Pressure gradients along whole culms and leaf sheaths, and other aspects of humidity-induced gas transport in Phragmites australis

Emergent aquatic macrophytes growing in waterlogged anaerobic sediments overlain by deep water require particularly efficient ventilating systems. In Phragmites australis (Cav.) Trin. ex Steud, pressurized gas flows, generated by humidity-induced diffusion of air into leaf sheaths, enhance oxygen transport to below-ground parts and aid in the removal of respiratory CO2 and sediment-generated CO2 and methane. Although modelling and flow measurements have pointed to the probable involvement of all leaf sheaths in the flow process and the development of pressure gradients along the whole lengths of living culm and leaf sheaths, direct measurements of pressure gradients have never been reported. The aim of this study was to search for pressure gradient development in Phragmites culms and leaf sheaths and to determine their magnitudes and distribution. In addition, dynamic (with gas flow) and static pressures (no flow condition) and their relationship to flows, leaf sheath areas, and living-to-dead culm ratios were further investigated. Dynamic pressures (DeltaPd) recorded in the pith cavities of intact (non-excised) leafy culms, pneumatically isolated from the below-ground parts and venting through an artificial bore-hole near the base, revealed a curvilinear gradient of pressure 'asymptoting' towards the tips of the culms. Similarly, DeltaPd in upper and lower parts of leaf sheaths increased with distance from the base of the culm, with values in the upper parts always being greater. Curvilinear gradients of pressure were also found along pneumatically isolated individual leaf sheaths, but radial channels linking the leaf sheath aerenchyma with the pith cavity of the culm appeared to offer little resistance to flow. In keeping with predictions, static pressure differentials (DeltaPs) achieved in intact and excised culms and single leaf sheaths on intact culms proved to be relatively independent of leaf sheath area, whereas the potential for developing convective flows (pressure-driven flows) increased with increasing leaf sheath area. As measured by the ventilating coefficient [1-(DeltaPd/(DeltaPs)] the old dead (efflux) to living (influx) culm ratio of 1:12 compared with 1:25 raised ventilating efficiency from 31% to 71%, giving flows per tall culm into the rhizome system of c. 2.8 cm3 and 6.5 cm3 min-1, respectively. It was concluded that dynamic pressure gradients probably extend along the whole length of the leafy culms and leaf sheaths of Phragmites and that all leaf sheaths and all exposed points along the leaf sheaths can contribute convective gas-flow to the rhizome system.     

Effects of oil on internal gas transport, radial oxygen loss, gas films and bud growth in Phragmites australis

Background and Aims

Oil pollution of wetlands is a world-wide problem but, to date, research has concentrated on its influences on salt marsh rather than freshwater plant communities. The effects of water-borne light oils (liquid paraffin and diesel) were investigated on the fresh/brackish wetland species Phragmites australis in terms of routes of oil infiltration, internal gas transport, radial O₂ loss (ROL), underwater gas films and bud growth.

Methods

Pressure flow resistances of pith cavities of nodes and aerenchyma of leaf sheaths, with or without previous exposure to oil, were recorded from flow rates under applied pressure. Convective flows were measured from living excised culms with oiled and non-oiled nodes and leaf sheaths. The effect of oil around culm basal nodes on ROL from rhizome and root apices was measured polarographically. Surface gas films on submerged shoots with and without oil treatment were recorded photographically. Growth and emergence of buds through water with and without an oil film were measured.

Key Results

Internodes are virtually impermeable, but nodes of senesced and living culms are permeable to oils which can block pith cavity diaphragms, preventing flows at applied pressures of 1 kPa, natural convective transport to the rhizome, and greatly decreasing ROL to phyllospheres and rhizospheres. Oil infiltrating or covering living leaf sheaths prevents humidity-induced convection. Oil displaces surface gas films from laminae and leaf sheaths. Buds emerge only a few centimetres through oil and die.

Conclusions

Oil infiltrates the gas space system via nodal and leaf sheath stomata, reducing O₂ diffusion and convective flows into the rhizome system and decreasing oxygenation of phyllospheres and rhizospheres; underwater gas exchange via gas films will be impeded. Plants can be weakened by oil-induced failure of emerging buds. Plants will be most at risk during the growing season.Key words: Phragmites australis, oil pollution, convective flow, pressure flow resistance, phyllosphere oxygenation, rhizosphere oxygenation, underwater gas films, bud emergence, stomata, pith cavity diaphragms, leaf sheath aerenchyma, rhizome aeration     

Carbon dioxide emission from bamboo culms

Bamboos are one of the fastest growing plants on Earth, and are widely considered to have high ability to capture and sequester atmospheric carbon, and consequently to mitigate climate change. We tested this hypothesis by measuring carbon dioxide (CO₂) emissions from bamboo culms and comparing them with their biomass sequestration potential. We analysed diurnal effluxes from Bambusa vulgaris culm surface and gas mixtures inside hollow sections of various bamboos using gas chromatography. Corresponding variations in gas pressure inside the bamboo section and culm surface temperature were measured. SEM micrographs of rhizome and bud portions of bamboo culms were also recorded. We found very high CO₂ effluxes from culm surface, nodes and buds of bamboos. Positive gas pressure and very high concentrations of CO₂ were observed inside hollow sections of bamboos. The CO₂ effluxes observed from bamboos were very high compared to their carbon sequestration potential. Our measurements suggest that bamboos are net emitters of CO₂ during their lifespan.     

Implications of missing efflux sites on convective ventilation and amino acid metabolism in Phragmites australis

Three stands of Phragmites australis (Cav.) Trin. ex Steudel were investigated regarding the relationship between the number of efflux culms and convective ventilation efficiency affecting the hypoxic status of roots and rhizomes. The lack of old (efflux) culms after mowing the preceding winter caused a significantly higher counterpressure within the rhizome, thereby diminishing air flushing rate, i.e. oxygen supply, of rhizomes. The levels of alanine and c-aminobutyric acid in basal culm internodes increased significantly. Both amino acids indicate the hypoxic status of the root and rhizome metabolism of P. australis . Amino acid patterns of the basal culm internodes are discussed with respect to the maintenance of aerobic root metabolism and nutrient availability.     

Assessment of Aboveground and Belowground Vegetative Fragments as Propagules in the Bioenergy Crops Arundo donax and Miscanthus × giganteus

Giant miscanthus (Miscanthus × giganteus) and giant reed (Arundo donax) are leading bioenergy crops. Both exhibit many invasive characteristics, though only giant reed is known to be invasive. Despite this, neither produces viable seed, limiting movement to vegetative propagules. Therefore, to assess vegetative fragments as potential propagules, we quantified seasonal changes in culm node viability and performance in giant miscanthus and giant reed under greenhouse conditions. Giant miscanthus culms were collected in spring, summer, fall, and winter from established fields, while giant reed culms were collected in summer, fall, and winter from feral stands. Treatments at each timing consisted of whole culms and single-node culm fragments planted in soil or placed in standing water for an 8-week period. Giant miscanthus whole culms and fragments produced shoots and roots in both soil and standing water immediately following cutting from spring to summer, but failed to produce shoots and roots after fall and winter cutting dates. All rhizome fragments survived and generated shoots and roots after burial. By comparison, giant reed produced shoots and roots in both soil and standing water throughout the year, regardless of cutting date. With giant miscanthus, precautions should be taken when living culms or rhizome fragments are harvested and transported through riparian habitats during the summer months. By comparison, giant reed showed a remarkable increase in propagule generation and productivity throughout the year and, thus, escaped propagules present a far greater risk of unintentional establishment compared to giant miscanthus.     

An overview of the effects of phytotoxins on Phragmites australis in relation to die-back

The accumulation in sediments of the low molecular weight, volatile, monocarboxylic acids and/or sulphide, generated by the decay of (i) the underground parts of the reed and/or (ii) organic deposits produced under eutrophic conditions, may play a crucial part in Phragmites die-back. In the field high levels of some of these phytotoxins have been detected at certain die-back sites and in sediments containing the rotting underground parts of the plant.Symptoms of die-back include a clumped habit, stunting and death of roots and shoots, bud death, premature senescence of shoots, weakened stems, impeded aeration of the underground parts of the plant due to callus development, blockages within the vascular systems, lignification and suberisation of laterals and apical regions of adventitious roots and lower levels of starch in rhizomes. Poor convective aeration of the rhizome system has also been associated with higher proportions of alanine, γ-amino butyric acid and serine in culm bases, indicating hypoxic metabolism in the underground system.In laboratory experiments plants developed almost all of the growth, morphological and anatomical symptoms of die-back when treated with various single phytotoxins, e.g. acetic, propionic, n- and iso-butyric and n-caproic acids and sulphide at concentrations reported for die-back sites and/or associated with the decaying underground parts of the plant. For each acid alone, a concentration of ca. 1 mM, was highly toxic at pH 4.5, but relatively non-toxic at pH 6.0. However, when a cocktail of the five acids was applied (where each acid was 1 mM) the mixture proved to be toxic even at pH 6.     

Density regulation during the regeneration of two monocarpic bamboos: self-thinning or intraclonal regulation?

Abstract. The population dynamics of two monocarpic bamboos, Sasa kurilensis and S. tsuboiana, were studied for more than 10 years after establishment following mass flowering. Both species show vigorous rhizomatous vegetative reproduction after growing up to maturity, but horizontal expansion in the seedling stage was much more vigorous in S. tsuboiana than in S. kurilensis. The pattern of changes in culm density in the two species was strikingly similar: culm densities of both species increased until they reached full-density states, after which they decreased in accordance with seedling growth. However, the mode of regulation in culm density was different. S. kurilensis seedlings were composed of only a few culms and scarcely extended their rhizomes during the observation period. Such poor lateral expansion resulted in asymmetric competition as observed in many non-clonal plants, and consequently their culm density decreased as a result of the mortality of genets due to self-thinning. In S. tsuboiana seedlings, the number of culms per genet increased considerably by frequent tillering and sprouting from rhizomes. However, after reaching full density state, the Bud Utility Ratio (BUR), (the proportion of the rhizome nodes with culms to the total number of rhizome nodes), decreased drastically. In this manner, S. tsuboiana regulated culm density intraclonally as is observed in the stable states of many clonal plants. Hence it is important for the understanding of the regeneration process in clonal species to clarify when and how their seedlings extend rhizomes during their growth.     

Characteristics of culm structure and carbon and nitrogen concentrations in dead bamboo culms of two Phyllostachys species

The tissue structure of a bamboo stem (culm) differs from that of woody species, exhibiting a large cavity in the internode surrounded by a cortex with high silica concentration. Thus, to obtain an accurate estimation of the necromass, as well as carbon (C) and nitrogen (N) stocks in dead bamboo culms, we examined the basic characteristics of culm structure as well as the C and N concentrations of decomposing culms of Phyllostachys bambusoides and P. heterocycla. We collected dead culms of the two bamboo species from 15 bamboo stands in central and southwestern Japan, and analyzed the relationship between the ratio of wall volume to culm disk volume and culm diameter, as well as the changes of C and N concentrations in dead culms with wall density, which can be used as an indicator of the degree of decomposition. The ratio of wall volume to culm volume tended to decrease with increasing culm diameter for both species. The C concentration did not change, but the N concentration increased with decreasing wall density. The wall density was related to the C/N ratio, which is a chemical parameter of the degree of decomposition. The culm structure should be considered when estimating culm density. The mean C concentration can be used for estimating the C stock of decomposing culms irrespective of decomposition level. N concentration, however, should be determined according to decomposition level for N stock estimation.     

Spatial and temporal flowering patterns of the monocarpic dwarf bamboo Sasa veitchii var. hirsuta

In order to clarify spatial and temporal patterns in the flowering process of S. veitchii var. hirsuta, the authors investigated the flowering of the species at a regional level in a mountainous area of Kyoto City, and at population/individual culm levels at experimental forest stations within the same municipal region. S. veitchii var. hirsuta flowered in several separate areas spanning ranges between several and more than 10?km north?Csouth and east?Cwest from 2004 to 2007. Sporadic flowering occurred the year before mass flowering. In the mass-flowering year, 98% of the culms in an S. veitchii var. hirsuta population flowered, and the other 2% or so flowered in the years before and after mass flowering. These results show a spatiotemporal synchronous flowering pattern (with most culms of populations gregariously flowering in a year over a large area) and an asynchronous flowering pattern (with a small percentage of the culms of populations flowering before and after the mass-flowering year) for S. veitchii var. hirsuta. It was also clarified that the year and scale of mass flowering differ by area, and that mass flowering occurs in individual areas in successive years. The survey performed at the individual culm level revealed that the mean numbers of open florets and attached seeds per culm showed two peaks from April to July, suggesting that S. veitchii var. hirsuta flowering events occur twice a year.     

Effect of eutrophication on culm architecture of lakeshore Phragmites reeds

The eutrophication of lakes in central Europe has been assumed to be at least partly responsible for a widespread die-back of fringing Phragmites australis reeds. To test the ‘eutrophication hypothesis’ on a broad data basis, lakeshore reed stands at 41 lakes of known trophic status and 10 stands in waste water and sludge treatment plants in Denmark and north Germany (North data subset, n=26), and south Germany, Switzerland and Austria (South data subset, n=25) were investigated. A total of 24 culm performance variables in three different shoot classes were analyzed by two-way ANOVA with the geographic origin and the ‘relative trophic index’ as factors. The geographic origin had a pronounced influence on culm architecture, whereas the effect of the trophic level mostly failed to be significant in the ANOVA. The culms from the North reed stands showed a weaker performance throughout than those from the South stands for a given trophic level. However, some of the morphometric traits in the North stands were significant positively correlated with the trophic level but very few significant cases were observed in the South data subset. Three hypotheses are discussed to explain the geographic effect: climatic effects, geochemistry of lake water and sediments, and trophic history of the lakes. It is concluded that lake eutrophication does not influence the culm performance negatively and that eutrophication cannot be regarded as a general cause in reed decline.     

Characterising the morphological characters and carbohydrate metabolism of oat culms and their association with lodging resistance

• Lodging resistance can be improved by enhancing the mechanical strength of culms, and culm carbohydrates could improve this mechanical strength. Culm carbohydrates can regulate development of the culm and affect its toughness.
• The present study determines the relationship between lodging and carbohydrate content in oat culms. Field experiments were conducted in alpine regions in 2017 and 2018 using three oat varieties with different lodging resistance. Lodging‐related morphological characteristics were directly determined and culm carbohydrate content and enzyme activity related to cellulose synthesis and sucrose metabolism were evaluated with ultraviolet spectrophotometry.
• Results showed that the lower the gravity height or the lower ratio of gravity height to plant height, the stronger the lodging resistance of the varieties. Higher culm nonstructural (NSC) and structural (SC) carbohydrate content contributed to the ability of culms to resist lodging, especially the content of cellulose and sucrose. PCA showed that sucrose metabolism and SC content were closely related to lodging resistance. Correlation analysis showed that the lodging index (LI) was significantly negatively correlated with NSC. Sucrose content was highly and significantly positively correlated with NSC. Additionally, the activities of sucrose phosphate synthase (SPS) and sucrose synthase (SS) were highly and significantly positively correlated with sucrose and cellulose content.
• The relationship between field characters and oat lodging, as well as the regulatory mechanism of carbohydrate content on lodging resistance of the culm are discussed.

     

Culm strength of barley : correlation among maximum bending stress, cell wall dimensions, and cellulose content

Grass culms are known to differ in breaking strength, but there is little physicochemical data to explain the response. The fourth internode of four brittle and two nonbrittle barley (Hordeum vulgare L.) strains were used for physical and chemical studies of culm strength. Inner and outer culm diameters of brittle strains (3.6 ± 0.2 and 5.0 ± 0.1 millimeters) were not significantly different from those of nonbrittle strains (3.9 ± 0.2 and 5.2 ± 0.2 millimeters). Maximum bending stress, at which the culm was broken, was 192 ± 34 g/mm² for brittle and 490 ± 38 g/mm² for nonbrittle strains. Wall thickness and cell dimensions of epidermal, sclerenchyma, and parenchyma cells were measured in culm cross sections. The area of cell wall per unit cell area for each tissue was significantly correlated with the maximum bending stress (r = 0.93 for epidermis, 0.90 for sclerenchyma, and 0.84 for parenchyma). Cell walls of brittle culms had 6 to 64% as much cellulose content as those of nonbrittle culms. Maximum bending stress correlated significantly with cellulose content of the cell walls (r = 0.93), but not with the contents of noncellulosic compounds. The lower cellulose content of the brittle culm was significantly correlated with brittleness.     

Root System Dynamics of Miscanthus × giganteus and Panicum virgatum in Response to Rainfed and Irrigated Conditions in California

Miscanthus (Miscanthus × giganteus) and switchgrass (Panicum virgatum) are large perennial grass bioenergy crops in the USA and Europe. Despite much research into their agronomic potential, few studies have examined in situ root growth dynamics under irrigation and soil water deficits, particularly as they relate to shoot performance. We grew miscanthus and switchgrass in outdoor mesocosms under irrigated and rainfed conditions and assessed the spatial distribution and abundance of roots using minirhizotron images and whole root system sampling. Despite surviving an extended period of drought, shoot and root biomass, root length density, numbers of culms, and culm height were reduced in both species under rainfed (dry) conditions. However, rainfed switchgrass far outperformed rainfed miscanthus in all shoot and root growth metrics. The rainfed (drought) treatment reduced switchgrass and miscanthus whole plant biomass by 83 and 98 %, culm production by 67 and 90 %, and root length density by 67 and 94 % compared to irrigated plants, respectively. Root nitrogen concentration was higher for miscanthus (3-fold) and switchgrass (4-fold) in the rainfed treatment compared to irrigated plants and did not significantly differ between species. Unlike miscanthus, switchgrass grew roots continuously into regions of available soil moisture as surface soil layers grew increasingly dry, indicating a drought avoidance strategy. Our study suggests that switchgrass is more likely to tolerate drought by mining deep wet soils, while miscanthus relies on shallow rhizome production to tolerate dry soils.     

STRUCTURE AND DYNAMICS OF BAMBOOS IN THE WOLONG NATURAL RESERVE,CHINA

Culm (ramet) demography of two bamboos, Fargesia robusta and Bashania fangiana, was studied over 7 yr in southwest China to determine if: 1) ramet numbers remain relatively constant; 2) ramet mortality was constant; 3) density response was greater in spreading vs. compact clones; 4) extrinsic factors (e.g., predation, climate) influenced ramet dynamics; and 5) ramets that die are smaller than ones that live. Culm numbers remained relatively constant (±20%) in most plots, but culm numbers increased (>;20%) on some sites. Mortality of culms >;6 mo old over 7 yr was low and constant, but mortality of younger culms (≤6 mo) was high and variable due to predation. Spreading clones had greater recruitment response to density than compact clones, but predation and climate may have diminished or amplified the observed demographic patterns. Predators and perhaps climate influenced culm demography, and culms that died were smaller than remaining live culms. The demography of the two bamboos we studied is compared to other clonal plants, including bamboos, to identify common processes controlling ramet demography.     

Biology of the bamboo Chusquea culeou (Poaceae: Bambusoideae) in southern Argentina

Over a period of 7 years the biology and phenotypic variability of Chusquea culeou were studied at 5 locations in cool temperate forests of southern Argentina. Excavated rhizomes had an average of 1.1 successful rhizome buds, and an average of 2.1 years elapsed between successive generations of rhizomes. Rhizome buds usually develop within the first four years after a rhizome forms. Height, volume and weight of a culm can be calculated from its diameter 1 m above the ground. Culm size, length of foliage leaf blades, and pattern of secondary branching differed among study sites. Dead culms were numerous and commonly remained erect for more than 7 years after dying. New culm shoots appear in spring and reach full size within a few months. Shoots can grow more than 9 cm/day. Less than half of the shoots survived a year; most were killed by moth larvae. Multiple primary branch buds emerge through the culm leaf sheaths in the second spring. The mean number of branch buds at mid-culm nodes varied between 34.8 and 81.5, and the mean number of primary branches was between 22.8 and 40.8. Number and length of branches, and number and length of foliage leaf blades at each node is related to the position of the node on a culm. Most branches grow about 3 cm and produce 1 to 3 foliage leaves annually. Foliage leaf blades generally live 2 years or more; few survive 6 years. Relative lengths of foliage leaf blades and their spacing along a branch permit recognition of annual cohorts.Both gregarious and sporadic flowering have been reported, and every year a few isolated plants flower and die. Length of the life cycle is unknown. Seedlings require up to 15 years to produce culms of mature size. Foliage branches may live more than 23 years, and culms may survive 33 years. Extensive loss of new shoots to predation suggests that gregarious flowering may be driven by a need to escape parasitism. C. culeou clumps expand slowly. Average annual rate of increase of the number of live culms in a clump was 4.6%. Methods of seed dispersal are undocumented. A dense stand of Chusquea culeou had an estimated phytomass of 179 tons/hectare (dry weight), 28% of which was underground. Net annual production was about 16 t/ha dry weight.     

Comparing the methods for quantifying the culm surface area of bamboo, <Emphasis Type="Italic">Phyllostachys pubescens</Emphasis>

We compared several methods for quantifying the culm surface area (S) of one of the most common bamboos in Japan, Phyllostachys pubescens Mazel ex Houz. Nine sample culms of P. pubescens were felled, and the true S was determined by the fine resolution analysis of the culm form (S _FRA). The S was then calculated independently with the sectional measurement method from the successive diameters measured at equal intervals of one-twentieth (S ₂₀), one-tenth (S ₁₀), one-fifth (S ₅) and one-half (S ₂) of the total culm length. The S was also quantified geometrically from the total culm length and the diameter at breast height or at base by assuming that the culm form could be approximated by a cone (S _DBH and S _DAB). The S _FRA was compared with each of the computed S values. For S ₁₀, S ₅ and S ₂, both the mean relative bias (%BIAS) and relative root mean square error (%RMSE) decreased with an increase in the number of measured diameters. The %BIAS and %RMSE of the S ₁₀ were, respectively, comparable and smaller compared to those of S ₂₀. The bias of the S _DBH and S _DAB suggested that the cone assumption of the culm form was violated. In conclusion, we recommend that the S should be quantified from the successive diameters measured at equal intervals of one-tenth of the total culm length using the sectional measurement method.     

Deriving a reduction factor for woody part of culm for bamboo Phyllostachys pubescens

We determined the reduction factor for the woody part of culm for one of the largest bamboo species, Phyllostachys pubescens Mazel ex Houz. The determined reduction factor enables us to convert the cross-sectional area of the whole culm into that of the culm wall. We collected 650 cross-cutting sample culms from a stand of P. pubescens in Mt. Toshima, Kumamoto Prefecture, western Japan. For the cut-end surface, the external culm diameter and culm wall thickness were measured, and then the cross-sectional area of the whole culm and the culm wall were computed. The cross-sectional area of the culm wall was strongly correlated with that of the whole culm. The regression analysis between these cross-sectional area indicated that the reduction factor for P. pubescens was 0.311, independent of the magnitude of the cross-sectional area. The independence implied that the determined reduction factor could be directly applied to convert the apparent culm volume into wood volume. Implications of the reduction factor for estimating transpiration and carbon stock of P. pubescens stands were discussed. The Monte Carlo simulation revealed that at least, but not more than, 60–70 cross-cutting culms collected from 20 culms are necessary for the reduction factor when estimating the stand level transpiration and carbon stock.     

Indirect method to estimate convective gas flow through culms of a <Emphasis Type="Italic">Phragmites australis</Emphasis> stand

We propose an indirect method to estimate continuously the rate of convective gas flow in a Phragmites australis stand. In this method, the rate of gas flow is estimated using the dynamic pressure differential in a culm and the convective conductance of the culm. The rate of gas flow obtained by this indirect method coincided well with that obtained by the direct method in which a culm is detached and then reconnected to the stubble using a mass flow meter. We monitored the total gas flux through a P. australis stand in a field and found that it fluctuated diurnally with the dynamic pressure differential in culms, showing a highest rate of 26lairm^–2 ground area h^–1 at noon. The total daily gas flux was about 170lairm^–2. Our indirect method has advantages in simultaneous and continuous measurements for a cluster of culms. This method will be of use not only to quantify various gas dynamics through aquatic plants in aquatic ecosystems but also to elucidate the ecosystem processes and properties that regulate these gas dynamics.     

A role for phytotoxins in thePhragmites die-back syndrome?

Die-back and healthy stands ofPhragmites australis (Cav.) Trin. exSteud., in the U.K. and Hungary, were compared in terms of plant morphology and anatomy, sediment redox potential and sulphide levels and plant resistance to internal Poiseuille gas flow. In laboratory experiments rhizome cuttings were exposed to acetic acid or dissolved sulphide in unstirred solution cultures in order to determine whether the die-back symptoms found in the field could be induced by these phytotoxins. Most of the die-back symptoms, namely stunting of adventitious roots and laterals, bud death, callus blockages of the gas-pathways, and vascular blockages (both xylem and phloem), were produced by each of the phytotoxin treatments. These symptoms were largely absent from healthy field sites and from the experimental controls. In a greenhouse experiment, plants were grown in waterlogged sand or loam, with or without a sub-surface organic layer composed of chopped up rhizomes and roots mixed with the soil base. Especially during the first 70 days, redox levels were considerably lowered, and shoot numbers and shoot growth much reduced by the presence of the organic layers; the effects were most pronounced in the sand plus organic matter treatment. It is suggested that accumulated phytotoxins, e.g. orgnaic acids and/or sulphide, whether produced from the death and decay of the plant, or from excessive organic loading or as an indirect results of eutrophication, will perpetuate the die-back ofPhragmites and prevent the recovery of the plant in the short term.     

Flowering in grassland predicted by CO2 and resource effects on species aboveground biomass

Continuing enrichment of atmospheric CO₂ may change plant community composition, in part by altering the availability of other limiting resources including soil water, nutrients, or light. The combined effects of CO₂ enrichment and altered resource availability on species flowering remain poorly understood. We quantified flowering culm and ramet production and biomass allocation to flowering culms/ramets for 10 years in C₄‐dominated grassland communities on contrasting soils along a CO₂ concentration gradient spanning pre‐industrial to expected mid‐21st century levels (250–500 μl/L). CO₂ enrichment explained up to 77% of the variation in flowering culm count across soils for three of the five species, and was correlated with flowering culm count on at least one soil for four of five species. In contrast, allocation to flowering culms was only weakly correlated with CO₂ enrichment for two species. Flowering culm counts were strongly correlated with species aboveground biomass (AGB; R² = .34–.74), a measure of species abundance. CO₂ enrichment also increased soil moisture and decreased light levels within the canopy but did not affect soil inorganic nitrogen availability. Structural equation models fit across the soils suggested species‐specific controls on flowering in two general forms: (1) CO₂ effects on flowering culm count mediated by canopy light level and relative species AGB (species AGB/total AGB) or by soil moisture effects on flowering culm count; (2) effects of canopy light level or soil inorganic nitrogen on flowering and/or relative species AGB, but with no significant CO₂ effect. Understanding the heterogeneity in species responses to CO₂ enrichment in plant communities across soils in edaphically variable landscapes is critical to predict CO₂ effects on flowering and other plant fitness components, and species potential to adapt to future environmental changes.