Variation in P-content in aquatic plant tissues offers an efficient tool for determining plant growth strategies along a resource gradient

1. The application of functional ecology models to aquatic plants often relies on morphological and life‐history traits which may reflect, in part, the phenotypic plasticity displayed by aquatic plants. The present study was designed to evaluate the use of physiological traits, such as nutrition patterns, to describe aquatic plant strategies along a gradient of increasing resource availablity. 2. Taking phosphorus (P) as an example, nutrition‐use efficiencies were evaluated in five species, through the P‐content in plant tissues, the variations in P‐content according to nutrient availability and the perenniality of P‐storage. Plasticity in P‐storage was also investigated in Ranunculus peltatus, a morphologically highly plastic species. 3. In 2001, P‐content was analysed in Callitriche hamulata, C. obtusangula, C. platycarpa, Elodea nuttallii and R. peltatus tissue samples. These five species were sampled at nine different sites in streams along an increasing resource gradient in the Northern Vosges Biosphere Reserve (NE France). Variations of P‐content in the roots, stems and dissected and floating leaves of R. peltatus were also studied. 4. Only C. platycarpa and R. peltatus were found to occur in low nutrient availability conditions. Callitriche hamulata, C. obtusangula and E. nuttallii were restricted to mesotrophic and eutrophic sites. The highest nutrient‐use efficiency was found for E. nuttallii which was able to adapt its P‐storage to varying resource availabilities. Ranunculus peltatus was able to store high concentrations of P, but its P‐integration within the vegetative structure was less efficient under eutrophic conditions. Callitriche spp. appeared to have relatively low nutrient‐use efficiencies, although C. obtusangula displayed a high P‐content. While P was stored preferentially in roots in R. peltatus populations occurring in nutrient‐rich sites, there was no particular P‐storage organ for populations from nutrient‐poor sites. 5. On the basis of P‐usage, R. peltatus and E. nuttallii presented competitor traits, C. hamulata and C. platycarpa displayed stress‐tolerant nutrient signatures and for C. obtusangula, ruderal or competitor characteristics dominated. The use of physiological traits, such as nutrition patterns, may provide valuable, complementary information about aquatic plant strategies, independent from the influence of morphological trait plasticity often displayed by these plants.     

Morphological plasticity of a spreading aquatic macrophyte,Ranunculus peltatus,in response to environmental variables

Ranunculus peltatus Schrank is an aquatic macrophyte spreading in northeastern France. As with other Ranunculus species, its growth demonstrates considerable plasticity. A preliminary understanding of its development and precise adaptive strategy is possible through functional ecology. The present study took morphological traits into account in order to analyse R. peltatus' development and morphological plasticity in relation to environmental parameters. During the five-month growing season, field measurements of ten different morphological traits were performed monthly at twelve sampling sites presenting various physical and chemical characteristics. Three overall growth stages occurred between April and August 2000: a stage of elongation of the main stem, a stage of branching and flowering and a stage of decline associated with vegetative dispersion. The sampling sites were defined according to environmental parameters and divided into three groups corresponding to three morphologically different R. peltatus populations. Plants in upstream, nutrient-poor, undisturbed sites were small and achieved little sexual reproduction. Plants in nutrient-rich, undisturbed sites had long shoots and were branched. Plants in weakly shaded, disturbed sites were small but produced many flowers. Correlations between morphological traits and environmental parameters showed significant relationships between chemical parameters and some characteristic vegetative growth traits. Physical environmental parameters were found to be less well correlated with morphological traits than the chemical parameters. The phenology and morphological plasticity of R. peltatus confer competitive advantages that could explain its ability to spread in some circumstances. Finally, the previous classification of R. peltatus as a Grime CSR species is discussed in relation to its plasticity. According to its morphological characteristics, R. peltatus tended indeed to adopt a C-strategy in nutrient-rich undisturbed sites, a S-strategy in nutrient-poor undisturbed sites and a R-strategy in disturbed sites.     

An experimental study of the plastic responses of Ranunculus peltatus Schrank to four environmental parameters

The impact of four environmental parameters (water depth, type of substratum, current velocity and light intensity), on Ranunculus peltatus morphology and reproduction was tested in four 1 month semi- controlled experiments. Four development stages were underlined from April to August 2001 in R. peltatus: an elongation stage (April–June), a flowering stage (May–June), a fragmentation stage (June–July) and a potential regenerative stage (July–August). Water depth was therefore tested on R. peltatus elongation, type of substratum on R. peltatus elongation and flowering, current velocity on R. peltatus fragmentation and light intensity on its possible regeneration. The maximum development was measured for a 32 cm water depth. Current velocity did not have a significant effect on R. peltatus fragmentation. Regeneration depended strongly on light availability. This stage occurred only for unshaded or 50% shaded plants. Darkness prevented plants from regrowing.     

Microhabitat and morphometric variation in two species of <Emphasis Type="Italic">Prasiola</Emphasis> (Prasiolales,Chlorophyta) from streams in central Mexico

Prasiolales are characterized by high morphological plasticity. This problem in taxonomic delimitation of the species may be a result of environment heterogeneity. Habitat characteristics and morphological variation of P. mexicana and P. nevadensis, two species of freshwater leaf-like thallus green algae, were investigated in five sites in central México where the morphological traits of these species overlap. Comparisons were made between habitat characteristics among streams and transects with and without Prasiola samples. Although no consistent correlation was found between environmental variables and morphological traits, a significantly wider range of habitat characteristics of current velocity and irradiance and morphometric variation (lamina length, lamina diameter, cell length, surface of the thallus, thickness and number of layers) suggested that P. mexicana was more plastic than P. nevadensis in its physicochemical requirements. Our results suggest that small variation of microhabitat conditions in P. nevadensis (irradiance and current velocity) could explain its very restricted geographic range in streams in central Mexico.     

Growth optimization of the invasive cyanobacterium Cylindrospermopsis raciborskii in response to phosphate fluctuations

The role of ecophysiological traits in the success and expansion of the toxic cyanobacterium Cylindrospermopsis raciborskii is still under debate. One key factor appears to be the high physiological flexibility of this organism when obtaining limiting resources. Recent studies have found that filamentous bloom-forming cyanobacteria are able to optimize their growth by adjusting phosphate uptake during fluctuating nutrient conditions. We investigated the growth response of two phosphate-deficient C. raciborskii isolates (MVCC19 from Uruguay and CCMP1973 from USA) to short-term fluctuations in phosphate supply. These isolates were exposed to five phosphate concentrations which were provided in two supply modes: a single pulse (SingleP) versus the same amount divided in 10 pulses (TenP), with one pulse applied every 6 min. Morphological traits and changes in chlorophyll a and phycocyanin fluorescence were also evaluated. Growth rates of CCMP1973 and MVCC19 almost doubled and tripled, respectively, when exposed to multiple rather than single phosphate additions. Different growth rates were observed with the same total added resource, thus contradicting the classical model of dependence of growth rate on absolute external concentration. Phosphate-deficient C. raciborskii showed a remarkable physiological flexibility in adapting to phosphate availability on a timescale from minutes to hours. The TenP mode provided an extension of phosphate exposure time that allowed the energetic optimization of uptake and growth. The morphological plasticity of the species in response to phosphate supply mode was also shown by differences in trichome length and individual size. Our results are the first evidence of growth optimization of phosphate-deficient C. raciborskii to short-term nutrient fluctuations, revealing its physiological flexibility. This adaptive behaviour may help to explain the invasive success of this diazotrophic cyanobacterium in a wide range of aquatic ecosystems where phosphorus is frequently the limiting resource.     

Environmental Heterogeneity and Population Differentiation in Plasticity to Drought in <Emphasis Type="Italic">Convolvulus Chilensis</Emphasis> (Convolvulaceae)

Plant populations may show differentiation in phenotypic plasticity, and theory predicts that greater levels of environmental heterogeneity should select for higher magnitudes of phenotypic plasticity. We evaluated phenotypic responses to reduced soil moisture in plants of Convolvulus chilensis grown in a greenhouse from seeds collected in three natural populations that differ in environmental heterogeneity (precipitation regime). Among several morphological and ecophysiological traits evaluated, only four traits showed differentiation among populations in plasticity to soil moisture: leaf area, leaf shape, leaf area ratio (LAR), and foliar trichome density. In all of these traits plasticity to drought was greatest in plants from the population with the highest interannual variation in precipitation. We further tested the adaptive nature of these plastic responses by evaluating the relationship between phenotypic traits and total biomass, as a proxy for plant fitness, in the low water environment. Foliar trichome density appears to be the only trait that shows adaptive patterns of plasticity to drought. Plants from populations showing plasticity had higher trichome density when growing in soils with reduced moisture, and foliar trichome density was positively associated with total biomass. Co-ordinating editor: F. Stuefer     

Leaf morphological responses to variation in water availability for plants in the <Emphasis Type="Italic">Piriqueta caroliniana</Emphasis> complex

Distribution of plants and the expression of traits associated with environmental variation can be affected by both average conditions and the variance in conditions including extreme climatic events. We expect that these same factors should affect the distribution of plants in hybrid zones between ecologically distinct species where the hybrids should occupy ecotones or intermediate habitats. We evaluated water availability and leaf morphological differences among parental and hybrid populations of herbaceous perennial plants in the Piriqueta caroliniana complex along environmental gradients in Southeastern North America. We focus on two taxa in this group; the viridis morphotype, which occurs in southern Florida, and the caroliniana morphotype, which is distributed from northern Florida to southern Georgia. Advanced-generation hybrid derivatives of these morphotypes occupy a broad geographic region that extends across much of central Florida. Overall, we found that hybrid populations occurred in significantly drier locations, indicating that their habitat requirements are transgressive (i.e., exceeding parental values) rather than intermediate to the parental morphotypes. Water availability differed between the two sampling years, and plants displayed morphological changes in response to these changes in moisture. During the drier year, leaves were narrower and more hirsute, corroborating experimental results that these leaf traits are plastic, and confirming that plasticity occurs in natural habitats. Hybrids exhibited intermediate leaf traits (shape and size) across both years, and displayed transgressive (hair density) leaf traits during the drier year. The apparent canalization of the hybrids’ leaf morphological traits may contribute to their tolerance of variable environmental conditions and may partially explain why they have displaced the caroliniana morphotype in central Florida.     

<Emphasis Type="Italic">Prorocentrum rivalis</Emphasis> sp. nov. (Dinophyceae) and its phylogenetic affinities inferred from analysis of a mixed morphological and LSU rRNA data set

A new freshwater epiphytic Prorocentrum species, Prorocentrum rivalis, from the temperate region of the Haute-Vienne, France, is described. This species is the third freshwater species identified among approximately 60 marine Prorocentrum species. This new species is described using scanning electron microscope and phylogenetic analyses by a polyphasic approach (LSU rRNA sequences combined with 9 morphological characters). The phylogenetic analysis attests that P. rivalis is close to other planktonic freshwater species and the freshwater Prorocentrum clade is evolutionarily derived from an epiphytic freshwater prorocentroid ancestor. The unique marine species in the freshwater clade results from an ecophysiological reversion. P. rivalis differs from other epiphytic taxa by its rarity, its temperate distribution and its ecophysiological needs. The phylogeny confirms also that all planktonic Prorocentrum species are evolutionarily derived from epiphytic/benthic ancestors.     

Long-term physiological and morphological acclimation by the evergreen shrub <Emphasis Type="Italic">Buxus sempervirens</Emphasis> L. to understory and canopy gap light intensities

Physiological and morphological plasticity are essential for growth and reproduction in contrasting light environments. In dry forest ecosystems, light generalists must also cope with the trade-offs involved in synchronous acclimation to light availability and drought. To understand how the broadleaf evergreen tree-shrub Buxus sempervirens L. (common box) inhabits both understory and successional terrain of Mediterranean forest, we measured photosynthesis–fluorescence light response, morphological traits and architectural characteristics across a light gradient. Our results show that B. sempervirens exhibits stress resistance syndrome, with little change in net photosynthesis rate across a light availability gradient, due to compensatory physiological and morphological acclimation. Light energy processing and dissipation potential were highest in leaves of well-illuminated plants, with higher electron transport rate, fraction of open photosystem II reaction centres, non-photochemical quenching, photorespiration and dark respiration. In contrast, traits reducing light capture efficiency were observed in high light shrubs, including higher leaf mass per unit area, leaf clumping, leaf inclination and branch inclination. We suggest that both physiological and morphological plasticity are required for B. sempervirens to survive across a light gradient in a dry forest ecosystem, while exhibiting homoeostasis in photosynthetic gas exchange. We further speculate that the low growth rate of B. sempervirens is effective in full sun only due to a lack of competition in low resource microsites.     

Amphibian phenotypic variation along a gradient in canopy cover: species differences and plasticity

The distributions of many freshwater organisms correlate with a gradient in canopy cover, ranging from sunny wetlands to closed woodland ponds. Little is known about mechanisms that exclude species from some sections of the gradient while allowing persistence in others. I addressed this question by manipulating shading in 740‐l outdoor mesocosms and measuring several ecologically‐relevant traits in three species of amphibian larva (Rana temporaria and Triturus alpestris, generalists occupying the entire gradient; and Hyla arborea, a specialist in open habitats). Shading caused delayed development, but had no effect on survival and increased the growth rate of R. temporaria. Body and tail color were darker in the shade. Plasticity in morphological shape, consisting of reduced gut width and increased tail size under shaded conditions, may reflect poor food availability and low dissolved oxygen. The canopy generalist R. temporaria increased activity in the shade, spent more time basking in shallow water, and maintained high larval performance. Unexpectedly, the specialist H. arborea was also highly plastic. These results describe extensive phenotypic plasticity induced by shade, and highlight traits that may influence performance along the canopy gradient.     

Competitive ability of two Brassica varieties in relation to biomass allocation and morphological plasticity under varying nutrient availability

Green cabbage (Brassica campestris, leafy variety) and turnip (Brassica campestris var. rapifera, rooty variety) were grown in both monocultures and mixtures at three nutrient levels to investigate their responses to nutrient availability with respect to biomass allocation, morphological plasticity and competitive ability. Their allocation parameters and leaf morphological traits were affected by both nutrient availability and developmental stage. Both of the varieties had a smaller biomass allocation to leaf blades, but a greater allocation to petioles at high nutrient levels. Root:shoot ratio (RSR) of green cabbage decreased with increasing nutrient availability, whereas that of turnip increased. Turnip had a smaller leaf blade weight ratio (LBWR) than cabbage, being compensated for by larger leaf area ratio (LAR) and specific leaf area (SLA). Leaf area ratio and SLA of both the varieties increased with increasing nutrient availability as did their mean dry weights. The mean dry weight of turnip was slightly greater than that of green cabbage in their respective monocultures, while that of green cabbage was greater than that of turnip in their 1:1 mixture. Therefore, green cabbage, having inherently greater biomass allocation to leaves, was generally more competitive than turnip with more biomass allocation to roots, especially at higher nutrient levels. However, within a variety, morphological plasticity (variation in LAR and SLA) was more important than the plasticity in biomass allocation (e.g. variation in RSR and LBWR) in determining competitive ability. The implication of our results is that competition models based on biomass allocation pattern alone may fail to predict competitive outcomes and that such models should also take morphological plasticity into full account.     

Shaped by the past,acting in the present: transgenerational plasticity of anti‐predatory traits

Phenotypic expression can be altered by direct perception of environmental cues (within‐generation phenotypic plasticity) and by the environmental cues experienced by previous generations (transgenerational plasticity). Few studies, however, have investigated how the characteristics of phenotypic traits affect their propensity to exhibit plasticity within and across generations. We tested whether plasticity differed within and across generations between morphological and behavioral anti‐predator traits of Physa acuta, a freshwater snail. We reared 18 maternal lineages of P. acuta snails over two generations using a full factorial design of exposure to predator or control cues and quantified adult F₂ shell size, shape, crush resistance, and anti‐predator behavior – all traits which potentially affect their ability to avoid or survive predation attempts. We found that most morphological traits exhibited transgenerational plasticity, with parental exposure to predator cues resulting in larger and more crush‐resistant offspring, but shell shape demonstrated within‐generation plasticity. In contrast, we found that anti‐predator behavior expressed only within‐generation plasticity such that offspring reared in predator cues responded less to the threat of predation than control offspring. We discuss the consequences of this variation in plasticity for trait evolution and ecological dynamics. Overall, our study suggests that further empirical and theoretical investigation is needed in what types of traits are more likely to be affected by within‐generational and transgenerational plasticity.     

Differential population response of allocation,phenology, and tissue chemistry in <Emphasis Type="Italic">Spartina alterniflora</Emphasis>

Phenotypic variation within species is widespread among salt marsh plants. For Spartina alterniflora, the dominant species of low intertidal wetlands across the Altantic and Gulf coasts of the US, distinct phenological and morphological differences among populations from different latitudes have been found. To determine whether S. alterniflora plants from lower latitudes and those regenerated from Delaware tissue cultures would maintain differences from that of native plants, we conducted a field study in a natural salt marsh in Delaware, US. After two growing seasons, plant height, stem density, above- and belowground biomass, elemental composition, and nutrient resorption were measured. Natural variation in porewater salinity influenced physiological traits of Na⁺/K⁺ ratio regulation and nitrogen resorption efficiency similarly across populations. While plant height exhibited plasticity where populations tended to converge to a similar height, several other traits remained distinct. Delaware plants had a greater rate of rhizome growth than Georgia and Louisiana plants, which correlated with a greater magnitude of fall senescence. If traits such as seasonal translocation are plastic and can change with the length of the growing season, climate warming may alter belowground biomass production of S. alterniflora in wetlands of the mid-Atlantic.     

VARIATION AMONG POPULATIONS OF XANTHIUM STRUMARIUM (COMPOSITAE) FROM NATURAL AND RUDERAL HABITATS

We assessed the mechanisms underlying the ability of cocklebur to spread from its natural riverside habitats and establish weedy populations in urban waste areas. We collected fruits from plants growing in natural and urban ruderal habitats and planted 2 maternal families from each of 9 habitat populations in 3 experimental gardens. The gardens were all in full sunlight but differed in the availability of water and nutrient resources. Plant performance in the gardens was measured by numbers and size of fruits produced. Traits known to be associated with cocklebur reproductive success were also measured: times to emergence and anthesis, photosynthetic capacity, mean stomatal conductance, relative growth rate, and biomass allocation to leaves and stems. Although there were significant differences among populations in the tradeoff between rate of growth in height and timing of anthesis, these population differences were not associated with habitat. Apart from a tendency to produce larger fruits in ruderal populations, there were no detectable differences in the characteristics of plants from natural vs. ruderal habitats. Plants from both habitats did have substantial and significant plastic responses to growth environment. In the three experimental gardens, fruit numbers increased with resource availability but fruit size did not differ significantly. As resource availability increased, plants from both habitats sustained growth longer and became more branchy. Canonical discriminant analysis of all the somatic and reproductive traits together supported the idea that natural and ruderal populations do differ in their overall plastic response to resource availability. The subtly different plastic responses of plants from the two habitats do not arise by substantial adjustments in a few dominant traits, but instead by relatively minor adjustments in a host of functionally interrelated phenological, morphological, and physiological traits. It is these small but coordinated differences in the plastic responses of many traits that appear to differentiate cocklebur from natural vs. weedy urban habitats.     

Plastic bet-hedging in an amphicarpic annual: an integrated strategy under variable conditions

Amphicarpy is a form of diversified bet-hedging expressed mostly in annual plants, where two types of offspring are produced with two distinct ecological roles: long-range aerial dispersers and highly competitive subterranean, sedentary fruit. Emex spinosa is a semi-arid, amphicarpic annual, inhabiting habitats with different levels of environmental variation. We tested the hypothesis that, in E. spinosa, bet-hedging may be “fine-tuned” by plasticity in the phenotype ratio (aerial/subterranean fruit mass) as a function of environmental conditions. We conducted a greenhouse experiment, manipulating nutrient availability and intraspecific density, to determine the pattern of ratio shifts. In order to determine whether the integrated strategy is an adaptation to variable habitats, a similar common garden experiment was conducted, comparing two natural populations differing in environmental variability. The offspring ratio shifted in response to both nutrient availability and plant density. In pots containing single plants the ratio increased steeply with nutrient availability, while in pots containing eight plants a more moderate increase occurred. These shifts were the result of plasticity in allocation to both achene types, as well as ontogenetic effects on aerial achene production. The degree of response increased with the heterogeneity of the habitat of origin. We found evidence for an adaptive integrated strategy, with bet-hedging “fine-tuned” by phenotypic plasticity. Strenuous conditions tended to shift the offspring ratio towards securing subterranean reproductive success, while favorable conditions resulted in a shift towards dispersible achenes. The authors Asaf Sadeh and Hagai Guterman contributed equally to this study.     

On the fate of seasonally plastic traits in a rainforest butterfly under relaxed selection

Many organisms display phenotypic plasticity as adaptation to seasonal environmental fluctuations. Often, such seasonal responses entails plasticity of a whole suite of morphological and life‐history traits that together contribute to the adaptive phenotypes in the alternative environments. While phenotypic plasticity in general is a well‐studied phenomenon, little is known about the evolutionary fate of plastic responses if natural selection on plasticity is relaxed. Here, we study whether the presumed ancestral seasonal plasticity of the rainforest butterfly Bicyclus sanaos (Fabricius, 1793) is still retained despite the fact that this species inhabits an environmentally stable habitat. Being exposed to an atypical range of temperatures in the laboratory revealed hidden reaction norms for several traits, including wing pattern. In contrast, reproductive body allocation has lost the plastic response. In the savannah butterfly, B. anynana (Butler, 1879), these traits show strong developmental plasticity as an adaptation to the contrasting environments of its seasonal habitat and they are coordinated via a common developmental hormonal system. Our results for B. sanaos indicate that such integration of plastic traits – as a result of past selection on expressing a coordinated environmental response – can be broken when the optimal reaction norms for those traits diverge in a new environment.     

Very fine roots respond to soil depth: biomass allocation,morphology, and physiology in a broad-leaved temperate forest

Very fine roots (<0.5 mm in diameter) of forest trees may serve as better indicators of root function than the traditional category of <2 mm, but how these roots will exhibit the plasticity of species-specific traits in response to heterogeneous soil nutrients is unknown. Here, we examined the vertical distribution of biomass and morphological and physiological traits of fine roots across three narrow diameter classes (<0.5, 0.5–1.0, and 1.0–2.0 mm) of Quercus serrata and Ilex pedunculosa at five soil depths down to 50 cm in a broad-leaved temperate forest. In both species, biomass and the allocation of very fine roots were higher in the surface soil but lower below 10-cm soil depth compared to values for larger roots (0.5–2.0 mm). When we applied these diameter classes, only very fine roots of Q. serrata exhibited significant changes in specific root length (SRL; m g⁻¹) and root nitrogen (N) concentrations with soil depth, whereas the N concentrations only changed significantly in I. pedunculosa. The SRL and root N concentrations of larger roots in the two species did not significantly differ among soil depths. Thus, very fine roots may exhibit species-specific traits and change their potential for nutrient and water uptake in response to soil depth by plasticity in root biomass, the length, and the N in response to available resources.     

Root morphological plasticity and nutrient acquisition of perennial grass species from habitats of different nutrient availability

We studied the root foraging ability and its consequences for the nutrient acquisition of five grass species that differ in relative growth rate and that occur in habitats that differ widely in nutrient availability. Foraging responses were quantified, based on the performance of the plants in homogeneous and heterogeneous soil environments of the same overall nutrient availability. Although all species tended to produce a significantly higher root length density in a nutrient-rich patch, this response was significant only for the faster-growing species. The increased root length density resulted from small, though not significant, changes in root biomass and specific root length. The effectiveness of root proliferation was determined by quantifying the total amount of nutrients (N and P) accumulated by the plants over the course of the experiment. Plants acquired more N in a heterogeneous environment than in a homogeneous environment, although the total nutrient availability was the same. The ability to acquire nutrients (N or P) in the heterogeneous environment was not related to the ability of species to increase root length density in response to local nutrient enrichment. In contrast to other studies, our results suggest that the role of morphological plasticity of roots in acquiring patchily distributed resources is limited. Possible reasons for this discrepancy are discussed. Received: 11 September 1997 / Accepted: 28 February 1998     

Implication of nutrient and salinity interaction on the productivity of <Emphasis Type="Italic">Spartina patens</Emphasis>

Reintroduction of fresh water to coastal systems with altered hydrologic regimes is a management option for restoring degraded wetland habitats. Plant production in these systems is believed to be enhanced by increased nutrient availability and reduced salinity. Although studies have documented nutrient limitation and salinity stress in coastal marshes, interpreting the effects of freshwater reintroduction on plant production is difficult because high nutrient availability often is confounded with low salinity. We tested the hypothesis that plant growth response to nutrients does not vary with salinity in a greenhouse study. Treatments consisted of four nutrient concentrations and four non-lethal salinity levels; plant response was measured as biomass accumulation after 144 days of exposure. The significant interaction between salinity and nutrient concentrations indicates that response of Spartina patens marshes to freshwater inflows would vary by site-specific soil conditions. Biomass decreased with increased salinity at all four nutrient concentrations with variation among the nutrient concentrations decreasing as salinity increased. We demonstrate the importance of considering ambient salinity and nutrient soil conditions in restoration planning involving freshwater inflow. We propose salinity should remain a primary concern in restoration plans targeted at improving degraded S. patens-dominated marsh habitat.     

Constraints on the evolution of phenotypic plasticity in the clonal plant Hydrocotyle vulgaris

The evolution of phenotypic plasticity of plant traits may be constrained by costs and limits. However, the precise constraints are still unclear for many traits under different ecological contexts. In a glasshouse experiment, we grew ramets of 12 genotypes of a clonal plant Hydrocotyle vulgaris under the control (full light and no flood), shade and flood conditions and tested the potential costs and limits of plasticity in 13 morphological and physiological traits in response to light availability and flood variation. In particular, we used multiple regression and correlation analyses to evaluate potential plasticity costs, developmental instability costs and developmental range limits of each trait. We detected significant costs of plasticity in specific petiole length and specific leaf area in response to shade under the full light condition and developmental range limits in specific internode length and intercellular CO₂ concentration in response to light availability variation. However, we did not observe significant costs or limits of plasticity in any of the 13 traits in response to flood variation. Our results suggest that the evolution of phenotypic plasticity in plant traits can be constrained by costs and limits, but such constraints may be infrequent and differ under different environmental contexts.