Effects Of Nutrient Availability Research Articles

Effects of nutrient availability and Ochromonas sp. predation on size and composition of a simplified aquatic bacterial community

Predation and competition are two main factors that determine the size and composition of aquatic bacterial populations. Using a simplified bacterial community, composed of three strains characterized by different responses to predation, a short-term laboratory experiment was performed to evaluate adaptations and relative success in communities with experimentally controlled levels of predation and nutrient availability. A strain with a short generation time (Pseudomonas putida), one with high plasticity in cell morphology (Flectobacillus sp. GC5), and one that develops microcolonies (Pseudomonas sp. CM10), were selected. The voracious flagellate Ochromonas sp. was chosen as a predator. To describe adaptations against grazing and starvation, abundance, biomass and relative heterogeneity of bacteria were measured. On the whole, the strains in the predation-free cultures exhibited unicellular growth, and P. putida represented the largest group. The presence of Ochromonas strongly reduced bacterial abundance, but not always the total biomass. The activity of grazers changed the morphological composition of the bacterial communities. Under grazing pressure the relative composition of the community depended on the substrate availability. In the presence of predators, P. putida abundance declined in both high and low nutrient treatments, and Pseudomonas CM10 developed colonies. Flectobacillus was only numerically codominant in the nutrient-rich environments.

Bacterioplankton production, abundance, and nutrient limitation among lakes of the Mackenzie Delta (western Canadian arctic)

The effects of nutrient availability and quality of dissolved organic carbon (DOC) on bacterioplankton production were assessed in six lakes with differing frequencies of river flooding. Bacterial productivity, dissolved nutrients, and DOC were tracked weekly throughout the open-water period of 2001. Inorganic nutrient (N and P) enrichment microcosm experiments were conducted to directly assess the effects of DOC quality (i.e., mixtures of colored and noncolored DOC) and inorganic nutrient limitation on bacterial productivity among the lakes. Averaged over the open-water season, both abundance and production of bacterioplankton increased with decreasing flood frequency (R2 = 0.61 and R2 = 0.78, respectively). Reduced bacterial production occurred in frequently flooded lakes, where colored DOC, light attenuation, and phosphate were high but ammonium was low. Bacterial production was greatest in infrequently flooded lakes, where noncolored DOC and ammonium were high but phosphate was low. Bacterial production was enhanced by amendments of inorganic nutrients in duplicate experiments (two-factor analyses of variance). Production was also enhanced in response to higher concentrations of either colored or noncolored DOC following release from inorganic nutrient limitation. Size fractionated (<1 µm versus >1 µm) N-debt and P-debt bioassays typically showed demand for P and release of N by bacteria in all study lakes.

Controls on herbaceous litter decomposition in the estuarine ecotones of the Florida Everglades

The effects of nutrient availability and litter quality on litter decomposition were measured in two oligotrophic phosphorus (P)-limited Florida Everglades esturies, United States. The two estuaries differ, in that one (Shark River estuary) is directly connected to the Gulf of Mexico and receives marine P, while the other (Taylor Slough estuary) does not receive marine P because Florida Bay separates it from the Gulf of Mexico. Decomposition of three macrophytes.Cladium jamaicense, Eleochaaris spp., andJuncus roemerianus, was studied using a litter bag technique over 18 mo. Litter was exposed to three treatments: soil surface+macroinvertebrates (=macro), soil surface without macroinvertebrates (=wet), and above the soil and water (=aerial). The third treatment replicated the decomposition of standing dead leaves. Decomposition rates showed that litter exposed to the wet and macro treatments decomposed significantly faster than the aerial treatment, where atmospheric deposition was the only source of nutrients. Macroinvertebrates had no influence on litter decompostion rates.C. jamaicense decomposed faster at sites, with higher P, andEleocharis spp. decomposed significantly faster at sites with higher nitrogen (N). Initial tissue C:N and C:P molar ratios revealed that the nutrient quality of litter of bothEleocharis spp. andJ. roemerianus was higher thanC. jamaicense, but onlyEleocharis spp. decomposed faster thanC. jamaicense. C. jamaicense litter tended to immobilize P, whileEleocharis spp. litter showed net remineralization of N and P. A comparison with other estuarine and wetland systems revealed the dependence of litter decomposition on nutrient availability and litter quality. The results from this experiment suggest that Everglades restoration may have an important effect on key ecosystem processes in the estuarine ecotone of this landscape.

Effect of nutrient availability, grazer assemblage and seagrass source population on the interaction between Thalassia testudinum (turtle grass) and its algal epiphytes

Seagrass leaves are often densely covered by epiphytic algae which can suppress seagrass productivity and has been implicated in declines of seagrass meadows worldwide. The net effect of epiphytes on seagrass growth and morphology depends on the independent and interactive effects of a variety of factors, including nutrient availability and the intensity of grazing on epiphytes. Here I report the results of a mesocosm experiment designed to test the effects of nutrient addition and within-functional group variation (grazer species composition and the source population of seagrass) on the strength of the interactions among grazers, epiphytes, and turtle grass ( Thalassia testudinum). Turtle grass ramets from two sites in the northern Gulf of Mexico were cleared of epiphytes and transplanted into common-garden mesocosms. Replicate ramets were grown in a split–split plot design with two levels of dissolved nutrients and four different grazer species combinations ( Tozeuma carolinense alone, Pagurus maclaughlinae alone, both species together, and no grazers present). As expected, grazers had a significant negative effect on epiphyte biomass/leaf area and a significant positive effect on turtle grass growth in the mesocosms. The two species were more similar in their direct effects on epiphyte biomass than in their indirect effects on turtle grass growth; this may reflect differences in epiphyte community composition under different grazer treatments. The effect of nutrient addition on turtle grass growth depended critically on the intensity of grazing: in the presence of grazers, turtle grass tended to produce a greater biomass of new leaf tissue in the tanks with nutrients added than in the control tanks. However, when grazers were absent, the direction of the effect was reversed, and plants with nutrients added grew less than the control plants. The two source populations of turtle grass differed significantly in epiphyte biomass/leaf area accrued in the mesocosms as well as in the strength of the effect of grazers on turtle grass growth. This suggests that population differentiation in seagrass interactions with epiphytes, as well as spatial and temporal variation in resources and grazer community composition, can greatly effect the role of epiphytes in limiting seagrass productivity.

Effects of nutrient availability on the production of pentaynene, a secondary compound related to defense, in Rudbeckia hirta

AbstractThis study focused on the effect of different availabilities of mineral resources in hydroponic culture on the production of polyacetylenic defences of Rudbeckia hirta L. Specifically, we investigated if there was any difference concerning the amount of pentaynene, measured with high performance liquid chromatography (HPLC), produced in flowers of plants grown at high light intensity (500 µmol m−2 s−1 photosynthetic photon flux density) and two levels of mineral resources (full strength and 1/5 dilution) in a hydroponic solution. A working hypothesis was that the concentration of secondary compounds would increase with decreasing nutrient supply, in accordance with the carbon:nutrient balance hypothesis. The amount of pentaynene produced in the flowers of R. hirta decreased with increasing flower age but increased significantly when the plant was subjected to a reduced nutrient availability. The decrease in pentaynene concentrations with flower age was also less pronounced in the nutrient‐stressed plants.

Pseudomonas syringae Responds to the Environment on Leaves by Cell Size Reduction

ABSTRACT The length and volume of cells of the plant-pathogenic bacterium Pseudomonas syringae strain B728a were measured in vitro and with time after inoculation on bean leaf surfaces to assess both the effect of nutrient availability on the cell size of P. syringae and, by inference, the variability in nutrient availability in the leaf surface habitat. Cells of P. syringae harboring a green fluorescent protein marker gene were visualized by epifluorescence microscopy after recovery from leaves or culture and their size was estimated by analysis of captured digital images. The average cell length of bacteria grown on leaves was significantly smaller than that of cultured cells, and approached that of cells starved in phosphate buffer for 24 h. The average length of cells originally grown on King's medium B decreased from approximately 2.5 to approximately 1.2 mum by 7 days after inoculation on plants. Some decrease in cell size occurred during growth of cells on leaves and continued for up to 13 days after cell multiplication ceased. Although cultured cells exhibited a normal size distribution, the size of cells recovered from bean plants at various times after inoculation was strongly right-hand skewed and was described by a log-normal distribution. The skewness of the size distribution tended to increase with time after inoculation. The reduced cell size of P. syringae B728a on plants was readily reversible when recovered cells were grown in culture. Direct in situ measurements of cell sizes on leaves confirmed that most cells of P. syringae respond to the leaf environment by reducing their size. The spatial heterogeneity of cell sizes observed on leaves suggest that nutrient availability is quite variable on the leaf surface environment.

SOIL RESOURCES REGULATE PRODUCTIVITY AND DIVERSITY IN NEWLY ESTABLISHED TALLGRASS PRAIRIE

In native tallgrass prairie, soil depth and nitrogen (N) availability strongly influence aboveground net primary productivity (ANPP) and plant species composition. We manipulated these factors in a newly restored grassland to determine if these resources similarly constrain productivity and diversity during the initial three years of grassland establishment. Four types of experimental plots with six treatment combinations of deep and shallow soil at reduced-, ambient-, and enriched-N availability formed the basis of this study. The soil responses to the experimental treatments were examined over three years, and patterns in diversity and productivity were examined in year 3. The soil depth treatment did not significantly affect soil carbon (C) and N pools or ANPP and diversity. A pulse amendment of C added to the soil prior to planting increased soil microbial biomass and decreased potential net N mineralization rates to effectively reduce N availability throughout the study. Nitrogen availability declined over time in nonamended soils as a result of plant establishment, but adding fertilizer N alleviated the increasing immobilization potential of the soil. The level of ANPP was lowest and diversity highest in the reduced-N treatment, whereas the enriched-N treatment resulted in high productivity, but low diversity. As a result, diversity was inversely correlated with productivity in these newly established com- munities. The same mechanism invoked to explain decreased diversity under nutrient en- richment in old-field ecosystems and native grasslands (e.g., reduced light availability with increased production) was supported in the restored prairie by the positive relationship between ANPP and intercepted light, and a strong correlation between light availability and diversity. The effects of nutrient availability on plant community composition (diversity and richness) were due primarily to the responses of prairie species, as the productivity of early successional, nonprairie species was less than 1% of total ANPP after three years of establishment. These results show that the effects of resource availability on productivity and diversity are similar in young and mature grasslands, and that manipulation of a limiting nutrient during grassland establishment can influence floristic composition, with conse- quences for long-term patterns of diversity in restored ecosystems.

Dental Plaque as a Biofilm: A Pilot Study of the Effects of Nutrients on Plaque pH and Dentin Demineralization

Direct or in situ methods (e.g., confocal microscopy, microsensors) are used to study non-oral biofilms for almost two decades, and they have recently been introduced in the research of dental plaque. We combined a pH microsensor technique and transversal microradiography (TMR) in a pilot study where the effects of nutrient availability on plaque acidogenicity and on the development of caries-like lesions were assessed. One volunteer accumulated dental plaque for 7 days in 0.2-mm-wide and 0.8-mm-deep dentin grooves at four conditions: (1) saliva only, (2) 8 × 5 min/day dipping in 10% sucrose solution, (3) 7 × 5 min/day consumption of sweet cookies plus one meal/day, and (4) subject’s regular diet. Plaque pH versus time and depth profiles in the grooves were recorded ex vivo before and after sugar challenge. ‘Saliva’ plaque responded to sugar with slow pH decrease – minimum pH 5.6–5.8 was reached after 30 min, while sugar dipping resulted in metabolically active plaque (minimum pH 5.3–5.5 within 4–8 min). TMR analysis revealed no demineralization after these two periods. Metabolically active plaque leading to distinct lesions resulted from frequent plaque exposure to diets rich in starch and carbohydrates (groups 3 and 4). These findings strengthen the view that the plaque acidogenicity does not necessarily reflect cariogenicity, and that retention of food components may account for increased cariogenicity at plaque retention sites in the mouth.

Nitrogen vs. phosphorus limitation across an ecotonal gradient in a mangrove forest

Mangrove forests are characterized by distinctive tree-height gradientsthat reflect complex spatial, within-stand differences in environmentalfactors,including nutrient dynamics, salinity, and tidal inundation, across narrowgradients. To determine patterns of nutrient limitation and the effects ofnutrient availability on plant growth and within-stand nutrient dynamics, weused a factorial experiment with three nutrient treatment levels (control, N,P)and three zones along a tree-height gradient (fringe, transition, dwarf) onoffshore islands in Belize. Transects were laid out perpendicular to theshoreline across a mangrove forest from a fringe stand along the seaward edge,through a stand of intermediate height, into a dwarf stand in the interior ofthe island. At three sites, three trees were fertilized per zone for 2yr. Although there was spatial variability in response, growth byR. mangle was generally nitrogen (N) -limited in thefringe zone;phosphorus (P) -limited in the dwarf zone; and, N- and/or P-limited in thetransition zone. Phosphorus-resorption efficiency decreased in all three zones,and N-resorption efficiency increased in the dwarf zone in response to Penrichment. The addition of N had no effect on either P or N resorptionefficiencies. Belowground decomposition was increased by P enrichment in allzones, whereas N enrichment had no effect. This study demonstrated thatessential nutrients are not uniformly distributed within mangrove ecosystems;that soil fertility can switch from conditions of N to P limitation acrossnarrow ecotonal gradients; and, that not all ecological processes respondsimilarly to, or are limited by, the same nutrient.

Within‐Alga Integration and Compensation: Effects of Simulated Herbivory on Growth and Reproduction of the Brown Alga, Fucus vesiculosus

We investigated the level of within‐alga integration, the consequent ability to compensate for simulated herbivory, and the effect of nutrient availability on the ability to compensate in Fucus vesiculosus (L.). Although F. vesiculosus lacks vascular connections, resource translocation occurs on a local scale. Vegetative apical parts and reproductive structures were dependent on the resources provided by the mature thallus. Thus, damage from herbivory has the potential to decrease the growth and reproductive success of F. vesiculosus. In contrast to vascular plants, F. vesiculosus did not show any plasticity in response to different types of damage. Removal of an apical part terminated the growth of the focal branch but had no effect on the growth of the neighboring branch. Removal of the older part of the thallus had no effect on the branching of the apical meristems. Thus, the within‐alga growth pattern remained similar whether the old thallus or an apical part was damaged. Increased resource availability did not enhance the ability of algae to compensate for lost thallus. The strong dependence of the apical part on the older thallus and the low plasticity in the type of response to damage indicate that in F. vesiculosus the tolerance of herbivory may be low.

Factors affecting seasonal variation of membrane filtration resistance caused by Chlorella algae

A seasonal fluctuation pattern was observed in membrane filtration resistance by Chlorella algae cultured in open ponds in the tropical environment. In order to investigate the causes of this phenomenon, Chlorella was cultivated under controlled conditions and the cake resistance was measured by batch filtration in dead-end mode. The filtration resistance was found to be a function of environmental conditions. Algae could grow favourably and offered low specific cake resistance (Rcs) on the order of 1011m/g for the culture temperature from 28°C to 35°C. The algal growth was inhibited and the specific cake resistance increased to the order of 1012m/g below or above this optimum temperature range. Strong solar radiation, coupled with high temperatures, also inhibited the growth of algae and resulted in higher specific cake resistance. The specific cake resistance of algae cultured at different temperatures increased with the amount of the extracellular organic matter (EOM) extracted by 0.1N NaOH. Hence EOM, rather than bacteria present in the mono-algal culture, was considered to be the primary factor affecting the cake resistance. The specific cake resistance increased drastically after actively growing cells were stored in nutrient-free water under dark conditions. However, the resistance was slightly decreased when the algal cells were stored in NSIII nutrient media in a dark room, indicating the effect of nutrient availability on the change of the specific cake resistance under the light-limiting conditions. EOM extracted from the cells kept in the nutrient-free water contained less sugar than the fresh culture, whereas the EOM extracted from the cells stored in the NSIII media contained more sugar. The molecular distribution of the EOM shifted from below 1,000kDa before storage to more than 2,000kDa after storage in both the nutrient-free and NSIII media.

Effect of Nutrient Availability on Macrocystis pyrifera Recruitment and Survival near Its Southern Limit off Baja California

The effect of nutrient availability on Macrocystis pyrifera recruitment and survival of juvenile sporophytes was examined at its southern limit off Baja California, Mexico. Nitrate levels were inversely correlated with water temperature (r2 = 0.70) at the experimental location. Low concentrations of nitrate occurred during most of 1998 (< 3 μM) except in spring (4.8–18.8 μM ). Experimental seeding with sporophylls and fertilizing the plants with a slow-release inorganic fertilizer produced successful recruitment when the water temperature was declining (18.7 °C). Survival of transplanted juveniles was significantly higher when fertilized during a period of particularly warm water and low nutrients in 1998 caused by El Niño. Analysis of tissue nitrogen suggests that plants were nutrient-limited during this period, and possibly affected synergistically by temperature stress. The results indicate that nutrients directly affect recruitment and survival of giant kelp at its southern limit, particularly during El Niño conditions.

Effects of nutrient availability and other elevational changes on bromeliad populations and their invertebrate communities in a humid tropical forest in Puerto Rico

Nutrient inputs into tank bromeliads were studied in relation to growth and productivity, and the abundance, diversity and biomass of their animal inhabitants, in three forest types along an elevational gradient. Concentrations of phosphorus, potassium and calcium in canopy-derived debris, and nitrogen and phosphorus in phytotelm water, declined with increasing elevation. Dwarf forest bromeliads contained the smallest amounts of debris/plant and lowest concentrations of nutrients in plant tissue. Their leaf turnover rate and productivity were highest and, because of high plant density, they comprised 12.8% of forest net primary productivity (0.47 t ha−1 y−1), and contained 3.3 t ha−1 of water. Annual nutrient budgets indicated that these microcosms were nutrient-abundant and accumulated &lt; 5% of most nutrients passing through them. Exceptions were K and P in the dwarf forest, where accumulation was c. 25% of inputs. Animal and bromeliad biomass/plant peaked in the intermediate elevation forest, and were positively correlated with the debris content/bromeliad across all forest types. Animal species richness showed a significant mid-elevational peak, whereas abundance was independent of species richness and debris quantities, and declined with elevation as forest net primary productivity declined. The unimodal pattern of species richness was not correlated with nutrient concentrations, and relationships among faunal abundance, species richness, nutrient inputs and environment are too complex to warrant simple generalizations about nutrient resources and diversity, even in apparently simple microhabitats.

The effect of nutrient availability on biomass allocation patterns in 27 species of herbaceous plants

We investigated allocation to roots, stems and leaves of 27 species of herbaceous clonal plants grown at two nutrient levels. Allocation was analyzed as biomass ratios and also allometrically. As in other studies, the fraction of biomass in stems and, to a lesser extent, in leaves, was usually higher in the high-nutrient treatment than in the low-nutrient treatment, and the fraction of biomass in roots was usually higher under low-nutrient conditions. The relationship between the biomass of plant structures fits the general allometric equation, with an exponent 1 in most of the species. The different biomass ratios under the two nutrient conditions represented points on simple allometric trajectories, indicating that natural selection has resulted in allometric strategies rather than plastic responses to nutrient level. In other words, in most of the species that changed allocation in response to the nutrient treatment, these changes were largely a consequence of plant size. Our data suggest that some allocation patterns that have been interpreted as plastic responses to different resource availabilities may be more parsimoniously explained as allometric strategies.

Early stages of root and leaf decomposition in Hawaiian forests: effects of nutrient availability.

We examined the effects of soil nutrient availability and tissue chemistry on decomposition of both fine roots (<2 mm diameter) and leaves in three sites along a forest chronosequence in the Hawaiian Islands. These sites form a natural fertility gradient, with the youngest and oldest sites having lower nutrient availability than the intermediate-aged site. Nitrogen (N) limits aboveground net primary productivity (ANPP) in the youngest site, while phosphorus (P) limits ANPP in the oldest site. Both root and leaf litter decomposed most slowly in the 4.1-Myear-old site. We also investigated root decomposition in fertilized plots at the youngest and oldest sites; when roots were produced and decomposed in fertilized plots, root decomposition rates increased with N and P additions at the 4.1-Myear-old site. At the 300-year-old site, however, root decomposition rates did not respond to N or P additions. Roots decomposed faster than leaves at the more infertile sites, in part because of lower lignin-to-nitrogen ratios in roots than in leaf litter. Decomposing roots immobilized more nutrients than did decomposing leaves, and may serve an important role in retaining nutrients in these forests.

The effect of feed deprivation on gastrointestinal respiration and Na+, K+-ATPase in chicks

In order to investigate the effect of nutrient availability on energy metabolism of the gastrointestinal tract (GIT) of chicks, two experiments were conducted. In exp. 1, jejunal slices from male White Leghorn chicks at 14 d were incubated in salt-based balanced medium (SBM) or complex medium (CM) for 30 min. The total O2 (TO2) and ouabain-sensitive O2 (OSO2) consumption in jejunal slices were significantly decreased in SBM. The results from exp. 1 imply that when the GIT mucosa is subjected to nutrient deprivation, the utilization of oxygen consumption is altered primarily by shifting active Na+ pump activity. Changes in the TO2, OSO2 and amiloride-sensitive O2 (ASO2) uptake in response to feed deprivation in the small intestine of male White Leghorn chicks were investigated in exp. 2. There was little change in TO2, OSO2 and ASO2 per unit of dry matter of the small intestine from birds subjected to fasting, except for a TO2 decrease in the jejunum of 48 h fasted chicks. However, there were significant reductions in TO2, OSO2 and ASO2 in the jejunum when data were expressed per unit DNA. Na+, K+-ATPase-dependent respiration represented 31–37% of TO2 in the avian jejunoileum. Amiloride-sensitive respiration accounted for 10–18% of TO2 in the jejunoileum. The maximal amount of 3Houabain binding sites of jejunal enterocytes was significantly decreased in chicks fasted for 48 h (P &lt; 0.05). Key words: Oxygen consumption, fasting, Na+, K+-ATPase, ouabain, jejunum, ileum, chicks

A gap dynamic model of mangrove forest development along gradients of soil salinity and nutrient resources

1 A gap dynamic model ( FORMAN) was developed as a first synthesis of available data for three Caribbean mangrove species into an individual‐based model that simulates the long‐term dynamics of mangrove forest development. 2 Field observations at three sites along the Shark River estuary were compared with simulation results, assuming development following Hurricane Donna in 1960. Total basal area simulated for each site was within ±10% of that observed, although species‐specific basal area predictions were less accurate. 3 A decrease in nutrient availability from marine to mesohaline sites modelled the reduced basal area of Avicennia germinans and Laguncularia racemosa. However, at the lower nutrient site a 83% reduction in maximum sapling recruitment of both A. germinans and L. racemosa was required to fit field results. Increased basal area of A. germinans and L. racemosa limited the development of Rhizophora mangle at higher nutrient downstream sites, apparently due to competition for light resources. Both observed and simulated results indicated that R. mangle had higher frequencies in the smaller size classes at all three sites, compared to a bell‐shaped size‐class distribution of L. racemosa, particularly at the higher nutrient sites. 4 Succession was projected for the next 500 years at a site in the lower estuary. Long‐term forest dynamics were sensitive to species‐specific maximum sapling recruitment rates. In the absence of large‐scale disturbance, initial dominance by L. racemosa was predicted to be replaced eventually by A. germinans, even when maximum sapling recruitment rate of A. germinans was set at half of L. racemosa and R. mangle. 5 Response curves for each species along gradients of soil nutrient resource and salinity illustrated their relative competitive balance over time (up to 300 years). Laguncularia racemosa dominated in fertile soils with low salinity at early stages of recovery, but its abundance decreased over time while A. germinans increased. The dominance of R. mangle was limited to regions with low nutrient availability and low salinity. Avicennia germinans dominated at higher salinities, where the effect of nutrient availability was overwhelmed by the tolerance of individual species to salt stress.

Short-term effects of nutrient enrichment of the sediment and interactions between the seagrass Cymodocea nodosa and the introduced green alga Caulerpa taxifolia in a Mediterranean bay

Nutrient addition experiments provide a means for testing ecological theories concerning the effects of nutrient availability on community composition and development. Here we present the results of two reciprocal short-term (4 months) experiments testing for the effects of nutrient addition in the sediment and competitive interactions between the seagrass Cymodocea nodosa and the tropical green alga Caulerpa taxifolia. This study was conducted from 1 June to 30 September 1995 in a bay on the south coast of Elba Island (Italy) recently impacted by the alga. Each experiment consisted of the manipulation of the level of nutrients (addition vs. control) and the manipulation of neighbours (presence vs. removal). Response variables were blade density and length of C. taxifolia in one experiment and shoot density and leaf length of C. nodosa in the other. Results indicate that during the growing season the canopy of C. nodosa did not have any effect on C. taxifolia growth while nutrient addition in the sediment favoured colonization (higher blade density) of this alga independently of the presence of the seagrass. Conversely, C. taxifolia had a negative effect on shoot density of C. nodosa independently of nutrient availability. Hence, colonization by the introduced alga C. taxifolia may provide an explanation for the regression of C. nodosa beds occurring at the site. Furthermore, we demonstrated that C. taxifolia was favoured by high nutrient loads in the sediment, while C. nodosa was not; this might affect the outcome of the interactions among these species in the long-term.

A 5-Year Study of the Effects of Nutrient Availability and Herbivory on Two Boreal Forest Herbs

1) The responses of populations of Mertensia paniculata (bluebells or lungwort) and Anemone parviflora (small-flowered anemone) to herbivore exclosure and fertilization in a factorial experiment were monitored over a 5-year period beginning at peak herbivore (hare) densities during the snowshoe hare population cycle. For each species the population density, number and size of leaves and the number of flowers were measured. 2) Both species responded more strongly to fertilizer addition than to the exclusion of herbivores. Mertensia produced more flowering stems, more leaves per stem, and stem density increased in the fertilized plots. Fertilizing increased total leaf length per plant for non-flowering stems but this was not observed for flowering stems. However, the net effect of having more flowering stems and having greater leaf area on nonflowering stems was to increase the total leaf area of the population. The responses of Mertensia make it likely to become a stronger competitor in a more productive plant community. 3) Anemone showed contrasting responses at both individual and population levels. While individual stems produced slightly more leaves in fertilized plots, the density of stems declined. There were no strong effects on either leaf size or flowering. There was evidence of higher leaf turnover in fertilized plants. Meanwhile, control and exclosed unfertilized plots showed an increase in population density. 4) The weak responses to herbivory may be explained by the timing; this part of the experiment was run during a period of declining herbivore activity. However, observed interaction effects suggest that those herbivores remaining in the system may be attracted to fertilized plots. It is planned to continue the experiment for at least another 5 years through, and beyond, the next hare peak.

Growth, biomass allocation and plant nitrogen concentration in Chilean temperate rainforest tree seedlings: effects of nutrient availability.

Seedlings of nine southern Chilean trees were grown at three nutrient supply rates, to examine the roles of growth rate, biomass distribution and nutrient use traits in determining species natural distributions on resource gradients. Relative growth rate (RGR) showed no overall relationship with species site requirements, although RGR of fertile-site species tended to be more responsive to nutrient supply. In the low-nutrient treatment, infertility-tolerant Fitzroya cupressoides showed a higher RGR rank than a fertility-demanding species (Laurelia philippiana) which outgrew it substantially at the highest supply rate. This reversal of RGR ranks was associated with divergent nutrient use responses: at high nutrient supply both spp. had similar plant nitrogen concentrations (PNC), whereas at the low supply rate Fitzroya's production of biomass per unit of assimilated N was twice that of Laurelia's. However, this pattern does not appear to serve as a general explanation of the respective distributions of the study species, as RGR ranks of most species were unaltered by nutrient supply. At low nutrient availability, no clear differences in shoot:root ratio (SRR) were apparent between poor-site and fertile-site species. However, at high nutrient availability, SRR was markedly higher in the latter, resulting from differences in biomass allocation to stems (not leaves). Leaf area ratios (LAR) were higher in fertile-site species than in those tolerant of low fertility, because of differences in specific leaf area rather than leaf weight ratio. Very high LAR at high nutrient supply was characteristic of most shade-tolerant angiosperms, but not of shade-tolerant conifers. Although PNC showed no overall differences between poor- and fertile-site species, sensitivity of PNC to external supply rate was greatest in two infertility-tolerant conifers. In contrast, the angiosperm Weinmannia trichosperma, although tolerant of low fertility, responded to increased nutrient supply with greatly increased RGR and little change in PNC. Results show little trait convergence between conifers and angiosperms in adaptation both to shade and to infertile soils; i.e. fitness of different taxa in a given environment may hinge on different trait combinations.