Forest Ecology and Management 305 (2013) 77–87

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Forest Ecology and Management

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Restoration potential of native forests after removal of Picea abiesplantations

0378-1127/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.foreco.2013.05.032

⇑ Corresponding author at: NLA University College, PO Box 74 Sandviken, N-5812Bergen, Norway. Tel.: +47 48294938.

E-mail addresses: heidi.saure@nla.no (H.I. Saure), ole.vetaas@geog.uib.no (O.R.Vetaas), arvid.odland@hit.no (A. Odland), vigdis.vandvik@bio.uib.no (V. Vandvik).

Heidi Iren Saure a,d,⇑, Ole Reidar Vetaas b, Arvid Odland c, Vigdis Vandvik d

a NLA University College, PO Box 74 Sandviken, N-5812 Bergen, Norwayb Department of Geography, University of Bergen, PO Box 7802, N-5020 Bergen, Norwayc Telemark University College, N-3800 Bergen, Norwayd Department of Biology, University of Bergen, PO Box 7803, N-5020 Bergen, Norway

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 January 2013Received in revised form 10 May 2013Accepted 17 May 2013Available online 10 June 2013

Keywords:Alpha diversityBeta diversityBryophytesOrdinationSoil conditionsVascular plants

Coniferous plantations may reduce biodiversity and homogenise environmental conditions but there is alack of knowledge on the restoration potential of such sites. We assess whether first generation planta-tion impacts on soil and biodiversity are reversible. The study was carried out in western Norway and wecompared species composition, alpha and beta diversity of vascular plants and bryophytes, and soil con-ditions on five sites of 4-year old wind-felled clearings and adjacent, remnant Norway spruce (Picea abies)plantations. Local native birch (Betula pubescens) forests provided a reference point for assessing the res-toration potential of the Norway spruce plantations. We found that species composition in the wind-felled clearings quickly developed similarities to the local birch forests. A rise in humus pH, calcium con-centrations and available nitrogen (total N in percentage of loss on ignition), indicates that one rotation ofNorway spruce plantations has not imposed long-term impairment of soil conditions. After removal ofthe plantation tree layer, mean species number per plot (alpha diversity) increased for vascular plantsbut remained unchanged for bryophytes. Heterogeneity, in terms of beta diversity, and the variance ofsome soil elements (calcium and magnesium) increased, and beta diversity trends were similar for bothvascular plants and bryophytes. During the course of succession, we predict that species composition andvascular plant alpha and beta diversity in wind-felled clearings of Norway spruce plantations may stabi-lise at levels similar to native birch forests.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Coniferous plantation trees are often non-native, and extensiveplantations of introduced conifers are found in the Southern Hemi-sphere and in Europe (Richardson and Rejmánek, 2004; Carrillo-Gavilán and Vilà, 2010; Simberloff et al., 2010). Plantation specieshave been shown to change and homogenise local environmentalconditions (Augusto et al., 2002; Magura et al., 2005) and to reduceand homogenise native plant diversity (Hill, 1979a; Vellend et al.,2007) and are hence considered as potential ‘‘ecosystem engi-neers’’ (sensu Jones et al., 1997). Norway spruce (Picea abies) isthe most important plantation species throughout northern andcentral Europe (Augusto et al., 2002; Spiecker, 2003; Richardsonand Rejmánek, 2004; Øyen and Nygård, 2007). It is common inplantations far beyond its natural range; in particular, its westernborder has expanded considerably (Spiecker, 2000). Norway spruce

has many characteristics of an ecosystem engineer: shade fromdense even-aged monocultures may dramatically reduce understo-rey plant richness (Hill, 1979a; Kirby, 1988) and its litter decom-poses slowly and acidifies the soil to a greater extent than forpine and deciduous trees (Nihlgård, 1971; Horntvedt, 1989; Can-nell, 1999). The overall impact of Norway spruce plantations onsoil and biodiversity may be greater than the planted area suggestsas plantations are often located on relatively productive and poten-tially species-rich sites (Gjerde, 1993; Øyen and Nygård, 2007). Inrecent years, increased emphasis has been put on restoring nativewoodlands on sites with coniferous plantations (Zerbe, 2002;Spiecker, 2003), yet little is known about the restoration potentialof such sites (Zerbe, 2002). Clear-felling represents a severe pertur-bation of the ecosystem, yet it holds potential for facilitating resto-ration of the native forest vegetation (Heinrichs and Schmidt,2009). Removal of the tree layer induces great changes in light-availability, temperature, wind-exposure, precipitation, soil chem-istry and hydrology (Braathe, 1956; Nihlgård, 1970; Likens et al.,1978; Atlegrim and Sjöberg, 1996), and vegetation and soils aredisturbed by forestry machines. The many changes result in anunstable ecosystem, with little control on export of water and soil

Fig. 1. Map showing the study area in the municipalities of Ørsta and Volda, in thecounty Møre og Romsdal, western Norway. d = Study sites (1–3) from pairs ofNorway spruce plantations and wind-felled clearings in Ørsta (site 1) and Volda(sites 2–3). x = Areas where native birch forests are investigated in Ørsta.

78 H.I. Saure et al. / Forest Ecology and Management 305 (2013) 77–87

nutrients (Likens et al., 1978), and it is difficult to predict short-and long-term changes in biodiversity (Schoonmaker and McKee,1988; Esseen et al., 1997). Shade- and moisture-demanding speciesthat grow in the forests are often reduced (Hannerz and Hånell,1997), new species may enter the clearings via seeds dispersedby forestry machines, animals and wind, from propagule bankswithin the soil, and by vegetative spread from adjacent vegetation(Grime et al., 1988; Jonsson, 1993; Rydgren and Hestmark, 1997;Zamora et al., 2011). With respect to the restoration potential, akey question is whether plantation effects prevail or if the fell-ing-initiated succession progresses towards pre-plantation condi-tions of native forest communities.

Bryophytes form a high proportion of understorey vegetationand terrestrial biodiversity at high latitudes and in oceanic regions(Esseen et al., 1997; Mutke and Barthlott, 2005), and are of conser-vation concern (Nelson and Halpern, 2005; Frego, 2007). Bryo-phytes differ from vascular plants in size, microhabitat,reproduction, and water and nutrient uptake (During, 1979; McCu-ne and Antos, 1981). For instance, bryophytes are often sensitive tochanges in moisture conditions, and they might therefore be ex-pected to differ from vascular plants in their response to severeenvironmental perturbations such as clear-felling (Lee and La Roi,1979; Carleton, 1990). Dominant forest-floor bryophytes (e.g.Sphagnum, Polytrichum, feather mosses) may play important rolesin the successional dynamics as they are able to suppress establish-ment and growth of vascular plants by reducing access to light,water, nutrients and space (Grime et al., 1990; Økland et al.,2004). This effect could be enhanced in coastal/oceanic climateswith long, moist growing seasons (Økland et al., 2004). While spe-cies richness of brypohytes sometimes covaries with vascular plantrichness (Fensham and Streimann, 1997; Pharo et al., 1999; Inger-puu et al., 2001), the patterns in species turnover are often poorlycorrelated (Nekola and White, 1999; Pharo et al., 1999; Sætersdalet al., 2004). This suggests that the community composition is gov-erned by different external or intrinsic drivers, and we explore thisby comparing successional dynamics in the bryophyte vs. higherplant communities following clear-felling.

The first aim of this study is to assess whether plantation im-pacts on the soil and biodiversity of western Norwegian woodlandsare reversible. Our reference point is data from local native forestvegetation (Odland, 1981) and we compare soils and vegetationof Norway spruce plantations and adjacent wind-felled clearings4 years after removal of the tree layer, and hypothesise (HA testedvs. H0 of no change): (i) higher species richness and a communitycomposition more similar to natural woodland habitats, and (ii) in-creased heterogeneity in plant communities and soil parameters.The second aim is to assess whether the successional trends differbetween understorey vascular plants and bryophytes. We expect:(iii) slower increase in bryophyte species richness after wind-fell-ing and subsequent clearing, and (iv) similar effects on beta diver-sity in bryophytes and vascular plants following clear-felling (i.e.they respond similarly to environmental heterogenisation).

2. Materials and methods

2.1. Study area

The study was carried out within two adjacent municipalities,Volda and Ørsta (Fig. 1), western Norway (62 �100N, 6 �90E). Climateis oceanic, with July and January mean temperatures of 14 �C and�0.3 �C, respectively (Aune, 1993), a relatively long growing season(�200 days above 5 �C) and annual precipitation of �1900 mm(Førland, 1993). Bedrock consists of gneiss (Tveten et al., 1998a,1998b). The area is in the south-boreal vegetation zone (Moen,1998), with native forests dominated by Betula pubescens with a

Vaccinium myrtillus, Chamaepericlymenum suecicum or fern under-storey (e.g. Gymnocarpium dryopteris, Phegopteris connectilis, Oreop-teris limbosperma and Dryopteris filix-mas) (Odland, 1981). Thenative forests have been heavily influenced by anthropogenicactivity, such as grazing (Odland, 1981), but grazing pressure hasbeen reduced in recent decades and secondary deciduous forestsare now expanding (Gjerde, 1993). Pinus sylvestris is the only nativeconiferous tree species in this region (Gjerde, 1993; Farjon, 2008),but large-scale afforestation of western Norway took place in thelate 1950s and early 1960s (Øyen and Nygård, 2007). Norwayspruce is the most common plantation species, and by 2005 Nor-way spruce covered 165,000 ha in western Norway (Øyen andNygård, 2007).

The investigated Norway spruce plantations are 40–60 yearsold and of first generation. Prior to plantation, the investigatedsites were formerly grazed and mostly dominated by birch forestsand encroaching juniper (Juniperus communis). The Norway spruceplantation forests had a well-developed bryophyte layer (cover 25–50%), dominated by feather mosses (Hylocomium splendens andThuidium tamariscinum) and Rhytidiadelphus loreus, and many otherbryophyte species were common in the different forests. A shrublayer was generally lacking and the field layer was poorly devel-oped (5–10% cover), but small ferns were commonly found to-gether with sterile Avenella flexuosa, V. myrtillus, Oxalis acetosella,Anemone nemorosa and Luzula pilosa, for example. Appendix A pre-sents a complete species list of the investigated vegetation types.

2.2. Sampling

This paper combines data from native birch forests that wereinvestigated by Odland in 1979 (Odland, 1981) and data collectedby Saure within first generation Norway spruce plantations (here-after spruce plantations) and adjacent (i.e. on the same site, eitherparallel to, or below the remnant spruce plantations) wind-felledclearings (Fig. 1). The wind-felled clearings resulted from a stormover New Year 1991/1992, which felled about 30,000 m3 of spruceplantations in Ørsta/Volda (Table 1 and Fig. 1). The wind-felledclearings and adjacent plantations were investigated by Saure insummer 1996 and 1997, 4 years after windthrow and mechanical

Table 1Background information on investigated spruce plantations (spruce) and wind-felled clearings (clearing). Available information is presented for the investigated birch forests. Noremnant spruce plantation was available at site 1b. Site numbers reflect the closeness of the sites (see Fig. 1). a.s.l. = above sea level. N = number. N/E = latitude north, longitudeeast, respectively. A range of latitude and longitude is presented for the birch forests (birch), total area of birch forests is not estimated. Birch 1 = Understorey dominated byVaccinium myrtillus. Birch 2 = Understorey dominated by ferns.

Site 1a Site 1b Site 2 Site 3a Site 3b Birch 1 Birch 2

Planted (year) 1951 1933 1954 1933 1933Windthrow/cleared (year) 1992/93 1992/93 1992/92 1992/93 1992/92Latitude, longitude (N, E) 62�17, 6�08 62�18, 6�11 62�12, 5�92 62�14, 6�10 62�14, 6�10 62�12–62�19 6�10–6�35Altitude (m a.s.l.) 120–250 120–230 150–300 70–150 50 110–530 200–530Aspect (�) N (349–40) N (325–338) S (168–143) N (4–27) N (15–30) All AllSlope (�) 20 22 33 27 7 5–30 20–30Currently grazed (+) + + +Slash (% cover) 1–5 5–10 5–10 1–5 10–25N samples (spruce/clearing) 8/9 0/6 11/11 7/5 2/8 11 12Area of spruce (ha) 17.8 0 1.8 2.1 0.1Area of clearing (ha) 3.3 4.1 1.5 0.2 0.5

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clearing of the damaged sites. Currently, the wind-felled clearingsare covered by replanted Norway spruce, precluding the possibilityto resurvey the successional trajectory on these sites.

Study sites were selected to represent spruce plantations ofsimilar productivity and to represent variation in the floristic com-position of the oligotrophic and mesotrophic birch forests in thearea. One wind-felled clearing (site 1b) lacks a remnant spruceplantation, but is situated near the spruce plantation at site 1a (dis-tance ca. 1.5 km).

All investigated sites are located within an area of 20 km (N–S)� 25 km (E–W) (Fig. 1). Following the tenet of space-for-time sub-stitution commonly used in successional studies (Matthews, 1992),we find it ecologically sound to use the remnant spruce plantationsas examples of pre-disturbance conditions of the adjacent wind-felled clearings. The close proximity between pairs of spruce plan-tations and wind-felled clearings to the investigated native forests,suggests that the native forests are relevant reference points forrestoration.

Five-by-five-meter plots (25 m2) were sampled for vegetationin wind-felled clearings, adjacent spruce plantations, and nativebirch forests. Vegetation analyses were carried out in four pairsof spruce plantations and wind-felled clearings (28 and 39 plots,respectively), and the native birch forests were sampled (20 plots)throughout a larger region of the study area (i.e. few vegetationanalyses performed within each site). Plots were placed randomlywithin the study sites. The native birch forest plots were posi-tioned to cover the study area so as to represent the variation with-in the local vegetation. Very wet patches, areas heavily influencedby forestry machinery, and forest edges were avoided in the wind-felled clearings. Within each plot, percent cover was determinedfor all vascular plants, bryophytes, and vegetation layers. Onlythe most dominant bryophytes were identified by Odland (1981).Lichens were rare. Cover of bare rock, soil/litter and slash (twigs,stumps and lying trunks) was registered in wind-felled clearingsand spruce plantations. We used a modified Hult-Sernander’s scale(classes 1 = <1–5%, 2 = 5–10%, 3 = 10–25%, 4 = 25–50%, 5 = >50%),to estimate species abundance in sample plots. Plant species (espe-cially bryophytes) that were not determined in the field were col-lected and thereafter determined in a laboratory. Speciesnomenclature follows Lid and Lid (2005) for vascular plants andHill et al. (2006) and Söderström et al. (2002) for bryophytes.

The ability to detect and identify plant species may differamong persons (Magurran et al., 2010; Grandin, 2011), and thiscould possibly have affected the results from the two vegetationsurveys combined in this study. We believe that the survey of vas-cular plants is comparable between the different vegetation types,but only the most common bryophyte species were registeredwithin the native forests. This affects total species richness and

mean bryophyte richness per plot, and these measurements arethus not included when discussing richness in the birch forests. Be-cause bryophytes constitute a major part of the ground floor vege-tation, and because the common mosses and liverworts weredetermined in the native birch forests, we included the commonbryophytes in the analyses of species composition and speciesturnover. In particular, only bryophyte species that were registeredat least once within native birch forests, and that we can thereforeassume were identified and noted by the original investigatorwhen present, were included in the overall analyses of speciescomposition to ensure that data are comparable between theinvestigated vegetation types.

Impact of logging residues (slash, Table 1) on species composi-tion in the wind-felled clearings was investigated in Saure (2000)and was found not to be a significant environmental variable (for-ward selection with 999 permutations in a Redundancy Analysis(RDA)). Slash cover is thus not treated any further here.

Samples of humus (upper 5 cm, excluding litter) and mineralsoil layer (upper 10–20 cm) were taken from one soil profile at aposition chosen to be representative of the conditions in every plotwithin the wind-felled clearings and spruce plantations, but itwould have been better to combine samples from several pointswithin the plots to better represent the variation of the soil condi-tions. Shallow and rocky soils sometimes precluded sampling ofthe mineral soil. Soil samples (ca. 0.5 L) were stored frozen.

2.3. Soil chemistry

Prior to chemical analyses, the soil samples were air dried andsieved through 2 mm sieves. The following variables were ana-lysed; (1) Loss on ignition (loi) by burning at 550 �C for 6 h, (2)pH in soil:water suspension (1:2) using a Metrohm 691 pH meter,(3) total nitrogen by the Kjeldahl digestion/modified procedureusing a Kjeltec System 1002 Distilling Unit and Metrohm 715 dos-imat, and (4) cations by extraction in ammonium acetate (NH4Ac);Potassium (K) (dilution 1:5) was analysed by Corning ClinicalFlame Photometer 410C, and Calcium (Ca) and Magnesium (Mg)(dilution 1:10) were analysed by Perkin–Elmer 1100 B AtomicAbsorption Spectrophotometer. All measurements followed theprocedures of Røsberg (1984), with some adjustments to dilutionsdue to highly absorptive humus (Saure, 2000). Total nitrogen wasexpressed as N as a fraction of loi (N/loi), indicating the amountof available nitrogen in the soils (Schroeder, 1984).

2.4. Data analysis

Overall patterns in species composition (hypothesis (i)) withinand among the spruce plantations, wind-felled clearings and native

Fig. 2. Two-dimensional NMDS ordination of (a) samples and (b) vascular plants and common bryophytes in spruce plantations, wind-felled clearings and native birchforests. Planted Picea abies is not included in the analysis. Uncommon bryophyte species are not included in the analyses as these were not sampled in sufficient detail in thenative birch forests. Species that are shared between native forests (Odland, 1981) and spruce plantations and/or wind-felled clearings, are printed in bold letters. See Table 1and Appendix A for information of the different groups within each vegetation type. Legend: d = spruce plantations, s = wind-felled clearings and = native birch forests (1;dominated by Vaccinium myrtillus, 2; dominated by ferns).

80 H.I. Saure et al. / Forest Ecology and Management 305 (2013) 77–87

forest vegetation are portrayed using a Non-metric Multidimen-sional Scaling ordination (NMDS; Kruskal, 1964). Species data arespecies cover classes (1-5, see Section 2.2). NMDS is based on a

Bray–Curtis distance matrix (Legendre and Legendre, 1998) be-tween sampled plots (n = 87). Default NMDS ordination was runin Canoco 5 for Windows (ter Braak and Šmilauer, 2012), choosing

H.I. Saure et al. / Forest Ecology and Management 305 (2013) 77–87 81

a solution based on 2 axes. These analyses were performed on dataon all vascular plants and the most common and/or dominant bry-ophytes and liverworts (see above). Initially, we ran parallel NMDSand Detrended Correspondance Analysis (DCA) ordinations as thisis often recommended (Liu et al., 2008). Both ordination methodsshowed similar patterns but the NMDS was the most robust meth-od (i.e. maintaining similar ordination patterns under different set-tings and with different data transformations). Planted Norwayspruce was deleted from the datasets. To improve the visual assess-ment, rare species (<4%) are not shown in the ordination diagram(Fig. 2b).

Except for the NMDS-ordination, all analyses and tests werecarried out in R (R Development Core Team, 2010). Species datafrom the wind-felled clearings and spruce plantations were for-merly classified by two-way indicator species analysis (TWIN-SPAN) (Saure, 2000), using the program TWINSPAN version 2.1(Hill, 1979b-modified by C.J.F. ter Braak and H.J.B. Birks 1991).The presentation of vegetation types in Fig. 2a and Appendix A fol-lows this classification.

Species richness (alpha diversity) (H (i) & (iii)) is measured asmean species number per sample plot (25 m2), and was calculatedfor vascular plants, bryophytes and for characteristic vascularplants of the native birch forest species (excluding species thatwere not recorded in the birch forests). Differences in species rich-ness among the vegetation types were tested by Welch two-sam-ple t-tests.

We tested differences in plant community heterogeneity (H (ii)& H (iv)), by comparing beta diversity, defined as: ‘‘the variability inspecies composition among sampling units for a given area’’ (Ander-son et al., 2006). We used multivariate dispersion as a measureof beta diversity, as multivariate dispersion measures the ‘‘theaverage dissimilarity from individual observation units to their groupcentroid in multivariate space, using an appropriate dissimilarity mea-sure’’ (Anderson et al., 2006, p. 683). We used the TukeyHSD.beta-disper to test the significance of differences in beta diversity, whichwas assessed using the Bray–Curtis index in the vegan library(Oksanen et al., 2011). Analyses for beta diversity were performedfor the ground- and field layer only; all tree species and J. commu-nis were deleted from the datasets. Unit of analysis is vegetationsample plots (25 m2), and species cover classes were transformedto the midpoint percentage cover of each class to linearise the re-sponse: 1 = 2.5%, 2 = 7.5%, 3 = 17.5%, 4 = 37.5% and 5 = 75% (Vetaas,1997).

The homogeneity of soil variance (H (ii)) between spruce plan-tations and wind-felled clearings was tested by F-tests for pH,nutrients, loi, and N/loi. Differences in soil conditions were furtheranalysed by Welch two sample t-tests, which are appropriate forcomparison of groups with significantly different variances (Dalg-aard, 2008).

3. Results

3.1. Species Composition

The three sampled vegetation types differed in understorey spe-cies composition (Fig. 2a). The first NMDS-axis explains most of thespecies composition variation (59.3%), and separates spruce plan-tations from birch forests with wind-felled clearings occupying amiddle position. There is a consistent trend for each wind-felledclearing to be shifted along NMDS 1 away from the adjacent spruceplantations and towards the native birch forests (Fig. 2a). This indi-cates that only 4 years after wind-felling, the vegetation has begunto attain similarity to species composition in the native forests. Lo-cal conditions also influenced species composition, as seen by theseparation of plots within spruce plantations, wind-felled clear-

ings, and birch forests (Fig. 2a). NMDS-axis 2 separates wind-felledclearings from the spruce plantations and native birch forests, indi-cating an ecological gradient from light-demanding pioneer spe-cies at the negative end (e.g. Rubus idaeus, Digitalis purpurea andCarex pilulifera) to more shade tolerant species (e.g. Plagiotheciumundulatum, O. acetosella and G. dryopteris) at the positive end(Fig. 2b).

All three habitats share a number of species (Figs. 2b and 3,Appendix A), such as B. pubescens and Sorbus aucuparia, and a num-ber of native forest understorey species including V. myrtillus, smallferns (P. connectilis, G. dryopteris, Blechnum spicant), O. limbosperma,A. nemorosa, O. acetosella, L. pilosa and A. flexuosa. Some of thesespecies were less common in spruce plantations, however. Wind-felled clearings and birch forests share a number of native forestspecies that were not registered or less common in the spruceplantations, e.g. Vaccinium vitis-idaea, Calluna vulgaris, Trientaliseuropaea, Potentilla erecta, Viola riviniana, V. palustris, Epilobiummontanum, Hypericum maculatum, Circaea alpina, Linnaea borealis,C suecicum, Anthoxantum odoratum and Calamagrostis purpurea. Afew species are unique to the native forests, e.g. Empetrum nigrum,Melampyrum pratense and Rubus saxatilis (Figs. 2b and 3, AppendixA).

There are also similarities within the moss layer, and the mostabundant native birch forest mosses are also abundant in spruceplantations and in wind-felled clearings (e.g. H. splendens, Dicra-num scoparium, R. loreus and R. triquetrus). However, Pleuroziumschreberi is more common in the birch forests, whilst Hypnumcupressiforme and P. undulatum are more common in spruce plan-tations and on wind-felled clearings. In addition to characteristicnative forest species, clearings are characterised by many pioneerspecies and human impact indicators such as Agrostis spp., C. pilu-lifera, Luzula multiflora, D. purpurea, R. idaeus, Rhytidiadelphussquarrosus and Pogonatum urnigerum.

3.2. Species diversity

Mean species richness of vascular plants (species number perplot) is higher in wind-felled clearings than in native birch forestsor spruce plantations (Fig. 4a), and native birch forests have highervascular species richness than spruce plantations. Mean richness ofthe species that are characteristic of the native birch forests is sim-ilar in the native birch forests and the wind-felled clearings, but issignificantly lower in the spruce plantations (Fig. 4b). For bryo-phytes, mean species richness is similar in clearings and planta-tions, but lower in the native birch forests (Fig. 4a, birch forestsnot shown). Estimates of beta diversity by multivariate dispersionshowed that species composition is more heterogeneous (betadiversity is higher) in the wind-felled clearings and birch foreststhan in the intact spruce plantations (Table 2).

Bryophytes and vascular plants show similar trends in betadiversity change following removal of the spruce tree layer (Ta-ble 2), although vascular plant beta diversity is not significantlylower than in the native birch forests. In contrast to vascularplants, a fairly high proportion of bryophyte species (20%) are un-ique to the spruce plantations (Fig. 3). None of the registered birchforest bryophytes were unique to these forests (Appendix A). Liver-worts are more abundant in the spruce plantations than in theclearings. A high proportion of vascular plant species (45%) andbryophytes (38%) are registered only in the clearings (Fig. 3).

3.3. Soil heterogeneity in edaphic conditions

Wind-felled clearings have higher hetereogeneity in soil cal-cium and magnesium than the adjacent spruce plantations (Ta-ble 3). Humus from wind-felled clearings also has higher meanvalues of pH, calcium and available nitrogen (N/loi), whereas

0

5

10

15

20

25

30

BryophytesVascular plants

Mea

n sp

ecie

s ri

chne

ss

Spruce ClearingBirch

a

aa

b

c

Vascular plants

0

5

10

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BirchClearingSpruce

Mea

n sp

ecie

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chne

ss

Other species

Native forest species

Aa

Bb

Bc

(a)

(b)

Fig. 4. Species richness (mean number of species per plot) for the three vegetation types: Norway spruce plantations (spruce), wind-felled clearings (clearing) and nativebirch forests (birch). (a) Species richness is compared within plant groups for bryophytes and vascular plants. Bryophyte richness for native birch forests is not shown. (b)Vascular plant richness divided into the mean number of such species that were present in native birch forests (native forest species) and mean number of species that werenot recorded in the birch forest (other species). Vegetation types with significantly different species richness, within each plant group, do not share a letter: capitalletters = mean species richness of native birch forest species, small letters: total vascular plant species richness. Bars are means with standard errors.

Fig. 3. Venn diagrams representing unique and shared fractions of the flora of vascular plants and bryophytes occurring in spruce plantations, wind-felled clearings andnative birch forests. Areas of circles and overlaps represent total number of species found in the three habitat types, and species shared between the different vegetationtypes, respectively. (a) vascular plants, (b) bryophytes without birch forests. Venn diagrams were plotted using software from Pacific Northwest National Laboratory (PNNL)http://omics.pnl.gov.

82 H.I. Saure et al. / Forest Ecology and Management 305 (2013) 77–87

spruce plantations have higher mean values of loi and potassium.Mineral soils are not significantly different (p < 0.01, Appendix B),

Table 2Beta diversity in the ground- and field layer is assessed by multivariate dispersion(mvdspn) for bryophytes and for vascular plants. Within datasets, shared superscriptssignify that mvdspn values are not significantly different from each other.

Vascular plants Bryophytes

Spruce plantations 0.38a 0.37a

Clearings 0.41ab 0.39a

Birch forests 0.45b 0.47b

with the exception of potassium concentrations which are higherin spruce plantations than in clearings.

4. Discussion

4.1. Restoration potential following wind-felling of spruce plantations

Four years after wind-felling, species composition in the clear-ings is clearly different from the understorey vegetation in spruceplantations. In concordance with our results, other studies find ra-

Table 3Mean values and variances of humus soil variables in spruce plantations (spruce) and wind-felled clearings (clearing). Differences in mean values and variances are tested for thetwo vegetation types. Loi = loss on ignition. N/loi = nitrogen in percentage of loi. N = 28 for spruce plantations, N = 39 for wind-felled clearings. Levels of significance: ���

p = <0.001, �� p = <0.01, � p = <0.05.

Variable Units Mean p-Value Variance p-Value Range

Spruce Clearing Spruce Clearing Spruce Clearing

pH 4.6 4.9 0.001 0.16 0.14 0.651 4.1–5.4 4.4–6.2Ca mg/100 g dry soil 182.0 258.0 0.033 12724.5 29933.6 0.023 0.0–496.5 13.0–763.6K mg/100 g dry soil 76.8 32.9 <0.001 471.8 290.6 0.167 39.9–128.1 10.8–81.6Mg mg/100 g dry soil 44.8 46.6 0.704 237.5 540.8 0.028 16.2–83.2 9.1–89.4loi g/100 g of dry soil 75.4 58.8 0.002 286.8 588.2 0.054 36.2–94.3 21.0–93.7N g/100 g of dry soil 1.4 1.3 0.416 0.15 0.27 0.119 0.7–2.3 0.4–2.2N/loi % of loi 1.9 2.3 <0.001 0.06 0.11 0.129 1.4–2.5 1.8–2.9

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pid changes in species composition in clear-felled conifer planta-tions (Zobel, 1989; Hannerz and Hånell, 1997; Heinrichs andSchmidt, 2009) but see Newmaster et al. (2007) and Bock andVan Rees (2002). As predicted (hypothesis i), the species composi-tion of vascular plants in former spruce plantations is developingsimilarities to native birch forests; newcomers in wind-felledclearings are native birch forest species, many birch forest specieshave increased in frequency and abundance, and almost all under-storey species of the native birch forests are now present in theclearings. Interestingly, some shade tolerant species characteristicof local birch forests were common (although not abundant) evenwithin the spruce plantations, and it is possible that e.g. ferns and afew forest herbs (e.g. O. acetosella, V. myrtillus and A. flexuosa)might be remnants from pre-plantation birch forests or that theyhave re-entered the spruce plantations following thinning andbeginning of structural maturation, which is in line with otherstudies of even-aged spruce plantations (Hill, 1979a; Peterken,2001). By large, these species maintained their high frequency onthe wind-felled clearings. Differences in altitude between the na-tive birch forests and the wind-felled clearings seems not to havehad any marked influence on the species composition; there isno altitudinal gradient within the NMDS-ordination (i.e. birch for-est plots with lower altitudes are positioned at all ends of theNMDS-axis 1 and 2) and the only mountainous species in the birchforests, Athyrium distentifolium, is established within wind-felledclearings too. The increase in abundance of B. pubescens and themaintenance of Sorbus aucuparia on the clearings facilitates theestablishment of a tree layer that resembles the local birch forests.Provided that characteristic birch forest species present in thewind-felled clearings are able to persist during secondary succes-sion towards forest, and that new forest species are able to colonisewith time, it is likely that new forests may develop an understoreyvegetation similar to the local, native birch forests. Only the mostdominant bryophyte species were determined in the investigatedbirch forests (Odland, 1981), which makes it very difficult to com-pare birch forest bryophyte composition to spruce plantations andclearings. Still, similarities in the ground cover and species compo-sition of dominant bryophytes in spruce plantations, wind-felledclearings and native birch forests indicate a potential for theemerging birch forests to attain a well-developed bryophyte layer.It may be worth noting that we have not investigated epiphytes,which may include rare species of bryophytes and lichens in oce-anic forests in this part of western Norway (Bendiksen et al.,2008). Despite its name, Norway spruce does not grow naturallyin western Norway (Giesecke and Bennett, 2004; Ohlson et al.,2011), which probably reflects a still-expanding post-glacial range(see Odland et al., 1992; Ohlson et al., 2011). The boreal deciduousforests in Norway, such as the native birch forests investigated inthis study, were believed to be pioneer stages that were graduallyreplaced by conifers (P. abies or Pinus sylvestris), but today they areoften classified as semi-stabile climax forest in areas where Nor-

way spruce does not grow naturally (Sverdrup-Thygesen et al.,2002). Although a rather common forest type, they are not muchinvestigated, and these birch forests have a value for conservationas characteristic forests of the cultural landscapes with many ele-ments of oceanic flora (Odland, 1981; Sverdrup-Thygesen et al.,2002).

Well in concordance with our hypothesis (i), we found a consid-erable increase in alpha diversity of vascular plants 4 years afterwindthrow, such that species richness was higher than within na-tive birch forests. The increase in alpha diversity is in line withother studies of young clear-felled sites (Zobel, 1989; Heinrichsand Schmidt, 2009), but see Hannerz and Hånell (1997), and canbe partly attributed to the establishment of light-demanding pio-neer species, as reflected along NMDS-axis 2. As shrub and treespecies grow taller on the wind-felled clearings, pioneer specieswill decrease in abundance or disappear. Species richness is there-fore likely to be reduced after canopy closure (Schoonmaker andMcKee, 1988; Zobel, 1989; Huston, 1994) and we assume that vas-cular plant species richness could stabilise at a level that is similarto the local, native birch forests. Moreover, the native birch forestsare not mature ‘‘old growth’’ forests as they are largely influencedby anthropogenic disturbance (e.g. grazing and fuel-harvest), acharacteristic feature of boreal deciduous forests in Norway (Od-land, 1981; Bendiksen et al., 2008). Thus, the almost immediatere-establishment of mean species richness of characteristic birchforest species on clearings may reflect that the study species aremostly common, generalist plants that have wide ecologicalniches. Furthermore, close proximity between native birch forestsand the wind-felled clearings, might have aided colonisation byforest species (French et al., 2008).

Soil conditions were not investigated in the native birch forests,thus we cannot assess the change that might have taken place fol-lowing the extensive planting of Norway spruce on former nativebirch forests. On the other hand, afforestation by Norway spruce(P. abies) is known to change soil conditions through acidificationand loss of soil nutrient elements (Nihlgård, 1971; Messenger,1980; Augusto et al., 2002), and Holme (2002) demonstrates suchsoil acidification when comparing soil conditions in Norway spruceplantations and native birch forests within this study region (Voldamunicipality). Differences in soil conditions may therefore indicatethat rapid changes, in soil pH, for example, have taken place fol-lowing wind-felling of the Norway spruce plantations (i.e. within4 years since disturbance). It has been feared that the ‘‘engineeringeffect’’ of Norway spruce plantations could cause long-termimpairment of soil conditions (Horntvedt, 1989), but this seemsunfounded for our study sites, as we found a relatively rapid risein pH, calcium (and magnesium) concentrations, and an enrich-ment of nitrogen in organic matter (N/loi) in wind-felled clearingscompared to spruce plantations (see also Covington, 1981 (Ca);Nykvist and Rosén, 1985 (pH); Johnson, 1995 (N/loi)). Potassiumconcentrations were lower, suggesting leaching from the system

84 H.I. Saure et al. / Forest Ecology and Management 305 (2013) 77–87

(Olsson et al., 1996), but fairly well-developed understorey vegeta-tion in the wind-felled clearings may have helped retain other soilnutrient elements (Vitousek and Reiners, 1975).

As a natural succession proceeds, soil-nutrient availability maydecrease due to larger amounts of elements being stored withinthe field-, shrub,- and tree vegetation, less favourable microcli-mates for soil mineralisation and less nutrients being releasedfrom logging residues left on site (Vitousek and Reiners, 1975;Hornbeck et al., 1987). On the other hand, nutrient-rich litter fromB. pubescens and other deciduous pioneer trees and understoreyvegetation (e.g. A. flexuosa and R. idaeus), may further amelioratethe soil conditions; i.e. increase pH, available nutrients, and biolog-ical activity, and hence facilitate increased understorey speciesrichness when compared to spruce plantations (Emmer et al.,1998). Our results could indicate that the impact of Norway spruceplantations is not static, and may in fact be reversible. This wouldbe in line with Horntvedt (1989). Unfortunately, replanting clear-felled sites is mandatory in Norway (Forestry Act, 2005), prevent-ing long-term surveys of soil development for investigation of res-toration potential.

Heterogeneity in plant community composition and some soilnutrient elements increased following windthrow and mechanicalclearing of the spruce plantations, confirming hypothesis (ii). Thisfairly rapid increase of heterogeneity further supports our inter-pretation that the ‘‘engineering effects’’ of Norway spruce planta-tions may be reversible. Although plant communityheterogeneity within wind-felled clearings is still lower than with-in the birch forests, there is no significant difference between thetwo vegetation types for vascular plants. In the perspective of res-toration of native birch forests, our results are consistent with atrend of increasing beta diversity in wind-felled clearings withtime, eventually reaching the levels of the native forests.

In order to restore native forests, clear-felling of plantation for-ests could be a potential strategy (Heinrichs and Schmidt, 2009).After removal of the tree layer, resources such as light, soil nutri-ents, and water become more available to plants (Braathe, 1956;Likens et al., 1978) but understorey regeneration could suffer fromthe increased exposure to wind and drought events (Braathe, 1956;Hannerz and Hånell, 1997). We assume that wind-felling andmechanical clearing of conifer plantations is comparable to moreconventional clear-felling, although windthrow infers a higher de-gree of soil disturbance. Forest floor disturbance creates microhab-itats that could facilitate establishment of new species (Jonssonand Esseen, 1990; Rydgren et al., 1998; Newmaster et al., 2007;Heinrichs and Schmidt, 2009), and as found on our wind-felledclearings, B. pubescens is a pioneer tree species which is highlycapable of natural regeneration (Holgén and Hånell, 2000; Cooperet al., 2008). The introduced conifer P. abies might establishthrough seeding from remnant, adjacent plantations (Hanssen,2003). The key species in local birch forests, V. myrtillus, is knownto respond negatively to clear-felling (Kardell and Eriksson, 1983;Atlegrim and Sjöberg, 1996), but we observed no overall negativechange. This might be due to lower soil moisture deficiency onnorthernly-exposed clearings and a humid regional climate (Inge-lög, 1974; Atlegrim and Sjöberg, 1996), but could also reflect arather rapid regeneration of V. myrtillus (Kooijman et al., 2000; Pal-viainen et al., 2005). The highly abundant A. flexuosa could havespread vegetatively (Rydgren et al., 1998), as it was common with-in the spruce plantations. Long-distance dispersal is often not veryefficient for forest species (Brunet and Von Oheimb, 1998), but dis-turbance from windthrow and forestry machinery may have stim-ulated germination from the soil propagule bank (Jonsson, 1993;Mcgee and Feller, 1993; Pywell et al., 2002). A viable soil seed bankmay explain the abundance of R. idaeus and C. vulgaris on the clear-ings (Kardell and Eriksson, 1983; Rydgren et al., 1998). We did notinvestigate soil seed bank composition, but we know that prior to

plantation, vegetation was similar to the investigated native for-ests (Odland, 1981, Havåg, Pers.comm., Walseth, Pers.comm).Seeds from former vegetation are probably stored in the soil (Plueet al., 2010), although the most shade-tolerant forest herbs may beabsent (Hermy et al., 1999). The presence of many light-demand-ing early-successional species in the native birch forests could en-hance the restoration potential from seed banks, but there is littleconsensus whether woodland restoration based on soil seed banksis likely to be successful (Augusto et al., 2001; Bossuyt and Honnay,2008).

4.2. Vascular plants as a surrogate measure for bryophytes

Bryophytes are under-represented in environmental assess-ment studies (Pharo et al., 1999; Fenton et al., 2003), and unfortu-nately this is also reflected in our investigation of native birchforests. An important question is whether more well-knowngroups, like vascular plants, can be used as surrogate measures ofbryophyte diversity (Pharo et al., 1999; Sætersdal et al., 2004; Chi-arucci et al., 2007). As proposed in hypothesis (iii), we found thatbryophyte species richness displayed a slower increase afterclear-felling than vascular plants. The dissimilar trends in speciesrichness after wind-felling of spruce plantations indicate that vas-cular plants are a poor surrogate measure for bryophyte speciesrichness. In comparison to vascular plants, bryophytes may displayslower (Åström et al., 2005, this study) or faster (Rydgren et al.,2011) increases in species richness following disturbance, as thetwo plant groups differ in demands of (air and soil) moisture con-ditions, colonisation substrate and in dispersal abilities.

Mean bryophyte species richness did not change followingwind-felling and clearing. An early-succession reduction in bryo-phyte richness following clear-felling is common (Roberts andZhu, 2002; Hylander et al., 2005; Palviainen et al., 2005), whichmight indicate that several bryophyte species had recovered dur-ing the 4 years before investigation of the wind-felled clearings.This partly corresponds to Palviainen et al. (2005) who found thatcertain feather mosses recovered 3–5 years following clear-felling.An additional explanation of a stable bryophyte richness followingclearance might be that the establishment of many new bryophytespecies was balanced by local species losses. Species that were lostwere mostly rare (<3 registrations, data not shown) and there wasa net loss in liverwort species. Forest bryophytes are adapted toshady microhabitats with relatively constant moisture conditions(cf. Palviainen et al., 2005), and liverworts are particularlydrought-intolerant (cf. Hylander et al., 2005). Clear-felling resultsin an abrupt change in life conditions with increased sunlight,higher near-ground temperatures and drought events (cf. Palviai-nen et al., 2005), which would explain why rare bryophyte species(Roberts and Zhu, 2002) and especially liverworts are vulnerable toclearance. Bryophyte species numbers were underestimated in thenative birch forests (Odland, 1981), and are hence not discussedhere.

In contrast to the different alpha diversity responses, beta diver-sity values for vascular plants and bryophytes show similar trendsfollowing clearance of the spruce plantations, and in relation to thenative birch forests. This is in concordance to hypothesis (iv). Ourstudy includes a limited number of sites and we cannot preclude thatour results are region-specific. Still, the trends that we find suggestthat there may be a higher potential for using vascular plants as sur-rogate measures for bryophyte beta diversity than for alpha diversityin spruce plantation clearings. The weak increase in species turnoverfor both plant groups is likely to be a response to the environmentalheterogenisation that has taken place in the wind-felled clearings.To our knowledge, there are relatively few studies investigating betadiversity trends for both understorey vascular plants and bryo-phytes following clear-felling, and contrasting results are reported

H.I. Saure et al. / Forest Ecology and Management 305 (2013) 77–87 85

regarding whether species turnover for the two plant groups is sim-ilar (Lee and La Roi, 1979; Chiarucci et al., 2007) or not (Nekola andWhite, 1999; Sætersdal et al., 2004).

5. Conclusions

Four years after wind-felling and removal of the tree layer infirst-generation spruce plantations, species composition in wind-felled clearings has developed many similarities to native birch for-ests. Wind-felling has further induced a significant increase in vas-cular plant species richness, soil nutrient status has changed, andheterogeneity in terms of soil conditions has increased. The pres-ence of characteristic woodland species reveals a restoration po-tential towards native birch forests. Altogether, it is possible thatthe plantation effects on soils and plant communities of the spruceplantations are reversible.

Vascular plants and bryophytes show dissimilar trends in alphadiversity following removal of the spruce tree layer, indicating thatvascular plants may be a poor surrogate measure for bryophyterichness in this system. This is particularly likely for liverworts.Trends in beta diversity change are similar for the two plantgroups, suggesting that there are underlying similarities in the eco-logical responses across the two species groups.

Implications for practice

� Clear-felling of spruce plantations in combination with

passive management, i.e. allowing natural successional

processes to run following clearance, may reverse impacts

of spruce plantations on richness, heterogeneity and spe-

cies composition of native forest communities.

� Assessment of plant community re-assembly after

removal of spruce plantations should include sampling

of bryophytes in cases where this group is important for

ecological function and/or biodiversity, as vascular plants

failed to be a good surrogate measure of bryophyte spe-

cies richness change.

Acknowledgements

This project was initiated by late Arnfinn Skogen (University ofBergen) who also assisted in identifying bryophytes after fieldwork 1996/97. Information on the study sites was provided bythe Department of Agriculture in Volda and Ørsta. We thank Odd-veig Saure and Jeanette Hansen for field-work assistance and tech-nical work, Gerald Jurasinski for useful advice on analyses for betadiversity, Beate Helle for art-work and Cathy Jenks for linguisticcorrections. This project was supported by NLA University Collegeand the Norwegian Research Council, Project No. 184099.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.foreco.2013.05.032.

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