Growth, tolerance and zinc and cadmium hyperaccumulation of Thlaspi caerulescens populations from three metal contaminated soils and three normal soils were compared under controlled conditions. Individuals of six populations were cultivated on five soils with increasing concentrations of zinc (50-25 000 µg g −" ) and cadmium (1-170 µg g −" ). There was no mortality of normal soil populations in the four metal-contaminated soils, but plant growth was reduced to half that of populations from metal-contaminated soils. However, in noncontaminated soil, the growth of individuals from normal soils was greater than that of individuals from metal-contaminated soils. Individuals from normal soils concentrated three times more zinc in the aboveground biomass than those from metal-contaminated soils, but the latter accumulated twice as much cadmium. We conclude that populations of T. caerulescens from both normal and metal-contaminated soils are interesting material for phytoextraction of zinc and cadmium, but to optimize the process of phytoextraction it is necessary to combine the extraction potentials of both type of populations.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. This content downloaded from 138.Abstract. Researchers have suggested that species-community interactions determine invasion success. Therefore, it is likely that small biological differences between species interact with habitat characteristics to produce distinct patterns of distribution and abundance throughout a new range. In this study we test the hypothesis that differences in the distribution and abundance of species sharing an identical set of "ideal weed characteristics" are explicable in terms of species-specific responses to environmental variation within their new range. Using multifactor experiments, we investigated some of the ecological interactions influencing reproductive success in two very closely related species of annuals having invaded the French Mediterranean region for > 150 yr and showing marked differences in their local distribution and abundance patterns. We transplanted seedlings of Conyza canadensis (a species restricted to recently disturbed areas) and C. sumatrensis (a species colonizing early-to mid-successional old fields) at equal densities in three contrasting old fields (6 mo, 4 yr, 17-yr abandonment, respectively) during 1991-1992, a growing season with average rainfall. Individual performances (measured as survivorship, reproductive timing, and reproductive output) were evaluated with respect to: (1) competition with plant neighbors (tested with a weeding treatment), (2) resource availability (tested with nutrient and water addition), and (3) herbivory (tested with chemical limitation). Manipulated factors interacted in a rather complex fashion to influence survivorship and reproduction in both species. However, patterns of relative performance were consistent with relative distribution patterns across Mediterranean landscapes: C. sumatrensis performed better than C. canadensis in all fields, including the youngest one (6 mo old).Herbivory only slightly affected transplant performances. In contrast, competition with plant neighbors had substantial effects on either Conyza species and may be the most important determinant of performance in Mediterranean old fields. The experiment showed unambiguously that the two species differ markedly in their competitive ability, with C. sumatrensis performing better than C. canadensis in the presence of neighboring vegetation.In addition, C. sumatrensis displayed a superior ability to take up and/or to use water and nutrient resources when they become available in competitive environments. We argue that potential physiological or anatomical species differences responsible for this differential susceptibility to local resource reduction by neighbors could involve differences in constructional organization, ...

Forests on the Haida Gwaii (HG) archipelago (British Columbia, Canada) evolved for about 10,000 years in the absence of large-mammal browsing. The introduction of black-tailed deer (Odocoileus hemionus sitkensis) from the mainland prior to 1901 provides an opportunity to evaluate changes in the adaptive defensive responses of plants to herbivory. We compared (1) food choice by deer and (2) chemical defence (terpene concentrations) between HG and mainland red cedars (Thuja plicata) using (1) nursery-grown seedlings never exposed to deer, (2) branches from trees that grew before the introduction of deer ("old trees") and (3) saplings exposed to deer herbivory on the mainland and on HG. We used the first two plant categories to test the hypothesis that plants that evolve under low herbivory levels have lower anti-herbivore defences. We used saplings to study the consequences of the dramatic increase in browsing on HG. During food experiments, deer preferred HG seedlings and old tree branches compared to those from the mainland. Total monoterpene concentrations were much higher than diterpene concentrations in all plant categories. Within plant categories, multivariate analysis showed that terpene profiles differed significantly between HG and mainland red cedars: HG seedlings and old trees had lower monoterpene levels. These results suggest that some monoterpenes may be determinants of deer food choice and that the defences of HG plants are less effective than those of mainland plants. The deer used branches from HG and mainland saplings indiscriminately. However, terpene profiles differed significantly between HG and mainland saplings, with multivariate analysis suggesting a higher defensive response in browsed HG saplings. Monoterpene profiles were different in lightly and heavily browsed saplings from HG, suggesting that under the current browsing regime, individuals with the greatest constitutive defences, or with greatest potential for induced defences, grow better and are selected on HG.

Summary 1.When a plant species is introduced into a new range, it may differentiate genetically from the original populations in the home range. This genetic differentiation may influence the extent to which the invasion of the new range is successful. We tested this hypothesis by examining Senecio pterophorus , a South African shrub that was introduced into NE Spain about 40 years ago. We predicted that in the introduced range invasive populations would perform better and show greater plasticity than native populations. 2. Individuals of S. pterophorus from four Spanish (invasive) and four South African (native) populations were grown in Catalonia, Spain, in a common garden in which disturbance and water availability were manipulated. Fitness traits and several ecophysiological parameters were measured. 3. The invasive populations of S. pterophorus survived better throughout the summer drought in a disturbed (unvegetated) environment than native South African populations. This success may be attributable to the lower specific leaf area (SLA) and better water content regulation of the invasive populations in this treatment. 4. Invasive populations displayed up to three times higher relative growth rate than native populations under conditions of disturbance and non-limiting water availability. 5. The reproductive performance of the invasive populations was higher in all treatments except under the most stressful conditions (i.e. in non-watered undisturbed plots), where no plant from either population flowered. 6. The results for leaf parameters and chlorophyll fluorescence measurements suggested that the greater fitness of the invasive populations could be attributed to more favourable ecophysiological responses. 7. Synthesis . Spanish invasive populations of S. pterophorus performed better in the presence of high levels of disturbance, and displayed higher plasticity of fitness traits in response to resource availability than native South African populations. Our results suggest that genetic differentiation from source populations associated with founding may play a role in invasion success.