Showing papers on "Growing season published in 1978"

The effects of water- and nitrogen-induced stresses on plant community structure in a semiarid grassland.

Abstract: A replicated factorial experiment was designed to test the hypothesis that manipulating inputs of water and mineral nitrogen to a semiarid grassland would disrupt existing interactions resulting in alteration of the structure of the primary producer community. Alteration of community structure was measured as either changes in growing season average biomass of 6 functional groups of plants or their relative contribution to total biomass.Additions of water greatly increased total biomass and resulted in the replacement of one of the dominant functional groups by a subordinate group. The water plus nitrogen treatment resulted in large biomass increases in two of the dominant functional groups, elimination of succulents as an important component of community structure, and establishment of several introduced weedy species. Continuation of the experiment will likely result in complete dominance of the water plus nitrogen treatment by these introduced species.Despite the large changes in community structure observed as a result of water- and nitrogen-induced stresses we conclude that the shortgrass prairie in northcentral Colorado is asymptotically stable with respect to these influences.

Responses of Annual Vegetation to Temperature and Rainfall Patterns in Northern California

Abstract: Weather patterns significantly influence annual vegetation, both within and between years. The impact of these weather patterns on standing crop, cover, and botanical composition were investigated during the years 1955—1973 inclusive. Standing crop increased as the growing season progressed from March to June, while cover declined. Erodium spp. Carduus pycnocephalus, Geranium spp. and Hypochoeris glabra all declined between these 2 calendar dates. Other annual species increased in botanical composition as the growing season progressed. Weather patterns primarily influenced total standing crop during the initial period of plant growth. Following germination, temperatures were typically warm and conducive to plant growth; drought at this time suppressed total standing crop. Consistent precipitation following fall germination, and again during the period of rapid plant growth in spring, contributes to a "grass year" in the annual type. The sequence of freezing temperatures in relation to phenology of develop...

The structure and function of ten Western North American grasslands: II. Intra-Seasonal dynamics in primary producer compartments

Abstract: (1) Intra-seasonal dynamics of the various above-ground and below-ground primary producer compartments for ten central and western North American grassland sites are presented. (2) The seasonal peak values of the primary producer compartments are examined, as indicative of the net accumulation of organic material, and the relationships of these peak values to various abiotic regimes at the sites are investigated. (3) Seasonal live biomass followed either a unimodal or a bimodal growth pattern. In general, grasslands with only cool-season or only warm-season plants showed a unimodal pattern, while grasslands dominated by both cool- and warm-season species had a bimodal seasonal growth pattern. There were no significant differences between grazed and ungrazed treatments in seasonal live biomass, although there was a significant site × treatment interaction. (4) Peak live biomass ranged from 84 to 336 g m -2 , and showed a linear increase with increasing amounts of growing-season precipitation up to 450 mm; at higher values of precipitation increases in live biomass tended to level out. (5) Maximum rates of accumulation of live biomass ranged from 0.4 to 6.5 g m -2 day -1 . Ungrazed grasslands had a peak rate of 4.2 g m -2 day -1 compared with 3.2 g m -2 day -1 for grazed grasslands. (6) Generally the recent-dead compartment showed maximum values soon after the peak in the live compartment. Material in the old-dead compartment, however, was at a maximum early in the growing season, and a general decline in the standing crop of old dead material followed as material was transferred to the litter compartment. (7) Litter dynamics responded closely to precipitation events, and showed a rather erratic pattern. (8) Root biomass generally reached a maximum about midway through the growing season. On the cooler grasslands, grazed treatments typically had a larger peak in root biomass; in contrast, the warmer grasslands did not show a marked response in root biomass to grazing.

Seasonal changes in the epiphyte community of natural and artificial macrophytes in Lake Memphremagog (Que. & Vt.)

Abstract: Seasonal changes in the epiphyte biomass, measured both as chlorophyll a and as cell volume, and species composition were compared on Potamogeton richardsonii and on a similar plastic plant ‘grown’ together in a shallow bay of Lake Memphremagog (46°06'N, 72°16′W). Both substrates exhibited two periods of high biomass during the June to September growing season; one in June, when the community was dominated by loosely attached species with a strong planktonic component (up to 37%), and one in September, when the epiphytes were characterized by species tightly attached to the leaves. Although this seasonal trend was similar, the loosely and tightly attached communities were best developed on the natural and artificial plants, respectively. The diversity of the epiphytes was significantly higher on the natural than on the artificial leaves from July on. Both the diversity differences and differences in community structure appear to be the result of the summer accumulation of CaCO3 observed only on the upper leaf surfaces of the natural plants. Consequently, P. richardsonii appears to affect epiphyte development largely by its precipitation of CaCO3, with no evidence for either direct inhibition or stimulation of the epiphytes by the natural plants. The reduced epiphyte biomass on growing tips was no different from that on artificial plants of the same age and exposure and is attributable to an insufficient time for colonization rather than to inhibition by the macrophyte.

Reproductive strategies in herbaceous plant communities during succession

Abstract: Reproductive strategies of herbaceous plants were analyzed in three successional com- munities. The communities were a 1-yr old-field, 10-yr old-field, and forest. Analyses included mea- surement of community-wide reproductive effort (reproductive dry wt/total dry wt) at 3-wk intervals throughout one growing season. Reproductive effort was highest in the 1-yr field. There was essen- tially no difference in reproductive effort between plants in the 10-yr old-field and forest communities; both exhibited relatively low reproductive effort. Reproduction in the 1-yr and 10-yr fields was highly synchronous; there was little synchrony of reproduction in the forest. This may indicate a more efficient utilization of resources throughout the growing season in the forest. Analyses also included measurement of numbers of seeds per plant produced by dominant species in each community. Number of seeds per plant in the 1-yr and 10-yr fields was relatively high, while number of seeds per plant was comparatively low in the forest. The ratio of root:total biomass increased with successional age, reflecting a proportionately larger expenditure of energy on competitive structures in the forest. Also, goldenrods, dominants in the 10-yr field, were found to possess 1 characteristic of an r-strategy (i.e., large numbers of small seeds) and a characteristic of a K-strategy (i.e., low repro- ductive effort). This apparent paradox was explained by relating these characteristics to the selective pressures that might produce them. In addition, the empirical data provide support for expanded concepts of reproductive strategies.

Contribution of overwintering leaves to the growth of three broad-leaved, evergreen shrubs belonging to the Ericaceae family

Abstract: Individual leaves of three bog ericads, leatherleaf (Chamaedaphne calyculata), bog laurel (Kalmia polifolia), and Labrador tea (Ledum groenlandicum), were retained for a maximum of two growing seasons in a peat bog in southern Ontario. The premature loss of mature leaves, resulting from artificial defoliation, significantly reduced the growth of new shoots of L. groenlandicum and K. polifolia but not of C. calyculata. Defoliation effects were directly proportional to the normal retention time for overwintering leaves. Mature leaves probably translocate photosynthate, nitrogen, and phosphorus to other plant parts. This would explain why leaf dry weights were greatest at the start, rather than at the end, of the leaves' second growing season. Net photosynthetic rates decreased with leafage, but in terms of leaf nitrogen content, new and old leaves fixed equal amounts of carbon.

Nitrogen budget of a shortgrass prairie ecosystem.

Abstract: A N budget is presented for a shortgrass prairie ecosystem. The grassland was ungrazed by domestic herbivores. The quantities of N in various plant, animal, microorganism, and soil components of the ecosystem are estimated for the date when aboveground living biomass was at its maximum for the growing season of 1973. Annual transfers of N between the various compartments were also estimated.

Causes of decreases in productivity in undisturbed tall grass prairie

Abstract: There is abundant evidence that protection of the tall grass prairie from grazing, mowing, or burning for even a few years results in accumulation of much litter and reduced productivity. This reduced productivity has been attributed primarily to either the insulating effect of the litter, a lowered rate of mineral recycling, or a presence of toxins in the litter. Considerable evidence exists in the literature concerning the first suggestion, but no studies have been reported concerning the last two in grasslands. A comprehensive project was conducted during the growing season of 1977 in a tall grass prairie in Oklahoma to investigate the three suggestions. It was clear from our data that increased productivity as a result of mowing and burning was not due to the elimination of toxins nor to an increased rate of turnover of organic matter. Soil moisture stress was greater in the mowed and burned plots than in the controls. Thus, the single factor examined which was consistently different, and likely to explain the increased productivity, was the higher soil temperature in the burned and mowed plots early in the growing season. The four dominant grasses in the study plots are non-Festucoid species which are known to have high optimum temperatures for growth. SEVERAL investigators have reported that protection of the tall grass prairie from grazing, mowing, or burning for even a few years results in an accumulation of considerable litter and reduced productivity (Weaver and Fitzpatrick, 1934; Weaver and Rowland, 1952; Kelting, 1954; Kucera and Koelling, 1964). Mechanical removal of prairie litter by clipping or mowing usually results in increased productivity and increased numbers of flowering culms (Curtis and Partch, 1950; Penfound, 1964b; Hulbert, 1969; Richards, 1969). Many investigators in many parts of the tall grass prairie also have reported increases in dry matter production as the result of burning (Curtis and Partch, 1948; Kelting, 1957; Kucera and Ehrenreich, 1962; Ehrenreich and Aikman, 1963; Hadley and Kieckhefer, 1963; Kucera and Koelling, 1964; Kucera and Dahlman, 1968; Hulbert, 1969; Richards, 1969). Many of these reported marked increases in flowering are also due to burning. There have been three basic reasons suggested for the decrease in productivity of an undisturbed prairie, and a fourth implied. Several investigators have suggested that the standing plant remains and litter serve as an insulation which keeps the ground cooler until later in the spring, thus shortening the growing season (Penfound and Kelting, 1950; Weaver and Rowland, 1952; Ehrenreich and Aikman, 1963; Hulbert, 1969; Peet, Anderson, and Adams, 1975). Others 1 Received for publication 24 April 1978; revision accepted 21 July 1978. have suggested that the accumulation of litter may result in a decreased rate of mineral cycling (Daubenmire, 1968). It has been reported that changes in numbers of soil microorganisms occur after burning of forests and other woody areas, but there are no such reports for the tall grass prairie (Ahlgren, 1974). Vogl (1974) and Peet et al. (1975) suggested that the litter may contain toxins which leach out and inhibit growth of the prairie plants. Some investigators have implied that the primary effect of the heavy litter layer is that it serves as a mechanical barrier to seedlings and culms of the grasses and decreases light energy which reaches them (Weaver and Fitzpatrick, 1934; Penfound, 1964a). There is considerable evidence in the literature to support the first suggestion concerning the insulating effect of the litter, but virtually no research has been done in grasslands to evaluate the other suggestions (Daubenmire, 1968; Ahlgren, 1974; Vogl, 1974). We decided, therefore, to investigate the three basic suggestions-insulating effect, lowering of rate of mineral recycling, production of toxins-during the same growing season in a tall grass prairie in Oklahoma. MATERIALS AND METHODS-Nine 3 x 3-m plots with 1-m borders were measured and staked in October 1976 in the protected tall grass prairie section of the University of Oklahoma Grasslands Investigations Plots. These plots are located approximately 12 km southwest of Norman along State Highway 9 in McClain

Productivity and production efficiency of an upper sonoran desert ephemeral community

Abstract: During years with good winter rainfall, ephemeral plants can contribute considerably to primary production in the upper Sonoran Desert. This study was designed to compare ephemeral community productivity and production efficiency within the different microhabitats created by trees, shrubs and open spaces in a Sonoran Desert ecosystem. Ephemerals were periodically harvested in the different habitats and dried for biomass and caloric determinations. The caloric data were compared to total solar input into the different microhabitats in order to determine production efficiencies of each stand. Productivity rates were also determined for the total community for the sample period, winters of 1972-74. Both productivity and production efficiency were highest under the shade of Cercidium. The highest production efficiency occurring in the shaded sites was 5.03% which approaches the theoretical maximum photosynthetic efficiency level. The production efficiency of the entire ephemeral community for the whole growing season was only 0.17% during an optimal year, demonstrating the influence of the environment in the interspaces on productivity and solar conversion in the desert. THE DESERT ecosystem is often considered to be bleak and unproductive when compared to more mesic or agricultural systems. This judgment is quite true if one measures productivity during long periods of drought; however, moderate changes in rainfall enhance desert productivity, while long periods of moisture input dramatically increase productivity of all desert plants. In the Sonoran Desert the "normal" periods of rainfall follow a bimodal pattern, occurring as convective thundershowers in the summer and long cyclonic storms in the winter. In the upper Sonoran Desert in central Arizona, the winter storms are the primary stimulant of primary productivity, much of it in the form of winter ephemeral plant growth. Although all desert plants respond to moisture input, the ephemerals, composed primarily of annual plants, are triggered by rainfall (Went, 1949) and are usually productive only for a very short period of time (Beatley, 1967). Production of desert annual plants has been shown to be closely related to amounts of precipitation (Norton, 1974).

Alfalfa Water Use and Production on Dryland and Irrigated Sandy Loam1

Abstract: Field studies were conducted during a 4-year period to determine “Vernal“ alfalfa (Medicago satva L.) dry matter yield in response to irrigation and fertilization variables. In the Northern Great Plants alfalfa forage production contributes significantly to overall agricultural land use; consequently optimization of resources use is desirable. Limited available information has created a need for such research. This study was carried out on a Maddock sandy loam soil, a member of the sandy, mixed frigid Udorthentic Haploboralls. Four irrigation levels, ranging from dryland to excessive irrigation, were established as whole plots in a randomized block, split plot design. Eight different fertilizer treatments of P, K, and S were applied to split plots during the 1st year of the study. Under a three harvest per season management system dry matter yield was significantly affected by harvest number and irrigation treatment each year. Within the dryland and deficient irrigation treatment by year, yields decreased with each cutting; with optimum and excessive irrigation, yields varied inconstantly with cutting; yields increased 14 to 330% over dryland by increasing irrigation applications. Total seasonal crop water use, or evapotranspiration (ET), was determined using a water balance approach. Drainage (DRAIN) was calculated, using flux gradients calculated from tensiometer data, and appropriate soil properties. Other inputs, including irrigation (IRR), growing season precipitation (PPT), and soil water depletion (DEPL), were measured. In addition, water use efficiency (WUE) was calculated by treatment by year. Multiple linear regression analyses were used to relate actual dry matter yield (YIELD) and relative yield (RELV) to several independent variables. When plant water stress existed throughout the growing season, RELY correlated with relative evapotranspiration (RELET) in a near perfect, linear manner. Under nonstress conditions IRR, PPT, and DEPL required consideration to attain nearly perfect correlation of RELY with RELET. The results of this study indicate that under plant water stress conditions, alfalfa dry matter yield is a linear function of plant water use.

Biological changes in northern spruce forest soil after clear-cutting.

Abstract: In northern Finland, reforestation has frequently failed in clear-cut areas of coniferous forests. These failures prompted an investigation of the changes in the invertebrate fauna and in the bacterial populations of such forest soils after clear-cutting. Clear-cut areas in four neighbouring sites of various ages were sampled monthly, and the results were compared with those for samples taken simultaneously from a control site in an untreated spruce stand. After clear-cutting, the total biomass of the invertebrate fauna showed a strong increase, mainly due to an increase of the Enchytraeidae populations. The biomass reached its maximum after 7 yr and returned to the original level by year 13 after clear-cutting. Bacterial counts showed the same pattern as the faunal biomass; the numbers at first increased after clear-cutting, then regressed, and approached the control value after 13 yr. Cellulose degradation and soil respiration behaved in the same way. Bacterial numbers showed a significant seasonal variation at all sites, including the control; counts were maximal after the snow melt in June and decreased during the growing season.

Plant and Soil Water Potentials Following Fire in a Northern Mixed Grassland

Abstract: Highlight: Leaf water potential, osmotic potential, and soil moisture were measured in mixed grassland during the growing season following an October fire. Plant and soil water stress increased near the end of the May-August period in both burned and control sites. Potentials in nothern wheatgrass and junegrass become lower in burned than in control plots as the growing season progressed. Increased plant water stress was judged sufficient to account for the reductions in productivity which have been observed after fire.

Nitrogen transformations in soil-plant systems in three years of field experiments using tracer and non-tracer methods on an ammonium-fixing soil

Abstract: Three years of field experiments showed the interplay of plant uptake of N, N movement, denitrification, fixation of fertilizer NH4+ and its release, and N mineralization in soil–plant systems. The N uptake by barley (Hordeum vulgare L.), averaged over the growing season, ranged between 0.97 and 2.02 kg N/ha/day and the rate depended on initial extractable inorganic N in the soil, and form and timing of N fertilization. The net mineralization rate of this soil, averaged over the growing season, ranged between 0.16 and 1.80 kg N/ha/day and varied with year and N fertilization practices. However, detailed monitoring of plant uptake showed that a maximum rate of uptake occurred early in its growth, decreasing to a negligible rate later in the season. The N mineralization rate was more uniform over the growing season. A pool of inorganic N in the soil at seeding or within the first half of the growing season overcame the seasonal deficit in N supply and resulted in increased crop growth and/or N uptake. Ferti...

The Establishment of Bracken Following Fire in Tropical Habitats

Abstract: dominant weed is well recognized in many vegetation types throughout the world (see Braid, 1959, for a review). It has been demonstrated that this dominance is a consequence of the fern's rapid extension of underground stems and abundant vegetative reproduction (Watt, 1943, 1947), its strong allelopathic potential (Gliessman & Muller, 1972), and its resistance to fire (Vogl, 1964). Very little attention has been directed to spore germination, gametophyte formation, and sporeling establishment in relation to the dominance of Bracken. Bracken is capable of producing large numbers of spores on each frond, and the time of spore release can extend through much of the growing season (Conway, 1957). In temperate regions, however, most spore dispersal takes place during the drier part of the year or just before the winter begins, times of the year that do not particularly favor sporeling establishment. This would explain, at least in part, reports of the small number of Bracken sporelings which become established under natural field conditions (Conway, 1953). In the tropics, where conditions of temperature and humidity are much more equable, such climatic control of spore germination and early growth presumably is less important. I have observed continual growth of new Bracken fronds throughout the year in several locations in Costa Rica (Gliessman, 1976). It is possible, then, that spore release is not restricted to a certain period, as it is in temperate regions (Conway, 1957), but may be much more haphazard over the entire year. Thus, a constant source of spores could be available for any potentially habitable area. Observations were made in the field in Costa Rica to determine the conditions under which Bracken sporelings become established, which has possible implications for Bracken control. In Costa Rica, Bracken is encountered frequently from just above sea level on well-drained soils up to more than 3000 m elevation. As in other areas of the tropics (Richards, 1966, pp. 391-399), Bracken most often forms a type of deflected succession in regions formerly covered with dense, tropical forest. These are areas that have an annual rainfall in excess of 2500 mm, the majority of which is concentrated in the wet season that extends from mid-May to late December. Due to frequent cloud cover (especially at higher elevations) and the occurrence of sporadic rainfall even in the dry season, humidity at the soil level is quite favorable for sporeling establishment all year around. The pattern of land use in this part of the tropics appears to lend itself quite well to the establishment of Bracken. Most forest clearing, using the well known system of "slash and burn," takes place towards the end of the wet season (December) until late in the dry season (late April). The felled material is allowed to dry as much as possible. Before the more frequent rainfall begins in early May, the slash is burned. As a consequence, when the wet season rains begin in earnest,

Seasonal Changes in Reflectance of Two Wheat Cultivars1

Abstract: Accurate crop identification from remote sensing signals is dependent on a knowledge of differences in characteristics of reflectance from various crop canopies. The objective of the present work was to follow changes in the reflectance of two dissimilar wheat [Triticum aestivum (L.)] cultivars through a growing season to determine which reflectance characteristics were suitable for use in the analysis and interpretation of spectral reflectance curves. Reflectance over the wavelength interval of 0.45 to 2.50 μm was measured with a ground-based spectroradiometer on nine cloud-free days between planting and maturity of wheat planted on Hidalgo sandy clay loam (Typic Calculstols) soil. Seeding rate affected the rate of ground cover, but not the spectral reflectance of either a winter or a spring wheat once about 25% of the area was covered by vegetation. All reflectance curves had the characteristic shape for vegetated surfaces by 4 weeks after emergence. The proportion of the ground covered by plants was more important than development stages of the plants in determining spectral responses, except at the end of the season when the plants senesced and lost pigmentation. Through the season soil was much less reflective than green vegetation at 0.75, 0.90, and 1.10 μm and much more reflective at 1.65 and 2.20 μm, making these wavelengths valuable for distinguishing vegetation from soil background and for assessing vegetation cover or density.

The effects of tillage, direct drilling and nitrogen fertiliser on soil temperature under a barley crop

Abstract: Summary Continuous hourly records of soil temperature were collected at 1, 5 and 20 cm, throughout two growing seasons, 1973 and 1974, under crops of spring barley in the east of Scotland. Measurements were obtained from three cultivation treatments, deep ploughing, normal ploughing and direct drilling, at two nitrogen fertiliser levels, 0 and 150kg/ha. Compared with ploughed soil, direct-drilled soil had a higher surface reflection coefficient and a higher thermal diffusivity between 5 and 20 cm; overall, this resulted in lower heat sums (°C hours/day over 5 °C and 10 °C) in direct-drilled soil at 1 cm and 5 cm during the first 20 days after sowing. The thermal regimes of normally and deeply ploughed plots were very similar. Nitrogen fertiliser caused significant lowering of soil temperature at all three depths in the latter half of the growing season, due to increased shading of the soil surface by the crop canopy. Some implications of these findings for cultivations for cereals are discussed.

Biophysics of a natural grassland: evaporation

Abstract: SUMMARY (1) Microclimatic measurements and flux determinations were made on natural Agropyron-Koeleria grassland during two growing seasons. (2) The calculated evaporation fluxes were analyzed in terms of the potential evaporation, soil moisture content, and plant water potential. The analysis fitted Monteith's canopyresistance concept. (3) The fraction of net radiation involved in evaporation was closely correlated with green leaf area index during the first half of the growing season, and with soil water content (0-30 cm) during the latter half of the growing season. (4) Seasonal water-use estimates, by micrometeorological and soil moisture balance approaches, agreed to within 10%.

A Latitudinal Gradient of Wood and Litter Production, and Its Implication Regarding Competition and Species Diversity in Trees

Abstract: Rates of wood and litter production in mesic forests along a latitudinal gradient are surveyed, and hypotheses concerning the eco- logical and evolutionary implications of emergent patterns are presented. Wood production is relatively constant along a latitudinal gradient, whereas total amount of light available during the growing season de- creases with increasing latitude. Therefore, efficiency of wood production is higher at temperate latitudes. We hypothesize that the higher wood production efficiencies at temperate latitudes came about as a result of selection for tree species with wood growth rates that are high during the growing season. Since rapid wood growth provides a competitive advan- tage in trees, we hypothesize that competition among trees is greater at higher latitudes. High efficiency of wood production apparently is a prerequisite for survival of forest trees at high latitudes. Only those tree taxa that had high wood production efficiency, or were able to evolve a high efficiency, survived when the primarily tropical or subtropical environment at higher latitudes turned much colder during the late Tertiary. This situation contributed to the present low diversity of trees at high latitudes relative to that at lower latitudes.

A Comparison of the Radiation and Energy Balance during the Growing Season for an Arctic and Alpine Tundra

Abstract: For arctic tundra at Barrow, Alaska, and alpine tundra on Niwot Ridge in the Front Range of Colorado, the components of the radiation and energy balance for the growing season from June through August are examined. Where sufficient data are available, the radiation fluxes are also compared for the other seasons. There is a remarkable similarity in the mean daily totals of net radiation for the growing season. Insolation receipt is greater in the alpine tundra but the net longwave radiative loss is also greater so that the two effects nearly compensate each other. The difference in net radiation between the two sites is only 3% of the daily total. When hourly data are examined, however, maximum intensities and diurnal ranges are considerably greater in the Alpine than in the Arctic. Differences in the energy balance regimes are apparent both in the daily totals and hourly fluxes. At Barrow there is generally little or no moisture stress due to a continual standing water supply. Most of the available radiant energy at the surface is used in the latent heat flux rather than the enthalpy flux.

A comparison of the radiation and energy balance during the growing season

Abstract: For arctic tundra at Barrow, Alaska, and alpine tundra on Niwot Ridge in the Front Range of Colorado, the components of the radiation and energy balance for the growing season from June through August are examined. Where sufficient data are available, the radiation fluxes are also compared for the other seasons. There is a remarkable similarity in the mean daily totals of net radiation for the growing season. Insolation receipt is greater in the alpine tundra but the net longwave radiative loss is also greater so that the two effects nearly compensate each other. The difference in net radiation between the two sites is only 3% of the daily total. When hourly data are examined, however, maximum intensities and diurnal ranges are considerably greater in the Alpine than in the Arctic. Differences in the energy balance regimes are apparent both in the daily totals and hourly fluxes. At Barrow there is generally little or no moisture stress due to a continual standing water supply. Most of the available radiant energy at the surface is used in the latent heat flux rather than the enthalpy flux.

Fertilization decision model – a synthesis of soil and plant parameters in a computerized program

Abstract: A fertilization decision model is suggested, based on soil and plant parameters. The optimum concentration range of nitrate nitrogen in the soil solution, during the uptake period of the growing plant, is 50–250 ppm N-NO3 for all soils and plants. The optimum bicarbonate-soluble P varies with plant and soil type, and is determined by response experiments. Percent recovery of added fertilizer by bicarbonate extraction is used to calculate the amount of fertilizer needed to raise the extraction value to the optimum level. Exchangeable potassium is considered to be the only source important during the growing season of an annual crop. The main plant parameters are the uptake rate of nutrients during the growing season, depth of root penetration, and sensitivity to ammonium. The model calculates, at preset time intervals, the balance between the quantity of nutrients in the root zone and uptake by plants. A decision to add fertilizer is made if a deviation from the optimum exceeds a predetermined value.

Method for analyzing seasonal growing conditions of crops

Abstract: This method uses electronic measurement of color values in aerial infrared photographic transparencies to analyze the growing condition of a crop. A series of infrared photographs are taken during the growing season, and average color values from the resulting image are plotted and compared to optimum growing conditions. Electronic measurements are made of color densities about the total field area as represented on each photographic image. Each measurement is corrected to eliminate exposure variations in the images due to photographic factors, weather factors, lighting and film conditions, and the date and time at which the photograph was taken.

Seasonality in New Zealand Plants

Abstract: Summary Leaf and shoot flush, leaf-fall and cambial gro~vth in ~voody plants, seasonal begetative growth in herbaceous plants, flowering, fruiting, and periodicity are discussed. Although information is somewhat fragmentary, it appears that it? the lowlands most aspects of seasonal behaviour are less well defined and morc protracted than in regions 1%-ith strongly seasonal climates. .It high altitudes, short growing seasons impose a more rigid phenological cycle on most species.

Diurnal and Seasonal Changes in Water Balance of Acer saccharum and Betula papyrifera

Abstract: Diurnal and seasonal changes in plant water potential, leaf diffusion resistance, and stem radial changes of Acer saccharum and Betula papyrifera trees were studied in northern Wisconsin during the 1974 and 1975 growing seasons. Water potential decreased during the day, following relatively high values in the morning, and increased in the late afternoon and evening. Diurnal patterns and actual values of water potential varied with species, soil water availability, and factors influencing transpiration (e.g., solar radiation, vapor pressure deficit, and transpiration flux density). When plant water deficits were not severe, leaf resistance of both species was rather stable during the day. During severe droughts, however, leaf resistance increased (stomata closed) during the day when light intensity was high. Leaf resistance at high light intensity was higher in Acer than in Betula. Stomatal closure with decreasing light intensity varied between species and among Acer trees. Tree stems of both species shrank during the day, as internal water deficits developed, and they expanded as trees rehydrated during the night. Stems of Acer shrank more than those of Betula. The amount of daily stem shrinkage increased as the season progressed if the trees were not under severe water deficits. During severe droughts the amount of diurnal stem shrinkage decreased. Shrinkage of stems lagged behind water potential changes by 1 to 2 h in Acer and less than 1 h in Betula. The relationship between stem radius and leaf water potential was not constant throughout the growing season.

The interactions of water and fertilizer nitrogen in effects on growth pattern and yield of potatoes

Abstract: Three field experiments in 2 years were made in S.E. Scotland to investigate the effect of water (normal rainfall or irrigation to maintain soil moisture tension between pF 2·0 and 2·5) on the response of the potato crop to nitrogen fertilizer (0, 100, 200 or 300 kg N/ha).Final tuber yield was closely related to leaf area duration. Addition of water reduced the delay in early haulm growth resulting from fertilizer nitrogen application. The beneficial effect of reducing moisture tension could not be related to water per se, nitrate, ammonium or conductivity levels in the fertilizer band or nitrogen uptake. Accumulation of nitrate in the roots and stems + petioles early in the season indicated that the rate of protein synthesis was reduced at high soil moisture tension in the presence of fertilizer nitrogen.In a long growing season (1971, 153 days) there was a significant response of tuber yield to nitrogen, with an increase in yield even at the highest rate tested (300 kg N/ha) and irrigation enhanced this response.In a normal season (1970, ca.130 days) there was a significant irrigation x nitrogen interaction on tuber yield. Irrigation increased the optimum rate of nitrogen from 100 to 200 kg N/ha in one experiment and from nil to 200 kg N/ha in another.Addition of water increased the number of tubers at harvest but the effect of nitrogen was less consistent. Yield increases resulting from application of nitrogen or reduction in soil moisture tension were mainly due to increases in the proportion of large tubers.

Seasonal dynamics of carbon dioxide exchange in a mixed grassland ecosystem

Abstract: Carbon dioxide exchange of a mixed grassland sward dominated by Agropyron dasystachyum and A. smithii was measured over two growing seasons using field chambers and infrared gas analysis. Net photosynthesis of the sward began in late April and rose to a peak in early July ( ground area h−1). Drought during the later part of the summer, a typical feature of the climate of the region, caused net photosynthesis to decline drastically. Air temperature and reduced green leaf area inhibited photosynthesis early and late in the growing season. A multiple regression analysis showed that soil moisture and air temperature together accounted for 44% of the variability in net photosynthesis. Solar radiation and dew-point temperature explained an additional 16%. The ability of the dominant grasses to adapt to fluctuations in water availability is discussed.

Evaporation from a themeda grassland

Abstract: SUMMARY nearby. (3) For short periods, following dew or rain, evaporation was controlled solely by meteorological influences; this accounted for about 20% of annual water loss. Transpiration which was potentially subject to stomatal and other control mechanisms accounted for about 80 % of water loss. (4) Soil water supply and phenology both influenced transpiration. During winter and spring, when plants were dormant, plant mechanisms imposed a large resistance to vapour transport, with as little as 30% of monthly net radiation being used for evaporation. This proportion was even less for short periods when soil water supply was deficient. In the summer growing season about half the net radiation was used for evaporation, when soil moisture levels were adequate, but this fraction decreased as soil moisture was depleted and demand was large.

Water-Table Depth and Irrigation Effects on Applied-Water-Use Efficiencies of Three Crops

Abstract: INFORMATION is needed on crop response to com-binations of water-table depths and irrigation treat-ment to properly design irrigation and drainage systems. A 3-yr field experiment was conducted on a Hecla sandy loam soil with three water-table depths (treatments)— shallow, medium, and deep—and with four weekly sprinkler irrigation application amounts from 0 (precipi-tation only) to 1.5 times calculated irrigation require-ment. Production of corn (grain and total dry matter), sugarbeets (sucrose), and alfalfa, and applied-water-use efficiencies (AWUE) were highest from the shallow water-table treatment without irrigation. Plant yield responses from deeper water tables increased with ir-rigation amounts but never equalled those for the shal-low water table without irrigation. The probable reason for highest AWUE in the shallow water-table treat-ment is that crop roots obtained water from the slowly receding water table during the growing season. Deep percolation losses seemed to increase with irrigation amounts and were greatest for the shallow water table. These results challenge the agricultural industry to de-sign drainage and irrigation systems to fit the needs of the crop.

A growth model for Trifolium subterraneum L. Swards

Abstract: A simple deterministic model to simulate the time course of potential dry matter growth by subterranean clover swards in the field is described. Relationships used in the model were obtained mainly from experiments in temperature-controlled glasshouses and from measurements of rate of carbon dioxide exchange in an assimilation chamber. Canopy carbon dioxide exchange rates in the light and in the dark are calculated in the model from leaf area index, total dry matter, air temperature, irradiance and the crop growth rate of the sward. Photosynthates are distributed among different parts of plants according to empirical relationships. The model can estimate the potential dry matter growth of swards grown at different levels of irradiance and at different temperatures. Dry matter yield of a crop growing in the field without limitation of water and mineral nutrients can be predicted to within 20% for 100 days of growth. Potential dry matter yield of pure subterranean clover swards at Adelaide is predicted by the model to be strongly influenced by the time of cessation of growth. If the growth is terminated in the middle of October, an early start to growth as well as a high plant density will be advantageous for a high final yield. On the one hand, if the growing season extends until late November, there will be only a small effect of time of commencement of growth on final yield. The model suggests that leaf area index is an important determinant of dry matter production up to about 200 g m-2, and that increased maintenance respiration at a dry matter yield above about 600 g m-2 results in a decreased growth rate. The effects of variation in irradiance and temperature on dry matter production at different growth stages are assessed. It is concluded from use of the model that the effects of temperature on crop growth rate depend on the amount of dry matter present and on the level of solar radiation.