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Facilitation of afforestation by Lupinus nootkatensis and by black plastic mulch in south-west Iceland Dennis A. Riege a; Adalsteinn Sigurgeirsson b a University of Maryland University College, USA b Icelandic Forest Research, Reykjavík, Iceland

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To cite this Article Riege, Dennis A. and Sigurgeirsson, Adalsteinn(2009) 'Facilitation of afforestation by Lupinus nootkatensis and by black plastic mulch in south-west Iceland', Scandinavian Journal of Forest Research, 24: 5, 384 — 393, First published on: 30 November 2009 (iFirst) To link to this Article: DOI: 10.1080/02827580903117404 URL: http://dx.doi.org/10.1080/02827580903117404

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ORIGINAL ARTICLE

Facilitation of afforestation by Lupinus nootkatensis and by black plastic mulch in south-west Iceland

DENNIS A. RIEGE 1

& ADALSTEINN SIGURGEIRSSON 2

1 University of Maryland University College, PSC 482, Box 178, FPO AP 96362, USA, and

2 Icelandic Forest Research,

Mógilsá, IS-116 Reykjavı́k, Iceland

Abstract Afforestation has proven difficult in south-west Iceland in a region of degraded soils and high winds. Experiments at Keflavik International Airport began in 2002 to examine whether Nootka lupine (lupin; Lupinus nootkatensis) or black plastic mulch facilitates establishment of Sitka spruce (Picea sitchensis), Hooker willow (Salix hookeriana) or downy birch (Betula pubescens) by ameliorating microsite conditions. By 2008, both lupine and black plastic mulch facilitated growth of all species at most plots. However, survival of spruce and birch seedlings decreased where dense lupine was accompanied by dense grass (but not in dense lupine alone). This indirect mechanism (nurse plant stimulation of competitor species) differs from prior models of shifts in balance from facilitation to competition under the stress-gradient hypothesis. Hooker willow performed best in both lupine and plastic mulch. However, in areas without dense grass, Sitka spruce continued successful growth and has potential for longer term afforestation. Planting seedlings into shallow excavations in lupine improved growth of willow and birch but not spruce. For afforestation in south-west Iceland, it is recommended that a mix of tree seedlings be transplanted directly into young lupine stands with sparse grass cover, with shelterbelts of seedlings planted into black plastic mulch along the stand edges.

Keywords: Betula pubescens, competition, Nootka lupine, Picea sitchensis, Salix hookeriana, stress-gradient hypothesis.

Introduction

One plant species may have both facilitative and

inhibitory effects on another species, depending on

densities and circumstances (Callaway & Walker,

1997; Holmgren et al., 1997). Callaway and Walker

(1997) proposed a model that stated that facilitative

effects will increase under conditions of abiotic stress

and will decrease in a natural succession as stress

diminishes over time. Subsequent studies generally

support this stress-gradient hypothesis (Gómez-

Aparicio et al., 2004; Lortie & Callaway, 2005;

Michalet, 2005; Veblen, 2008). In Iceland, Aradóttir

(2004) found that Nootka lupine (lupin; Lupinus

nootkatensis) had competitive as well as facilitative

effects on tree seedling establishment of native

downy birch (Betula pubescens) and that the compe-

titive effects increased as lupine cover expanded. In

the present study, experiments were initiated at

Keflavik International Airport, Iceland in 2002 to

exploit a window of opportunity to plant tree

seedlings in young lupine�grass meadows. The timing allowed for examination of whether lupine

facilitation might be superseded by competition as

lupine density increased, in accord with the stress-

gradient hypothesis.

Iceland faces a formidable set of difficulties

compared with other countries practicing afforesta-

tion (Óskarsson & Sigurgeirsson, 2001). The climate

is windy with low temperatures during the growing

season. Soils are cold, wet and deficient in nitrogen.

Frost heaving is common. Icelandic soils are of

volcanic origin and most are andisols (Arnalds

et al., 1995). Because of the volcanic origin and

fine textures, the soils are highly susceptible to wind

erosion. Human activity since the settlement of

Iceland 1100 years ago has resulted in soil erosion

and desertification of large areas (Magnússon,

1997). The Sudurnes peninsula, protruding into

the Atlantic Ocean, is an area of strong winds that

has lost much of its original topsoil. The winds,

Correspondence: D. A. Riege, University of Maryland University College, PSC 482, Box 178, FPO AP 96362, USA. E-mail: [email protected]

Scandinavian Journal of Forest Research, 2009; 24: 384�393

(Received 21 October 2008; accepted 11 June 2009)

ISSN 0282-7581 print/ISSN 1651-1891 online # 2009 Taylor & Francis

DOI: 10.1080/02827580903117404

D o w n l o a d e d B y : [ R i e g e , D e n n i s A . ] A t : 1 4 : 1 9 3 0 N o v e m b e r 2 0 0 9

eroded soils, salt spray and paucity of soil nitrogen

have made attempts at afforestation particularly

difficult. Today, most of Sudurnes is semi-barren

or has a low cover of moss or heath. For several

decades, Nootka lupine, introduced from Alaska,

has been widely and successfully used for revegeta-

tion of desertified lands in Iceland (Arnalds &

Runolfsson, 2004). At Keflavik Airport in Sudurnes

a vegetation restoration program has resulted in

establishment of several hundred hectares of lupine

and grass cover. Nootka lupine improves sites for

seedling establishment by nitrogen fixation, addition

of organic material and amelioration of microclimate

(Myrold & Huss-Danell 2003; Magnússon et al.,

2004; Mattson et al. 2007). Magnússon et al. (2004)

reported that lupine stands in Iceland may start to

degenerate after 15�20 years, to be replaced by grassland. Establishment of forest cover at the

airport will accomplish a management goal to re-

place low ground cover with taller vegetation, in

order to reduce habitat for colonial nesting birds that

are an aircraft strike hazard. Development of meth-

ods that promote afforestation in south-west Iceland

will be useful to counter regional soil erosion and to

establish windbreaks around human habitation.

This study also examined the facilitation potential

of black plastic mulch, which has been shown to be

effective in promoting tree seedling establishment,

especially in low-quality sites (Green et al., 2003).

Plastic mulch inhibits competition from herbaceous

cover, increases soil temperature and moisture, and

ameliorates the problem of frost heaving (Flint &

Childs, 1987; Tarara, 2000; Green et al., 2003). In

field trials in Iceland, plastic mulch has benefited

seedling establishment of willow (Salix) species

(Sigurgeirsson, 2000).

Sitka spruce (Picea sitchensis), Hooker willow

(Salix hookeriana) and downy birch have all shown

potential for afforestation in south-west Iceland and

were chosen for these experiments. However, with-

out facilitation under the climatically harsh condi-

tions near the coast and on nutrient-poor soils (as

are common in Sudurnes), plantings of these species

often fail (Riege & Sigurgeirsson, unpublished data).

Sitka spruce should be competitive with lupine,

owing to its shade tolerance (Burns & Honkala,

1990) and evergreen habit. In southern Iceland,

Sitka spruce seedlings have performed well within

lupine cover (Óskarsson & Sigurgeirsson, 2004). In

the USA, growth of Sitka spruce seedlings in old

fields was better under bracken fern (Pteridium

aquilinum) cover than in the open (Riege & del

Moral, 2004). Both lupine and bracken die back

over winter. Native Salix species (S. phylicifolia and

S. lanata) in Iceland are inherently slow growing and

seldom reach more than 1�2 m in height except

under favorable, sheltered conditions. Hence, fast

growing introduced species, such as Hooker willow

and feltleaf willow (S. alaxensis) from Alaska, are

favored in Iceland when the goal is to achieve

rapid forest cover, shelter and soil protection

(Sigurgeirsson, 2000). At present, clones of Hooker

and feltleaf willow are predominantly used for

shelterbelts in southern Iceland. Downy birch (Be-

tula pubescens) is the only native tree species that

forms natural woodlands in Iceland. In some regions

of Iceland, it can be an aggressive pioneer that can

rapidly colonize denuded soils and derelict land, if

protected from sheep grazing. Lupine may be a more

effective facilitator of spruce colonization than Hoo-

ker willow or downy birch, which are pioneer

deciduous trees that are adapted to open growth

(Franklin & Dyrness, 1973; Sveinbjörnsson et al.,

1993). Plastic mulch may be more effective than

lupine for establishment of willow and birch. The

present experimental sites included patches of lupine

of varying density that were established in both semi-

barren and moss�heath cover. This allowed the effects of cover type and lupine density on facilita-

tion to be investigated.

The regional aim of this study was to improve

afforestation methods in a difficult boreal, maritime

environment. Experiments were designed to test the

hypotheses that survival and growth of transplanted

tree seedlings are facilitated by lupine and by black

plastic mulch under variable site conditions. The

study also examined whether planting seedlings in a

shallow excavation within lupine further improves

facilitation. A broader goal was to utilize the varying

cover densities of lupine and grass to investigate

whether results followed the stress-gradient hypoth-

esis, in which competitive effects increase to the

detriment of facilitative effects as plant density

increases.

Materials and methods

Study sites

Keflavik International Airport is located in the

south-west corner of Iceland on the Sudurnes

Peninsula (Figure 1). Climate is moderated by the

Gulf Stream and can be characterized as temperate,

maritime and windy. Mean January temperature is

08C, July 108C (US Naval Air Station Keflavik Meterologic Office Climate Summary, 1949�1995). Mean annual precipitation is 1074 mm, dispersed

throughout the year. Mean wind speed is 6 m s �1

with extreme winds to 39 m s �1

. Soils at the airport

are highly eroded and vegetation is sparse as a result

of centuries of human destruction of woodlands and

overgrazing on the peninsula (Steindórsson, 1957).

Afforestation facilitation by lupine 385

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A vegetation restoration program at Keflavik

International Airport has resulted in many areas of

lupine cover. Five locations were chosen in 2002 for

experimental sites and identified as FB, F5, F2, F3

and FA in the Airport Geographic Information

System. FB, F5 and F2 were mostly barren before

revegetation. F3 was covered with grass�moss before the restoration treatment, with a thin sod developed

in some areas. FA was representative of large tracts

of moss�heath cover in the region, dominated by mixtures of Racomitrium moss, crowberry (Empetrum

nigrum) and heather (Calluna vulgaris). F5, F2 and

F3 received a restoration treatment in 1998�1999 of seeding with a 25 kg ha

�1 mixture of 45% tufted

hairgrass (Deschampsia caespitosa), 45% Bering hair-

grass (D. beringensis), 10% annual ryegrass (Lolium

multiflorum) and 3.5 kg ha �1

lupine. This was fol-

lowed by an application of chicken manure at

30 t ha �1

. FB and FA were pilot sites in the restora-

tion program where lupine was not sown as seed, but

transplanted as seedlings in 1996 at a density of

1500 ha �1

, along with a 15 kg ha �1

hairgrass mix,

followed by an application of 20 t ha �1

of organic

materials. During July�August 2002, the percentage cover of major vegetation types within the lupine

patches at each site was estimated by a sample of

nine 1 m 2

quadrats located in a stratified-random

pattern.

Experimental design

Tree seedlings appropriate to the region were sup-

plied by the Sudurlandsskógar afforestation project

of Selfoss, Iceland: containerized seedlings of 1-year-

old Hooker willow (clone Katla, Iceland), downy

birch (provenance Reykjarhóll, Iceland) and 1-year

and 2-year Sitka spruce (provenance Taraldsøy,

Norway). Salix can be easily propagated vegetatively

with unrooted cuttings. Hence, individual Salix

‘‘cultivars’’ are defined as genetically identical

‘‘clones’’. The clone Katla was originally collected

in Yakutat, Alaska, in 1985 but selected for wider use

in the mid-1990s owing to its good performance in

wind-exposed areas along the south coast of Iceland.

Katla performed best among 118 willow clones over

five growing seasons in trial plots at Keflavik Airport

(Riege & Sigurgeirsson, manuscript in preparation

for Icelandic Agricultural Sciences).

One method in Iceland for large-scale afforesta-

tion uses a rotating three-armed star machine to

transplant seedlings into depressions excavated every

2 m. This treatment was simulated manually in

lupine stands by the use of hand picks, with

excavations 2 m apart, approx. 50 cm in diameter

and approx. 20 cm deep with all vegetation cleared

from the excavations. The depressions were intended

to help shelter the seedlings from wind and to reduce

competition. To test whether the depression treat-

ment improved tree establishment, seedlings were

also transplanted directly into the lupine cover.

Containerized seedlings (150 ml for willow, 100 ml

for others) were transplanted with a Finnish Potti-

putki planting tube. A teabag of RTI Silva-Pak slow-

release fertilizer was inserted next to each plant. This

fertilizer was found to be effective for tree seedling

establishment in trials in southern Iceland

(Óskarsson & Sigurgeirsson, 2004). Afforestation

on barren land without fertilization usually yields

high mortality and stagnant growth in Iceland

(Óskarsson & Sigurgeirsson, 2001).

During June 2002, seedlings were transplanted in

adjacent blocks within (1) lupine cover, (2) depres-

sions in lupine cover, (3) black plastic mulch in non-

lupine cover, and (4) non-lupine cover as control.

The number of seedlings of each species dedicated to

each treatment is given in Table I. A complete,

balanced factorial design for the treatments was not

practical within the logistical constraints for the

experiments. Since a main goal of the project was

to evaluate the long-term effect of lupine facilitation,

the number of control seedlings was limited, so that

most seedlings could be placed within lupine cover.

Other experiments initiated in the region in 1998

will provide long-term results for seedlings under

treatments in non-lupine cover. Seedlings were

planted 2 m apart in alternating groups of five to

ensure heterogeneous dispersion of types (not for

Figure 1. Location of Keflavik International Airport in Iceland.

Table I. Experimental design: number of plants of each species

per treatment at each site.

Picea Salix Betula

Control 10 10 10

Plastic mulch 20 10 10

Lupine 50 20 20

Depression�lupinea 50 20 20

Note: a no treatment at site FA.

386 D. A. Riege & A. Sigurgeirsson

D o w n l o a d e d B y : [ R i e g e , D e n n i s A . ] A t : 1 4 : 1 9 3 0 N o v e m b e r 2 0 0 9

use as replications in analysis). FA was too rocky for

the depression treatment. Lupine plants at FA were

aligned in rows. There, seedlings were planted

approx. 20 cm south-west of the lupine plants along

the rows (leeward of the more damaging north-east

winds). Seedlings were placed 1 m apart in holes

punched in the midlines of two parallel strips of

photodegradable black plastic mulch that were 1.5 m

wide and 30 m long, with a 2 m gap separating the

strips. The plastic strips were covered with a thin

layer of gravel.

Seedlings were scored for survival and heights (at

highest living part) were measured during 11�14 June 2003, 20�22 May 2004, 22�27 June 2005 and 11�12 May 2008. Three hypotheses on survival and seedling heights were tested. Survival and heights of

Site: FB F5 F2 F3 FA

Picea sitchensis

Salix hookeriana

Betula pubescens

D = lupine + depression P = black plastic mulch L = lupineC = control

PC L

100

% s

u rv

iv a l

L D

100

150

200

h e ig

h t (c

m ) P

L D

100

% s

u rv

iv a l PC

L D

P C L D

P C L

D PC

L D P

C L

D P

100

h e ig

h t

(c m

)

P C

2002 2005 2008

P C

L D

D P

L D

P C

h e ig

h t (c

m )

C L

C L D

P C

C L D

100

% s

u rv

iv a l

P C

L D

Figure 2. Survival and mean height of Picea sitchensis, Salix hookeriana and Betula pubescens seedlings by site and treatment, 2002�2008. (Standard error bars omitted for clarity; standard errors for 2008 given in Table IV.)

Afforestation facilitation by lupine 387

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seedlings in: (H1) lupine are greater than in control

plots, (H2) depressions in lupine are greater than

those planted directly into lupine, and (H3) black

plastic mulch are greater than in control plots. As

cover varied among the experimental sites, site

effects were also examined. Thus, a randomized

factorial design for hypotheses on seedling heights,

with each plant randomly assigned to treatment and

site, was used. The ANOVA model was:

Yijk �m�ai �bj �abij �oijk

where m is the grand mean, a is the treatment effect,

b is the site effect, ab is the interaction effect, and o is

the error. For analysis of survival, values for percen-

tage survival per treatment per site were arcsin

transformed, with the interaction effect removed

from the ANOVA model since there was no replica-

tion. For analysis of H1 and H3, 2005 data were

used, because lupine spread over the FB and F5

control plots thereafter; 2008 data were used for H2.

Data were analyzed with Statistix 9.0 software

(www.statistix.com). Results of H2 were considered

significant if pB0.05. Results for H1 and H3 were

considered significant if pB0.025, adjusted by a/2 as

the control data were in two comparisons (Dawson

& Trapp, 2001). Some analytical results include

treatments that have a relatively low number of

seedlings. However, the major findings of the study

were supported by p values50.005.

Results

Site cover

In 2002 the lupine plots at F5 and F2 had almost

total lupine cover (Table II). Grasses, primarily

seeded hairgrass, were abundant at these two sites,

with growth facilitated by the nitrogen-fixing lupine.

From 2002 to 2008, the grass cover within the lupine

stands continued to thicken. The stand at FB, where

lupine was transplanted rather than sown, had only

72% lupine cover in 2002 and still had small barren

patches (Table II). However, by 2005 lupine cover

had increased to almost 100% at FB, although grass

cover was less and of different composition than in

F5 and F2. Unseeded fescue species (Festuca vivi-

para and F. rubra) rather than seeded hairgrass were

dominant at FB. In 2002, lupine covered only about

half of its plot at F3, illustrating the slower spread

when faced with competitive grass and moss (Table

II). By 2008, lupine covered approx. 70% of F3.

Lupine spread has also been slower (2008 cover

�70%) in the FA moss�heath than at other sites.

Survival

Survival ranged from 10% for downy birch in

depressions in lupine at F2 to 100% for several

species�treatment�site combinations (Figure 2). Neither significant treatment nor site effects on

survival of the three species were found by ANOVA

(results not shown), with the one exception of a site

effect in the depression in lupine treatment on Sitka

spruce ( p�0.02). However, the two-way ANOVA without replication was unable to measure interac-

tion effects. Survival of Sitka spruce seedlings varied

considerably by site and treatment (Figure 2). The

1-year spruce seedlings suffered greater mortality

than 2-year seedlings at all sites, had very few

survivors in dense grass, and are not recommended

for planting into dense lupine (data not shown). The

1-year spruce seedlings were therefore not further

dealt with in the present analysis. Survival of Sitka

spruce seedlings remained high in black plastic

mulch at all sites. The highest mortality of spruce

seedlings occurred within lupine and depressions in

lupine at F5 and F2. Many spruce seedlings at these

sites were found beneath thick grass, suggesting that

competition from grass caused mortality. In contrast,

spruce mortality was very low at FB (100% survival

for seedlings in lupine). FB had almost total lupine

cover in 2008, like F5 and F2, but less grass cover

than those two sites. Grass cover (mostly fescue) had

increased since 2002 in lupine at FB, F3 and FA, but

Table II. Cover characteristics in lupine stands at afforestation experimental sites during year of tree seedling transplants, July�August 2002.

Percentage cover by site

FB F5 F2 F3 FA

Lupine 72 96 99 52 48

Grass a

21 (F) 69 (D) 89 (D) 65 (F) 32 (F)

Moss 6 3 B1 33 34

Dwarf shrubs b

12 0 0 11 43

Forbs 11 4 4 15 6

Bare 10 B1 0 B1 B1

Note: before restoration treatment in 1996�1999, FB, F5 and F2 were barren, F3 was grass�moss, and FA was dwarf shrub�moss heath. a F�dominated by unseeded Festuca vivipara and F. rubra; D�dominated by seeded Deschampsia beringensis and D. caespitosa; bprimarily

Empetrum nigrum and Calluna vulgaris.

388 D. A. Riege & A. Sigurgeirsson

D o w n l o a d e d B y : [ R i e g e , D e n n i s A . ] A t : 1 4 : 1 9 3 0 N o v e m b e r 2 0 0 9

www.statistix.com

was less dense than the hairgrass cover of F5 and F2.

Similar to Sitka spruce, downy birch seedlings

suffered high mortality when in competition with

thick grasses under lupine (Figure 2: F5, F2).

Hooker willow seedlings survived better than spruce

or birch in lupine cover (Figure 2). Many willow

seedlings emerged above the lupine plants by 2004� 2005 (lupine�60�80 cm tall). Competition from the lupine and tall grasses was less detrimental to

the willows. Although in many cases survival of

plants in control plots exceeded that of treatment

plots after 6 years, most control plants exhibited very

poor growth in comparison to plants in lupine or

black plastic mulch.

Growth

Lupine effects. Overall, seedling heights of Sitka

spruce seedlings were significantly greater in lupine

than in control plots (Table III). However, there

were significant site and interaction effects, as spruce

heights were not greater in the two sites (F5, F2)

with denser grasses (Figure 2). Mean willow heights

were greater in lupine than in control plots at all

sites (Figure 2), although site and interaction effects

were still significant (Table III). Performance of

Hooker willow seedlings was poorest in the moss� heath cover of site FA, but even at FA, survival

and growth of willow seedlings were boosted in

lupine cover (Figure 2). Heights of surviving birch

seedlings were taller in lupine than in control plots at

all sites (Figure 2), with no significant site or

interaction effects. However, the sample sizes of

birch heights were low at F5 and F2 as few seedlings

survived.

Depression in lupine effects. The excavation of depres-

sions within lupine significantly increased the

heights of Hooker willow and downy birch, but

not Sitka spruce (Table III). Depressions particu-

larly did not improve spruce heights in F5 and F2,

where grasses rapidly recolonized the excavations

(Table IV, Figure 2). The depression effect on

willow heights was not as strong as the black

plastic or lupine effects (Table III). Again, the low

sample size of birch in depressions at F5 and F2

Table III. Effects of site and treatment on height of seedlings under three different hypotheses (H1�H3).

Picea Salix Betula

df F p df F p df F p

Lupine (L) 1 24.52 B0.001 1 42.01 B0.001 1 10.42 0.002

Site (S) 4 3.91 0.005 4 10.17 B0.001 4 0.90 0.469

L�S 4 5.63 B0.001 4 3.17 0.017 4 1.11 0.358 Error 196 111 84

Depression (D) 1 1.41 0.237 1 4.51 0.036 1 7.99 0.006

Site (S) 3 3.30 0.022 3 10.71 B0.001 3 0.85 0.470

D�S 3 0.84 0.475 3 2.98 0.034 3 2.63 0.058 Error 181 129 60

Plastic (P) 1 27.91 B0.001 1 134.85 B0.001 1 8.45 0.005

Site (S) 4 10.34 B0.001 4 23.80 B0.001 4 9.43 B0.001

P�S 4 4.12 0.004 4 4.09 0.005 4 8.29 B0.001 Error 128 77 70

Note: H1�lupine�control (2005); H2�depressions in lupine�lupine (2008); H3�black plastic mulch�control (2005).

Table IV. Height (cm) by treatment, species and site in May 2008.

Site Control Lupine

Depression� lupine

Plastic

mulch

Picea

FB N/A 42911 42918 2597

F5 N/A 40910 42913 70930

F2 31911 29911 31911 47914

F3 2497 37914 45921 58915

FA 1598 51915 N/A 26911

Salix

FB N/A 167936 200946 93928

F5 N/A 131937 148942 183912

F2 96950 145945 133942 206930

F3 77942 122947 154933 195918

FA 1996 122945 N/A 128949

Betula

FB N/A 65935 105931 40913

F5 N/A 54920 89912 90925

F2 41918 64927 91927 83927

F3 38910 75926 74926 72926

FA 3098 95935 N/A 3699

Note: data are shown as mean9SE.

N/A: not applicable. Controls at sites FB and F5 overgrown by

lupine and no longer valid.

Afforestation facilitation by lupine 389

D o w n l o a d e d B y : [ R i e g e , D e n n i s A . ] A t : 1 4 : 1 9 3 0 N o v e m b e r 2 0 0 9

tempers interpretation of the increase in birch

heights (Table IV, Figure 2).

Black plastic mulch effects. Seedling heights for all

species very significantly increased in black plastic

mulch versus controls, although accompanied by site

and interaction effects (Table III). Spruce heights

were greater in plastic mulch than in controls at all

sites (Figure 2). Spruce seedlings at three sites (F5,

F2, F3) made a burst of growth from 2004 to 2008

(Figure 2), such that average heights in plastic at

these sites came to exceed those in lupine and in

depressions within lupine (Table IV). However, at

FB and FA all three species mostly performed better

in lupine than in plastic mulch (Table IV, Figure 2).

This may reflect lower grass competition or less

fertile soil at FB and FA. Hooker willow seedlings

showed a substantial increase in height in black

plastic mulch at F5, F2 and F3, with many plants

�2 m tall. Downy birch seedlings survived well in

black plastic mulch, with the exception of the heath

site FA (Figure 2), but birch heights in plastic mulch

generally did not exceed those in the lupine treat-

ments.

Discussion

Facilitation and competition by lupine

Nootka lupine, which is used extensively for land

reclamation in Iceland, shows strong promise as a

nurse crop to facilitate establishment of trees (Ara-

dóttir, 2004; Óskarsson & Sigurgeirsson, 2004;

Mattson et al., 2007). In the experiments reported

here, lupine facilitated growth of Hooker willow,

Sitka spruce and downy birch in most cases. For

Hooker willow, seedling survival was high at all sites.

Most willow seedlings emerged above the lupine

plants. However, for Sitka spruce and downy birch,

survival of seedlings decreased in areas where dense

grass accompanied lupine, indicating a competitive

effect. In these more competitive sites, Sitka spruce

also did not show a height increase in lupine.

It is not uncommon for one plant species to show

both positive and negative effects on another species.

The net effect may be positive (facilitation) or

negative (competition), depending on densities, age

and abiotic factors. According to the stress-gradient

hypothesis, facilitative effects are stronger when

environmental stress is greater (Callaway & Walker,

1997; Michalet, 2005; Veblen, 2008). Competitive

intensity increases as nurse plants become denser.

The environment of south-west Iceland is physically

stressful with infertile soils, high winds and recurrent

frost heaving. This is reflected by the vegetation,

which is either semi-barren or ground-hugging.

Lupine cover ameliorates these conditions. The

results indicated that lupine generally facilitated

tree seedling establishment in this stressful environ-

ment. However, as lupine stands increase in density,

the physical environment becomes less stressful and

competition increases. Prevalence of competition

over facilitation can be seen in the results on spruce

and birch seedling performance at the two sites of

highest plant density (Figure 2: F5 and F2). In

southern Iceland Aradóttir (2004) noted a switch

from facilitation to competition with increasing

density of lupine. She found that downy birch

seedlings were facilitated by low-density lupine

cover, but survival of birch seedlings was inversely

correlated with further increase in lupine density.

However, Óskarsson and Sigurgeirsson (2004)

found high survival and better growth of Sitka spruce

and downy birch seedlings in denser lupine in a

gravelly outwash plain in southern Iceland.

These studies of lupine facilitation of tree seed-

lings in Iceland partially support the stress-gradient

model of Callaway and Walker (1997) that facilita-

tion will decrease and competition increase with

density of the nurse plants. However, an indirect

mechanism, and not nurse-plant density itself, may

be behind the possible switch from lupine facilitation

to lupine competition. The inhibitor of tree seedling

establishment appeared to be the increased density

of grasses that were stimulated by the fertility

provided by lupine. Dense grass cover has been

reported to inhibit establishment and growth of Sitka

spruce seedlings in North America (Coates et al.,

1993; Riege & del Moral, 2004). In the absence of

dense grass, dense lupine facilitated tree seedling

establishment at site FB and at the site of Óskarsson

and Sigurgeirsson (2004). Callaway and Walker

(1997) and Callaway and Pennings (2000) empha-

sized the importance of indirect interactions on the

balance of facilitation versus competition, but in

their examples plant species A usually indirectly

facilitates species B by inhibiting species C, which is

a competitor of species B. In the current case, the

indirect mechanism differs. Species A (lupine) may

indirectly inhibit seedlings of tree species B by

facilitating species C (hairgrass), which then out-

competes species B.

Facilitation by black plastic mulch

Black plastic mulch was an effective facilitator of tree

seedling establishment in this study, increasing

growth of Sitka spruce, Hooker willow and downy

birch (at most sites), while maintaining high survival

rates. The expectation that shade-tolerant Sitka

spruce would perform less well in the open black

390 D. A. Riege & A. Sigurgeirsson

D o w n l o a d e d B y : [ R i e g e , D e n n i s A . ] A t : 1 4 : 1 9 3 0 N o v e m b e r 2 0 0 9

plastic than in lupine was not supported at the sites

with dense grass competition. Here, spruce seedlings

exhibited more growth in black plastic than under

other treatments (Figure 2). This may indicate that

the chief benefit of the black plastic is to decrease

competition, although the plastic cover also increases

soil temperature and moisture (Tarara, 2000; Green

et al., 2003). Black plastic was least effective at FA.

The thick moss cover at FA may have counteracted

the soil benefits by the ability of moss to insulate

temperature change and to absorb moisture.

Although many studies show that plastic mulch can

promote tree seedling establishment, other studies

show no benefit to mulching (see review in Green

et al., 2003). For example, Houle and Babeaux

(1994) found that plastic mulch had no effect on

growth or survival of four species of tree seedlings

(including Salix planifolia and Picea glauca) after 4

years in subarctic Quebec. Continued monitoring of

the present plastic-mulch facilitation experiment and

others in the region (Sigurgeirsson, 2000) will be

critical to evaluate its utility for afforestation in

Iceland. There is a dearth of literature on the long-

term success of plastic mulch in establishing forests.

Management implications

Both nurse crops of Nootka lupine and black plastic

mulch may be recommended as treatments that can

facilitate tree seedling establishment in stressful

environments similar to those of Iceland. However,

lupine in combination with dense grass cover may

competitively inhibit tree seedlings. It is recom-

mended that lupine be seeded without grass seeding

or fertilization that promotes grasses. Tree seedlings

should be planted in young, open lupine stands. If

seedlings are planted into older, dense lupine, they

may benefit from site preparation that reduces

competition, such as mowing (Aradóttir, 2004), the

plowing of trenches within the lupine (Óskarsson &

Sigurgeirsson, 2004) or the use of taller plant

material (Sigurdsson, 2005). The results present

caution, however, that excavated ground can be

rapidly recovered by neighboring vegetation in the

fertile lupine stands. In these experiments the extra

cost of excavation did not improve establishment of

Sitka spruce. On barren or moss�heath sites, tree seedlings can be planted directly into lupine without

excavations, accompanied by packets of slow-release

fertilizer. Results at the moss�heath site indicated that lupine facilitated growth of spruce, willow and

birch seedlings without a scarification treatment.

Contrary to the prediction, Hooker willow seed-

lings may be more successful than Sitka spruce or

downy birch in afforestation via lupine, owing to the

fast growth of willow to overtop the lupine. How-

ever, in sites without dense grass, it appears that the

facilitative effects on spruce seedlings will outweigh

the competitive effects to allow the seedlings to

emerge above the lupine. Although Hooker willow

showed better initial performance and should be a

major component of multispecies afforestation,

methods that promote Sitka spruce are desirable.

Spruce trees will provide greater height and longevity

to the future forest. Facilitation of tree seedling

establishment by black plastic mulch exceeded that

of lupine in this study in areas of thick grass, where

the plastic suppressed competition. A benefit in the

use of plastic mulch is that tree seedlings can be

planted immediately without a lag of 2�3 years after lupine seeding. However, it is possible that

performance of the seedlings in plastic mulch will

decrease in the long term in comparison to lupine,

owing to the lack of nitrogen input that lupine

provides.

Two caveats to the findings are that some results

are based on low numbers of seedlings and that long-

term success will not be determined for several years.

The authors believe that these three species are

capable of long-term establishment in the region,

based on the presence of mature stands in a few,

relatively sheltered locales (Riege & Sigurgeirsson,

personal observation). Use of shelterbelts and plant-

ing in large blocks should enhance survival and

growth in the more open areas.

An afforestation technique is proposed for south-

west Iceland that combines the benefits of lupine and

black plastic mulch. In the first year, tree seedlings

are planted into black plastic along the edges of the

site, while the interior of the site is seeded with

lupine. The establishment of shelterbelts, particu-

larly on edges that face prevailing winds, will help to

protect the interior. Hooker willow will be the

primary species of the shelterbelt, but it will also

include a mix of Sitka spruce, downy birch and

shrubby willow species. The shelterbelt will be

modeled on the system of Robertson and Eysteins-

son (2002), where a mixture of deciduous and

evergreen trees and shrubs provides surface rough-

ness that is efficient in reducing winds and trapping

snow. After 2�3 years of lupine growth, a mixture of appropriate varieties of Sitka spruce, willow and

birch will be transplanted into the site interior. This

combination of species mimics natural succession

where the pioneer willows and birches grow more

rapidly as early dominants to be eventually super-

seded by spruce (Reed & Harms, 1956; Slettjord,

1993). Over the years lupine will expand into the

shelterbelts in concert with the deterioration of the

plastic, where it will facilitate continuation of shel-

terbelt growth.

Afforestation facilitation by lupine 391

D o w n l o a d e d B y : [ R i e g e , D e n n i s A . ] A t : 1 4 : 1 9 3 0 N o v e m b e r 2 0 0 9

Conclusions

Nootka lupine can be an effective nurse crop for tree

seedling establishment under harsh conditions in

northern maritime climates with low soil fertility.

Lupine may also promote growth of competitive

plants, particularly grasses, that can inhibit tree

seedlings. Tipping the balance from facilitation to

competition by a nurse plant promoting a competi-

tive species that inhibits the target species is a

mechanism that differs from common models of

the stress-gradient hypothesis. For best results, tree

seedlings should be planted into young, open lupine

stands with low amounts of competitive cover.

Planting seedlings into excavations within lupine

can improve early growth but, in sites of high

competition, grasses and lupine can rapidly spread

over the depressions and deter slower growing

species, such as Sitka spruce. Black plastic mulch

also shows potential as a facilitator of establishment

of tree seedlings in these regions. For creation of tree

plantations in these environmentally stressful areas,

it is recommended that a mix of species be planted

directly into young lupine stands that are bordered

by shelterbelts of seedlings, primarily willows,

planted into black plastic mulch.

Acknowledgements

We express thanks to David James of the Atlantic

Division, US Naval Facilities Engineering Com-

mand, for his vision in promoting vegetation restora-

tion and afforestation research on the Keflavik

International Airport. Funding, field and office

support was generously provided by the Icelandic

Forest Research Service, US Naval Facilities En-

gineering Command, the Environmental Division of

the former Keflavik NATO Base, and Sudurlandss-

kógar. We thank Mats Hannerz and two anonymous

reviewers for suggestions that improved the manu-

script.

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