Search

We examined recruitment, survival, life cycle and fecundity of two metallicolous (M, on metalliferous calamine soils) and two non-metallicolous (NM, on normal soils) populations of Thlaspi caerulescens in Belgium and Luxemburg. In each population, permanent plots were monitored over two reproductive seasons. In M populations, plots were located in two contrasting environments (grass versus grove) in order to test the influence of vegetation cover on life strategy. Our results show that the monocarpic life cycle is dominant in all populations of T. caerulescens. However the length of the pre-reproductive period varies from several months (winter annuals) to 1 year or more (perennials), and is partly related to plant origin (M versus NM). Most plants growing in metalliferous environments were annuals, whereas NM plants were mostly perennials. These differences in life cycle were related to differences in survival during summer, which was better in NM than in M populations. Within each M population, different survival conditions and life cycles were observed according to vegetation cover. Plants growing in grass areas were mostly annuals and had a low survival rate in summer whereas grove plants were mostly perennials and survived better in summer. Our results suggest the selection of stress avoiders (shortening of life cycle) in M populations of T. caerulescens but only for individuals growing in grass areas. Summer survival seems to play a key role in selection of life strategy in T. caerulescens.

Drought is common in Mediterranean-type climates. Water stress can have serious physiological consequences for plant fitness. Here we analysed the response of two alien invasive species, Senecio inaequidens DC. and S. pterophorus DC., and one native non-invasive, Senecio malacitanus Huter, in terms of photosynthesis, water relations and growth. The proportional reduction in growth as a result of water stress was smaller in S. malacitanus, followed by S. inaequidens and finally S. pterophorus. Variations in relative growth rate were related to differences in unit leaf rates, which are strongly correlated with photosynthesis. At a similar level of leaf relative water content (RWC), photosynthesis in S. inaequidens and S. malacitanus did not differ, whereas it was lower in S. pterophorus. S. malacitanus started to show a reduction in RWC later than the other species. The hypothesis that alien invaders have greater physiological tolerance to drought than native non-invaders is not supported by our results since S. malacitanus showed a more adaptive response to drought than the aliens and was also the most resistant of the three species to water shortage. Differences in invasiveness would therefore be explained by a combination of traits, including establishment capacity, competitive capacity and drought resistance, among others.

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.

A good way to check hypotheses explaining the invasion of ecosystems by exotic plants is to compare alien and native congeneric species. To test the hypothesis that invasive alien plants are more competitive than natives, we designed a replacement series experiment to evaluate interspecific competition between three Senecio species representing the same bushy life form: two alien species (S. inaequidens and S. pterophorus, both from South Africa) and a native species from the south-east of the Iberian Peninsula and Maghreb (S. malacitanus). While S. inaequidens is widespread throughout western Europe and is expanding towards the south of Spanish–French border, the geographical distribution of the recently introduced S. pterophorus is still limited to north-eastern Spain. Plants from each species were grown in pure and in mixed cultures with one of their congeners, and water availability was manipulated to evaluate the effects of water stress on competitive abilities. Our results show that the alien S. inaequidens is the most competitive species for all water conditions. The native S. malacitanus is more competitive that the alien S. pterophorus in water stress conditions, but this situation is reversed when water availability is not limiting.

A good way to check hypotheses that explain the invasion of ecosystems by exotic plants is to compare congeneric alien and native species. To test the hypothesis that alien invaders grow faster than natives, the maximum relative growth rate and its components were compared in controlled growth conditions between four Senecio species, two aliens introduced from southern Africa (S. inaequidens and S. pterophorus) and two European natives (S. malacitanus and S. jacobaea). The four species colonize similar habitats, but the frequency and abundance of their populations and their distribution ranges differ. The two aliens showed a higher relative growth rate than the natives, and although there were differences between species for leaf area ratio, leaf dry matter content, and dry matter partition between stems, leaves, and roots, no clear pattern was detected to explain the differences in growth rates: several combinations of the components of the relative growth rate can give similar results. The higher relative growth rate of the alien species, combined with other ecological and life-history traits, may enhance their invasive capacity.

Three series of seed addition experiments in natural sites and in common experimental conditions are described to compare the effect of disturbance, water availability, and habitat type on the emergence and establishment of the two alien congeners Senecio inaequidens D.C. and Senecio pterophorus D.C. and the native Senecio malacitanus Huter. The emergence of the three species was favored by disturbance, while establishment was enhanced by water availability. Habitat also had a dramatic effect on emergence, with more seedlings germinating under the shrubs and in the forest. However, only the plants that emerged in the open annual grassland survived until the adult stage, some of which flowered, mostly S. inaequidens. The emergence of the alien S. pterophorus was lower than that of the other species, but once established it produced significantly more biomass than the others. The effect of disturbance and water availability on emergence and establishment does not explain the differences in invasive capacity between the two aliens and the native; however, some traits, such as the biomass production of S. pterophorus and the early flowering of S. inaequidens, can contribute to the capacity of the two aliens to colonize new habitats.Key words: Senecio, alien invasive, emergence, disturbance, water addition, seed or site limitation.

A field experiment was designed to evaluate the importance offacilitative and competitive interactions in Picrishieracioides, a facultative biennial that colonises the early andthemid-stages of secondary succession in the Mediterranean region. Seedlings ofPicris hieracioides from populations of the early- (1year)and the mid-stages of field abandonment (15–40 years) were transplantedintwo adjacent old fields, abandoned for 4 (F4) and 20 years (F20) and thatdiffered markedly in floristic composition and vegetation structure. For twoyears, we experimentally manipulated competition (no-neighbours vs. naturalvegetation) and resource availability (addition of water and fertiliser vs.controls) in an attempt to evaluate their influence on survival, reproductivetiming, growth and reproductive output throughout the life cycle. Earlymortality was higher in non vegetated plots in both fields. Mortality ofseedlings was mainly due to herbivory by larvae of genusAgriotes. Flowering throughout the whole experiment wasalso facilitated by vegetation in the F4 field as a result of the positiveeffect of annual vegetation and remained unaffected in the F20 field because ofthe high competitive effect of established perennial vegetation. The additionofresources altered the effect of facilitation and competition on late seedlingsurvival. Survival was enhanced in the vegetated plots of the F4 field, becauseresource addition increased the shade provided by the canopy of vegetation,protected seedlings from temperature extremes and reduced water loss. Seedlingmortality also decreased in the F20 field but in a similar manner to vegetatedand non-vegetated subplots, and consequently the outcome of positive andnegative interactions remained neutral. The net effect of facilitation andcompetition resulted in interference later in the life cycle and appearedthrough final lower growth and reproduction in both fields. However, thegreatercompetition in 1994 than in 1995 in both fields, probably because the size ofthe rosettes, makes them less susceptible to competition, illustrates thedifficulty in predicting the outcome of competition solely of one season forfacultative biennial plants. The relative competition intensity calculatedusingonly survivors (RCI1) was unaffected by habitat fertility in bothfields. In striking contrast, the relative competition intensity calculatedusing seedling mortality (RCI2) was significantly higher in subplotswithout resource addition in both fields because of high seedling mortality invegetated subplots. Finally, there were no differences in the net effect offacilitation and interference processes among populations from early and midsuccessional stages showing that phenotypic plasticity buffers theenvironmentalselective pressures linked to successional processes.

Patterns in flowering and biomass allocation in seedlings of Rumex acetosella L. collected from five successional old fields, from 1 year old to 15 years old, have been examined in experimental pots varying in density (1, 2, and 4 plants/pot) and neighbor relatedness (sib, nonsib, other population) to determine the effects of successional habitat variation on patterns of resource allocation. The flowering of seedlings from the successionally young populations was not affected by density, whereas for seedlings from the older populations, increased density was correlated with increased variation in flower and seed production both within and between populations. At high density, seedlings from the successionally youngest population showed the greatest allocation of resources to flowering. As a result, differentiation along the successional gradient was such that the younger populations invested a greater proportion of resources to aerial biomass, while the older populations allocated relatively more resources to vegetative propagation. These trends were maintained at high density. The results of the sibling competition treatment showed no consistent trend related to the age of the populations but were density dependent. At high density, sexual biomass was higher between nonsibs than between sibs of the same population, suggesting greater competition among related plants. Finally, the sexual biomass of individual plants was less in competition with seedlings from a different population than with seedlings from the same population. Sprout biomass showed the reverse trend. The observed differentiation between successionally different populations in resource allocation irrespective of density may account for these results. Overall, our results provide evidence for biotic specialization of R. acetosella in relation to successional habitat change. Key words: density, sibling competition, succession, trade-off, Rumex acetosella, resource allocation.

Two sets of plants of Rumex acetosella L. of known sex (males and females in the same proportions), belonging to populations from two fields that were abandoned 2 and 15 years ago, were grown on soil regularly watered to field capacity or to half of field capacity. Four harvests of six plants for each sex, population and treatment were performed to follow the changes in biomass allocation to aerial and subterranean organs as related to plant water potential. In well-watered plants, the old population had a higher rate of water use than the young one because of rapid leaf surface area development and a higher root/shoot biomass ratio, which should make it more competitive in situations of strong interspecific competition, such as the late stages of succession. When plants were subjected to water stress (half of field capacity), clear-cut differences appeared between males and females at the vegetative stage. Female plants showed a good regulation of water balance and reached higher biomass values than male plants. However, flowering and fruiting induced a decrease in the root/shoot biomass ratio and in the transpiration rate of female plants, which would tend to exclude them with time. These results may explain the fact the the oldest Rumex acetosella communities consist predominantly of males.

International audience; At present, no efficient technique is available for cleaning up soils which are highly polluted by heavy metals. Limiting the movement of pollutants out of the contaminated area by creating a dense and persistent plant cover appears to be the more reasonable approach. In this context, phytostabilization is a technique that uses metallicolous plants to revegetate highly polluted soils.This paper presents the results of an experiment performed in situ using metallicolous ecotypes of four plant species native to the Mediterranean French region, and grown in different combinations at a polluted site over two years. The soils were highly polluted with zinc, cadmium and lead. The aim was to find the best species mixture in terms of cover, biomass and duration. The four species used were the biennial legume Anthyllis vulneraria, two perennial grasses, Festuca arvernensis and Koeleria vallesiana, and the perennial forb Armeria arenaria. Mixtures which included A. vulneraria, and especially when in combination with F. arvernensis, showed the highest values of cover and biomass. After flowering, the biennial individuals of A. vulneraria disappeared but subsequent germination and survival of seedlings occurred abundantly under the two grasses. Mixtures with A. arenaria showed the lowest values of cover and biomass. Soil nitrogen increased in the plots with A. vulneraria as well as the concentration of essential nutrients (N P K) in the aerial parts of the two grasses. In contrast, the concentration of metals (Zn Pb Cd) decreased in the aboveground biomass of the latter in the same plots.These results show that reciprocal facilitation effects can act in heavy metal polluted environments, and that phytostabilization efforts in the Mediterranean region can be improved by using mixtures including local metallicolous legume and grass species.

International audience; The relationship between zinc (Zn) tolerance and hyperaccumulation in Thlaspi caerulescens was investigated from F-1 and F-2 crosses within and among metallicolous and nonmetallicolous Mediterranean populations. F-1 offspring were grown on increasingly Zn-enriched soils to test Zn enrichment effects, and many families of F-2 offspring were grown on a Zn-rich soil. Tolerance of F-1 offspring depended on stress intensity. Tolerance of interecotype crosses was intermediate between that of the intraecotype crosses. No difference in Zn accumulation appeared among the F-1 offspring from the three crosses involving metallicolous parents. Otherwise, none of these offspring exceeded the Zn hyperaccumulation threshold (10 000 mg kg(-1)), unlike the nonmetallicolous ones. The latter also showed the highest mortality. In some F-2 families from interecotype crosses, hyperaccumulation values exceeded 15 000 mg kg(-1) in nontolerant offspring, whereas tolerant offspring displayed lower values (c. 10 000 mg kg(-1)). There was no difference between tolerant and nontolerant offspring when they showed low hyperaccumulation. Therefore, the relationship between tolerance and hyperaccumulation in F-1 and F-2 crosses depended on the hyperaccumulation level of plants.

International audience; # Genetic variation structure and breeding system were investigated in metallicolous (MET) and nonmetallicolous (NONMET) populations of the heavy-metal hyperaccumulator Thlaspi caerulescens from Belgium, Luxembourg and the French Mediterranean region. # • Allozyme variation showed a clear differentiation between the two ecotypes in Belgium and Luxembourg but not in southern France, in line with the lower degree of geographical isolation between the two ecotypes in this region. # • In both regions inbreeding coefficient (FIS) of NONMET populations was significantly higher compared to MET populations. Pollen/Ovule ratios were significantly higher in MET compared with NONMET populations. # • These results suggest that NONMET populations of T. caerulescens are more self-fertile than their MET counterparts. This contrasts with earlier studies on other metal-tolerant species in which selfing rates were higher in MET populations. This pattern may be explained by the fact that both ecotypes are not in sympatry in the populations studied, and therefore reproductive isolation has not been selected to maintain the adaptations to metal-contaminated soils. In addition, higher selfing rates in NONMET populations may have evolved as a mechanism of reproductive assurance, because these populations generally are at low densities.

Alien invasive species undergo genetic bottlenecks during the colonisation of new areas that can lead to a loss of genetic variation within populations and to subsequent reproductive constraints. We analysed the self-compatibility and the effects of inbreeding and inter-population gene flow in the fitness of one native and one introduced population of the invasive Senecio pterophorus D.C. Plants were self-pollinated and outcrossed within families, within populations, between local populations and between populations located in the native and introduced range. Self-pollinated individuals from both populations produced almost no seeds, thus revealing self-incompatibility. High family-level variation was observed in the effect of pollination treatment on seed set and total fitness. Overall, in the Spanish population, related crosses produced fewer seeds and lower germination rates than unrelated crosses. In the South African population, inbreeding depression affected the probability of flowering. Heterosis was found to affect seed set in both populations and growth and mean pre-reproductive time in the Spanish population. We discuss the effects of the incompatibility system, inbreeding depression and long-distance gene flow within the introduced population with respect to the invasive potential of S. pterophorus in north-eastern Spain.

Question: Plant invasions result from complex interactions between species traits, community characteristics and environmental variations. We examined the effect of these interactions on the invasion potential of two invasive Senecio species, S. inaequidens and S. pterophorus, across three Mediterranean plant communities in a natural park. Location: Catalonia, NE Spain. Methods: We carried out two series of experimental seedling transplantations, in the spring and fall of 2003, in grassland, shrubland and Quercus ilex forest. Competition with neighbouring plants and water availability were manipulated. We evaluated the survival, growth and reproduction with respect to each treatment combination. Results: Any habitat can be colonised if disturbance occurs. In the absence of disturbance, shrubland enhanced the survival of seedlings. Competition with resident vegetation dramatically reduced survival in grassland and forest when establishment occurred in the spring. However, establishment in the fall...

Populations of the dioecious herb Rumex acetosella show variation in sex ratio along an old-field successional gradient in central France, where female-biased or 50:50 ratios occur in young populations and male-biased ratios occur in old populations. In order to test the hypothesis that differences in environmental sensitivity and/or reproductive biology account for sex related differences in habitat occupation of this species, two complementary experiments were performed. First, the environmental sensitivity (or reaction norms) of biomass allocation for male and female plants was examined over an experimental gradient of density and nutrients to test whether the sexes differ in habitat preference (...)

The effects of time since abandonment and date of flowering on the number and biomass of seeds of Rumex acetosella were studied at monthly intervals over a single growing season in a successional series of abandoned fields at two sites near Paris, France. Overall, a decrease in filled-seed production per plant was observed with increasing successional age, but only for the early summer period. A similar trend was observed for reproductive effort, i.e. filled-seed production per unit of vegetative biomass (...)

(1) Rumex acetosella is a dioecious perennial species which reproduces both by seeds and by adventitious shoots. The progression of the floral sex ratio was monitored for four years in experimental populations grown from seeds collected from five abandoned fields located south of Paris, France. At each census, all ramets in flower were scored for sex type. (2) In the first year, sex ratios were balanced, or slightly female biased. In the second year, sex ratios were seen to vary quite widely, depending on census date, population and also on the replicate sample within the population. In the third and fourth years male ramets greatly outnumbered females. Thus in the long term, male ramets showed greater clonal growth, i.e. recovered better than females following the repeated energetic cost of sexual reproduction. (3) The results of the present study suggest the importance of examining the age and past history of a population, as the relative contributions of sexual vs. vegetative reproduction may lead to different sex ratios depending on the time of establishment of the population.

The sex ratios of natural populations of Rumex acetosella L. have been recorded during two growing seasons, along a successional gradient of abandoned agricultural fields of various ages (from 6 months to 15 years old), south of Paris. At the same time, male and female ramets have been collected to determine the resource allocation pattern of the different organs. We have observed that females of Rumex acetosella L. were more frequent than males at the younger stages, and that males predominated at the older ones. Concomitantly, there was a decrease in aboveground biomass and in the reproductive effort of the two sexes in the field, with increasing community maturity. The decrease in reproductive effort was confirmed, in controlled conditions, in the female genets only. Females always produced a higher reproductive effort and a higher aboveground biomass than males, which may explain why they were more abundant in the younger successional stages. Otherwise, our results show that sexual reproduction represented a high energetic cost which tended to consume root resources. Females, and mainly those of the younger stages which presented a high reproductive effort, may suffer more than males. The latter, which were more vigorous in vegetative reproduction and less affected by flower production than females, prevailed at older stages.