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Elevational distribution limits of non-native species:combining observational and experimental evidenceSylvia Haider a , Jake M. Alexander b & Christoph Kueffer ba Department für Ökologie und Ökosystemmanagement, Lehrstuhl fürRenaturierungsökologie, Technische Universität München, Freising, Deutschlandb Institute of Integrative Biology, Plant Ecology Group, ETH Zurich, Zurich, Switzerland

Accepted author version posted online: 09 Nov 2011. Version of record first published:18 Apr 2012

To cite this article: Sylvia Haider, Jake M. Alexander & Christoph Kueffer (2011): Elevational distribution limits of non-native species: combining observational and experimental evidence, Plant Ecology & Diversity, 4:4, 363-371

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Plant Ecology & DiversityVol. 4, No. 4, December 2011, 363–371

Elevational distribution limits of non-native species: combining observational and experimentalevidence

Sylvia Haidera*, Jake M. Alexanderb and Christoph Kuefferb

aDepartment für Ökologie und Ökosystemmanagement, Lehrstuhl für Renaturierungsökologie, Technische Universität München,Freising, Deutschland; bInstitute of Integrative Biology, Plant Ecology Group, ETH Zurich, Zurich, Switzerland

(Received 25 March 2011; final version received 1 November 2011)

Background: In temperate mountains, most non-native plant species reach their distributional limit somewhere along theelevational gradient. However, it is unclear if growth limitations can explain upper range limits and whether phenotypicplasticity or genetic changes allow species to occupy a broad elevational gradient.Aims: We investigated how non-native plant individuals from different elevations responded to growing season temperatures,which represented conditions at the core and margin of the elevational distributions of the species.Methods: We recorded the occurrence of nine non-native species in the Swiss Alps and subsequently conducted a climatechamber experiment to assess growth rates of plants from different elevations under different temperature treatments.Results: The elevational limit observed in the field was not related to the species’ temperature response in the climate chamberexperiment. Almost all species showed a similar level of reduction in growth rates under lower temperatures independent ofthe upper elevational limit of the species’ distribution. For two species we found indications for genetic differentiation amongplants from different elevations.Conclusions: We conclude that factors other than growing season temperatures, such as extreme events or winter mortality,might shape the elevational limit of non-native species, and that ecological filtering might select for genotypes that arephenotypically plastic.

Keywords: alien species; biological invasions; climate chamber experiment; elevational gradient; European Alps; geneticdifferentiation; growth rates; phenotypic plasticity; plant invasions

Introduction

Determining the factors that limit the spread of non-native plant species is important as a basis for predictingtheir current and future distribution ranges (Richardsonand Pyšek 2006). Climate in particular has been shownto have an important influence on native and non-nativespecies’ growth and reproduction and thus their distribu-tion (Beerling 1993; Thuiller et al. 2006; Ross et al. 2008).In the face of climate change, it is becoming increasinglyimportant to understand the role of climatic factors in limit-ing non-native species (Walther et al. 2009). Here, we focuson mountains, where generally fewer non-native plants arepresent. However, the risk of plant invasions into moun-tain ecosystems may increase under warmer conditions(Walther et al. 2002; Pauchard et al. 2009).

Mountains are a particularly good model system toanalyse the effects of climatic factors on species rangelimits, since they offer steep climatic gradients over veryshort distances (Peterson et al. 1997; Dietz and Edwards2006; Pauchard et al. 2009). The elevational limits of non-native species in temperate regions, and in particular in theEuropean Alps, are often very well defined (Becker et al.2005; Seipel et al. 2011), with species consistently presentup to a certain elevation but rarely found above that ele-vation. Accordingly, statistical models have found a strongcorrelation between elevational limits and climate in the

*Corresponding author. Email: sylvia.haider@wzw.tum.de

European Alps (e.g. Marini et al. 2009; Petitpierre et al.2010). These patterns suggest that, for most non-nativespecies, the margin of their climatic niche is close to thehighest elevation where they currently occur. This interpre-tation is also supported by other evidence: the elevationallimits of a number of species have been shown to be at sim-ilar thermal thresholds in mountain areas in their introducedand native ranges (Alexander et al. 2009b), and the distribu-tion limits of non-native species can remain constant overtime, as was found in the Swiss Alps between 2003 and2009 (T. Seipel, C. Kueffer, J. Alexander and P. Edwards,unpublished data).

Until now, only few studies have experimentally relatedgrowth or reproduction of non-native plants to their distri-bution limits along elevational gradients (e.g. Monty andMahy 2009; Trtikova et al. 2010). In this study, we inves-tigate how growing season temperatures affect the growthrates (as an estimate of biomass production) of several non-native herbaceous species, and if these growth responsesare related to the elevational limits of non-native species.Growth rate is commonly used to assess the fitness ofshort-lived plant species because there is a linear relation-ship between vegetative growth and reproductive output(Weiner et al. 2009). In addition, sufficiently rapid growthis necessary to enable annual plants to flower and pro-duce seeds before the end of their first growing season

ISSN 1755-0874 print/ISSN 1755-1668 online© 2012 Botanical Society of Scotland and Taylor & Francishttp://dx.doi.org/10.1080/17550874.2011.637973http://www.tandfonline.com

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(Olsson and Ågren 2002) and thus establish self-sustainingpopulations.

Most non-native species in mountains first arrive atlow elevations, and this lowland non-native species pool isthen successively filtered as species spread towards higherelevations (Alexander et al. 2011). Thus, to reach highelevations species need either a broad environmental tol-erance or they need to change genetically. Indeed, invasionsuccess has been attributed to a species’ ability to adaptrapidly in response to selective pressures in the new range(Lee 2002), and a number of studies have reported differ-entiation in morphology, physiology and phenology amongindividuals from different latitudes grown under commonenvironmental conditions, which is interpreted as an indi-cation of genetic differentiation (e.g. Weber and Schmid1998 for two non-native Solidago species). Such putativelyadaptive responses can include increased phenotypic plas-ticity, which may also support invasion success by enablingpopulations to rapidly respond to changing environmen-tal conditions (Richards et al. 2006). However, the relativeimportance of genetic differentiation and phenotypic plas-ticity along elevational gradients and thus their relevancefor shaping non-native species distribution patterns is lessclear (Alexander et al. 2009a), and evidence has beenreported for both types of response (e.g. Parker et al. 2003for phenotypic plasticity and Monty and Mahy 2009 forgenetic clines).

Here we investigate if species’ growth responses togrowing season temperature can provide an explanationfor the limits of non-native species at high elevation,and whether non-native species respond to broad temper-ature gradients from low-elevation introduction sites tohigh elevations through phenotypic plasticity or geneticdifferentiation.

We recorded the occurrence of nine non-native herba-ceous species along several mountain pass roads in theSwiss Alps (Canton Valais) and subsequently tested thegrowth response of these species in a climate cham-ber experiment. For the latter we compared plant growth(biomass production per day) between low-elevation plantsand plants from the high-elevation limit under low andhigh-elevation temperature conditions during the growingseason. Our main research questions were: (1) To whatextent is the growth of non-native plants affected by grow-ing season temperatures at their elevational range margins?(2) Is there evidence for genetic differentiation in growthresponses between plants from different elevations?

Methods

Species

We included nine herbaceous species that are non-nativeto the Canton Valais in Switzerland and represent a rangeof families, life histories, bioclimatic origins, introductiontimes and elevational limits (Table 1). Each species was suf-ficiently abundant in the field so that seed material for theexperiment could be sampled from a number of individualsand populations.

Field data and seed collection

The field survey and seed collection were conductedbetween June and August 2006 in the central EuropeanAlps, in canton Valais in southern Switzerland (46◦ 10’ N,7◦ 20’ E). The Valais is characterised by a temperate andcontinental climate and is densely populated with industry,agriculture, tourism and settlements, especially at the bot-tom of the valley (for details see Alexander et al. 2009a).The occurrence of the nine species was recorded alongtwo mountain roads (elevational transects; Sion to Arollaand Martigny to the Grand-St-Bernard pass) ranging from500–2000 m a.s.l. We recorded the presence–absence ofthe study species on both sides of the road at 100 melevational intervals (i.e. at 16 elevations) in 2 m × 250 mplots that were directly adjacent to the road (longer sideparallel to the road). The survey was repeated in Augustand September 2008 to account for differences betweenyears and to increase the chance of recording species withboth early and late phenologies. Data from both years andfrom both roadsides were pooled, i.e. a species was con-sidered present if it was recorded at least in one year inone of the two plots of the same elevation in one road.In sum, our survey included 32 sampling sites (16 sam-pled elevations along two roads). In addition, we extracteddata from Becker et al. (2005) for 13 sampling sites at rail-way stations along four elevational transects (Monthey toChampéry, Hohtenn to Goppenstein, Visp to Ausserbergand Visp to Zermatt) which were situated in the same cli-matic region with a maximum distance of 50 km to ourown sampled elevational transects. These additional sitescovered an elevational range from 405–1620 m a.s.l. withslightly fewer sites above 1200 m a.s.l. In total we used datafrom 45 sampling sites.

Seeds were collected from low-elevation populations atthe valley bottom (ca. 500 m a.s.l.) and from high-elevationpopulations at ca. 1400 m a.s.l. (hereafter elevational ori-gins), which was for most of the species the upperelevational limit where they were sufficiently abundant tocollect seed from replicated populations. Seeds were typ-ically collected from several individuals of multiple wildpopulations (for both elevational origins: one to five pop-ulations per species, mean: four populations). The seedswere bulked by species and elevation.

Climate chamber experiment

Seed were germinated in October 2006 on fine compostin a greenhouse, located at the Technische UniversitätMünchen in Freising-Weihenstephan, and maintainedat a constant temperature of 22 ◦C. Fifteen days aftersowing, we transplanted the seedlings individually into14 cm pots containing 2 L of nutrient-poor garden soiland 2 mm sand (1:1 mixture by volume). We balancednitrogen (N), phosphorus (P) and potassium (K) and keptit at a constant moderate level (each with ca. 100 mg/L)during the experiment. The plants were kept for a further19 days in the greenhouse (without supplementary light),by which time they had developed their first true leaves,

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Elevational distribution limits of non-native species 365

Table 1. Species included in the study and their family, year of introduction in the Swiss Canton Valais, bioclimatic (temp, tem-perate; Med, Mediterranean) and geographic origin, longevity (ann, annual; perenn, perennial), breeding system (in, inbreeding; out,out-crossing), ploidy level (di, diploid; poly, polyploid) and the upper elevational limit in our data set and according to the literature(700–1500 m a.s.l. = montane, 1500–2300 m a.s.l. = subalpine).

Taxon FamilyYear of

introductionBioclimatic

originGeographic

origin1 Longevity2Breeding2

systemPloidy2

level

Elev. limit inour data set

(m a.s.l.)

Max. elev.literature3,7

(m a.s.l.)

Bromusinermis

Poaceae 19204 temp Eurasia, Asia,N. America

perenn out poly 2000 1500 − 2300

Buniasorientalis

Brassicaceae 19385 temp Eurasia, Asia perenn in di 1500 700 − 1500

Centranthusruber

Valerianaceae 19186 Med Eurasia perenn out poly 1077 700 − 1500

Conyzacanadensis

Asteraceae 17505 temp Americas ann in di 1600 1500 − 2300

Daucus carota Apiaceae < 14927 Med Eurasia, Asia perenn out di 2000 1500 − 2300Eragrostis

minorPoaceae < 14927 Med Eurasia, Asia ann out poly 1200 700 − 1500

Lepidiumvirginicum

Brassicaceae 19068 temp Americas ann in poly 1445 700 − 1500

Sonchusoleraceus

Asteraceae < 14927 Med Eurasia, Asia ann in poly 1600 1500 − 2300

Trifoliumhybridum

Fabaceae > 14927 temp Americas perenn out di 2000 1500 − 2300

1USDA, ARS, National Genetic Resources Program (online database); 2Klotz et al. (2004); 3Aeschimann et al. (2004); 4Conert et al. (1998); 5Waldis(1987); 6Hegi (1918); 7Lauber and Wagner (2001); 8Thellung (1906).

before they were moved into climate chambers (one foreach temperature treatment) also located in Freising. Afteranother 14 days during which conditions were maintainedas in the greenhouse to enable plants to acclimatise to thenew environment, the temperature treatments were started.We used monthly mean values of June, which were chosento simulate temperature and humidity conditions duringthe growing season at low (ca. 500 m a.s.l.) and high ele-vation (ca. 1400 m a.s.l.); these are referred to here as thehigh-temperature (at low elevation) and low-temperature(at high elevation) treatments. The daily temperaturesum was 453 degree hours (18.9 ◦C daily mean temper-ature; 12.5–25.0 ◦C daily temperature range; 50–80%relative humidity) for the high-temperature treatmentand 254 degree hours (10.6 ◦C daily mean temperature;6.0–16.0 ◦C daily temperature range; 70–80% relativehumidity) for the low-temperature treatment. Data werebased on temperature values from MeteoSwiss, a climatemodel developed at the University of Lausanne and temper-ature loggers in the field. The day length in all temperaturetreatments was 14.5 h. Plants were watered daily as needed.Within the climate chambers we arranged plants accordingto a randomised block design with each block containingone plant per elevational origin per species. We changedthe arrangement of plants within each block during theexperiment to reduce differences in growth resulting fromdifferent locations in the climate chamber or neighbouringplants.

With the exception of Bunias orientalis (three missingplants of low elevational origin under high-temperaturetreatment) and Trifolium hybridum (two missing plants

of high elevational origin under high-temperature treat-ment) we had six replicates per species, temperaturetreatment, and elevational origin, giving a total of 211individuals (9 species × 2 temperature treatments × 2elevational origins × 6 replicates, less five missingplants).

Low numbers of replicates did not allow an initial har-vest to destructively measure the initial biomass. Thus,with the exception of Eragrostis minor, initial size of eachplant was estimated by counting the initial number ofleaves (relation of final leaf number to biomass at har-vest for all species together except E. minor: R2 = 0.24,P < 0.001). Since all E. minor plants had one leaf at thebeginning of the climate simulation, we instead used initialheight for this species (relation of final height to biomassat harvest for E. minor: R2 = 0.67, P < 0.001). At thebeginning of the experiment all plants had similar sizesand grew substantially during the experiment. Therefore,although initial biomass could not be measured, we donot believe that it introduced any systematic bias to theresults.

The time under temperature treatments were speciesspecific (according to a species’ phenology and also forpractical reasons) and ranged from 51–58 days. All plantsof a given species were harvested at the same time, withone exception: Sonchus oleraceus was under the low-temperature treatment for 4 days longer. Harvest wasmade either at the onset of flowering or, for those thatdid not flower, at the latest after 58 days under tempera-ture treatment. The above-ground biomass of each plantwas determined after drying at 70 ◦C for 48 h. These

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data were then converted to mean biomass productionper day over the time under temperature treatment of theplant.

Statistical analyses

All statistical analyses were made with R (version 2.10.1 forWindows, The R Foundation for Statistical Computing).Based on the presence–absence data for each species,we used generalised linear models (glm) with a bino-mial error and logit link to calculate its probability ofoccurrence along the elevational gradient. To evaluate theeffects of temperature treatment and elevational origin onmean biomass production per day under temperature treat-ment, linear mixed-effects models (function lmer in theR-package lme4) were used separately for each species,including temperature treatment and elevational origin asfixed effects and block as a random effect. The models werefitted without the interaction term, because models includ-ing the interaction of the two main effects showed that theinteraction was only significant for two species (Table 2).The initial leaf number and the initial height, respectively,were included as a covariate to account for differences inplant size before the beginning of the temperature treat-ment (cf. Rose et al. 2009). Model-checking plots wereinspected to ensure that model assumptions were met andno transformations were necessary. The response variablewas standardized (z-transformation) to allow direct com-parisons of coefficients between species. The coefficientsfor the temperature treatment and elevational origin effectswere extracted from the mixed models for each species andrepresent the magnitude and direction of the phenotypic andgenetic component of a species’ growth (hereafter growthresponse). For example, positive coefficients indicate thatplants grown under the high-temperature treatment, orcollected from low elevation, grew faster.

Results

Elevational limits

Of the nine sampled species, four species (Conyzacanadensis, Eragrostis minor, Lepidium virginicum andSonchus oleraceus) have their elevational limit between1200 and 1600 m a.s.l. (Table 1; Figure 1). Centranthusruber and Bunias orientalis were found only in few sites,but also occurred only up to mid-elevations (1077 and1500 m a.s.l., respectively; Figure 1). Three species reachedthe highest sampled elevation of 2000 m a.s.l., but eachwith a distinct distribution pattern: Daucus carota waspresent in almost all sites along the whole elevationalgradient, the occurrence of Bromus inermis slightlydeclined towards the highest elevations, and the occur-rence of Trifolium hybridum increased from low to highelevations.

In our sampled elevational transects, all species flow-ered at their highest site of occurrence with the exceptionof B. inermis and D. carota. Both these species werepresent up to 2000 m a.s.l., but flowered only up to 1900(B. inermis) and 1800 m a.s.l. (D. carota).

Climate chamber experiment

Of the 211 plant individuals included in the experiment,nine plants did not survive. These were eight individuals ofEragrostis minor, of which with one exception all died inthe low-temperature treatment (four plants of low and threeplants of high elevational origin). In addition, one Conyzacanadensis plant of low elevational origin died under thehigh-temperature treatment.

With the exception of Sonchus oleraceus, all speciesreduced their growth significantly at lower temperatures(Figures 2, 3a; Table 2). None of the species apart fromE. minor flowered under the low-temperature treatmentat the point of harvest. In the high-temperature treat-ment all plants of Centranthus ruber, Sonchus oleraceusand Trifolium hybridum (except one individual) of bothelevational origins flowered. For Bromus inermis approx-imately half of the plants of each elevational origin hadflowers under the high-temperature treatment.

There was no consistent difference in mean biomassproduction per day between plants of different elevationalorigins. Trifolium hybridum was the only exception, forwhich low elevational origin plants grew consistentlysmaller than plants of high elevational origin (signifi-cant elevational origin effect; Figures 2, 3b, Table 2).For Daucus carota it was the contrary, and plants ofhigh elevational origin grew consistently smaller than lowelevational origin plants (marginally significant elevationalorigin effect; Figure 3b; Table 2).

For all species, plants of low elevational origin had amore constant growth under the different temperature treat-ments than plants of high elevational origin, indicating amore plastic growth response of high elevational originplants. For Lepidium virginicum and Trifolium hybridumthe slope of high elevational origin plants was significantlysteeper than for low elevational origin plants (significantinteraction of temperature treatment and elevational origin;Figure 2; Table 2).

Neither the upper elevational limit nor the elevationalrange of a species in the field had a relationship with thespecies’ growth response in the climate chamber (linearmodels for temperature treatment effect explained by theupper elevational limit: F1,7 = 0.075, P = 0.792 and bythe elevational range: F1,7 = 0.890, P = 0.377; elevationalorigin effect explained by the upper elevational limit:F1,7 = 0.254, P = 0.630 and by the elevational range:F1,7 = 0.034, P = 0.859).

Discussion

We found that in the field study most species reached a well-defined elevational limit, i.e. they were consistently presentup to a certain elevation but rarely found above that ele-vation, whilst in the climate chamber experiment all butone species showed a general reduction in growth rates inresponse to the low-temperature treatment. However, therewas no relationship between the temperature response andthe elevational limit of a species. In fact, all species reducedtheir growth rate to the same extent between the high- andlow-temperature treatments. Even species for which the

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Elevational distribution limits of non-native species 367

Table 2. Results of the mixed effects models with temperature treatment (high- or low-temperature treatment) and elevational origin(low or high elevational origin) as main effects, block as random effect and initial number of leaves (for Eragrostis minor initial height,respectively) as covariate to control for differences in growth before the beginning of the temperature treatments. Additionally P andF-values are given for the same model fitted with the interaction of temperature treatment and elevational origin. Positive effects indicatefaster growth under the high-temperature treatment and for plants of the low elevational origin, respectively. n = 24 for all species apartfrom Bunias orientalis (n = 21) and Trifolium hybridum (n = 22). Numerator degrees of freedom is 1 for both main effects and theinteraction. Significant effects are highlighted in bold.

Temperature treatment Elevational originTemp. treat. × Elev.

orig.

Effect P F Effect P F P F

Bromus inermis 0.806 0.022 3.811 0.307 0.329 2.771 0.930 0.013Bunias orientalis 1.893 <0.001 19.570 0.662 0.344 1.358 0.086 3.724Centranthus ruber 1.498 <0.001 23.956 −0.004 0.997 <0.001 0.161 2.926Conyza canadensis 0.878 0.016 6.165 0.377 0.285 1.269 0.647 0.222Daucus carota 1.381 <0.001 28.528 0.522 0.054 4.076 0.431 0.626Eragrostis minor 1.428 <0.001 22.384 −0.421 0.094 5.132 0.065 11.439Lepidium virginicum 1.228 0.003 13.323 −0.451 0.190 1.817 0.016 8.364Sonchus oleraceus −1.385 <0.001 18.836 0.274 0.401 0.772 0.241 1.614Trifolium hybridum 1.612 <0.001 47.473 −0.631 0.005 11.081 0.039 5.930

Figure 1. Presence and absence of nine non-native herbaceous species along an elevational gradient in the Swiss Alps. Data were gatheredin a field survey and complemented by data from Becker et al. (2005) collected in transects in the study region (Canton Valais, Switzerland);n = 45.

low-temperature treatment represented the elevational limitretained positive growth rates under the low-temperaturetreatment.

Experimental temperature effect independent ofelevational limit

Growth rates are often used as proxies for the fitness of non-native or invasive species under particular environmental

conditions or in comparison with native species (Daehler2003; Schumacher et al. 2008; Funk 2008; Schumacheret al. 2009). To associate growth rates with fitness is jus-tifiable if there is a linear relationship between vegetativegrowth and the production of reproductive biomass as wellas mortality rates. This has been shown, for instance byWeiner et al. (2009), particularly for short-lived, herba-ceous plants (see also Primack 1979; Thompson et al. 1991;Kollmann and Bañuelos 2004). For the spread of normally

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Figure 2. Biomass production per day (z-standardised) under high- and low-temperature treatments (mean +/- SE). Plants of lowelevational origin are represented by the solid line, plants of high elevational origin by the dashed line. All species had a significant climatetreatment effect (P < 0.05; Table 2). Significant effects for elevational origin (E) and the interaction between temperature treatment andelevational origin (T × E) are indicated. Significant main effects refer to the models fitted without the interaction term.

Figure 3. Effect sizes and 95% confidence intervals of each species for (a) temperature treatment and (b) elevational origin effect inbiomass production per day. An effect is significant (P < 0.05), if the confidence interval does not cross zero (grey horizontal line). Positiveeffects indicate faster growth under the high-temperature treatment and for plants of the low elevational origin, respectively (Table 2).

warm-adapted annual species into cold climates, suffi-ciently rapid growth is needed to complete the life cycleduring a short growing season (Olsson and Ågren 2002;Griffith and Watson 2006; Alexander 2010). In fact, in ourfield survey we found individuals of almost all species flow-ering and fruiting at their distribution limit, which indicatesthat reproductive output may not be the limiting factor.

In the climate chamber experiment, all species apartfrom Sonchus oleraceus reduced their growth significantlyunder the low-temperature treatment, but the magnitude of

the temperature treatment effect could not be related to thespecies’ upper elevational limit in the field survey. Reducedgrowth rates in response to cold temperature may in fact beadaptive, i.e. a phenotypically plastic response rather thana sign of reduced growth capacity due to stress. A numberof authors interpret a plastic response of biomass (amongother parameters) to temperature as an indicator of the phe-notypic plasticity of a species (e.g. Sexton et al. 2002).A reduced growth under lower temperatures has been inter-preted as beneficial because, for instance, it might enable

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Elevational distribution limits of non-native species 369

plants to avoid damage caused by strong wind (Woodward1986), hold their photosynthetic tissues closer to the soilsurface (Fitter and Hay 1987), and be less likely to exhaustthe available resources (Grime and Hunt 1975). A reducedgrowth at cold temperatures has been frequently observedalong elevational as well as latitudinal gradients (e.g. Liet al. 1998; Olsson and Ågren 2002; Willis and Hulme2002; Bastlová et al. 2006).

Alternative explanations for species distribution limitsare extreme events (Parmesan et al. 2000; Jentsch et al.2009), and especially mortality related to extreme events,such as severe frost during winter (Trtikova et al. 2010).The limits might also represent limits to dispersal duringexpansive spread. However, the non-native species in ourstudy, which have an elevational limit at mid-elevations,have been present in the Valais for at least 70 years(Table 1). So we assume that there has been enough time forthem to be transported towards higher elevations, particu-larly because propagule dispersal towards higher elevationsmight have become easier over the last decades because ofincreasing human disturbance and climate warming (Pyšeket al. 2011). We also think that habitat availability is notlikely to be a limiting factor since disturbed, ruderal sitesare not a rare habitat type along roads.

Lack of consistent genetic differentiation

We asked whether phenotypic plasticity or genetic differen-tiation was more important for the species’ growth responseto changing environmental conditions along the elevationalgradient. For the majority of species we did not find indica-tions of differentiation with regard to growth rates betweenplants from different elevations, which is in line with pre-vious studies performed in the Swiss Alps. For instance,Trtikova et al. (2010) found no evidence for adaptationsto elevation for Erigeron annuus in a common gardenexperiment. Also Poll et al. (2009) found no evidence forgenetic adaptations in a transplant experiment along anelevational gradient with multiple non-native Asteraceaespecies.

With our experimental design we cannot exclude thatthere is genetic differentiation at the upper elevational mar-gin of the three species that reached higher elevations than1400 m a.s.l. (Bromus inermis, Daucus carota, Trifoliumhybridum), which corresponds to our low-temperaturetreatment. However, many species show clinal genetic dif-ferentiation along elevational gradients (e.g. Monty andMahy 2009) and it is not necessarily the case that adaptationis more likely at the upper limit of a species than at anyother point along the environmental gradient.

In contrast to these results from the Swiss Alps, geneticadaptation has been found to play an important role in non-native plant expansion along elevational gradients in otherareas (Monty and Mahy 2009; Haider 2011). For instance,in a common garden experiment, Molina-Montenegro et al.(2011) showed that local adaptation to drought is likelyto be the explanation for the successful spread of non-native Taraxacum officinale to high elevations in the Andes.Genetic changes are particularly expected for species with

traits that contain high genetic variation within popula-tions, and recombine existing genetic material from dif-ferent source populations (Sakai et al. 2001; Gilchristand Lee 2007). However, despite including polyploid andout-crossing species, we did not detect any consistentgenetic clines in our study. A possible explanation mightbe that genetic differentiation is hindered through highanthropogenic influence, propagule pressure and gene flow(Kawecki 2008; Alexander and Edwards 2010). In a pre-vious experiment with non-native species on the island ofTenerife, we found a consistent pattern with regard to thedirection and magnitude of genetic differentiations betweenmid-elevation and high-elevation plants, but not betweenplants of mid and low elevation (Haider 2011). The lowerhalf of the elevational gradient in Tenerife is also char-acterised by intense anthropogenic land use (Haider et al.2010). High gene flow due to high anthropogenic activitymay explain the lack of genetic differentiation in the caseof both the lower half of the elevational gradient in Tenerifeand the Valais. Indeed, plants of the invasive Solidagocanadensis in the Valais are not genetically differentiatedby elevation at marker loci (Alexander et al. 2009c).

One of the species for which we found an effectof elevational origin was Trifolium hybridum. The lowergrowth of T. hybridum plants with a low elevational ori-gin needs careful interpretation because this species, aswell as Bromus inermis, has been deliberately sown for ero-sion control at higher elevations (Schmid et al. 2007). Thisis in contrast with most other non-native species that areintroduced at low elevations and spread from there (Beckeret al. 2005; Pauchard et al. 2009; Alexander et al. 2011).However, in the case of T. hybridum, cold-adapted geno-types might have been pre-selected and introduced directlyto high-elevation sites. This might also explain why thisspecies is more abundant at higher elevations.

For all species, the plastic growth response to temper-ature was more pronounced for plants of high elevationalorigin. For the species where this difference was significant(Lepidium virginicum and Trifolium hybridum), plants fromhigh elevations overall had a higher growth rate, while inthe majority of the other species low-elevation plants grewfaster (Figure 2). This suggests that phenotypic plasticityrather than adaptation to local conditions enables speciesto establish at higher elevations, and that ecological filter-ing might select for those genotypes that are phenotypicallymore plastic. This also matches with the assumption thatspecies need to establish under the climatic conditions atlow elevations and spread from there to high elevations(Becker et al. 2005; Alexander et al. 2011; McDougall et al.2011).

Conclusions

Despite well-defined elevational distribution limits of manynon-native plant species in the Swiss Alps, we did notfind that cold growing season temperature at high ele-vation affected species’ growth rates strongly enough toexplain their distribution limits. Factors other than growing

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season climate may be implicated in the well-defined cli-matic limitation of these species, including mortality dueto severe winter conditions or inter-annual variation inextreme climate events.

AcknowledgementsWe thank P.J. Edwards, H. Dietz and L. Trepl for helpful com-ments during the project and four anonymous reviewers for theircomments that improved earlier drafts of the manuscript. We aregrateful to the Grassland Group and the Chair of Plant Nutritionof the Technische Universität München for providing their climatechambers and giving helpful advice. SH was funded by gradu-ate scholar ships from the German Academic Exchange Service,Universität Bayern e. V. and the HWP-program of the TechnischeUniversität München.

Notes on contributorsSylvia Haider is a postdoctoral researcher at the TechnischeUniversität München. Her research focuses on biological inva-sions and she is investigating factors that determine distributionranges and limits of non-native plant species. She is particularlyinterested in the mechanisms that enable non-native plant speciesto respond to variable environmental conditions and to spreadalong broad environmental gradients.

Jake M. Alexander is a postdoctoral researcher at the ETH Zurichwith an interest in evolutionary ecology and biogeography. Hiswork focuses on adaptation of plant species along environmen-tal gradients, as well as the factors limiting plant invasions andspecies range margins, with a particular emphasis on processes inmountains.

Christoph Kueffer is a senior scientist at the Institute of IntegrativeBiology, ETH Zurich, Switzerland. His research interests areinvasion biology, plant functional ecology, restoration ecology,mountains, oceanic islands and transdisciplinary research onanthropogenic ecological change.

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