Physiological constraints on the spread of Alliaria petiolata populations in Massachusetts

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1 Physiological constraints on the spread of Alliaria petiolata populations in Massachusetts KRISTINA A. STINSON 1, AND TRISTRAM G. SEIDLER 2 Harvard Forest, Petersham, Massachusetts USA Citation: Stinson, K. A., and T. G. Seidler Physiological constraints on the spread of Alliaria petiolata populations in Massachusetts. Ecosphere 5(8):96. Abstract. The expansion of plant species into new sites is limited by a combination of environmental conditions and the capacity for adaptive variability in trait expression. Here, we investigated whether and how adaptation to forest edge conditions might be limiting the spread of the invasive plant Alliaria petiolata (garlic mustard) into the forest interior in eastern Massachusetts. We conducted a common garden experiment to test whether plants from forest edge vs. forest interior microhabitats differ in their plasticity and physiological responses to experimental shading. All plants in the experiment responded to shading with reductions in growth, photosynthetic activity, and reproduction regardless of the source environment, indicating a high degree of plasticity and a strong likelihood that most seeds in our study populations are produced at the forest edge. Effects of the source habitat on physiological function were detectable, but small compared to the magnitude in growth and reproductive responses to light. Plants from the forest edge physiologically outperformed those from the interior when grown in the shade, but had equally low reproductive success. Plants originating in the forest interior, in contrast, demonstrated greater allocation to growth in relation to photosynthesis and reproduction compared to plants originating at the forest edge. We compare our findings to earlier work on the importance of plasticity for invasive spread in this species, and conclude that failure by garlic mustard to invade some forest interior sites is due in part to overwhelming reproductive and physiological disadvantages in low light. We further suggest that in some cases shade tolerance in this species is constrained in favor of plastic responses that optimize fitness in high light conditions. The implications for geographic variation in the spread and management of this species are discussed. Key words: Alliaria petiolata; garlic mustard; invasive species; light; New England; plasticity; range limits; shade. Received 30 May 2014; accepted 13 June 2014; final version received 17 July 2014; published 27 August Corresponding Editor: J. Nippert. Copyright: Ó 2014 Stinson and Seidler. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. 1 Present address: Department of Environmental Conservation, University of Massachusetts, Amherst, Massachusetts USA. 2 Present address: Biology Department, University of Massachusetts, Amherst, Massachusetts USA. kstinson@eco.umass.edu INTRODUCTION A number of factors are thought to facilitate invasion by exotic organisms, including global climatic perturbations, environmental disturbance, loss of native biodiversity, and the absence of natural enemies (e.g., Lodge 1993, Vitousek et al. 1997, Dukes and Mooney 1999, Levine and D Antonio 1999, Hastings et al. 2005). However, the large majority of introduced organisms do not persist beyond their native ranges, presumably because the newly encountered habitats are not suitable (Lodge 1993). Understanding why some organisms fail to invade can provide v 1 August 2014 v Volume 5(8) v Article 96

2 insight into invasion biology and support efforts to pre-empt or control the spread of exotics. Here we report that physiological constraints to shade tolerance limit invasion by the phytotoxic invasive plant Alliaria petiolata (garlic mustard) in a subset of forest interior habitats in Massachusetts. Because many invasive plants are associated with areas of frequent disturbance, plasticity in response to light is thought to be a major driver of invasion success (Meekins and McCarthy 2000, Meekins and McCarthy 2001, Parker et al. 2003, Myers et al. 2005). Traits related to light capture and photosynthesis are often extremely flexible, and many plant species demonstrate adaptive phenotypic plasticity in leaf morphology and physiology in response to variable light (Cavers et al. 1979, Sultan and Bazzaz 1993, Arntz and Delph 2001). However, resource allocation tradeoffs in contrasting habitats can also lead to ecological divergence between light-adapted and shade-tolerant ecotypes (Arntz and Delph 2001). Garlic mustard (Alliaria petiolata (Bieb.) Cavara & Grande) is a frequently studied invasive Eurasian biennial herb in the Brassicaceae that has become problematic in North America. In its native range, it primarily occupies open sites such as hedges, roadsides, and forest edges, preferring partial or full sun to extremely shaded sites. Unlike most invasive plants, garlic mustard can become very abundant in shaded forest interior sites in North America where it now threatens native plant community composition (Anderson et al. 1996, Meekins and McCarthy 1999, Stinson et al. 2006, Stinson et al. 2007, Van Riper et al. 2010) and disrupts mutualisms between soil mycorrhizal fungi and native tree seedlings (Stinson et al. 2006, Callaway et al. 2008, Wolfe et al. 2008). However, we have observed that in some parts of the invaded range, garlic mustard establishes at high densities in edge habitats, but performs poorly within forest habitats despite a long history of presence on the regional landscape (Stinson and Carpenter 2009; K. Stinson, personal observation). Others have also noted that local garlic mustard populations decline over time in some parts of the invaded range (Lankau et al. 2009). The widespread success of this species within the forest interior is not well understood, but plasticity in response to light levels has been cited as one important factor (Byers and Quinn 1998, Myers et al. 2005). Another possibility is that physiological adaptations to shade give rise to a divergence of forest and edge ecotypes, each best suited to the particular conditions of the micro-environment (Sakai et al. 2001, Richards et al. 2006). On the other hand, it is possible that genetic constraints to shade adaptation, arising from specialization of garlic mustard to edge habitats in the home range, persist in some populations within the new range (Sakai et al. 2001, Richards et al. 2006). Here, we tested for variation in morphological, physiological and reproductive responses to light in seedlings from the edge and interior of three Massachusetts mixed deciduous forests. We also tested whether physiological and allocation traits conferred differential growth and reproductive success, as a possible explanation for this species variable performance in edge versus shaded habitats. METHODS Garlic mustard is a biennial herbaceous plant, producing semi-evergreen rosette leaves in the first year, followed by the reproductive adult phase in the second year (Rodgers et al. 2008a). Plants persist as rosettes beneath the snow and begin to bolt, or send up one or more primary flowering stems in late spring/early summer. All viable second year plants produce siliques containing þ seeds each, and by mid-july most seeds have ripened and plants begin to senesce (Cavers et al. 1979, Baskin and Baskin 1992). We examined garlic mustard invasions in three forested sites located on public conservation lands spanning a 12-km 2 area, within the towns of Concord, Acton, and Carlisle in Middlesex County, Massachusetts. Invasion originates along roadsides and other areas of significant disturbance due to development in the surrounding environment. Each site represents typical secondary growth forests, comprised of mixed hardwood canopy trees. Similar forests to the west of this area have up to 62% incursion into the forest interior, at densities of adult plants m 2 (Stinson et al. 2007). At the current study sites, edge populations have persisted for v 2 August 2014 v Volume 5(8) v Article 96

3 at least three decades and can reach up to 100 adult plants m 2. While adjacent understory habitats are sparsely invaded, garlic mustard reaches a maximum of only,20 plants m 2 in the forest interior. We collected a total of ;320 garlic mustard seedlings from each location in the early stage of emergence (cotyledons present with 0 2 true leaves). Half of these were collected from forest edge, and the other half from m within the forest interior. Seedlings were excavated in early March, and transferred to individual 9 cm diameter pots containing Sunshine Mix #5 (Sun Gro Horticulture Canada, Agwam, MA, USA), a mixture of peat moss, coarse perlite, organics, and dolomitic limestone. Potted plants were grown together in controlled light conditions for two seasons, within an outdoor study area adjacent to the experimental glasshouse facility at Harvard University, Cambridge, Massachusetts. We constructed four replicate shade houses from m metal tube framing to replicate two light treatments. Half of each shade house was covered with knitted black nylon shade cloth (85% light blocking) to simulate low light forest interior conditions; the other half was covered with white nylon mosquito tent netting (10% light blocking) to simulate high light, edge conditions (Green-Tek, Janesville, WI, USA). A total of 960 individual seedlings (160 individuals per population) were arranged in a factorial design in each shade house with the following structure: 2 habitats (edge or interior) 3 3 populations 3 20 plants/population 3 4 houses 3 2 light treatments ¼ 960 plants. Plants were watered with a drip irrigation system during the first and second year of growth. Watering to pot saturation occurred daily for the first two weeks, and then occasionally for the remainder of each growing season. Plants were fertilized with a slow-release NPK fertilizer (Osmocote Smart-Release Plant Food, Scotts, Marysville, OH, USA) in June of each year of the study. In year one, we measured the number of rosette leaves and area of the rosette. We used a LI-6400XT portable photosynthesis and fluorescence system (LI-COR, Lincoln, NE, USA) equipped with a cm leaf cuvette and an artificial light source ( B LED; LI-COR). The gas exchange system was programmed to provide photosynthetic photon flux density of 1500 lmol m 2 s 1. Photosynthetic light response was measured under a carbon dioxide concentration of 400lmol mol 1, air temperature of 238C, air flow of 300 mol s 1 and relative humidity of 40 50%. We recorded data to obtain maximum photosynthetic rate (A max ; lmol CO 2 m 2 s 1 ), water use efficiency (WUE; lmol CO 2 g H 2 O 1 ), and stomatal conductance (ml H 2 Om 2 s 1 ) on each of two randomly chosen rosette leaves per plant. All physiological measurements were stratified across treatments and replicates, and were made between the hours of 9:00 am and 11:00 am within a three-day period of sunny weather during the month of June. To estimate biomass allocation to rosettes, we harvested and weighed 20 first-year plants from each treatment 3 source habitat combination at the end of the growing season, and used an allometric equation based on the linear regression of rosette biomass on final rosette leaf area for the experiment. In year two, adult plants were monitored weekly and the number of siliques was counted until all siliques had ripened. All ripe reproductive structures were harvested, dried, and weighed. The average seed mass from ten seeds, selected randomly from different siliques, was used as a measure of seed provisioning. All plants were harvested, dried and weighed to obtain measures of final shoot and root biomass. We constructed a general linear model in JMP version 10.1 (SAS Institute, Cary, NC, USA) to test the effects of the source habitat and experimental light treatment on the measured morphological, physiological, and reproductive plant characters. The model included treatment and source habitat as the main effects, and the full interaction term. Morphological and reproductive data were analyzed together in a single MANOVA. A separate analysis was conducted for physiological variables together in a single MANOVA. RESULTS Growth Both the source habitat and experimental light treatment accounted for differences in rosette leaf number and shoot mass, whereas light treatment alone accounted for differences in leaf area and specific leaf area (Table 1, Fig. 1). Plants v 3 August 2014 v Volume 5(8) v Article 96

4 Table 1. Univariate ANOVAs for effects of light treatment and habitat of origin on rosette morphological variables. Leaf number Leaf mass Leaf area Specific leaf area Treatment F P F P F P F P Light treatment ,0.0001* * ,0.0001* * Source habitat * * Light 3 habitat Notes: All dependent variables were ln-transformed for analysis. An asterisk (*) indicates significant differences between treatments (P, 0.05). For all treatments, df ¼ 3, 322. originating in the forest interior maintained more rosette leaves and higher rosette leaf mass than those originating in edge habitat, but these differences were small compared to overall responses to our experimental light treatments (Fig. 1A, B). Plants had greater leaf area overall when grown in high light than in low light (Fig. 1C). Specific leaf area increased in response to shade but was not affected by source habitat (Fig. 1D). Adult shoot mass was similar in all plants, except for a nearly significant decrease in response to the shade treatment (Table 2, Fig. 2A). Light treatment, source habitat, and the interaction term all affected root mass: In high light treatments, adult root mass was greater than in the shade treatments, and plants from the forest interior had greater root mass in high light than plants from the edge habitat (Table 2, Fig. Fig. 1. Morphological responses of rosette-stage plants grown from seed originating in forest edge (dotted line) versus forest understory (solid line) and grown in contrasting light treatments. Response variables are (A) leaf number, (B) leaf dry mass, (C) leaf area, and (D) specific leaf area. Error bars 61 SE. v 4 August 2014 v Volume 5(8) v Article 96

5 Table 2. Univariate ANOVAs for effects of light treatment and habitat of origin on adult allocation to shoot and root biomass. Shoot mass Root mass Treatment df F P df F P Light treatment 3, , ,0.0001* Source habitat 3, , ,0.0001* Light 3 habitat 3, , ,0.0001* Notes: Degrees of freedom indicated separately due to differences between usable data for shoot and root biomass. An asterisk (*) indicates significant differences (P, 0.05). 2B). Physiology Maximum photosynthetic rates, water use efficiency, and stomatal conductance all decreased significantly in response to shading. Maximum photosynthesis, water use efficiency, and stomatal conductance were also affected by the source habitat, with plants originating in the forest edge performing significantly better under shaded conditions than plants originating in the forest understory (Table 3, Fig. 3A C). There was a significant interaction between light treatment and source habitat for maximum photosynthesis (Table 3, Fig. 3A). Reproduction The light environment strongly determined reproductive success, despite the source effects on morphology and physiology noted above. Plants grown in high light had significantly higher reproductive output in terms of number of fruits (siliques), total mass of siliques, and mean seed mass, regardless of the source habitat. There were no effects of source habitat, nor were there interactions between source habitat and light treatment, on reproduction (Table 4, Fig. 4). DISCUSSION Garlic mustard is an aggressive invasive plant that dominates many forest interior habitats in North America, but in some areas remains restricted largely to forest edge habitat. We investigated possible ecological explanations for relatively unsuccessful invasion by this species at three forested sites in Massachusetts, where it has been present in the surrounding landscape for several decades but remains at low densities in the forest interior. Since proliferation into a new habitat requires locally adaptive responses to new environmental conditions, we were interested in traits that might confer future success in the understory environment. Based on field observa- Fig. 2. Morphological responses of adult plants grown from seed originating in forest edge (dotted line) versus forest understory (solid line) and grown in contrasting light treatments. Response variables are (A) total shoot dry mass and (B) root dry mass. Error bars 61 SE. v 5 August 2014 v Volume 5(8) v Article 96

6 Table 3. Univariate ANOVAs for effects of light treatment, habitat of origin on physiological variables. Maximum photosynthetic rate Water use efficiency Stomatal conductance Treatment df F P F P F P Light treatment 3, ,0.0001* ,0.0001* ,0.0001* Source habitat 3, * * * Light 3 habitat 3, * Notes: Maximum photosynthetic rates were ln-transformed for analysis. An asterisk (*) indicates significant differences between treatments (P, 0.05). tions and prior work showing variable responses of this species to light availability, we hypothesized that overwhelmingly high reproductive success at the forest edge in our study area currently deters ecotypic differentiation into shade ecotypes in the adjacent forest understory. By experimentally growing plants from the forest edge and interior in contrasting light conditions, we tested for ecotypic variation and plasticity in a number of plant traits related to light capture, growth and reproduction. Our data demonstrated that: (1) plants from both habitat sources achieve similarly high reproduction in high light conditions, and therefore most seeds are likely to originate at the forest edge; (2) all plants are similarly inhibited in size and reproductive success in the shade; and (3) plants from both edge and interior habitat sources achieve equal reproductive success at a given light level through slightly Fig. 3. Physiological responses of plants grown from seed originating in forest edge (dotted line) versus forest understory (solid line) and grown in contrasting light treatments. Response variables are (A) maximum photosynthetic rate, (B) water use efficiency, and (C) stomatal conductance. Error bars 61 SE. v 6 August 2014 v Volume 5(8) v Article 96

7 Table 4. Univariate ANOVAs for effects of light treatment, habitat of origin on reproductive variables. Fruit number Total fruit mass Mean seed mass Treatment F P F P F P Light treatment * * * Source habitat Light 3 habitat Notes: Fruit number was ln-transformed and total fruit mass was 1 þ ln-transformed for analysis. An asterisk (*) indicates significant differences between treatments (P, 0.05). For all treatments, df ¼ 3, 47. different physio-morphological mechanisms. Taken together, our findings suggest that expansion into the forest understory is currently limited by a combination of source-sink propagule flow from edge to understory habitat, as well as traits that maximize reproduction in high light. These same processes may mask and/or mitigate the underlying potential for locally adaptive divergence into distinct physiological ecotypes at our research sites. Below we discuss and interpret these findings in more detail, and suggest directions for further research. Edge populations as a primary seed source Garlic mustard is largely restricted to edge habitat and open forests in the home range and, like many other invasive plants, is affiliated with disturbance (Cavers et al. 1979, Welk et al. 2002, Rodgers et al. 2008a). Its unusual ability to invade shaded forest habitat in many parts of Fig. 4. Reproductive responses of plants grown from seed originating in forest edge (dotted line) versus forest understory (solid line) and grown in contrasting light treatments. Response variables are (A) total number of siliques per plant, (B) total mass of siliques per plant, and (C) mean seed mass. Error bars 61 SE. v 7 August 2014 v Volume 5(8) v Article 96

8 North America and not in others is therefore very interesting and a number of hypotheses have been put forth to explain it (e.g., Rodgers et al. 2008a). Less frequently addressed are reasons that garlic mustard fails to invade forest habitats in parts of the invaded range. Here, overwhelmingly greater performance by all plants in the high light treatment clearly indicates the importance of light for growth and reproduction in our populations and suggests that the more favorable forest edge habitat is where most seeds are produced. While plants from the forest interior were generally larger than those from the forest edge, these size differences were not associated with a fitness tradeoff in the home environment, and were minor in magnitude compared to the growth and reproductive gains by all plants in the high light treatment. Figs. 1 3 demonstrate that growth and reproduction were highly plastic and were maximized at high light, irrespective of the source-habitat effects on size. Our findings concur with prior work citing superior performance at the forest edge, and the importance of light availability for determining invasion success in this species (Meekins and McCarthy 2000, Myers et al. 2000, Meekins and McCarthy 2001, Myers and Anderson 2003). Moreover, in this silique-bearing species, both seed production and dispersal distances are congruent with plant size (Cavers et al. 1979; K. Stinson, personal observation). Strong propagule pressure from the more favorable environment is therefore likely to create a source-sink dynamic (Sakai et al. 2001), whereby seed from high light conditions outnumber those from the forest understory. Asymmetric reproductive success that is biased toward performance in high light may therefore slow or preclude evolutionary divergence into shade ecotypes at our study locations (García- Ramos and Kirkpatrick 1997, Parker et al. 2003, Bossdorf et al. 2005). Additional constraints to invasion reduced performance in shade Despite some growth and physiological differences amongst plants from different sources (discussed further below), we found little evidence to suggest locally adaptive responses to shade. In many plant species, tradeoffs in contrasting environments lead to distinct suites of traits that maximize fitness through local adaptation and/or adaptive plasticity (e.g., Bradshaw and Hardwick 1989). Although plasticity is frequently cited as important in the invasion process, evolutionary divergence may also arise when different physiological strategies are favored in contrasting habitats within the new range (e.g., Clausen et al. 1940, Bradshaw and Hardwick 1989, Parker et al. 2003, e.g., Bossdorf et al. 2005). While we know of no other studies addressing divergence between edge and understory sub-populations, other researchers have reported some degree of adaptive plasticity in photosynthetic rates and leaf morphology (Dhillion and Anderson 1999, Myers et al. 2005). Here, however, consistently low reproductive success by all plants in the shaded treatment suggests that neither ecotypic divergence nor adaptive plasticity facilitate invasion into the forest understory at our study sites. We interpret this as further support for the idea that local adaptations to shade at our sites are constrained, at least in part, by the overwhelming reproductive advantages to plants performing well in high light, and consistent influx of large numbers of seeds from edge habitats. The contrast between our study locations and other areas where garlic mustard has become an aggressive invader of forest interior habitat (Stinson et al. 2007, Rodgers et al. 2008a, Pardini et al. 2009, Van Riper et al. 2010) requires additional research. One possible explanation is that plants at our sites retain an overwhelming preference for high light sites because they frequently encounter edge habitat and/or other disturbance-related patches of high light. For instance, Burls and McClaugherty (2008) noted differences in forest incursion rates by garlic mustard in neighboring sites with different land use histories. Whether and how our forests differ from others in rates of fragmentation and other disturbances, together with species composition and gap dynamics of canopy trees, would help clarify the importance of underlying environmental differences that might drive the apparent source-sink dynamic we observed here. In addition, it is possible that founder effects determine the diversity of traits and genotypes available at a given site. Molecular data indicate that garlic mustard has been introduced multiple times, creating the potential for landscape-level genetic structure and potential bottlenecks (Bossv 8 August 2014 v Volume 5(8) v Article 96

9 dorf et al. 2005, Mullarkey et al. 2013). Ecological evidence for population-level variation includes Meekins and McCarthy s (2001) work on populations from lowland versus upland forests. Others have quantified site-specific demographic variation in seed maturation, survival, size, and allocation to (Byers and Quinn 1998), expression of phytochemical and physiological traits (Cipollini and Lieurance 2012, Lankau 2012b), and concurrent patterns of molecular and phenotypic variation that suggest evolutionary differences among geographically distinct populations (Bossdorf et al. 2005, Mullarkey et al. 2013). An avenue of future research is whether genetic composition and/or diversity differs at our sites from those where understory invasion has succeeded, and whether this is correlated with observed geographic and temporal environmental conditions. Population-level differences in mechanisms of light response Despite a general lack of evidence for locally adaptive responses to shade, we did find small but significant differences in morphological and physiological traits of plants originating in edge versus interior habitats. As these traits did not confer differential reproductive success to plants from a given source habitat, we do not interpret this finding as ecotypic divergence per se. However, distinct resource allocation patterns do seem to underlie the mechanisms by which reproduction is achieved by plants originating in different habitats. Specifically, we found that edge-sourced plants retained higher photosynthetic rates, stomatal conductance, and water use efficiencies in the shade treatments than those originating in the forest interior, allowing them to achieve similar fecundity to forest interior plants despite lower allocation to leaves and shoots. This somewhat surprising result suggests there may be genetic and/or maternal environmental influences that predispose plants from the forest edge to inherently high photosynthetic activity. For instance, higher stomatal densities and/or chlorophyll levels typically contribute to higher metabolic rates in plants from high light environments (Meekins and McCarthy 2000, Myers et al. 2005) and may be retained by plants originating at the forest edge regardless of the light conditions in the habitat of offspring growth. Such characteristics may be particularly important in forest edge habitats for competition in this species (e.g., Winterer et al. 2005, Rebek and O Neil 2006), and likely reflects selection on traits that will continue to maximize growth and reproduction in high light habitats. As mentioned above, we hypothesize that frequent dispersal into higher light conditions in the highly disturbed forests at our current study sites can further influence selection on the retention of intrinsically high photosynthetic rates; this requires more targeted landscape-level analyses and is an area for further research. In contrast, plants originating in shade appear to compensate for their lower metabolic function via more vigorous leaf growth, particularly in high light conditions (Fig. 1A C). Interior plants also showed a sharper decline in root biomass in the shade, as is expected when light becomes more limiting (Tilman 1988). As indicated by rosette size and final reproductive output (Figs. 1 and 4), forest interior plants also produced greater rosette mass per unit of reproductive effort than plants from the forest edge. A plausible explanation for production of larger rosettes and more leaves is that correlations between size and fecundity select for larger plants in the forest interior (Susko and Lovett- Doust 2000a, Smith et al. 2003). It is also possible that greater maternal provisioning to seed in lower light habitat creates more robust seedlings in the understory (Susko and Lovett-Doust 2000b, Susko and Cavers 2008). Thus it appears that photosynthetic efficiency amongst plants from edge-sourced plants and compensatory growth in forest interior plants led to similar reproductive success at a given light level. This suggests there may be latent evolutionary potential within shade populations that is currently masked by the large magnitude of growth and reproductive responses to higher light levels. Conclusions We conclude that low levels of understory invasion by garlic mustard in this study represent instances of insufficient performance in the forest interior compared to that of performance at the forest edge, as well as a strong possibility that regular influx of seeds from the forest edge overwhelm offspring from forest interior plants. This type of source-sink scenario may help to v 9 August 2014 v Volume 5(8) v Article 96

10 explain why some garlic mustard invasions persist in a lag phase (Sakai et al. 2001) prior to successful incursion into forested habitats. From a management perspective, this work suggests that some populations in the forest interior are self-limiting and may be controlled by simply eradicating the more fecund populations at the forest edge. We found little evidence for differential adaptation to edge and understory conditions, countering the idea that evolutionary changes are facilitating invasion (Parker et al. 2003, Bossdorf et al. 2005, Phillimore et al. 2012), at least into the studied forest habitats in Massachusetts. Although there were strong effects of source habitat on physiological responses to low light, these were very small in magnitude compared to growth and reproductive enhancements at high light. Overwhelmingly greater seed production in the high light treatments suggests instead that plants at the forest edge provide the primary source of propagules, outnumbering those originating in the understory. Strong propagule pressure from the more favorable environment may therefore preclude genetic divergence into shade ecotypes (García-Ramos and Kirkpatrick 1997, Parker et al. 2003, Bossdorf et al. 2005). Our findings suggest that in our study area, garlic mustard is still master-of-some, better able to increase fitness in edge habitats than in the forest interior, while elsewhere it appears to be Jackof-all-trades (Richards et al. 2006). Nevertheless, distinct resource allocation patterns in the different habitats we studied do suggest the potential for future adaptations to the forest interior habitat. Indeed, elsewhere in the invaded range including sites near the present study s locations garlic mustard reaches high densities in forested sites that are difficult to eradicate and have documented ecological effects on plant community and soil microbial dynamics (Hochstedler et al. 2007, Stinson et al. 2007, Burke 2008, Rodgers et al. 2008b, Barto et al. 2011, Lankau 2011a, Lankau 2011b, Morris et al. 2012). We thus interpret our findings as an example of geographic differences in landscape-level invasion success by this species within forested habitats (Burls and McClaugherty 2008, Carpenter 2008). Abiotic factors may contribute to this variation, including nutrient status (Herrmann et al. 2009), moisture (Byers and Quinn 1998, Meekins et al. 2001, Hochstedler and Gorchov 2007), and landscape position (Burls and McClaugherty 2008). Earthworm densities (Maerz et al. 2009, Nuzzo et al. 2009), powdery mildew infection (Ciola and Cipollini 2011), deer browsing (Knight et al. 2009), and competition with native forest understory plants (Rebek and O Neil 2006, Stinson et al. 2006) have also been associated with garlic mustard invasions and may co-vary with its relative success in shaded sites. Temporal influences on the success of invasion are also possible, given that recent studies have attributed geographic variation in invasion severity to temporal evolutionary changes in physiological and phytochemical traits (Evans et al. 2012, Lankau 2012). Thus we stress that not all populations respond equally to variation in light availability, and that a combination of factors is likely to determine both local and landscape patterns of expansion by garlic mustard from edge sites into the forest interior. Future work is needed to better understand the ecological and evolutionary differences between successful and unsuccessful forest invasions in this species. ACKNOWLEDGMENTS This work was conducted under NSF grant #DBI to K. A. Stinson, with additional support from the Nature Conservancy and the Harvard Forest. K. A. Stinson designed and conducted the experiment, collected the data, managed the research team and did most of the writing and editing of the manuscript. T. G. Seidler consolidated and managed the datasets, conducted most of the analyses, generated the graphs, and edited the manuscript. We thank L. Carley for meticulous assistance with manuscript preparation; K. Donohue and T. Sipe for discussion of the results; R. Stomberg for assistance with experimental design; and K. Glennon, L. Griffen, E. Mathai and H. Wasson for research assistance. LITERATURE CITED Anderson, R., S. Dhillion, and T. Kelley Aspects of the ecology of an invasive plant, garlic mustard (Alliaria petiolata), in central Illinois. Restoration Ecology 4: Arntz, A., and L. Delph Pattern and process: evidence for the evolution of photosynthetic traits in natural populations. Oecologia 127: Barto, E. K., P. M. Antunes, K. Stinson, A. M. Koch, J. N. Klironomos, and D. Cipollini Differencv 10 August 2014 v Volume 5(8) v Article 96

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