Contrasting Weather Conditions Research Articles

Sensitivities of crop models to extreme weather conditions during flowering period demonstrated for maize and winter wheat in Austria

SUMMARYThe objective of the present study was to compare the performance of seven different, widely applied crop models in predicting heat and drought stress effects. The study was part of a recent suite of model inter-comparisons initiated at European level and constitutes a component that has been lacking in the analysis of sources of uncertainties in crop models used to study the impacts of climate change. There was a specific focus on the sensitivity of models for winter wheat and maize to extreme weather conditions (heat and drought) during the short but critical period of 2 weeks after the start of flowering. Two locations in Austria, representing different agro-climatic zones and soil conditions, were included in the simulations over 2 years, 2003 and 2004, exhibiting contrasting weather conditions. In addition, soil management was modified at both sites by following either ploughing or minimum tillage. Since no comprehensive field experimental data sets were available, a relative comparison of simulated grain yields and soil moisture contents under defined weather scenarios with modified temperatures and precipitation was performed for a 2-week period after flowering. The results may help to reduce the uncertainty of simulated crop yields to extreme weather conditions through better understanding of the models’ behaviour. Although the crop models considered (DSSAT, EPIC, WOFOST, AQUACROP, FASSET, HERMES and CROPSYST) mostly showed similar trends in simulated grain yields for the different weather scenarios, it was obvious that heat and drought stress caused by changes in temperature and/or precipitation for a short period of 2 weeks resulted in different grain yields simulated by different models. The present study also revealed that the models responded differently to changes in soil tillage practices, which affected soil water storage capacity.

Significance of spring barley variety in energy-saving cultivation technologies

Five spring barley varieties bred by the institute were studied during growth with minimum tillage and application of mineral fertilizers for a planned yield of 2.5, 3.0, and 3.5 t/ha. High resistance to stress factors under sharply contrasting weather conditions and the best grain yield and quality were noted in highly plastic varieties.

Aerosol properties at gosan in Korea during two pollution episodes caused by contrasting weather conditions

We analyzed aerosol optical and chemical properties over Northeast Asia for two pollution episodes caused by contrasting weather conditions, stagnant anticyclone (November, 2011) and fastmoving continental outflow associated with migratory cyclone/anticyclone (February, 2003). Pollution levels were significantly high and even comparable with heavily polluted urban cities in China and Korea even though these levels were from the episodic measurements since Gosan is an internationally well-known remote background site. Space-borne MODIS measurements clearly show that the pollution plume with high aerosol optical depth (AOD) overlaid and very slowly moved over Northeast Asia during the stagnation episode. On the other hand, a strong synoptic wind transported the plume from eastern China to its downwind regions during the continental outflow episode. The two pollution episodes showed discriminative aerosol chemical compositions associated with different source characteristics. Concentrations of nss (non-sea-salt)-sulfates and ammonium in the continental outflow episode were almost two times higher than those in the stagnation episode due to the influence of anthropogenic emissions from China. A higher fraction of nitrate, accompanied with an increase of carbonaceous species in the stagnation episode, was attributable to vehicular emissions originated from Korea.

Variation between near-surface and columnar aerosol characteristics during the winter and summer at Delhi in the Indo-Gangetic Basin

Aerosol characteristics were studied over Delhi, a typical urban station in the Ganga basin in Northern India, during two contrasting weather conditions: winter and summer, to explain the changes in columnar and surface aerosol characteristics with the help of ground based measurements and CALIPSO satellite data. The near-surface mean aerosol mass concentrations of PM10 and PM2.5 (±standard deviation) were observed to be ∼200 (±24) and 118 (±33)μgm−3, respectively, during the winter and ∼168 (±31) and 55 (±12)μgm−3, respectively, during the summer. PM2.5 was found to be about two times higher than the PM10 concentration during the winter period. Aerosol mass size distribution showed bi-modal nature during both the periods, with relative dominance of fine-particle mass concentrations during the winter, having low Reff value (0.63±0.05μm) and coarse-particle mass concentrations during the summer, having large Reff value (1.52±0.60μm). The concurrent measurement of columnar aerosol optical depth (AOD) showed high values (>0.60 at 500nm) during both the periods. The Ångström exponent (α) over the station, however, also suggests relatively large contribution of fine-mode particles during the winter (α∼1.02) and coarse-mode dust particles during the summer (α∼0.51). The observed features in the surface and columnar measured aerosol characteristics during two different seasons are explained using the vertical winds coupled with the vertical profile of aerosols.

Location-specific weather predictions for Sriharikota (13.72°N, 80.22°E) through numerical atmospheric models during satellite launch campaigns

Accurate knowledge of different meteorological parameters over a launch site is very crucial for efficient management of satellite launch operations. Local weather over the Indian satellite launch site located at Sriharikota High Altitude Range (SHAR: 13.72°N, 80.22°E) is very much dependent on the atmospheric circulation prevailing over the Bay of Bengal oceanic region and topography-induced convective activities. With a view to providing severe weather threat prediction in terms of launch commit criteria (LCC), two numerical atmospheric models namely high-resolution regional model (HRM) and advanced regional prediction system (ARPS) are made operational over SHAR in a synoptic and mesoscale domain, respectively. In the present research article, two launch campaigns through Polar Satellite Launch Vehicle (PSLV-C11 and PSLV-C12) when contrasting weather conditions prevailed over the launch site are chosen for demonstration of potential of two models in providing location-specific short-to-medium-range weather predictions meeting the needs of LCC. In the case of PSLV-C11 campaign, when the launch site underwent frequent thundershower-associated rainfall, ARPS model–derived meteorological fields were effectively used in prediction of probability of the wet spells. On the other hand, Bay of Bengal underwent severe cyclonic storm during PSLV-C12 campaign, and its formation was reasonable captured through HRM simulations. It is concluded that a combination of HRM and ARPS provide reliable short-to-medium-range weather prediction over SHAR, which has got profound importance in launch-related activities.

Effects of Fine-Scale Topography on CO2 Flux Components of Alaskan Coastal Plain Tundra: Response to Contrasting Growing Seasons

Arctic regions hold considerable reservoirs of soil organic carbon. However, most of this carbon is in a potential labile state, and expected changes in temperature and water availability could strongly affect the carbon balance of tundra ecosystems. Plant community composition and soil carbon are closely tied to microtopography and position relative to the water table. We evaluated CO2 fluxes and moss contribution to ecosystem photosynthesis in response to fine-scale topography across a drained lake bed in Barrow, Alaska, during two contrasting growing seasons. CO2 exchange was assessed through static chamber measurements in three vegetation classes distinguished by plant dominance and topographic position within low-centered polygons. Gross primary production (GPP) and ecosystem respiration (ER) were the lowest under high soil moisture conditions in 2006. ER responded more strongly to wet conditions, resulting in a larger summer sink in 2006 than in 2005 (64 vs. 17g CO2 m−2, respectively). Microsites responded differently to contrasting weather conditions. Low elevation microsites presented a strong reduction in ER as a result of increased water availability. A maximum of 48% of daytime GPP and 33% of seasonal daytime GPP was contributed by moss on average across microtopographic positions. The interaction between fine-scale microtopography and variation in temperature and water availability can result in considerable differences in CO2 sink activity of the polygonal tundra.

Characteristics of adaptive reactions of lupine varieties under contrasting metrological condition

It is shown that realization of the productive potential of blue and white lupine varieties under contrasting weather conditions occurs as a result of adaptive reactions at the energy level. The highest seed productivity was found in the combination of maximum nitrogen-fixing and photosynthetic activity.

Pollinator‐mediated selfing erodes the flexibility of the best‐of‐both‐worlds mating strategy inBulbine vagans

Summary1. The best‐of‐both‐worlds hypothesis proposes that delayed autonomous selfing assures reproduction when pollinators are absent, but allows outcrossing when pollinators are present without incurring seed and pollen discounting. However, because self‐compatibility is necessary for delayed selfing, discounting can be incurred if pollinators mediate selfing concurrently with outcrossing. The extent that pollinator‐mediated selfing erodes the best‐of‐both‐worlds mating strategy is largely unknown.2. In this study, we assess how delayed selfing and pollinator‐mediated selfing contribute to reproductive output under contrasting pollination environments inBulbine vagans(Asphodelaceae).3. We manipulated floral phenotype under contrasting weather conditions, plant densities and floral display. We emasculated flowers at early‐flowering (allowing only outcrossing) or late‐flowering (eliminating delayed selfing) stages and compared reproductive output with intact flowers (outcrossing + pollinator‐mediated selfing + delayed selfing).4. During inclement weather, few or no pollinators visited flowers. Reproductive output was severely limited by cross pollen, and delayed selfing provided reproductive assurance. During fine weather, cross pollen was also limiting, but pollinator‐mediated selfing boosted reproductive output before delayed selfing could occur.5. On isolated plants, flowers received fewer pollinator visits and reproductive output was limited more by cross pollen compared with grouped plants. Despite these differences, reproductive output of both isolated and grouped plants was boosted to similar levels by pollinator‐mediated selfing before delayed selfing could occur.6. One‐ and three‐flowered plants were similarly limited by cross pollen. On three‐flowered plants, facilitated and geitonogamous selfing boosted reproductive output before delayed selfing could occur. By contrast, on one‐flowered plants geitonogamy was eliminated, and pollinator‐mediated selfing was reduced. This provided mating flexibility by retaining opportunities for subsequent outcrossing, although delayed selfing boosted reproductive output.7. Reproductive output was consistently limited by cross‐pollination. Yet, delayed selfing provided reproductive assurance only during inclement weather and on one‐flowered plants. Under other conditions, pollinators mediated selfing that could render ovules unavailable for outcrossing as selfing occurred and for outcrossing that might occur subsequently. We conclude that best‐of‐both‐worlds flexibility will often be eroded by pollinator‐mediated selfing that inevitability occurs with outcrossing.

Modelling net ecosystem carbon and water exchange of a temperate Eucalyptus delegatensis forest using multiple constraints

This study examined the carbon budget of a Eucalyptus delegatensis forest over four years under contrasting weather conditions by using a comprehensive forest-growth model (CenW 3.1). Model parameterisation was constrained through multiple measurements, including daytime eddy flux measurements of CO 2 and water vapour exchange, estimates of above- and below-ground biomass pools, growth rates measured through stem-diameter increments and measurement of specific ecosystem processes, such as litter fall and soil respiration rates. The resultant estimates of net ecosystem exchange ranged from an uptake of 4.0 t C ha −1 year −1 in a year with adequate rainfall to a loss of 0.4 t C ha −1 year −1 in a year affected by drought and insect damage. The simulations indicated that this reduction in carbon gain was primarily due to insect damage rather than a direct consequence of water shortage. Under good growing conditions, carbon gain accumulated equally in stem wood and other biomass pools while soil organic carbon and woody litter were gradually decreasing following harvesting that had occurred 20 years earlier. Under more stressful conditions, net biomass increments decreased, while soil organic carbon and woody litter increased because of greater litter inputs and because drier conditions inhibited decomposition. Modelled estimates of photosynthesis and net ecosystem exchange were similar to estimates from eddy flux observations alone provided that it used a novel routine for deriving night-time carbon fluxes. Estimates of net ecosystem exchange derived in the present work were, however, substantially lower than previous estimates that had been derived using a more traditional analysis of eddy flux data. This study showed that detailed physiological modelling is a valuable technique for combining all available site information as well as further constraints based on broader scientific principles and considerations of the conservation of mass. It can thus provide a powerful constraint on the overall site carbon budget of an ecosystem.

Degradation of endophyte alkaloids in field-dried cut ryegrass herbage

The stability of endophyte alkaloids was investigated in ryegrass herbage cut and then dried in the field under a simulated hay drying regime in early and mid-summer. Ryegrass (Lolium perenne) infected with typical 'wild-type' and the 'AR37' strain of endophyte (Neotyphodium lolii) was measured for peramine, ergovaline and lolitrem B for wild-type, and epoxy-janthitrems for AR37. Despite very contrasting weather conditions, relative decline in alkaloid concentrations was similar in both early and mid-summer. Concentrations had halved within 5 to 7 days, except for lolitrem B in mid-summer which had no decline until after 4 days of drying. Ergovaline showed quite variable and erratic responses over time possibly due to the low concentrations in the cut herbage. At least in the mid-summer experiment, if herbage had been conserved at the time of normal haying making, alkaloid concentrations would have been similar to those of the herbage at the time of cutting. Keywords: endophyte, Neotyphodium lolii, Lolium perenne, peramine, lolitrem, ergovaline, janthitrems

A biometric and ecologic comparison between Artemia from Mexico and Chile

BackgroundA preliminary biometric and ecologic database for the brine shrimp Artemia from Mexico and Chile is presented. The area abounds in small and seasonal ponds and large inland lakes, the latter mainly located in Mexico, although relatively large and isolated lakes are found in complex hydrological settings in pre-high plateau areas of Chile. This paper summarizes research efforts aimed at the localization, characterization, and evaluation of the aquaculture potential of Artemia populations in Mexico and Chile, which exhibit great habitat diversity (ponds, salterns, coastal lagoons, sea arms, coastal and inland lakes), contrasting weather conditions and different levels of isolation and human intervention.ResultsThis study covered locations between 29° north latitude (Baja California, Mexico) to 50° south latitude (Puerto Natales, Chile). Biological characteristics considered are species name, reproductive mode, cyst diameter, chorion thickness, and nauplius length, whereas ecological data include pond size, pH, salinity, temperature, and water ionic composition. Artemia franciscana is the only species found in Mexico, it exists together with A. persimilis in Chile, though separated geographically. Ecological differences in habitat exist between both regions but also within countries, a pattern particularly clear with regard to water composition. Surprisingly, a Mexican (Cuatro Ciénegas, A. franciscana) and a Chilean location (Torres del Paine, A. persimilis) share habitat characteristics, at least for the period when data were collected. The discriminat analysis for cyst diameter and nauplius length shows that Artemia from only one location match in cyst diameter with those from San Francisco Bay (SFB) (Point Lobos), and one (Marquez) is far apart from SFB and all the others. The Chilean locations (Pampilla, Cejar, Cahuil, Llamara, Yape) share cyst diameter, but tend to differ from SFB. The remaining Mexican locations (Juchitan, Ohuira, Yavaros) are well separated from all the others. With regard to nauplii length, populations tend to distribute in a relatively random manner, being Marquez the location differing the most in cyst diameter from SFB.ConclusionThis database will contribute to the knowledge of radiation centers and serves as a baseline for further biogeographic studies, population characterization, management, and monitoring of Artemia biodiversity. Likewise, the impact of colonization and translocations for aquaculture purposes can be better assessed with a baseline for reference. Mexico and Chile exemplify the need to increase and further integrate regional information to tackle fundamental problems underlying practical utilization of Artemia.

Variations in Maize Pollen Emission and Deposition in Relation to Microclimate

The coexistence of genetically modified (GM) crops with conventional crops has become a subject of debate and inquiry. Maize (Zea mays L.) is one of the most cultivated crop plants in the world and there is a need to assess the risks of cross-pollination. Concentration and deposition rate downwind from different-sized maize crops were measured during three flowering seasons, together with micrometeorological conditions in the surrounding environment. Pollen release started once the air vapor pressure deficit (VPD) increases above 0.2 to 0.5 kPa. Moreover, the dynamics of release was correlated with the dynamics of VPD surrounding the tassels. Horizontal deposition appeared to follow a power law over short distance downwind from the source, and the dispersal distance increased with the source canopy height and the roughness length of the downwind canopy. This work also provides a data set containing both pollen measurements and contrasting weather conditions to validate dispersal models and further investigate maize pollen dispersal processes.

Comparison of surface energy exchange models with eddy flux data in forest and grassland ecosystems of Germany

Latent heat, which plays a major role in the energy balance of ecosystems, is usually calculated in different types of models, e.g., water balance approaches, aggregated “big-leaf” models, multi-layer ecosystem process models, that can be generally catalogued as Soil–Vegetation–Atmosphere–Transport models (SVATs) in a broader sense, etc. In this study, four (multi-) layered models (PLant-ATmosphere INteraction model (PLATIN), MixFor-SVAT, SVAT-CN, PnET-N-DNDC) and an atmospheric boundary layer model including vegetation (HIgh Resolution Vegetation Atmosphere Coupler (HIRVAC)) were used to predict latent heat (LE) and sensible heat ( H) exchange in the target area of VERTIKO, a research project on vertical transports of energy and trace gases and their spatial/temporal extrapolation under complex natural conditions. Investigated vegetation types were a Norway spruce forest, and two grassland sites in Brandenburg and Saxony (Germany) during field campaigns in 2001 and 2002. Four models had half-hourly to hourly time steps, but PnET-N-DNDC had a daily time step. All models used meteorological conditions above the canopy as input, and computed LE and H independently, or H as the residual of the energy balance. Intercomparisons and discussions of model parameters focussed on vegetation type (or species), and LAI. Model predictions were compared to half-hourly and daily eddy covariance measurements of ecosystem LE and H during two periods (cold and wet fall 2001, warm and dry summer 2002) to include contrasting weather conditions in the model evaluation. The agreement between modeled and measured fluxes was reasonable for most models. Yet, as the models were principally based on energy balance, the lack of energy balance closure as found in the data made it difficult to evaluate models against data, indicating a need for more attention to measurement accuracy in future studies. Systematic differences between the models were obvious. In general, HIRVAC model predictions of LE were routinely the lowest, and PnET-N-DNDC the highest. The tendency of under- or overestimating LE fluxes was comparable in PLATIN, SVAT-CN and MixFor-SVAT. SVAT-CN especially underestimated LE and overestimated H of the forest site. Under- or overestimation by the models could not be attributed to vegetation type. Both, MixFor-SVAT and SVAT-CN underestimated LE of one of the grassland sites, and overestimated the other. When averaging all model outputs, daily fall fluxes reached between 75 and 115% of the data for the three sites (76–83% for the summer period), and between 65 and 90% for half-hourly fluxes (73–97% for summer). Sensitivity analysis for an important model parameter, LAI, revealed only small effects on modeled LE in most models, probably due to compensation in the relative amounts of evapotranspiration and soil evaporation, at least for the limited range of LAI we analyzed (mean ±1 m 2 m −2). The sensitivity to LAI was smaller for forest simulations than for grasslands. For a given site PLATIN and MixFor-SVAT showed lowest, SVAT-CN intermediate, and HIRVAC largest sensitivity to changes in LAI. For latent heat simulations of forests, the sensitivity of LE to changes in LAI was smaller than the differences in the LE predicted by the five models. Further, it is much more important to determine vegetation type and accurate leaf physiology than the LAI of the system. The models differed widely in their approach for canopy radiation, leaf physiology, energy balance, atmospheric transport or soil water modeling, yet the treatment of evaporative losses from soil and interception pools seemed to cause the larger differences between the models after rain events.

Environmental Factors Affecting Productivity, Indican Content, and Indigo Yield in Polygonum tinctorium Ait., a Subtropical Crop Grown under Temperate Conditions

Polygonum tinctorium Ait. is a herbaceous subtropical annual plant, belonging to the family Polygonaceae. Within the cells of its leaves P. tinctorium accumulates large amounts of a colorless glycoside, indican (indoxyl beta-d-glucoside), from which the blue dye indigo is synthesized. P. tinctorium is well-known in Japan, where it had been cultivated to produce natural indigo for textile dyeing, whereas it represents a potentially interesting new crop in Europe. To better understand the effects of environmental parameters on P. tinctorium crop production and indigo yield, field experiments were carried out in central Italy under temperate climate. Three lines were tested during the 2001 and 2002 growing seasons, and plant/leaf yields as well as indican contents were evaluated. The results showed that P. tinctorium grown in temperate climate conditions can be harvested three times a year. Yields of 82 and 120 t ha(-1) of fresh plant yield were obtained in 2001 and 2002, respectively. The contrasting weather conditions between the two years significantly affected biomass production, which was higher in the 2002 season, characterized by wet weather conditions. The cycle length from sowing to the last harvest was accomplished in 229-238 days when plants had accumulated 2017-2018 degrees C. Green leaves accounted for 40-45% by weight of fresh plant tissue and contained 11-20 g kg(-1) indican. The three lines did not significantly differ in the main productive parameters or in fresh leaf indican content (14.1 g kg(-1) mean value). Photosynthetic active radiation influences indican leaf production according to the model y = 0.0004x + 8.566 (P < 0.01, correlation coefficient = 0.818). Indican content ranged from 12 to 25 g kg(-1) of fresh leaves with PAR daily values from 10000 to 40000 mEinstein m(-2) (recorded in May and at the end of July-beginning of August, respectively). The results indicate that in nonlimiting rainfall conditions a very high indican content and a potentially high indigo yield can be obtained by cultivating P. tinctorium in this pioneer geographical area.

Seasonal changes in the photosynthetic capacity of canopy oak (Quercus robur) leaves: the impact of slow development on annual carbon uptake.

The photosynthetic development of pedunculate oak ( Quercus robur L.) sun leaves in a mature woodland canopy in Oxfordshire, southern England, was investigated in situ during 3 years with contrasting weather conditions. Development of full photosynthetic capacity (indicated by light-saturated net assimilation rates, A(max), typical of the summer period) took between approximately 50 and 70 days after budbreak in different years. This slow development means that these leaves do not utilise a substantial fraction of the seasonal peak of solar irradiance. During the late autumn senscence period the photosynthetic capacity declined over a 2-week period, but as this is a time of low irradiance, the loss of potential photosynthesis was relatively small. The consequences of these developmental changes and differences in bud break dates for daily and seasonal leaf carbon balance were investigated through a simple light-response photosynthetic model. Seasonal changes in photosynthetic capacity would decrease annual carbon uptake per unit leaf area by about 23% compared to that potentially possible if leaves photosynthesised at peak rates throughout the growing season. This difference is likely to be up to 30% larger in years with late budburst and as low as 18% in years with early budburst.

Organisation of the pastoral territory and grazing management: joint modelling of grazing management practices and plant cover dynamics

The authors propose an original modelling methodology to investigate grazing systems, which confronts and combines two conceptually and methodologically different approaches that link two contrasting points of view on the grazing system: a farmer's organisation of grazing and changes in plant cover states on the farm territory. The methodology consists of a multivariate analysis enabling the modelling of changes in the plant cover to be interpreted in the light of the farmer's grazing practices by applying a model of farmer behaviour for action. One objective was to determine the relative role of vegetation in the farmer's decisions on the grazing management, in particular by highlighting the farmer's strategic organisation rules and adaptation to change. The research is based on a 2-year case study of a sheep farm using chiefly native vegetation and some improved grass in the calcareous hills of inland Provence in southeastern France. The farm is representative of one of the meat sheep farming types of southeastern France producing mainly housed lambs raised exclusively on suckling and concentrate and sold at 100–120 days at a carcass weight of 15–18 kg. The two successive study years were selected for their greatly contrasting weather conditions. In this situation, adjustments made by the farmer were shown to be peripheral and consisted of either combining the use of certain fields, or securing the use of complementary fields from neighbours. Thus this paper aims to enable both scientists and agricultural advisers to better understand different aspects of the ways the system operates and the factors influencing its management.

Genetic and environmental effects on morphology and fluctuating asymmetry in nestling barn swallows

Abstract A barn swallow Hirundo rustica partial cross-fostering experiment with simultaneous brood size manipulation was conducted in two years with contrasting weather conditions, to estimate heritable variation in tarsus, tail and wing size and fluctuating asymmetry. Environmental stress had contrasting effects depending on trait type. Significant heritabilities for tarsus, tail and wing size were found only in enlarged broods irrespective of year effects, while tarsus asymmetry was significantly heritable in the year with benign weather conditions irrespective of brood size manipulation effects. Tail, wing and composite (multicharacter) asymmetry were never significantly heritable. The environment with the higher heritability generally had higher additive genetic variance and lower environmental variance, irrespective of trait type. Heritability was larger for trait size than for trait asymmetry. Patterns of genetic variation in nestlings do not necessarily translate to the juvenile or adult stage, as indicated by lack of correlation between nestling and fledgling traits.

Within-season grass yield and nitrogen uptake, and soil nitrogen as affected by nitrogen applied at various rates and distributions in a high rainfall environment

A study was conducted to assess the effect of rates of nitrogen (N) fertilizer, and to compare the effectiveness of single and split applications of N on yield and quality of forage grass and on the potential for nitrate leaching. Three field trials were conducted at different sites in successive years, with plant and soil measurements made at each of four harvests. Extractable inorganic N was measured to 0.6 m in three depth increments prior to spring N application and after each cut in order to evaluate immediate and residual effects of the N fertilizer on plant growth, and the environmental implications of the applications. Response of yield and N uptake to N applications differed in the three trials. In all trials, the effect of N rate was greater than the effect of N distribution during the growing season. Although there were only small, whole-season yield increases associated with distributing the N over the season, the distribution of yield within the season was changed considerably. Soil data showed relatively little leaching of N during the growing season under contrasting weather conditions of the three growing seasons. Retention of N within the soil root zone contributed to residual effects on yield and plant uptake, and these effects frequently lasted to the end of the growing season. Crop response to N applications was apparently influenced by the N supplying capacity of the soil and the effect of weather on crop growth rate. Soil nitrate at harvest did not vary consistently with N application treatments in the three trials, other than having highest concentrations at the highest fertilizer rate. Soil nitrate was greatest after cut 1 and decreased sharply toward the end of the season following the single spring applications, whereas plots receiving equal distributions of N through the season had relatively high concentrations at all sampling times during the season. Soil extractable ammonium concentration was influenced by high rates of N application, but the effect was small and largely confined to the sampling after cut 1. The soil always contained about 10–15 mg kg−1 extractable ammonium in surface 0.3 m depth, with a tendency for slightly greater concentrations in early spring. Soil ammonium appears to be involved in soil and plant processes, but the exact magnitude and significance of its involvement could not be determined from the measurements made. The redistribution of grass yield by splitting the application of fertilizer N within the growing season would be beneficial for grazing systems. Unfortunately, soil inorganic N measurements will not greatly assist in determining the precise rate and distribution of fertilizer for varying field conditions. Key words: N response, N uptake, residual N effect, soil extractable N, N leaching

Neighbour interaction and stability in subalpine meadow communities

Abstract. The ability of a plant community to maintain favourable conditions within its canopy was examined by means of removing the vegetation around individual plants (radius ca. 1 m) and measuring light and temperature relations and rates of net photosynthesis in both isolated and sward plants. The results suggest that neighbour interactions are important for the stabilization of environmental relations within the canopy, at least in hay meadows as compared to intensively grazed pastures. The canopy may maintain optimal conditions for plant life within a wider variation of environmental factors. At the same time, plants have to adapt to contrasting (sunny vs. cloudy) weather conditions occurring with about equal frequency during the vegetation period in the central Caucasus.

A transition in weather over the Agulhas Current

Aircraft observations were used to investigate contrasting weather conditions over the northern edge of the warm Agulhas Current south of George, South Africa, during June 1989. A sea surface temperature front of 7°C over 10 km was observed near 35°S, 23°E. Surface-sensible and latent heat fluxes increased by a factor of five on the warm side. A deepening of the moist unstable layer was indicated by increased equivalent potential temperatures over the warm side. Most significantly, convectively generated turbulence, as inferred from variances in air pressure, increased exponentially with sea surface temperature and surface layer winds doubled south of 35°S. The sea state, initially smooth over the continental shelf, became rough seawards of the northern edge of the Agulhas Current. Numerical model simulations were performed to assess air-sea interactions. Model-simulated thermodynamic variables responded in close agreement with observations, but the increase in cross-frontal winds was not fully resolved. ...