Macrophyte productivity and community development in created freshwater wetlands under experimental hydrological conditions

Macrophyte biomass production and species richness were monitored from 1988 through 1991 in four freshwater wetlands constructed on the floodpain of the Des Plaines River, Lake County, Illinois, USA. The wetlands were constructed in 1988 and pumping of river water began in 1989 under two differentd hydrologic regimes: two wetlands received high water inflow (equivalent to 40 cm wk⁻¹ of water depth) and two received low flow (11 cm wk⁻¹). Biomass production showed no relationship to the hydrologic inflows after two years of experimentation, with both the highest and lowest production occuring in low flow wetlands. Rates of primary production increased between 1990 and 1991 under low flow conditions and decreased under high flow conditions, primarily as a result of the initial composition of the plant community. The change from dry conditions in 1988 to flooded conditions in 1989 altered the species composition in each wetland to include almost 100% wetland-adapted species. Similarity in species composition among the four wetlands diverged from 1988 to 1989 as the plant community adjusted to flooded conditions and then converged in both 1990 and 1991 as the wetlands developed.     

不同水淹深度对鄱阳湖洲滩湿地植物生长及营养繁殖的影响

水淹深度是影响湿地植物生长和繁殖的关键因子,不同湿地植物对淹水深度存在着不同响应。然而,在水情不断变化的背景下,鄱阳湖洲滩湿地植物种群和群落如何变化还不清楚。为了探究淹水深度对湿地植物生长的影响,并预测鄱阳湖洲滩湿地植被分布的趋势,采用控制实验模拟了不同水淹深度(0、0.5、1 m和2 m)下鄱阳湖湿地3种优势植物(灰化薹草(Carex cinerascens)、南荻(Miscanthus lutarioriparius)和虉草(Phalaris arundinacea))的生长和繁殖情况。实验结果表明:1)水淹对灰化薹草总生物量的影响最显著。遭受水淹时,灰化薹草把大部分的生物量集中在地下部分;随着水淹深度逐渐增加,南荻的生物量逐渐减少;不同深度水淹对虉草生物量没有产生显著影响(P0.05)。就生物量而言,虉草对水淹的适应性强于其他两种植物。2)不同水淹深度下,灰化薹草的株高都显著降低;而南荻只在2 m水淹梯度下株高才显著降低。在枯水年时,下降的水位有利于南荻向较低高程迁移。3)不同深度水淹对灰化薹草的分株没有产生显著影响(P0.05);而虉草在经过2 m水淹后分株数显著高于其他水淹深度。在丰水年时,相比于灰化薹草和南荻,升高的水位对虉草的繁殖影响较小。在一个水位周期性变化的湿地生态系统中,不同深度的水淹对植物的生长及退水后的繁殖产生了严重影响,研究结果为预测水文变化对湿地植被的生存和分布提供了重要的依据。     

鄱阳湖洲滩湿地植物生长和营养繁殖对水淹时长的响应

水淹时长是影响湿地植物分布的重要因素。在水情不断变化的背景下,鄱阳湖洲滩湿地植物种群和群落的变化规律还不清楚。为了探究淹水时长对鄱阳湖洲滩3种优势植物生长和繁殖的影响,并预测在水文发生剧烈变化后,3种优势植物分布的趋势,采用控制实验模拟了不同水淹时长(0、60、90、120、150 d和180 d)下鄱阳湖湿地3种优势植物(灰化薹草(Carex cinerascens)、南荻(Miscanthus lutarioriparius)和虉草(Phalaris arundinacea))的生长和繁殖情况。研究结果表明:1)南荻在水淹超过120 d后,存活率开始降低,水淹到达180 d完全死亡。而灰化薹草和虉草在淹水180 d后仍全部存活。表明南荻耐淹水的能力弱于其他两种植物。2)水淹显著降低灰化薹草的总生物量,并且总生物量随水淹时长的增加而逐渐降低。而短时间(小于150 d)的水淹没有对虉草总生物量产生显著影响。3)退水初期,灰化薹草的恢复生长趋向于地上部分,而虉草表现为地下和地下部分同步生长。该研究结果可以为预测水文情势变化下鄱阳湖湿地植物种群和群落的动态变化提供依据。     

Responses of native and invasive wetland plants to hydroperiod and water depth

Monotypic stands of reed canary grass, Phalaris arundinacea, replace native wetland vegetation where stormwater runoff alters hydrologic conditions, nutrient inflows, and sedimentation rates. We asked if different hydrologic conditions could explain the dominance of Phalaris and/or loss of the native grass, Spartina pectinata, and we compared the growth of each species alone and together under four hydroperiods (varying inundation frequency and duration) each at two water depths (surface saturation and flooding to 15 cm). When grown alone, aboveground biomass was similar for the two species, but Phalaris produced twice the stem length of Spartina via its low tissue density. Per unit biomass, Phalaris distributed its leaves over a larger canopy volume. Flooding reduced belowground biomass and increased total shoot length and shoot:root biomass of each species. Phalaris produced the most biomass, shoots, and total shoot length when wetter and drier conditions alternated weekly, while Spartina grew best with prolonged (4-week) inundation. When grown with Spartina, Phalaris changed its morphology by increasing its total shoot length:biomass ratio by 50%. However, ratios of Spartina:Phalaris aboveground biomass, shoot number, and total shoot length in two-species pots were not significantly affected by water depth or hydroperiod. We conclude that two plant attributes facilitate Phalaris' dominance of wetlands: its high ratio of total shoot length:biomass and its adaptable morphology (characterized herein as increased total shoot length:biomass when grown with Spartina).     

Effect of minor water depth treatments on competitive effect and response of eight wetland plants

Two facets of plant competition, competitive effect (CE) and competitive response (CR), can be used to explain plant community composition but our understanding of abiotic factors that may differentially affect species’ competitive ability is incomplete. We tested whether water-depth affected CE (ability to suppress neighbour) and CR (avoid suppression from neighbour), and if so whether there was consistence in the rank order of both measures of competition under different water depth treatments. CE and CR were measured and compared for eight wetland plant species (Carex lurida, Carex tribuloides, Elymus virginicus, Juncus tenuis, Lythrum salicaria, Phalaris arundinacea, Rumex orbiculatus and Verbesina alternifolia) at five different water-depth treatments (+2, 0, −2, −4 and −6 cm relative to the substrate). Overall, we found that mean CE was at its lowest value at +2 cm water depth, while mean CR was highest at +2 and −6 cm compared to the other water treatments. There was a significant variation of CE between species, with a defined hierarchical order. Pairwise CE rank order correlations between water depth treatments were significant but CR correlations were generally not. There was no significant correlation between CE and CR. CE was significantly correlated with biomass of species grown alone but CR was not. These findings indicate that CE may be used as a general measure to predict wetland species performance, and thus community assemblage, across a range of water depths. CR does not seem to demonstrate predicable patterns between species and water depth treatments. Our results suggest that competition intensity may be reduced in a non-resource-stressed flooded environment by a reduction in CE, but the corresponding increase in CR could dampen this effect on overall competitive ability.     

Experimental growth responses to groundwater level variation and competition in five British wetland plant species

New experimental evidence from controlledglasshouse studies was used to testprevious empirical field observations aboutthe interactions of hydrological dynamicswith individual plant species in wetlandhabitats. Manipulation of water levels, andthe presence/ absence of competitorspecies, produced significant morphologicalresponses (e.g. resource allocation toabove- and below-ground structures; plantheight; leaf size; tiller production) inmature individuals of five freshwaterwetland plant species characteristic ofBritish northern poor-fen habitats. Theseresponses provide evidence for potentialadvantages in survival and ability tospread vegetatively. The data also providenew insight into factors controlling thedistribution of wetland plants alonggradients of water table level, in thepresence or absence of competing species.Although there were variations betweenspecies, the effects of competition were,in general, less than those attributable tohydrological regime.     

The influence of some environmental factors on the production of Carex vesicaria and Phalaris arundinacea

The study deals with the annual cycles of biomass, productivity and necrosis of the perennial wetland species Carex vesicaria and Phalaris arundinacea, and with the main growth parameters during the first part of 1979. The amplitudes of biomass variation differ, but other features of the development are largely the same. To detect the various climatic factors which determine the above-ground biomass dynamics, a procedure of multiple stepwise regression has been used. Regressions for annual trend and variation in production were obtained. They provide coarse models, which may be improved by including translocation to belowground organs.Nomenclature follows P. Fournier (1977): Les quatres flores de France. Paris.     

Impacts of invasive plant species on riparian plant assemblages: interactions with elevated atmospheric carbon dioxide and nitrogen deposition

Resource competition is commonly invoked to explain negative effects of invasive plants on native plant abundance. If invasives out-compete natives, global changes that elevate resource availability may interact with invasives to exacerbate impacts on native communities. Indeed, evidence is accumulating that elevated CO₂ and N deposition decrease native biomass and simultaneously increase invasive biomass. However, superior competitive ability, and a relative increase in the magnitude of invasive impacts under elevated resource availability, remain to be definitively proven. Using model, multi-species, multi-individual riparian plant communities, where planting density was maintained by replacement of native with exotic individuals, we conducted a greenhouse, competition experiment using native (to the UK) and invaded communities exposed to ambient and elevated CO₂ (CO₂ experiment) or N availability (N experiment). We tested two hypotheses: (1) invasives are superior competitors to natives at ambient atmospheric CO₂ and N deposition; (2) negative effects of invasives on natives are exacerbated under elevated CO₂ or N availability. Our results provide some support for the first hypothesis: in the CO₂ experiment native biomass was significantly lower in invaded communities. In the N experiment, native biomass was unaffected by the presence of exotics but other characteristics (e.g. root:shoot ratios) were altered. Differences in light availability between the experiments may have modified the effects of the invasives on the native assemblages but our design did not permit us to determine this definitively. The hypothesis that elevated CO₂ and N availability benefit invasives at the expense of natives was not supported by our results. This may be explained either because the invasives showed minor responses to the resource manipulations or because native and exotic species were differentially limited by CO₂ and N. Our results confirm the expectation that invasives alter the characteristics of native assemblages but lead us to question whether elevated resource availability will magnify these effects.     

Nitrogen enrichment of soil and plant byRhizobium phaseoli-Phaseolus vulgaris symbiosis

Summary Incubation of 25 and 30 day old nodulated plants ofPhaseolus vulgaris for periods of 24 h and 72 h under 15-dinitrogen atmosphere indicated nitrogen enrichment of soil compared with soil free of plants and with other plants under the same experimental conditions. This indication of release from the root-nodule system is discussed.UNDP/IAEA/Project BRA/71/556-CENA.     

Linking community and ecosystem development on Mount St. Helens

In the two decades following the 1980 eruption of Mount St. Helens in Washington State, the N₂-fixing colonizer Lupinus lepidus is associated with striking heterogeneity in plant community and soil development. We report on differences in nutrient availability and plant tissue chemistry between older, dense patches (core) of L. lepidus and more recently established low density patches (edge). In addition, we conducted a factorial nitrogen and phosphorus fertilization experiment in core patches to examine the degree of N and P limitation in early primary succession. We found that there were no significant differences in N or P availability between core and edge L. lepidus patches during the dry summer months, although nutrient availability is very low across the landscape. In the high density patches we found lower tissue N content and higher fiber content in L. lepidus tissue than in the younger edge patches. The addition of nutrients substantially altered plant community composition, with N addition causing an increase in other forb biomass and a corresponding competition-induced decline in L. lepidus biomass. The majority of the positive biomass response came from Hypochaeris radicata. In the second year of the fertilization experiment, the addition of N significantly increased total community biomass while L. lepidus biomass declined by more than 50%. The response of every species other than L. lepidus to N additions suggests that N may be the macronutrient most limiting plant production on Mount St. Helens but that the gains in productivity were somewhat offset by a decline of the dominant species. By the third year of the experiment, L. lepidus began to increase in abundance with P addition. This result suggests co-limitation of the community by N and P.     

Controls of nitrogen limitation in tallgrass prairie

Summary The relationship between fire frequency and N limitation to foliage production in tallgrass prairie was studied with a series of fire and N addition experiments. Results indicated that fire history affected the magnitude of the vegetation response to fire and to N additions. Sites not burned for over 15 years averaged only a 9% increase in foliage biomass in response to N enrichment. In contrast, foliage production increased an average of 68% in response to N additions on annually burned sites, while infrequently burned sites, burned in the year of the study, averaged a 45% increase. These findings are consistent with reports indicating that reduced plant growth on unburned prairie is due to shading and lower soil temperatures, while foliage production on frequently burned areas is constrained by N availability. Infrequent burning of unfertilized prairie therefore results in a maximum production response in the year of burning relative to either annually burned or long-term unburned sites.Foliage biomass of tallgrass prairie is dominated by C₄ grasses; however, forb species exhibited stronger production responses to nitrogen additions than did the grasses. After four years of annual N additions, forb biomass exceeded that of grass biomass on unburned plots, and grasses exhibited a negative response to fertilizer, probably due to competition from the forbs. The dominant C₄ grasses may out-compete forbs under frequent fire conditions not only because they are better adapted to direct effects of burning, but because they can grow better under low available N regimes created by frequent fire.     

水淹条件下灰化薹草和虉草活体、枯落物分解过程的比较

为了揭示鄱阳湖湿地活体植物和自然枯落物在水淹条件下分解过程的差异,以鄱阳湖湿地优势植被物种灰化薹草(Carex cinerascens)和虉草(Phalaris arundinacea)为研究对象,采用分解袋法开展室内模拟实验。植物样品设置新鲜和风干两种处理,分别模拟活体植物和自然枯落物在水下的分解过程。研究结果表明:(1)在180 d的淹水实验过程中,两种处理下的灰化薹草和虉草的分解过程都具有阶段性,前期分解速率较快,后期较慢。(2)两种植物枯落物的分解速率与植物C/N比有显著相关性,而活体植物的分解速率与植物C/N比相关性不强。(3)活体植物和自然枯落物的水下分解速率与物种有关,虉草活体比虉草枯落物分解速率快,而薹草活体比薹草枯落物分解速率慢。(4)模拟实验结束时活体植物仍有34%—43%未分解,推测鄱阳湖丰水期退水后大量洲滩植被消失的原因并非是植被在水淹下完全分解,而是一部分植被被水生动物啃噬或被水流冲走。     

Effects of plant breeding and selection on yields and nitrogen fixation in soybeans under two soil nitrogen regimes

Summary Soybeans (Glycine max (L.) Merr.) have a high N requirement which is fulfilled by soil N uptake and N₂-fixation. This study was concerned with the effects of past yield selection on N₂-fixation in soybeans.The soybean cultivars, Lincoln, Shelby, and Williams, which represent successive improvements in the Lincoln germplasm, and a non-nodulating control were planted in a soil containing¹⁵N labelled organic matter. Two replications occurred on soil previously cropped to alfalfa and two on soil previously cropped to soybeans. Plants were harvested at five growth stages and leaf area, plant weight, total N, and atom percent¹⁵N were determined. Mature grain was harvested and yield components were also determined, as well as the total N and¹⁵N content.Cultivar differences in total dry matter were only evident at physiological maturity, when Williams contained the greatest dry matter. Williams exhibited the longest period of seed formation and seed fill and also had the highest grain yield which resulted from a larger weight per seed.The N content of the cultivars did not vary until physiological maturity when Williams contained the highest percent N. The quantity of N fixed at physiological maturity was highest for Williams and lowest for Lincoln. Fixed N contained in the harvested grain was greater for Williams than for the other two cultivars. The fraction of the total plant N derived from fixation was not greatly affected by cultivar and all cultivars acquired an average of 50% of their total N through N₂-fixation.Previous cropping history greatly affected the quantity of N fixed and the fraction of the total plant N derived from fixation. Soybeans following soybeans were more dependent upon N₂-fixation than soybeans following alfalfa with the former deriving 65% of the total plant N from fixation and the latter only 32%. These soybean cultivars apparently utilized soil N first and then used N₂-fixation to satisfy their N requirement.The past selection for higher yield has resulted in soybean cultivars with improved capacities to fix atmospheric N₂ and an improved ability to take up available soil N.     

Impacts of Myriophyllum aquaticum invasion in a Mediterranean wetland on plant and macro-arthropod communities

The invasion by alien macrophytes in aquatic ecosystems may produce a strong alteration of the native aquatic vegetation leading to heavy impacts for both plant and faunal native diversity. Myriophyllum aquaticum is an aquatic plant native of Southern America, invasive in several part of the world. We studied the effects of M. aquaticum invasion on plant and macro-arthropod communities in the canals around a protected wetland in the Mediterranean basin. We sampled plant and macro-arthropod communities in 10 transects in invaded and non-invaded tracts of the canals. We assessed the differences in plant and macro-arthropod species richness, diversity, taxonomic diversity and species composition between invaded and non-invaded habitats by means of univariate and multivariate analyses. Our study shows a significant loss of plant diversity between non-invaded to invaded sites, leading to communities numerically and taxonomically impoverished and highly divergent in the species composition. We also detected significant differences in arthropod species composition between invaded and non-invaded transects. Some taxa such as mosquitoes and malacostraca were more frequent in the M. aquaticum-dominated stands. Furthermore, the study shows a positive relation between invaded habitats and juvenile individuals of the invasive alien crayfish Procambarus clarkii.     

Photosynthetic capacity and nitrogen partitioning among species in the canopy of a herbaceous plant community

Partitioning of nitrogen among species was determined in a stand of a tall herbaceous community. Total amount of nitrogen in the aboveground biomass was 261 mmol N m^–2, of which 92% was in three dominant species (Phragmites, Calamagrostis and Carex) and the rest was in the other eight subordinate species. Higher nitrogen concentrations per unit leaf area (n _L) with increasing photosynthetically active photon flux density (PPFD) were observed in all species except for three short species. The changes in n _L within species were mainly explained by the different nitrogen concentrations per unit leaf mass, while the differences in n _L between species were explained by the different SLM (leaf mass per unit leaf area). Photon absorption per unit leaf nitrogen ( _N ) was determined for each species. If photosynthetic activity was proportional to photon absorption, _N should indicate in situ PNUE (photosynthetic nitrogen use efficiency). High _N of Calamagrostis (dominant) resulted from high photon absorption per unit leaf area ( _area ), whereas high _N of Scutellaria (subordinate) resulted from low n _L although its _area was low. Species with cylinder-like leaves (Juncus and Equisetum) had low _N , which resulted from their high n _L. Light-saturated CO₂ exchange rates per unit leaf area (CER) and per unit leaf nitrogen (potential PNUE) were determined in seven species. Species with high CER and high n _L (Phragmites, Carex and Juncus) had low potential PNUE, while species with low CER and low n _L showed high potential PNUE. NUE (ratio of dry mass production to nitrogen uptake) was approximated as a reciprocal of plant nitrogen concentration. In most species, three measures of nitrogen use efficiency (NUE, _N and potential PNUE) showed strong conformity. Nitrogen use efficiency was high in Calamagrostis and Scutellaria, intermediate in Phragmites and relatively low in Carex. Nitrogen use efficiency of subordinate species was as high as or even higher than that of dominant species, which suggests that growth is co-limited by light and nitrogen in the subordinate species.     

Recording,processing and analysis of grass root images from a rhizotron

To develop and test a system for computer-assisted image analysis, repeated video recordings of reed canary-grass roots (Phalaris arundinacea L.) were made in an 18-window rhizotron. The images were digitized and processed using a Unix computer and the Khoros software development environment.Two image sizes, 126×95 mm and 61×46 mm, both comprising 650 × 490 pixels, were compared. Among image processing techniques used were median filtering, segmentation and skeletonization. Root area and length in both the topsoil and subsoil were estimated using the two image sizes. The resolution (image size) strongly affected the calculated root lengths. The results were compared with root length measurements obtained manually.Statistically significant differences in root length and area in the topsoil were detected between the sampling dates using the computer-assisted methods. Possible sources of error and methods for reducing them are discussed.     

Effects of aphid-tending Argentine ants, nitrogen enrichment and early-season herbivory on insects hosted by a coastal shrub

The presence of the exotic Argentine ant, Linepithema humile Mayr (Hymenoptera: Dolichoderinae), nitrogen enrichment, and early-season herbivory by the specialist beetle Trirhabda bacharidis (Coleoptera: Chrysomelidae) have been shown, through separate experiments, to affect the densities of insect herbivores of the coastal shrub Baccharis halimifolia (Asteraceae), in Florida. Using a fully-factorial field experiment, we examined the relative importance of all three of these factors to the six most common insect herbivore species utilizing this host plant in a West Central Florida coastal habitat. The presence of ants affected more herbivore species than either early-season herbivory by larval T. bacharidis or nitrogen enrichment. Experimental reductions of L. humile resulted in reductions of an aphid, its coccinellid predators, and adult T. bacharidis, and increases of two species of leafminers and one species of stemborer. Due to the strong negative effects of stemborer herbivory on host plant survival, the increase in stemborer abundance led to increased host plant mortality. Early-season herbivory by larval T. bacharidis only affected the abundance of aphids and their predators, both of which were more abundant on trees with reduced early-season herbivory. Nitrogen fertilization had the most limited effects and only T. bacharidis larvae achieved higher densities on fertilized trees. Our results indicate that aphid tending by the exotic L. humile affects other insects on B. halimifolia more so than herbivory by the exploitative competitor T. bacharidis or nitrogen as a limiting nutrient.     

Influence of temporal heterogeneity in nitrogen supply on competitive interactions in a desert shrub community

Soil nutrients in arid systems are supplied to plants in brief pulses following precipitation inputs. While these resource dynamics have been well documented, little is known about how this temporal heterogeneity influences competitive interactions. We examined the impacts of the temporal pattern of N supply on competitive intensity and ability in an N-limited desert shrub community. At our field site, the three codominant shrubs, Atriplex confertifolia, A. parryi, and Sarcobatus vermiculatus, differ in seasonal growth patterns, with A. confertifolia and S. vermiculatus achieving higher growth rates earlier in the growing season than A. parryi. We predicted that these timing differences in maximum growth rate may interact with temporal variation in N supply to alter competitive abilities over time. Seedlings of the two Atriplex species were planted either individually in field plots or as target plants surrounded by neighbor seedlings. After one year of establishment, the same amount of ¹⁵N was applied to plots either as early spring pulses, mid spring pulses or continuously through the second growing season. Competitive effects were observed under continuous and pulsed N supply. Averaged across all target–neighbor treatments, competitive intensity was ∼1.8-fold greater when N was pulsed compared to when N was supplied continuously, but overall, the outcome of competitive interactions was not influenced by N pulse timing. While the timing of resource supply did not differentially influence the competitive abilities of coexisting species in this system, the temporal pattern of resource supply did alter the intensity of competitive interactions among species. While additional studies in other systems are needed to evaluate the generality of these results, this study suggests that competitive intensity may not necessarily be a direct function of productivity or resource availability as traditionally assumed. Instead, the intensity of competitive interactions in resource-poor systems may depend upon the temporal pattern of resource supply.