Tree recruitment above the treeline and potential for climate-driven

treeline change

Hofgaard, Annika1�; Dalen, Linda

2,3& Hytteborn, Hakan

2,4

1Norwegian Institute for Nature Research, NO-7485 Trondheim, Norway; 2Department of Biology, Norwegian

University of Science and Technology, NO-7491 Trondheim, Norway; 3Present address: The Directorate for Nature

Management, NO-7485 Trondheim, Norway; E-mail linda.dalen@dirnat.no; 4Plant Ecology, Department of Ecology

and Evolution, Evolutionary Biology Centre, Uppsala University, Norbyvagen 18D, SE-752 36 Uppsala, Sweden;

E-mail hakan.hytteborn@gmail.com;�Corresponding author; Fax 147 73801401; E-mail annika.hofgaard@nina.no

Abstract

Questions: How do population structure and recruitmentcharacteristics of Betula saplings beyond the treeline varyamong climatic regions, and what is the potential fordevelopment into tree-sized individuals with interactinggrazing pressure?

Location: Scandes Mountains.

Methods: Sapling characteristics of Betula pubescenssubsp. tortuosa, their topographic position above the tree-line, growth habitat and evidence of recent grazing wasinvestigated in three areas with a long continuous grazinghistory, along a latitudinal gradient (62-691N).

Results: Saplings were common up to 100m above thetreeline in all areas. The northern areas were characterisedby small (o30 cm) and young (mean 14 years old) saplingsin exposed micro-topographic locations unfavourable tolong-term survival. In the southern area, broad height (2-183 cm) and age (4-95 years; mean 32 years) distributionswere found in sheltered locations. Age declined withaltitude in all areas. Sapling growth rate varied within andbetween areas, and the age�height interaction was signifi-cant only in the southern area. Growth rates decreasedfrom south to north and indicated a considerable timerequired to reach tree size under prevailing conditions.

Conclusions: Regional differences can be attributed toclimatic differences, however, interacting biotic and abioticfactors such as micro-topography, climate and herbivory,mutually affect the characteristics of birch saplings. Inview of the long time needed to reach tree size, thegenerally expected evident and fast treeline advance inresponse to climate warming may not be a likely short-term scenario. The sapling pool in the southern regionpossesses strongest potential for treeline advance.

Keywords: Alpine zone; Browsing; Climate change;Mountain birch; Saplings; Treeline dynamics.

Nomenclature: Lid & Lid (2005)

Introduction

The forest-tundra ecotone – the transition zonebetween closed forest and treeless tundra – is an im-portant ecological and environmental entity interms of regional to global vegetation, climate, bio-diversity and human land use (Callaghan et al. 2002;ACIA 2005). Within the forest-tundra ecotone, twodistribution limits, the treeline (i.e. northern oruppermost location of tree-sized tree species in-dividuals; 42m) and the tree species limit (i.e.northern uppermost location of tree seedlings), areof special importance when analysing ecotonal re-sponse to recent environmental changes, and inpredictions of future responses of the ecotone. It isin the zone between these limits where abiotic andbiotic processes are determining the pace and oc-currence of treeline change. The local climatechanges drastically as one moves upwards or north-wards across the ecotone. Tree cover influencesmicroclimate and creates a relatively benign climatewith regard to wind, radiation, temperature and soilmoisture fluctuations (e.g. Holtmeier 2009), favour-ing survival and height development of present treespecies. Beyond the forest, the communities are gra-dually more exposed, and micro-topography andgrowth forms, which modify the microclimate, be-come increasingly important in structuring thevegetation communities (Wilson et al. 1987; Hof-gaard & Wilmann 2002; Korner & Paulsen 2004).

Much effort has been put into defining majorexplanatory variables for treeline position, andtemperature is generally considered a common lim-iting factor at large spatial scales (Korner 1998;Korner & Paulsen 2004; Holtmeier 2009), whereas

Journal of Vegetation Science 20: 1133–1144, 2009& 2009 International Association for Vegetation Science

micro-topography, soil conditions and biotic fac-tors, like browsing and insect outbreaks, modify thepositions on a local to regional scale (Sveinbjorns-son et al. 2002; Dalen & Hofgaard 2005; Resler2006; Holtmeier & Broll 2007). Treeline positionoscillates through time and space, partly in ac-cordance with long- and short-term climate changes(Hofgaard 1997a; Seppa et al. 2002). Accordingly,ongoing climate changes and anticipated warmingfor the future decades and centuries (IPCC 2007) aregenerally expected to have a discernible impact ontreeline position (e.g. ACIA 2005). However, in-vestigations of treeline position and dynamicsduring recent decades show somewhat deviating re-sponses to climate warming, with clear treelineadvance in some studies (Lescop-Sinclair & Payette1995; Kullman 2002; Lloyd & Fastie 2003) and noor very little evidence of response in others (Kull-man 1993; Hattenschwiler & Korner 1995; Szeicz &MacDonald 1995; Gehrig-Fasel et al. 2007). Beyondthe treeline, seedlings, saplings or krummholz (i.e.tree species individuals with a prostrate growthform) may be common (Kullman 2002; Holtmeier2009), even though establishment and survival ofseedlings into sapling stage constitute a bottleneckin the recruitment process (Kullman 1993). As anadvance in treeline necessarily involves growth ofsaplings and/or krummholz into tree size at loca-tions beyond the current treeline, the present statusof sapling/krummholz populations (e.g. age struc-ture, physiognomic stature) above the treeline isindicative of the potential for treeline advance.

Alpine Scandinavia has a long and continuousgrazing history by semi-domestic reindeer, sheepand wild grazers/browsers, which have shaped theforest-tundra transition zone through time. Betulapubescens subsp. tortuosa (mountain birch) is themost common treeline species and constitutes theuppermost forest in large parts of the ScandesMountain chain (Moen 1999). The species is a fa-voured browse for many animal species (see below),which hampers both height growth and survival ofbirch. The tree population structure at the presentScandinavian birch treeline shows marked differ-ences in different regions of the Scandes. Youngtrees, which recently reached a height of 2m, makeup a large part of populations in the southernScandes. Somewhat older trees, already 2-m tall andhence treeline trees for a long period, dominate inthe north (Dalen &Hofgaard 2005). Presence of treeindividuals shorter than 2m (henceforward calledsaplings) above the treeline is a frequently usedmeasure of treeline advance, but their presence perse does not indicate an advance of tree distribution

into tundra areas. A change in recruitment, long-term survival and height development of saplings iscrucial for a change to occur. The rate of this processis under multiple environmental controls, and re-gion-specific abiotic (climate) and biotic (grazing)factors create regionally deviating encroachmentpatterns in alpine tundra.

The novelty of this study is the use of age struc-ture analyses combined with recorded staturemeasures and environmental factors for individualsaplings. In this study, the presence of saplings abovethe treeline and their frequency, location, spatial dis-tribution, height, age and evidence of recent browsingwas investigated during a period of evident regionaland global warming (late 1990s) in three regions of theScandes. The data are discussed with special focus onregional-specific potential for treeline advance. Twomain questions are addressed: (1) how do populationstructure and recruitment characteristics of naturallyoccurring birch saplings above the treeline varyamong climatic regions, and (2) what is the potentialfor development into tree-sized individuals in regionswith interacting grazing pressure?

Methods

Study areas

In this study we use a dataset collected in 1999and 2000 in three areas along the Scandinavianmountain chain: Dovre in southern Norway (621100-200N, 91300-400E), Abisko in northern Sweden(681100-300N, 181400-191000E) and Joatka in north-ern Norway (691400-500N, 231500-241000E) (Fig. 1).The Dovre and Abisko areas are situated in ratherrugged mountainous terrain, with peaks reaching42200 and 41800m, respectively, whereas in Joat-ka the landscape is characterised by an undulatingplateau at about 400-500m a.s.l., with the highestmountains reaching just above 1000m. The eleva-tion of sampling locations varies both with latitudeand within area according to variations in altitu-dinal treeline location (see below and Table 1). Allthree areas are dominated by slightly continentalclimate conditions characterised by low precipita-tion and pronounced differences between summerand winter temperatures (Hanssen-Bauer & Nordli1998; Moen 1999; Hanssen-Bauer & F�rland 2000).While the mean annual temperature decreases fromthe southern to the northern area (0.2, � 0.5 and� 2.31C at Dovre, Abisko and Joatka, respectively),the mean summer temperature increases slightly(9.2, 9.8 and 9.91C, respectively) (Fig. 2). However,

1134 Hofgaard, A. et al.

Fig. 1. Study areas along the Scandes Mountain chain: Dovre in central Norway, Abisko in northern Sweden and Joatka innorthern Norway.

Table 1. Sapling occurrence and their altitudinal distribution along the four investigated sites per study area, mean treelinelocation and summit height at individual sites. Presence of saplings at summit position and within the upper 50 altitudinalmeters of sampled mountain sites is marked in bold.

Study area Aspect Number of saplings Altitudinal range(ma.s.l.)

Treeline location(ma.s.l.)

Summit height(m a.s.l.)

Dovre N 9 1160-1448 1110 1453E 20 1150-1250 1100 1335S 29 1185-1325 1135 1403W 20 1160-1260 1110 1690

Abisko N 27 752-820 700 1206SE 34 772-1000 720 1191SW 8 815-840 760 880NW 1 870-870 820 945

Joatka N 22 468-513 410 513E 21 490-522 440 522S 38 530-570 480 570W 20 438-482 380 532

- Tree recruitment above the treeline - 1135

Fig. 2. Ten-year running mean of seasonal and annual temperatures for the period 1880-2000 in the three study areas. Datafrom the Norwegian Meteorological Institute; http://www.met.no. Note: all y-axes display a temperature range of 131C toemphasise differences in temperature variability between seasons.

1136 Hofgaard, A. et al.

the length of growing season decreases from southto north (cf. Dalen & Hofgaard 2005, for more de-tails). Temperature development since the mid-20thcentury (Hanssen-Bauer & Nordli 1998; Fig. 2)shows no or very little change for summer and au-tumn and slightly increasing temperatures duringspring in the Dovre and Abisko areas. However,there has been an apparent increase in winter tem-perature since late 1980s at all three locations.Despite this warming trend, the growing seasonlength in the northernmost region has reduced dur-ing the last two decades of the 20th century as aconsequence of delayed snow melt (Skaugen &Tveito 2002; Karlsen et al. 2007).

Gneiss is the dominant bedrock in Dovre, andhard-shale in Abisko and Joatka, and thin spodosolgenerally characterises the soil in all three areas.Herb-rich mountain birch forests dominate the sub-alpine zones, with some scattered pine, Pinussylvestris L., at lower altitudes. Closed birch forestgrows generally up to about 1090ma.s.l. in Dovre,670m a.s.l. in Abisko and 420ma.s.l. in Joatka, butwith somewhat varying position depending on slopeaspect and topography. Beyond the forest, dwarfshrub heath and lichen heath dominate in the low-alpine zone that continues up to ca. 1550m inDovre, 1050m in Abisko and about 650ma.s.l. inJoatka.

The areas have no records of stand-clearing fire,but in all three areas herbivores, such as semi-domestic reindeer, sheep (only Dovre), moose, hare,grouse and defoliating insects, are present. Birchleaves, buds and twigs constitute a significant part ofthe diet of these animals. There is no known majortrend in reindeer and sheep grazing pressure over thedecades predating the study, but there are some dif-ferences in timing. The Joatka area is generally usedas a grazing ground in early summer and autumn,and the Abisko and Dovre areas throughout thewhole summer. Major regional insect outbreaks(mainly Epirrita autumnata Borkhausen) occur atca. 12-20-year intervals, but less frequently in in-dividual areas. The Abisko area had a majoroutbreak in 1955-1956 that strongly affected treestand structure at altitudes below the forest line(Tenow & Bylund 2000) but with less or no impactat the treeline (cf. Emanuelsson 1987). There wereno signs of recent stand-replacing outbreaks in anyof the studied areas.

Study design

In each of the three areas, four sites, each con-sisting of a single mountain slope, were selected.

Each slope faced in a different direction. At Dovreand Joatka, the sites faced north, west, east andsouth (N, W, E, S). At Abisko, it was not possible tofind suitable sites with these aspects, and thus thefollowing aspects were chosen: N, SE, SW and NW(Table 1). The distance among sites within each ofthe three areas ranged from 5 to 30 km. Each sitewas selected so that its slope facing the chosen as-pect would run at least 1 km along the treelinecontour. The treeline position for individual slopeswas defined as the mean altitude of the 20 upper-most birch trees (42m tall), situated at least 50mapart along the slope (see Dalen & Hofgaard 2005).A thorough search for birch saplings (i.e. birch in-dividuals o2m; no lower limit of birch sapling sizewas set, but individuals less than a few cm are verydifficult to locate and thus are likely to be under-re-presented) was conducted within a horizontal strip(1 km�20m altitude) at 50-m steps above the meantreeline. The numbers of strips surveyed within sitesvaried from two to 10 depending on summit heightof the mountains on which the sites were situated.Up to 20 saplings were sampled in each strip, wherepresent (saplings were not found at every elevation).Due to scarcity of saplings, the total sapling numbervaried between areas (range 70-101), sites (range 1-38) and altitudes (range 0-20).

For each sapling, the following variables wererecorded in the field: height, growth form (poly-cormic, monocormic), number of stems ifpolycormic, percentage of main branches beingbrowsed in the current season, altitude above sealevel, local aspect of growing position, slope in-clination and micro-topography (1m2 radiusclassified as concave, convex or flat). Further, allsaplings were cored or cut as close to the ground aspossible. The age was determined in the lab, wherethe surface of the cores and cross-sections wassmoothed with a scalpel. Zinc ointment was appliedto increase the contrast between early and late woodof annual rings. The rings were counted using a ste-reomicroscope (6-40�). For three saplings (Dovre),age determination was not possible because of rot inthe centre of the stem. For the saplings sampled in2000 (63 in Dovre, 60 in Abisko and 43 in Joatka),position along the ridge-snow bed (i.e. patch withlong-lasting snow cover) gradient was recorded byusing five categories classified as: ridge, ridge-lee-side, leeside, leeside-snow bed and snow bed. Totaldegree of browsing (i.e. without regard to time sincethe event) was not recorded, as in general all birchsaplings in the alpine zone are browsed (A. Hof-gaard, unpubl. data) and consequently the measurehas low predictable strength.

- Tree recruitment above the treeline - 1137

Three additional variables were calculated foreach sapling: altitudinal distance to treeline andaltitudinal distance to mountain summit was calcu-lated from map data and individually recordedaltitude above sea level; and from the slope and as-pect recordings, a variable representing relativeradiation (RR) was calculated. We used the formulaRR 5 cos(90� sopt� S � cos(aopt�A)), where sopt isthe optimal slope, 451, aopt is the optimal angle ofaspect, set to 2051, S is measured slope in degreesand A is measured aspect in degrees (after Dargie1984).

Data treatment and analyses

Sapling data from all four sampled slopes ineach area were pooled to form one dataset per area.Differences between study areas were tested fornon-categorical variables using ANOVA and Tu-key’s honest significant difference (HSD) test formultiple comparisons. The variables sapling age,sapling height and distance to treeline wereln-transformed to gain better approximations tonormal distributions. For the categorical variablesof micro-topography and the ridge-snowbed gra-dient, w2-tests were performed to test for overalldifferences between study areas. Correlation ana-lyses between the different variables were performedseparately for each study area, with Spearman’scorrelation coefficient for the micro-topography vari-

ables and Pearson’s coefficient for non-categoricalvariables. Correlation analyses were not used for theridge-snowbed variables, as some of the categorieshad too few samples, and not between micro-topo-graphy classes because they are mutually exclusive.

Results

In total, 78, 70 and 101 birch saplings weresampled in Dovre, Abisko and Joatka, respectively.Saplings were relatively abundant up to 150m abovethe treeline at Dovre and Abisko, and up to 100m atJoatka. At higher altitudes, only two saplings werefound: one at 250m above the treeline in Abisko andone at the 300m level on a summit at Dovre (cf.Table 1). At all the sites in Joatka, saplings werefound up to, or close to, the summit level. The max-imum altitude at which saplings were found variedfrom 570m a.s.l. (summit level) in the northernmostarea to 1448m in the southernmost.

In all three areas, the highest densities of sap-lings and also generally the highest altitude forsaplings were found on south-facing (SE in Abisko)slopes (Table 1). In Dovre and Abisko, most sap-lings were generally found on lee sides on concave orflat ground, but with some occurrence (mainlyDovre) on convex surfaces. However, at Joatkamost saplings were found on ridges with a flat sur-face (Table 2). The type of micro-topography was

Table 2. Mean values and standard deviation for recorded and calculated abiotic and biotic birch sapling variables in thethree study areas, Dovre, Abisko and Joatka. Different letters in superscript indicate significant differences at Po0.05between investigated areas (ANOVA and Tukey’s HSD test). � and �� indicate significant overall differences between areasat Po0.05 and Po0.01, respectively, for the categorical variables (w2-tests). ‘‘% of cases’’ means the percentage of saplingsrepresenting the classified category within each study area.

Variable Dovre Abisko Joatka

Altitude of growing location (ma.s.l.) 1208 � 5.30a 808 � 5.62b 510 � 3.70c

Altitude above treeline (m) 69.2 � 5.35a 80.4 � 4.64b 53.3 � 1.60a

Slope inclination (o) 18 � 1.88a 29 � 1.53b 12 � 1.06c

Microtopography:Concave (% of cases)�� 29.5 51.4 11.9Flat (% of cases)�� 39.7 32.9 87.1Convex (% of cases)�� 30.8 15.7 1

Ridge-Snow bed gradientRidge (% of cases)� 9.5 6.7 60.5Ridge-Lee side (% of cases)�� 9.5 25 0Lee side (% of cases)�� 63.5 66.7 16.3Lee side-Snow bed (% of cases)�� 17.5 0 23.3Snow bed (% of cases) 0 1.7 0

Sapling age (y) 32 � 2.20a 17 � 1.31b 9 � 0.81b

Sapling height (m) 0.6 � 0.05a 0.3 � 0.02ab 0.1 � 0.02b

PolycormicyPolycormic (# per sapling) 2.62 � 0.34a 1.7 � 0.16a 1.14 � 0.65a

BrowsingMean % browsed per sapling 21.8 � 3.31a 5.9 � 1.09b 1.7 � 0.94b

1138 Hofgaard, A. et al.

not associated with altitude or distance to treeline(App. 1). The altitudinal position of saplings waspositively correlated with relative radiation at bothAbisko and Joatka, reflecting increasing importanceof southerly exposure with altitude in the twonorthern areas.

The sapling populations in the southern areawere generally both older and taller than the morenorthern populations (Table 2; Figs. 3 and 4). Theage distribution in the south showed a peak around20 years and a gradual decline in frequency of in-dividuals in older age classes to about 70 years. Fewindividuals belonged to the 80-100-year classes (Fig.3). In the Abisko area, most saplings were around 10years old and the age distribution showed a strongdecline to about 40 years. However, one individualwas 81years old. In the northernmost area, thedominant age was less than 10 years, and only a fewsaplings were older than 15 years.

The height distribution generally followed a si-milar declining trend from the southern to thenorthern sampling areas (Fig. 4), with a mean heightof 0.63, 0.27 and 0.13m, respectively (Table 2). Inthe southern area, a large part of the sapling popu-lation was up to ca. 1m, and 13% were taller than1.2m (Fig. 4). In the two northern areas, however,only a few saplings were taller than 0.5m (9% and3%, respectively). Generally, sapling age and heightdeclined with altitude above treeline, although thiscorrelation was significant only in the south (App.1). Browsing frequency was higher in the southernarea compared to the two northern sites. However,browsing was positively correlated with saplingheight (App. 1), thus possibly weakening the latitu-dinal difference. The overall growth rate of thesaplings, i.e. the relation between sapling height andage (Fig. 5), varied considerably within and betweenstudy areas, ranging from 0.12 to 22.5 cm/yr, withmeans of 2.1, 1.8 and 1.6 cm/yr for areas from southto north, respectively. Age and height showed a sig-nificant positive correlation in the southernmost andAbisko areas, whereas no such relationship wasfound for saplings in the northernmost area. Poly-cormic structure among the saplings was low: it wasmore common in convex than concave topographyand showed a significant increase with age at all sites(App. 1).

Discussion

The more frequent presence of large saplings(41m) beyond the treeline in the southern areacompared to the northern areas provides potential

for marked treeline advance in response to climaticimprovement. However, this requires that otherabiotic and biotic factors are concomitantly ad-vantageous (Moen et al. 2008). In the northernareas, the potential for treeline advance seem to bemore restricted, given the size structure of the sap-ling cohort, dominated by small saplings. The

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Fig. 3. Age distributions of birch saplings (heighto2m) atthe three study areas, Joatka, Abisko and Dovre, ag-gregated into 5-year age classes. Note different scaling ofy-axes; and that the youngest age class is likely to be un-der-represented (cf. Methods).

- Tree recruitment above the treeline - 1139

pattern is emphasised in the northernmost area,where the treeline tree population has had a low re-cruitment rate during recent decades (Dalen &

Hofgaard 2005). This difference between the south-ern and northern areas may partly be related toclimatic differences mediated by proximity to theArctic and the thereby different relative influence ofArctic versus Atlantic air masses between areas(Hanssen-Bauer & F�rland 2000), but also possiblyby differences in herbivore pressure (Cairns &Moen2004).

Survival of small birch saplings is strongly cor-related with the length of the growing season(Kullman 1986); hence, the lack of older individualsin the sapling population of the northern areaspoints towards confined survival capacity that mightbe related to the short growth periods. Higher num-bers of saplings and larger altitudinal distributionon slopes with greater relative radiation also pointsto the importance of temperature and growing sea-son length. This pattern generally agrees with treeoccurrence patterns shown for forest limit and tree-line positions in the Scandes Mountains (Kjallgren& Kullman 1998; Dalen & Hofgaard 2005; Truonget al. 2007). South-facing slopes are snow-free longbefore northerly slopes and hence have longergrowing seasons. The higher radiation loads andsubsequent higher temperatures throughoutthe growing season promote growth (cf. Junttila &Nilsen 1993; Weih & Karlsson 2001). The meansummer temperature was lowest in the southern-most studied area and highest in the northernmostareas, indicating that length of the growing period isa more important controlling factor for birch treeestablishment than temperature per se.

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Fig. 4. Height distributions of birch saplings (heighto2m) at the three study areas, Joatka, Abisko and Dovre,aggregated into 10-cm height classes. Note different scal-ing of y-axes; and that the smallest size class is likely to beunder-represented (cf. Methods).

Fig. 5. Relationship between age and height of birch re-cruits in the alpine zone, overall nonlinear growth rates forDovre, Abisko and Joatka. Log scale for age is used tovisualise details in the range 1-20 years. Please note theoutlier from the northernmost area (Joatka), which wasfound in a sheltered location.

1140 Hofgaard, A. et al.

The number, age and height of saplings de-creased with altitude and distance to treeline, asexpected. In addition to temperature decline, ger-mination ability of mountain birch seeds decreaseswith altitude of mother trees (Holm 1994), and dis-tance to potential high-quality seed sources willconsequently increase with increasing altitude.However, birch seeds are light and easily dispersed(Molau & Larsson 2000), and seed availability andquality is possibly less important as a limiting factorin this zone (Truong et al. 2007), but increasedmortality at and beyond the treeline due to prevail-ing harsh environmental conditions may severelylimit survival in early stages of life (Karlsson &Weih2001).

Establishment and growth of saplings in the al-pine region varies with micro-topography (Dalen &Hofgaard 2005), with concavities, flat and shelteredpatches and lee sides being advantageous habitats.These habitats are beneficial as they provide shelterfrom wind, deeper snow cover in winter, higherdaytime temperatures, more stable soil moistureconditions and higher winter soil temperatures fa-vouring nutrient uptake capacity in the followingsummer (Karlsson & Weih 2001; Sturm et al. 2001;Sveinbjornsson et al. 2002). Snow cover also pro-tects saplings against winter browsing, strong windsand abrasion (Scott et al. 1993; Sveinbjornsson et al.2002). A rise in tree limit during the mid-20th cen-tury in the Swedish Scandes was related to anincrease in snow depth (cf. Kjallgren & Kullman1998). However, long-lasting snowbed conditionsare unfavourable to birch sapling survival due todecreased length of the growth period (Kullman1986).

Birch germination and seedling appearance isfavoured by bare soil (Holm 1994), common onwind-exposed ridges in alpine zones. However, thishabitat is less suitable for long-term survival andheight growth, as indicated by the dominance ofsmall saplings (i.e. high turnover rate) located in thishabitat. Higher polycormic structure in convexwind-exposed habitats compared to concave habi-tats, and increase in polycormic structure with age,indicate repeated destruction of the leading shoot.This pattern is highlighted in the northernmost area,where sub-arctic, harsh environmental conditionsprevail.

Browsing restricts the occurrence and develop-ment of seedlings, saplings and small trees in thealpine environment and may lower treeline positionsin relation to climatically defined positions (Oksa-nen et al. 1995; Hofgaard 1997b; Cairns & Moen2004; Olofsson et al. 2005). Consequently, climati-

cally-driven sapling development and treelineadvance will be modified by land use (Gehrig-Faselet al. 2007; Holtmeier & Broll 2007). The recordedeffect of browsing animals was only significant in thesouth. However, the pattern is linked to the size ofthe saplings, which impedes possible interpretation.The low-alpine zone represents the most importantsummer grazing ground for both sheep and reindeer,where herbivory has structured the vegetationthrough time (Emanuelsson 1987; Holtmeier 2009)and is an important ecosystem driver in the ScandesMountains (Olofsson et al. 2001; Holtmeier 2009),imparting biotic control to the system (Adler et al.2001). Effects of browsing populations are notstraightforward as the aggregated effect of climateand browsing comprises a complex set of interac-tions (cf. Ayres & Lombardero 2000). This involveschanges in animal population size; the importanceof snow for sapling occurrence and winter browsingpressure; and feedback effects on snow depth andwind redistribution of snow through changes insapling/tree stature and tree cohort density (Sturmet al. 2001; Holtmeier 2009).

Given the mean growth rate of � 2 cm/yr,growing from the recorded mean height of 10-30 cmin northern areas to a height of 2m would, on aver-age, take close to 100 years if conditions controllinggrowth are held at the level that shaped the sampledsapling population. At the same time, the recordedmaximum growth rates of 420 cm/yr suggest that alimited part of the sapling population could poten-tially become tree-sized individuals within a fewyears. This is particularly the case in the south.However, due to scarcity and spatial variation insaplings along the slopes, the potential for treelineadvance is highly variable at the landscape level.Even though the most pronounced climate warmingis predicted for northern regions (IPCC 2007), fac-tors such as changed snow conditions, growingseason length and browsing pressure might lead tocounter-intuitive effects on tree recruitment. In ad-dition, in view of the fact that in all investigatedareas birch saplings were common even before thewarm late 1990s, and considering the complexity ofconditions needed for successful establishment andgrowth from seedling to tree, a massive rapid inva-sion of trees at and beyond their present upper limitseems less likely.

Acknowledgements. We would like to thank Eva Romell,

Erik Heiman, Anna Sj�stedt, Heidi Myklebust and Frode

Morken for help in the search for birch saplings; Kari

Sivertsen for help with Fig. 1; and three anonymous

- Tree recruitment above the treeline - 1141

reviewers for thoughtful and constructive comments. We

are grateful for the logistic support from Kongsvoll,

Abisko and Joatka research stations and for financial

support from the European Commission (ENV4-CT97-

0586) and the Research Council of Norway (IPY grant

176065/S30 to ‘‘PPS Arctic’’).

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- Tree recruitment above the treeline - 1143

TableA1.Correlationcoefficientsbetweeninvestigatedsaplingvariablesin

thethreestudyareas.�and��

indicatesignificance

atPo0.05andPo0.01,respectively.Study

areasare

D5Dovre,centralNorw

ay,A

5Abisko,northernSweden

andJ

5Joatka,northernNorw

ay.

Study

area

Altitude

a.s.l.

Distance

totreeline

Deviation

from

NE

Distance

tosummit

Sapling

height

Polycorm

icBrowsing

Slope

Age

Relative

radiation

Concave

Convex

Flat

Distance

totreeline

D0.940�

Distance

totreeline

A0.825��

J0.681��

Deviationfrom

northeast

D0.128

�0.177

Deviationfrom

North-East

A0.562��

0.096

J0.514��

0.425�

Distance

tosummit

D�0.319��

�0.344��

0.326��

Distance

tosummit

A�0.565��

�0.127

�0.887��

J�0.812��

�0.723��

�0.003

Saplingheight

D�0.450��

�0.485��

0.168

0.480��

Sapling

height

A�0.085

�0.126

0.059

�0.255�

J0.088

�0.042

0.104

0.108

Polycorm

icD

0.130

0.133

�0.063

0.039

�0.170

A�0.211

�0.071

�0.137

0.181

�0.071

Polycorm

icJ

0.144

�0.019

0.141

�0.031

0.159

Browsing

D�0.112

�0.183

0.322��

0.569��

0.258�

0.250�

A�0.160

�0.271�

0.081

�0.155

�0.068

0.139

Browsing

J0.034

0.070

0.117

0.005

0.011

�0.007

Slope

D0.078

0.102

�0.150

�0.136

�0.166

0.097

�0.021

A0.091

�0.119

0.600��

�0.538��

�0.264�

�0.050

0.131

Slope

J�0.027

�0.319��

0.225�

0.303��

0.027

0.204�

0.073

Age

D�0.269�

�0.358��

0.379�

0.587��

0.450��

0.255�

0.477��

�0.143

A�0.210

�0.161

�0.042

�0.024

0.252�

0.652��

0.182

0.029

Age

J0.149

�0.010

�0.031

�0.181

0.027

0.217�

�0.033

0.288��

Relativeradiation

D�0.169

�0.387��

0.762��

0.193

0.170

�0.078

0.210

�0.498��

0.359��

Relative

radiation

A0.669��

0.188

0.871��

�0.756��

0.022

�0.209

0.050

0.312��

�0.163

J0.470��

0.091

0.765��

0.045

0.041

0.192

0.075

0.413��

�0.061

Concave

D�0.042

�0.053

�0.072

0.169

0.295��

�0.045

0.265�

0.045

0.244�

�0.014

A0.124

0.152

0.219

�0.158

0.193

�0.096

�0.061

�0.048

0.099

0.188

Concave

J�0.072

0.023

0.398��

0.328��

0.182

0.023

0.009

0.319��

0.073

0.380��

Convex

D�0.150

�0.056

�0.136

0.08

0.046

0.203

�0.067

0.237�

0.138

�0.074

-A

�0.039

�0.015

0.001

0.036

�0.027

0.019

0.074

0.025

�0.035

0.027

-Convex

J�0.031

�0.027

�0.038

0.036

0.027

0.213�

�0.042

0.108

0.104

0.016

-Flat

D0.181

0.102

0.196

�0.233�

�0.319��

�0.150

�0.183

�0.265�

�0.352�

0.108

--

A�0.101

�0.150

�0.233

0.140

�0.184

0.088

0.008

0.032

�0.078

0.178

--

Flat

J0.080

�0.010

�0.354��

�0.320��

�0.181

�0.110

0.009

�0.341��

�0.111

�0.360��

--

1144 Hofgaard, A. et al.