Influence of light and nutrient conditions on seedling growth of native and invasive trees in the Seychelles

Several recent studies have shown that plant invasions can occur in resource-poor and relatively undisturbed habitats. It is, therefore, important to investigate whether and how life-history traits of species invasive in such habitats differ from those of species that are only invasive in disturbed and resource rich habitats. We compared the growth of seedlings of native and invasive tree species from nutrient-poor secondary forests in the tropical Seychelles. We hypothesised that the relative performance of the two groups would change predictably along resource gradients, with native species performing better at low levels of resource availability and invasive species performing better at higher levels. To test this hypothesis, we performed a common garden experiment using seedlings of six invasive and seven native tree species grown under three levels of light (65, 11 and 3.5% of ambient light) and two of nutrients (low and high). Due to large variation among species, differences in growth rates (RGR) were not significant among seedlings of the native and the invasive species. However, seedlings of the invasive species showed higher specific leaf areas (SLA) and higher leaf nutrient contents than seedlings of the native species. They also exhibited greater plasticity in biomass and nutrient allocation (i.e., greater plasticity in LAR, RSR and leaf nutrient contents) in response to varying resource availability. However, differences between the mean values of these parameters were generally small compared with variation within groups. We conclude that successful invaders on nutrient-poor soils in the Seychelles are either stress-tolerant, possessing growth traits similar to those of the native species, or fast-growing but adapted to nutrient-poor soils. In contrast, the more typical, fast-growing alien species with no particular adaptations to nutrient-poor soils seem to be restricted to relative nutrient-rich sites in the lowlands. The finding—that some introduced species thrive in resource-poor habitats—suggests that undisturbed habitats with low resource availability may be less resistant to plant invasions than was previously supposed.     

Biomass partitioning,architecture and turnover of six herbaceous species from habitats with different nutrient supply

Three grasses (Holcus lanatus, Anthoxanthum odoratum and Festuca ovina) and three herbs (Rumex obtusifolius, Plantago lanceolata and Hieracium pilosella) were grown in a greenhouse at 3 nutrient levels in order to evaluate plant allocation, architecture and biomass turnover in relation to fertility level of their habitats.Four harvests were done at intervals of 4 weeks. Various plant traits related to biomass partitioning, plant architecture, biomass turnover and performance were determined. Differences in nutrient supply induced a strong functional response in the species shoot:root allocation, but architecture and turnover showed little or no response. Architectural parameters like specific leaf area and specific root length, however, in general decreased during plant development.Species from more nutrient-rich successional stages were characterized by a larger specific leaf area and longer specific shoot height (height/shoot biomass), resulting in a higher RGR and total biomass in all nutrient conditions. There was no evidence that species from nutrient-poor environments had a longer specific root length or any other superior growth characteristic. The only advantage displayed by these species was a lower leaf turnover when expressed as the fraction of dead leaves and a shorter specific shoot height (SSH) which might prevent herbivory and mowing losses.The dead leaf fraction, which is a good indicator for biomass and nutrient loss, appeared to be not only determined by the leaf longevity, but was also found to be directly related to the RGR of the species. This new fact might explain the slow relative growth rates in species from a nutrient-poor habitat and should be considered in future discussions about turnover.     

Comparison of life history traits between invasive and native populations of purple loosestrife (Lythrum salicaria) using nonlinear mixed effects model

We compared growth patterns of invasive and native populations of purple loosestrife (Lythrum salicaria) while varying water and nutrient levels. We examined three life-history traits (height, number of branches, and the size of largest leaf) during the growth period adopting a nonlinear mixed effects model. Invasive populations were found to be slower in shoot elongation but grew to be taller than native populations. Invasive populations produced more branches than natives only in the high water, high nutrient treatment. Invasive populations had a similar increase in the size of the largest leaf compared to natives, but ultimately produced a greater size of largest leaf than natives. Invasive populations were found to display a greater vegetative expansion, but this was not strongly affected by our treatments.     

Comparison of life history traits between invasive and native populations of purple loosestrife (Lythrum salicaria) using nonlinear mixed effects model

We compared growth patterns of invasive and native populations of purple loosestrife (Lythrum salicaria) while varying water and nutrient levels. We examined three life-history traits (height, number of branches, and the size of largest leaf) during the growth period adopting a nonlinear mixed effects model. Invasive populations were found to be slower in shoot elongation but grew to be taller than native populations. Invasive populations produced more branches than natives only in the high water, high nutrient treatment. Invasive populations had a similar increase in the size of the largest leaf compared to natives, but ultimately produced a greater size of largest leaf than natives. Invasive populations were found to display a greater vegetative expansion, but this was not strongly affected by our treatments.     

Asexual propagations of introduced exotic macrophytes <Emphasis Type="Italic">Elodea nuttallii</Emphasis>, <Emphasis Type="Italic">Myriophyllum aquaticum</Emphasis>, and <Emphasis Type="Italic">M. propinquum</Emphasis> are improved by nutrient-rich sediments in China

An increasing number of recent studies indicate that multiple interacting factors can affect the invasion of plants. However, few studies have focused on asexual propagation and the interaction of propagation with environmental factors that regulate the invasive potential of introduced exotic species in aquatic habitats. This study was designed to investigate the differences in asexual propagation between introduced exotic and non-invasive native aquatic macrophytes in nutrient-poor and nutrient-rich sediments and to test the hypothesis that differences in asexual propagation (stem fragment production) and propagule establishment between introduced exotic and non-invasive native macrophytes are driven by sediment nutrient levels. Three exotic aquatic macrophytes (Elodea nuttallii, Myriophyllum aquaticum, and M. propinquum) recently introduced to China and their non-invasive native counterparts (Hydrilla verticillata, M. oguraense, and M. ussuriense) were used for comparison in nutrient-poor (TN 0.59 and TP 0.03 mg g⁻¹) and nutrient-rich (TN 2.35 and TP 0.10 mg g⁻¹) sediments. After 8 weeks of growth, the exotic species tended to produce more total biomass, branch biomass and apical shoots and have higher relative growth rate (RGR) than their native counterparts in nutrient-rich sediment. Rooting efficiency and root growth of exotic fragments were higher than that of native counterparts in nutrient-rich sediment, although the survival rates of fragments did not differ between native and exotic species. In addition, superior traits (rooting efficiency and root growth) of exotic species were also observed in nutrient-poor sediment, but to a lesser degree than in nutrient-rich sediment. These results suggest that asexual propagation of these three introduced exotic macrophytes is more effective in nutrient-rich sediment than in nutrient-poor sediment in China.     

Pollination and reproduction enhance the invasive potential of an early invader: the case of Lythrum salicaria (purple loosetrife) in South Africa

The potential of an alien plant to spread rapidly and colonize new habitat may be related to the mode of reproduction and the ability to attract pollinators. Most studies focus on widespread invasive plants, in which pollinators are rarely limiting. Here, we assess the ability of a recent invader in South Africa, the tristylous Lythrum salicaria to self-reproduce and whether this can explain the delay between introduction and spread. This study was conducted in one of the largest known populations (a total of 7 populations in South Africa) of L. salicaria in the Liesbeek river in the fynbos biome. We assessed the importance of pollinators and autonomous selfing in L. salicaria by comparing seed set between pollinator excluded and naturally pollinated flowers. Overall, 5 pollinators (4 native and 1 alien) were recorded with Cape honeybees and Africa Monarch butterflies the most prominent. Seed and fruit set were significantly higher in open pollinated flowers compared to pollinator excluded flowers. Also, seed and fruit set in pollinator excluded flowers were higher in long and medium morphs compared to short morphs. Germination was high for seeds from pollinator, but also from pollinator excluded treatments. This shows that L. salicaria in South Africa is self-compatible to some extent, but it is frequented by pollinators, significantly increasing seed production. Despite L. salicaria being tristylous, all 3 morphs are present in South Africa and with a huge seed production, this species has the potential to become a major invader of rivers and wetlands in South Africa.

     

Can Gas-Exchange Characteristics help Explain the Invasive Success of Lythrum salicaria?

Since its introduction to North America, Lythrum salicaria (L.) (purple loosestrife) has become invasive in marshy and riparian habitats. We compared gas-exchange responses to external CO₂ partial pressure and light, as well as related leaf structural and biochemical characteristics, of L. salicaria with those of co-occurring native Asclepias syriaca (common milkweed) and Solidago graminifolia (lance-leaved goldenrod) along a pond bank in the Black Rock Forest, Cornwall, New York, USA to examine if the invasive success of L. salicaria may be influenced by robust leaf gas-exchange characteristics, including relatively high rates of photosynthesis and low rates of respiration, compared with those of less successful co-occurring native plant species. Neither the mean rate of net photosynthesis measured at ambient CO₂ and saturating photon flux density (A) nor the mean dark respiration rate (R_D) differed significantly between L. salicaria and either of the native species, while both the mean maximum rate of photosynthesis at saturating CO₂ concentration and photon flux density (A _max) and the mean rate of respiration measured in light (R_L) were significantly higher in L. salicaria than A. syriaca, but no different between L. salicaria and S. graminifolia. Likewise, photosynthetic nitrogen-use efficiency was greater in L. salicaria than A. syriaca only, while photosynthetic water-use efficiency was significantly less in both L. salicaria and S. graminifolia than in A. syriaca. Despite limited interspecific differences in leaf photosynthesis, respiration, and resource-use efficiency, particularly between L. salicaria and S. graminifolia, we found that L. salicaria assimilated 208% more carbon per unit of energy invested in leaf biomass than either of the co-occurring native species, suggesting that increased photosynthetic energy-use efficiency may influence its observed invasive success. This revised version was published online in July 2006 with corrections to the Cover Date.     

Increased relative abundance of an invasive competitor for pollination, Lythrum salicaria, reduces seed number in Mimulus ringens

When exotic plant species share pollinators with native species, competition for pollination may lower the reproductive success of natives by reducing the frequency and/or quality of visits they receive. Exotic species often become numerically dominant in plant communities, and the relative abundance of these potential competitors for pollination may be an important determinant of their effects on the pollination and reproductive success of co-occurring native species. Our study experimentally tests whether the presence and abundance of an invasive exotic, Lythrum salicaria L. (Lythraceae), influences reproductive success of a co-flowering native species, Mimulus ringens L. (Phrymaceae). We also examine the mechanisms of competition for pollination and how they may be altered by changes in competitor abundance. We found that the presence of Lythrum salicaria lowered mean seed number in Mimulus ringens fruits. This effect was most pronounced when the invasive competitor was highly abundant, decreasing the number of seeds per fruit by 40% in 2006 and 33% in 2007. Reductions in the number of seeds per fruit were likely due to reduced visit quality resulting from Mimulus pollen loss when bees foraged on neighboring Lythrum plants. This study suggests that visit quality to natives may be influenced by the presence and abundance of invasive flowering plants.     

Phenolic and mineral content of leaves influences decomposition in European forest ecosystems

Summary Factors influencing decomposition in European forests growing on different soils were studied in stands dominated by the European beechFagus sylvatica L. Phenolic contents of freshly fallen leaves ofF. sylvatica growing on nutrient-poor soils (acid sandy soil) were higher than those of similar leaves on nutrient-rich soils (calcareous mull soil). Analysis of fallen leaves of different ages showed rapid decay of phenolics during the first winter on the ground. After 1 year the phenolic content of leaves ofF. sylvatica growing on nutrient-poor soils was still twice as high as in similar leaves on nutrient-rich soils. Field and laboratory experiments showed that a major decomposer (Oniscus asellus, Isopoda) preferred leaves from trees on nutrient-rich soils. Mineral contents of leaves ofF. sylvatica growing on different soils differed: on rich soils leaves had higher contents of Ca, Mg, Na, and K. These elements are important nutrients for decomposers. The distribution of major decomposers reflects the mineral content of their diet, which in turn reflects soil type. Different rates of leaf turnover and nutrient turnover in different forest ecosystems (even when the same tree species is dominant) are due to the decomposing system, which is influenced by the phenolic and mineral contents of the leaves.     

Site fertility and soil water-table level affect fungal biomass production and community composition in boreal peatland forests

A substantial amount of below-ground carbon (C) is suggested to be associated with fungi, which may significantly affect the soil C balance in forested ecosystems. Ergosterol from in-growth mesh bags and litterbags was used to estimate fungal biomass production and community composition in drained peatland forests with differing fertility. Extramatrical mycelia (EMM) biomass production was generally higher in the nutrient-poor site, increased with deeper water table level and decreased along the length of the recovery time. EMM biomass production was of the same magnitude as in mineral-soil forests. Saprotrophic fungal biomass production was higher in the nutrient-rich site. Both ectomycorrhizal (ECM) and saprotrophic fungal community composition changed according to site fertility and water table level. ECM fungal community composition with different exploration types may explain the differences in fungal biomass production between peatland forests. Melanin-rich Hyaloscypha may indicate decreased turnover of biomass in nutrient-rich young peatland forest. Genera Lactarius and Laccaria may be important in nutrient rich and Piloderma in the nutrient-poor conditions, respectively. Furthermore, Paxillus involutus and Cortinarius sp. may be important generalists in all sites and responsible for EMM biomass production during the first summer months. Saprotrophs showed a functionally more diverse fungal community in the nutrient-rich site.     

Rapid growth and early flowering in an invasive plant,purple loosestrife (<Emphasis Type="Italic">Lythrum salicaria</Emphasis> L.) during an El Niño spring

Phenological shifts may play a role in the success of invasive species, especially in association with climatic variability. We studied the response of a North American population of the invasive plant, Lythrum salicaria L., to changes in local climate associated with the El Niño/Southern Oscillation Event (ENSO) of 1997–1998. For L. salicaria plants at two wetland sites near North Bay, Ontario, Canada, we made weekly observations of flowering phenology and monthly measurements of aboveground biomass during the 1997 and 1998 growing seasons (April–October). Reproductive output was measured as cumulative length and biomass of inflorescences at the end of the growing season. Temperature and precipitation during the 1997 growing season were typical for the region and provided good baseline data for comparison to the full effects of the ENSO event in 1998, which increased spring temperatures and reduced precipitation in the study area. In response to these conditions, populations of L. salicaria began to flower 14 days earlier (Julian day = 181 ± 10) in 1998 than in 1997 (Julian day = 195 ± 12), and accumulated more aboveground biomass early in the growing season (P < 0.05). However, by the end of the growing season, there were no significant differences between years in aboveground biomass or total inflorescence lengths, and senescence of plants occurred at similar times for both growing seasons. Advances in spring phenology during ENSO events offer several potential advantages to L. salicaria, and could have a significant impact on biological control programs initiated for this species in North America.     

Ecological implications of seed characteristics of the native Prosopis cineraria and the alien P. juliflora

Seed weight, percentage germination, seedling growth, and nutrient concentrations (Mg, Na, K, Zn, Cu and P) of whole seeds, and of seed coats and embryos separately of two tree species, the native Prosopis cineraria and the invasive alien P. juliflora from semi-arid and arid areas of north and north-west India, were analysed to understand the differences in their ecology. Seeds of P. cineraria were heavier than those of P. juliflora. Percent germination was similar in the two species, but seedling growth was faster in P. juliflora than in P. cineraria. Nutrient concentrations of seeds of the two species were similar (except Cu). Nutrient concentrations in the embryo were higher in P. cineraria, while those in the seed coat were higher in P. juliflora. The relative allocation of nutrients to seed coat was higher in P. juliflora than in P. cineraria. Nutrient-rich embryos and slow growth, along with a staggered seed germination pattern in the native P. cineraria could be linked to delayed establishment as well, in the substratum. Faster growth of the nutrient-poor embryos in P. juliflora along with its simultaneous seed germination pattern, and creation of a favourable microenvironment through leaching of nutrients from a nutrient-rich seed coat can facilitate immediate and successful establishment of this alien species in the invaded habitats.     

Competition among native and invasive Phragmites australis populations: An experimental test of the effects of invasion status,genome size,and ploidy level

Among the traits whose relevance for plant invasions has recently been suggested are genome size (the amount of nuclear DNA) and ploidy level. So far, research on the role of genome size in invasiveness has been mostly based on indirect evidence by comparing species with different genome sizes, but how karyological traits influence competition at the intraspecific level remains unknown. We addressed these questions in a common‐garden experiment evaluating the outcome of direct intraspecific competition among 20 populations of Phragmites australis, represented by clones collected in North America and Europe, and differing in their status (native and invasive), genome size (small and large), and ploidy levels (tetraploid, hexaploid, or octoploid). Each clone was planted in competition with one of the others in all possible combinations with three replicates in 45‐L pots. Upon harvest, the identity of 21 shoots sampled per pot was revealed by flow cytometry and DNA analysis. Differences in performance were examined using relative proportions of shoots of each clone, ratios of their aboveground biomass, and relative yield total (RYT). The performance of the clones in competition primarily depended on the clone status (native vs. invasive). Measured in terms of shoot number or aboveground biomass, the strongest signal observed was that North American native clones always lost in competition to the other two groups. In addition, North American native clones were suppressed by European natives to a similar degree as by North American invasives. North American invasive clones had the largest average shoot biomass, but only by a limited, nonsignificant difference due to genome size. There was no effect of ploidy on competition. Since the North American invaders of European origin are able to outcompete the native North American clones, we suggest that their high competitiveness acts as an important driver in the early stages of their invasion.     

Phenotypic plasticity of introduced versus native purple loosestrife: univariate and multivariate reaction norm approaches

The plastic responses to environmental change by Lythrum salicaria (purple loosestrife) were compared between native plants derived from seeds collected in Europe and those introduced into North America. Plants from nine populations each were grown under two levels of water and nutrient conditions. At the end of the growing season, samples were evaluated for eight traits related to their life history, plant size/architecture, and reproduction. Genetic (G), environmental (E), and G × E interactions were assessed by restricted maximum likelihood (REML) analysis of covariance (ANCOVA) and multivariate analysis of covariance (MANCOVA). Both univariate and multivariate reaction norm analyses were used to test for differences in the magnitude and direction of phenotypic plasticity between introduced and native plants. Under high-nutrient conditions, introduced plants were taller and had more branches and greater aboveground biomass. They also exhibited significantly greater amounts of phenotypic plasticity for aboveground biomass than did the natives in response to changing nutrient levels in standing water. This difference in univariate plasticity contributed to the general contrast in multivariate plasticity between introduced and native plants. These results support the idea that introduced plants may successfully invade a habitat and grow better than native plants in response to increased resources.     

Invasive Plant and Experimental Venue Affect Tadpole Performance

Introductions of non-native predators and competitors appear to contribute to worldwide amphibian declines; however, potential negative impacts of invasive plants on habitat quality and amphibian populations have not been examined. Loss of diversity and alterations in ecosystem function associated with plant invasions may disrupt food webs, potentially leading to further declines of already threatened amphibian populations. We used a combination of small bins, mesocosms, and field experiments to examine the impacts of Eurasian purple loosestrife (Lythrum salicaria) replacing native cattails (Typha latifolia) in North American freshwater wetlands on survival, developmental rate, and diet (freshwater algae) of American toad (Bufo americanus) tadpoles. Tadpoles developed slower in L. salicaria compared to tadpoles developing in T. latifolia. This effect was consistent across experimental venues, although mesocosms showed this effect only in the second year of our study. Survival and development rates were always more variable in purple loosestrife than in cattail. In bins, tadpoles showed significantly reduced survival when raised in purple loosestrife extract and addition of leaf litter exacerbated this negative effect. Tadpole survival rates in mesocosms and field cages were not significantly different between plant species, most likely an effect of high variability among replicates. We suspect a combination of direct toxicity of high tannin concentrations in L. salicaria leaves and their indirect negative impacts on aquatic food webs are responsible for these results. Tadpole gut analyses revealed differences in algal communities among venues and between L. salicaria and T. latifolia suggesting that alterations in tadpole food quality and quantity contribute to the observed reduced tadpole performance. The replacement of native wetland plant species by L. salicaria does not represent a simple exchange of ecological equivalents and the function of invaded habitats for native species has clearly changed. While we were investigating only a single amphibian species, our results suggest that the impact of L. salicaria on ecosystem processes and aquatic food webs may be more general and likely to negatively affect other wetland species. The threats non-indigenous plants represent for amphibian populations and food webs may be underestimated, and warrant further investigation.     

Impacts of major predators on tropical agroforest arthropods: comparisons within and across taxa

Non-native plants can have adverse effects on ecosystem structure and processes by invading and out-competing native plants. I examined the hypothesis that mature plants of non-native and native species exert differential effects on the growth of conspecific and heterospecific seedlings by testing predictions that (1) invasive vegetation has a stronger suppressive effect on seedlings than does native vegetation, (2) seedlings of invasive species are better able to grow in established vegetation than are native seedlings, and (3) invasive species facilitate conspecific and inhibit heterospecific seedling growth. I measured growth rates and interaction intensities for seedlings of four species that were transplanted into five wetland monoculture types: invasive Lythrum salicaria; native L. alatum, Typha angustifolia, T. latifolia; unvegetated control. Invasive L. salicaria had the strongest suppressive effect on actual and per-individual bases, but not on a per-gram basis. Seedlings of T. latifolia were better able to grow in established vegetation than were those of L. salicaria and T. angustifolia. These results suggest that L. salicaria is not a good invader of established vegetation, but once established, it is fairly resistant to invasion. Thus, it is likely that disturbance of established vegetation facilitates invasion by L. salicaria, allowing it to compete with other species in even-aged stands where its high growth rate and consequent production of aboveground biomass confer a competitive advantage.     

Plasticity of Lythrum salicaria and Phragmites australis growth characteristics across a European geographical gradient

Plants of Lythrum salicaria and Phragmites australis originating from localities across the European north–south geographical gradient were cultivated in parallel in an outdoor tub experiment. A strong correlation was found between growth and morphometric characteristics related to plant size (plant height, basal diameter, aboveground- and belowground plant biomass, etc.) and the position of the respective populations along the north–south gradient. Plants of both L. salicaria and P. australis from the southern localities grew taller and more vigorously and flowered later than plants from relatively more northern localities. From this point of view, the plants originating from south European populations were comparable to invasive North American plants. Our study indicates that explanation of the competitive success of populations invading new geographical areas may involve the role of geographic gradients within the species native range.     

Differences in Seed Production Between Stellaria media Populations from Different Habitat Types

Seed production in several populations of Stellaria media fromdifferent types of habitat was examined. Two populations, onefrom a flower garden and the other from a herring gull colonywere compared in detail by growing them at three soil fertilitylevels under similar conditions in a greenhouse. Three important differences between the two populations independentof soil fertility emerged: the garden plants flowered on averageeleven weeks earlier, they produced much smaller seeds, andthey produced more seeds per capsule. Other differences were influenced by soil fertility: at highfertility the garden plants produced about 50 per cent moreseeds than the gull colony plants, but on a seed weight basis,the gull colony plants were the more productive. However forboth populations at high fertility the proportion of shoot biomassallocated to seed production was similar. Some of these differences can be accounted for in terms of conditionsoccurring in the two habitats, in particular the relative importanceof disturbance and competition at the sites. Chickweed, Stellaria media (L.) Vill., ecotypic differentiation, time to flowering, seed production, resource allocation     

Meristem activity and biomass production as response mechanisms in two forest herbs

Actaea spicata and Geranium sylvaticum are deciduous rosette herbs, frequently coexisting in nutrient-rich forests of boreal Sweden. A. spicata is restricted to this habitat whereas G. sylvaticum has a wider ecological amplitude. To explore the different distributions of these species we studied the growth mechanisms by which they responded to nutrient and light supply. Plants raised from seeds were grown under four combinations of light and nutrient supplies and growth was followed for 20 weeks. Growth responses were measured as biomass accumulation and meristem activity, i.e. leaf development by shoot meristems. Biomass accumulation and meristem activity were affected by nutrient and light treatments. However, A. spicata responded more to nutrients, whereas G. sylvaticum responded more to light, and low resource supply shortened the time during which shoot meristems of A. spicata developed leaves without a corresponding effect on biomass accumulation. In G. sylvaticum, the periods of leaf development and biomass accumulation were the same, but a relationship between high development rates and peak allocation of biomass to shoot meristems indicated a storage function of these during leaf development. We conclude that the inflexible growth of A. spicata, mainly affected by nutrient supply, makes it less competitive in open habitats and restricts it to shaded, nutrient-rich habitats. G. sylvaticum should be more successful in habitats subjected to unpredictable disturbances, due to a flexible growth morphology and a dynamic function of meristems that buffers against variation in nutrient supply and facilitates positive light responses. Meristematic data should be included in interspecific comparisons on growth responses to different resource supplies. Unless species have the same growth mechanism, it is not satisfactory to study biomass-related traits alone, as internal constraints concerning plant development might confuse the interpretation.     

Evidence for a shift in life-history strategy during the secondary phase of a plant invasion

We investigated the correlated response of several key traits of Lythrum salicaria L. to water availability gradients in introduced (Iowa, USA) and native (Switzerland, Europe) populations. This was done to investigate whether plants exhibit a shift in life-history strategy during expansion into more stressful habitats during the secondary phase of invasion, as has recently been hypothesized by Dietz and Edwards (Ecology 87(6):1359, 2006). Plants in invaded habitats exhibited a correlated increase in longevity and decrease in overall size in the transition into more stressful mesic habitats. In contrast, plants in the native range only exhibited a decrease in height. Our findings are consistent with the hypothesis that secondary invasion is taking place in L. salicaria, allowing it to be more successful under the more stressful mesic conditions in the invaded range. If this trend continues, L. salicaria may become a more problematic species in the future.