Showing papers on "Growing season published in 1986"

Light and Nutrient Effects on the Relative Biomass of Blue-Green Algae in Lake Phytoplankton

Abstract: The factors determining the relative biomass of blue-green algae during the growing season were studied using data from 22 lakes worldwide. Multiple linear regression analyses suggest that total ni...

Input of carbon to soil from wheat plants

Abstract: The growth of wheat plants and the distribution of labelled photosynthate from pulse-labelling with 14CO2 were measured periodically during the growing season in the field. During early growth there was approximately the same proportion of photosynthate translocated below ground and retained in the shoots. Of the 14C below ground about a half was respired and a quarter each was in the soil and roots. This distribution changed exponentially during growth with an increasing proportion of 14C remaining in the shoots and a corresponding decreasing proportion being translated below ground, which was only a few percent by flowering. From this information the total input of carbon to the soil from the crop was calculated to be 1305 kg Cha−1. Comparison of these estimates with those from previous experiments suggest that differences do occur due to the stage of growth of the plant, the environmental conditions, soil type and microbial activity.

The response of plants to elevated CO2 : IV. Two deciduous-forest tree communities.

Abstract: Tree saplings, two groups of three species from each of two deciduous tree communities, were grown in competition at three CO2 concentrations and two light levels. After one growing season, biomass was measured to assess the effect of CO2 on community structure, and nitrogen and phosphorus concentrations were measured for leaves, stems, and roots of all trees. Gas-exchange measurements were made on the same species grown under the same CO2 concentrations. Photosynthetic capacity (rate of photosynthesis at saturating CO2 and light) tended to decline as CO2 concentration increased, but differences were not statistically significant. Stomatal conductance declined significantly as CO2 increased. Nitrogen and phosphorus concentrations generally declined as CO2 increased, but there were some unexpected patterns in roots and stems. CO2 concentration did not significantly affect the overall growth of either community after one season, but the relative biomass of each species changed in a complex way, depending on CO2 light level, and community.

Monitoring the grasslands of the Sahel using NOAA AVHRR data: Niger 1983

Abstract: Normalized difference vegetation index data derived from the Advanced Very High Resolution Radiometer on board the NOAA-7 satellite for the 1983 growing season for the Sahelian Zone of Niger are compared with biomass estimates derived from an empirical grassland productivity model. The model used daily rainfall data to estimate the potential biomass production for fourteen meteorological stations through the growing season. A good general correspondence (r = 0·75) was seen between the productivity model and the satellite-derived integrated NDV1, although specific differences were apparent between actual and potential biomass. The study shows the utility of high-temporal-resolution satellite data for monitoring grassland conditions at a local and regional scale and emphasizes the importance of a maximum value compositing approach to the analysis. The study also shows the potential of the satellite data for quantifying phenological characteristics of vegetation

Physical control of the horizontal patchiness of sea-ice microalgae

Abstract: Factors controlling the horizontal distribution of sea-ice microalgae were studied in Southeastern Hudson Bay and adjacent Manitounuk Sound (Canadian Arctic). Both large (-30 km) and small (0.3 to 500m) scales of variability were investigated. Results showed that salinity was the most important factor controlling large scale distribution of the ice-microalgal biomass, through its effect on the structure of the ice (surface available for colonization). Variation in the thickness of the snow-ice cover, which determines irradiance at the bottom of the ice, was the factor controlling distribution of the algal biomass at smaller scale (diameter of patches of microalgal biomass ranging between 20 and 90 m). The relation between ice-algal abundance and snow-ice thickness changed however over the season. At the beginning of the growing season (in April when the bottom-ice irradiance was higher than a minimum critical irradiance), maximum algal biomass was observed under areas covered by the smallest snow depths. Towards the end of the season, when light transmitted through the snow-lce cover increased, maximum algal biomass was observed under areas covered by the deepest snow. This suggests that ice algae have both minimum and maximum critical light levels. The minimum level is the irradiance below which there is no photosynthetic activity (Imi, 7.6 CL Einst m-2 S-') and the maximum level corresponds to the inhibiting light intensity, which may vary during the growth season. The horizontal heterogeneity of the snow-ice cover, whlch is influenced by wind at the air-ice interface, thus provides diversified bottom-ice habitats where irradiance is compatible or not with the physiological limits of the ice microalgals cells. This results in a strong patchiness in distribution of ice-bottom microalgae.

Influence of Climatic Conditions on Production of Stipa-Bouteloua Prairie Over a 50-Year Period

Abstract: Range forage yields obtained over a W-year period at the Research Substation near Manyberries in southeastern Alberta were analyzed in relation to several climatic factors. The basic variables were precipitation, pan evaporation, temperature, hours of sunlight, and wind velocity. The precipitation from April through July was highly correlated with range forage production and this relationship could be utilized to predict the annual forage production by 1 August each year. A slightly better correlation wns obMned when range forage production was related to the total of the previous September plus the current April through July preeip itation. Pan evaporation totals, mean temperature, and hours of sunlight were negatively correlated with forage production, while wind velocity during the growing season showed a low relationship to forage production. Stepwise regression analysis showed that the inclusion of May and June mean temperatures with June and July precipitation accounted for 63% of the variation in range forage production. The predicted forage yield would be useful in making management decisions or adjustments, especially during drought periods, while the long-term forage yield data can be utilized in range forage models or in validating their effectiveness. Studies on the response of mixed prairie to weather and climatic fluctuations have been mostly concerned with changes in floristic composition, with less attention to the relationship between climatic variations and range forage yields. Such relationships could provide ranchers with some method of predicting suitable stocking rates on native rangelands and aid ecologists in understanding short-term and long-term rangeland ecosystem dynamics. The relationship between precipitation ahd yield of range vegetation was investigated as early as 1922 in north-central Montana by Patton (1927). More recent studies by Rogler and Haas (1947) in North Dakota, Smoliak (1956) in southeastern Alberta, Rauzi (1964) in Wyoming, Hulett and Tomanek (1969) in western Kansas, and Ballard (1974) in Montana explored the relationship between precipitation during the growing season and range forage yields. The good correlations that they found could be utilized to predict seasonal range forage production as early as 1 July. Other studies showed that fall-through-summer precipitation better explained the variation in total forage production but spring precipitation best predicted grass production (Noller 1968, Whitman and Haugse 1972). Range forage production has also been related to soil type. In a study of 14 sites, Cannon (1983) found that thickness of mollic epipedon of range soils was significantly related to forage production and that using thickness and mean annual precipitation improved the estimate of range forage production. Weather fluctuations on Mixed Prairie grassland in the Northern Great Plains were shown to result in a dominance of xeric species during drought periods and mesic species under more favorable growing conditions (Coupland 1958,1959). Provision of more favorable growing conditions through weather modification may be a possibility with cloud seeding to increase precipitation. Hausle (1972) concluded that the amount of forage production that could result from additional precipitation could be predicted by statistical methods where long-term production and climatological data are available. Ballard and Ryerson (1973) indicated that increased precipitation resulting from weather modification will probably have a significant effect on range forage production only when combined with good livestock and grazing management

Factors Influencing Soil Moisture and Plant Community Distribution on Niwot Ridge, Front Range, Colorado, U.S.A.

Abstract: Slope-aspect influenced evapotranspiration and soil desiccation in the Colorado alpine tundra via control over radiation and wind speed. South- and east-facing sites upon a fellfield knoll received 18 and 14% more net radiation than the north-facing slope on clear sky and more typical summer days, respectively. West-facing slopes experienced moderate radiation loads but highest wind speeds. Water loss during two drying sequences for east-, west-, and northfacing slopes was 80, 80, and 600%o, respectively, of evapotranspiration from the south-facing slope. Consequently, soil at the south-facing site dried faster than soil on other slopes of the knoll. Dryas octopetala prefers more moist northern slopes on Niwot Ridge, illustrating the importance of topoclimatic controls over soil moisture to vegetation distribution in the fellfield. Water relations during the growing season influence vegetation distribution within topographic depressions on Niwot Ridge. Where meltwater is supplied throughout the growing season wet meadow and moist shrub tundra occur. Evapotranspiration from the wet meadow was 1.5 times greater than from other communities in the Saddle area during a drying period. Contrasts in snow cover depth between windward and leeward slopes is an important location factor for dry fellfield, dry meadow, moist meadow, and snowbed vegetation.

Long-term growth and development of transplants of the salt-marsh grass Spartina alterniflora

Abstract: The effect of transplant spacings (45, 60, and 90 cm) on establishment ofSpartina alterniflora along an eroding shoreline in North Carolina was evaluated and annual biomass production of the planted marsh was compared to a natural marsh. The 45- and 60-cm spacings were more successful for establishment on marginal sites that were near the lower elevation limits ofS. alterniflora. The 90-cm spacing was adequate where growing conditions were favorable. Measurements of aboveground growth indicated that there were no differences due to spacing by the end of the second growing season. Differences between spacing treatments in belowground dry weight persisted through three growing seasons. Annual aboveground and belowground standing crop of the transplanted marsh and a nearby natural marsh were compared over a ten-year period. During the early years of development, several characteristics of the transplanted vegetation differed from the natural marsh, but these differences diminished with time. Development of the aerial portion of the transplanted vegetation was rapid, with the most vigorous growth occurring in the second growing season. At that stage of development, the transplants were taller with more flowering stems and a greater standing crop. There were fewer but larger stems than in later years or in the natural marsh. Belowground standing crop increased over the first 3 growing seasons, reached an equilibrium level in 4 growing seasons, and remained constant during the remainder of the study. This indicated that annual production and decomposition of belowground material were about equal. Annual belowground production was estimated to be about 1.1 times the October standing crop of aboveground material. The results indicated that vegetation in a man-initiatedS. alterniflora marsh was effective in reducing shoreline erosion and was comparable to a natural marsh growing under similar environmental conditions. The ten-year sampling period was adequate to document that the transplanted marsh was equal in primary productivity and that it was persistent and self-sustaining.

The effects of chrysomelid beetle grazing and plant competition on the growth of Rumex obtusifolius.

Abstract: The interaction between grazing by Gastrophysa viridula and interspecific plant competition was investigated for Rumex obtusifolius growing in the field. During an eight-month growing season non-competing R. obtusifolius grew larger than competing plants although herbivore loads (beetle productivity per unit leaf area) were similar for competing and non-competing plants. However, grazing significantly reduced the growth of R. obtusifolius only in competing plants thus demonstrating a synergism between plant competition and invertebrate herbivory. The relevance of these findings for the distribution of plant species is discussed.

Growth and flowering in Eriophorum vaginatum: annual and latitudinal variation

Abstract: Growth, flowering, and nutrient content of Eriophorum vaginatum were observed an- nually over 4 yr at 34 sites spanning 5.50 latitude and 1050 m elevation in northern and central Alaska. We found a strong correlation between the average number of thawing degree-days during the growing season and the peak-season leaf mass per tiller. However, the results from reciprocal transplant gardens established at five sites suggested the correlation was due more to genetic differences among populations than to direct climatic effects. Other variables showed little correlation with long-term degree-day averages, and leaf N and P concentrations were site specific. A year of high inflorescence density at any site along the latitudinal transect was likely to be a high flowering year at all other sites. However, a year of above-average growth north of the Brooks Range did not always coincide with above-average growth to the south, and vice versa. The greatest year-to-year variation was in inflorescence density; least variation was in leaf mass per tiller and N and P concentrations. We conclude that yearly variation in weather affects plants on a broad regional scale, but specific controls over flowering versus growth differ in their geographic extent.

Temperature and leaf osmotic potential as factors in the acclimation of photosynthesis to high temperature in desert plants.

Abstract: Seasonal changes in the high temperature limit for photosynthesis of desert winter annuals growing under natural conditions in Death Valley, California were studied using an assay based upon chlorophyll fluorescence. All species of this group were 6 to 9°C more tolerant of high temperature at the end of the growing season (May) than at its beginning (February). Over this same time period, the mean daily maximum air temperatures increased by 12°C. Laboratory studies have demonstrated that increases in thermal tolerance could be induced by increasing growth temperature alone. For plants growing under field conditions there was also a good correlation between the thermal tolerance of leaves and the osmotic potential of leaf water, indicating that increases in the concentrations of some small molecules might also confer increased thermal tolerance. Isolated chloroplast thylakoids subjected to increasing concentrations of sorbitol could be demonstrated to have increased thermal tolerance.

Soil aeration and growth of forest trees (review article)

Abstract: Many forest trees exhibit reduced growth or are killed when the soil is low in oxygen. Anaerobic soil conditions are associated with flooding or compaction of soil but also occur commonly in soils of heavy texture. Reductions in height growth, leaf growth, cambial growth and reproductive growth of trees growing on poorly aerated soils are well documented. The amount of growth reduction varies widely among species and duration of anaerobic soil conditions during the growing season. Inhibition of growth is preceded by changes in physiological processes, including food, water, hormone, and mineral relations. Some species can adapt to soil anaerobiosis by (1) producing hypertrophied lenticels which assist in aeration of the stem and release of toxic compounds, and (2) growing new roots to replace loss of original roots under anaerobic conditions. The replacement roots assist in absorption of water and mineral nutrients and in oxidizing the rhizosphere and detoxifying soil toxins. Ethylene, together with other...

Carbon and nitrogen partitioning in the biennial monocarp Arctium tomentosum Mill.

Abstract: Growth and nitrogen partitioning were investigated in the biennial monocarp Arctium tomentosum in the field, in plants growing at natural light conditions, in plants in which approximately half the leaf area was removed and in plants growing under 20% of incident irradiation. Growth quantities were derived from splined cubic polynomial exponential functions fitted to dry matter, leaf area and nitrogen data.Main emphasis was made to understanding of the significance of carbohydrate and nitrogen storage of a large tuber during a 2-years' life cycle, especially the effect of storage on biomass and seed yield in the second season. Biomass partitioning favours growth of leaves in the first year rosette stage. Roots store carbohydrates at a constant rate and increase storage of carbohydrates and nitrogen when the leaves decay at the end of the first season. In the second season the reallocation of carbohydrates from storage is relatively small, but reallocation of nitrogen is very large. Carbohydrate storage just primes the growth of the first leaves in the early growing season, nitrogen storage contributes 20% to the total nitrogen requirement during the 2nd season. The efficiency of carbohydrate storage for conversion into new biomass is about 40%. Nitrogen is reallocated 3 times in the second year, namely from the tuber to rosette leaves and further to flower stem leaves and eventually into seeds. The harvest index for nitrogen is 0.73, whereas for biomass it is only 0.19.

Lodgepole Pine Ecosystems

Abstract: Despite the low productivity of forests in the Rocky Mountain environment, biological activity plays a critical role in regulating the fluxes of water and major ions. Moreover, although much of the landscape is dominated by monospecific stands of lodgepole pine, these fluxes vary markedly among sites. Concomitant differences in soil texture and forest structure appear to be important factors that induce this variation, a result of their influence on soil water storage capacity and the mobility of ionic solutes in mineral soil. Water and nitrogen availability, along with a cool, short growing season, limit the amount of leaves and roots that can develop, which in turn regulate water flows, the generation of protons consumed in soil weathering reactions, and the supplies of organic matter for heterotrophic organisms. New environmental stresses and increasing pressures for intensive forest management dictate that a more complete understanding of the controls of material fluxes in natural and man-dominated ecosystems is needed. The lodgepole pine ecosystem, while relatively simple, provides a representative example of the complexity that can be involved.

Quantitative Phenology and Water Relations of an Evergreen and a Deciduous Chaparral Shrub

Abstract: Quantitative phenology and pre—dawn xylem pressure potentials (XPP) of an evergreen shrub, Ceanothus megacarpus, and a deciduous shrub, Salvia mellifera, were compared on pairs of plants in shared microsites in southern Californian chaparral for 2 yr. Shoot elongation of both species occurred at essentially the same time during both growing seasons. Leaf maturation also occurred at essentially the same time for both species during the 1977—1978 season, but commenced ≈1 mo earlier on S. mellifera than on C. megacarpus during the 1978—1979 season. No substantial differences in phenology were found among leaves of C. megacarpus canopy, subcanopy, or short shoots. On S. mellifera, however, leaf maturation was more rapid and was completed earlier in the season on flowering canopy shoots than on nonflowering canopy and subcanopy shoots, and although no mature leaves on flowering canopy shoots persisted beyond June during either year, a substantial number of mature leaves on subcanopy shoots remained well into their second growing season. Leaf curling, associated with changes in XPP, was significant in both species, and allowed rapid adjustment of leaf area without the abscission or production of leaves. Flowering and seed production occurred early in the growing season for C. megacarpus and late in the growing season for S. mellifera. This difference in flowering phenology corresponded to the more rapid vegetative development of S. mellifera flowering canopy shoots. In both species subcanopy or short shoots made up a much larger fraction of the total number of shoots per plant, and consequently contributed relatively more to whole—plant leaf phenology and annual shoot production, than did canopy shoots. There were only subtle differences between species in the timing of increases in whole—plant mature leaf area (MLA). Maximum rates of increase of whole—plant MLA occurred in both species when soil temperature (ST) was ≈13.5°C and XPPs were ≈—0.60 MPa, and these rates declined sharply at STs above 15° and XPPs below —1.5 MPa. Ceanothus megacarpus and S. mellifera had nearly indistinguishable XPPs throughout both years. Leaf shedding by S. mellifera was more related to photoperiod and flowering than to low XPP, and was clearly not a successful drought—avoiding mechanism. Like some evergreen species, S. mellifera tolerated long periods of extremely low XPP. Important differences between the species were that over a 12—mo period the integrated total of MLA—days for S. mellifera was 73% of that for C. megacarpus, and leaves produced during the current growing season contributed 71% of the total MLA—days in S. mellifera and only 37% in C. megacarpus. The differences between the results of this study and several accepted generalizations regarding growth and water relations of deciduous and evergreen chaparral shrubs may be due to the observation of these growth forms in different microsites and the lack of truly quantitative phenological measurements in previous studies.

Sources and transformations of organic nitrogen in intensively managed grassland soils

Abstract: Large amounts of nitrogen (200–350 kgN ha−1 yr−1) are returned to the soil under intensively managed grassland through the death and decay of unharvested herbage and of roots. With grazed swards, the return of animal excreta may contribute an additional 150–300kgN ha−1 yr−1, but this is distributed unevenly. Earthworms are important in the early stages of decomposition of plant residues and faeces. The balance between the mineralisation and immobilisation of N during decomposition is influenced largely by the C:N ratio. In many grassland soils, the rate of addition of organic matter exceeds the rate of decomposition but, when long-term grassland is subjected to intensive management, this pattern may be reversed. Soils differ considerably in the proportion of the total soil N that is mineralised, and hence becomes available for uptake, during a growing season. Available soil N may be predicted by chemical analysis based on the measurement of NH+ 4-N + NO− 3-N extracted by boiling with l M KCl. In the field, prediction is improved when the results of soil analysis are adjusted by a factor, calculated for the individual site, derived from mean values of temperature and either soil water status or rainfall.

Soil-plant relationships in the tropics

Abstract: As commonly defined, the tropics comprise that region of the earth lying between the Tropics of Cancer and Capricorn, constituting 38% of the land surface (5 x 109 ha). The tropics may also be defined as those areas with mean temperatures greater than 18?C in all months of the year. A third definition of the tropics specifies those areas with soils in which the temperature at a depth of 50 cm varies less than 5?C between summer and winter (39). Compared to the geographical definition, these latter definitions reduce the area considered to be tropical. In contrast to the temperate region where low temperature restricts plant growth at some period of the year, in the tropical regions the length of the growing season is determined by the amount of rainfall and its temporal distribution. Dudal (13) has separated the tropics into five major ecological zones based on growing periods: humid, subhumid, semiarid, arid, and wetland. The growing season varies from 12 months in the humid zone to nearly 0 months in the arid regions. In the tropics a wide range of climate and vegetation exists, owing mainly to variations in amount and duration of rainfall.

Corn Yield Prediction Using Climatology

Abstract: A method is developed to predict corn yield during the growing season using a plant process model (CERES-Maize), current weather data and climatological data. The procedure is to place the current year's daily weather (temperature and precipitation) into the model up to the time the yield prediction is to be made and sequences of historical data (one sequence per year) after that time until the end of the growing season to produce yield estimates. The mean of the distribution of yield estimates is taken as the prediction. The variance associated with a prediction is relatively constant until the time of tassel initiation and then decreases toward zero as the season progresses. As a consequence, perfect weather forecasts reach their peak value between the beginning of ear growth and the beginning of grain fill. The change in the predicted yield in response to weather as the growing season progresses is discussed for 1983 and 1976 at Peoria, Illinois. Results are given of an attempt to incorporate 30-day Climate Analytic Center outlooks into the predictive scheme. 21 references, 14 figures, 1 table.

Timing of Seed Production and Dispersal Geranium Carolinianum: Effects on Fitness

Abstract: The importance of the timing of seed production and of seed dispersal to the fitness of an annual plant was examined using Geranium carolinianum in the piedmont of North Carolina. Mature seeds were collected from naturally growing plants on four dates in May and June and were then sown back into the population on five dates in late May, June, and July. The mean mass of seed produced varied during the growing season and this had important consequences for fitness. Seeds produced early were heavier, but lighter seeds germinated earlier. Seedlings that emerged earlier developed into larger plants with relatively higher reproductive output. Timing of seed dispersal had no effect on date of emergence, plant size, or fecundity.

Seasonal patterns of net photosynthesis of loblolly pine from diverse origins

Abstract: Growth and photosynthesis were examined in seedlings of Pinustaeda L. (loblolly pine) from six widely separated provenances. The seasonal patterns of net photosynthesis were similar for each. Seedling net photosynthesis peaked in late October and net photosynthesis per unit leaf area peaked before midsummer and again in late October. When averaged over the entire season, seedling photosynthesis was greatest for Florida seedlings and least for Arkansas–Oklahoma and Texas seedlings. Seedling photosynthesis was strongly correlated with final size on all sampling dates and when averaged over the entire season. Both leaf area and net photosynthesis per unit leaf area contributed to the differences among provenances. Late season growth and photosynthesis, observed in the Florida provenance, increased provenance differences established early in the growing season.

Seasonal variation in root growth capacity during cultivation of container grown Pinus sylvestris seedlings

Abstract: Root growth capacity (RGC) in Scots pine seedlings was studied from the time of sowing and during the following two growing seasons. The method used for measuring RGC is also described. In the firs...

Carbon and nitrogen allocation patterns of Douglas-fir seedlings fertilized with nitrogen in autumn. II. Field performance

Abstract: October-fertilized and unfertilized 2-0 Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) seedlings were outplanted the following February. Half of each planting block was seeded with grass to induce water stress during the typical summer drought. Sucrose was applied to soil around each seedling to limit availability of nitrogen to tree roots. Fertilized seedlings broke bud 9–10 days earlier, produced more shoot growth, and, as shown in later harvests, had higher relative growth rates than unfertilized seedlings. However, initial differences in growth response were due primarily to the earlier budbreak. Seedlings growing with grass had predawn water potentials of −1.5 MPa by early August; by September 3, unfertilized seedlings growing with grass were significantly more stressed than any others. Although free amino acid and total nitrogen concentrations were higher in fertilized than unfertilized seedlings when planted, they became equal by the end of one growing season. However, fertilized seedlings conta...

Effects of salal understory removal on photosynthetic rate and stomatal conductance of young Douglas-fir trees

Abstract: The effects of salal (Gaultheriashallon Pursh.) understory removal on the growth of thinned 32-year-old Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) trees were determined in a stand subject to growing season soil water deficits. Four pairs of similar trees were selected and the understory was completely removed from around one of each pair, the root zones of which were both isolated using plastic sheeting buried to bedrock. Photosynthesis, stomatal conductance, soil water potential and canopy microclimate were measured intensively in one pair on 4 clear days during an extended dry period in June 1982. Basal area increment of the four pairs of trees was measured over three growing seasons. To determine the effect of soil water potential on tree photosynthesis, the same variables were intensively measured over 3 consecutive days in late August 1982 for another tree initially subjected to a soil water potential of approximately −1.6 MPa, but irrigated to approximately −0.02 MPa between the 1st and 2nd d...

Site fertility affects seasonal carbon reserves in loblolly pine

Abstract: Summary Concentrations of nitrogen, phosphorus and total non-structural carbohydrates were examined in loblolly pine foliage from control stands and stands fertilized with sludge containing 450 kg N ha-‘. Needles from the fertilized sites had higher N concentrations regardless of needle age. Fertilization increased P concentration in needles relative to the control sites. Seasonal patterns of change in sugar concentrations were similar among sites for both current-year and one-year-old needles, the highest concentration (126 mg g-l) occurring in February. Sugar concentrations were lowest in June and were generally lower in current needles than year-old needles. Starch concentrations and the amount of starch per fascicle varied both seasonally and among sites. Fertilization increased starch reserves in current needles at the end of the growing season (7 mg g-r and 14 mg g-r for the control and fertilized sites, respectively), and increased the rate of starch mobilization during the initiation of needle growth. In the growing season, starch concentrations were lower in current foliage from high fertility sites than low fertility sites. Carbon reserves accumulated during the growing season in mature oneyear-old needles from control sites indicating a lower sink strength for carbon under low nutrient conditions. There was a positive relationship between starch and nitrogen concentrations during the dormant season whereas the relationship was negative during the growing season. The magnitude and direction of change in starch reserves with the onset of growth following the dormant season may indicate nutrient limitation and potential growth response to fertilization.

Dynamique de la strate herbacee des paturages de la zone sud-sahelienne

Abstract: The present state of the Sahelian rangelands and their overexploitation requires control of their use and the development of other management practices. This is the more so, because of the devastating effects of the droughts during the last 15 years. For this purpose description and evaluation of the species composition of the herbage vegetation is needed, which is especially difficult because the predominance of annuals contributes to large differences from year and from place to place.The present thesis concerns research on the interaction between the properties of species, rainfall, substrate and way of exploitation, as reflected in the dynamics of the rangeland vegetation. This research was executed in Niono in Mali in the South of the Sahel at an average rainfall of 600 mm per year. Experiments were done on sand, loam and clay soils. Natural rainfall was varied by shielding against rain and additional sprinkling.The main annual species were studied throughout the growing season from germination to seed production. Auto-ecological experiments in the field and under controlled conditions provided additional information about the species response to environmental differences. The observed changes in the absolute and relative contribution of the species in terms of number and biomass clarified the role of various plant properties in conjunction with rainfall amount and distribution, substrate properties and management.Important properties of the species that govern the dynamics of the vegetation are germination rate, drought resistance of the young seedlings, type of photosynthesis (C 3 or C 4 ), day length, length of the growing cycle, degree in which the plants continue vegetative growth after the onset of flowering and the production of seeds. The main effect of the substrate is its influence on the rain-dependent availability of water throughout the growing season, as affected by physical soil properties and topography. Exploitation by grazing adversely affects this rain-dependent availability of water, but also has a direct effect on height, length of the growing cycle and seed production of the species. Rapid germination, drought resistance of seedlings, C 4 -photosynthesis, a long growing cycle and extension in height are in general advantageous properties to survive, but plants can neither have it all ways. Moreover, slow germination and a short growing cycle are apparently advantageous in long periods of drought, whether these are due to irregular rainfall as such or to substrate properties or overexploitation.Vegetation surveys to support the improvement of rangeland use are of more lasting value, if the dynamics of the herbage cover is interpreted against the background of the results that are presented in this study.

The effect of high nutrient addition upon seasonal patterns of mycorrhizal development, host growth, and root phosphorus and carbohydrate content in fraxinus pennsylvanica marsh

Abstract: SUMMARY The effects of Glomus macrocarpum and a high rate of nutrient addition upon growth, root carbohydrate and P content, and mycorrhizal development in green ash (Fraxinus pennsylvanica) seedlings were studied over two growing seasons. Mycorrhizal and simitaly growing non-mycorrhizal seedlings had similar seasonal patterns in growth and in root carbohydrate and P content. No clear difference in mycorrhizal development as as result of nutrient treatment occurred until the middle of the second growing season when the percent root infection of seedlings that received low nutrient increased and that of seedlings that received high nutrient decreased. A seasonal cycle with two phases in the interaction of the VA mycorrhiza and host growth, root shoot ratios, root infection, root content of soluble sugars, starch and P, arbuscule and vesicle numbers, and spore population is proposed.