Journal of Vegetation Science && (2012)

Effects of invasion by introduced versus native coniferson coastal heathland vegetation

Heidi I. Saure, Vigdis Vandvik, Kristian Hassel & Ole R. Vetaas

Keywords

Aspect; Canopy effect; Natural afforestation;

Picea sitchensis; Pinus sylvestris; PRC;

Semi-natural landscapes; Single tree impact;

Species composition; Species richness

Abbreviations

DCA = detrended correspondence analysis;

CCA = constrained correspondence analysis;

GLMM = generalized linear mixed model;

PRC = principal response curves.

Nomenclature

Lid & Lid (2005) ; Hill et al. (2006) ; Soderstrom

et al. (2002)

Received 7 November 2011

Accepted 26 September 2012

Co-ordinating Editor: Sam Scheiner

Saure, H.I. (corresponding author, heidi.

saure@nla.no): NLA University College, PO

Box 74 Sandviken, N-5812, Bergen,Norway

Saure, H.I. & Vandvik, V. (vigdis.

vandvik@bio.uib.no): Department of Biology,

University of Bergen, PO Box 7803, N-5020,

Bergen, Norway

Hassel, K. (kristian.hassel@ntnu.no):

Systematics and Evolution Group, Museum of

Natural History and Archaeology, Section of

Natural History, Norwegian University of

Science and Technology, N-7491, Trondheim,

Norway

Vetaas, O.R. (ole.vetaas@geog.uib.no):

Department of Geography, University of

Bergen, PO Box 7802, N-5020, Bergen, Norway

Abstract

Questions: What are the effects of conifer invasion on plant species composi-

tion and richness of coastal heathlands; do effects differ between heathlands

invaded by introduced or native coniferous tree species?

Location: Threatened coastal Calluna heathlands, western Norway.

Methods:We compared the effects of invasion by introduced Sitka spruce (Picea

sitchensis (Bong.) Carr.) and native Scots pine (Pinus sylvestris L.) on heathland

plant communities by focusing on the local impacts of single coniferous tree can-

opies. For each of 58 target coniferous trees (2–4-m tall), 2-m transects were laid

out in a north and south orientation from the tree stem to beyond the tree can-

opy, and control transects were laid out in open heathland adjacent to the target

trees. Vascular plants and bryophytes were recorded within 0.0625 m2 contigu-

ous quadrats along the transects. Principal response curves (PRC) and general-

ized linear mixed models (GLMMs) were used to investigate tree canopy effects

on species composition and richness.

Results: Invasion by coniferous trees induced considerable changes in the

coastal heathland vegetation: distinct microcommunitites developed beneath

the canopies of relatively young, single coniferous trees in the heathlands. There

was amarked difference in subcanopy species composition under the introduced

Sitka spruce relative to the native Scots pine; forest floor species had colonized

to a greater extent under Sitka spruce, replacing the light-demanding species

characteristic of the coastal heathland vegetation.

Conclusion: Our study showed that conifer invasion induces considerable and

rapid changes in heathland vegetation, and that the effects were stronger under

the introduced conifer. This suggests that Sitka spruce invasionmay pose a more

immediate threat to the characteristic coastal heathland vegetation and flora

than succession based on native conifers. The open coastal heathlands along the

Atlantic coast of Western Europe are a threatened landscape of high conserva-

tion value, and our results suggest that the spread of Sitka spruce into these

landscapes should be controlled, especially in heathlands that are targeted for

conservation.

Introduction

Coastal heathlands are semi-natural, treeless landscapes

that extend almost 3600 km along the Atlantic coast of

Western Europe, from Portugal in the south to northern

Norway in the north. The heathlands are dominated by

ericaceous species, in particular Calluna vulgaris, and they

harbour a characteristic light-demanding oceanic flora, but

are often relatively species-poor (Gimingham 1961;

Fremstad 1997; Loidi et al. 2010). As key cultural land-

scapes of Western Europe that harbour characteristic habi-

tats and biodiversity, coastal heathlands are important

targets for nature conservation (Webb 1998; Council of

Europe 2000).

Journal of Vegetation ScienceDoi: 10.1111/jvs.12010© 2012 International Association for Vegetation Science 1

The coastal heathlands are man-made landscapes that

developed gradually from 6000 to about 1000 yrs ago, fol-

lowing forest clearance (Kaland 1986; Prøsch-Danielsen &

Simonsen 2000; Holden et al. 2007; Hjelle et al. 2010).

Mild winters allowed outdoor grazing on the evergreen

C. vulgaris year round, and traditional management

included regular burning to improve the pasture value of

C. vulgaris, as well as cutting of vegetation and peat for fod-

der and fuel. This land-use regime effectively prevented

forest regeneration and maintained the dynamics of the

heathland ecosystem (Webb 1998; Kvamme et al. 2004).

At their greatest extent, coastal heathlands covered sev-

eral million hectares (Webb 1998), but more than 80% of

the European heathlands have been lost over the last

150 yrs and heathlands are now classified as threatened

(UK BAP 1999; Kvamme et al. 2004; Lindgaard & Henrik-

sen 2011). Land conversion, such as commercial forest

plantations or pasture improvements, and airborne nitro-

gen deposition contribute to degeneration or loss of coastal

heathlands (van Breemen & van Dijk 1988; Webb 1998;

Holden et al. 2007), but a key driver is abandonment of

the traditional land-use regime (Webb 1998; Plieninger

et al. 2006). Abandoned heathlands are invaded by native

tree species such as birch (Betula pubescens and B. pendula)

and Scots pine (Pinus sylvestris L.; Mitchell et al. 1999), and

the resulting succession has strong impacts on understorey

species composition and diversity (Mitchell et al. 1997;

Curt et al. 2003). As the canopy closes, the characteristic

heathland flora may decline or be gradually replaced by

more shade-tolerant forest species (Hester et al. 1991;

Sørensen & Tybirk 2000).

Invasion by introduced coniferous tree species can

accelerate successional rates in heathlands (Christensen &

Johnsen 2001; Catling & Carbyn 2005), and may decrease

native species diversity (Richardson 1998; Higgins et al.

1999). Sitka spruce (Picea sitchensis (Bong.) Carr.), native to

the west coast of North America, is widely planted

throughout Europe (Carrillo-Gavilan & Vila 2010). It starts

reproducing early (5–20 yrs) (Artsdatabanken 2009;

Vikane unpublished data) and can thus quickly invade

abandoned heathlands. Such invasions are currently

observed in Norway (Maren & Nilsen 2008; Thorvaldsen

2011). Sitka spruce forms dense stands that are likely to

have stronger effects on understorey species composition

and diversity than the more open native Scots pine stands

(Ferris et al. 2000), but we are not aware of any studies

that have investigated the effect of invasion of Sitka spruce

on coastal heathland vegetation.

This study investigates the impact of invasion by two

conifers, the introduced Sitka spruce and the native Scots

pine, on plant species richness and composition of coastal

heathlands in four sites in western Norway. Tree invasion

is still at an early stage in our study area, with young coni-

fers scattered within otherwise open heathlands, creating

a “patchy” environment. Single tree canopies significantly

impact environmental conditions, such as light, soil nutri-

ents and moisture conditions, and thus affect species

composition (Zinke 1962; Vetaas 1992; Wallrup et al.

2006; Jager et al. 2007), and the impact of single trees

may be used to forecast consequences of future canopy

closure (Jager et al. 2007). In the early stage of invasion,

effects operate at fine spatial scales (Zinke 1962; Amiotti

et al. 2000). We therefore study the fine-scale effects of

single coniferous trees on heathland vegetation. This

study was performed at 60 °N, where the southern sides

of the coniferous tree canopies receive more solar radia-

tion than the northern side, resulting in differences in

light, temperature and soil moisture conditions (Geier-

Hayes et al. 1995; Hutchinson et al. 1999). To assess the

effects of this fine-scale variability we laid out transects

oriented north and south from the stem of each target

tree. We ask: (1) what are the effects of coniferous inva-

sion on plant species composition and richness of coastal

heathlands; and (2) are there differences in the effects

depending on whether the heathlands are invaded by

introduced or native coniferous tree species? In particular,

we assess how far along the gradient from the tree stems

the canopy effects can be detected, and we explore how

these effects are related to local environmental effects of

the canopy by comparing responses along the north- and

south-oriented transects.

Methods

Study area

Four study sites were selected within coastal heathlands in

western Norway (Fig. 1, Table 1). The sites are within the

boreonemoral bioclimatic vegetation zone and the hyper-

oceanic vegetation section (Moen 1998). The climate is

oceanic with mild winters (average temperatures

of the coldest mo>1 °C), high precipitation

(1300–2000 mm�yr�1) and a relatively long growing sea-

son (more than 200 d>5 °C; DNMI 2011). The prevailing

wind direction is from south to southeast (Flesland meteo-

rological station near sites 1–3; DNMI 2011) and south to

west (Skudenes II meteorological station, near site 4;

DNMI 2011). Bedrock consists of dioritic and granitic

gneisses (site 1 and 3), diorite and monzodiorite (site 2)

andmonzonite (site 4; NGU 2012).

The investigated sites have characteristic coastal Calluna

heathland vegetation (Fremstad 1997) with a dense field

layer dominated by Calluna vulgaris and characterized by

ericaceous species (e.g. Vaccinium spp., Empetrum nigrum

and Erica tetralix) and graminoids (e.g. Molinia caerulea,

Eriophorum vaginatum, Trichophorum cespitosum, Avenella

flexuosa and Carex spp.). Juniperus communis is the most

Journal of Vegetation Science2 Doi: 10.1111/jvs.12010© 2012 International Association for Vegetation Science

Effects of introduced versus native conifer invasion H.I.Saure et al.

common species of a scanty shrub layer. Bryophyte cover

is high, and is dominated by Hypnum cupressiforme/jutlandi-

cum (coll.), Pleurozium schreberi, Hylocomium splendens,

Sphagnum species, Rhytidiadelphus loreus and R. squarrosus.

The study sites showed no sign of recent burning or cut-

ting, and grazing pressure is low; outwintered sheep were

present in site 2 and there were signs of low-intensity deer

grazing in site 3. Natural woodlands are present in the sur-

rounding landscape, and Appendix S1 provides informa-

tion on the local woodland species pool (municipalities

near the city of Bergen, western Norway). At all sites, Sitka

spruce is spreading into heathlands from adjacent planta-

tions (large plantations or narrow strips of shelter plant-

ing). Mean height of Sitka spruce trees in the heathlands

was 4.6 m, and mean age of fertile Sitka spruce (i.e. carry-

ing cones) was 14.7 yrs. Seedling density in a 100-m zone

bordering plantations was 0.55 individuals m�2 (Vikane,

unpubl.). Seedlings of other shrubs and trees, e.g. Sorbus

acuparia and native Scots pine, were also recorded, but at

much lower densities than Sitka spruce.

Sampling

Fieldwork was carried out in July–September 2009.

Vegetation was sampled within 5 9 5 m blocks centred on

2–4-m tall target trees of Sitka spruce or Scots pine in Callu-

na wet heath vegetation (Fremstad 1997), avoiding

slopes>12°, bare rock and wet depressions, and with dis-

tance between target coniferous trees >10 m. These criteria

resulted in selection of blocks with tree cover of ca. 15%

and where the target coniferous tree was the only

tree>2 m.

Within each block, we placed three 2-m transects; two

canopy transects and one open heathland control transect

(hereafter referred toas control).Canopy transectswere laid

out 180 °S and 360 °N from the target coniferous tree stem

(hereafter referred to as the south and north transects). The

control was placed parallell to the canopy transects, with

the mid-point in line with the stem and at a distance from

the edge of the target coniferous tree canopy of at least

0.5 m. Controlswere randomly allocated to the east orwest

side of the target tree, unless one of the sideswas unsuitable

according to the rejection criteria givenabove.

Transects were divided into eight sample plots of

0.25 9 0.25 m, which were analysed for vascular plant

and bryophyte abundance (recorded as frequency in four

subplots). Similarly, we recorded cover of the target conif-

erous tree in each plot. Target coniferous trees varied

somewhat in size, but all canopy transects ran at least

0.5 m (two sample plots) beyond the target coniferous tree

canopy. This transect length was assumed to be long

enough for the outer sample plots to be unaffected by the

target tree canopy.

We sampled 38 blocks with Sitka spruce as the target

coniferous tree, and 20 blocks with Scots pine, which

resulted in 174 transects and 1392 transect plots.

Table 1. Study sites on large, coastal islands of western Norway.

Site 1 Site 2 Site 3 Site 4

Local name Palsvarden Hille Stura Kvalheimsneset

Municipality Karmøy Austevoll Øygarden Radøy

County Rogaland Hordaland Hordaland Hordaland

Latitude, longitude 59 °10 ′N, 5 °13 ′E 60 °7 ′N, 5 °4 ′E 60 °37 ′N, 4 °50 ′E 60 °41 ′N, 4 °54 ′E

Climate station Skudenes II & III Storebø Tjelstø Manger

Mean annual temperature (°C) 7.6 7.0 7.2 7.2

Mean annual precipitation (mm) 1295 1875 1975 1540

Area (ha) 16.95 10.87 2.97 5.58

Currently grazed (+) (“+”) (“+”)

Mean age fertile sitka spruce (yr/SD) 18.0 (4.7) 12.9 (3.5) 12.4 (3.0) 12.0 (2.1)

Blocks sitka spruce/Scots pine (number) (10/5) (10/5) (8/5) (10/5)

Fig. 1. Maps showing former distribution (ca. 1850) of the European

heathlands (Diemont & Kaland, in Haaland 2002) and the study sites within

the coastal heathland zone (1–4) in western Norway. Table 1 provides

further information on the investigated areas.

Journal of Vegetation ScienceDoi: 10.1111/jvs.12010© 2012 International Association for Vegetation Science 3

H.I.Saure et al. Effects of introduced versus native conifer invasion

Data analysis

A detrended correspondence analysis (DCA; Hill & Gauch

1980) with down-weighting of rare species showed that

the first gradient was rather long (3.9 SD), and constrained

correspondence analysis (CCA) was chosen to investigate

relationships between species composition and environ-

ment. We used partial CCA in conjunction with principal

response curves (PRC; van den Brink & ter Braak 1999) to

test and quantify the effects of the target trees on the

heathland vegetation. PRC was originally developed for

analysing repeated-measures design experiments, and is

commonly used in ecological research (see e.g. Frampton

et al. 2000; Savadogo et al. 2009; Nakatani et al. 2011).

We use PRC in a spatial context, i.e. comparing commu-

nity composition along spatial transects, rather than over

time, following the general procedures of Heegaard &

Vandvik (2004), adapted for our data as follows: to analyse

spatial data using the PRC approach, we need two sets of

categorical variables, one describing the spatial gradient (in

our case the eight distances along the trasects), and one

describing the “controls” and “treatments” we want to

compare (in our case control transects vs. the four different

categories of canopy transects represented by tree species

9 aspect). In the spatial PRC analysis, blocks and distance

classes are coded as “covariables”, and the interactions

between distance and each of the treatment variables are

coded as “environmental variables”. The resulting PRC

analysis thus estimates the compositional differences of

each of the treatments from the control transects at each of

the eight distances from target tree stems. Within this gen-

eral framework, effects of different coniferous tree species

were tested by coding canopy transect distances under

each of Sitka spruce and Scots pine as different treatments

and partialling out the main effect of canopy transects.

Effect of aspect was tested by coding canopy transect dis-

tances for each aspect (north and south) as treatment and

partialling out the main effect of canopy transect distance

and blocks. Finally, the interactive effect of coniferous tree

species and aspect was tested by coding canopy transect

distances for each combination of aspect (north and south)

and coniferous tree species (Sitka spruce and Scots pine) as

treatments, and partialling out the main effects of canopy

transects, coniferous tree species, aspect and blocks.

Important features of the method are the PRC diagram

and the species scores. The coding used in PRC standardizes

the control to be zero-valued for all times, i.e. a horizontal

line in the PRC diagram. The PRC scores for each of the

“treatments” through time represent compositional differ-

ences from the control, and the species scores reflect the

influence of particular species on the overall community

response described by the PRC scores through time. Species

with high positive scores are positively correlated, species

with negative scores respond oppositely, and species with

near zero scores are indifferent to the effects reflected by

the PRC axes (Microcomputer Power, Ithaca, NY, USA).

The PRC diagram,which represents the total effect of conif-

erous trees (including differences betweeen tree species

and aspect) was tested by coding each combination of

coniferous tree species and aspect along the canopy tran-

sect as treatments, while partialling out the effect of blocks.

A generalized linear mixedmodel (GLMM) tested differ-

ences in the dependent variable, species richness (species

number per plot, including vascular plants and bryo-

phytes), in response to the fixed factorial effects of trees vs.

controls, native vs. introduced conifers, north vs south

canopy transects, and distance to stem. The GLMM was

run with the function lmer (package lme4) and fitted by

the Laplace approximation. Blocks were included as ran-

dom effects, and the model was simplified by backwards

elimination starting from the full model with three-way

interactions. A Poisson distribution of errors was assumed

for the species richness response variable, thus the GLMM

was runwith a log-link function. Following the PRC analy-

ses, the effects were plotted as contrasts between canopy

transects and their respective controls with distance to

stem.Welch two-sample t-tests were run post-hoc to assess

the significance of differences in species richness between

canopy transects and controls, and the different transect

categories (tree species, aspect) at each specific distance

from stems.

The number of species unique to the canopy transects

(Sitka spruce north transect, Sitka spruce south

transect, Scots pine north transect and Scots pine south

transect) and their respective controls are displayed to indi-

cate howmany species are likely to have established or dis-

appeared after development of coniferous tree canopy.

Ordinations were run using CANOCO 4.5 for Windows.

All other analyses were run by using R (R Foundation for

Statistical Computing, Vienna, AT).

Results

Species composition

The DCA ordination displays a continuous variation in spe-

cies composition among the sites, blocks and plots in our

data (Fig. 2), and indicates that species composition varies

both between and within sites. Within each site, both

coniferous tree species and distance from stem seem to

impact species composition, whereas south and north tran-

sects point in the same direction for each coniferous tree

species, indicating relatively low impact of aspect on spe-

cies composition (Fig. 2), which is also confirmed in the

PRC analyses (Table 2).

The overall impact of coniferous tree canopies on the

heathland community is reflected along the first PRC axis

Journal of Vegetation Science4 Doi: 10.1111/jvs.12010© 2012 International Association for Vegetation Science

Effects of introduced versus native conifer invasion H.I.Saure et al.

(Fig. 3a), and vegetation under canopies is characterized

by increased abundances of shade-tolerant species such as

Plagiothecium succulentum, Avenella flexuosa, Oxalis acetosella,

Racomitrium fasciculare, Isothecium myosuroides and seedlings

of woody species, relative to controls (Fig. 3b), and also by

increased proportions of graminoids and ferns, and a lack

of liverworts (Fig. 3c). However, relatively few species are

encountered exclusively in the innermost canopy transect

plots (Fig. 4), when compared to the open controls. In con-

trast, Rhytidiadelphus loreus and Racomitrium lanuginosum

respond negatively to canopy cover (are less abundant

than the control at distances <1 m from stems), as do

species such as Pedicularis sylvatica, Carex nigra, Dactylorhiza

maculata, Succisa pratensis, Aulacomnium palustre, Diplophyl-

lum albicans, Paraleucobryum glaucum and Frullania tamarisci

(Fig. 3b).

The introduced and the native coniferous tree species

differ in their impact on species composition (Table 2), as

reflected by the second PRC axis (Fig. 3d). Eriophorum

angustifolium, Andromeda polifolia, Chamaepericlymenum

suecica and Racomitrium fasciculare (Fig. 3e), and graminoid

cover (Fig. 3f) increased under Scots pine canopies,

whereas Mnium hornum, Isothecium myosuroides, Sphagnum

fallax, Sciuro-hypnum reflexum, Oxalis acetosella, Plagiothecium

succulentum, P. undulatum, Polytrichastrum formosum and Ca-

lypogeia muelleriana (Fig. 3d) and ferns and liverworts

(Fig. 3f) increased under Sitka spruce canopies. In general,

more species increased under Sitka spruce (had negative

PRC 2 scores) than under Scots pine (positive PRC 2

scores), and fewer species disappear from the innermost

(0.25 m and 0.5 m) Sitka spruce plots than from Scots pine

plots (Fig. 4).

Species richness

Species richness decreases towards the tree stems (Table 3,

Fig. 5). Post-hoc t-tests show that plots near the coniferous

tree stem have significantly lower species richness (3–5

species 0.25 m from stems; Fig. 5) than the controls (mean

11.0 ± 0.2 species�plot�1, ±SE), but at 0.75 m the effect

remains only for the south transects (Fig. 5). Near and

beyond the canopy edge (1.0–1.25 m from coniferous tree

stem) there is no significant difference in species richness

between canopy transects and controls (Fig. 5). Canopy

Table 2. Summary of the partial CCA and PRC analyses of species composition along canopy transects from stems (0 m) beyond the tree canopy (2 m)

and controls (0–2 m) in open heathland.

Variance component

of all sites

Variables Covariables N Variance P (999)

Sites S 1392 11.04 0.001**

Blocks within sites B S 1392 26.73 0.001**

Sitka spruce vs. scots pine blocks S+P 1392 1.92 0.001**

Distance from tree TD B 1392 0.73 0.001**

Sitka spruce vs. scots pine trees TD*CS + TD*CP TD + B 1392 0.49 0.004**

Aspect: south vs. north transects TD*AS+TD*AN TD + B 1392 0.45 0.124

Differences in aspect responses

between trees

TD*ASCS+ TD*ANCS +

TD*ASCP + TD*ANCP

TD*CS + TD*CP +

TD*AS + TD*AN + B

1392 0.38 0.485

Total effect of trees, incl. aspect and

tree species (PRC)

TD*ASCS+ TD*ANCS +

TD*ASCP + TD*ANCP

B 1392 2.06 0.001**

“Variables” specify the effects tested in each model after accounting for the effects specified in “Covariables”. Sample size of the data set used (N), percent-

age of the total floristic variation accounted for by eachmodel (Variance) and the respective significance (P (999)), in a Monte Carlo permutation test are pre-

sented. S, 4 sites; B, 58 blocks; S, blocks with introduced Sitka spruce; P, blocks with native Scots pine; TD, 8 canopy transect distances; CS, canopy

transects under Sitka spruce, CP; canopy transects under Scots pine; AS, aspect south; AN, aspect north; *P<0.05; **P<0.01.

–2.0 5.0

–1.0

5.0

distance from stem

Sitka southSitka north

Pine south

Pine north

Site 1

Site 3

Site 2

Site 4

DCA-axis 1 (10.0%)

DC

A-a

xis

2 (8

.1%

)

Fig. 2. DCA ordination of 0.25 9 0.25 m sample plots in coastal

heathlands. Envelopes are drawn around each of the four sites.

Environmental variables are overlain onto the samples. Sitka south, Sitka

north, Pine south and Pine north refer to the south and north transects

under Sitka spruce and Scots pine, respectively. Distance from stem =

canopy transect distances (0–2 m) and controls are coded as 2 m.

Journal of Vegetation ScienceDoi: 10.1111/jvs.12010© 2012 International Association for Vegetation Science 5

H.I.Saure et al. Effects of introduced versus native conifer invasion

effects on species richness were more pronounced in the

south transects than in north transects (Table 3); south

transects have lower species richness than north transects

at all distances (often >1.0 species; Table 3), and the nega-

tive effect of canopy on species richness also persisted

somewhat further from the stems in the south transects

(Fig. 5). There were no differences in species richness

between vegetation under Sitka spruce and Scots pine can-

opies (Table 3).

Discussion

We found that distinct microcommunities had emerged

beneath relatively young and isolated coniferous trees

PRC-1 Positive species scores

9%6%

6%

9%

26%

44%

0%

PRC-1 Negative species scores

1%4%

8%

11%

42%

32%

2%

PRC-2 Positive species scores0%

5%

2%

12%

21%

43%

17%

PRC-2 Negative species scores

6% 6%

2%

13%

13%

37%

23%

Ferns Woody Low shrub Herbs Graminoids Mosses Liverworts

(a) (b) (c)

(d) (e) (f)

Fig. 3. PRC diagram (a & d) visualising the total effect of trees on species composition in coastal heathlands, relative to controls, along a transect from the

stems (0 m) and beyond the canopy (2 m). Species scores of the most strongly affected species (species scores >|0.2|) are shown to the right (b & e). Rare

species (<8 occurrences) and indifferent species are not displayed. The relative abundance of plant groups (c & f) along PRC axes 1 and 2 is displayed,

counting all species with scores >|0.1|. Plant group ‘ferns’ also include Lycopodiaceae.

Journal of Vegetation Science6 Doi: 10.1111/jvs.12010© 2012 International Association for Vegetation Science

Effects of introduced versus native conifer invasion H.I.Saure et al.

(2–4-m tall) that have invaded coastal heathlands. Canopy

effects differed between the introduced and the native

coniferous tree species, and between the north and south

side of the trees. Whereas coniferous tree species had

strong effects on understorey species composition, transect

orientation primarily affected species richness.

Community dynamics beneath coniferous canopies

The pronounced changes in vegetation beneath canopies

of the invading conifers, relative to open heaths, are seen

mainly as an increased occurrence and abundance of

shade-tolerant forest species and a reduction in light-

demanding heathland species. Species sorting (sensu

Leibold et al. 2004) along the environmental gradient

imposed by the developing canopies is likely to be the

dominant process in the emergence of distinct vegetation

beneath the invading coniferous tree. Shading is known to

be an important explanatory factor for species richness

reductions towards tree stems (Jager et al. 2007), and even

small single trees, like the conifers in this study, may

induce shade light gradients on heathland vegetation

(Piessens et al. 2006). Spruce canopies are known to be

denser than Scots pine canopies (Hill 1979; Ferris et al.

2000), and the degree of species sorting (i.e. the degree of

environmental control of spatial dynamics) is clearly stron-

ger beneath Sitka spruce than beneath Scots pine. Thus,

contrasting microcommunities have developed beneath

the two conifers; species associated with Scots pine are

light-demanding heathland species (e.g. Eriophorum angus-

tifolium and Andromeda polifolia), while species correlated

with Sitka spruce canopies are typical, shade-tolerant for-

est species (e.g. Sciuro-hypnum reflexum, Oxalis acetosella,

Isotheciummyosuroides and Plagiothecium undulatum).

Species richness decreased beneath canopies of the

invading conifers. This corresponds to other studies where

species richness is reduced in formerly treeless habitats

after invasion of introduced tree species (Richardson et al.

Sitk

a sp

ruce

Northern transects

0

5

10

15

20

25

Can

opy

tran

sect

s

25

20

15

10

5

0

Con

trol

sSouthern transects

0

5

10

15

20

25

30

30

25

20

15

10

5

0

Scot

s pi

ne

0

5

10

15

20

25

30

35

Can

opy

tran

sect

s

35

3025201510

5

0

0.25 0.5 0.75 1 1.25 1.5 1.75 2

Con

trol

s

05

10152025303540

4035302520151050

0.25 0.5 0.75 1 1.25 1.5 1.75 2

Distance from stem (m)Distance from stem (m)

Fig. 4. Number of species (vascular plants and bryophytes) unique to canopy transects or controls at different distances along the transects.

Journal of Vegetation ScienceDoi: 10.1111/jvs.12010© 2012 International Association for Vegetation Science 7

H.I.Saure et al. Effects of introduced versus native conifer invasion

1989; Jager et al. 2007), but Dickie et al. (2011) found

increasing species richness in grasslands that were invaded

by an introduced and a native tree species. In plantation

forests, species richness has been found to be lower under

Norway spruce (Picea abies) than under Scots pine and

birch (Wallrup et al. (2006). Interestingly, we found no

difference in subcanopy species richness between the two

coniferous tree species. This can partially be accounted for

by increased colonization by forest species under Sitka

spruce, indicating that dispersal conditions must be good

within the heathland landscape. The importance of dis-

persal is supported by the the higher proportions of forest

ferns and liverworts under Sitka spruce, as the small and

numerous propagules may give these groups a dispersal

advantage relative to other forest species (Tryon 1970; Ha-

jek et al. 2011).

South-facing aspect seems to enhance the canopy effect

on species richness as significant differences from controls

extend further away from the stems in the south transects.

Other studies from northern latitudes frequently report

increased species richness in southern canopy transects

(Lopez-Pintor et al. 2006) and southern forest edges

(Honnay et al. 2002), which is typically attributed to

higher levels of solar radiation and thus warmer and drier

microclimates on the south-facing sides of the trees or

forests at these latitudes (Geier-Hayes et al. 1995). In

contrast, we found lower species richness on the south side

of the coniferous trees. This may be accounted for by the

cool and humid regional climate, resulting in a regional

species pool that is dominated by species that are adapted

to the cooler and more humid environments found on the

north side of the coniferous trees (Vevle 2000; Lid & Lid

2005; Atherton et al. 2010).

From a landscape perspective, the vegetation under

these invading conifers may be regarded as small, develop-

ing, but already quite compositionally distinct “forest un-

derstorey patches” within the matrix of coastal heathlands,

or as forest understorey metacommunites (sensu Leibold

et al. 2004). Similarly, small-scale metacommunities in

boreal forest ecosystems has been documented on rock

boulders that host a set of cryptogam species not found in

the forest matrix (Virtanen & Oksanen 2007). The devel-

opment of these distinct metacommunities is driven by

strong species sorting combined with dispersal of shade-

tolerant forest specialist species (especially bryophytes)

from nearby forest patches (see above), but could poten-

tially have been cancelled out by strong mass effects (sensu

Shmida & Wilson 1985; Leibold et al. 2004), as the propa-

gule rain from the surrounding heathland vegetation is

likely to be substantial. Evidently, the species sorting

resulting from microenvironmental and other effects of

the young conifers are too strong to be obscured by propa-

gule rain from the heathland matrix. Mass effects may still

be buffering the impact of the invading conifers however

(see also Shmida & Ellner 1984; Lichstein & Pacala 2011;

Alexander et al. 2012), which would imply that the rate of

compositional change under trees may increase as the tree

density in the landscape increases and the heathland

matrix becomes relatively less abundant.

Implications for heathland vegetation following conifer

invasion

Scots pine is the only native conifer in western Norway

(Gjerde 1993), thus the spread of Sitka spruce from planta-

Table 3. Generalized linear mixed model testing the fixed effects of tree

invasion, tree species, canopy transect aspect and distance to stem on

total species richness.

Estimate SE z-value P-value

Intercept 2.39 0.05 48.01 <0.001***

Distance along transects �0.01 0.02 �0.41 0.69 ns

Sitka spruce (vs. scots pine) 0.01 0.05 0.15 0.88 ns

North transects

(vs. control)

�0.27 0.05 �5.94 <0.001***

South transects

(vs. control)

�0.43 0.05 �9.22 <0.001***

Distance along

north transects

0.20 0.04 5.58 <0.001***

Distance along

south transects

0.25 0.04 7.04 <0.001***

Blocks were included as random variables and a Poisson distribution of

errors was assumed. Y = number of species, D = distance along tran-

sects, tree = introduced Sitka spruce and native Scots pine, treatment

(treat) = south transects, north transects and open controls, B = blocks

each containing a target tree and the associated treatment transects. The

model was simplified by backward elimination Y~D*tree*treat + (1|B).

Bold highlights significant effects.

–6.00

–5.00

–4.00

–3.00

–2.00

–1.00

0.00

1.00

2.00

0.25 0.5 0.75 1 1.25 1.5 1.75 2

Distance from stem (m)

Mea

n sp

ecie

s no

. dev

ianc

e fr

om c

ontr

ol

Sitka southSitka northPine southPine northControl

Fig. 5. Species richness in canopy transects, shown as difference in mean

species number at each transect type and distance, relative to the

controls. Controls are not significantly different from each other, and

overall mean control species number is 11. Large symbols have

significantly different species richness (P < 0.05) than their respective

controls. See Table 3 for tests of significance for the different main effects

and interactions.

Journal of Vegetation Science8 Doi: 10.1111/jvs.12010© 2012 International Association for Vegetation Science

Effects of introduced versus native conifer invasion H.I.Saure et al.

tions means that a “new” type of conifer with a denser and

lower canopy has entered the heathlands. Sitka spruce

invasion into semi-natural landscapes, such as the coastal

heathlands investigated in this study, is too recent to have

resulted in closed-canopy Sitka spruce forests (Peterken

2001).We therefore do not knowhow the introduced Sitka

spruce will impact characteristic heathland plant species

and diversity as forest succession proceeds, but impacts by

single trees at an early invasion stage can be used to predict

consequences of later invasion stages (Jager et al. 2007).

The establishment of shade-tolerant forest species,

which was more pronounced beneath the introduced Sitka

spruce than beneath the native Scots pine, is at the cost of

abundance or even presence of typical heathland species

(Hester et al. 1991). Dense Sitka spruce plantations rapidly

out-shade most vascular plants, but ferns and bryophytes

survive the low-light regime at varying levels of cover and

diversity (Hill 1979; French et al. 2008). After canopy

closure, the understorey within naturalized Sitka spruce

forest is therefore likely to show much less resemblance to

the coastal heathlands.

Old-growth, natural Sitka spruce forests along the Paci-

fic coast of North America have a well-developed under-

storey, with shrubs, ferns, herbs and a very well-developed

bryophyte layer (Peterson et al. 1997). Similarly, thinning

of Sitka spruce plantations allows some colonization of

woodland species (Wallace & Good 1995; French et al.

2008). As naturalized Sitka spruce forests in western Nor-

way mature, probably 50–120 yrs after canopy closure

(Banner et al. 1989; Ferris et al. 2000), the stands may

therefore become more structurally diverse, and are likely

to facilitate a more diverse understorey vegetation and

more native tree species than are found within current

Sitka spruce plantations, reflecting the local species pool of

woodland species.

In the investigated heathlands, vegetation beneath

native Scots pine was characterized by open heathland

species, indicating that future changes in understorey com-

position might be less dramatic in Scots pine forests com-

pared to Sitka spruce forests. This is supported by other

studies that report similarities in understorey species com-

position between heathland and Scots pine forests (Peter-

ken 2001; Curt et al. 2003), and suggests that heathland

invaded by Scots pine may have a higher restoration

potential than those invaded by Sitka spruce.

The West European coastal heathlands are important

for conservation (Council of Europe 2000), not only for

their characteristic biodiversity, but also for their historical

and aesthetic values (Norwegian Ministry of the Environ-

ment 2006–2007). We have shown that the invader

species makes a difference to vegetation development after

coniferous tree invasion in the coastal heathlands. To facil-

itate future restoration of coastal heathlands, Sitka spruce

invasion should be controlled in areas of potential

restoration interest. Short generation time (5–20 yrs) and

high seed production (Richardson & Rejmanek 2004)

means that Sitka spruce has a considerable invasion poten-

tial. Removal of adjacent Sitka spruce plantations and

invading individuals within the coastal heathlands will

therefore be necessary to control its spread.

Acknowledgements

We thank J.H. Vikane for sharing data on Sitka spruce

populations, S. Vogler, R. Smith, L. Hagenlund, R. Ramirez

and S. Rumpf for fieldwork and technical assistance; J H.

Vikane, J.-A. Grytnes and J. Kapfer for useful advice on

data analyses; B. Helle, C. Aasebø and L.G. Velle for art-

work and C. Jenks for linguistic correction. The project

was supported by NLA University College and the Norwe-

gian Research Council, project no.184099.

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Supporting Information

Additional supporting information may be found in the

online version of this article:

Appendix S1. Species list from coastal heathlands,

Sitka spruce plantations and Scots pine forests.

Journal of Vegetation ScienceDoi: 10.1111/jvs.12010© 2012 International Association for Vegetation Science 11

H.I.Saure et al. Effects of introduced versus native conifer invasion