Flooding effects on rapid responses of the invasive plant Alternanthera philoxeroides to defoliation

In experiments under controlled growth conditions it was examined how flooding affected the responses of the invasive plant Alternanthera philoxeroides to defoliation. In drained and flooded conditions, plants were subjected to five defoliation levels: 0, 10, 50, 90% removal of leaf tissue and apex removal (90% leaf tissue plus apical bud removal). Plants were harvested weekly for five weeks. In drained conditions, plant biomasses including total biomass, shoot biomass and root biomass after 50% defoliation rapidly recovered to the control plant level. They were significantly lower for the 90% defoliation and apex removal treatments compared to control plants throughout the experiment. In flooded conditions, total biomass and shoot biomass after 50% defoliation, 90% defoliation, and apex removal treatments could return to control plant levels before the end of the experiment. In 90% defoliation and apex removal treatments root to shoot biomass ratios of both drained and flooded plants were initially much higher than in control plants, but the difference disappeared rapidly. The final biomasses decreased with increased defoliation intensity in drained conditions, but no significant difference was generally found in any of the defoliation treatments in flooded conditions. The rapid re-growth of A. philoxeroides plants after defoliation may partly be responsible for its invasion success. However, defoliation capable of removing 90% of the leaf tissue may be desirable in restricting the growth of this invasive species in drained conditions.     

Phenotypic plasticity of invasive Alternanthera philoxeroides in relation to different water availability, compared to its native congener

Phenotypic plasticity and genetic differentiation are two possible mechanisms that plants use to cope with varying environments. Although alligator weed (Alternanthera philoxeroides) possesses very low genetic diversity, this alien weed has successfully invaded diverse habitats with considerably varying water availability (from swamps to dry lands) in China. In contrast, its native congener (Alternanthera sessilis) has a much narrower ecological breadth, and is usually found in moist habitats. To understand the mechanisms underlying the contrasting pattern, we performed a greenhouse experiment to compare the reaction norms of alligator weed with those of its native congener, in which water availability was manipulated. Our results revealed that the two congeners had similar direction of phenotypic plasticity. However, A. philoxeroides showed greater plasticity in amount than did A. sessilis in many traits examined during the switch from wet to drought treatment. Nearly all of the phenotypic variance in A. philoxeroides could be ascribed to plasticity, while A. sessilis had a much higher fraction of phenotypic variance that could be explained by genotypic variation. These interspecific differences in plastic responses to variable water availability partially explained the difference in spatial distribution of the two congeners.     

The bud and root sprouting capacity of Alternanthera philoxeroides after over-wintering on sediments of a drained canal

Alternanthera philoxeroides (Mart.) Griseb. is one of many aggressive invasive plants that can grow in diverse habitats. Aquatic A. philoxeroides forms dense floating mats over the water surface. However, when water levels decrease during winter, some mats become stranded on exposed sediments and are thus exposed to air. Do the stems of these mats possess the capacity to develop new shoots during the next growing season? In this study, we examined the sprouting of sediment-stranded over-wintering mats of A. philoxeroides. Stems of the over-wintering mats were divided into three types (dry, withered, and fresh stems) depending on moisture content and were immersed in water for 4 weeks to observe the sprouting of axillary buds and roots. The results showed that withered stems yielded much more biomass than dry or fresh stems. Stem moisture content significantly affected the sprouting rate and the length growth rate of buds and roots. Dry stems lacked reproductive capacity. The sprouting rate and length growth rate of the buds and roots were higher in fresh stems than in withered stems. Furthermore, the mean values of the bud sprouting rate and the bud length growth rate were highest during the first week, i.e., most of buds sprouted within 1 week or less. Our results suggest that more than 70% (on a dry weight basis) of the stems in stranded mats possessed rapid sprouting capacity even after over-wintering on the sediment for more than 2 months. This strategy may be an adaptation to the fluctuations inherent in many aquatic habitats, and it possibly explains why A. philoxeroides can flourish even after a dry winter. Handling editor: S. M. Thomaz     

Impact of simulated herbivory on Alliaria petiolata survival, growth, and reproduction

In weed biological control, insect damage to target weeds can be simulated in invaded habitats to study potential responses of the plant to introduced natural enemies. In the present study, we investigated the impact of two levels of manual flower-shoot damage (shoots cut at tip or base) on Alliaria petiolata (garlic mustard) survival, size, and reproduction. Experiments were conducted in 2002 and 2003 using invasive field populations of A. petiolata under naturally varying plant densities. Plant survival was recorded, and size and reproduction parameters were measured. Manual flower-shoot damage had a significant effect on plant survival. In both years, fewer plants survived in the basal-cut treatment than in either the control (un-cut) or tip-cut treatment. Plant size and reproductive output were likewise reduced in the basal-cut treatment. In both years, total seed production was significantly lower in the basal-cut treatment than either the control or tip-cut treatment. When combined, increased mortality and reduced seed production of basal-cut plants greatly reduced the contribution these plants made to the seed bank. Plant density did not affect reproduction or plant size. The impacts of cutting were consistent across years and sites with distinct biotic and abiotic conditions, and A. petiolata densities. We anticipate that herbivore damage to A. petiolata populations by introduced biological control agents will likewise remain consistent under varying biotic and abiotic conditions if the agents are equally adapted to these.     

Effects of soil nutrient heterogeneity on intraspecific competition in the invasive, clonal plant Alternanthera philoxeroides

Background and Aims

Fine-scale, spatial heterogeneity in soil nutrient availability can increase the growth of individual plants, the productivity of plant communities and interspecific competition. If this is due to the ability of plants to concentrate their roots where nutrient levels are high, then nutrient heterogeneity should have little effect on intraspecific competition, especially when there are no genotypic differences between individuals in root plasticity. We tested this hypothesis in a widespread, clonal species in which individual plants are known to respond to nutrient heterogeneity.

Methods

Plants derived from a single clone of Alternanthera philoxeroides were grown in the greenhouse at low or high density (four or 16 plants per 27·5 × 27·5-cm container) with homogeneous or heterogeneous availability of soil nutrients, keeping total nutrient availability per container constant. After 9 weeks, measurements of size, dry mass and morphology were taken.

Key Results

Plants grew more in the heterogeneous than in the homogeneous treatment, showing that heterogeneity promoted performance; they grew less in the high- than in the low-density treatment, showing that plants competed. There was no interactive effect of nutrient heterogeneity and plant density, supporting the hypothesis that heterogeneity does not affect intraspecific competition in the absence of genotypic differences in plasticity. Treatments did not affect morphological characteristics such as specific leaf area or root/shoot ratio.

Conclusions

Results indicate that fine-scale, spatial heterogeneity in the availability of soil nutrients does not increase competition when plants are genetically identical, consistent with the suggestion that effects of heterogeneity on competition depend upon differences in plasticity between individuals. Heterogeneity is only likely to increase the spread of monoclonal, invasive populations such as that of A. philoxeroides in China.     

The alkaloidal responses of wild tobacco to real and simulated herbivory

Summary I compared the induced alkaloidal response in undamaged leaves of plants subjected to herbivory by the larvae of Manduca sexta and to different simulations of this herbivory; all herbivory treatments removed similar amounts of leaf mass. Although larval feeding induced a significant increase (2.2x) in alkaloid concentrations compared to undamaged plants, the alkaloid responses to larval feeding were significantly lower than the responses to an herbivory simulation (4x controls) which involved removing the same amount of leaf area from the same positions on the leaf, over a similar time period. Moreover, another herbivory simulation, identical in amount of leaf mass removed and duration of damage to the larval feeding, but without regard to spatial array of leaf damage, resulted in an alkaloidal response (5.5x controls) higher still than the previous herbivory simulation. In a second experiment the importance of leaf vein damage on the induced alkaloidal response was examined. Here, leaf removal that involved cutting leaf tissues from between secondary veins before removing the midrib, resulted in alkaloidal responses that were significantly lower (1.7x controls) than responses from leaf removal that involved cutting both veins and midribs along with the intervein tissues (2.6x controls). Vein damage alone did not produce a significant response. These results indicate that herbivory is difficult to simulate: that how a leaf is damaged can be as important as the magnitude of leaf damage in determining a plant's response to damage.     

Effects of simulated herbivory on tillering and reproduction in an annual ryegrass,Lolium remotum

This study examined the overall impact of simulated herbivory on tillering and reproductive performance of an annual ryegrass,Lolium remotum. The interaction between herbivore damage and intraspecific competition and the effect of the timing of damage were also studied. The experimental plants were sown at two densities and were randomly assigned to eight different damage treatments consisting of artificial leaf area removal by clipping with scissors or removal of one-third of the ripening seeds. The treatments were executed at two flowering stages. The pattern of tiller development differed significantly among treatments and between densities. At the lower density, earlier treatments delayed tiller development more than the same treatments executed later. At the higher density, all treatments delayed tiller development. The density effect was significant for all reproductive traits measured. The reproductive output of plants grown at the higher density was lower and the negative treatment effects were stronger than at the lower density. The treatment effect was significant for seed dry weight per plant and individual seed weight but not for number of seeds per plant. There were no statistically significant interaction effects between the damage treatments and density, suggesting that the plants responded to the damage similarly, irrespective of the density. The plants did not totally compensate for losses due to damage at either density, even though they slightly increased their resource allocation to sexual reproduction at the higher density.     

Genetic diversity of alligator weed ecotypes is not the reason for their different responses to biological control

Biological control of alligator weed Alternanthera philoxeroides (Mart.) Griseb. using Agasicles hygrophila, a Chrysomelid beetle, has been successful in limiting growth in water, but not on land. In order to understand a possible genetic basis of this difference, technique using inter-simple sequence repeats (ISSR) markers was applied to analyse genetic diversity of this invasive weed. No genetic variation was detected not only within or between populations growing in the same habitats, but also between land- and water-grown populations. Thus we consider that the genetic variation is not the baseline factor resulting in the biological control difference in China. The differential success of pupation by the beetle may be related to the phenotypic plasticity of the plant stem diameter, rather than to genotypic factors.     

Effect of herbivory by Teleonemia scrupulosa on the performance of Longitarsus bethae on their shared host, Lantana camara

Herbivory by insects may change the characteristics of nutrients and secondary plant chemicals of the foliage, thereby altering the acceptability and suitability of the plant for oviposition, feeding and development for subsequent herbivores. In the current study, the effect of herbivory by the sap-sucking lace bug, Teleonemia scrupulosa Stäl (Heteroptera: Tingidae), on the suitability of Lantana camara L. (Verbenaceae) for the root-feeding flea beetle, Longitarsus bethae Savini & Escalona (Chrysomelidae: Alticinae), was investigated under laboratory conditions. Preference of adult L. bethae was not influenced by the intensity of feeding damage caused by T. scrupulosa adults. However, high densities of T. scrupulosa nymphs and their feeding damage caused L. bethae adults to emigrate and colonize less infested or uninfested plants. Oviposition by L. bethae was significantly reduced at high densities of T. scrupulosa nymphs. While low infestation of T. scrupulosa had no effect the survival of L. bethae, moderate and high infestations caused significant reduction in percentage survival of L. bethae. The number of T. scrupulosa nymphs was negatively correlated with the percentage survival of L. bethae. Neither the duration of development nor the body size of L. bethae was influenced by the intensity of T. scrupulosa infestation. Overall, undamaged or slightly damaged plants that allowed better survival of L. bethae were often chosen as oviposition sites in preference to those that were highly infested, and on which survival was poor. Although the present study indicates the likelihood of inter-specific competition between L. bethae larvae and T. scrupulosa, this is likely to be mitigated by female flea beetles choosing to oviposit on less infested or uninfested plants in the field.     

Effects of herbivory simulated by clipping and jasmonic acid on Solidago canadensis

Herbivory is likely to be an important factor in the evolutionary ecology of plants and especially of aggressive invasive species. Because in the introduced range experimental treatment with non-native herbivores is laborious and may be risky for native plants, simulated herbivory by removal of plant tissue and treatment with jasmonic acid, which is a natural elicitor of defences against herbivores, may be a suitable alternative in experimental studies. We studied the effects of removal of 50% of the leaf area by clipping and of spraying with jasmonic acid on growth, morphology, leaf physiology and reproduction of the invasive Solidago canadensis in Europe. The relative height growth rate was reduced (–11.9%) by clipping in the period of day 0–20 but increased (+13.5%) in the period of day 42–138 after start of the treatments. As a consequence, final heights did not differ between treatment and control plants. Clipped plants, however, had thinner stems (–12.2%) than unclipped ones. Plants that had been sprayed with jasmonic acid tended to have shorter stem internodes (–14.7%), a lower specific leaf area (–4.6%), and to delay flowering (+4.4 days) than plants that had not been sprayed with jasmonic acid. The biomass of inflorescences was reduced by both clipping (–43.2%) and spraying with jasmonic acid (–32.2%). Because each, clipping and jasmonic acid, alone only induced some but partly different responses observed in previous studies with natural herbivores, the combined application of both covers the full response spectrum better and should therefore be used as the most realistic simulation of herbivory.Fehlende oder geringe Herbivorie ist ein wahrscheinlicher Erklärungsfaktor für den Erfolg invasiver Pflanzenarten. Da das Einführen von Herbivoren aus dem Ursprungsgebiet zu Versuchszwecken oder zur biologischen Kontrolle mit Gefahren verbunden ist, kann der Einfluss von Herbivoren durch mechanische Entfernung von Pflanzenteilen oder durch die Behandlung mit Jasmonsäure nachgeahmt werden. Jasmonsäure ist ein natürlicher Auslöser von Abwehrreaktionen gegen Herbivoren bei Pflanzen. Wir untersuchten den Einfluss des Entfernens von 50% der Blattfläche, des Besprühens mit Jasmonsäure oder einer kombinierten Behandlung auf das Wachstum und die physiologischen Eigenschaften der Blätter der invasiven Art Solidago canadensis in Europa. Blattentfernung reduzierte die relative Höhenwachstumsrate der Sprosse um 11.9% in der Periode von 0–20 Tagen nach der Behandlung aber führte zu einer anschließenden Steigerung um 13.5% in der Periode von 42–138 Tagen, so dass die Endhöhe der Pflanzen durch diese simulierte Herbivorie nicht beeinflusst war. Behandlung mit Jasmonsäure führte zu verkürzten Stengelinternodien (–14.7%), verkleinerter Blattfläche (–4.6%) und verzögerter Blütezeit (+4.4 Tage). Die Infloreszenzbiomasse wurde durch die Blattentfernung um 43.2% und die Jasmonatbehandlung um 32.2% reduziert. Da die beiden Behandlungsfaktoren das Pflanzenwachstum unabhängig voneinander und in unterschiedlicher Weise beeinflussten und die beobachteten Pflanzenreaktionen in der Summe den in der Literatur beschriebenen Reaktionen auf natürliche Herbivorie entsprachen, kann die Kombination von mechanischer und chemischer Behandlung als optimale Simulation von Herbivorie bei S. canadensis und möglicherweise anderen invasiven Arten betrachtet werden.     

Genetic diversity of alligator weed in China by RAPD analysis

Random amplified polymorphic DNA (RAPD) was applied to analyze geneticdiversity of an invasive weedy species, alligator weed (Alternantheraphiloxeroides (Martius) Grisebach), collected from eight differentsites in southern China. Amplified by 108 RAPD primers, 391 bands wereidentified from samples collected from three of the eight sites withconsiderably large spatial intervals, but no genetic variation was detectedamong the samples. A total number of 196 RAPD fragments were amplified from allsamples collected in the eight sites by 31 primers that produced the mostconsistent results, but no genetic variation was detected within or betweenpopulations. The molecular data indicated extremely low genetic diversity in thealligator weed. Given the fact that the alligator weed is a serious invasiveweed, and widely found in China, we consider that the low genetic diversity ofthe alligator weed does not affect the success of its expansion in China, andlow genetic diversity does not necessarily lead to endangered status of a plantspecies. In addition, molecular evidence from this study suggests that thealligator weed in southern China might originate from a very few clones or evenonly from a single clone. Therefore, the rapid range expansion of the alligatorweed is most likely the result of a massive vegetative propagation since it wasintroduced in China.     

Fusarium nygamai, a potential bioherbicide for Striga hermonthica control in sorghum

Glasshouse trials were performed to investigate the control of the parasitic weed Striga hermonthica by Fusarium nygamai and the performance of the host plant sorghum (Sorghum bicolor) using different inoculum substrates and inoculum amounts of the fungus. Optimal constant and alternating temperatures for the growth of the fungus were 25°C and 30/20°C, respectively. Striga incidence was decreased up to 100% when the fungus was incorporated into the soil preplanting. Emerged Striga plants at different stages of growth up to the flowering stage were killed by the fungus when the fungus was applied postemergent. In root-chamber trials none of the Striga seeds germinated when 10 ml inoculum suspension of 8 × 10⁶ spores/ml of F. nygamai was applied on seeds of the parasitic weed sprinkled on the surface of filter paper. F. nygamai has potential as a bioherbicide for Striga control. Further studies regarding its performance under field conditions and its safety to the environment and humans should be assessed.     

Effects of simulated herbivory in three old field Compositae with different inflorescence architectures

The effects of simulated herbivory (early or late defoliation and cutting of the flowering shoot) on the growth and reproduction of three species of monocarpic composite forbs (Crepis pulchra, Picris hieracioides and C. foetida) with different inflorescence architectures were studied in experimental plots. For the three species studied, early defoliation had no significant effect on subsequent growth. In contrast, late defoliation, occurring at the start of the season of drought, had a negative effect on growth and reproduction in the two Crepis species, particularly C. foetida, but had less effect on P. hieracioides. Sexual biomass was more clearly affected by late defoliation than was vegetative biomass, although the effects differed markedly among species possibly as a result of differences in phenology. Clipping the flowering shoot removed about 3 times less biomass than late defoliation and had little effect on vegetative biomass. It had much greater effects on the sexual biomass in P. hieracioides and C. pulchra, and resulted in the production of many shoots sprouting from the rosette, allowing the treated plants to regain a vegetative biomass close to that of control plants. Clipping did however lead to the production of shorter shoots and a reduction in the number of capitula formed. In C. foetida, much branching occurred even when the main shoot was not cut; the architecture of individual plants was therefore only slightly changed by clipping the apical bud and the sexual biomass of this species was not affected by ablation of the flowering shoot. Overcompensation was found in only two families of C. pulchra for vegetative biomass. No over-compensation was found for sexual biomass, despite an increase in the number of flowering shoots in C. pulchra and P. hieracioides following clipping. However situations close to compensation for the vegetative biomass in the three species and in P. hieracioides for the sexual biomass were recorded. The response of the three study species to simulated herbivory were related to their architecture and to the time of defoliation.     

Grazing tolerance and mycorrhizal colonization: Effects of resource manipulation and plant size in biennial Gentianella campestris

As herbivory usually leads to loss of photosynthesizing biomass, its consequences for plants are often negative. However, in favorable conditions, effects of herbivory on plants may be neutral or even beneficial. According to the compensatory continuum hypothesis plants can tolerate herbivory best in resource-rich conditions. Besides herbivory, also primarily positive biotic interactions like mycorrhizal symbiosis, bear carbon costs. Tritrophic plant–fungus–herbivore interaction further complicates plant's cost-benefit balance, because herbivory of the host plant is expected to cause decline in mycorrhizal colonization under high availability of soil nutrients when benefits of symbiosis decline in relation to costs. To gain insight into above interactions we tested the effects of plant size and resource manipulation (simulated herbivory and fertilization) on both above-ground performance and on root fungal colonization of the biennial Gentianella campestris.Clipping caused allocation shift from height growth to branches in all groups except in large and fertilized plants. For large plants nutrient addition may have come too late, as the number of meristems was most likely determined already before the fertilization. Clipping decreased the amount of DSE (dark septate endophytic) fungi which generally are not considered to be mycorrhizal. The effect of clipping on total fungal colonization and colonization by arbuscular mycorrhizal (AM) fungal coils were found to depend on host size and resource level. Dissimilar mycorrhizal response to simulated herbivory in small vs. large plants could be due to more intensive light competition in case of small plants. Carbon limited small plants may not be able to maintain high mycorrhizal colonization, whereas large clipped plants allocate extra resources to roots and mycorrhizal fungi at the expense of above-ground parts. Our results suggest that herbivory may increase carbon limitation that leads re-growing shoots and fungal symbionts to function as competing sinks for the limited carbon reserves.     

Effects of simulated root herbivory and fertilizer application on growth and biomass allocation in the clonal perennialSolidago canadensis

Summary Compensatory growth in response to simulated belowground herbivory was studied in the old-field clonal perennialSolidago canadensis. We grew rootpruned plants and plants with intact root systems in soil with or without fertilizer. For individual current shoots (aerial shoot with rhizome and roots) and for whole clones the following predictions were tested: a) root removal is compensated by increased root growth, b) fertilizer application leads to increased allocation to aboveground plant organs and increased leaf turnover, c) effects of fertilizer application are reduced in rootpruned plants. When most roots (90%) were removed current shoots quickly restored equilibrium between above-and belowground parts by compensatory belowground growth whereas the whole clone responded with reduced aboveground growth. This suggests that parts of a clone which are shared by actively growing shoots act as a buffer that can be used as source of material for compensatory growth in response to herbivory. Current shoots increased aboveground mass and whole clones reduced belowground mass in response to fertilizer application, both leading to increased allocation to aboverground parts. Also with fertilizer application both root-pruned and not root-pruned plants increased leaf and shoot turnover. Unfertilized plants, whether rootpruned or not, showed practically no aboveground growth and very little leaf and shoot turnover. Effects of root removal were as severe or more severe under conditions of high as under conditions of low nutrients, suggesting that negative effects of belowground herbivory are not ameliorated by abundant nutrients. Root removal may negate some effects of fertilizer application on the growth of current shoots and whole clones.     

Establishment and dispersal of the biological control weevil Rhinoncomimus latipes on mile-a-minute weed, Persicaria perfoliata

Mile-a-minute weed, Persicaria perfoliata (L.) H. Gross (Polygonaceae), is an annual vine from Asia that has invaded the eastern US where it can form dense monocultures and outcompete other vegetation in a variety of habitats. The host-specific Asian weevil Rhinoncomimus latipes Korotyaev (Coleoptera: Curculionidae) was first released in the US in 2004 as part of a classical biological control program. The weevil was intensively monitored in three release arrays over 4 years, and field cages at each site were used to determine the number of generations produced. The weevil established at all three sites and produced three to four generations before entering a reproductive diapause in late summer. Weevils dispersed at an average rate of 1.5–2.9 m wk⁻¹ through the 50 m diameter arrays, which had fairly contiguous mile-a-minute cover. Weevils dispersing in the broader, more variable landscape located both large monocultures and small isolated patches of mile-a-minute 600–760 m from the release within 14 months. Weevil density ranged from fewer than 10 to nearly 200 weevils m⁻² mile-a-minute weed. Mile-a-minute cover decreased at the site with the highest weevil density. The production of P. perfoliata seed clusters decreased with increasing weevil populations at two sites, and seedling production declined over time at two sites by 75% and 87%. The ability of the weevil to establish, produce multiple generations per season, disperse to new patches, and likelihood of having an impact on plants in the field suggests that R. latipes has the potential to be a successful biological control agent.     

Effects of light and nutrient availability on the growth,allocation, carbon/nitrogen balance,phenolic chemistry,and resistance to herbivory of two freshwater macrophytes

Phenotypic responses of Potamogeton amplifolius and Nuphar advena to different light (7% and 35% of surface irradiance) and nutrient environments were assessed with field manipulation experiments. Higher light and nutrient availability enhanced the growth of P. amplifolius by 154% and 255%, respectively. Additionally, biomass was allocated differently depending on the resource: high light availability resulted in a higher root/shoot ratio, whereas high nutrient availability resulted in a lower root/shoot ratio. Low light availability and high nutrient availability increased the nitrogen content of leaf tissue by 53% and 40% respectively, resulting in a 37% and 31% decrease in the C/N ratio. Root nitrogen content was also increased by low light and high nutrient availability, by 50% (P=0.0807) and 77% respectively, resulting in a 20% and 40% decrease in root C/N ratio. Leaf phenolics were significantly increased 72% by high light and 31% by high nutrient availability, but root phenolic concentrations were not altered significantly. None of these changes in tissue constituents resulted in altered palatability to crayfish. N. advena was killed by the same high nutrient treatment that stimulated growth in P. amplifolius, preventing assessment of phenotypic responses to nutrient availability. However, high light availability increased overall growth by 24%, but this was mainly due to increased growth of the rhizome (increased 100%), resulting in a higher root/shoot ratio. High light tended to increase the production of floating leaves (P=0.09) and significantly decreased the production of submersed leaves. High light availability decreased the nitrogen content by 15% and 25% and increased the phenolic concentration by 88% and 255% in floating and submersed leaves, respectively. These differences in leaf traits did not result in detectable differences in damage by herbivores.     

Effects of simultaneous changes in light, nutrients, and herbivory levels, on the structure and function of a subtropical turtlegrass meadow

The interactive effects of light, nutrients, and simulated herbivory on the structure and functioning of a subtropical turtlegrass bed were analyzed monthly from May to October 2001 in Perdido Bay, FL. For each of the three factors, two levels were evaluated in a factorial design with four replicates per treatment. The variables included: light, at ambient and 40% reduction; nutrients, at ambient and 2× ambient concentrations; and herbivory, with no herbivory and simulated effects of a density of 15 sea urchins/m². In practice, light levels turned out to be 40% of surface PAR for ambient conditions, and 16% for shaded plots. Biomass removed as herbivory represented, on average, slightly less than 20% of the above-ground biomass. Separate three-way ANOVAs found no significant three-way interactions for any of the response variables, and few two-way interactions. There were no significant nutrient effects on turtlegrass above-ground biomass, although nutrient additions produced significant decreases in epibiont biomass, and net above-ground primary production (NAPP); significant increases in below-ground biomass during the peak of the growing season. Shoot density and average number of leaves per shoot increased significantly, while the C/N ratio of the oldest leaf in the enriched plots decreased significantly. Light reduction significantly negatively affected all response variables, except below-ground biomass, shoot density and leaf length. Herbivory had isolated and inconsistent significant effects on below-ground biomass, shoot density, average number of leaves per shoot, and leaf length and width. Overall, our results indicate that nutrients are not limiting in Perdido Bay, and that nutrient additions had mostly detrimental effects. Light appeared to be the most important variable limiting seagrasses growth and abundance, and as with terrestrial plants, seagrasses seemed to respond more to light and nutrients than to herbivory. However, it is essential that additional tests of the single and interactive effects of the three key factors of light, nutrients and herbivory be done to evaluate the generality of our work, since our study is the first of its kind in seagrass meadows.     

Phenotypic plasticity rather than locally adapted ecotypes allows the invasive alligator weed to colonize a wide range of habitats

Both phenotypic plasticity and locally adapted ecotypes may contribute to the success of invasive species in a wide range of habitats. Here, we conducted common garden experiments and molecular marker analysis to test the two alternative hypotheses in invasive alligator weed (Alternanthera philoxeroides), which colonizes both aquatic and terrestrial habitats. Ninety individuals from three pairs of aquatic versus terrestrial populations across southern China were analyzed, using inter simple sequence repeat (ISSR) marker, to examine population differentiation in neutral loci. Two common gardens simulating aquatic and terrestrial habitats were set up to examine population differentiation in quantitative traits. We found no evidence of population differentiation in both neutral loci and quantitative traits. Most individuals shared the same ISSR genotype. Meanwhile, plants from different habitats showed similar reaction norms across the two common gardens. In particular, plants allocated much more biomass to the belowground roots in the terrestrial environment, where alligator weed may lose part or all of the aboveground shoots because of periodical or accidental disturbances, than those in the aquatic environment. The combined evidence from molecular marker analysis and common garden experiments support the plasticity hypothesis rather than the ecotype hypothesis in explaining the adaptation of alligator weed in a wide range of habitats.