Net carbon exchange and evapotranspiration in postfire and intact sagebrush communities in the Great Basin

Invasion of non-native annuals across the Intermountain West is causing a widespread transition from perennial sagebrush communities to fire-prone annual herbaceous communities and grasslands. To determine how this invasion affects ecosystem function, carbon and water fluxes were quantified in three, paired sagebrush and adjacent postfire communities in the northern Great Basin using a 1-m³ gas exchange chamber. Most of the plant cover in the postfire communities was invasive species including Bromus tectorum L., Agropyron cristatum (L.) Gaertn and Sisymbrium altissimum L. Instantaneous morning net carbon exchange (NCE) and evapotranspiration (ET) in native shrub plots were greater than either intershrub or postfire plots. Native sagebrush communities were net carbon sinks (mean NCE 0.2–4.3 μmol m⁻² s⁻¹) throughout the growing season. The magnitude and seasonal variation of NCE in the postfire communities were controlled by the dominant species and availability of soil moisture. Net C exchange in postfire communities dominated by perennial bunchgrasses was similar to sagebrush. However, communities dominated by annuals (cheatgrass and mustard) had significantly lower NCE than sagebrush and became net sources of carbon to the atmosphere (NCE declined to −0.5 μmol m⁻² s⁻¹) with increased severity of the summer drought. Differences in the patterns of ET led to lower surface soil moisture content and increased soil temperatures during summer in the cheatgrass-dominated community compared to the adjacent sagebrush community. Intensive measurements at one site revealed that temporal and spatial patterns of NCE and ET were correlated most closely with changes in leaf area in each community. By altering the patterns of carbon and water exchange, conversion of native sagebrush to postfire invasive communities may disrupt surface-atmosphere exchange and degrade the carbon storage capacity of these systems.     

Annual cycle of gross primary production and respiration in the Viroin River (Belgium)

Gross primary production, community respiration and reaeration coefficient were determined during an annual cycle on the Viroin River (South Belgium), based on the daily variations of dissolved oxygen concentration. Reaeration coefficient remains remarkably constant (0.26 h⁻¹) during the year in spite of discharge variations. The autotrophic community is dominated by ‘Ranunculus fluitans’. Primary production parallels the variations of total solar radiations. It ranges from 0 in winter to 8 g O₂ m⁻² d⁻¹ in summer. In spring and summer, respiration variations parallel those of primary production (average value: 10 g O₂ m⁻² d⁻¹); in the dry autumn, decomposition of dying macrophytes considerably enhances the community respiration (15 g O₂ m⁻² d⁻¹). A P/R diagram is used to characterize the trophic state of the Viroin.     

Life cycle,population dynamics,growth and production of <Emphasis Type="Italic">Abra segmentum</Emphasis> (Mollusca,Bivalvia) at low salinities in a Mediterranean lagoon

Aspects of the biology of Abra segmentum were investigated at low salinities in a Mediterranean coastal lagoon (Monolimni Lagoon, Northern Aegean Sea). Monthly samples were collected during the period from February 1998 to January 1999. Recruitment occurred from mid-spring to early autumn (0.3–5.7 psu) and recruits grew during summer and autumn (1.2–5.7 psu), while a major part vanished during next autumn, displaying a maximum life span of about 20 months. A positive correlation was found between the percentage of individuals having a shell length of ≤3.5 mm and temperature; age group 0 showed a growth rate of 0.97 mm per month, and the largest individual collected had a 19.76 mm shell length. The population density sharply increased during late spring (0.3–1.2 psu); this increase was followed by a decline during summer and, afterwards, a gradual increase up to late autumn. Secondary production calculated by the size–frequency method gave a mean annual density (n) of 3,357 individuals m⁻², a mean annual biomass (B) of 21.98 g DW m⁻², an annual production (P) of 73.72 g DW m⁻² and a P:B ratio of 3.35. A comparison of the present data with available data of A. segmentum populations from higher salinity habitats revealed that this bivalve in the study area showed a life history pattern similar to that of other populations of the species and a comparatively high growth rate, maximum body size, n, B, P and P:B ratio. Our findings suggest that the studied aspects of A. segmentum biology could not be markedly affected by low salinities.     

Effects of residual soil salinity resulting from irrigation with sulphate waters on lettuce

Summary The response of lettuce (Lactuca sativa L.) to residual soil salinity as influenced by the ionic composition of two different saline waters (EC_w=3.1 dS/m, referenced at 25°C) and rain water, was investigated in a greenhouse experiment with three successive plantings of lettuce in the same soil. One of the saline waters was saturated with gypsum (SO₄=35 mol (−)m⁻³) and the other contained SO₄ at 15 mol (−)m⁻³ and Na and Cl at 18 and 14 mol (±)m⁻³, respectively (mixed water). All waters were applied with a 0.3 leaching fraction. Soil water salinity and sodium adsorption ratio (SAR) increased in both cases using saline waters. The effect of mixed saline water was higher and became more marked after each planting, resulting from higher contribution of Na and Cl to soil salinity. With both saline waters, soil solution became saturated with gypsum. At first planting, gypsum saturated and mixed waters produced fresh yield increases of 15 and 24%, respectively, relative to rain water. At second planting, however, there was reduction in yield of 11 and 22%, respectively, relative to rain water; at third planting yield reduced by 22 and 76% with gypsum saturated and mixed water, respectively.     

Gas exchange in the salt marsh species <Emphasis Type="Italic">Atriplex portulacoides</Emphasis> L. and <Emphasis Type="Italic">Limoniastrum monopetalum</Emphasis> L. in Southern Portugal

Salt marshes are ecosystems subjected to a variety of environmental stresses like high salinity, water deficit, intense radiation or high temperatures. Field measurements were conduced in two halophyte species, Atriplex portulacoides L. and Limoniastrum monopetalum L., in the Reserva Natural do Sapal de Castro Marim, to compare their physiological response, i.e., water potential (ψ), net photosynthetic rate (A), stomatal conductance (gs) under natural conditions. Both species demonstrated marked variations in ψ throughout the year, with very low values in the summer, the period of higher salinity, drought and temperature. Deficit water potential (Δψ = ψ_midday − ψ_predawn) was lower in the summer than in other seasons in A. portulacoides but not in L. monopetalum. The highest values for A and gs in L. monopetalum were observed in autumn and for A. portulacoides in winter, presenting both lowest values in spring and summer. A_max was particularly high for L. monopetalum than for A. portulacoides in summer and autumn, despite gs_max was similar in both species. Diurnal pattern of A and gs were similar in both species, with higher values in the morning, decreasing throughout the day.     

Can water responses in Stipa tenacissima L. during the summer season be promoted by non-rainfall water gains in soil?

This article reports on quantified soil water gains and their possible effects on summer water relationships in a semiarid Stipa tenacissima L. grasslands located in SE Spain. We believe that the net soil water gains detected using minilysimeters could be from soil water vapour adsorption (WVA). Our study of high water-stress showed stomatal conductance (21.8–43.1 mmol H₂O m⁻² s⁻¹) in S. tenacissima leaves unusual for the summer season, and the evapotranspiration from S. tenacissima grassland, estimated by a multi-source sparse evapotranspiration model, closely corresponding to total WVA. This highlights the importance of summer soil WVA to stomatal conductance and vital transpiration in S. tenacissima. This study measured pre-dawn leaf water potential (ψ) response to sporadic light rainfall, finding that a light summer rainfall (1.59 mm day⁻¹) was sufficient to vary ψ in S. tenacissima from −3.8 (close to the turgour loss point) to −2.7 MPa. We hypothesize that soil WVA can supply vegetation with water vital to its survival in seasons with a severe water deficit, giving rise to a close relationship between soil water dynamics and plant water response.     

Role of desert annuals in nutrient flow in arid area of Northwestern China: a nutrient reservoir and provider

Previous studies have tested the “vernal dam” hypothesis of spring ephemeral herbs in hardwood forests. The desert annual is a component of the desert ecosystem that takes advantage of water resources and temperature conditions during the rainy season to rapidly complete its life cycle within several months. To understand the role desert annual/ephemeral plants play in nutrient flow, we studied vegetation cover, nitrogen content and litter production of annual plants and litter decomposition rate in plant communities dominated by four shrubs (Haloxlon ammodendron, Hedysarum scoparium, Calligonum mongolicum, and Nitraria tangutorum) and two dominant annuals (Agriophyllum squarrosum and Halogeton arachnoideus Moq) in Minqin, northwestern China. Results indicate that over half of the total vegetation cover was provided by annuals. Annuals also took up a large amount of nitrogen (0.46–3.78 g N m⁻²) along the oasis–desert ecotone. Litter production and nutrient content were higher in areas dominated by annual plants than in areas dominated by shrubs. Furthermore, the litter decomposition rate of the annuals was higher than that of the shrubs, except for the shrub H. ammodendron, although almost all of the litter’s carbon (C) and nitrogen (N) remained after 6 months of decomposition. Without the annuals, more nutrients and rainwater might be lost through leaching or dust transfer caused by the wind erosion. In addition, green twigs of the annuals are the food for some animals, we found some green twigs and litter from annuals left in front of gerbil and rabbit burrows, sometimes even blocking these burrows. Thus, desert summer annuals, like nutrient reservoirs and providers, take up nutrients during the rainy season, providing some animals and microbes with food, and finally release these nutrients after death. Bao-Ming Chen and Gen-Xuan Wang contributed equally to this work.     

Short-term soil inorganic N pulse after experimental fire alters invasive and native annual plant production in a Mojave Desert shrubland

Post-fire changes in desert vegetation patterns are known, but the mechanisms are poorly understood. Theory suggests that pulse dynamics of resource availability confer advantages to invasive annual species, and that pulse timing can influence survival and competition among species. Precipitation patterns in the American Southwest are predicted to shift toward a drier climate, potentially altering post-fire resource availability and consequent vegetation dynamics. We quantified post-fire inorganic N dynamics and determined how annual plants respond to soil inorganic nitrogen variability following experimental fires in a Mojave Desert shrub community. Soil inorganic N, soil net N mineralization, and production of annual plants were measured beneath shrubs and in interspaces during 6 months following fire. Soil inorganic N pools in burned plots were up to 1 g m⁻² greater than unburned plots for several weeks and increased under shrubs (0.5–1.0 g m⁻²) more than interspaces (0.1–0.2 g m⁻²). Soil NO₃ ⁻−N (nitrate−N) increased more and persisted longer than soil NH₄ ⁺−N (ammonium−N). Laboratory incubations simulating low soil moisture conditions, and consistent with field moisture during the study, suggest that soil net ammonification and net nitrification were low and mostly unaffected by shrub canopy or burning. After late season rains, and where soil inorganic N pools were elevated after fire, productivity of the predominant invasive Schismus spp. increased and native annuals declined. Results suggest that increased N availability following wildfire can favor invasive annuals over natives. Whether the short-term success of invasive species following fire will direct long-term species composition changes remains to be seen, yet predicted changes in precipitation variability will likely interact with N cycling to affect invasive annual plant dominance following wildfire.     

Salinity effect on plant growth and leaf demography of the mangrove, Avicennia germinans L.

We assessed the effect of salinity on plant growth and leaf expansion rates, as well as the leaf life span and the dynamics of leaf production and mortality in seedlings of Avicennia germinans L. grown at 0, 170, 430, 680, and 940 mol m⁻³ NaCl. The relative growth rates (RGR) after 27 weeks reached a maximum (10.4 mg g⁻¹ d⁻¹) in 170 mol m⁻³ NaCl and decreased by 47 and 44% in plants grown at 680 and 940 mol m⁻³ NaCl. The relative leaf expansion rate (RLER) was maximal at 170 mol m⁻³ NaCl (120 cm m⁻² d⁻¹) and decreased by 57 and 52% in plants grown at 680 and 940 mol m⁻³ NaCl, respectively. In the same manner as RGR and RLER, the leaf production (P) and leaf death (D) decreased in 81 and 67% when salinity increased from 170 to 940 mol m⁻³ NaCl, respectively. Since the decrease in P with salinity was more pronounced than the decrease in D, the net accumulation of leaves per plant decreased with salinity. Additionally, an evident increase in annual mortality rates (λ) and death probability was observed with salinity. Leaf half-life (t _0.5) was 425 days in plants grown at 0 mol m⁻³ NaCl, and decreased to 75 days at 940 mol m⁻³ NaCl. Thus, increasing salinity caused an increase in mortality rate whereas production of new leaves and leaf longevity decreased and, finally, the leaf area was reduced.     

Exotic plant communities shift water-use timing in a shrub-steppe ecosystem

Semiarid areas in the US have realized extensive and persistent exotic plant invasions. Exotics may succeed in arid regions by extracting soil water at different times or from different depths than native plants, but little data is available to test this hypothesis. Using estimates of root mass, gravimetric soil water, soil-water potential, and stable isotope ratios in soil and plant tissues, we determined water-use patterns of exotic and native plant species in exotic- and native-dominated communities in Washington State, USA. Exotic and native communities both extracted 12 ± 2 cm of water from the top 120 cm of soil during the growing season. Exotic communities, however, shifted the timing of water use by extracting surface (0–15 cm) soil water early in the growing season (i.e., April to May) before native plants were active, and by extracting deep (0–120 cm) soil water late in the growing season (i.e., June to July) after natives had undergone seasonal senescence. We found that δ ¹⁸O values of water in exotic annuals (e.g., −11.8 ± 0.4 ‰ for Bromus tectorum L.) were similar to δ ¹⁸O values of surface soil water (e.g., −13.3 ± 1.4 ‰ at −15 cm) suggesting that transpiration by these species explained early season, surface water use in exotic communities. We also found that δ ¹⁸O values of water in taprooted exotics (e.g., −17.4 ± 0.3 ‰ for Centaurea diffusa Lam.) were similar to δ ¹⁸O values of deep soil water (e.g., −18.4 ± 0.1 ‰ at −120 cm) suggesting that transpiration by these species explained late season, deep water use. The combination of early-season, shallow water-use by exotic winter-actives and late-season, deep water-use by taprooted perennials potentially explains how exotic communities resist establishment of native species that largely extracted soil water only in the middle of the growing season (i.e., May to June). Early season irrigation or the planting of natives with established root systems may allow native plant restoration.     

The role of aquatic macrophytes in microhabitatual transformation of physical-chemical features of small water bodies

The paper presents the results of an examination of the phycical-chemical parameters of water together with an analysis of the chlorophyll a concentration of 12 small water bodies situated within urban and suburban areas of the city of Poznań (mid-west Poland)—typical mid-forest, strongly anthropogenically modified in the urban landscape, strongly antropogenically modified in an agricultural area and clay-pits. There were zones of open water (Unvegetated Zone) as well as zones of rush and aquatic vegetation (Vegetated Zone) distinguished in the examined ponds. The influence of the rush vegetation, nymphaeids and elodeids on the abiotic parameters of an aquatic environment was examined. Water samples were taken during the summer of 2004 from 12 stations within the open water and 24 within macrophytes. The plant matter was randomly collected in triplicate from the central part of the vegetated stand. The influence of macrophytes on the abiotic features of water was estimated using the parameter of the plant length (cm l⁻¹) and the plant biomass (g l⁻¹). In the studied ponds 12 aquatic macrophyte communities were distinguished. A salient feature of submerged macrophytes was a great density of plant stems along with considerebly low biomass, however, the rush vegetation (Phragmitetum communis, Typhetum latifoliae) when compared to nymphaeids (Polygonetum natantis, Potametum natantis) and elodeids (Potametum lucentis) was characterised by lower stem densities and higher biomass. The water bodies were alkaline and of pronounced hardness. In most of them high trophy conditions were found with especially high concentrations of phosphorus (96 μg l⁻¹ on average). There was significant differentiation in the water chemistry (mainly in respect to mineral compounds) between the Vegetated and Unvegetated Zones as well as between particular aquatic macrophyte communities.     

Macrophyte presence is an indicator of enhanced denitrification and nitrification in sediments of a temperate restored agricultural stream

Stream macrophytes are often removed with their sediments to deepen stream channels, stabilize channel banks, or provide habitat for target species. These sediments may support enhanced nitrogen processing. To evaluate sediment nitrogen processing, identify seasonal patterns, and assess sediment processes relative to stream load, we measured denitrification and nitrification rates in a restored third- to fourth-order agricultural stream, Black Earth Creek, Wisconsin, and estimated processing over a 10 km reach. Our results show that sediments with submerged and emergent macrophytes (e.g., Potomageton spp. and Phalaris arudinacea) support greater denitrification rates than bare sediments (1.12 μmol N g⁻¹ h⁻¹ vs. 0.29). Sediments with macrophytes were not carbon limited and organic matter fraction was weakly correlated to denitrification. The highest denitrification potential occurred in macrophyte beds (5.19 μmol N g⁻¹ h⁻¹). Nitrification rates were greater in emergent beds than bare sediments (1.07 μg N ml⁻¹day⁻¹ vs. 0.35) with the greatest nitrification rates during the summer. Total denitrification removal in sediments with macrophytes was equivalent to 43% of the nitrate stream load (463.7 kg N day⁻¹) during spring and nitrification in sediments with macrophytes was equivalent to 247% of summer ammonium load (3.5 kg N day⁻¹). Although the in-channel connectivity to nitrogen rich water was limited, actual stream nitrogen loads could increase with removal of macrophytes. Macrophyte beds and supporting fringing wetted areas are important if nitrogen management is a concern for riparian stream restoration efforts.     

Microphytoplankton assemblages in shallow waters at Admiralty Bay (King George Island,Antarctica) during the summer 2002–2003

Microphytoplankton populations were studied in shallow coastal water (<60 m) near the Brazilian Antarctic Station Comandante Ferraz (EACF) and three reference areas in Admiralty Bay in early and late summer (2002–2003). Phytoplankton was diverse (113 taxa), but not abundant (10³ cells l⁻¹). The highest abundances (>10⁴ cells l⁻¹) were caused by pennate benthic diatoms (Fragilaria striatula Lyngbye) that occurred mainly in early summer, associated with the presence of ice. In late summer, when the water temperature (−0.4 to 1.5°C), salinity (34 to 35), and phosphate (2.6 to 4.5 μmol l⁻¹) were highest and the dissolved oxygen was lowest (6.4 to 2.9 ml l⁻¹), centric diatoms (Thalassiosira spp.) were more abundant, suggesting an influence of oceanic waters. Phytoplankton abundance (≤10² cells l⁻¹) and chlorophyll a concentrations (0.22 μg l⁻¹) were lowest close to EACF. Pennate diatoms were dominant close to shore and in surface waters elsewhere, probably because of ice melting or sediment resuspension caused by water mixing.     

Seasonal changes in production and nutrient content ofCynodon dactylon (L.) Pers. subjected to water deficits

Seasonal changes in water relations, production and mineral composition were studied in a sward ofCynodon dactylon (L). Pers. subjected to water deficits during a dry summer, and at recovery in autumn. The experiment was carried out under Mediterranean field conditions. Water deficits during summer reduced total dry matter production by 60%, but in autumn there were no differences between treatments. Compared to well-watered sward, the sward grown under drought showed an increase in potassium, calcium and nitrogen of 55, 10 and 10% respectively. These differences decreased with the arrival of autumn rains. Leaf osmotic potential (Ψ_Π) fell during the dry summer to −2.8 MPa in well-watered plants and to −4.2 MPa in stressed plants. In autumn there were no differences between treatments. Nevertheless, relative water content (RWC) only decreased to 0.86 in droughted plants. In summer potassium contributed to the osmotic adjustment. In contrast, under water deficits a decrease of 71% in sodium and, to a lesser but significant extent decreases in phosphorus, magnesium and chlorine was observed. Nitrogen, phosphorus and sulphur showed low concentrations during summer and increased in autumn.     

Seasonal changes in abundance and spatial distribution of the soil arthropods in a Japanese cedar (Cryptomeria japonica D.Don) plantation,with special reference to collembola and acarina

Seasonal changes in abundance and distribution pattern of soil micro-arthropods were studied in connection with a few environmental factors in a Japanese cedar (Cryptomeria japonica D. Don) plantation. The soil arthropods were sampled from three different depths at intervals of two months for two years. Of the collected animals (total 51000–155000 m⁻²), Collembola (20000–76000 m⁻²), oribatid mites (19000–55000 m⁻²) and carnivorous mites (6200–21000 m⁻²) were the numerically dominant animal groups. Low seasonal variations in abundance indicated their seasonal stability in population levels. The trends in seasonal fluctuation were similar among these groups and between the two years, showing bimodal pattern with little peaks in early summer and winter. The pattern of seasonal fluctuation in abundance of carnivorous mites (P _d) was significantly synchronized with that in the total abundance of Collembola and oribatid mites (P _τ). Thus, the number-ratios (P _d/P _τ) were fairly constant, ranging from 0.10 to 0.25. Seasonal changes in vertical distribution of the three animal groups showed a similar pattern for both years. The downward migrations were shown to be more affected by low temperatures in winter accompanied by snow coverage rather than by the desiccation of the surface soil in summer. All the three groups demonstrated as a whole slightly aggregated patterns of horizontal distribution throughout the two years. Temporal increases in the patchiness indices were observed from summer to autumn when the moisture content of the surface soil was low.     

The allelopathic potential ofCoridothymus capitatus L. (Labiatae). Preliminary studies on the roles of the shrub in the inhibition of annuals germination and/or to promote allelopathically active actinomycetes

Summary Suppression of annuals at various intensities was observed around some shrubs ofCoridothymus capitatus growing on kurkar formation in the coastal hills of Israel. The phenomenon was clearly observed as annuals-free belts of 15–20 cm around ‘aggressive’ shrubs. Quantitatively, density of annuals decreased by 16 fold in the annual-free belts as compared to a distance of 60–80 cm from the canopies of the shrubs. Their dry matter was decreased by 5.4 fold around the shrubs. Suppression rate of emergence of planted seeds of annuals (Plantago psyllium andErucaria hispanica) early in the season was 45% higher around ‘aggressive’C. capitatus than that around ‘non-aggressive’ ones. In the laboratory, seed germination of the annuals was strongly suppressed by diffusates and volatiles from shoots, as well as from their water extracts and their essential oils. Incubation of fresh shoots ofC. capitatus in soil collected from around ‘non-aggressive’ shrubs, for 7 days, increased population levels of actinomycetes by 9.6 fold and by 36.7 fold when soil was collected from around ‘aggressive’ shrubs. Isolates of some soil-borne actinomycetes inhibited germination of the test plantsLactuca sativa andAnastatica hierochuntica on agar plates (4–98%). The preliminary results indicate a possible synergistic inhibitory effect induced by essential oils of the aromatic shrub and the phytototic activity of actinomycetes.     

Carbon and hydrogen isotope fractionation under continuous light: implications for paleoenvironmental interpretations of the High Arctic during Paleogene warming

The effect of low intensity continuous light, e.g., in the High Arctic summer, on plant carbon and hydrogen isotope fractionations is unknown. We conducted greenhouse experiments to test the impact of light quantity and duration on both carbon and hydrogen isotope compositions of three deciduous conifers whose fossil counterparts were components of Paleogene Arctic floras: Metasequoia glyptostroboides, Taxodium distichum, and Larix laricina. We found that plant leaf bulk carbon isotopic values of the examined species were 1.75–4.63‰ more negative under continuous light (CL) than under diurnal light (DL). Hydrogen isotope values of leaf n-alkanes under continuous light conditions revealed a D-enriched hydrogen isotope composition of up to 40‰ higher than in diurnal light conditions. The isotope offsets between the two light regimes is explained by a higher ratio of intercellular to atmospheric CO₂ concentration (C _i/C _a) and more water loss for plants under continuous light conditions during a 24-h transpiration cycle. Apparent hydrogen isotope fractionations between source water and individual lipids (ε_{lipid–water}) range from −62‰ (Metasequoia C₂₇ and C₂₉) to −87‰ (Larix C₂₉) in leaves under continuous light. We applied these hydrogen fractionation factors to hydrogen isotope compositions of in situ n-alkanes from well-preserved Paleogene deciduous conifer fossils from the Arctic region to estimate the δD value in ancient precipitation. Precipitation in the summer growing season yielded a δD of −186‰ for late Paleocene, −157‰ for early middle Eocene, and −182‰ for late middle Eocene. We propose that high-latitude summer precipitation in this region was supplemented by moisture derived from regionally recycled transpiration of the polar forests that grew during the Paleogene warming.     

The Influence of Spatial Patterns of Soil Moisture on the Grass and Shrub Responses to a Summer Rainstorm in a Chihuahuan Desert Ecotone

The cycling of surface water, energy, nutrients, and carbon is different between semiarid grassland and shrubland ecosystems. Although differences are evident when grasslands are compared to shrublands, the processes that contribute to this transition are more challenging to document. We evaluate how surface redistribution of precipitation and plant responses to the resulting infiltration patterns could contribute to the changes that occur during the transition from grassland to shrubland. We measured soil water potential under grasses (Bouteloua eriopoda), shrubs (Larrea tridentata) and bare soil and changes in plant water relations and gas exchange following a 15 mm summer storm in the grassland–shrubland ecotone at the Sevilleta National Wildlife Refuge in central New Mexico USA. Following the storm, soil water potential (Ψ_s) increased to 30 cm depth beneath both grass and shrub canopies, with the greatest change observed in the top 15 cm of the soil. The increase in Ψ_s was greater beneath grass canopies than beneath shrub canopies. Ψ_s under bare soil increased only to 5 cm depth. The substantial redistribution of rainfall and different rooting depths of the vegetation resulted in high Ψ_s throughout most of the rooting volume of the grasses whereas soil moisture was unchanged throughout a large portion of the shrub rooting volume. Consistent with this pattern, predawn water potential (Ψ_PD) of grasses increased more than 5 MPa to greater than −1 MPa whereas Ψ_PD of shrubs increased to −2.5 MPa, a change of less than 2 MPa. Transpiration increased roughly linearly with Ψ_PD in both grasses and shrubs. In grasses, assimilation was strongly correlated with Ψ_PD whereas there was no relationship in shrubs where assimilation showed no significant response to the pulse of soil moisture following the storm. These data show that preferential redistribution of water to grass canopies enhances transpiration and assimilation by grasses following large summer storms. This process may inhibit shrubland expansion at the ecotone during periods without extreme drought.     

Population dynamics and production of the freshwater snail Chilina gibbosa Sowerby 1841 (Chilinidae,Pulmonata) in a North-Patagonian reservoir

Chilina gibbosa is an endemic snail widely distributed in Patagonia, Argentina. Due to its importance in the benthic fauna and in the diet of some fish in the oligo-mesotrophic reservoir Ezequiel Ramos Mexía (39° 30′ S, 69° 00′ W), special attention has been given to its life cycle, growth patterns and annual production. Samples were taken monthly at five littoral stations between June 1983 and July 1984. Mean abundance and biomass of C. gibbosa were much higher in vegetated stations dominated by Potamogeton berteroanus (Station 1 : 583 ind. m⁻², 5.95 g AFDM m⁻²) or by Nitella clavata (Station 5 : 275 ind. m⁻², 4.18 g AFDM m⁻²) than bare stations with low transparency or stations with other macrophytes. The snails presented a clustered spatial pattern and their abundance was significantly correlated with macrophyte wet biomass only when this was above 250 g m⁻². Analysis of size distributions showed an annual life cycle with a reproductive period in the summer. However, differences in recruitment and growth occurred probably due to differences in water temperature and food availability. Growth was maximum in summer and almost absent during winter. Hence, shell growth data fit a sigmoid curve well, and growth was somewhat higher at Station 1. Annual production at Stations 1 and 5, estimated by the ‘growth increment summation’ method (28.8 g AFDM m⁻² and 14.18 g AFDM m⁻² respectively), was among the highest recorded for pulmonate gastropods, possibly due to a low interspecific competition. The P : B ratio values were within the range for univoltine gastropods (4.84 and 3.39). The high productivity and turnover rate of these snails grant a high availability of food for the abundant molluscivore, the silverside Patagonina hatcheri.     

Decomposition of dominant submerged macrophytes: implications for nutrient release in Myall Lake, NSW, Australia

Breakdown and nutrient dynamics of submerged macrophytes were studied in Myall Lake, Australia. Mass loss of Myriophyllum sulsagineum was the lowest (64.90%) among the studied macrophytes during the 322 days followed by charophytes (60.79%), whereas Najas marina and Vallisneria gigantea lost 91.15 and 86.02% of their respective initial mass during that time. The overall exponential breakdown rates of Najas marina and Vallisneria gigantea were similar, with k-values of 0.24 and 0.23 day⁻¹, respectively. These rates were significantly higher than the break down rates of charophytes (0.007 day⁻¹) and M. sulsagineum (0.008 day⁻¹). During growth phase, water column depicted lower nutrient concentrations while during decay period, significant increase in water column nutrients resulted. Release of nutrients from decomposing macrophytes and incorporation of these nutrients into sedimentary phase as well as uptake of nutrients by the growing macrophytes, can present a considerable cycling pathway of nutrients in Myall lake system. The results of this study suggest that different submerged macrophytes may differ appreciably in quality and may exhibit different decomposition rates, patterns and nutrient dynamics in aquatic ecosystems in general, and Myall lakes in particular.