Factors affecting vegetation dieback of an oligohaline marsh in coastal Louisiana: Field manipulation of salinity and submergence

Vegetation dieback is an important component of wetland loss in low salinity marshes of coastal Louisiana. A field experiment was conducted to determine the factors responsible for vegetation dieback within oligohaline marshes of Louisiana. Sections of marsh, dominated by Sagittaria lancifolia L., were transplanted into one of four locations depending on the treatment: (1) increased submergence—sods were lowered 15 cm below the donor marsh surface, (2) increased salinity—sods were transplanted into a higher salinity marsh and adjacent dieback pond, (3) increased salinity and submergence—sods were transplanted into a higher salinity marsh and adjacent dieback pond at 15 cm below the marsh surface, and (4) control—sods were exhumed and replaced at the ambient elevation of the donor marsh. Plant biomass and edaphic characteristics were measured after 5 mo. An increase in submergence caused decreased plant growth of the S. lancifolia-dominated marsh community. An increase in salinities to 4–5 g/kg were not detrimental to plant growth. Although saltwater intrusion alone did not cause decreased growth of the S. lancifolia-dominnled plant community, the combination of saltwater intrusion and increased plant submergence caused the greatest decrease in plant growth due to increased toxic sulfides and a likely reduction in the uptake of NH₄-N by the wetland vegetation. This illustrates that the dieback of oligohaline marsh vegetation can be alleviated by decreasing plant submergence even at salinities as high as 4.6 g/kg.     

Can community structure track sea‐level rise? Stress and competitive controls in tidal wetlands

Climate change impacts, such as accelerated sea‐level rise, will affect stress gradients, yet impacts on competition/stress tolerance trade‐offs and shifts in distributions are unclear. Ecosystems with strong stress gradients, such as estuaries, allow for space‐for‐time substitutions of stress factors and can give insight into future climate‐related shifts in both resource and nonresource stresses. We tested the stress gradient hypothesis and examined the effect of increased inundation stress and biotic interactions on growth and survival of two congeneric wetland sedges, Schoenoplectus acutus and Schoenoplectus americanus. We simulated sea‐level rise across existing marsh elevations and those not currently found to reflect potential future sea‐level rise conditions in two tidal wetlands differing in salinity. Plants were grown individually and together at five tidal elevations, the lowest simulating an 80‐cm increase in sea level, and harvested to assess differences in biomass after one growing season. Inundation time, salinity, sulfides, and redox potential were measured concurrently. As predicted, increasing inundation reduced biomass of the species commonly found at higher marsh elevations, with little effect on the species found along channel margins. The presence of neighbors reduced total biomass of both species, particularly at the highest elevation; facilitation did not occur at any elevation. Contrary to predictions, we documented the competitive superiority of the stress tolerator under increased inundation, which was not predicted by the stress gradient hypothesis. Multifactor manipulation experiments addressing plant response to accelerated climate change are integral to creating a more realistic, valuable, and needed assessment of potential ecosystem response. Our results point to the important and unpredicted synergies between physical stressors, which are predicted to increase in intensity with climate change, and competitive forces on biomass as stresses increase.     

The influence of vegetation,salinity, and inundation on seed banks of oligohaline coastal marshes

Sea level rise may alter salinity and inundation regimes and create patches of open water in oligohaline coastal marshes, potentially affecting the composition and germination of seed bank species. We conducted seedling emergence experiments to: (1) examine the effects of standing vegetation on the seed banks of three oligohaline marsh communities in coastal Louisiana (dominated by Paspalum vaginatum Sw., Sagittaria lancifolia L., or Spartina patens (Ait.) Muhl., respectively); and (2) investigate the effects of salinity and inundation regime on germination of seed bank species. We also studied the effect of a temporary increase in salinity (to simulate a salt water intrusion event) on the viability of buried seeds. We found that the presence or absence of vegetation within a community affected the abundance of some species in the seed bank but had little effect on species composition. Also, the seed banks of the three communities exhibited considerable overlap in species composition and had similar species richness (10–11) and diversity (antilog Shannon-Weaver diversity index = 6.5–7.1), despite differences in vegetation type. Higher salinities and flooding reduced seedling emergence for most species; few species emerged at salinities above four parts per thousand (ppt), and only Sagittaria lancifolia and Eleocharis parvula germinated well under flooded conditions. A temporary increase in salinity did not affect species richness or seedling emergence of most species. Our results suggest that differences in vegetation may have little effect on the composition of seed banks of oligohaline marshes. However, higher salinities and greater depth and duration of inundation (anticipated as global sea level continues to rise) may decrease recruitment of seed bank species, reducing their abundance in oligohaline marsh communities.     

Sea‐level rise,habitat loss,and potential extirpation of a salt marsh specialist bird in urbanized landscapes

Sea‐level rise (SLR) impacts on intertidal habitat depend on coastal topology, accretion, and constraints from surrounding development. Such habitat changes might affect species like Belding's savannah sparrows (Passerculus sandwichensis beldingi; BSSP), which live in high‐elevation salt marsh in the Southern California Bight. To predict how BSSP habitat might change under various SLR scenarios, we first constructed a suitability model by matching bird observations with elevation. We then mapped current BSSP breeding and foraging habitat at six estuarine sites by applying the elevation‐suitability model to digital elevation models. To estimate changes in digital elevation models under different SLR scenarios, we used a site‐specific, one‐dimensional elevation model (wetland accretion rate model of ecosystem resilience). We then applied our elevation‐suitability model to the projected digital elevation models. The resulting maps suggest that suitable breeding and foraging habitat could decline as increased inundation converts middle‐ and high‐elevation suitable habitat to mudflat and subtidal zones. As a result, the highest SLR scenario predicted that no suitable breeding or foraging habitat would remain at any site by 2100 and 2110. Removing development constraints to facilitate landward migration of high salt marsh, or redistributing dredge spoils to replace submerged habitat, might create future high salt marsh habitat, thereby reducing extirpation risk for BSSP in southern California.     

Net ecosystem carbon exchange and the greenhouse gas balance of tidal marshes along an estuarine salinity gradient

Tidal wetlands are productive ecosystems with the capacity to sequester large amounts of carbon (C), but we know relatively little about the impact of climate change on wetland C cycling in lower salinity (oligohaline and tidal freshwater) coastal marshes. In this study we assessed plant production, C cycling and sequestration, and microbial organic matter mineralization at tidal freshwater, oligohaline, and salt marsh sites along the salinity gradient in the Delaware River Estuary over four years. We measured aboveground plant biomass, carbon dioxide (CO₂) and methane (CH₄) exchange between the marsh and atmosphere, microbial sulfate reduction and methanogenesis in marsh soils, soil biogeochemistry, and C sequestration with radiodating of soils. A simple model was constructed to estimate monthly and annually integrated rates of gross ecosystem production (GEP), ecosystem respiration (ER) to carbon dioxide ( \( {\text{ER}}_{{{\text{CO}}_{2} }} \) ) or methane ( \( {\text{ER}}_{{{\text{CH}}_{4} }} \) ), net ecosystem production (NEP), the contribution of sulfate reduction and methanogenesis to ER, and the greenhouse gas (GHG) source or sink status of the wetland for 2 years (2007 and 2008). All three marsh types were highly productive but evidenced different patterns of C sequestration and GHG source/sink status. The contribution of sulfate reduction to total ER increased along the salinity gradient from tidal freshwater to salt marsh. The Spartina alterniflora dominated salt marsh was a C sink as indicated by both NEP (~140 g C m^?2 year^?1) and ²¹⁰Pb radiodating (336 g C m^?2 year^?1), a minor sink for atmospheric CH₄, and a GHG sink (~620 g CO_2-eq m^?2 year^?1). The tidal freshwater marsh was a source of CH₄ to the atmosphere (~22 g C–CH₄ m^?2 year^?1). There were large interannual differences in plant production and therefore C and GHG source/sink status at the tidal freshwater marsh, though ²¹⁰Pb radiodating indicated modest C accretion (110 g C m^?2 year^?1). The oligohaline marsh site experienced seasonal saltwater intrusion in the late summer and fall (up to 10 mS cm^?1) and the Zizania aquatica monoculture at this site responded with sharp declines in biomass and GEP in late summer. Salinity intrusion was also linked to large effluxes of CH₄ at the oligohaline site (>80 g C–CH₄ m^?2 year^?1), making this site a significant GHG source (>2,000 g CO_2-eq m^?2 year^?1). The oligohaline site did not accumulate C over the 2 year study period, though ²¹⁰Pb dating indicated long term C accumulation (250 g C m^?2 year^?1), suggesting seasonal salt-water intrusion can significantly alter C cycling and GHG exchange dynamics in tidal marsh ecosystems.     

Influence of Mycorrhizal Inoculation, Inundation Period, Salinity, and Phosphorus Availability on the Growth of Two Salt Marsh Grasses, Spartina alterniflora Lois. and Spartina cynosuroides (L.) Roth., in Nursery Systems

Restoration of salt marsh ecosystems is an important concern in the eastern United States to mitigate damage caused by industrial development. Little attention has been directed to the mycorrhizal influence on plantings of salt marsh species to stabilize estuarine sediments and establish cover. In our study, seedlings of two salt marsh grasses, Spartina alterniflora and Spartina cynosuroides, were grown in soil with a commercial, mixed species inoculum of arbuscular mycorrhizal fungi. Plants were grown in experimental “ebb and flow” boxes, simulating three levels of tidal inundation, to which two levels of applied phosphorus (P) and two levels of salinity were imposed. After 2.5 months, S. alterniflora was poorly colonized by arbuscular mycorrhizae, developing only fungal hyphae and no arbuscules, but S. cynosuroides became moderately colonized. Mycorrhizal inoculation marginally improved growth and P and nitrogen (N) content of both plant species at low levels of P supply but significantly increased tillering in both plant species. This factor could be beneficial in enhancing ground cover during restoration procedures. Greater P availability increased the mycorrhizal status of S. cynosuroides and improved P nutrition of both plant species, despite a reduction in the root‐to‐shoot ratio. Increasing salinity reduced mycorrhizal colonization of S. alterniflora but not of S. cynosuroides. Growth and nutrient content of S. alterniflora was improved at higher levels of salinity, but only increased nutrient content in S. cynosuroides. Increased duration of tidal inundation decreased plant growth in both species, but tissue P and N concentrations were highest with the longest time of inundation in both species.     

The interactive effects of created salt marsh substrate type,hydrology, and nutrient regime on <Emphasis Type="Italic">Spartina alterniflora</Emphasis> and <Emphasis Type="Italic">Avicennia germinans</Emphasis> productivity and soil development

Recent salt marsh and barrier island restoration efforts in the northern Gulf of Mexico have focused on optimizing self-sustaining attributes of restored marshes to provide maximum habitat value and storm protection to vulnerable coastal communities. Salt marshes in this region are dominated by Spartina alterniflora and Avicennia germinans, two species that are valued for their ability to stabilize soils in intertidal salt marshes. We conducted a controlled greenhouse study to investigate the influences of substrate type, nutrient level, and marsh elevation on the growth and biomass allocation of S. alterniflora and A. germinans, and the consequent effects on soil development and stability. S. alterniflora exhibited optimal growth and survival at the lowest elevation (? 15 cm below the water surface) and was sensitive to high soil salinities at higher elevations (+ 15 cm above the water surface). A. germinans performed best at intermediate elevations but was negatively affected by prolonged inundation at lower elevations. We found that although there was not a strong effect of substrate type on plant growth, the development of stressful conditions due to the use of suboptimal materials would likely be exacerbated by placing the soil at extreme elevations. Soil shear strength was significantly higher in experimental units containing either S. alterniflora or A. germinans compared to unvegetated soils, suggesting that plants effectively contribute to soil strength in newly placed soils of restored marshes. As marsh vegetation plays a critical role in stabilizing shorelines, salt marsh restoration efforts in the northern Gulf of Mexico and other storm impacted coasts should be designed at optimal elevations to facilitate the establishment and growth of key marsh species.     

Stability of Bacterial Composition and Activity in Different Salinity Waters in the Dynamic Patos Lagoon Estuary: Evidence from a Lagrangian-Like Approach

We employed a Lagrangian-like sampling design to evaluate bacterial community composition (BCC—using temporal temperature gel gradient electrophoresis), community-level physiological profiles (CLPP—using the EcoPlate? assay), and influencing factors in different salinity waters in the highly dynamic Patos Lagoon estuary (southern Brazil) and adjacent coastal zone. Samples were collected monthly by following limnetic–oligohaline (0–1), mesohaline (14–16), and polyhaline (28–31) waters for 1 year. The BCC was specific for each salinity range, whereas the CLPPs were similar for mesohaline and polyhaline waters, and both were different from the limnetic–oligohaline samples. The limnetic–oligohaline waters displayed an oxidation capacity for almost all organic substrates tested, whereas the mesohaline and polyhaline waters presented lower numbers of oxidized substrates, suggesting that potential activities of bacteria increased from the polyhaline to oligohaline waters. However, the polyhaline samples showed a higher utilization of some simple carbohydrates, amino acids, and polymers, indicating a shortage of inorganic nutrients (especially nitrogen) and organic substrates in coastal saltwater. The hypothesis of bacterial nitrogen limitation was corroborated by the higher Nuse index (an EcoPlate?-based nitrogen limitation indicator) in the polyhaline waters and the importance of NO₂ ^?, NO₃ ^?, low-molecular-weight substances, and the low-molecular-weight:high-molecular-weight substances ratio, indicated by the canonical correspondence analyses (CCAs). Our results demonstrate the important stability of microbial community composition and potential metabolic activity in the different water salinity ranges, which are independent of the region and time of the year of sample collection in the estuary. This is a quite unexpected result for a dynamic environment such as the Patos Lagoon estuary.     

Above- and below-ground competition between the liana <Emphasis Type="Italic">Acacia kamerunensis</Emphasis> and tree seedlings in contrasting light environments

Proliferation of lianas in canopy gaps can restrict tree regeneration in tropical forests through competition. Liana effects may differ between tree species, depending on tree requirements for above- and below-ground resources. We conducted an experiment in a shade house over 12 months to test the effect of light (7 and 27% external irradiance) on the competitive interactions between seedlings of one liana species and three tree species and the contribution of both above- and below-ground competition. Seedlings of the liana Acacia kamerunensis were grown with tree seedlings differing in shade tolerance: Nauclea diderrichii (Pioneer), Khaya anthotheca (Non-Pioneer Light Demander) and Garcinia afzelii (Non-Pioneer Shade Bearer). Trees were grown in four competition treatments with the liana: no competition, root competition, shoot competition and root and shoot competition. Both root and root–shoot competition significantly reduced relative growth rates in all three tree species. After one year, root–shoot competition reduced growth in biomass to 58% of those (all species) grown in no competition. The root competition treatment had a more important contribution in the effect of the liana on tree growth. Tree seedlings did not respond to competition with the liana by altering their patterns of biomass allocation. Although irradiance had a great effect on tree growth and allocation of biomass, the interaction between competition treatments and irradiance was not significant. Nauclea diderrichii, the tree species which responded most to the effects of competition, showed signs of being pot-bound, the stress of which may have augmented the competition effects. The understanding of the interaction of above- and below-ground competition between lianas and trees and its moderation by the light environment is important for a proper appreciation of the influence of lianas on tropical forest regeneration.     

Fifteen Years of Vegetation and Soil Development after Brackish-Water Marsh Creation

Aboveground biomass, macro‐organic matter (MOM), and wetland soil characteristics were measured periodically between 1983 and 1998 in a created brackish‐water marsh and a nearby natural marsh along the Pamlico River estuary, North Carolina to evaluate the development of wetland vegetation and soil dependent functions after marsh creation. Development of aboveground biomass and MOM was dependent on elevation and frequency of tidal inundation. Aboveground biomass of Spartina alterniflora, which occupied low elevations along tidal creeks and was inundated frequently, developed to levels similar to the natural marsh (750 to 1,300 g/m²) within three years after creation. Spartina cynosuroides, which dominated interior areas of the marsh and was flooded less frequently, required 9 years to consistently achieve aboveground biomass equivalent to the natural marsh (600 to 1,560 g/m²). Aboveground biomass of Spartina patens, which was planted at the highest elevations along the terrestrial margin and seldom flooded, never consistently developed aboveground biomass comparable with the natural marsh during the 15 years after marsh creation. MOM (0 to 10 cm) generally developed at the same rate as aboveground biomass. Between 1988 and 1998, soil bulk density decreased and porosity and organic C and N pools increased in the created marsh. Like vegetation, wetland soil development proceeded faster in response to increased inundation, especially in the streamside zone dominated by S. alterniflora. We estimated that in the streamside and interior zones, an additional 30 years (nitrogen) to 90 years (organic C, porosity) are needed for the upper 30 cm of created marsh soil to become equivalent to the natural marsh. Wetland soil characteristics of the S. patens community along upland fringe will take longer to develop, more than 200 years. Development of the benthic invertebrate‐based food web, which depends on organic matter enrichment of the upper 5 to 10 cm of soil, is expected to take less time. Wetland soil characteristics and functions of created irregularly flooded brackish marshes require longer to develop compared with regularly flooded salt marshes because reduced tidal inundation slows wetland vegetation and soil development. The hydrologic regime (regularly vs. irregularly flooded) of the “target” wetland should be considered when setting realistic expectations for success criteria of created and restored wetlands.     

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).     

Field assessment of environmental factors constraining the development and expansion of <Emphasis Type="Italic">Schoenoplectus californicus</Emphasis> marsh at a California tidal freshwater restoration site

The effective restoration of wetland habitats requires understanding the establishment requirements, growth responses, and expansion dynamics of targeted plant species. This is particularly true when restoring areas that have been previously managed for other activities, such as agriculture, which can have legacy effects on the local environment. We investigated environmental factors (specifically hydrology and soil physicochemical conditions) that may influence the establishment, growth and expansion of Schoenoplectus californicus in a tidal freshwater marsh restoration site in the Sacramento–San Joaquin Delta, California, USA. This study site was previously leveed, drained, and utilized for agricultural production. A 1997 levee breach restored tidal connectivity and wetland vegetation has re-established in portions of the area. Our approach coupled an intensively-sampled transect study in S. californicus-dominated marshes with a spatially-extensive survey of S. californicus lateral expansion rates and elevation. Lateral expansion of S. californicus marsh edge was significantly less in lower elevation areas (0.61 ± 0.04 m NAVD88), whereas the marsh edge at higher elevations (0.84 ± 0.03 m NAVD88) exhibited greater expansion, often at rates greater than 1.0 m year^?1. These elevation means correspond to percentages of time that the marsh surface was flooded of 100 and 94 %, respectively. Although marsh edge expansion was influenced by elevation and the resultant hydrology, other factors, such as physical exposure of marsh shorelines and compacted agricultural soils also appear to be important. However, once established, S. californicus appears to be able to ameliorate high soil bulk densities over time as the advancing marsh platform develops.     

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.     

Cadmium tolerance and phytoremediation potential of acacia (Acacia nilotica L.) under salinity stress

In this study, we explored the effect of salinity on cadmium (Cd) tolerance and phytoremediation potential of Acacia nilotica. Two-month-old uniform plants of A. nilotica were grown in pots contaminated with various levels of Cd (0, 5, 10, and 15 mg kg^?1), NaCl (0%, 0.5%, 1.0% (hereafter referred as salinity), and all possible combinations of Cd + salinity for a period of six months. Results showed that shoot and root growth, biomass, tissue water content and chlorophyll (chl a, chl b, and total chl a+b) contents decreased more in response to salinity and combination of Cd + salinity compared to Cd alone. Shoot and root K concentrations significantly decreased with increasing soil Cd levels, whereas Na and Cl concentrations were not affected significantly. Shoot and root Cd concentrations, bioconcentration factor (BCF) and translocation factor (TF) increased with increasing soil Cd and Cd + salinity levels. At low level of salinity (0.5%), shoot and root Cd uptake enhanced, while it decreased at high level of salinity (1.0%). Due to Cd tolerance, high shoot biomass and shoot Cd uptake, this tree species has some potential for phytoremediation of Cd from the metal contaminated saline and nonsaline soils.     

本地和外来草本物种对水分条件时间异质性的可塑响应

极端气候导致的干旱和水淹事件频发，影响了外来植物和本地植物的生长。为了解外来种和本地种植物对干旱和水淹事件发生顺序的响应，探讨草本植物适应水分时间异质性的策略，该文以美国蒙大拿州西部4种本地植物和4种外来植物为研究对象，将所有植物分别进行持续湿润(对照，CK)、水淹-干旱(I-D)和干旱-水淹(D-I)处理，并观测一系列形态和生物量特征的变化。结果表明：(1)与CK相比，D-I和I-D处理均显著降低了外来种的总生物量(P<0.05)。(2)D-I显著降低了本地种早期总生物量、后期地下生物量和根冠比，但显著提高了其后期的相对生长(P<0.05)。(3)D-I处理显著降低了所有植物的地下-地上生物量关系的异速指数，外来种异速指数显著高于本地种(P<0.05)。综上认为，极端事件(水淹和干旱)的发生顺序能改变外来植物和本地植物的生物量分配，早期干旱比后期干旱更容易减少植物生物量的积累，但能促进本地种后期的生长；本地种在环境胁迫下不被降低的总生物量表现说明维持表型稳定的能力较强；D-I处理下本地种和外来种地上和地下生物量关系的分配方式不同。     

The species richness–biomass relationship in herbaceous plant communities: what difference does the incorporation of root biomass data make?

So far, in all studies on the much-discussed hump-backed relationship between plant community productivity and species richness, productivity has been assessed through plant shoot biomass, i.e. it has been ignored that frequently most of the biomass is produced below ground. We revisited the 27 grassland and forest field-layer communities, studied earlier by Zobel and Liira, to sample root biomass, plant total biomass and root/shoot allocation, and learn how the incorporation of below-ground biomass data would affect the shape of the hump-backed relationship. In order to avoid scaling artefacts we estimated richness as the average count of species per 500 plant ramets (absolute richness). We also included relative richness measures. Relative richness was defined as richness per 500 ramets/size of the actual species pool (the set of species present in the community), relative pool size was defined as size of the actual species pool/size of the regional species pool (the set of species available in the region and capable of growing in the given community).
The biomass-absolute richness relationship was humped, irrespective of the biomass measure used, the hump being most obvious when plant total biomass was used as the independent variable. Evidently, the unimodal richness–productivity curve is not a sampling artefact, as suspected by Oksanen. However, relative richness was not related to community biomass (above-ground, below-ground or total). The hump-backed curve is shaped by the sizes of actual species pools in communities, implying that processes which are responsible for small-scale diversity pattern mainly operate on the community level.
Neither absolute nor relative richness were significantly related to root/shoot allocation. The presumably stronger (asymmetric) shoot competition at greater allocation to shoots appears not to suppress small-scale richness. However, there is a significant relationship between relative pool size and root/shoot allocation. Relatively more species from regional species pools are able to enter and persist in communities with more biomass allocated into roots.     

Vegetation change from chronic stress events: Detection of the effects of tide gate removal and long‐term drought on a tidal marsh

Question: Chronic stress events are defined as disturbance events that exceed the lifespan of the dominant plant species, fluctuate in intensity and lack abruptness or physical destruction of biomass. Can the effects of chronic stress events be measured on vegetation communities? Did two chronic stress events, the removal of a tide gate and a four year drought, cause a temporary or permanent shift in the vegetation communities of a tidal marsh? Location: Tidal marsh in southeastern United States. Methods: Change in species composition and dominance and community change on a landscape level salinity gradient were measured between time periods ranging from four months to seven years to construct a statistical baseline reference community at freshwater, oligohaline, and mesohaline sections of a tidal marsh. Statistical shifts in the plant community were defined as changes in the plant community that fell outside of the defined baseline reference community. Results: Plant community changes outside of the reference community occurred in 13 out of 378 community comparisons. Removal of the tide gate had a greater effect on interstitial salinity levels than the drought and was most intense in the oligohaline marsh, where between 20 to 45% of the freshwa‐ter/oligohaline community types permanently converted to oligohaline community types. However, community shifts in the freshwater and oligohaline marsh induced by the drought were temporary, lasting from 1 to 3+ years. Neither chronic stress event permanently altered the mesohaline plant communities. Conclusion: The effects of chronic stress events could be detected; an extended historical record of vegetation change (18 years) was necessary to identify community shifts outside of a reference condition of the community and to determine if those shifts were permanent or temporary.     

Responses of Fraxinus excelsior seedlings to grass-induced above- and below-ground competition

The competitive interactions between woody seedlings and herbaceous vegetation have received increasing interest in recent years. However, little is known about the relative contributions and underlying mechanisms of above- and below-ground competition between species. We used a novel experimental approach to assess the responses of Fraxinus excelsior seedlings to different combinations of root and shoot competition imposed by the grass Dactylis glomerata under greenhouse conditions. Seedling growth was significantly reduced by competition for soil resources, but neither biomass nor height were significantly affected by shoot competition for light. Competitive response indices based on biomass confirmed that below-ground competition was more important than above-ground competition, and indicated that root and shoot competition did not interact to influence plant growth. Fraxinus biomass allocation and seedling traits were almost all significantly affected by root competition; these responses varied depending on the trait examined. In contrast, morphological responses to shoot competition were limited. In the absence of root competition, seedlings showed a significant increase in the biomass allocated to leaves and a greater leaf area ratio in response to shoot competition. Our findings suggest that morphological modifications help to mitigate the negative effects of competition, but the expression of plasticity may be suboptimal due to resource constraints. The present study also highlights the importance of appropriate experimental controls and analysis to avoid confounding effects of experimental design and ontogeny on the interpretation of competitive responses.     

Seasonal flooding,soil salinity and primary production in northern prairie marshes

Hydrologic regime is an important control of primary production in wetland ecosystems. I investigated the coupling of flooding, soil salinity and plant production in northern prairie marshes that experience shallow spring flooding. Field experiments compared whitetop (Scolochloa festucacea) marsh that was: (1) nonflooded, (2) flooded during spring with 25 cm water and (3) nonflooded but irrigated with 1 cm water · day^–1. Pot culture experiments examined whitetop growth response to salinity treatments. The electrical conductivity of soil interstitial water (EC_e) at 15 cm depth was 4 to 8 dS· m^–1 lower in flooded marsh compared with nonflooded marsh during 2 years. Whitetop aboveground biomass in flooded marsh (937 g · m^–2, year 1; 969 g · m^–2, year 2) exceeded that of nonflooded marsh (117 g · m^–2 year 1; 475 g · m^–2, year 2). Irrigated plots had lower EC_e and higher aboveground biomass than nonflooded marsh. In pot culture, EC_e of 4.3 dS · m^–1 (3 g · L^–1 NaCl) reduced total whitetop biomass by 29 to 44% and EC_e of 21.6 dS · m^–1 (15 g · L^–1 NaCl) reduced biomass by more than 75%. Large reductions of EC_e and increases of whitetop growth with irrigation indicated that plants responded to changes in soil salinity and not other potential environmental changes caused by inundation. The results suggest that spring flooding controls whitetop production by decreasing soil salinity during spring and by buffering surface soils against large increases of soil salinity after mid-summer water level declines. This mechanism can explain higher marsh plant production under more reducing flooded soil conditions and may be an important link between intermittent flooding and primary production in other wetland ecosystems.     

Root Morphology, Stem Growth and Field Performance of Seedlings of Two Mediterranean Evergreen Oak Species Raised in Different Container Types

Outplanting container-grown oak seedlings with undesirable shoot and root characteristics result in poor establishment and reduced field growth. The objective of this study was to determine the influence of container type on both above-and below-ground nursery growth and field performance of one-year old tap-rooted seedlings Quercus ilex L. and Quercus coccifera L. The experiment was conducted in an open-air nursery and the seedlings were grown in three container types. At the end of the nursery, growth period seedlings’ shoot height, diameter (5 mm above root collar), shoot and root biomass, root surface area, root volume and total root length were assessed. Then the seedlings were planted in the field and their survival and growth were recorded for two growing seasons after outplanting. The results showed a difference between the Quercus species in the effect of container type. Q. ilex seedlings raised in paper-pot had significantly greater height, diameter, shoot and root biomass and root volume than those raised in the other two container types. Similarly, Q. coccifera seedlings raised in paper-pot, had significantly greater above-and below-ground growth than those raised in the other two container types. Both oak species showed relatively low survival in the field; the mortality was mainly observed the first year after outplanting, especially after the summer dry period. However, 2 years after outplanting, the paper-pot seedlings of the two oak species showed better field performance.