Seedling growth and biomass allocation of endemic and threatened shrubs of rupestrian fields

The increasing anthropogenic pressure in the rare rupestrian fields in southeastern Brazil has led to the expansion of degraded areas on the extremely nutrient-deficient quartzitic soils. On the other hand, the use of rupestrian field native species in reclamation programmes has been hampered by the lack of studies involving seedling physiological ecology. The present study evaluated biomass allocation and seedling growth rate during early seedling growth of four Fabaceae shrubs: Collaea cipoensis, Calliandra fasciculata, Chamaecrista ramosa, and Mimosa foliolosa. The following hypotheses were tested: (i) species proportionally allocate higher biomass to the roots, presenting a high root/shoot ratio; and (ii) species exhibit low phenotypic variation because they have adapted to poor nutritional environments. A 12-month greenhouse experiment was carried out to evaluate seedling growth and biomass allocation performance in substrates with contrasting levels of soil fertility. The four species studied presented values of root/shoot ratio lower than one in both fertility conditions of the substrate. Growth parameters for Collaea and Calliandra increased with increasing soil fertility, while no differences were observed for Mimosa and Chamaecrista. Although the four species are naturally adapted to low nutritional quality soils, seedling development was not hindered by high fertility substrate conditions. Despite the remarkable differences in fertility between the substrates, the responsiveness in growth and allocation in Chamaecrista and Mimosa was lower than that expected if the species would exhibit high phenotypic variation. The implications for rupestrian field restoration are discussed.     

Endophytic bacteria isolated from orchid and their potential to promote plant growth

Twelve endophytic bacteria were isolated from the meristem of in vitro Cymbidium eburneum orchid, and screened according to indole yield quantified by colorimetric assay, in vitro phosphate solubilization, and potential for plant growth promotion under greenhouse conditions. Eight strains with positive results were classified into the genus Paenibacillus by FAME profile, and evaluated for their ability to increase survival and promote the growth of in vitro germinated Cattleya loddigesii seedlings during the acclimatization process. The obtained results showed that all strains produced detectable indole levels and did not exhibit potential for solubilizing inorganic phosphate. Particularly, an increase of the total biomass and number of leaves was observed. Two strains of Paenibacillus macerans promoted plant growth under greenhouse conditions. None of the treatments had a deleterious effect on growth of inoculated plants. These results suggest that these bacterial effects could be potentially useful to promote plant growth during seedling acclimatization in orchid species other than the species of origin.     

Establishing clones of Veratrum californicum,a native medicinal species,for micropropagation

Factors affecting the micropropagation of Veratrum californicum, a slow-growing species that is a potentially valuable source of cyclopamine, were investigated. Sterile cultures were initiated on modified Murashige and Skoog medium, and clones from individual donor plants were assigned to experimental conditions when approximately 100 shoots of each clone were available. The effects of temperature, light quality, and plant growth regulators on multiplication and survival were assessed. Four clones from which large greenhouse populations were obtained were selected for in-depth analysis. When shoots were cultured at 10°C and 16°C, multiplication ratios consistently >1 were observed from three of four clones and two of four clones, respectively, during the five-subculture cycles. None of the clones stably increased when cultured at 24°C, and plants from this treatment did not survive acclimatization in the greenhouse. Only one clone showed increased multiplication ratios in response to plant growth regulator treatments, with maximum multiplication when shoots were cultured with 9 μM benzyladenine and 0.5 μM naphthaleneacetic acid. Light quality in the laboratory did not affect multiplication ratio but did affect subsequent greenhouse survival. The size of plants derived from culture was most often equivalent (65% of 1,271) to 3-yr-old seed-derived plants. Although the growth of clones during acclimatization differed, plants derived from cultures incubated at 16°C had the best rates of overall greenhouse survival. Temperature and light treatments in vitro critical to long-term plant survival were demonstrated and will assist the establishment of a mass propagation system for V. californicum.     

Early growth,dry matter allocation and water use efficiency of two sympatric Populus species as affected by water stress

We exposed cuttings of two sympatric species of Sect. Tacamahaca Spach, Populus cathayana Rehder and Populus przewalskii Maximowicz, to two watering regimes in a greenhouse. In the semi-controlled environmental study, two watering treatments which were watered to 100 and 25% of field capacity were used, respectively. The effects of water deficit on early growth, biomass allocation and water use efficiency (WUE) were investigated. We found that there were significant interspecific differences in early growth, dry matter allocation and water use efficiency between two sympatric Populus species. Compared with P. cathayana, P. przewalskii showed higher shoot height, dry matter accumulation, number of leaves, total leaf area, fine root mass, fine root/total root ratio and water use efficiency under both well-watered and water-stressed treatments. On the other hand, P. przewalskii also showed higher root mass/foliage area ratio, root/shoot ratio and carbon isotope composition than P. cathayana under water-stressed treatment. The results suggested that there were different water-use strategies between two sympatric Populus species, P. przewalskii with higher drought tolerance may employ a conservative water-use strategy, whereas P. cathayana with lower drought tolerance may employ a prodigal water-use strategy. The findings confirm the existence of interspecific genetic differences in early growth, dry matter allocation and water use efficiency as affected by water stress, these variations in drought responses may be used as criteria for species selection and tree improvement.     

Density-dependent effects on the dieback of exotic species Spartina anglica in coastal China

The exotic species Spartina anglica C.E. Hubbard has been experiencing dieback in coastal China during the past decade, while it is invading other places in the world. Thus, an explanation for the dieback of the populations in coastal China would be helpful for controlling invasions of S. anglica in other places of the world. Both field and greenhouse experiments were conducted to determine the possible role of density in the dieback of the populations in China. The density effects were tested by maintaining six density levels in field experiments and three density levels in greenhouse experiments over 7 months. The results showed that growth, asexual reproduction and biomass accumulation in medium density were significantly higher than those in other density treatments for field experiments. The growth and asexual reproduction in high-density patches were lowest among the treatments in the field. Data showed the same pattern for the greenhouse experiments. The results implied that density regulation had a great effect on most of the studied parameters. Because the species is surviving at high density along the Chinese coast, and has a much lower growth rate due to strong intraspecific competition, and also because high-density populations may decline into low density populations due to resource exhaustion, high plant density might be one of the important factors in the dieback of S. anglica.     

Growth patterns in some indo-west-pacific seagrasses

The plastochrone interval of the rhizomes (PIR) was determined in the monopodially branching seagrasses Cymodocea rotundata Ehrenb. et Hempr. ex Aschers. (1981), Cymodocea serrulata (R.Br.) Aschers. et Magnus (1981), Syringodium isoetifolium (Aschers.) Dandy (1982), Halodule uninervis (Forssk.) Aschers. (1982) and Halophila ovalis (R.Br.) Hook.f. (1982). The growth rate during 1981 was in all species, including Thalassia hemprichii (Ehrenb.) Aschers., constant during the year and the PIR amounted to 6.7 days (Cymodocea serrulata) and 3.9 days (Cymodocea rotundata). However, the PIR observed in 1982 showed in all species a unimodal pattern and a decrease during June, July and August. The PIR was 6.3 days in Halodule uninervis while an interval of 5.2 days was calculated in Syringodium isoetifolium.The growth rates of the horizontal and vertical axes (rhizomes and shoots) were correlated. A regression between the number of leaves on the shoot and the number of nodes on the rhizome resulted for most species in a linear correlation. The slopes of the calculated correlation were rather similar in the tree species with a comparable morphology: 0.44 in Cymodocea serrulata, 0.43 in Cymodocea rotundata and 0.49 in Halodule uninervis. In Syringodium isoetifolium, the longevity of the leaves increased during ageing of the shoot.The proliferation ratio (the ratio shoots/rhizome apices) was measured in all species. A maximum of 0.41 was observed in Thalassia hemprichii. This species and Cymodocea rotundata showed a unimodal pattern. In the last species, the ratio ranged from 0.03 to 0.13. The ratio was constant during the year in Cymodocea serrulata and Syringodium isoetifolium. Proliferation by means of monopodially branching of the rhizome was of little importance in Halodule uninervis. The pattern of proliferative and reiterative branching in the Cymodoceoideae is illustrated by an image of 123 days of growth in the fastest growing specimen.Most species showed growth in fronts. The expansion of monospecific seagrass fringes was monitored by repetitive mapping. The pattern of succession in subtidal meadows was determined.     

Population Growth of Pratylenchus penetrans on Winter Cover Crops Grown in the Pacific Northwest

Population growth of Pratylenchus penetrans on 13 fall and winter cover crops was studied in the greenhouse and field. All crops except oat cv. Saia supported population growth of P. penetrans in greenhouse experiments, although the response of P. penetrans to oat cv. Saia varied considerably between experiments. The mean ratio of the final population density/initial population density (Pf/Pi) after 16 weeks for P. penetrans added to a greenhouse soil mix was 0.09, whereas Pf/Pi values after 10 weeks for two experiments with naturally infested soil were 0.95 and 2.3. Although P. penetrans increased on sudangrass cv. Trudan 8 and sudangrass × sorghum hybrid cv. SS 222, subsequent incorporation of sudangrass vegetation into soil reduced P. penetrans populations to preplant levels. Field experiments were inconclusive but suggested that oat cv. Saia or rye cv. Wheeler may be better choices for winter cover than weed-contaminated fallow or other crops on P. penetrans-infested sites in the Pacific Northwest.     

Application of Sequential Classification of Prey/Predator Ratio Test to Tetranychus urticae and Phytoseiulus persimilis System in Greenhouse Roses

A sequential sampling procedure for classifying the ratio of prey/predators developed by Nyrop (1988) was examined to implement for a biological control in the greenhouse roses. The procedure was combined with a sequential density classification procedure for use in monitoring a phytophagous mite, Tetranychus urticae Koch, and a phytoseiid predator, Phytoseiulus persimilis Athias-Henriot. This procedure was required four parameters: Means and variances for T. urticae and P. persimilis, correlation coefficient between densities of prey and predator and critical ratio of prey and predator. The parameter values used in this study were 0.725 for the correlation coefficient and 10 for the critical ratio. The variances for each species were estimated using a Taylor's power law model. The procedure is proven to be successfully applicable to T. urticae and P. persimilis system in greenhouse roses at two action threshold levels of 5 and 10 T. urticae per three-leaflet leave. The limitations and implementation of this procedure is also discussed.     

Effect of vesicular-arbuscular mycorrhizae on seedling growth of four tree species from the tropical deciduous forest in Mexico

The influence of vesicular-arbuscular mycorrhizae on the growth of seedlings of Caesalpinia eriostachys, Cordia alliodora, Ipomoea wolcottiana and Pithecellobium mangense was investigated in a greenhouse experiment conducted at the Biological Station of Chamela on the Pacific coast of Mexico. Dry biomass production, relative growth rate, root/shoot ratio and mycorrhizal dependency were quantified for 75-day-old seedlings. With the exception of the pioneer species I. wolcottiana, mycorrhizal infection resulted in increases in biomass production, relative growth rate and leaf area. The root/shoot ratios attained for the species, however, did not show a consistent trend with infection. Nevertheless, all species had root/shoot ratios below 1 with infection and only one, Cordia alliodora, had a ratio greater than 1 without infection. The two late successional species from the mature part of the forest, Caesalpinia eriostachys and P. mangense, showed a larger mycorrhizal dependency than the two associated with disturbed environments.     

Linking field based studies with greenhouse experiments: the impact of <Emphasis Type="Italic">Centaurea stoebe</Emphasis> (=<Emphasis Type="Italic">C. maculosa</Emphasis>) in British Columbia grasslands

The grassland community of Lac du Bois Provincial Park in the interior of British Columbia has become increasingly invaded by Centaurea stoebe (=Centaurea maculosa; spotted knapweed). Allelopathy, through the production of the phytotoxin (±)-catechin by C. stoebe roots is believed to be partly responsible for knapweed’s invasive success. We used field sampling and greenhouse experiments to examine two questions: (1) Is increasing abundance of C. stoebe negatively associated with the abundance of specific native grassland species? (2) Do species that exhibit a negative correlation with C. stoebe abundance in the field demonstrate increased levels of susceptibility to application of (±)-catechin during germination? Thirty-eight plots were sampled in the grassland, encompassing areas of low—high knapweed abundance. Seeds from eight native species, exhibiting positive, neutral or negative correlation with knapweed abundance, were treated with three concentrations (0, 0.5, 2.0 mg/mL) of (±)-catechin. Root growth and percent germination were measured over a 6-week period. The results indicate that C. stoebe abundance is negatively correlated with native plant species abundance and may alter plant community composition. Moreover root radical growth was significantly negatively affected by treatment with (±)-catechin in all four native plant species that exhibited a negative correlation with knapweed abundance in the field. Past studies have failed to conclusively link greenhouse results with plant community patterns. Here, we provide a correlative link between plant community composition and tolerance to a phytotoxin.     

The effect of phosphorus on growth and cluster-root formation in the Chilean Proteaceae: Embothrium coccineum (R. et J. Forst.)

One of the main factors that favours the formation of cluster roots is a low supply of phosphorus (P). The soils of southern Chile are mainly formed from volcanic ash, characterized by low levels of available P. Embothrium coccineum, a Chilean Proteaceae species produces cluster roots (CR). The factors that control CR formation in Chilean Proteaceae have not been extensively studied. The objective of this work was to assess the effects of P on the growth and cluster-root formation of E. coccineum. Plants were produced from seeds collected at two different locations: Valdivia and Pichicolo both at 39ºS. They were cultured under similar greenhouse conditions, from June to September, watered twice a week using: distilled water (W), full strength Hoagland’s nutrient solution (H) or Hoagland without P (H-P). At the end of the experiment, height, total dry biomass, number of cluster roots (CR) per plant, CR /total root weight, were measured. Also acid exudation of CR was assayed using bromocresol purple on sterile agar plates. Treatments significantly affected growth and proportion of CR, the highest growth was observed with H. Under all treatments plants produced a similar number of CR. However, the proportion of CR biomass was higher with W and H-P than with H. Plants under W exhibited the lowest growth and low shoot/root ratio. Acid exudation of CR was not detectable in our experiment. These results are discussed comparing CR formation in low P conditions on Lupinus albus and other Proteaceae species, and the possible role of CR formation in E. coccineum considering its wide geographical distribution.     

Arbuscular mycorrhizal fungi in two mangroves in South China

The symbiosis between arbuscular mycorrhizal fungi (AMF) and mangrove plant species was investigated in two mangrove swamps in south China. AMF were mostly found in the form of hyphae and were commonly associated with all the mangrove species we investigated. Six AMF species belonging to the genera Glomus or Acaulospora were identified. Multiple step-wise linear regression analyses showed that hydrological conditions and phosphorus levels in the rhizosphere were the main abiotic factors affecting the colonization of mangrove species by AMF. A greenhouse experiment was conducted to evaluate the effects of AMF inoculation on the growth and nutrient uptake of a true mangrove plant species, Sonneratia apetala B. Ham. The inoculated AMF significantly improved growth, resulting in greater plant height, diameter at ground level and plant biomass, as well as increased absorption of N, P and K. These findings suggest that AMF play important roles in mangrove ecosystems.     

Pathogenicity of Two Species of Entomopathogenic Nematodes Against the Greenhouse Whitefly,Trialeurodes vaporariorum (Hemiptera: Aleyrodidae), in Laboratory and Greenhouse Experiments

The greenhouse whitefly Trialeurodes vaporariorum (Hemiptera: Aleyrodidae) is a polyphagous pest in greenhouse crops. The efficacy of two entomopathogenic nematodes (EPN), Steinernema feltiae and Heterorhabditis bacteriophora, as biological control agents against T. vaporariorum was evaluated using two model crops typical of vegetable greenhouse productions: cucumber and pepper. Laboratory tests evaluated adults and second nymphal instars for pest susceptibility to different EPN species at different concentrations of infective juveniles (IJ; 0, 25, 50, 100, 150, 200, and 250 IJ per cm²); subsequent greenhouse trials against second nymphal instars on cucumber and pepper plants evaluated more natural conditions. Concentrations were applied in combination with Triton X-100 (0.1% v/v), an adjuvant for increasing nematode activity. In laboratory studies, both life stages were susceptible to infection by the two nematode species, but S. feltiae recorded a lower LC₅₀ than H. bacteriophora for both insect stages. Similarly, in greenhouse experiments, S. feltiae required lower concentrations of IJ than H. bacteriophora to reach the same mortality in nymphs. In greenhouse trials, a significant difference was observed in the triple interaction among nematode species × concentration × plant. Furthermore, the highest mortality rate of the second nymphal instars of the T. vaporariorum was obtained from the application of S. feltiae concentrated to 250 IJ/cm² on cucumber (49 ± 1.23%). The general mortality caused by nematodes was significantly higher in cucumber than in pepper. These promising results support further investigation for the optimization of the best EPN species/concentration in combination with insecticides or adjuvants to reach a profitable control of this greenhouse pest.     

Locating genes associated with root morphology and drought avoidance in rice via linkage to molecular markers

This research was undertaken to identify and map quantitative trait loci (QTLs) associated with five parameters of rice root morphology and to determine if these QTLs are located in the same chromosomal regions as QTLs associated with drought avoidance/tolerance. Root thickness, root:shoot ratio, root dry weight per tiller, deep root dry weight per tiller, and maximum root length were measured in three replicated experiments (runs) of 203 recombinant inbred lines grown in a greenhouse. The lines were from a cross between indica cultivar Co39 andjaponica cultivar Moroberekan. The 203 RI lines were also grown in three replicated field experiments where they were drought-stressed at the seedling, early vegetative, and late-vegetative growth stage and assigned a visual rating based on leaf rolling as to their degree of drought avoidance/tolerance. The QTL analysis of greenhouse and field data was done using single-marker analysis (ANOVA) and interval analysis (Mapmaker QTL). Most QTLs that were identified were associated with root thickness, root/shoot ratio, and root dry weight per tiller, and only a few with deep root weight. None were reliably associated with maximum root depth due to genotype-by-experiment interaction. Root thickness and root dry weight per tiller were the characters found to be the least influenced by environmental differences between greenhouse runs. Correlations of root parameters measured in greenhouse experiments with field drought avoidance/tolerance were significant but not highly predictive. Twelve of the fourteen chromosomal regions containing putative QTLs associated with field drought avoidance/tolerance also contained QTLs associated with root morphology. Thus, selecting for Moroberekan alleles at marker loci associated with the putative root QTLs identified in this study may be an effective strategy for altering the root phenotype of rice towards that commonly associated with drought-resistant cultivars.     

Plasticity of Sapium sebiferum seedling growth to light and water resources: Inter- and intraspecific comparisons

Two main hypotheses have been posed to explain the role of phenotypic plasticity in the invasive success of exotic plants: (1) invasive species may be more plastic than resident species in the introduced range, and (2) invasive populations of an exotic species may be more plastic relative to native populations due to evolutionary changes after introduction. To test the first hypothesis, we conducted a greenhouse pot experiment in which seedlings of invasive Sapium sebiferum competed against native Schizachyrium scoparium grasses under different light and water conditions. To test the second hypothesis, we performed an additional greenhouse pot experiment in which seedlings from native and invasive populations of S. sebiferum were grown under environmental treatments analogous to those in the first greenhouse experiment. Compared to native S. scoparium grasses, or to S. sebiferum seedlings from native populations, growth rates of S. sebiferum seedlings from invasive populations were generally higher. When they were competing with S. scoparium grasses, the greater response of S. sebiferum to light and water conditions reflected different patterns: S. sebiferum seedlings were better able to respond with increased growth in unflooded soils, whereas S. sebiferum had more robust growth in the shaded conditions. No difference in responses to change in water conditions, but a significant difference in responses to variation in light conditions was found between two population types of S. sebiferum. The results of this study suggest that relative to S. scoparium, the greater plasticity of S. sebiferum to variation in light conditions is evolved in the introduced range, while that to variation in water conditions reflects an innate property.     

Environmental,biological and mineralogical controls on strontium incorporation into skeletal carbonates in some intertidal gastropod species

Strontium concentrations have been determined in the outermost portions of shells of Patella coerulea, Monodonta articulata and M. turbinata.The incorporation of Sr into the carbonate seems to be influenced by the rate of growth of shells and by environmental temperatures, besides the Sr/Ca ratio in seawater.Temperature and growth rate may act either alternatively or concurrently; the latter is the case for Patella in the whole annual cycle, and for both Monodonta species during the higher temperature interval (18°–25°C).In the lower temperature interval (13°–17°C), the polymorphic composition of M. articulata shells exerts a primary control on Sr contents, whereas in M. turbinata the ratio Sr/Ca seems to be controlled by environmental temperatures, in a thermodynamic sense.     

黄土丘陵区燕沟流域典型植物叶片C、N、P化学计量特征季节变化

以黄土丘陵区燕沟流域为例,分析了流域8种典型植物叶片C、N、P化学计量特征的季节变化。结果发现,8种植物叶片C含量分布范围在370.2-566.9 mg/g,N含量在9.2-39.0 mg/g,P含量在0.81-2.35 mg/g,C:N在10.5-52.9,C:P在186.8-667.5,N:P在5.7-23.0。叶片平均C、C:N和C:P在5月小于7月和9月(P<0.05),而在7月和9月差异不显著;N在5月大于7月和9月(P<0.05),7月和9月差异不显著;P在7月小于5月和9月(P<0.05),5月和9月差异不显著;N:P在9月明显小于5月和7月(P<0.05),5月和7月差异不显著。叶片C含量受季节因素影响显著,而在物种间差异不显著;叶片N、P、C:N、C:P、N:P受物种和季节因素影响均显著。因此,8种植物中沙棘、黄刺梅和虎榛子采用防御性的生活史策略;刺槐、柠条和狼牙刺采用竞争性生活史策略,铁杆蒿和茭蒿的生活史策略介于上述二者之间;尽管叶片N:P随生长季节发生明显变化,但研究区植物生长的限制性元素未随生长季节变化而改变。     

Signal effects of the lectin from the associative nitrogen-fixing bacterium Azospirillum brasilense Sp7 in bacterial–plant root interactions

Aims

We compared the degree of arbuscular mycorrhizal fungi (AMF) colonization on an invasive, Triadica sebifera, and two native, Baccharis halimifolia and Morella cerifera, woody species that occur in coastal Mississippi, USA. Specifically, we investigated how the degree of colonization affects growth of these species and assessed potential allelopathic effects of T. sebifera on the two native species.

Methods

A field study and a greenhouse experiment were used to determine the degree of AMF colonization on focal woody species. Seedling growth of these species was compared between active (fungicide untreated) and AMF-suppressed (fungicide treated) soils in the greenhouse experiment. In a second greenhouse experiment, we tested the potential allelopathic effects of T. sebifera on the native species by growing the natives in soils from T. sebifera invaded or uninvaded regions, with and without activated carbon (AC).

Results

The invasive species, Triadica sebifera, exhibited a higher degree of AMF colonization compared to the native species and significantly higher total biomass in active soils compared to AMF-suppressed soils. Seedling biomass and AMF colonization of native B. halimifolia and M. cerifera did not differ between T. sebifera invaded and uninvaded soils, irrespective of the application of AC.

Conclusions

Results suggest that invasive T. sebifera benefits from the presence of AMF, which might facilitate establishment of this invader. Results also suggest that allelopathy is not a likely mechanism of T. sebifera invasion in coastal transition ecosystems. A higher degree of AMF colonization, relative to native co-occurring species, may partly explain the successful invasion of T. sebifera into coastal plant communities of the southeastern USA.     

Are rare species less shade tolerant than common species in fire-prone environments? A test with seven <Emphasis Type="Italic">Amorpha</Emphasis> (Fabaceae) species

Increases in tree density resulting from fire suppression have contributed considerably to the loss of savanna and grassland habitats in North America. Inability to tolerate shade is likely an important cause of species loss in areas that have not maintained historical burning regimes. We conducted an experiment to test whether differences in shade tolerance can explain rarity within the genus Amorpha. Four common and three rare species of Amorpha were grown in a greenhouse for 100 days in direct sun or under 90% shade. Overall, shading significantly reduced growth and survival and affected allocation among species, but these differences did not differ consistently between common and rare species. Ability to tolerate shade was best explained by phenotypic plasticity, with greater shade survival being exhibited by species with the largest changes in leaf area ratio. Furthermore, this study demonstrated that all three of the rare species can be readily cultivated under greenhouse conditions.     

Effects of salinity and clipping on biomass and competition between a halophyte and a glycophyte

Global climate change will likely result in the reduction of water levels in intermountain wetlands and ponds, and the vegetation communities associated with these wetlands are an important forage source for livestock. Lowered water levels will not only constrict wetland plant communities, it will potentially change aquatic and soil salt concentrations. Such an increase in salinity can reduce plant growth and potentially affect competitive interactions between plants. A greenhouse experiment examined the effects of salinity and competition on the growth of two wet meadow grass species, Poa pratensis (a glycophyte) and Puccinellia nuttalliana (a halophyte). The following hypotheses based on published data were tested: (1) Biomass of both species will decrease with increasing concentration of salt; (2) root:shoot (R:S) ratio of P. pratensis will decrease with increasing salt concentration while R:S ratio of P. pratensis and P. nuttalliana will increase with clipping; (3) competitive importance will decrease for P. pratensis and P. nuttalliana with increasing salt concentration because salt induces a stress response and competitive importance is reduced in stressed environments. A factorial design included 3 plant treatments (P. nuttalliana alone, P. pratentsis alone, P. nuttalliana + P. pratensis) × 4 salinity rates (control; 5, 10, 15 g/L NaCl) × 2 clipping intensities (plants clipped or not clipped) for a total of 24 combinations replicated 6 times over a period of 90 days. We found a reduction in dry biomass as salinity increased, and this effect was greatest for P. pratensis. (1.94 g (SE 0.13) at 0 g/L NaCl to 0.22 g (SE 0.11) at 15 g/L NaCl). The R:S ratio of P. pratensis was reduced by salinity, but not for P. nuttalliana. Competitive importance of both species was reduced by clipping and by salinity, but the effect was greater and more consistent for P. pratensis. We conclude that salt concentration reduces plant growth and the effect of competition.