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Ecological Engineering 37 (2011) 294–301

Contents lists available at ScienceDirect

Ecological Engineering

journa l homepage: www.e lsev ier .com/ locate /eco leng

estoration of alpine spoil heaps: Successional rates predict vegetation recoveryn 50 years

nut Rydgrena,∗, Rune Halvorsenb, Arvid Odlandc, Gudrun Skjerdala

Sogn og Fjordane University College, Faculty of Science, P.O. Box 133, N-6851 Sogndal, NorwaySection of Botany, Natural History Museum, University of Oslo, P.O. Box 1172 Blindern, N-0318 Oslo, NorwayTelemark University College, N-3800 Bø, Norway

r t i c l e i n f o

rticle history:eceived 24 August 2010eceived in revised form 29 October 2010ccepted 7 November 2010vailable online 14 December 2010

eywords:lpine restorationolonizationlant species compositionestoration ecologypoil heapsates of succession

a b s t r a c t

Human disturbances in the alpine region can have long-lasting ecosystem effects because biologicalrecovery in harsh environments proceeds slowly. As a by-product of the exploitation of hydroelectricity,surplus masses from tunnel excavations are deposited as spoil heaps in the alpine landscape. The typicalmanagement goal for such spoil heaps is that their species composition converges towards that of theirundisturbed surroundings. At present we lack knowledge of the rate as well as the direction of thesesuccessions. We examined the species composition of five alpine spoil heaps in western Norway at twopoints in time, after 6–16, and 24–34 years of succession, with regard to vegetation cover, species rich-ness, species composition and soil conditions. We also compared the vegetation of spoil heaps with thatof their surroundings. After ca. 30 years, bryophyte and lichen cover and species richness were similar tothose of their surroundings, while cover of vascular plants and species richness recovered more slowly.The vegetation development of spoil heaps appears safe-site limited rather than dispersal limited. Topsoildevelopment was slow, but some accumulation of organic matter was observed. The species compositionfollowed a successional trajectory in direction of the vegetation of the surroundings. Estimated linearsuccessional rates indicate that 35–48 years are needed from construction of spoil heaps till a species

composition more or less similar to their surroundings has been reached. However, these estimates arelikely to be over-optimistic because successional rates tend to decrease with time. Based on our resultswe propose three changes to the current spoil-heap construction practice that will improve their restora-tion: (1) to increase surface unevenness, by which the number of safe sites will increase and germinationand establishment success will be enhanced; (2) to increase substrate variability, by adding more of fine-grained materials more species will establish in shorter time; and (3) to use seed from local sources or to

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. Introduction

A steadily increasing human population continuously increasests demands for, and use of, energy. The signals of global warm-ng seriously motivate for use of energy from renewable sourcesnstead of fossil fuels. Hydroelectricity still plays the most impor-ant role among the non-carbon renewable energy sources (Lincoln,005), although sun and wind energy are becoming increasingly

mportant. However, exploitation of non-carbon renewable energyources of all kinds impacts the landscape severely, leaving largeams, dry rivers, penstocks, power lines and spoil heaps. Spoileaps, usually consisting of blasted rocks originating from tunnel

∗ Corresponding author.E-mail address: knut.rydgren@hisf.no (K. Rydgren).

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925-8574/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.ecoleng.2010.11.022

aturally.© 2010 Elsevier B.V. All rights reserved.

xcavations, represent newly created terrain in which restorationsually starts out as a primary succession on virgin ground with noiological legacy. A common management goal for spoil heaps ishat their species composition converges towards that of adjacentndisturbed vegetation in order to reduce their visual impact, andotentially, also increasing their natural value.

Many spoil heaps are located in the alpine region for whichestoration ecological knowledge is sparse compared to that of theowlands. This is unfortunate because arctic–alpine areas are par-icularly sensitive to human exploitation and because restorationf alpine ecosystems is slow and may last for decades or even cen-

uries (Harper and Kershaw, 1996; Jorgenson et al., 2010). This slowynamics call for development of indicators of restoration suc-ess that are useful long before recovery is complete. Ordinationethods are relevant for evaluating restoration success (Martínez-

uiz and Marrs, 2007) because they enable quantification of the

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irection and rate of vegetation change, i.e. successional rates, frompecies compositional data recorded at two (or more) time-pointsZedler and Callaway, 1999; Urban, 2006; Rydgren et al., 2010).evertheless, surprisingly few restoration studies have used ordi-ation to estimate successional rates and to calculate the expectedime before a restoration target is reached.

In the early 1990s Skjerdal (1993) and Skjerdal and Odland1995) analyzed vegetation and environmental conditions of sev-ral spoil heaps in southern and western Norway. In 2008, wee-visited five of these alpine spoil heaps, constructed between974 and 1984. We also collected data on the vegetation of theurroundings of the spoil heaps to be representative of localestoration targets. We analyzed these data by ordination meth-ds with the aim of summarizing the co-ordinated vegetationhange along gradients (Økland and Eilertsen, 1996; Rydgren et al.,004). By comparing the rate of change in species compositionetween two time-points we were able to estimate successionalates of spoil-heap vegetation. Recording of the vegetation of theurroundings made possible evaluation of the hypothesis that thepecies composition on spoil heaps converges towards that ofhe surroundings, and to estimate the time scales involved in theuccessions. Our aims are to evaluate the direction and rate of suc-ession of alpine spoil heaps by answering the following questions:1) How similar are the alpine spoil heaps to their surroundingsith respect to vegetation cover, species richness, species compo-

ition and soil conditions? (2) Does the species composition of thelpine spoil heaps follow a successional trajectory towards that ofhe surroundings? (3) Provided that the present successional ratesepresent a long-term average, how long time can be expectedefore spoil-heap vegetation is more or less similar to that of theurroundings?

. Materials and methods

.1. Study area

We studied five alpine spoil heaps in Sogn og Fjordane county,Norway (Fig. A.1, Table A.1). The extent of the study area was ca.

0 km (60◦44′–61◦03′N, 7◦24′–8◦08′E). The spoil heaps were situ-ted from 1000 to 1360 m a.s.l. (Table A.1). They were constructedetween 1974 and 1984, and varied in area from 10,000 m2 to1,000 m2. All spoil heaps consisted of blasted rocks except Kleå-alen that had a finer-grained substrate, resulting from tunnelonstruction by a drilling technique. The spoil-heap rock materialiffered in origin; comprising gneisses, granites, metamorphosedocks of Precambrian age in the Caledonian nappes and phylliteTable A.1), but were in all cases similar to the bedrock of theurroundings (Skjerdal and Odland, 1995).

Immediately after construction all spoil heaps were fertilizednd seeded yearly for at least three years (Skjerdal and Odland,995). 6 kg of extra seeds were added in 2002 at the Kleådalenite and 30 Salix shrubs were planted in 2005 at the Fossane site.dditional fertilization took place in 2002 and 2007 at Svartavatn,ossane and Kleådalen, while Sothaug and Øydalen were fertilizedearly in the period 1974–1980 and every second year in the period981–1988. The seed mixtures used at Øydalen and Sothaug con-ained Agrostis capillaris, Festuca ovina, Schedonorus pratensis andhleum pratense (Skjerdal and Odland, 1995) while, most likely,vartavatn, Fossane and Kleådalen were seeded with A. capillaris,

. ovina and Festuca rubra. Sheep, and on Sothaug also domesticeindeer, grazed the sites. All sites were situated in the alpine veg-tation zone and in the vegetation section intermediate betweenceanic and continental except Svartavatn that belonged to thelightly oceanic section (Moen, 1999). The climate is relatively wet

2

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eering 37 (2011) 294–301 295

nd cold; Svartavatn and Fossane received most of precipitationnd Sothaug least (Table A.1).

.2. Recording of species composition and derived variables

The vegetation of spoil heaps was recorded in July–August inll years. Svartavatn, Fossane and Kleådalen were first studied in991 (Skjerdal, 1993), Øydalen and Sothaug in 1994 (Skjerdal anddland, 1995). In 2008 we re-analyzed all five spoil heaps and

ecorded the vegetation of the undisturbed surroundings of all spoileaps. In the early 1990s we placed non-permanent sample plots,.5 m × 0.5 m each, by stratified random sampling using a baselinepproach (Skjerdal and Odland, 1995) to establish 10–20 samplelots per spoil heap. In 2008 we placed 15 sample plots per spoileap, 0.5 m × 0.5 m each, by randomization within selected blocks;hree sample plots in each of five blocks (5 m × 10 m). We avoidedlacing blocks and plots at very steep slopes (slope >40◦) due toafety reasons, since such spoil-heap slopes are unstable. All 2008lots were permanently marked by subterranean aluminium tubes

n all corners. Three blocks with a total of nine sample plots werestablished in the surroundings of each spoil heap. Since the spoil-eap environment is generally dry we avoided slightly paludifiednd wet environments in our sample. No plot was allowed to beituated closer to another plot than 1 m. The total data set con-isted of 194 sample plots. Each plot was divided into 16 equallyized subplots in each of which we recorded presence or absencef vascular plants, bryophytes and lichens (a species was consid-red to be present in a subplot when its living parts covered theubplot). In the 1990s most Cladonia specimens were determinedo the genus level only due to the poor development and minuteize of most specimens (see Skjerdal, 1993; Skjerdal and Odland,995). Frequency in the 16 subplots was used as measure of plotbundance. We also visually estimated the percentage total coverf vascular plants, bryophytes and lichens in each sample plot.

Species richness variables (the number of species of vascularlants, bryophytes, and lichens, respectively) were recorded bothor each 0.25-m2 sample plot and for the entire spoil heap byearching for additional species in the blocks (for approximately

h) and at the entire spoil heap (for 1–1½ h). By this proceduree probably recovered most of the vascular plant species whileinute bryophytes and lichens are likely to be under-represented

n the material from the entire spoil heaps.

.3. Soil variables

We collected samples from the upper 5 cm of the soil of everyample plot. In the early 1990s samples were taken in the centref the sample plot or, when the soil was shallow, from all overhe sample plot (Skjerdal and Odland, 1995). In 2008, we sam-led the soil by mixing four sub-samples collected just outsidehe mid-points of each sample-plot edge. The soil was dried at0 ◦C to constant weight and then sifted (2 mm mesh width). Lossn ignition in % (by ashing ca. 3–5 g of soil at 550 ◦C in at leasth) and pH (in aqueous solution; the proportion soil:water was:2 in 1990s and 1:2.5 in 2008) were analyzed in 2008 by theorwegian University of Life Sciences, Department of Plant andnvironmental Sciences and in the 1990s at the laboratory of Uni-ersity of Bergen, Botanical Institute, except for pH in soil samplesrom Svartavatn, Fossane and Kleådalen which was measured at aeld station (Skjerdal, 1993).

.4. Statistical analyses

R version 2.8.1 (R Development Core Team, 2008) was used fortatistical analyses.

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We used GNMDS ordination (Global Non-Metric Multidimen-ional Scaling; Minchin, 1987) in the vegan package version 1.15-1Oksanen, 2005) to find the main vegetation gradients in the totalata set (194 sample plots). GNMDS was run by use of functionsetaMDSdist, initMDS, isoMDS and postMDS in the vegan pack-

ge (Oksanen, 2005), with the Bray–Curtis dissimilarity index,aximum number of iterations = 500 and convergence tolerance

riterion = 10−5. GNMDS solutions were obtained from 300 differ-nt random starting configurations, of which more than two had toive identical minimum stress solutions. This best solution sub-ected to varimax rotation using Principal Component Analysis.he resulting axes were re-scaled to half-change units. We solvedor two-, three- and four-dimensional GNMDS solutions, and com-ared pairs of axes in the best solutions obtained for each numberf dimensions by calculating Kendall’s �. The first axes of all solu-ions were very similar (0.919 ≤ |�| ≤ 0.946, P < 0.0001), while theecond axes were highly correlated only for the three- and four-imensional solutions (|�| = 0.775; P < 0.0001). We therefore usedhe first two dimensions of the three-dimensional GNMDS as basisor our interpretation of vegetation gradients.

All cover and soil variables were zero-skewness transformedØkland et al., 2001) in order to improve homoscedasticity andhen brought onto a standard 0–1 scale by ranging, while countata (species richness) was untransformed (species richness ofryophyte and lichens were analyzed together because several sites

acked lichens in the early 1990s). Because of the clustered struc-ure of the data from 2008 we applied a linear mixed-effect modellme or lmer, Crawley, 2007) that accounted for the nested struc-ure of the plots. We used years and sites as factors in a two-waypproach, with random effects of block and plots nested withinlock. We also applied lme to test if sample plots from the spoileaps at the two different time-points and the surroundings dif-

ered in position along the first GNMDS axis, and to test if samplelots from the five different sites differed in position along theecond GNMDS axis. All models were simplified as much as pos-ible and in each case we reported the minimal adequate modelCrawley, 2007).

We calculated successional rates for the vegetation of each spoileap as the average yearly plot displacement along the first GNMDSxis (in half-change units) of plots from each spoil heap betweenhe two points in time (1990s and 2008). For each spoil heap wesed the obtained values to calculate the expected time in years forhe vegetation to reach a species composition more or less similaro their surroundings and to calculate the expected total numberf years since construction before the vegetation of a spoil heaps expected to be more or less similar to that of their surround-ngs. These calculations assume constant successional rates, equalo those observed between the 1990s and 2008.

. Results

.1. Vegetation cover

Vascular plant cover increased significantly from an average of3% in the early 1990s to 19% in 2008 (F1,89 = 6.26, P = 0.014; Fig. 1A).ascular plant cover development differed among the five spoileaps (time × spoil heap; F4,89 = 3.62, P = 0.009). The overall averageascular plant cover of the surroundings was 59%.

Bryophyte and lichen cover increased significantly from an aver-ge of 29% in the early 1990s to 48% in 2008 (F1,89 = 29.02, P < 0.001;ig. 1B); the same as the cover of the surroundings. Bryophytend lichen cover development differed among the spoil heapstime × spoil heap; F4,89 = 2.74, P = 0.033).

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990s (1991: Svartavatn (n = 19), Fossane (n = 20) and Kleådalen (n = 15); 1994:ydalen (n = 10) and Sothaug (n = 10)); and 2008 (n = 15 for all spoil heaps). (A)ascular plants and (B) bryophytes and lichens.

.2. Species richness

Bryophytes and lichens were the plant groups with highestncrease in species numbers per plot from the early 1990s to 2008Fig. 2B and C). The number of bryophyte species increased from.7 to 8.5, while the number of lichens increased from 1.6 to 7.7.his was a significant increase (P < 0.001; bryophytes and lichensnalyzed together, see Section 2.4).

The increase in number of bryophyte and lichen species wasignificantly higher at Fossane than at Kleådalen and SothaugP < 0.001 and P = 0.034, respectively). The number of vascular plantpecies also increased significantly (P < 0.001; Fig. 2A), but the pat-ern of development did not differ among spoil heaps (test ofnteraction term Chi.sq4 = 4.5 and P-value = 0.345). The number ofascular plant species per plot was almost half of the number

bserved in the surroundings (6.1 vs. 11.2), while the number ofryophyte and lichen species was as high on the spoil heaps as

n the surroundings (spoil heaps vs. surroundings; 8.5 vs. 8.3 forryophytes).

K. Rydgren et al. / Ecological Engineering 37 (2011) 294–301 297

Fig. 2. Box-plot of number of species at the five spoil heaps in the early 1990s (1991:Svartavatn (n = 19), Fossane (n = 20) and Kleådalen (n = 15); 1994: Øydalen (n = 10)and Sothaug (n = 10)) and 2008 (n = 15 for all spoil heaps). (A) Vascular plants, (B)bryophytes, and (C) lichens.

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ig. 3. Stacked bar plot of the total number of species of the three different speciesroups at the five different spoil heaps at the scale of the whole spoil heaps at thewo different time-points (1990s and 2008).

At the scale of entire spoil heaps, a considerable increase inpecies number was observed between the early 1990s and 2008,rom a total number always below 100 in the early 1990s to a totalumber always above 100 in 2008 (Fig. 3). Vascular plants made uphe most species-rich group in all spoil heaps in 2008. The small-st increase in species number, both with respect to relative andbsolute increase (59% and 58 species, respectively) was observedn Kleådalen, while the highest increase was observed in Fossane330% and 89 species, respectively).

.3. Species composition

The main gradient in species composition in the total data setGNMDS axis 1) ran from the species composition of new spoileaps (early 1990s) via that of established spoil heaps (2008) to

hat of the surroundings (Fig. 4). The change from the early 1990so 2008 was considerable (Fig. 4, Table 1) and the mean positionf these three groups of plots was significantly different alongxis 1 (F2,111 = 364.1, P < 0.001) but not along axis 2 (F2,111 = 0.501,

ig. 4. GNMDS ordination of 194 sample plots from five sites consisting of 74 samplelots of spoil heap vegetation in the early 1990s (white symbols), 75 sample plots ofpoil heap vegetation in 2008 (grey symbols), and 45 sample plots of surroundingegetation in 2008 (black symbols). The mean resultant displacement of each sitelong the two first GNMDS axes is shown as black symbols on connecting lines.

298 K. Rydgren et al. / Ecological Engineering 37 (2011) 294–301

Table 1Rate of change in species composition along the main vegetation gradient in GNMDSordination on five alpine spoil heaps. DistRt90s–08 – the average displacement in half-changes (H.C.) of the sample plots of a given spoil heap between the two points intime (1991 or 1994 and 2008). Yearly successional rate – the average yearly plot dis-placement along the first GNMDS axis between the two points in time, in half-changeunits. DistRt08–Su08 – the average distance in half-changes between the position ofthe spoil heaps in 2008 and their surroundings, in half-change units. ExpYr – theexpected time in years for the spoil heap vegetation to reach a species composi-tion more or less similar to their surroundings, given that a set of assumptions issatisfied. TotYr – the expected total number of years since construction before thevegetation of a spoil heap is more or less similar to that of their surroundings.

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(H.C.)Yearlysuccessionalrate (H.C.)

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Svartavatn 1.073 0.063 1.371 22 48Fossane 1.503 0.088 1.125 13 37

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= 0.607). This axis thus mainly represented a temporal (succes-ional) gradient, characterised by establishment and abundantncrease for many species, most notably bryophytes and lichens,etween the early 1990s and 2008. Examples of species that werebsent or had low frequency in the early 1990s while in 2008aving become among the most frequent species on the spoileaps are Barbilophozia lycopodioides agg., Lophozia ventricosa agg.,ophozia sudetica, Ptilidium ciliare, Cladonia arbuscula agg., Cladoniaellidiflora, Cladonia coccifera agg., Cladonia gracilis, Cladonia pyx-data agg., Cladonia uncialis agg. and Stereocaulon spp. (Table A.2).he fact that these species were also among the most frequentpecies in the surroundings, explained the pattern of displacementf spoil-heap plots towards plots from their surroundings in therdination diagram. The decrease in abundance of species thatere relatively frequent on the spoil heaps in the early 1990s (P.

ratense and Leptobryum pyriforme) but that lacked in surroundingslso contributed to this pattern.

In 2008 the vegetation of the spoil heaps still differed consid-rably from that of their surroundings, as indicated by positionslong GNMDS axis 1 (Fig. 4). The high frequency and abundance ofeveral species on the spoil heaps, e.g. two of the species used inhe seed mixtures A. capillaris, F. rubra, as well as Poa alpina, Saginaaginoides, and Ceratodon purpureus, that played a minor role in theurroundings contributed strongly to this pattern (Table A.2). Theame was true for common species in the surroundings that weretill rare or absent on the spoil heaps in 2008 (e.g. Vaccinium vitis-daea, Vaccinium uliginosum, Bistortia vivipara, Hieracium alpinum,nthoxanthum odoratum agg., Avenella flexuosa, Carex biegelowii,nd Juncus trifidus). Under the assumption that the rate of veg-tation change is constant, 7–22 more years (35–48 years sinceonstruction) are needed for the vegetation of the spoil heaps toecome indistinguishable from that of their surroundings withespect to position along the main axis (Table 1).

The second ordination axis separated the sites (F4,109 = 37.9,< 0.001), without regard to plots belonging to spoil heaps or theirurroundings. The mean plot scores of the sites to some extenteflected variation in annual precipitation, with the sites receiv-ng the least precipitation at the low-score end along GNMDS axis(Sothaug followed by Øydalen). However, Kleådalen, with inter-ediate precipitation (Table A.1), had the highest mean plot score

long GNMDS axis 2.

.4. Soil development

Soil organic matter increased significantly from 1.4% in the early990s to 2.7% in 2008 (F1,93 = 62.6, P < 0.001; Fig. 5A). This pattern

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as observed in all spoil heaps, i.e. the time × spoil heap inter-ction was not significant (F4,89 = 0.759, P = 0.555). The differentpoil heaps differed significantly with respect to pH (F4,94 = 49.1,< 0.001; Fig. 5B) but the decrease in pH from an average of 6.1 in

he early 1990s to 6.0 in 2008 was not significant.

. Discussion

.1. Interpretation of vegetation change on the spoil heaps:ethodological aspects

Our results clearly show that the studied alpine spoil heaps aree-vegetated relatively rapidly and, notably, that the vegetation ofhe spoil heaps develops towards similarity with the surroundingegetation on well-drained mineral ground. At present, after 24–34ears of succession, the cover and species richness of bryophytesnd lichens on the spoil heaps have reached the level of their sur-oundings, while establishment of vascular plants proceeds more

lowly. Soil formation also proceeds slowly although the increasen organic material content and decrease in pH accords with expec-ations for new soils (Matthews, 1992).

Based on ordination results, and assuming a constant rate of suc-ession, we estimate that the vegetation of the spoil heaps (except

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he unstable steep slopes, see material and methods) will becomeore or less similar to that of their surroundings in 50 years. The

pplication of ordination to calculate successional rates does, how-ver, rely on several assumptions, the most important of which are:hat the succession on spoil heaps continues (1) with the same ratend (2) in the same direction, as observed between the 1990s and008, (3) that the vegetation of the surroundings does not change

n a systematic manner, and (4) that the samples are representa-ive both of the successions at spoil heaps and for ‘comparable’onditions in the surroundings.

All of these assumptions can be questioned. Firstly, with record-ngs from only two points in time one has to assume a constantuccessional rate, i.e. that former and future behaviour is linear.his is not usually the case; successional rates usually slow downuring succession (Prach et al., 1993; Martínez-Ruiz et al., 2001;ydgren et al., 2004) and estimates based upon a linear temporalodel are therefore likely to be over-estimates.The second assumption, about the direction of vegetation

hange, is hard to evaluate from observations made at only twooints in time. The ordination diagram does, however, indicates ahange in species composition along the main, first, ordination axishat is clearly in direction of that of the surroundings. This cannot,owever, be taken as an indication that spoil-heap vegetation willecome similar to the vegetation of the surroundings in all respects.he first GNMDS axis clearly reflects variation along a successionalradient. Late-successional vegetation of spoil heaps is, however,ikely to differ from that of the surroundings along local complexnvironmental gradients that are important in alpine areas, suchs soil moisture and soil nutrients (Odland and Munkejord, 2008),hich will, in turn, be reflected in persistent vegetational differ-

nces.Thirdly, our calculation of successional rates is based upon the

ssumption that the vegetation of the surroundings will not changer that it will only change marginally in the future compared to theegetation of spoil heaps. With respect to displacement along theain successional gradient this assumption is likely to hold true,

ecause the species turnover on the spoil heaps is high comparedo the dynamics of mountain heath vegetation in the absence ofevere disturbances (Dahl, 1988). Anthropogenic influences such asitrogen deposition (Fremstad et al., 2005), acid rain (Dahl, 1988)nd the expected climate warming may cause vegetation changesKlanderud, 2008) that will, however, both affect the surroundingsnd the spoil heaps. The extent of vegetation change due to broad-cale impacting factors needs to be assessed by repeated recordingsn permanent plots, thereby opening for a ‘sliding’ restoration goal,f. the criticism of static restoration goals by Choi (2007).

The importance of the fourth point, that the three subjectivelyelected blocks and their total of nine randomly placed sample plotsn the surroundings adequately represent the target of the restora-ion process, is difficult to evaluate. We intentionally avoidedampling of plots on moist soil, which is not likely to appear onhe convex surface of the spoil heaps. However, the strong spreadf plots from the surroundings along ordination axis 2 observedor at least one spoil heap (Kleådalen, Fig. 4) may indicate that ourata cover unrealistically much of local environmental variationnd variation in species composition.

.2. Vegetation recovery on spoil heaps

Alpine regions are characterised by cold climate, short grow-

ng seasons and, hence, slow recovery from disturbance (Billings,973). For this reason, and because successions from bare sub-trates tend to proceed slowly anyhow, even in the lowlandsNovák and Konvicka, 2006; Tischew and Kirmer, 2007; Öster etl., 2009), the initial successional rates observed for vegetation on

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lpine spoil heaps are surprisingly high. In part, these relativelyigh successional rates result from inclusion of bryophytes and

ichens, which include many pioneer species, in the study. Sep-rate analyses of vascular plants would certainly have indicatedonsiderably slower successional rates.

In the early phase of restoration of vegetation on spoil heapsin our case represented by the situation in the early 1990s,–20 years after the spoil heaps were constructed), the covernd species richness of bryophytes and lichens were still low ando evident difference between bryophytes, lichens and vascularlants was observed. The strong increase in cover of these groupsbserved from the early 1990s till 2008 suggests a sigmoidal timerajectory, with rapid increase in the intermediate phase after con-itions for growth have been sufficiently improved (Rydgren et al.,001), and deceleration in later phases, due to negative densityffects on growth (Pedersen et al., 2001) and/or establishmentf a dynamic equilibrium between the species composition andhe environment. Our result that Sothaug, the site with driest cli-

ate among the investigated spoil heaps, also has the lowest totalover of bryophytes and lichens in 2008 (Fig. 1B), indicates thathumid climate, which is in general favourable for bryophyte

rowth, is likely to be an important determinant of successionalates.

Slow recovery of vascular plants on the spoil heaps may beaused by a suite of factors including low seed production and/orow diaspore influx (dispersal limitation) from the surroundingse.g. due to a harsh climate) and shortage of safe sites on the spoileaps that are suited for germination and establishment (microsite

imitation). Seed production varies considerably in arctic and alpinenvironments (Callaghan and Emanuelsson, 1985; Bliss and Gold,999) and decreases towards the species’ altitudinal distribution

imits (Callaghan and Emanuelsson, 1985).Dispersal limitation is often considered important in wood-

ands (Matlack, 1994; Holl, 2002) and agrarian lowlands (Nováknd Konvicka, 2006; Rehounková and Prach, 2008; Öster et al.,009). However, if the restoration sites are large and thus act asuge seed traps, a large proportion of the surrounding species poolay be able to colonize even from distances of more than 3 km

Kirmer et al., 2008). In alpine ecosystems, dispersal limitationor species that successfully produce seeds is likely less impor-ant than in the lowlands (cf. Elven and Ryvarden, 1975). In thepen alpine landscape, with occasionally strong winds, ‘everything’an be dispersed (Ryvarden, 1971). Interestingly, nine of the tenpecies that contribute most strongly (together more than 90%)o the total number of diaspores in the seed traps of Ryvarden1971) are recorded on the studied spoil heaps. These speciesre typical pioneer species on newly de-glaciated mineral soils inlpine glacier forelands (Ryvarden, 1971). These results and theact that the total number of vascular plants on entire spoil heapsncreased from 7–59 in the early 1990s to 57–108 in 2008 suggesthat many species are able to establish from seeds dispersed intohe spoil heaps from the surroundings, given enough time. Slowecovery of vascular plant species richness indicates that only amall fraction of successful dispersals result in successful establish-ent (Forbis, 2003). Between-year variation in seed viability (cf.

hambers, 1989) and high seedling mortality is typical of arctic andlpine areas (Bliss and Gold, 1999; Forbis, 2003). Formation of anrganic soil layer enhances germination from seeds and improveseedling survival (cf. Matthews, 1992). The slow establishment ofominant vascular plant species, e.g. Vaccinium spp. (Table A.2) in

he surroundings on the spoil heaps may be due to the slow forma-ion of an organic layer. On average the loss on ignition (%) was 2.7%n the spoil heaps, while it was 34.6 in the surroundings (results nothown). However, once the major species have established viableopulations on the spoil heaps and started to reproduce regularly,

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heir local abundance may increase rapidly. At this stage asexualeproduction may also contribute importantly to the restorationrocess, as evident for several dwarf-shrub (e.g. Empetrum nigrum)nd grass (e.g. F. rubra) species on spoil heaps.

.3. Management implications: spoil-heap construction influencehe restoration process

The way spoil heaps are constructed influences the restora-ion process in several ways. Usually, they are constructed withsmooth surface topography that incurs shortage of safe sites andakes microsite limitation likely. Furthermore, the coarse sub-

trate is unfavourable for establishment and growth (Chambers,995). For many vascular plant species successful establishment onxposed mineral soils depends on safe sites (Stöcklin and Bäumler,996) such as concave micro-surfaces (Jones and del Moral, 2005)

n which seeds are trapped and moisture conditions are moreavourable than elsewhere (Jumpponen et al., 1999). Accordingly,e-vegetation of spoil heaps is likely to be enhanced by creatingore uneven surfaces, with a varied microtopography.The coarse substrate of the studied spoil heaps has poor water

etention capacity and often dries out rapidly (Johnson, 1987). Thisoses a challenge for establishment and growth of plants, as indi-ated by the distribution of plots along GNMDS axis 2 which pointo regional (climatic) gradients (notably moisture) as an importantource of variation in species composition among sites. The depen-ency of vascular plants on water supply from the soil and their

nability to survive extreme drought spells make vascular plantsore susceptible to drought than bryophytes and lichens. Inter-

ctions between the climate of the study area and (adverse) soiloisture conditions explain the difference in successional rates

etween vascular plants and bryophytes on spoil heaps. Therefore,estoration of spoil heaps is likely to be speeded up by provid-ng a surface mineral soil layer with variation in particle size, alsoncluding finer particles than sand.

Seeding of spoil heaps is likely to influence restoration successcf. Tropek et al., 2010). Our data do not allow explicit quantificationf the importance of seeding, but several of the seeded species havestablished large populations; e.g. F. rubra, F. ovina and A. capillaris.. ovina is the only among these species which is common in the sur-oundings. A negative impact of sown species on colonization fromhe surroundings is reported in some studies (Johnson, 1987; Holl,002) while other studies conclude otherwise (e.g. Gretarsdottirt al., 2004). Lack of consistency in this respect may be due to vari-tion in environmental (e.g. climatic) stress among study areas.owards increasingly stressful habitats facilitation takes over forompetition and amensalism as the most important type of interac-ion between plants (Callaway et al., 2002). Under such conditions,eeding is likely to be beneficial (Gretarsdottir et al., 2004). Thenfluence of one species on others may, however, be species and/orabitat-specific (Olofsson et al., 1999).

Our results suggest that seeding of spoil heaps is likely to bef importance, in the short term for aesthetic reasons and in theong term for successful colonization. The composition of the seed

ixture does, however, matter. From an ecological point of view,pecies should be of local origin to avoid negative effects causedy non site adapted ecotypes (Moreno-de las Heras et al., 2008;ischoff et al., 2010), and they should be able cope with the locallimate and well-drained soils (Johnson, 1987). Such a seedingtrategy has, however, hardly ever been tried out for spoil heaps

nd careful field experimental research is needed. Most likely,areful consideration of substrate particle distribution and micro-opography (cf. González-Alday et al., 2008; Alday et al., 2010)s more important than seeding for restoration of vegetation onpoil heaps (or re-construct old ones). These measures enhance

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stablishment from diaspores and counteract safe-site limitationf species that live in the surroundings.

cknowledgements

We are grateful to Energy Norway and Norwegian Wateresources and Energy Directorate that contributed with finan-ial support, and to E-CO Energi and Borgund hydro power plant,stfold Energi AS, for logistic help. Inger Auestad, Liv Norunnamre, Ulrike Hanssen, Andrè F. Husabø, Bente Karin Husabø andilje F. Husabø participated in the field work and all did an out-tanding effort under sometimes wet and cold conditions. Einarimdal and Arne Pedersen gave excellent help in determining diffi-ult specimens of respectively lichens and bryophytes and Joachimpindelböck gave statistical advice. We thank two anonymous ref-rees for suggestions that improved the manuscript.

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at doi:10.1016/j.ecoleng.2010.11.022.

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