in montane grasslands, rocky mountain national...

Invasion by Sweet Clover (Melilotus) in Montane

Grasslands, Rocky Mountain National Park

Joy J. Wolf* , Susan W Beatty , and Greg Carey

***

*Depanment of Geography, University of Wisconsin, Parkside

**Department of Geography, University of Colorado, Boulder***Department of Psychology and Institute for Behavioral Genetics , University of Colorado, Boulder

Exotic (nonnative) invasion, often fostered by anthropogenic disturbances, can be detrimental to the biodiversity of

indigenous plant communities. We examined the impact of two exotic species Meliotus offcinalis and M. alba

(yellow and white sweet clover) in the montane grassland community in Rocky Mountain National Park to

determine (1) whether native and exotic plant diversity differs between patches within grasslands that are invaded

by Meliotus species and patches that are not invaded (control patches) and (2) whether the presence of Melilotus

species are associated with specific exotic species. Variables that were recorded included species richness, species

composition, and percent cover in invaded and control patches at multiple spatial scales. The results showed that

more exotic, annuallbiennial, and forb species occurred within patches invaded by Meliotus species, while more

native, perennial, and grass species occurred in nearby control patches. Moreover, the invaded patches had several

exotic species that were not found in control patches , whereas certain native species were only found in control

patches. In some instances, the presence ofMelilotus in the native community appeared unrelated to disturbance;

some colonies were in seemingly undisturbed meadows well beyond disturbance edges. The data also showed thatMeliotus species spread beyond original mapped boundaries in a span of two seasons.

Key Words: Colorado, exotic

invasion, M. alba, Melilotus offcinalis, montane zone grasslands.

he impact of invasion on species richness is fastbecoming a significant component of ecological

research. Exotic invasion, often fostered by

anthropogenic disturbances, can be detrimental to the

biodiversity of indigenous plant communities. As exotic(or nonnative) species spread into undisturbed areas

native plant habitats become fragmented from exoticspecies patches, and ultimately species composition . is

altered (Wiens, Crawford, and Gosz 1985; Vitousek andWalker 1989; Hobbs and Huenneke 1992).

The expansion and dynamics of exotic plants intonative communities are serious threats to plant-communitycomposition and dynamics. Exotic species can havecompetitive characteristics that facilitate invasion intodisrupted native communities (Baker 1974; Bazzaz 1986;Rejmanek and Richardson 1996). These characteristicsallow exotic species to displace natives or occupy newlycreated microsites (Allan 1936; Grubb 1977; Wiens,Crawford, and Gosz 1985; Fox and Fox 1986; Vitouseket al. 1997). When native species cannot survive followinga human-caused disturbance, competitive ruderal exoticspecies invade the plant community, affecting biologicaldiversity (Grubb 1977; Grime 1979; Drake et al. 1989;

Chapin 1991). In addition to influencing the nativespecies diversity, exotic invaders may alter ecosystemprocesses. For instance , individual invaders competing for

resources may grow quickly or invest in nitrogen-fiingmicrobes in anaerobically sealed root nodules. As a resultnatives may be outshaded, and nutrient levels may be

altered (Grime 1979; McGil and Cole 1981; Vitousek

and Walker 1989; Asner and Beatty 1996). Ultimately,possible successional stagnation can have a negative

impact on the abundance of native species (Pickett andMcDonnell 1989; Vitousek, Matson, and Van Cleve 1989;

Wedin and Tilman 1990; McLendon and Redente 1992;Tilman, Wedin, and Knops 1996).

Disturbance regimes are key elements that affect

vegetation patterns and dynamics and maintain hetero-geneity in plant communities (Grime 1979; Pickett andWhite 1985; Fox and Fox 1986). However, native species

displacement can occur when disturbances create condi-tions to which natives are not adapted. As exotic speciescolonize areas of elevated nitrogen levels in agriculture orlower nitrogen levels in disturbed soils, the ecologi-

cal consequence is generalized supercompetition (VanDriesche and Van Driesche 2000). Although exoticspecies colonization is often enhanced in disturbed openspaces (Gleason 1926; Beatty 1991), predicting invasionis a complicated goal. In one study by S. W. Beatty andD. L. Licari (1992), neither disturbance nor soil bound-aries were predictors of the invasion or spread of fennel(Foeniculum vulgare), nor were fennel life-history traits

Annals of the Association of Amerian Geographers 93(3), 2003, pp. 531-543cg 2003 by Association of American GeographersPublished by Blackwell Publishing, 350 Main Street, Malden, MA 02148 , and 9600 Garsington Road, Oxford OX4 2DQ, U.K.

532Wolf, Beatty, and Carey

similar to the native species in the community. Fennel'competitive mechanism was its different growing seasoncompared to those of native species (Beatty and Licari1992). In addition, understanding latitudinal range ofspecies can often help researchers predict areas of possiblefuture invasions (Rejmanek and Richardson 1996). Insome cases, a higher resistance to invasion can beassociated with species-rich communities (Elton 1958;

Fox and Fox 1986; Berger 1993; Lodge 1993) or functionalgroup richness, such as forbs, graminoids, and legumes(Symstad 2000). For example, Terr McLendon (1992)found that as the vigor of native grasslands in RockyMountain communities increased in the presence ofCanada thistle (Circium arvense), the invasion becameless significant.

In the absence of human intervention, biological

invasion is considered a natural process (Westman1990). However, anthropogenic activities facilitate exoticspecies establishment and persistence in native commu-nities by providing the propagules of exotic species. Evenactivities such as park visitors who hike along trailscontribute to exotic species establishment; seeds canhitchhike" on clothing or automobile tires and invade

new habitats. Thus, a community that includes anthro-pogenic disturbance-and many exist within nationalpark boundaries-is likely to be invaded by exotic species(Fox and Fox 1986; Pimm 1989).

Yellow and white sweet clover (Meliotus offcinalis andM. alba), the focus of this study, have competitive traitsin native communities. These species were introducedinto grassland communities in the mid-1800s, prior tothe establishment of Rocky Mountain National Park, tostabilize soil following human disturbance such as roadconstruction. As biennial legumes (Fabaceae) that expendenergy toward producing a long taproot during the firstyear, these species produce many seeds, fi nitrogen fromroot nodules, and have a long growing season. In thisstudy, yellow sweet clover was flowering May throughOctober and white sweet clover was flowering Junethrough October. This means that, although the otherspecies in our study had a shorter growing season (eitherearly summer or late summer), Meliotus species werepresent throughout the entire growing period. To furthercompete with other speciesMelilotus can develop dense

patches and grow to 90-300 cm in height. In our sites,Melilotus ranged from 120 cm in sites near Pinus ponderosato 300 cm in sites located in open grassland. Meliotus canalso harm foragers, due to their production of coumarin,which causes anticoagulation in blood: organisms mayexperience bloat when sweet clover is ingested (Duke1981; Spellenberg 1994; Burril et al. 1996). Thus,

Melilotus species may have the ability to invade undis-

turbed native grasslands and alter species richnessand composition.

According to the 1996 Colorado Noxious Weed Act, anoxious weed is an alien plant, as designated by the localadvisory board, and has the following criteria; aggressivelyinvades or is detrimental to economic crops or nativecommunities; is poisonous to livestock; is a carrier ofdetrimental insects, diseases, or parasites; or is detrimentalto the environmentally sound management of natural oragricultural ecosystems. Melilotus species have some ofthese characteristics, but they are not on the noxious weedlist, partly because the species is not perceived as anaggressive invader and partly because it is stil used in thehoney and agriculture industries.

This article focuses on the impact that Melilotus speciesmay have on invaded plant communities within RockyMountain National Park. Specifically, we address thefollowing questions to determine if Melilotus has an

influence on species composition (total species, uniquespecies, native/exotic richness) and an association withspecific exotic plant invasions: (1) Is there a difference inthe total number of species between invaded and control(Le., not invaded by Meliotus) patches? (2) Is there adifference in species composition: that is, is there a highernumber (abundance) and density (percent foliar cover) ofexotic versus native species, forb versus grass species,and annual versus perennial species in invaded patchescompared to the noninvaded community in controlpatches? (3) Do patches invaded by Meliotus species haveexotic species that are not found in control patches?Although exotic species are found in both invaded andcontrol patches, our aim is to determine whether speciescomposition of both exotic and native species are differentin the presence of Melilotus species. This association withunique exotic species may imply that Meliotus species canfacilitate other exotic invasions. Our study contributes topark management in decisions that focus on understand-ing exotic invasion in montane grasslands.

Study Area

The research was conducted in the montane zone ofRocky Mountain National Park in Colorado, U.S.(Figure 1). Rocky Mountain National Park was estab-lished on 26 January 1915 with the goal of maintainingpristine ecosystems for the benefit and enjoyment ofvisitors. Although human activity has a significant impact,

the National Park Service has historically strived tomaintain its land in its natural condition. Hence,

introduction, dispersal, and spread of nonnative speci

are a critical concern. One of the highest biodiversityzones in Rocky Mountain National Park, the montane

Invasion by Sweet Clover (Meliotus) in Montane Grasslands

10S' S8'W

2S'W

RockyMountainNational

ParkAlluvialFan

AspenglenCampground

HorseshoePark

Research Bldg./

Visitor Center'/' ,

Moraine .Park

Estes Park

40' 19'W

10 Kilometers

SMiles. Study Site

533

Larmer Co.

109' OO'W41' OO'N

Colorado, U.

Figure 1. Study-site locations within the eastern part of Rocky Mountain National Park, near Estes Park. Black dots represent study sites at whichMelilotus-invaded patches were located. The upper inset shows the Rocky Mountain National Park boundary area map and surrounding counties.The lower inset shows the park boundary within Colorado.

grasslands, is threatened by invading plant species due tofavorable growing conditions and relatively high humanimpact (Rapport 1995; Stohlgren, Bull, and Otsuki 1998).Montane grasslands have warm moist growing seasons anddry, well-drained soils, and include a variety of nativegrasses and forbs that are surrounded by conifer forests.This montane life-zone has an average precipitation of 54cm, with the highest rainfall occurring in July and August.

In this montane region, human-caused disturbancesinclude the accommodations that the park provides forits visitors, such as trails, roads, buildings , picnic areas, andcampsites. The park is located next to the gatewaycommunity of Estes Park, and visitation to the parkexceeded 3.1 milion in 1997. Evidence of early home-steading and grazing activities since the mid- 1800s stilexist within the park boundaries (Buchholtz 1983).

Nonnative plants initially became established in thepark in the late nineteenth century, when settlers expandedto the West. Of the over 1,000 documented species in thepark, 120 are nonnative (Hogan 1993). In this study,Melilotus species were located in disturbed sites in the

montane zone along roadsides and highly used trails andnear construction; however, they can exist in a variety ofecosystems (Cole 1991). Melilotus species, which produce apleasant scent resembling that of freshly mown hay, aredrought-resistant, yet they persist in high rainfall areas.

They grow well on oil shale and on bare mineral soil, wherethe seeds are scarifed for germination. However, seeds canalso be scaried by traveling through the digestive tract of aforaging animal, or by freezing and thawing of soil.

Methods

Sampling Design

To determine the effects of nonnative plant species onnative community structure , we compared patches in-vaded by Melilotus species to adjacent native communitiesthat were not invaded. We located twelve independentstudy sites within the eastern montane zone of the parkbetween 2 316 and 2,743 m, that had patches ofMelilotusspecies in May 1998 (Figure 1). Sites were selected based


on park-manager knowledge of prior invasion before 1998,and each site was located by disturbances such as trailsroads, utility corridors, and filled ditches. Given thesimilar surrounding environmental data, known history ofthe overlying disturbance, and similar soil texture and soilorganic matter, we adopted a chronosequence researchdesign to minimize variability between sites.

At each site, we permanently marked the patchinvaded by Meliotus species and measured the area ofthe patch. Within the same disturbance influence, a

control (noninvaded) patch of identical size. was alsomarked at least 12 meters away. The patch size ranged

from 25 m2 to 75 m. The total patch area was measuredmonthly from May through October to determine rate ofexpansion. At each site, elevation, slope, aspect, distur-bance type, and global positioning system (GPS) coordi-nates were recorded.

To accurately estimate species cover and compositionwithin a patch, six plots were randomly located and stakedin each patch. Each plot size was at least 1 m from theperieter of the patch. Three plots were 0.25 m x 0.5 m,

and three were 0.5 m x 1 m (Figure 2). Each plot was

divided into fifty equal boxes, to allow a more objective wayto measure species density and bare ground, because eachbox represented 2 percent ground cover. Thus, a count ofthree boxes of exotic species means that 6 percent of theground cover within the plot represented exotic species.

Boxes that were half or less filled got a designation ofO.5 box

or 1 percent. The design was nested: patch (invaded versuscontrol) was nested with site. Two plot sizes were used to testfor scale-dependent signatures of the vegetation patternsand determine the highest species richness and diversityrepresented in each patch (Stohlgren, Falkner, and Schell1995). In addition, we placed four large plots along the edges

and 1.5 m outside of invaded patch boundaries, to

Invaded Patch

..

1.5 metersNoninvaded Patch

.-.

+- 1.5 meters

Figure 2. Experimental design of a study site. Small plots (0.25 x 0.m) and large plots (0.5 x 1 m) were randomly staked inside invadedpatches; large plots were placed at the edges and outside of invadedpatches. Species data (e., composition, percent cover, height) werecollected from plots. Patch size ranged from 25 m2 to 75 m

determine if there were diferences in the number of speciesand vegetated percent cover along the edge of Melilotus

invasions and from inside invaded patches to outside them.

Species, Composition Observation

To determine the impact of Meliotus species on nativegrasslands, we compared the species composition andvegetated ground cover between invaded and controlpatches. The vegetation data recorded from each plotincluded species composition, vegetation, and bareground percent cover, flowering. stage, and whether theplant was an exotic or native, a grass or forb, and anannuallbiennial or perennial species. At the beginning ofthe study in May 1998, each invaded patch had anoticeable boundary of invasion. Although the originalpatch was probably following the patterns of pastdisturbance (Turner and Gardner 1991; Veblen et al.1994), the Melilotus patches may have expanded pastthese distinct boundaries.

We recorded the vegetation data during the growingseasons of June, July, and August in 1998 and 1999, todescribe temporal andspatial patterns. Because our 1999data confirmed the results of the 1998 data, only 1998 data

. are ilustrated in the tables and figures.

Data Analysis

ANOV A was used to analyze those variables measuredonly once, while a repeated-measures ANOV A was used

to test the means of variables measured at three sampledates (SAS Institute 1989). Vegetation variables analyzed

to describe species composition included total numberof species and abundance of exotic, native, grass, forb,

annuallbiennial, and perennial species. Abundance wasdetermined from the number of species of a variable ineach patch. Percent foliar cover variables included baresoil, exotic, native, grass, forb, annual/biennial, andperennial cover. Data were inspected for normality withthe Shapiro-Wilk statistic (SAS Institute 1989). In therepeated measures, a polynomial contrast was used to

assess the linear and quadratic changes over time. Thistechnique accounts for temporal autocorrelation that mayexist in data recorded at three times in the growing season

(10-14 June, 11-15 July, 12-16 August) and spatial

autocorrelation that may exist in the edge plots. Condi-

tions for type H matrix were met for all repeated measures

(Johnson 1998). In all analysesMeliotus species were

controlled for regarding exotic, forb, or annual/biennial data.Principal components analysis (PCA) was used to exam-ine species-composition similarities. In addition to prin-cipal components, we also examined multidimensional

Invasion by Sweet Clover (Meliotus) in Montane Grasslands

scaling (MDS) solutions. The MDS results were similarto those from the PCA and hence are not presented

here. Other studies using similar statistical models havebeen successful in determining differences in native andnonnative vegetation structure (Gauch 1982; Horvitzetal.1998).

Results

Total Richness, Native and Exotic Species

Using repeated measures, we found that total species

richness in both patches changed significantly throughoutthe season; the changes from June to July (peak growingseason) and from July to August were significant (n = 132

p': 0.002). However, overall, the total species richnessand bare soil did not differ between patches (5.24-6.45,

Total Number of Species

'* 0June July August

Exotic Species

&J 2

June July August

Native Species

&J 400 2

'* 0

June July August

Total Bare (Unvegetated) Ground

1) 10

rf 5

June July August

535

p': 0.07; see Figure 3A, D). For most variables , July had ahigher number of species and percent cover compared toJune and August. No differences were found betweensmaller versus larger plot sizes.

With Meliotus species removed from the analyses , ourresults showed significantly more exotic species in invadedpatches (Figure 3B), and significantly more native speciesin control patches (Figure 3C; = 132, p.:0.001);specifically, 3.29 (4.39 in 1999) natives in invaded patchescompared to 4.55 (5.44 in 1999) in control sites , and 2.

(2.44 in 1999) exotics in invaded patches compared to1.27 (0.89 in 1999) in control sites. Similar results were

found for percent cover for exotic and native species(n = 132 , p.: 0.001): there was higher exotic species coverin invaded patches (46-57 percent exotic cover versus17-28 percent native cover) and higher native species

cover in control patches (36-50 percent native cover

Grass Species

u 4

00 2

June July August

Forb Species

u 4

00 2

'* 0

June July August

Annual/Biennial Species

u 2

00 1

June July August

Perennial Species

00 2

'* 0

June July August

Figure 3. Mean number of species (mean + 1 SE) in plant community and bare soil in invaded and control patches throughout growing season in

1998. Black bars represent invaded patches, and white bars represent control patches. Percent cover = number of boxes covered in a plot: each box

represents 2 percent cover in a plot (total plot area to represent each patch = 5,250 cm ). n = 108 for bare ground; n = 132 for other variables.

Melilotus taken out of exotic, annuallbiennial, and forb analysis. Error bars represent + 1 pooled SE of the mean estimate. Total number of species(A) and bare soil (D) were not significant between patches (p :;0. 17). Exotic (B), annuallbiennial (G), and forb (F) species abundance was higherin invaded patches (p 001), native species (C) (p 007) and perennial (H) species abundance was significantly higher in control patches

(p

05) throughout the season, and grass species (E) was significantly higher in control patches in June and August (p 05).


versus 15-22 percent exotic cover). Whereas nativespecies richness changed over time (p.. 0.008) (Le., anincrease in July and a decrease in August), the number ofexotic species remained the same over the season(p.. 0.08) and did not have a significant linear or

quadratic change over time. In fact, a statistically higherpercent (p.. 0.05) cover of exotic species in invadedpatches was present for all sample dates. These temporaleffects were not due to bare soil, because the amount ofbare soil changed proportionally to vegetation cover(Figure 3D).

Grass and Forb Species

The number and percent cover of grasses and forbs

differed in Melilotus patches compared to control patchesthroughout the growing season. In both invaded andcontrol patches, we found a greater diversity of forbcompared to grass species overall. However, forb richness

was higher in invaded patches (n = 132, p": 0.02) at allsample dates (Figure 3F), even though grass richness was

higher in control patches in June and August (p.. 0.Figure 3E).

Overall, a statistically higher percent cover of grass

species was present in control patches compared toinvaded patches (54 percent compared to 28 percent in1998, and 38 percent compared to 17 percent in 1999).There was also a higher percent cover of forb species ininvaded patches compared to control patches (60 percentcompared to 32 percent in 1998 and 38 percent comparedto 14 percent in 1999). A significant interaction existedbetween time and percent cover for both grasses and forbs

that increased the percent cover in July compared to Juneand August.

Annual/Biennial and Perennial Species

To analyze annual and biennial species, annuals andbiennials were grouped together. Overall, in all patchesthere were more perennials than annuals/biennials,although a greater number of annuals/biennials werepresent in invaded patches, while a greater number ofperennials were present in control patches (p.. 0.00 1)

(Figure 3G, H). The ,ratio of perennials to annuals/biennials was significantly lower (p.. 0.05) in invadedpatches. This ratio was between 1.0 and 2.0 in invadedpatches, but was considerably greater than 2.0 incontrol patches at all sample dates. The higher ratio wasdue to a higher number of perennials and a lower numberof annuals/biennials in the control patches. Thus, itappeared that the effect of invasion lowered the ratio due

to a decreased number of perennials and an increasednumber of annuals/biennials in invaded patches.

An interaction occurred between the richness ofannuals/biennials between time and type of patch(F(18,180) = 3.45, p..O.OOOl) compared to perennialspecies. Whereas annual/biennial richness in controlpatches increased in July, compared to June, and thendecreased in August, the richness of annuals/biennials ininvaded patches did not change over time, creating alarger difference between patches at the end of the season(Figure '3G). Following the same trend as annuals/biennials in control patches, perennial species richness

increased in July and decreased in August in both invadedand control patches (Figure 3H). Also, perennial species

were significantly lower in richness and percent cover ininvaded patches compared to annual/biennial species.

To detect a relationship between exotics and annuals/biennials, a linear regression analysis was employedfor invaded and control. patches. The number of exoticspecies was positively correlated with the number annual/biennial species, = 132 (adjusted R2: 0.20,

F = 33.49, p..0.0001); however, the number of nativespecies showed a weak negative association with thenumber of annual/biennial species (adjusted R2: 0.03,

F = 5., p..0.02).

Principal Components Analysis

A principal components analysis (PCA) was performedusing all the species composition variables (exotic andnative species, grass and forb species, annual/biennial andperennial species, and bare ground cover for all threesample dates) to detect collinearity between these vari-ables (Table 1). The PCA was performed as a conservativemeasure for the large number of statistical tests conductedabove on correlated variables. Undoubtedly, some

reached significance by chance. To examine which aspectsof the data were robust and to control for correlationamong the variables, the species composition variableswere reduced using principal components. Four unrotatedprincipal components were extracted with eigenvaluesgreater than 1.0; together, they explained 80 percent ofthe variance. The first principal component (whichaccounted for 43 percent of the variance) was a measureof total species richness over time. The second factor (25percent) had high loadings on the number of exotic species

and annual/biennial species represented in all three

data-collection times. The third principal component(7 percent) was a contrast in grasses and forbs: highpositive loadings on grasses and high negative loading onforbs. The last factor (5 percent) was a clear temporal

component; it had high positive loadings on all types of

Invasion by Sweet Clover (Meliotus) in Montane Grasslands 537

Table 1. Principal Component Analysis Using 94 Observations and 21 Species Variables

Loadings PRINI PRIN2 PRIN3 PRIN4

JuneTotal species richness 288490 079401 - 0. 161435 303237

Grass species 186828 - 0.021432 275256 0.528107

Forb species 259396 115646 - 0,274503 116725

Annual;biennial species 059298 349459 - 0.034121 135844

Perennial species 259345 - 0. 145206 - 0.172567 260307

Native species 262444 - 0. 152784 - 0.266319 207386

Exotic species 012235 360546 131884 175432

JulyTotal species richness 303355 035675 152950 005521

Grass species 223872 - 0.0313 23 0.499338 046680

Forb species 249537 119122 - 0. 196659 - 0.015935

:!;

Annual;biennial species 061578 0.350503 042468 118162,'o

Perennial species 282706 - 0. 150880 083745 - 0,073506

Native species 274821 - 0. 152555 - 0,013444 - 0,010231

Exotic species 062353 0.356468 174111 049478

AugutTotal species richness 274416 135073 053559 - 0.340197

Grass species 179562 - 0. 114211 0.520449 - 0, 153818

Forb species 215484 223165 - 0,257587 - 0.314726

Annual;biennial species 015831 0.362796 - 0.032814 - 0. 103614

Perennial species 268104 - 0. 106057 013223 - 0,306380

Native species 284559 - 0.092669 - 0,055100 - 0,255757

Exotic species - 0.012445 371979 092009 - 0. 150607

Components Scores

Patch Type Variable Mean Std Dev Minimum Maximum

Invaded PRINI 0839047 0509739 - 4,8551640 9077528PRIN2 1.6178035 1.8637385 - 1.6848390 0292198PRIN3 - 0.0275765 1.655189 2512299 5593913PRIN4 - 0.0134759 0703634 - 2.6197864 5308653

Control PRINI - 0.0875527 0021340 - 5,8508411 7213610PRIN2 - 1.6881428 1.574396 - 4.3447575 1.644390PRIN3 0287755 1.725346 - 2, 1592417 8382263PRIN4 0:0140618 1.0778378 1.6760519 0673876

species counts at time one, zero loadings for all measures attime two, and high negative loadings at time three.

We scored the principal components and ran a nestedANOV A on component scores. There were significantdiferences between the invaded and control patches forthe scores on the first two principal components. WithMelilotus controlled for, the invaded patches had moreoverall total number of species (shown in Table 1 asPRIN 1) as well as more exotic and annual/biennial species(shown in Table 1 as PRIN2). There were no significantdifferences on the third and fourth components.

Edge Analysis

In July, we compared species composition and density inlarge plots (50 x 100 cm) in the invaded patches (center

plots), on the edge of these patches (edge plots), and at1.5 m outside ofthe patches (outside plots) (Figure 2). Weused analysis of variance, with the location of plots as themain factor, to analyze species-composition change alongthe transition zone. The variables analyzed included totalspecies richness, and individual richness and groundcover for exotic, native, grass , forb, annual/biennial, and

perennial species. Total species richness and individualrichness and ground cover for grasses, natives, andperennials had the highest values at the patch edges

(Figure 4, F(31,71) = 2.70, p.:0.01). In addition, rich-ness and cover values increased from inside to outside inperennials and natives, but decreased from inside tooutside in Meliotus, exotics, forbs, and annuals/biennials(Figure 5). Tukey s Studentized Range post hoc testsrevealed that species richness and cover values for natives


0 6

en 4

.. 2

TotalSpp NativesExotics GrassesForbs Ann/Bien Perennials

Figure 4. Mean number of species (mean + 1 SE) in plant commu-nity along the transition from inside to outside invaded patchesthroughout growing season. Transition is characterized by plotsinside, on the edge of, and 1.5 m outside invaded patches. n = Error bars represent + 1 pooled SE of the mean estimate. Melilotus was

taken out of exotic, annual/biennial, and forb analysis, Black barsrepresent mean values of species in plots inside invaded patches, greybars represent mean values of species in plots on the edges of invadedpatches, and white bars represent mean values of species in plotsplaced 1.5 m outside invaded patches. Total area that representsinside, edge, and outside for each invaded patch = 3500 cm, Theedges have higher numbers of total species, natives, grasses, andperennials, and exotics, forbs, and annuals/biennials decreased(p.:O.Ol for exotic and annual/biennial species) from inside .thepatch to outside the patch.

and perennial ground cover contained the greatestsignificant (p.: 0.05) change between center and edgeplots (native richness difference between means = 1.39,native ground cover difference between means = 14.40,perennial ground cover difference between means =12.98) whereas exotic, forb, and annual/biennial coverwere significantly different between all combinations ofplots, as was exotic richness. Ground cover of forbs andannual/biennial-species richness were significantly higher

SoilCloverExotic GrassForb Ann/Bien Perennial Native

Figure 5. Mean (+ 1 SE) vegetation cover of species in plantcommunity in plots that are inside, on the edge of, and outsideinvaded patches throughout the growing season. Mean values are foramount of ground cover along the transition from within invadedpatches to 1.5 m outside invaded patches for bare soil, Melilotusspecies, native, exotic, grass, forb, annual/biennial, and perennialspecies, Ground cover = the number of boxes (each plot = fifty equalboxes) that each species covered in a plot, Melilotus taken out of

exotic, annual, and forb analysis. Black bars represent mean values ofboxes in plots inside invaded patches, grey bars represent mean valuesof boxes in plots on the edges of invaded patches, and white barsrepresent mean values of boxes in plots placed 1.5 m outside invadedpatches, Edges show higher groundcover from natives, grasses, andperennials. Meliotus, exotic, forb, and annual/biennial speciesgroundcover decreased from inside to outside invasion (p.: 0.05),By the end of the season, and by year twoMeliotus had spread beyond1.5 m outside invasion in some cases.

inside compared to outside the patches (forb coverdifference = 15.65, annual/biennial richness difference= 1.22). Melilotus cover had significant differences be-tween center and edge plots (difference betweenmeans = 17., p.:0.05) and center and outside plots(difference between means = 17., p.:0.05). The posthoc tests showed that grass cover and perennial richnessincreased from center to edge plots. Also, higher grassperennial, and native richness corresponded to a lowerbare soil cover at the patch edges.

Invasion Spread

All invaded patches spread beyond their originallymapped range at the beginning of the season in May1998. This process was due to germinating seeds from

the previous season. Meliotus species expanded beyondthe originally mapped patch edges an average distance of

8 m from the original boundary by the end of 1998. Atthe beginning of 1999, the patches were stil the same size at the end of 1998; by the end of 1999, they extended anaverage distance of 1.8 m from the original boundary ofeach invaded patch. Both species of Melilotus occupiedinvaded patches throughout the growing season, M,offcinalis was the dominant tye at the beginning of the.season until the end ofJ uly, while M. alba was the dominant

tye at the end of the growing season in late August throughOctober, even though M, offcinalis was also present.

Discussion

Our hypothesis that plant-community compositiondiffered significantly between invaded and control pat-ches was confirmed. Our data did not confirm our firsthypothesis that the total number of species would differbetween patches, however, although the PCA revealedthat the total species richness changed in all patches overtime. Similarities in total species richness between patchesmay be due to the disturbance acting as facilitator forcolonizing species in all patches (Bazzaz 1986). Within thecomposition of exotic and native species between patches,more exotic and fewer native species were found ininvaded patches. Lower native-species richness could bedue to the life-history traits and competitive advantages ofMeliotus-for example, the tall, dense patches ofMelilotusthat exist from May through October, which can locallyshade out and spatially dominate a plant community.Thus, in time, only the hardiest shade-tolerant nativespecies may successfully compete and survive locally inMelilotus patches. In fact, in this study, native species didnot dominate any invaded patches in richness or cover.

Assuming that the invaded and control patches are

Invasion by Sweet Clover (Melilotus) in Montane Grasslands 539

equivalent, species-composition differences between patch-es may be influenced by Melilotus species. The cause andeffect of these dynamics need to be tested, however. Thesedata support the implication that Melilotus may affectspecies composition over time.

The data also confirmed that there were a significantlyhigher number of grass species in control patches and ahigher number of forb species in invaded patches. Themontane grasslands in Rocky Mountain National Parkthat are not invaded by Melilotus species have a naturallyhigh diversity of native grasses and forbs (Stohlgren et al.1999). In some cases , grasses and forbs both dominate thenative ecosystem, although forbs play an important role inthe community structure. The change in the proportion ofgrass and forb species in invaded patches could affectcommunity development in these grasslands , as moreexotic forb species invade anthropogenically disturbedareas within the park. In a study that tested functional

group richness and invisibility, Amy Symstad (2000)found that native functional diversity resulted in loweredsuccess of invasion. In our study, the differences betweengrasses and forbs may indicate that fragmentation is takingplace within the native community.

The fact that Melilotus is not on the noxious-weed listaddresses a core issue in geographic research. Several

factors support the addition of Melilotus to this list. First,the potential harm from coumarin is a risk to wildlife.Second, Melilotus are now found along roads, trails, oldfields, and other anthropogenic ally disturbed areas, sincethey are stil included in mixed seeds and planted for soilstability (Parker, Mertens, and Schemske 1993). ThirdMeliotus species may invade undisturbed native grass-lands and affect species composition. While the RockyMountains National Park is working toward controllingor eliminating nonnative species within its geographicboundaries Meliotus may continue to be seeded alongroadsides near park boundaries. This activity perpetuatesthe spread or reintroduction of Meliotus into disturbed

and undisturbed areas of the park. While Melilotuscontinues to be planted outside park boundaries, manage-ment should focus on disturbances that promote entry,since complete removal is unlikely because, besidescontaminated gravel used in construction, seeds dispersedfrom vehicles, wind, animals, and agriculture activity allcontinue to serve as sources of introduced species (Lodge1993). As a consideration of nondestructive defense,more research is needed to determine the competitiveability of the native flora against invasion.

Some native grass species in the montane zone are cool-season C species , but many of the invading grasses arelower-elevation exotic C grasses. Because C grassesutilize resources more effciently, they can also compete

with C species. In some cases, Melilotus may outcompete

native species. In one study based in Wisconsin, M, albawas negatively correlated with Potentilla and Solidagospecies (Parker, Mertens, and Schemske 1993). Theimplication of this correlation is that these native forbs arein the park and may, in time, also be negatively influencedby Meliotus species. At the time of our study, Solidagomissouriensis and Potentila hippiana were both in invadedpatches , but these species may be remnants within theexpanded range of the invasion.

Our results confirmed the hypothesis that annual!biennial-species richness was different between invadedand control patches; a greater annual/biennial-speciesrichness was found in invaded patches , accompanied byMeliotus invasion. In other studies , exotic invasion wascorrelated with an increased number of annuals/biennials(Pickett and White 1985; Beatty 1991; McLendon andRedente 1992). Our data correspond with other researchthat shows how exotic forbs are typically opportunisticannuals and biennials that thrive in open, disturbed areas.Areas with a high proportion of annuals/biennials aremore vulnerable to invasion, because the invading speciesdo not have to compete with perennials that possess anestablished root system and nutrient reserves.

Significantly higher foliar cover of annual/biennialspecies was found in invaded patches. On the other hand,perennials have a lower seed production and limiteddispersal, so perennials may colonize disturbed sites ata slower rate than do annuals/biennials (Parker, Mertens,and Schemske 1993). A plant community with lowernumbers of perennial species may provide an environmentthat increases community invasiveness. The implicationof an increase in annual/biennial species in invadedpatches is that Melilotus may disrupt or even haltsuccessional development (Bazzaz 1986; Chapin 1991;McLendon and Redente 1992). The ecological structuremay assist in facilitating Meliotus occurrence. For exam-ple, the amount of bare ground was always higher ininvaded patches, although it is not known whether higherbare ground existed at the time ofinvasion. Thus, nativeplant species may have a competitive edge in keepingMelilotus invasion away. More research is needed in thefuture to determine this possibility. Another implication ofexotic grass and forb invasion is that the quality of foragemay change for herbivores, at least locally: when exoticannuals/biennials invade , the montane grassland struc-ture may change to grasses and forbs that may be less.palatable to foraging animals.

The presence of exotic species may facilitate theinvasion of other exotic species, though it will not alwaysdo so. As a result, a heterogeneous mosaic landscapeof native species can include patches of exotic species


(Westman 1990). Indeed, although many native speciesare common to our sites, several exotic species are foundhere as well (Table 2). At the same time, some exoticspecies were present exclusively in invaded patches, whilesome native species were present exclusively in controlpatches (Table 3). Although these species may be presentwithin the larger landscape, this finding presents inter-esting patterns. These patterns may be influenced by thepresence of Melilotus species. More research is needed totest this possible scenario.

As a possible consequence of climate, a wetter growingseason in 1997 may have influenced a higher than normaloutbreak of first-year Meliotus in the park (Jeff Connor,conversation, June 1998) and throughout Colorado,Utah, and Arizona. Because of the biennial nature ofthese species, the second year of their cycle also showedvery high incidence of invasion. As a consequence, theability of Meliotus species to increase beyond disturbedareas may have been facilitated and then maintainedin 1998 and 1999 by favorable climate conditions. Inconjunction with visitor activity, climate increases thechance of new Melilotus introductions and spread. Thusthe perpetuation of Melilotus invasion could significantlyaffect the native-species composition in new places withinthe park (McLendon 1992). Long-term monitoring wilprovide information that wil help determine the criticalrole that climate variability plays in the dynamics ofMeliotus invasion.

For some variables, our data confirmed our finalhypothesis, which stated that a change in species

Table 2. Common Species in Study Sites in the MontaneZone in Rocky Mountain National Park

Grasses

Native Species

Forbs

Muhlenbergi montanaBouteloua gracilis

Stipa comata

Koeleri macranthaPoa agassizensis

Pseudoroegneri sPicata

Penstemon virgatusGrindelia subalpinaHeterotheca villosa

Helianthus pumilusChrysothamnus vicsidijlorusArtemesia frigidaEriogonum umbellatumPotentilla hippianaOenothera coronoPifolia

GrassesBromopsis inerris

Phleum pratenseBromus tectorum

Agopyron smithiiAgopyron cristatum

Exotic Species

ForbsLepidium virginium y virginicumArabis hirsutaAlyssum minus

Achilla millefolium

Table 3. Exotic Species Found Only in Invaded Patchesand Native Species Found Only in Control Patches, by

Scientific Nomenclature and Common Name (in Parentheses)

Exotic SpeciesPresent Only inInvaded Patches

rigeron jlgellari

(whiplash fleabane)Sisymbrium altissimum

(tumble mustard)Rumex cripus (curly dock)Verbascum thapsus

(wooly mullien)

Convolvulus arvensis(morning glory)

Tragopogon porrfolius (salsify)

Agostis stolonifera (redtop)

Native Species Present

Only in Control Patches

Selaginella densa (stiff club moss)

Astragalus f/euoses (milkvetch)

Phacelia hastate (scorpion weed)Gaillardia aristata (blanketflower)

Heterotheca vilosa (golden aster)

Oligosporu pacificus (sagewort)Stipa comata (needle and thread)Stipa viridula (green needlegrass)Geranium richardsonii

(white geranium)

composition would occur along the plots within, on theedge of, and outside the invaded area. The number ofexotic species and annuallbiennial species significantlydecreased from inside to outside the patches (p-cO.01).However, the pattern of change differed for the othervariables. All the other variables had the highest valuesalong the edge of invaded patches, compared to the insideor outside of the patch. The high diversity at the edge ofinvasion may be due to the dynamics of rapid initialcolonization that is often found in postdisturbance sites, toa more heterogeneous microsite, or to a combination ofunique species found on the edge of a patch. In addition,species in disturbed zones in which Melilotus species occurmay be competitive weedy species that displace nativespecies. It is important to note that several observations ofMeliotus in the native community appeared unrelated todisturbance; these patches were in seemingly undisturbedmeadows well beyond disturbance edges. In these cases,wind or animals may disperse the seeds from disturbed toundisturbed areas, and seeds may establish in bare groundwithin the microhabitat. Because Melilotus offcinalis is one

of the first species growing in early spring, it could spreadinto native habitats before the native species begin their

growth cycle. These data are important for implications ofnative competitiveness on the edge and Melilotus dom-inance in the center of a patch. Overshading could be amechanism for dominance within a patch, but shading didnot seem to be a factor on the edge.

We found a correlation between the presence ofMelilotus species and community development in thetwo growing seasons. By using repeated measures toinvestigate species composition changes, we uncovered

Invasion by Sweet Clover (Melilotus) in Montane Grasslands

several interesting patterns. First, in recording the speciescomposition and density at three times, we detected largerdifferences at the beginning and end of the season thanduring the middle or height of the growing season. Thesedifferences may reflect competitive growth patterns ofsome species , although this is not tested here. Anotherpattern uncovered by the use of repeated measures was anincrease in number and ground cover of species in July. Inour study, the amount of ground covered by each plantincreased as individual plants grew or more individualscolonized the plots. As ground cover of Meliotus speciesincreases, the shading and crowding of these species mayinfluence change in species composition. Long-termmonitoring of species composition in these patches wouldgive a better understanding of community development inthe presence of Meliotus over a longer period of time.

Biological invasion has been a dynamic factor inecological change worldwide (Vitousek et al. 1997). Manychanges in the distribution of exotic invasive species resultfrom introduction by humans. This research has addressedthe role of geography in exotic invasion, the importance ofnatural disturbance regies, and the biogeographicalpatterns of exotic invasion in native plant communities(Pickett and White 1985). Because disturbance variesspatially and temporally, the implication is that altereddisturbance may fail to maintain the native-plant hetero-geneity but may facilitate exotic invasion that could decreasethe complexity of the patch mosaic in the landscape.

This study contributes to the preservation of native

communities in the Rocky Mountain National Park fromMelilotus invasion. Future management strategies shouldemphasize the importance of monitoring or controllingMeliotus species along the roadsides, especially near theboundaries of the park, where this species can stil belegally planted after construction. These data should helpmanagers understand (1) the extent to which Melilotusspecies have invaded native habitats, (2) the implicationsof the introduction of Melilotus to native communities atsmall spatial scales , (3) the extent to which altered naturaldisturbances may lead to perpetuating exotic invasionand (4) the degree of public awareness needed to preservebiodiversity. As visitors become aware of exotic invasionthey may want to participate in controlling speciesintroduction in the park. Because each exotic specieshas unique characteristics that influence its invasivenesswhile certain habitats also do so, the need for scientificresearch on the role of invasive species continues to beimportant in understanding invasion effects on naturalcommunities. From this research, we can use the data togenerate hypotheses about the impact of Meliotus

native communities; however, future research is needed toinvestigate specific processes that cause vegetation pat-

541

terns. In addition to effects on vegetation, disturbancescan affect the physical and chemical processes of soildevelopment. Because soil can drive vegetation patternsfuture research should focus on investigating edaphicconditions as a driving force in Melilotus invasion. Futureresearch should also focus on exploring restorationtechniques , such as fire and nutrient manipulations, as away to control exotic species and protect native commu-nities from future invasion (Hobbs and Humphries 1995jMorghan and Seastedt 1999; D'Antonio 2000; VanDriesche and Van Driesche 2000). Many exotic speciesare ultimately damaging in natural areas, and control anderadication of exotic species are two of the most diffcultchallenges for park managers.

Acknowledgments

This research was supported by the Rocky MountainNature Association, the National Park Service, theNational Science Foundation, the Edna Bailey SussmanFellowship, the Colorado Mountain Club, and the Uni-versity of Colorado Museum. We thank Jeff Connor forhelp in facilitating this research in the park. Forconversation regarding sampling design, we thank TomStohlgren and Bil Bowman. For comments and editing onthe manuscript, we thank Margie Krest. For help inidentifying many of the species, we thank Ann Henson.For research assistance, we thank Jennifer Rohrs , Chris-tine Edwards, Carl Cordova, Dan Campbell, Jeff Lukas,Emily Friend, Connor Bailey, Brenton Wonders, andJeremy Goff.

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