High Shoot Density Research Articles

Effects of turbidity and habitat complexity on antipredator behavior of three-spined sticklebacks (Gasterosteus aculeatus)

Human-induced processes like eutrophication are increasing water turbidity and altering vegetated habitats in the Baltic Sea. Unfortunately, the influence of these environmental changes on predator–prey interactions remains poorly studied in mobile taxa of this region. We used three-spined stickleback (Gasterosteus aculeatus) as a model species to study the combined effects of turbidity and shoot density on habitat choice (shelter vs. open) in the presence and absence of piscivorous perch (Perca fluviatilis). Habitat choice of sticklebacks was video-monitored and compared between two paired observation periods: “control” (no predator) and “risk” (predator present). Though sticklebacks exhibited a general preference for sheltered habitat across treatments, repeated measures ANOVA found that sticklebacks responded to predator presence by significantly increasing use of the sheltered habitat. However, shoot density of the shelter interacted with observation period, where risk-induced shifts of sticklebacks into sheltered habitat were most apparent and sustained at higher shoot densities. Stickleback activity level was generally reduced with predator presence at higher turbidity and shoot density levels, suggesting a possible adaptation to reduce predator encounter rates in visually deteriorated conditions. Overall, our study demonstrates that relatively minute increases (1’s – 10’s of NTU) in algal turbidity intensify three-spined stickleback sheltered habitat use, but vegetation density may still play a larger role in avoiding predators at these turbidity levels. When compared with a similar study on 0+ perch, our work suggests that eutrophication (i.e., increased turbidity, loss of habitat complexity) may have variable, taxon- and/or habitat-specific effects on predator–prey interactions of the Baltic Sea.

Managing Tall Fescue and Zoysiagrass Mixtures as Turfgrass in the Transition Zone

Maintaining a mixture of cool‐ and warm‐season turfgrasses year‐round instead of overseeding into a perennial monoculture stand annually in the transition zone may be an effective way to combine the strengths of two species. Four mixtures of tall fescue (Festuca arundinacea L.) (Jaguar 3 and Bonsai) and zoysiagrass (Zoysia japonica Steud.) (Zenith and Cathy) were evaluated under 5‐ and 7.5‐cm mowing heights and N regimes of 400, 200, and 100 kg ha–1 yr–1 in Beijing, China, during 2011 and 2012. Turf quality was better at a 5‐cm mowing height than a 7.5‐cm mowing height in July and October and was equivalent between the two mowing heights for the other months. Visual quality was highest in plots receiving N at 400 kg ha–1 yr–1 in May, June, September, October, and November. In August, however, the highest visual turf quality was observed for an N rate of 100 kg ha–1 yr–1. Generally, the 5‐cm mowing height and low N application produced high shoot density and ground coverage by zoysiagrass, whereas the shoot density of tall fescue showed no difference between the two mowing heights. The higher N rate most often favored greater tall fescue shoot density and ground coverage. Based on the results, we recommend a 5‐cm mowing height and an N rate of 400 kg ha–1 yr–1, avoiding application in August, for the mixture.

Seagrass Restoration Enhances “Blue Carbon” Sequestration in Coastal Waters

Seagrass meadows are highly productive habitats that provide important ecosystem services in the coastal zone, including carbon and nutrient sequestration. Organic carbon in seagrass sediment, known as “blue carbon,” accumulates from both in situ production and sedimentation of particulate carbon from the water column. Using a large-scale restoration (>1700 ha) in the Virginia coastal bays as a model system, we evaluated the role of seagrass, Zostera marina , restoration in carbon storage in sediments of shallow coastal ecosystems. Sediments of replicate seagrass meadows representing different age treatments (as time since seeding: 0, 4, and 10 years), were analyzed for % carbon, % nitrogen, bulk density, organic matter content, and 210Pb for dating at 1-cm increments to a depth of 10 cm. Sediment nutrient and organic content, and carbon accumulation rates were higher in 10-year seagrass meadows relative to 4-year and bare sediment. These differences were consistent with higher shoot density in the older meadow. Carbon accumulation rates determined for the 10-year restored seagrass meadows were 36.68 g C m-2 yr-1. Within 12 years of seeding, the restored seagrass meadows are expected to accumulate carbon at a rate that is comparable to measured ranges in natural seagrass meadows. This the first study to provide evidence of the potential of seagrass habitat restoration to enhance carbon sequestration in the coastal zone.

Effects of the invasive plant Lupinus polyphyllus on vertical accretion of fine sediment and nutrient availability in bars of the gravel-bed Paloma river

Floodplain vegetation is fundamental in fluvial systems, controlling river corridor geomorphology and ecology through a series of hydraulic, sedimentological, and biological processes. Changes caused by introduced plant species can thus result in shifts in river regime, succession trajectories and nutrient availability, affecting native biodiversity. The exotic bigleaf or marsh lupine Lupinus polyphyllus, introduced in Patagonia in the last decades, is aggressively invading fluvial corridors. It fills unoccupied ecological niches in southern Chilean rivers, due to its capacity for nitrogen fixation, its perennial habit, and high shoot density and leaf surface area.

Pore water N:P and NH4 +:NO3 − alter the response of Phragmites australis and Glyceria maxima to extreme nutrient regimes

Phragmites australis and Glyceria maxima are fast-growing littoral grasses often competing for similar wetland habitats. Eutrophication affects their competitiveness, but the outcome is not generally predictable due to the complexity of interrelated factors. We hypotheses that pore water N:P and NH4 +:NO3 − modify their growth and metabolic responses to the trophic status of the habitat. The hypothesis was tested under standardized conditions of long-term sand cultures. Application of N + P up to extreme levels in combination with N:P < 10 and NH4 +:NO3 − 90 and NH4 +:NO3 − = 4 caused lower productivity, changes in resource allocation, morphology and metabolic relations (e.g. high shoot density, low shoot diameters and heights, reduced root and rhizome growth). Observed signs of stress resembled the factors associated with the reed retreat at the die-back sites. Unbalanced N levels obviously alter plant susceptibility to stresses (altering, e.g. ventilation efficiency, plant anchorage or below-ground storage capacity). The positive effect of sufficient P supply was pronounced in Glyceria. It might therefore favour Glyceria in competition with Phragmites at highly fertile habitats rich in P.

Particle trapping and retention by Zostera noltii: A flume and field study

The relative particle trapping ability of Zostera noltii Hornem. was examined in an 8m seawater flume. Two size classes of fluorescent tracer particles (150–250μm and <63μm) were used to determine the number of particles captured by bare sediment (control) and three densities of Z. noltii (200, 600 and 1000shootsm−2) at three flow velocities (0.04, 0.15 and 0.4ms−1). Particle sizes, plant densities and flow velocities were all within the range of conditions observed in the field. Fewer large particles were trapped at higher flow rates (ANOVA, F2,30=23.29, p<0.001, n=36), with the mean number of particles decreasing with increasing flow. The numbers of large particles trapped in both low and medium flows were similar in vegetated and bare sediments, suggesting that particles were deposited due to low energy conditions rather than due to the presence of vegetation. At low flow, low density seagrass beds trapped more particles of both sizes on the sediment surface than higher density beds. This result was explained in hydrodynamic terms because skimming flow develops at higher shoot densities, carrying suspended particles over the plants following leaf reconfiguration in flow. This work was extended to the field and sediment retention in high density Z. noltii (Tay Estuary, Scotland, U.K.) was compared to bare sediment over one week (14 tides). Tracer particles were added directly among natural seagrass beds exposed at low tide. After large losses due to initial particle resuspension, Z. noltii retained large particles (150–250μm) more effectively than bare plots over time. Particle retention for small particles (<63μm) was similar for both plot types. Mechanisms underlying the results are suggested and the findings discussed.

Wave attenuation by submerged vegetation: combining the effect of organism traits and tidal current

Accurate wave height prediction along the shore plays an important role in coastal protection and management. To account for the effect of submerged vegetation in wave-attenuation models, it is important to understand how the interaction between vegetation characteristics and hydrodynamic forcing affects wave attenuation. To determine the effect of vegetation characteristics, we used seagrass mimics that varied in (1) blade stiffness, (2) shoot density and (3) leaf length; to investigate the effect of hydrodynamic forcing, we studied wave attenuation in the absence and presence of a tidal current. Results show that wave attenuation is positively correlated with blade stiffness and for a given wave in shallow water, attenuation is dependent on a combination of shoot density and leaf length, which can be described by the leaf area index. The presence of a tidal current strongly reduced the wave-attenuating capacity of seagrass mimics, and this reduction was most pronounced at high shoot densities. Thus, most studies that have been carried out under waves only will structurally overestimate wave attenuation for tidal environments, emphasising that tidal currents need to be taken into account in future studies on wave attenuation by vegetation.

Spatial and seasonal variation in wave attenuation overZostera noltii

Wave attenuation is a recognized function of sea grass ecosystems which is believed to depend on plant characteristics. This paper presents field data on wave attenuance collected over a 13 month period in a Zostera noltii meadow. The meadow showed a strong seasonality with high shoot densities in summer (approximately 4,600 shoots/m2) and low densities in winter (approximately 600 shoots/m2). Wave heights and flow velocities were measured along a transect at regular intervals during which the site was exposed to wind waves and boat wakes that differ in wave period and steepness. This difference was used to investigate whether wave attenuation by sea grass changes with hydrodynamic conditions. A seasonal change in wave attenuation was observed from the data. Results suggest that a minimum shoot density is necessary to initiate wave attenuation by sea grass. Additionally, a dependence of wave attenuation on hydrodynamics was found. Results suggest that the threshold shoot density varies with wave period and a change in energy dissipation toward the shore was observed once this threshold was exceeded. An attempt was made to quantify the bed roughness of the meadow; the applicability of this roughness value in swaying vegetation is discussed. Finally, the drag coefficient for the meadow was computed: A relationship between wave attenuance and vegetation Reynolds number was found which allows comparing the wave attenuating effect of Zostera noltii to other plant species.

Seagrass genotypic diversity increases disturbance response via complementarity and dominance

Summary 1. Genetic diversity, like species diversity, can enhance resistance or resilience to perturbation. However, we know little about how disturbance intensity affects this relationship or what mechanisms underlie the positive effects of genetic diversity. 2. We experimentally tested the independent and interactive effects of seagrass genotypic diversity (two levels) and disturbance (three levels) on seagrass biomass in a 2‐year field experiment. 3. Our results indicate that genotypic diversity enhances seagrass resilience from experimental biomass removal, but only at the highest level of disturbance; in the absence of disturbance, monocultures out‐perform polycultures over the short term. 4. Following recovery from the planned experimental disturbance, a natural macroalgal bloom caused a loss of seagrass shoots in our plots. In this case polycultures lost fewer shoots than monocultures (i.e. were more resistant to the disturbance), and this positive effect of genetic diversity persisted until the end of the experiment (1 year in total, including 6 months after all plots had recovered to pre‐disturbance densities). At the end of the 2‐year experiment, polycultures had higher shoot density and above‐ground biomass than monocultures. 5. The positive effects of diversity on shoot density and biomass were caused by both trait‐independent complementarity (TIC; due to differential resource use among clones) and positive dominance (due to one genotype achieving high density in both monoculture and polyculture). 6. Synthesis. Our results confirm that genetic diversity, like species diversity, can influence disturbance response and does so via similar mechanisms. They also highlight that over longer time frames, these effects are likely to result from a complex mix of dominance and complementarity mechanisms that depend on the traits of the specific taxa involved and the response variables of interest.

Using high-density OBC seismic data to optimize the Andrew satellites development

The processed data from conventional towed-streamer seismic surveys over the Andrew satellite fields in the Central North Sea are of poor quality because of anomalously fast sand-filled channels in the Eocene overburden. The channels attenuate the primary energy and produce strong multiples. A step change in data quality has been achieved by acquiring a wide-azimuth seismic survey using ocean bottom cables and a high shot density. The new data have much higher signal-to-noise ratio and better resolution. The imaging under the Eocene channels is greatly improved in the processed results of the new survey. Furthermore, the acquisition and processing were completed in time to meet the tight development schedule for the 2008 Kinnoull discovery.

Cool-Season Turfgrass Species and Cultivars: Response to Simulated Traffic in Central Italy

Turfgrass species differ greatly in their ability to withstand the abrasion and compaction of traffic. Wear tolerance of turfgrass species and cultivars has been evaluated abroad by many researchers, while only few and partial studies have been conducted in Italy. Field experiment was carried out in Viterbo in 2001, 2002 and 2003 to evaluate the effect of the simulated traffic on 110 varieties belonging to four turfgrass cool-season species: tall fescue (Festuca arundinacea Schreb.), fine fescues (Festuca rubra L. ssp. rubra Gaud., ssp. commutata Gaud., ssp. tricophylla Gaud.), perennial ryegrass (Lolium perenne L.) and Kentucky bluegrass (Poa pratensis L.). Shoot density, visual turfgrass quality and thatch thickness were the major characters recorded to estimate wear tolerance. Traffic was simulated using a device containing three rollers pulled by a small tractor. The traffic simulator weighed 564 kg and applied a pressure of about 3 MPa. Results indicated that perennial ryegrass and tall fescue had high wear tolerance and low statistical variation among cultivars. Kentucky bluegrass showed an average wear tolerance owing to its shoot density and good recovery potential. In spite of their high shoot density, fine fescues exhibited poor wear tolerance because of their scarce resistance to high temperatures which are typical of the Mediterranean climate in late spring and summer. This study enabled a preliminary selection of the most suitable cool-season grasses and cultivars for trafficked and non-trafficked areas in Central Italy and highlighted that different turfgrass species have different wear tolerance mechanisms.

Growth dynamics and state of the seagrass Thalassia testudinum in the Gandoca-Manzanillo National Wildlife Refuge, Caribbean, Costa Rica

Seagrass beds are highly productive and valuable habitats, which fulfill a key role in coastal ecosystems. Spatial distribution, biomass, density, productivity and leaf dynamics of the dominant seagrass species Thalassia testudinum were studied at five locations in the Gandoca-Manzanillo National Wildlife Refuge, Caribbean coast of Costa Rica, from November 2005 until March 2006. Seagrass beds within the study area cover approximately 16 ha. Spatial and temporal differences in ecological seagrass parameters were examined along gradients of riverine inputs and related to a range of environmental parameters (depth, grain size distribution, nutrient concentrations, salinity and temperature). Average canopy cover and above-ground biomass of T. testudinum inversely correlated with depth, but did not vary significantly between study sites when compared along depth ranges. Considerable spatio-temporal variations in shoot densities, areal productivity and leaf sizes seem to be related to riverine inputs and wave energy. It appeared that T. testudinum at exposed sites respond to increased environmental disturbance related to the offset of the rainy season with clonal recruitment, whereby shoot densities increase and average leaf sizes are reduced. Lower shoot densities and greater leaf sizes, in contrast, are characteristic for locations with rather consistent environmental conditions, where seagrasses are sheltered. T. testudinum in the refuge has higher shoot densities but shorter leaves and lower productivity compared to sites in 15 other Caribbean countries. The seagrass beds appear to be in a relatively healthy state, however, observations of temporal increased stocks of filamentous epiphytes and macroalgae indicate temporal environmental stress in the system.

Effective Perforating and Gravel Placement: Key to Sand-Free Production

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper IPTC 12581, "Effective Perforating and Gravel Placement: Key to Low-Skin, Sand-Free Production in Gravel Packs," by Samyak Jain, SPE, Raymond Tibbles, SPE, and Jock Munro, Schlumberger, and Rajeswary Suppiah and Norhisham Safin, SPE, Petronas Carigali, originally prepared for the 2008 International Petroleum Technology Conference, Kuala Lumpur, 3-5 December. The paper has not been peer reviewed. Cased-hole gravel packing is used to control sand production from oil and gas wells. The success of a cased-hole gravel-pack job depends on the ability to pack perforation tunnels effectively. The full-length paper presents a systematic approach for perforation-damage removal, effective gravel placement, and packing of the perforation tunnels. Guidelines to surging the formation and executing the perforation-packing job are presented. Introduction Every year the petroleum industry spends USD millions cleaning sand out of wells and repairing problems related to sand production, and it loses additional dollars by restricted production rates. Sand production is a common occurrence in highly unconsolidated formations. In cased-hole applications, perforations act as a conduit for fluid flow between the formation and the wellbore. Production rates can be reduced severely if the packed perforations are damaged. The ability of the well to deliver its potential productivity depends on the inflow area these perforations provide for fluid flow. Perforations may be dam-aged for a number of reasons including perforation debris, a compacted or crushed zone consisting of pulverized sand grains, perforations filled with reservoir sand, and other drilling- and completion-related activities. Background Perforation Surging. Perforating is the critical process connecting the reservoir with the wellbore. The selection of the gun system is driven by many factors including wellbore dimensions and formation and reservoir properties. One of the most critical parameters for perforation strategy is the rock strength, measured as unconfined compressive strength. It is recognized throughout the industry that for sand-control completions, both high shot density and big-hole charges are the optimum gun/charge configuration. Increased area open to flow maximizes the productivity of the well while it reduces the fluid velocity per perforation, reducing the risk of sand production through the gravel pack and sand screens. High-velocity flow through isolated perforations can create "hot spots" that, with time, can erode through the screens, resulting in massive sand production and sometimes the loss of the zone or the well.

Toxicity of reduced nitrogen in eelgrass (Zostera marina) is highly dependent on shoot density and pH

In sheltered, eutrophicated estuaries, reduced nitrogen (NHx), and pH levels in the water layer can be greatly enhanced. In laboratory experiments, we studied the interactive effects of NHx, pH, and shoot density on the physiology and survival of eelgrass (Zostera marina). We tested long-term tolerance to NHx at pH 8 in a 5-week experiment. Short-term tolerance was tested for two shoot densities at both pH 8 and 9 in a 5-day experiment. At pH 8, eelgrass accumulated nitrogen as free amino acids when exposed to high loads of NHx, but showed no signs of necrosis. Low shoot density treatments became necrotic within days when exposed to NHx at pH 9. Increased NH3 intrusion and carbon limitation seemed to be the cause of this, as intracellular NHx could no longer be assimilated. Remarkably, experiments with high shoot densities at pH 9 showed hardly any necrosis, as the plants seemed to be able to alleviate the toxic effects of high NHx loads through joint NHx uptake. Our results suggest that NHx toxicity can be important in worldwide observed seagrass mass mortalities. We argue that the mitigating effect of high seagrass biomass on NHx toxicity is a positive feedback mechanism, potentially leading to alternative stable states in field conditions.

Drag forces on common plant species in temperate streams: consequences of morphology, velocity and biomass

Swift flow in streams may physically influence the morphology and distribution of plants. I quantified drag as a function of velocity, biomass and their interaction on the trailing canopy of seven European stream species in an experimental flume and evaluated its importance for species distribution. Drag increased at a power of 1.3–1.9 with velocity and 0.59–0.77 with biomass in 75% of the measurements. Velocity and biomass interacted because higher velocity causes reconfiguration and greater internal shelter to unimpeded flow and higher biomass enhances shelter among neighbouring shoots. Increase of drag with velocity did not differ systematically among inherently streamlined or non-streamlined species while increase of drag with biomass was smallest among non-streamlined shoots which provide greater mutual shelter. At low shoot density, inherently streamlined species usually experienced the lowest drag conducive to colonisation and growth in swift flow. At high shoot density, no systematic differences in drag existed between the two morphologies. No clear relationship existed between drag forces, morphology and field distribution of species as a function of current velocity probably because a variety of environmental conditions and plant traits influences distribution. Drag on the trailing canopy usually increased 15- to 35-fold for a 100-fold increase of biomass suggesting that an even distribution of plants at low density across the stream bed offers greater resistance to downstream flow than an uneven distribution with the same biomass confined to dense patches surrounded by open flow channels. Thus, management strategies to ensure a patchy plants distribution should be suitable for combining agricultural drainage and ecological stream quality.

Effects of seagrass beds (Zostera noltii and Z. marina) on near-bed hydrodynamics and sediment resuspension

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 358:125-136 (2008) - DOI: https://doi.org/10.3354/meps07338 Effects of seagrass beds (Zostera noltii and Z. marina) on near-bed hydrodynamics and sediment resuspension John Widdows1,*, Nick D. Pope1, Mary D. Brinsley1, Harald Asmus2, Ragnhild M. Asmus2 1Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL1 3DH, UK 2Alfred Wegener Institute for Polar and Marine Research, Wadden Sea Station Sylt, Hafenstrasse 43, 25992 List, Germany *Email: j.widdows@pml.ac.uk ABSTRACT: The main objectives of this flume study were to (1) quantify density dependent effects of the short-leaf seagrass Zostera nolti on hydrodynamics and sediment resuspension from a sandy bed, and (2) measure the erodability of 2 contrasting sediments (sandy and muddy) and the extent to which this is modified by the presence of 2 seagrass species, Z. noltii (sandy) and Z. marina (muddy). Field measurements of near-bed tidal currents, turbulence and suspended particulate matter at 2 different Z. noltii locations (low energy [sheltered] and higher energy [exposed] environments) were interpreted in the context of the flume results. Skimming flow above the high density bed of Z. noltii was accompanied by a 40% reduction in near-bed flow, but this was offset by a 2-fold increase in turbulent kinetic energy (TKE) and bed shear stress (τ0). Despite this increase in τ0 there was an increase in sediment stabilisation with increasing seagrass density (10-fold increase in critical bed shear stress for erosion [τe] from 0.1 [bare sediment] to 1.0 Pa at the highest shoot density). This was largely explained by the increased microphytobenthos abundance (reflected in the higher chlorophyll a and carbohydrate contents) and a lower density of the grazer and bio-destabiliser Hydrobia ulvae. In contrast, the muddy site was more easily eroded (10-fold higher), with Z. marina having little effect on sediment erodability (bare: τe = 0.05 Pa; Z. marina: τe = 0.07 Pa). This higher erodability was due to differences in hydrodynamics and the physical/biological properties of the sediment. KEY WORDS: Seagrass beds · Zostera noltii · Zostera marina · Hydrodynamics · Sediment resuspension Full text in pdf format PreviousNextCite this article as: Widdows J, Pope ND, Brinsley MD, Asmus H, Asmus RM (2008) Effects of seagrass beds (Zostera noltii and Z. marina) on near-bed hydrodynamics and sediment resuspension. Mar Ecol Prog Ser 358:125-136. https://doi.org/10.3354/meps07338 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 358. Online publication date: April 21, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Dynamics of biomass production in a poplar coppice culture over three rotations (11 years)

Short rotation coppice culture systems are characterized by a high stool and shoot density, and by rotation durations between 2 and 3 years for species showing an extremely fast growth such as poplar. With the objective to study the long-term biomass production evolution and dynamic of such systems, a high-density plantation with 17 poplar ( Populus) clones belonging to six parentages was established in April 1996 and coppiced in December 1996, January 2001, and February 2004. At the end of each growing season, stool survival, shoot diameter, and the number of shoots per stool were measured. Biomass production was estimated directly by harvesting a number of shoots or indirectly via allometric power equations. Stool survival rates differed among parentages and from year to year irrespective of number of rotations; the highest rates were for P. nigra and the lowest rates were for P. deltoides × P. nigra and P. deltoides × P. trichocarpa parentages. In the second and third rotations, the P. trichocarpa × P. deltoides hybrids showed very high mortality rates. The number of shoots per stool varied among rotations and parentages, and shoot elimination was parentage specific. The number of shoots per stool increased with increasing rotation number, while they decreased from year to year within each rotation. A single general allometric equation linking shoot diameter to shoot biomass could be used to estimate biomass production for most years and most clones. Exceptions were found for the year 2001 and for clone Hazendans. Biomass production differed among parentages and among rotations. The different parentages also differed in their strategy of biomass accumulation. Although P. nigra had the highest biomass production over 11 years, the parentage ranking of biomass production shifted over rotations. The P. trichocarpa × P. deltoides hybrids showed high biomass production in the first rotation, but low in the second and third rotations. Biomass production increased from year to year within one rotation, while it decreased with rotation number. Among the parentages examined, P. nigra and P. trichocarpa were the best performing ones and the best adapted to the multiple coppice rotations.

Species richness and identity affect the use of aboveground space in experimental grasslands

Complementary resource use is regarded as a mechanism that contributes to positive relationships between biodiversity and ecosystem functioning. Here, we used a biodiversity experiment composed of nine potentially dominant species (grasses: Alopecurus pratensis, Arrhenatherum elatius, Dactylis glomerata, Phleum pratense, Poa trivialis; legumes: Trifolium pratense, T. repens; non-legume herbs: Anthriscus sylvestris, Geranium pratense) to test for differences among monocultures and mixtures and for effects of species richness and the presence of particular species on the use of aboveground space. The number of rooting shoots determined in a line transect increased from monocultures to mixtures. Particularly, the presence of A. elatius in mixtures caused a higher shoot density at the community level. The number of pin contacts per sampling point (cumulative cover) at the community level, analysed with the point intercept method, was higher in mixtures than monocultures, and higher in mixtures with than without A. elatius. The effect was attributable to increased densities across the strata of the vertical stand profile as well as to an increase in community height. The impact of species richness on the use of aboveground space differed considerably between individual species. A. elatius achieved increased densities across all strata of the stand profile, while D. glomerata reached higher densities with a more pronounced use of space in the upper strata with increasing species richness of mixtures. Cumulative cover of P. pratense and A. pratensis was not affected by species richness, while the remaining species decreased space use mostly in the upper strata with increasing species richness or in mixtures with the competitively superior A. elatius. Our study shows that potentially dominant species are limited in their ability for adaptive responses to canopy shading. Nevertheless, the differential responses to species richness of individual species with regard to vertical niche occupation resulted in positive diversity effects on aboveground space use at the community level.

Bentgrass Cultivar and Annual Nitrogen Regime Affects Seasonal Shoot Density

Creeping bentgrass (Agrostis stolonifera var. palustris Huds. Farw.) is the preferred turfgrass species for golf greens. During summer months, however, shoot density (SD) often declines, resulting in poor stand quality. Turf managers utilize several management practices to maintain turf vigor. One practice is light, frequent N fertilization. However, annual N rates vary widely. Recently, high shoot density (HSD) bentgrasses bred to provide superior appearance and stress tolerance compared to the industry standard, ‘Penncross,’ have been widely planted. The effect of variable N rates on these cultivars is unclear. This field study measured seasonal SD changes of three contrasting cultivars maintained with low and high N regimes, 112 and 196 kg of N per ha/year, respectively. Significant seasonal SD differences were measured and cultivars ranked ‘A-4’ (1400 to 2160 shoots/dm2) > ‘L-93’ (1230 to 1780 shoots/dm2) > ‘Penncross’ (760 to 1470 shoots/dm2). Generally, increasing N resulted in only slight differences in SD. In August, however, when N regime was averaged across cultivars, higher N significantly increased SD in both study years. For the densest, most persistent turf during summer months, a HSD cultivar should be planted and moderate N should be applied to sustain appearance and ensure adequate recovery should turf damage occur.

Positive contribution of stem growth to grain number per spike in wheat

This paper challenges the common belief that spike:stem competition determines grain number in wheat. A field experiment was conducted using three contrasting treatments (high shoot density, shading, and extended photoperiod) and six genotypes of varying earliness and spike:stem ratio. Floret primordia were counted in medium and top spikelets of main spikes throughout stem elongation. Floret death occurred simultaneously in both spikelets, and the corresponding critical time was used for comparison with growth analysis. Whole shoot growth rate was highest at the time of floret death, but both stem and spike growth continued to accelerate for a period of time thereafter. Spike respiration measurements did not indicate special requirements at the critical time. Spike carbohydrate concentration started to decline more or less before floret death, depending on genotype, but exactly at the inflexion time for spike elongation, thus suggesting a developmental process rather than a trophic limitation. Multiple correlation indicated that grain number positively correlated not only with spike growth rate at the time of floret death, but with both growth rates of spike and non-spike (stem + leaves) shoot parts. Grain number was poorly predicted by partitioning to spike of either shoot dry mass or shoot growth rate at critical time. In contrast, floret surviving proportion was highly correlated to partitioning and multiple correlation did not indicate a significant effect on survival of non-spike growth rate. It is concluded that even though spike:stem competition decreased floret survival, it was only loosely correlated to grain number.