is thought to be the main reason
for such spatial patterns (Rebele 1992). This variability
creates the required conditions for development of forest
understory (shrubs and tree saplings) and ground vegetation. Sparse tree distribution was the main reason for the
small volume of growing stock in the natural stands, indicating that their economic value is relatively low compared
with plantations. In the plantations, contrary to the natural
stands, the tree layer was dense and dominance of the
Vegetation Restoration on Opencast Oil Shale Mines
Table 3. Continued
The values are percentages of total abundance for tree layer and understory, and percentages of total aboveground biomass for ground vegetation (only values .0.1%
are shown). For each stand type, number of species in tree layer, in understory, and in ground vegetation as well as total number of species are shown in bold. Asterisks
indicate planted species. Nomenclature of species according to Kukk (1999).
planted species was very high (.90%). As previous studies
(Kaar et al. 1971; Kaar 2002) have shown, environmental
conditions on the oil shale opencast spoil are ideal for
growth of Pinus sylvestris and Betula pendula as well as
Alnus glutinosa. Thus, dense stands of these species suppress the growth of other trees, which otherwise could
spontaneously establish on mined areas.
Although in some cases establishment of plantations on
degraded lands may promote growth of other species
(Lugo 1997; Singh et al. 2002), our results indicate that
on calcareous and stony spoils of the opencast oil shale
mines, spontaneous succession may have several advantages in terms of increased plant diversity. This increase
in plant diversity may result in an increase in the diversity
of other organisms, as variability in stand structure and
species composition creates habitats for a variety of organisms. Our results also revealed differences in the ability of
the tree species to modify growing conditions for other
plants as measured by the effect of woody species on
ground vegetation. Alnus glutinosa, a species that forms
symbiotic relationships with nitrogen-fixing bacteria from
the genus Frankia (Wall Huss-Danell 1997; Sprent
Parsons 2000), promotes the most ground vegetation growth
(Table 2). Vigorously growing herbaceous vegetation in
turn suppresses woody seedlings, which seems to be the
main reason why the understory density was lowest in the
alder stands. Low-ground vegetation biomass in the pine
stands is probably related to poor soil-forming capabilities
of coniferous monocultures as shown by Kilian (1998).
The greatest biomass of ground vegetation observed in
the alder stands was related to high species richness.
Although mean species richness in the natural stands was
also 12 species/m2, it varied widely due to the high spatial
heterogeneity of the stand structure. The variation in species richness of the ground vegetation among plots in the
natural stands, however, followed the same pattern as in
forest plantations. In plots with high dominance of Pinus
sylvestris, the number of species was low, while within the
plots where deciduous tree species dominated, species
richness of the ground vegetation was higher. Similar
results were obtained by Pitkanen (1998), who found that
the number of coniferous and broad-leaved tree species
had a significant effect on the diversity of vegetation in
managed boreal forests. The high dominance of Betula
pendula in the tree layer was accompanied by high
dominance of Calamagrostis arundinacea in the ground
vegetation. This was obviously the main reason for the
low-diversity index of the ground vegetation in the birch
stands, while in the other stand types no one species
accounted for 50% or more of the total aboveground
biomass (Table 3).
We can conclude that spontaneous succession is a useful
technique for restoration of small areas of the calcareous
and stony spoils of opencast mines and may replace the
typical reclamation technique of planting tree monocultures where diversity is the goal. Spontaneous succession
enhances establishment of diverse vegetation and may
therefore create habitats for a wide range of organisms.
If economic value is set as a priority, however, creation of
plantations or scattered colonization foci may assist in
overcoming dispersal barriers and direct succession toward
defined targets (Robinson Handel 2000). Selection of
tree species for planting has a significant impact on development of the rest of the plant community. Among active
reclamation practices in oil shale opencast mines, planting
of Alnus glutinosa gave the best results in terms of total
number of vascular species and aboveground biomass of
ground vegetation. Planting density may also affect the
development of vegetation in plantations. Further studies
are needed to reveal whether the within-stand variability
in diversity is connected to the density of the tree layer and
whether there are temporal fluctuations in diversity and
biomass of ground vegetation in oil shale opencast mines.
We are grateful to Elga and Ene Rull who helped us with the
field sampling. Sarah Wilkinson and Michael Dunderdale
checked the English language of the manuscript. The
company Estonian Oil Shale is thanked for the permission to
carry out the study in the Narva opencast. Plant Biochemistry
Laboratory of the Estonian Agricultural University analyzed
the chemistry of spoil samples. Prof. Karel Prach and two
anonymous reviewers made valuable comments and annotations on the manuscript. The study was funded by the Ministry
of Education, Republic of Estonia (project no. 0282119s02).
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