Showing papers on "Growing season published in 1991"

Leaf age and season influence the relationships between leaf nitrogen, leaf mass per area and photosynthesis in maple and oak trees

Abstract: . Seasonal changes in photosynthesis, leaf nitrogen (N) contents and leaf mass per area (LMA) were observed over three growing seasons in open-grown sun-lit leaves of red maple (Acer rubrum), sugar maple (A. sacchamm) and northern pin oak (Quereus ellipsoidalis) trees in southern Wisconsin. Net photosynthesis and leaf N were highly linearly correlated on both mass and area bases within all species from late spring until leaf senescence in fall. Very early in the growing season leaves had high N concentrations, but low photosynthetic rates per unit leaf N, suggesting that leaves were not fully functionally developed at that time. Leaf N per unit area and LMA had nonparallel seasonal patterns, resulting in differing relationships between leaf N/area and LMA in the “early versus late growing season. As a result of differences in seasonal patterns between leaf N/area and LMA, net photosynthesis/area was higher for a given LMA in the spring than fall, and the overall relationships between these two parameters were poor.

Positive plant interactions in tundra vegetation and the importance of shelter.

Abstract: (1) The hypothesis that positive plant interactions, arising from species aggregations, are important in harsh environments was tested on the sedge Carex bigelowii in the alpine/subarctic tundra of Swedish Lapland. (2) Association analyses showed that, in contrast to records for the High Arctic, aggregations of the low-growing species ( 2 cm away from the dwarf shrubs or outside the moss mat. (3) The effect of the dwarf shrubs was interpreted as a result of above-ground shelter because shoot height, as well as number of shoots, also increased when plywood shelters were erected on the windward side of C. bigelowii. The effects of this above-ground shelter on shoot growth could result from increased snow cover, protection against strong desiccating winds early in the growing season, and a warmer micro-climate. (4) An alternative explanation for the increased growth of C. bigelowii growing near other species is that their patchy distributions, particularly those of the dwarf shrubs, may be related to localized increased soil nutrient status. When fertilizer was added to C. bigelowii, there were significant increases in shoot height and number. The benefits to the growth of C. bigelowii when growing close to other species may be a combination of shelter effects and a patchy distribution of nutrients.

Mean and inter-year variation of growing-season normalized difference vegetation index for the Sahel 1981-1989

Abstract: Images are presented that show the mean and coefficient of variation of nine years (1981-1989) of NOAA AVHRR normalized difference vegetation index (NDVI) data for the growing season (July-October) in Africa, north of the equator. The variation in the growing season NDVI is represented by the coefficient of variation image that shows the large variation in the Sahelian growing season between years. It is concluded that these images illustrate some aspects of the perspective being brought to regional and continental scale processes by coarse resolution satellite sensors and the potential of these sensors to provide consistent, long-term datasets.

Seasonal carbon isotope discrimination in a grassland community

Abstract: Grassland communities of arid western North America are often characterized by a seasonal increase in ambient temperature and evaporative demand and a corresponding decline in soil moisture availability. As the environment changes, particular species could respond differently, which should be reflected in a number of physiological processes. Carbon isotope discrimination varies during photosynthetic activity as a function of both stomatal aperture and the biochemistry of the fixation process, and provides an integrated measure of plant response to seasonal changes in the environment. We measured the seasonal course of carbon isotope discrimination in 42 grassland species to evaluate changes in gas exchange processes in response to these varying environmental factors. The seasonal courses were then used to identify community-wide patterns associated with life form, with phenology and with differences between grasses and forbs. Significant differences were detected in the following comparisons: (1) Carbon isotope discrimination decreased throughout the growing season; (2) perennial species discriminated less than annual species; (3) grasses discriminated less than forbs; and (4) early flowering species discriminated more than the later flowering ones. These comparisons suggested that (1) species active only during the initial, less stressful months of the growing season used water less efficiently, and (2) that physiological responses increasing the ratio of carbon fixed to water lost were common in these grassland species, and were correlated with the increase in evaporative demand and the decrease in soil moisture.

Ecophysiological differences among juvenile and reproductive plants of several woody species.

Abstract: Photosynthetic and water relations characteristics of small juvenile and large reproductive plants were investigated during one growing season for four woody species native to Red Butte Canyon, Utah, USA: Acer negundo, Artemisia tridentata, Chrysothamnus nauseosus, and Salix exigua. For all species, juvenile plants differed from reproductive plants in at least one of the following characters: water potential, stomatal conductance, photosynthetic rate, or water-use efficiency. Late in the growing season, mortality occurred within juvenile plants (apparently due to a lack of water), but not within reproductive plants. The observed differences between juvenile and reproductive classes are discussed in terms of environment, development, and mortality selection.

Canopy photosynthesis of crops and native plant communities exposed to long‐term elevated CO2

Abstract: . There have been seven studies of canopy photosynthesis of plants grown in elevated atmospheric CO2: three of seed crops, two of forage crops and two of native plant ecosystems. Growth in elevated CO2 increased canopy photosynthesis in all cases. The relative effect of CO2 was correlated with increasing temperature: the least stimulation occurred in tundra vegetation grown at an average temperature near 10°C and the greatest in rice grown at 43°C. In soybean, effects of CO2 were greater during leaf expansion and pod fill than at other stages of crop maturation. In the longest running experiment with elevated CO2 treatment to date, monospecific stands of a C3 sedge, Scirpus olneyi (Grey), and a C4 grass, Spartina patens (Ait.) Muhl., have been exposed to twice normal ambient CO2 concentrations for four growing seasons, in open top chambers on a Chesapeake Bay salt marsh. Net ecosystem CO2 exchange per unit green biomass (NCEb) increased by an average of 48% throughout the growing season of 1988, the second year of treatment. Elevated CO2 increased net ecosystem carbon assimilation by 88% in the Scirpus olneyi community and 40% in the Spartina patens community.

Regional hydrologic and carbon balance responses of forests resulting from potential climate change

Abstract: The projected response of coniferous forests to a climatic change scenario of doubled atmospheric CO2, air temperature of +4 °C, and +10% precipitation was studied using a computer simulation model of forest ecosystem processes. A topographically complex forested region of Montana was simulated to study regional climate change induced forest responses. In general, increases of 10–20% in LAI, and 20–30% in evapotranspiration (ET) and photosynthesis (PSN) were projected. Snowpack duration decreased by 19–69 days depending on location, and growing season length increased proportionally. However, hydrologic outflow, primarily fed by snowmelt in this region, was projected to decrease by as much as 30%, which could virtually dry up rivers and irrigation water in the future.

Long-Term Dynamics of Standing Crop and Species Composition after the Cessation of Fertilizer Application to Mown Grassland

Abstract: (1) Current agricultural overproduction in Western Europe has led to an increase in the area of unfertilized grassland. This paper reports an experiment where fertilizer application was stopped to try to restore former species-rich vegetation. (2) Standing crop and species composition of three different fields were studied during the first 14 years after cessation of fertilizer application. In two fields on peaty soil a strong decrease in standing crop (from 800 to 300g m-2) was observed but in a field on sandy soil, the initial production of 300g m-2 did not decrease. The deviations from the mean between years were significantly correlated to the potential water surplus in the growing season in this latter field, but not for the two fields on peaty soil. The species composition changed gradually in all fields. (3) In the two fields with decreasing standing crop, species-richness per field and per 4-m2 plot increased markedly during the 14 years. A maximum of forty species was observed after 8 years in the field on sandy soil. (4) Several species reached their maximum cover during these successions. The dynamics of species replacement was described using a Gaussian response model for each species. Species indicating nutrient-poor conditions entered earlier in the successional sequence with cutting twice a year and in the field on sandy soil. The results are discussed with regard to diversity-productivity relationships, nutrient cycling and adaptations of species to nutrient-poor conditions.

Host‐pathogen interactions in natural populations of linum marginale and melampsora lini: i. patterns of resistance and racial variation in a large host population

Abstract: Populations of wild flax, Linum marginale and its associated rust fungus Melampsora lini growing at Kiandra, New South Wales, Australia, were sampled during the 1986-1987 growing season. Thirteen different races of M. lini were detected in a sample of 96 isolates. The distribution of isolates was uneven: race A comprised 73% of the samples; race N, 8%; and race H, 5%; while the remaining races were represented by only one or two samples. The dominance of race A increased over the course of the growing season, comprising 67% of the early season samples and increasing to 78% for those collected late in the season. The overall diversity of the pathogen population decreased late in the growing season, but this trend was not statistically significant. The average virulence of individual isolates of the pathogen population increased during the growing season. This trend was most pronounced among the minor races, where the mean number of differential hosts infected increased from 4.58 for early season samples to 5.12 and 5.08 for mid and late season samples, respectively. In contrast to the virulence pattern in the pathogen, the L. marginale population displayed a more even distribution of resistance. In a sample of 67 plants 10 resistance phenotypes were detected from their pattern of resistance/susceptibility to seven pathogen isolates. No phenotype had a frequency that exceeded 30%. Resistance phenotypes were randomly distributed on both a population level and on a fine scale.

Water holding capacity of litter and soil organic matter in mixed prairie and fescue grassland ecosystems of Alberta.

Abstract: Litter and organic matter accumuiations can reduce soil water through interception of precipitation and subsequent evaporation of absorbed water. Interception varies with mass and water hoiding capacity (WHC) of litter and organic matter, and is highest from small precipitation events. WHC varies with vegetation type, which is affected by grazing regime. Thus long-term grazing could affect WHC of litter and organic matter and would be important in the hydrologic assessment of rangelands subjected to many small precipitation events throughout the growing season. The study was conducted in mixed prairie, parkland fescue, and foothills fescue grasslands in Alberta, Canada. Grazing regimes were of light to very heavy intensities, grazed early, Me, and continuously during the growing season. Litter and organic matter were sorted by sieving into various sized categories. Litter-soil cores were also evaluated. WHC of litter and organic matter was lower in mixed prairie than in fescue grasslands. WHC increased with increazed particle size, being higher for roots and standing and fallen litter than for organic matter. WHC of iarge particle-sized material decreased with heavy intensity and/or early season grazing. WHC was affected more by intensity than season of grazing. Grazing affected WHC through species composition changes, since species have different WHC, and through trampling which affected particle size. It was concluded that litter and organic matter WHC were important in rangeland hydrologic assessments.

Duration of snow cover and its influence on life-history variation in yellow-bellied marmots

Abstract: We evaluated the relationship between growing-season phenology, as indicated by time of snow melt, and intrapopulation variation in reproduction and growth of yellow-bellied marmots. The time of snow melt explained significant proportions of the variation in frequency of reproduction (78%), litter size (79%), and estimated body mass of young of the year (68%), but not growth rate. We suggest that the duration of snow cover, through its effect on the length of the growing season, influences habitat quality; marmots living at localities with prolonged snow cover have a shorter season of access to food. The variation in life-history traits is attributed to phenotypic plasticity and not to local genetic variation.

Atmospheric CO_2 Concentrations Within a Mixed Forest: Implications for Seedling Growth

Abstract: We studied carbon dioxide concentrations in a mixed deciduous forest in New England, USA by making continuous measurements at 0.05, 0.2, 3, and 12 m above the soil surface. The measurements began in early March and continued until the end of November 1985; therefore, they spanned the growing season and parts of the dormant seasons both before and afterwards. The data were compared with those from Mauna Loa, Hawaii, which represent global atmospheric CO2 levels in the Northern Hemisphere. The results show strong seasonal and daily variations in CO2 concentrations at all heights in the forest. On average, during the growing season, CO2 levels were generally higher in the forest than in bulk air at Mauna Loa. The highest level of CO2 was found near the forest floor and the lowest at the 12-m level. Daily levels of CO2 were constant throughout the day in the dormant season and were the same for all heights in the forest. However, during the growing season, the CO2 concentrations were lowest during the middle of the day, especially at the 12 m height. Thus, this study shows that the CO2 concentrations in the forest may be quite different than those in bulk air and that seedlings, saplings, and mature trees may experience different CO2 environments for which they may show different re- sponses in photosynthesis, growth, and water use. Moreover, a tree may experience different CO2 environments as it grows towards the canopy, and different modules of an individual may also be growing in different CO2 atmospheres.

Environmental effects on co2 efflux from water track and tussock tundra in arctic alaska, u.s.a.

Abstract: CO2 efflux and variation in soil environmental characteristics were examined in two tundra vegetation communities, water track (a small drainage of intermittent water flow) and tussock tundra, in the northern foothills of the Philip Smith Mountains in arctic Alaska Correlation analyses were performed on the observations made at six times during the growing season in order to evaluate the relationships between system CO2 loss and soil moisture, soil temperature, depth of thaw, and depth to the water table The two sites differed significantly in terms of soil moisture, soil temperature, depth of thaw, and water table depth on several of the sampling dates During four of the six measurement periods, the rate of CO2 efflux differed significantly between sites Early in the season, respiration was greater in tussock tundra than at the water track, but later in the season, rates at the water track exceeded those at the tussock site Highest rates were measured at the water track near mid-season Efflux of CO2 at both sites was positively correlated with soil temperature Soil surface (0-5 cm depth) environmental conditions were better predictors of CO2 efflux than were conditions measured at greater depth (5-10 cm) Soil moisture appeared to increase respiration between 100 and 700% of soil dry weight and decrease soil respiration at higher water contents The effects of soil moisture were stronger in tussock tundra than in the water track community These data suggest that both soil temperature and soil moisture limit C02 efflux in water track and tussock tundra communities and that the relative importance of these factors changes throughout the growing season

Estimating primary productivity of forested wetland communities in different hydrologic landscapes

Abstract: Five forested wetland sites in western Kentucky with hydrologic regimes varying from seasonally to continuously flooded were investigated for net above-ground biomass productivity (litterfall plus biomass growth) and for possible indicators of that productivity, including abiotic (flooding frequency and depth, phosphorus concentrations in water and sediments) and biotic (biomass, tree density, basal area, structural complexity, and mean height) indices. Net biomass productivity ranged from 205 g m-2y-1 for a stagnant semipermanently flooded Taxodium swamp to 1,334 g m-2y-1 in a bottomland forest along the Ohio River. Productivity was highest in wetlands with pulsing hydroperiods, intermediate with slowly flowing systems, and lowest with stagnant conditions. Surface water flooding of the wetlands during the growing season ranged from 17 to 100 percent of the year and did not predict productivity. Phosphorus concentrations in water and in sediments were not correlated to one another and did not, by themselves, predict productivity. No single abiotic variable predicted the exact ranking of productivity of the sites. Of the biotic variables, average tree diameter was inversely related to productivity.

Grazing impacts on soil water in mixed prairie and fescue grassland ecosystems of Alberta

Abstract: Reduced soil water under grazing is generally attributed to reduced infiltration as livestock trampling compacts the soil surface. Grazing can also have the opposite effect on soil water through reduced evapotranspiration when vegetation is removed. On the Canadian Prairies, grazing impacts on soil water have been assessed in short-term studies but impacts of long-term grazing have not been documented. In this study, impacts of long-term grazing on soil water were assessed in mixed prairie, parkland fescue grassland, and foothills fescue grassland ecosystems of southern and central Alberta. Grazing regimes were of light to very heavy intensities, grazed early, late, and continuously during the growing season. Soil water was measured with a neutron probe to a depth of 1 m from April through October over three growing seasons. Normal patterns of soil water recharge in autumn and spring and soil water depletion in summer due to evapotranspiration were not altered by grazing. Fluctuations in soil water were m...

Response of Mediterranean Grassland Species to Changing Rainfall

Abstract: (1) Long-term changes (over 14 years) were investigated in the composition of a species-rich Mediterranean grassland (`dehesa') in south-western Spain. The frequency of every species of herbaceous vascular plant was recorded annually at the end of the growing season, from 1977 to 1986 and again in 1990, in permanent plots along a transect. Monthly rainfall data over the same period were typical of the Mediterranean climate: there was virtually no rainfall in summer (June-September), when potential evapotranspiration was high; rainfall in the cooler part of the year was unpredictable in amount and distribution. (2) The vegetation was dominated by winter annuals, which germinated after the autumn rains and fruited in late spring or early summer; they comprised 86 of the 99 species recorded. Most of these species displayed considerable variations in abundance over the period of study and the abundance of some fluctuated violently from one year to another. (3) Correlations were examined between the logit of total annual frequency of each species (n = 11 years) and rainfall totals over the growing season (October-May) and its three components: autumn (October-November), winter (December-February) and spring (March-May). Evidence for trends in logit frequency that might represent successional changes was sought from similar correlations with time. (4) The abundance of many species was highly correlated with rainfall, which accounted for up to 83% of year-to-year variance (in Juncus capitatus). Rainfall over the whole growing season was a good predictor of abundance for more species than any of its component periods (25 species with significant associations out of 99). Cluster analysis resolved five groups of species with similar year-to-year variations in abundance: two groups contained most of the species showing significant positive correlations with rainfall, and another group contained a small number of generally infrequent species that showed significant negative correlations. The species in the remaining two groups had few significant correlations with rainfall. Only three moderately abundant species were significantly correlated with time. Successional change was apparently much less important than rainfall as a source of variation in plant frequency. (5) The demographic mechanisms underlying these relationships, their implications for the regulation of community structure and their possible value in detecting the consequences of climatic change in the Mediterranean region are discussed.

Effects of established perennial grasses on yields of associated annual weeds

Abstract: Perennial grasses are needed for seeding annual grasslands in the Mediterranean/maritime climatic regime of southwest Oregon. Selection of plants for reseeding purposes would be facilitated by identification of perennial grasses that, once established, are able to suppress resident annual plant production. Perennial grasses were transplanted and allowed to establish in the absence of competition for the first growing season at 2 sites in the foothills of southwest Oregon. After the first growing season, resident annual plants were allowed to reinvade. Perennial grasses such as Berber orchardgrass (Dactylis glomerata L. var. Berber) and Idaho fescue (Festuca idahoensis Elmer) that begin growth early suppressed annuals more effectively than later growing perennial grasses such as intermediate and tall wheatgrasses (Agropyron intermedium (Host.) Beauv. and A. elongatum (Host.) Beauv., respectively). Of the perennial grasses adapted to these sites, those which initiated growth earliest, maintained some growth through winter months, and matured earliest were the best competitors.

Site-specific growth and nutrition of planted Piceamariana in the Ontario Clay Belt. IV. Nitrogen loading response

Abstract: Black spruce (Piceamariana (Mill.) B.S.P.) containerized seedlings were raised at high fertilizer regimes, i.e., "nutrient loaded," during greenhouse culture to assess whether increased preplant nutrient reserves improved site-specific outplanting performance. Growing media electrical conductivity levels during the greenhouse phase ranged between 0.9–1.2 and 0.3–0.6 dS/m for loaded and nonloaded seedlings, respectively. Although similar in total biomass and P and K content at rotation end, the loaded seedlings contained 78% more N than the nonloaded seedlings, demonstrating induced luxury consumption of N from loading. After overwintering, the seedlings were planted on intact potted surface soils from three ecological site types (upland Feathermoss, lowland Alnus, and lowland Ledum) of contrasting fertility. In the first growing season, N loading significantly increased height growth and dry matter production on all substrates; relative response was higher on the more N-deficient lowland sites. Plant nutr...

The growing season.

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Mowing and Fertilization Effect on Productivity and Spectral Reflectance in Bromus Inermis Plots.

Abstract: Experiments were conducted to examine the potential role of grazing on ecosystem-level parameters as part of the NASA-sponsored First ISLSCP (International Satellite Land Surface Climatology Programme) Field Experiment (FIFE) conducted at Konza Prairie Research Natural Area in 1987. Here we report results of one experiment conducted in a field consisting primarily of Bromus inermis, a cool season C3 grass. The experiment involved four simulated grazing components (unmowed control, 20-, 10-, and 5-cm mowing heights) and fertilization (untreated control and ammonium nitrate appli- cation). The plots were mowed to ground level and raked in April, following which they were mowed seven times during the growing season from May to October. Biomass pro- duction, N production, and spectral reflectance data were collected with a hand-held ra- diometer throughout the growing season, with standing crop estimates taken at two periods (7 August (day 219) and 27 October (day 300)) to correlate with the remote sensing in- formation base. Standing crop values of mowed plots were as much as 67-70% lower than controls, but they produced significantly larger amounts of both biomass and total N. Maximum season- long production values in the mowed plots were -43% above controls, with major differ- ences developing as a result of fertilization. Fertilized plots produced 67% more foliage than unfertilized plots. Our data show over-compensatory growth as a result ofthe simulated grazing treatments. Indexes (NDVI (normalized difference vegetation index) and greenness) derived from the reflectance data were poorly correlated with biomass. The correlation of NDVI with N content of the canopy foliage was somewhat stronger, particularly if stratified by mowing class. NDVI was a better predictor of vegetation status than the greenness indexes, but in plots simulating heavily grazed areas where leafy vegetation was sparse and soil became more visible from above the canopy its utility decreased significantly.

Analysis of growth and light interception of balsam fir and white birch saplings following precommercial thinning

Abstract: during both the first and the second growing season. This increase in growth was attributed to a greater net assimilation rate (NAR) which was associated with a higher level of light availability. Thinning tended to positively affect the RGR of white birch during the first summer following treatment but not during the second growing season. Similar fluctuations were noted for NAR even though light levels remained high for thinned white birch trees during both the first and the second growing season. Balsam fir produced more sapwood per unit of additional leaf area than controls during the first summer following treatment but no differences were observed during the second. The sapwood area growth to leaf area growth ratios of thinned and control white birches were similar during both the first and the second summer following thinning. Thus the sapwood area-leaf area relationship appears to be more stable following abrupt changes in environmental conditions for the indeterminate growth species, white birch, than for the determinate growth species, balsam fir. growth analysis / thinning / net assimilation rate / light use efficiency / sapwood area-leaf area ratio

Emergence Periodicity of Six Summer Annual Weed Species

Abstract: Emergence periodicity of common pursiane, hemp sesbania, horse purslane, prickly sida, spurred anoda, and velvetleaf was determined from March to October during 5 yr at Stoneville, MS, in the absence of reseeding. In early March of each year prior to emergence determinations, plots were not tilled or tilled to depths of 0, 5, 10, or 15 cm. Yearly emergence periodicity of the six weed species was not affected by tillage. In each year, weeds of all species emerged in identifiable flushes and very few emerged at other times during the growing season. Velvetleaf emerged mainly during the early growing season from March through May. Peak periods of emergence for the other species varied slightly over the 5 yr, but conformed generally to the early or the midseason peak period (June, July, and August) or both. In most years, common purslane, horse purslane, hemp sesbania, prickly sida, and spurred anoda emerged during the midseason period. Annual patterns of temperature fluctuations were similar during the 5 yr, but relatively warm soil temperatures during the early growing season of 1977 may have caused earlier emergence of velvetleaf, prickly sida, and hemp sesbania. Weekly water balance (rainfall minus open-pan evaporation) varied during March through May of each year but was negative during each June, July, and August. Apparently the main effect of a positive water balance during most of the year was to cause merger of the early and midseason peak emergence periods in 1979. Early and midseason emergence peaks

Influence of fertilizer, irrigation, and non-growing season precipitation on soil nitrate-nitrogen under corn

Abstract: (...) A field experiment was conducted to investigate the effects of fertilization, irrigation, and non-growing season precipitation on soil NO 3 −N under corn in southwestern Quebec from 1984 to 1987. The high rate of fertilization (400 kg N ha −1 , 300 kg P 2 O 5 ha −1 , 400 kg K 2 O ha −1 ) over the normal rate (170 kg N ha −1 , 100 kg P 2 O 5 ha −1 , 170 kg K 2 O ha −1 ) significantly increased soil NO 3 -N during the growing season, and a residual effect of fertilizer rate on soil NO 3 -N levels was noted in 2 of 3 yr during the non-growing season. (...)

Soil denitrification in three cropping systems characterized by differences in nitrogen and carbon supply

Abstract: Rates of denitrification measured over a growing season, both within and between rows of plants, in grass and lucerne leys and in barley plots were related to soil moisture and nitrate levels using bivariate, nonlinear regression models. Both within and between rows in all three crops, moisture explained a significant fraction of the variation in denitrification rates when the rates were regressed as increasing exponentially with increasing soil water content under moist conditions. Only soil moisture explained a significant fraction of the variation of the denitrification rates measured in the soil cores taken in (19.5%) and between (46.3%) plant rows at the barley field and in (42.4%) the plant rows in the lucerne ley. In some treatments, the rates appeared to be unrelated to moisture below a critical moisture threshold.

Out-of-season Prickly Pear: Fruit Characteristics and Effect of Fertilization and Short Droughts on Productivity

Abstract: Field experiments were conducted to examine the effect of fertilization and short periods of drought on the out-of-season winter crop in prickly pear (Opuntia ficus-indica (L.) Mill.). In addition, the winter and summer crops were compared regarding floral bud production and fruit characteristics. Under both continuous fer- tigation (N, P, K applied with the irrigation water) and continuous irrigation, the number of floral buds per plant was much lower in the winter than in the summer crop. Fertilization increased production of floral buds in both crops, but to a greater extent in the winter crop. The increase in floral bud production in fertilized plants was associated with an increase in NO3-N content in the cladodes. Suspension of fertigation for 4 or 8 weeks immediately after the summer harvest decreased cladode water content and delayed and reduced floral bud emergence as compared with continuous fertigation (control) or late drought (4 or 8 weeks) applied 4 weeks after the summer harvest. The plants subjected to early drought suffered from high mortality of floral buds. The fruits of the winter crop ripened in early spring, following the pattern of floral bud emergence the previous autumn. Mean fresh weight and peel : pulp ratio (w/w) were higher in fruits that ripened in the spring (winter crop) than in fruits that ripened in the summer. Plantations of prickly pear in the Medi- terranean region are not irrigated or are only slightly irrigated during fruit development and flower in the spring (Barbera, 1984; Porto- lano, 1969). An autumn period of flowering has been reported for prickly pear grown un- der irrigation in the Santa Clara Valley, Calif. (Curtis, 1977), and a similar phenomenon was observed recently in Israel in plantations irrigated in the late summer and in the au- tumn. Induction of flowering by applying ir- rigation after a short drought period has been reported for some tropical and subtropical trees, such as coffee and lemon (Magalhaes and Angelocci, 1976; Monselise, 1985; Southwick and Davenport, 1986). There- fore, we believed that irrigation followed by a drought period might induce autumn flow- ering in prickly pear. Data concerning floral bud production and flowering in prickly pear are very scarce (Gibson and Nobel, 1986), but recently we

Population distributions of plant size and light environment of giant ragweed (Ambrosia trifida L.) at three densities

Abstract: Plots in a naturally occurring population of giant ragweed (Ambrosia trifida L.) near Ames, Iowa, USA were left unthinned (high density,=693 plants/m2) or were thinned in early June 1989 to create low and medium densities of 10 and 50 plants/m2. Size and light environment of individual plants were measured at monthly intervals from June to September. By September, low density plants had 15 times greater biomass/plant and 30 times greater leaf area/plant than high density plants, although biomass and leaf area per unit land area decreased with decreasing density. Plants at high density allocated more biomass to stem growth, but plants at medium and low density had successively higher leaf area ratios, higher potential photosynthetic rates, higher allocation to leaves, and higher growth rates. Average light on leaves decreased with increasing density and also decreased over the growing season in the low and medium densities. The distribution of light environments of individual plants was non-normal and skewed to the left in most months, in contrast to the rightwards skew of distributions of plant size parameters. Inequality in the distributions, as measured by coefficient of variation and Gini coefficients, increased over most of the growing season. There was little effect of density on inequality of stem diameter, height, or estimated dry weight, but inequality in reproductive output greatly increased with density. There was greater inequality in number of staminate flowers produced than in number of pistillate flowers and seeds produced. Path analysis indicated that early plant size was the most important predictor of final plant size and reproductive output; photosynthesis, conductance, and light environment were also significantly correlated with size and reproduction but usually were of minor importance. Variation in growth rate apparently increased inequality in plant size at low density, whereas belowground competition and death of smaller plants may have limited increases in inequality at high density.