Showing papers on "Growing season published in 1992"

Productivity and compensatory responses of yellow-poplar trees in elevated C0 2

Abstract: INCREASED forest growth in response to globally rising CO2 concentrations could provide an additional sink for the excess carbon added to the atmosphere from fossil fuels1,2. The response of trees to increased CO2, however, can be expected to be modified by the interactions of other environmental resources and stresses, higher-order ecological interactions and internal feedbacks inherent in the growth of large, perennial organisms3,4. To test whether short-term stimulation of tree growth by elevated CO2 can be sustained without inputs from other environmental resources, we grew yellow-poplar (Liriodendron tulipifera L.) saplings for most of three growing seasons with continuous exposure to ambient or elevated concentrations of atmospheric CO2. Despite a sustained increase in leaf-level photosynthesis and lower rates of foliar respiration in CO2-enriched trees, whole-plant carbon storage did not increase. The absence of a significant growth response is explained by changes in carbon allocation patterns, specifically a relative decrease in leaf production and an increase in fine root production. Although these compensatory responses reduced the potential increase in carbon storage in increased CO2 concentrations, they also favour the efficient use of resources over the longer term.

Seasonality and the scheduling of life history at different latitudes

Abstract: For many ectotherms, the annual cycle is partitioned into ‘growing’ (summer) and ‘non-growing’ (winter) seasons, and the lengths of these seasons are inversely related across a latitudinal gradient. This pattern of variation has the potential to affect diverse life-history traits profoundly. A key selective agent is size-dependent winter mortality which, with increasing latitude, places an increasing premium on attainment of large body size before the first winter of life. Winter body size is determined primarily by two factors: (1) birth date, which defines the beginning of the first growing season and (2) somatic growth rate within the first season. Using examples drawn from the Atlantic silverside, Menidia menidia (L.), and other fishes, I show how latitudinal variation in the scheduling of the spawning season, countergradient variation in the capacity for growth and reproduction, and mode of sex determination (environmental v. genetic) represent adaptations to seasonality.

Plant-dependent CH4 emission in a subarctic Canadian fen

Abstract: The importance of vegetation in affecting CH4 emissions was investigated in a Carex-dominated fen located near Schefferville, Quebec, and in the Experimental Lakes Area, Ontario. Comparison of emission rates with and without the presence of aboveground vegetation indicated that over 90% of the emission was plant-associated transport. Further evidence of this association was found in a linear correlation of CH4 emission with aboveground plant biomass (R=0.93). To test the importance of aboveground plant photosynthetic production on methane production, aboveground vegetation was clipped from sites continually over the growing season. Both emissions and dissolved pore water CH4 were reduced relative to adjacent vegetated areas. A significant correlation (R=0.93) of CH4 emissions with net CO2 exchange in this fen gives evidence of the close association between new plant production and methanogenesis.

Effects of flooding on root and shoot production of bald cypress in large experimental enclosures

Abstract: Effects of hydroperiod on the root production of bald cypress (Taxodium distichum) saplings were determined in large (8.0 m2 x 1.5 m deep) watertight enclosures over three growing seasons. Our objectives were to determine the effect of continuous and periodic flooding regimes on biomass production, carbon allocation to roots and shoots, and root—system morphology. The effect of the flooding treatments on plant biomass was different for 1—yr—old seedlings and 3—yr—old saplings. After one growing season, root and shoot biomass was highest in the periodically flooded (PF) treatment. After three growing seasons there were no significant differences in total biomass but there were differences in root—to—shoot ratios. Improved growth in the continuously flooded (CF) treatment began in the second growing season and coincided with morphological adaptations to flooding. Such adaptations include the production of water roots, development of intercellular air spaces, and distinctly different root—system morphologies. Periodically flooded cypress allocated more carbon to roots than did continuously flooded cypress and developed deeper root systems. A relatively deep rooting zone may have provided the PF saplings access to water and dissolved nutrients within the water table (50—60 cm deep during summer). Continuously flooded plants had low root—to—shoot ratios and shallow root systems. A relatively shallow rooting zone with ample water and nutrients allowed CF cypress to allocate relatively more biomass to leaves. After 3 yr, total productivity in the two treatments was not significantly different, yet belowground production was greater in periodically flooded saplings (P = .05) and there was a tendency for higher aboveground production in continuously flooded saplings (P = .14). Without the belowground production estimates we might have concluded that CF plants were more productive than PF plants. Most plants can respond to changing resource availabilities by shifting the allocation of carbohydrates to roots or shoots. Because resource availability in freshwater forested wetland ecosystems can be highly variable, studies of production should include estimates of root production.

Plant adaptation in the Great Basin and Colorado Plateau

Abstract: Adaptive features of plants of the Great Basin are reviewed. The combination of cold winters and an arid to semiarid precipitation regime results in the distinguishing features of the vegetation in the Great Basin and Colorado Plateau. The primary effects of these climatic features arise from how they structure the hydrologic regime. Water is the most limiting factor to plant growth, and water is most reliably available in the early spring after winter recharge of soil moisture. This factor determines many characteristics of root morphology, growth phenology of roots and shoots, and photosynthetic physiology. Since winters are typically cold enough to suppress growth, and drought limits growth during the summer, the cool temperatures characteristic of the peak growing season are the second most important climatic factor influencing plant habit and performance. The combination of several distinct stress periods, including low-temperature stress in winter and spring and high-temperature stress combined with drought in summer, appears to have limited plant habit to a greater degree than found in the warm deserts to the south. Nonetheless, cool growing conditions and a more reliable spring growing season result in higher water-use efficiency and productivity in the vegetation of the cold desert than in warm deserts with equivalent total rainfall amounts. Edaphic factors are also important in structuring communities in these regions, and halophytic communities dominate many landscapes. These halophytic communities of the cold desert share more species in common with warm deserts than do the nonsaline communities. The Colorado Plateau differs from the Great Basin in having greater amounts of summer rainfall, in some regions less predictable rainfall, sandier soils, and streams which drain into river systems rather than closed basins and salt playas. One result of these climatic and edaphic differences is a more important summer growing season on the Colorado Plateau and a somewhat greater diversification of plant habit, phenology, and physiology.

Correlating genetic variation in carbon isotopic composition with complex climatic gradients.

Abstract: Genetic variation in both carbon isotope discrimination and the proportions of leaf and photosynthetic twig tissues were observed in ecotypes of Hymenoclea salsola T.G., a common shrub in the deserts of the western United States, when grown under common garden conditions. These variations were correlated with climatic conditions in the habitats of origin through a model that described the leaf-to-air water vapor gradients experienced by plants during the growing season. Both carbon isotope discrimination and the proportion of leaves in the canopy were lower in plants derived from habitats with higher leaf-to-air vapor gradients, despite the fact that some of these sites received relatively high amounts of annual precipitation. These patterns were consistent with the notion that plants are able to maintain substantial control of water-use efficiency over large environmental gradients of temperature and moisture availability.

Rhizome dynamics and resource storage in Phragmites australis

Abstract: Seasonal changes in rhizome concentrations of total nonstructural carbohydrates (TNC), water soluble carbohydrates (WSC), and mineral nutrients (N, P and K) were monitored in two Phragmites australis stands in southern Sweden. Rhizome biomass, rhizome length per unit ground area, and specific weight (weight/ length ratio) of the rhizomes were monitored in one of the stands. Rhizome biomass decreased during spring, increased during summer and decreased during winter. However, changes in spring and summer were small ( 1000 g m−2). Concentrations and standing stocks of mineral nutrients decreased during spring/ early summer and increased during summer/ fall. Only N, however, showed a pattern consistent with a spring depletion caused by translocation to shoots. This pattern indicates sufficient root uptake of P and K to support spring growth, and supports other evidence that N is generally the limiting mineral nutrient for Phragmites. The biomass data, as well as increased rhizome specific weight and TNC concentrations, clearly suggests that “reloading” of rhizomes with energy reserves starts in June, not towards the end of the growing season as has been suggested previously. This resource allocation strategy of Phragmites has consequences for vegetation management. Our data indicate that carbohydrate reserves are much larger than needed to support spring growth. We propose that large stores are needed to ensure establishment of spring shoots when deep water or stochastic environmental events, such as high rhizome mortality in winter or loss of spring shoots due to late season frost, increase the demand for reserves.

Exchange of N2O and CH4 between the atmosphere and soils in spruce-fir forests in the northeastern United States

Abstract: We measured the exchange of N2O and CH4 between the atmosphere and soils in 5 spruce-fir stands located along a transect from New York to Maine. Nitrous oxide emissions averaged over the 1990 growing season (May–September) ranged from 2.1 ug N2O-N/m2-hr in New York to 0.4 ug N2O-N/m2-hr in Maine. The westernmost sites, Whiteface Mtn., New York and Mt. Mansfield, Vermont, had the highest nitrogen-deposition, net nitrification and N2O emissions. Soils at all sites were net sinks for atmospheric CH4 Methane uptake averaged over the 1990 growing season ranged from 0.02 mg CH4-C/M2-hr in Maine to 0.05 mg CH4-C/m2-hr in Vermont. Regional differences in CH4 uptake could not be explained by differences in nitrogen-deposition, soil nitrogen dynamics, soil moisture or soil temperature. We estimate that soils in spruce-fir forests at our study sites released ca. 0.02 to 0.08 kg N2O-N/ha and consumed ca. 0.74 to 1.85 kg CH4 C/ha in the 1990 growing season.

Methane emissions from California rice paddies with varied treatments

Abstract: Two field experiments in California rice paddies are reported, one with a single treatment of a research plot and the other with varied treatments in a typical commercial rice field. Small total methane emissions, only 11 g CH4/m2, were measured for the entire growing season in the first experiment. In the second experiment, the addition of exogenous organic matter (rice straw), the presence or absence of vegetation, and the nitrogen fertilizer amounts were examined for their influence on methane emissions. The total methane emission over the growing season varied from 1.2 g CH4/m2 (with no added organic matter) to 58.2 g CH4/m2 (with largest organic matter treatments). Added organic matter was the major factor affecting methane emissions. Vegetation did not greatly affect total methane fluxes, but it did influence the mode and timing of release. Nitrogen fertilizer did not greatly affect the amount of methane emitted, but it influenced slightly the time course of the process. A diurnal effect in methane emission was observed during the early ontogeny of the crop. The variation of methane emission with time during the course of the growing season was very unusual in this experiment; only one peak was observed, and it was early in the season. During the period of largest emissions, δ13C values of the methane were measured to be −55.7 ±1.8‰ in plots with added organic matter.

Season length and cultivar determine the optimum evapotranspiration deficit in cotton

Abstract: In areas where the growing-season length limits cotton (Gossypium hirsutum L.) production, irrigation practices that meet the maximum evapotranspiration (ET) demand have been shown to promote excessive vegetative growth and to reduce yield. Past experiments using limited irrigation in cotton do not provide generalized information on the magnitude of ET deficits that optimize lint yields, however. A study was conducted to determine how season length and cultivar maturity affected the response functions to ET deficits in cotton. Four cultivars (Acala SJ-C1, GC-510, Coker-310, and Jaen) were studied at Cordoba (southwestern Spain) on a Typic Xerofluvent soil under variable irrigation (...)

Simulating Corn Yield Response to Irrigation Timings: Validation of the Epic Model

Abstract: Models simulating crop growth offer an opportunity to fill important data gaps from experimental plots. Examples include incorporation of many weather conditions as well as alternative management strategies. The simulated data must be reliable, however. This study examined the application of the EPIC growth model to simulate corn (Zea mays L.) yield on the Southern High Plains when water stress was imposed on various dates in the growing season. The EPIC model produced simulated yield distributions with means not significantly (P=0.05) different from those of the actual data in 2 of the 3 yr considered, and standard deviations of simulated yields similar to those of the actual yields for all 3 yr 5 (...)

Effects of genotype, habitat, and seasonal variation on iridoid glycoside content of Plantago lanceolata (Plantaginaceae) and the implications for insect herbivores

Abstract: We investigated the effects of genotype, habitat, and seasonal variation on production of the iridoid glycosides, aucubin and catalpol, in leaves of the common weed Plantago lanceolata. Two genotypes, one each from a lawn and an adjacent abandoned hayfield population, were clonally replicated in the greenhouse, and then planted back into the two habitats. One quarter of the plants from each treatment were harvested on each of four dates, at approximately two-week intervals. Over the course of the growing season, and in both habitats, we found a significant increase in the concentration of both aucubin and catalpol in P. lanceolata leaves. The genotypes differed in their response to environmental variation, both in time and between sites, as indicated by significant genotype x date and genotype x site interactions. Early in the season, habitat (lawn or field) had a greater effect on iridoid glycoside concentration than did plant genotype, but later in the season, plant genotype was more influential in determining the iridoid glycoside concentration. Thus, the relative palatability of Plantago genotypes to specialist and generalist herbivores may vary in time and space.

Water and nutrient relations of woody perennials from tropical dry forests

Abstract: . Dry forests occupy a larger area in the tropics than rain forests. They grow under a wide range of rainfall conditions; the determining characteristics are the occurrence of a dry season of 2–6 months duration, and the dominance of deciduous woody perennials. The knowledge of the ecophysiological properties of woody perennials from these forests, essential for the development of forest restoration strategies, is still scanty. This paper describes the ecophysiological behaviour of 10 species of woody perennials growing in a secondary dry forest, which has been recovering since 1944 on the hills of the Botanical Garden of Caracas. Four species, Astronium graveolens, Bauhinia megalandra, Sapindus saponaria and Tabebuia chrysantha, were initially planted while the others, Bursera simaruba, Capparis flexuosa, Erythroxylon cumanense, E. densum, E. orinocense and Eugenia casearioides, reproduced from seed sources existing in forest remnants growing nearby. Specific leaf areas measured are relatively high, covering a range from 11 to 34 m2/kg. Nitrogen and phosphorus levels are also high compared to data reported from dry forests elsewhere, therefore no nutrient limitation for photosynthetic productivity is apparent. Osmotic pressure (π) of leaf sap, extracted from frozen samples taken during the growing season, is correlated with the corresponding content of soluble sugars and the ions Mg and K; Ca-ions do not play a significant role in explaining the variance of π. Most species maintained a diluted leaf sap during the rainy season, characterized by π values between 5–15 bar. The osmotic pressure increased strongly in older leaves and during the dry season. Four species showed more stable π values throughout the growing season, with C. flexuosa and B. simaruba characterized by higher and lower π values, respectively. Proline was found to be a reliable indicator of water stress in these woody species, the amount of proline measured in leaf sap being logarithmically correlated with the corresponding π value. Osmotic pressure of leaf sap and leaf xylem tension was higher during the dry season for all species, while the contrary was true for leaf conductance. Leaf conductance was better correlated with leaf-air vapour pressure deficit than with leaf xylem tension. The most drought-tolerant species were C. flexuosa, E. casearioides and the three Erythroxylon species. Drought resistance of B. simaruba, B. megalandra and A. graveolens was associated with their high sensitivity to leaf-air VPD and lower leaf conductances. The other two species occupied intermediate positions.

Climate change and implications for agriculture in niger

Abstract: Five-year moving averages of annual rainfall for 21 locations in Niger showed a decline in the annual rainfall after 1960. Correlation coefficients of the moving averages of monthly rainfall with annual rainfall showed significant correlations between the decline in the annual rainfall with decreased rainfall in August. Analysis of daily rainfall data for rainy season parameters of interest to agriculture suggested that from 1965 there was a significant decrease in the amount of rainfall and in the number of rainy days in the months of July and August, resulting in a decreased volume of rainfall for each rainstorm. In comparison to the period 1945–64, major shifts have occurred in the average dates of onset and ending of rains during 1965–88. The length of the growing season was reduced by 5–20 days across different locations in Niger. The standard deviation for the onset and ending of the rains as well as the length of the growing season has increased, implying that cropping has become more risky. Water balance calculations also demonstrated that the probability of rainfall exceeding potential evapotranspiration decreased during the growing season. The implications of these changes for agriculture in Niger are discussed using field data.

Managing native and legume-fixed nitrogen in lowland rice-based cropping systems

Abstract: Lowlands comprise 87% of the 145 M ha of world rice area. Lowland rice-based cropping systems are characterized by soil flooding during most of the rice growing season. Rainfall distribution, availability of irrigation water and prevailing temperatures determine when rice or other crops are grown. Nitrogen is the most required nutrient in lowland rice-based cropping systems. Reducing fertilizer N use in these cropping systems, while maintaining or enhancing crop output, is desirable from both environmental and economic perspectives. This may be possible by producing N on the land through legume biological nitrogen fixation (BNF), minimizing soil N losses, and by improved recycling of N through plant residues. At the end of a flooded rice crop, organic- and NH4-N dominate in the soil, with negligible amounts of NO3. Subsequent drying of the soil favors aerobic N transformations. Organic N mineralizes to NH4, which is rapidly nitrified into NO3. As a result, NO3 accumulates in soil during the aerobic phase. Recent evidence indicates that large amounts of accumulated soil NO3 may be lost from rice lowlands upon the flooding of aerobic soil for rice production. Plant uptake during the aerobic phase can conserve soil NO3 from potential loss. Legumes grown during the aerobic phase additionally capture atmospheric N through BNF. The length of the nonflooded season, water availability, soil properties, and prevailing temperatures determine when and where legumes are, or can be, grown. The amount of N derived by legumes through BNF depends on the interaction of microbial, plant, and environmental determinants. Suitable legumes for lowland rice soils are those that can deplete soil NO3 while deriving large amounts of N through BNF. Reducing soil N supply to the legume by suitable soil and crop management can increase BNF. Much of the N in legume biomass might be removed from the land in an economic crop produce. As biomass is removed, the likelihood of obtaining a positive soil N balance diminishes. Nonetheless, use of legumes rather than non-legumes is likely to contribute higher quantities of N to a subsequent rice crop. A whole-system approach to N management will be necessary to capture and effectively use soil and atmospheric sources of N in the lowland rice ecosystem.

Small-scale heterogeneity in a semi-arid North American grassland. I : Tillering, N uptake and retranslocation in simulated urine patches

Abstract: 1. We examined the response of two grass species differing in growth form and photosynthetic pathway to simulated cattle urine deposition in a native semi-arid grassland. The species were Agropyron smithii, a spreading, rhizomatous C3 grass, and Bouteloua gracilis, a cespitose C4 grass. They are codominants in this grassland and A. smithii is the most important forage species in the study site. 2. Simulated urine increased tiller density of both species, with B. gracilis increasing in the first growing season while A. smithii was not increased until the second year. Both species had increased tiller densities and above-ground N concentrations in a ring outside the area wetted by urine. The percentage of retranslocated leaf N decreased, and standing dead litter N concentration increased in both species after urine deposition. 3. Above-ground biomass, N yield, and tiller height of A. smithii increased in urine patches. Leaf production was not affected by treatment. Both the increase in the live:dead biomass ratio and the higher number of live leaves at the end of the first growing season suggested urine deposition delayed senescence of A. smithii. 4. The implications of plant growth form in the spread of urine, the potential duration of urine effects in semi-arid grasslands, and the role of herbivore urine deposition in promoting grassland structural and functional heterogeneity are discussed.

Altitudinal Trends in Leaf Nutrient Contents, Leaf Size and | delta 13 C of Alchemilla alpina

Abstract: 1. At the peak of the growing season, nitrogen and phosphorus concentrations increased with altitude particularly above 500 m. Leaf dry mass and area decreased with altitude, this trend being more pronounced above 500 m. The total amount of nitrogen per leaf, specific leaf area (SLA) and potassium concentration did not change with altitude. δ 13 C increased linearly with altitude. 2. Nitrogen, phosphorus and chlorophyll concentrations decreased throughout the growing season from a maximum in May; the decline at the end of the growing season was most rapid at high altitude. δ 13 C also decreased with time but uniformly at all altitudes

Adaptive significance of nitrogen storage in Bistorta bistortoides, an alpine herb

Abstract: Studies were conducted to examine the importance of nitrogen storage to seasonal aboveground growth in the alpine herb Bistorta bistortoides. Stored reserves accounted for 60% of the total nitrogen allocated to the shoot during the growing season. The stored nitrogen was equally partitioned between preformed buds of the shoot and the roots/rhizome. Reliance on stored N was similar in populations of a 105-day growing season site and of a 75-day growing season site. Contrary to our initial hypothesis, stored nitrogen reserves were not used to extend the growing season of this species into the late-spring when soils are still cold, and saturated with snow-melt water. The time at which stored nitrogen was used to initiate shoot growth coincided with the time of root initiation, rapid soil warming, and near maximum soil concentrations of NO inf3sup- and NH inf4sup+ . Thus, nitrogen demand and soil nitrogen supply were both high at the same time. The importance of nitrogen storage in this species appeared to be in satisfying the high demand of simultaneous vegetative and reproductive growth during the early-growing season after soils thawed. The initiation of rapid leaf and inflorescence growth occurred in mid-June in both sites. The maximum pool size of shoot nitrogen (maximum nitrogen demand) occurred only 12 days later in the long season site, and 28 days later in the short season site. The early-season utilization of nitrogen stores allows plants of this species to initiate reproductive allocation at the same time vegetative tissues are exhibiting maximal growth rates. By releasing vegetative and reproductive growth from competition for nitrogen, seeds could mature early in the alpine growing season, before the frost probability sharply increases in mid-August.

Limitations to growth and yield of cereal and lupin crops on duplex soils

Abstract: The combination of soil physical and chemical properties in duplex soils can depress crop growth and yield, and cause considerable variability in yield within a paddock and from year to year. At an intensively monitored site at East Beverley, Western Australia, the sandy A horizon was found to be nutrient deficient, low in pH, and contained hard layers which impeded root growth. The clayey B horizon had a higher pH, and variable permeability which influenced waterlogging in winter, and root growth and water availability from the subsoil in spring. Growth of wheat and lupin crops reflected response to these stresses. Wheat yields in particular were very variable (0.7-4.2 t/ha in 1988), and as all components of yield were affected, this suggests that the crop in low yielding areas experienced stress throughout the growing season. Lupin yields varied more widely from year to year than wheat yields, but were less variable within a year, possibly because of a more limited exploration of the subsoil by roots. Treatments to overcome the chemical constraints to growth raised yields and reduced variability; wheat and lupin yields were increased by about 25% after application of potassium, and wheat yields increased by about 10% after application of lime. Ripping lowered soil strength in the A horizon, but had little effect on yield, suggesting that the subsoil plays a larger role in determining yield on this soil. Attempts to improve the permeability of the subsoil using gypsum were inconclusive in the year of application. Similar physical and chemical problems have been identified on other duplex soils throughout Australia. Responses to ameliorative treatments have been variable, and there have often been interactions between treatments. Further improvements in yield on these difficult soils requires a better understanding of the interaction of soil physical, chemical and biological properties, and their interactions with crop physiology.

Nitrogen Uptake of Mesquite Seedlings at Conventional and Exponential Fertilization Schedules

Abstract: Exponential nutrient additions have been shown to induce steady-state nutrition in plants and enhance seedling growth and nutrient status, which suggests improved nutrient efficiencies during the growing season. The objective of this study was to compare periodic growth, plant nutrient status, and N uptake of desert mesquite (Prosopis chilensis Mol.) seedlings that received 200 mg N seedling⁻¹ as complete fertilizer delivered either conventionally (as single dose or constant top dressing) or exponentially (as pure or modified exponentially increasing additions) during a 12-wk rotation. Steady-state nutrition (characterized by a stable tissue nutrient concentration) was induced for the cultural period in the modified-exponential fertility regime. The nutrients were supplied exponentially except for higher initial amounts that compensated for the incomplete accessibility of growing medium to the small root systems. These seedlings showed consistently higher N accumulation that matched dry-matter production, reflecting higher fertilizer-N uptake efficiency throughout the growing season. During the same growing season, pure exponential nutrient-addition rates appeared to achieve steady-state conditions 4 wk later because of incomplete interception of applied nutrients by small root systems during establishment. Conventional fertility regimes caused nutrient dilution in plants with time. Nutrient uptake did not match growth in these seedlings, resulting in nutrient stress at varying stages of seedling development.

Altitudinal and seasonal trends in soil nitrogen mineralization rate in the Scottish Highlands

Abstract: Soil nitrogen mineralization rate and nitrification rates were determined in situ and in the laboratory for a number of sites along two altitudinal transects in the scottish Highlands for four periods during the growing season. Available nitrogen, total nitrogen, ignition loss, water content and pH were determined on the same soil samples. Mineralization rates in both field and laboratory either did not change with altitude or were highest in samples from high altitudes, despite the altitudinal decline in temperature. Mineralization and nitrification rates were highest at the beginning and end of the growing season. It is suggested that the mineralization rate is influenced by the concentration or readily decomposed nitrogenous compounds and that at high altitude these are not broken down winter as much as at low altitudes, because of the incidence of freezing temperatures (...)

Nitrogenase Activity, Nitrogen Fixation, and Nitrogen Inputs by Lupines at Mount St. Helens

Abstract: We measured the timing and magnitude of nitrogenase activity and N2 fixation by lupines colonizing early successional volcanic sites at Mount St. Helens. Ni- trogenase activity (measured by acetylene reduction) in Lupinus lepidus growing at a py- roclastic site exhibited significant diurnal trends, with lowest ethylene evolution rates at night. Nitrogenase activity also followed seasonal trends, with high rates in June, very low levels in August, the dry warm part of the season, and a partial recovery of nitrogenase activity in September after precipitation resumed. A comparison of typical nitrogenase activities measured at several sites suggested that rates of N2 fixation were highest in L. lepidus growing at disturbed low N sites. Adult lupine C and N composition also varied during the growing season, with trends correlated with seasonal patterns of nitrogenase activity. Seasonal N2 fixation in L. lepidus and L. latifolius was measured using 15N isotope. Both species fixed ;60% of their N during the first season of growth with some evidence of preferential allocation to aboveground biomass. N fixation by Lupinus lepidus individuals was - 18.1 mg/g biomass or an average of 15.4 mg per plant, while L. latifolius fixed an average of 16.3 mg/g biomass, equivalent to 22.9 mg per plant. Average net C fixation during the same period was 355 and 589 mg per plant for L. lepidus and L. latifolius, respectively. Despite these rates, the current distribution of L. lepidus into a few, small patches that occupy < 1% of the surface area indicates that annual N inputs by lupines are <0.05 kg/ha and thus probably not the primary source of N input into developing Mount St. Helens pyroclastic sites except at a local scale.

Impacts of summer warming on the energy and water balance of wetland tundra

Abstract: Measurements, made at a high subarctic, maritime, wetland tundra site, are presented for three different growing seasons. These are divided into hot-dry, normal-dry and normal-wet years and the behaviour of their surface energy and water balances is examined within the framework of a combination model. For periods of comparable energy availability, evapotranspiration during hot-dry conditions can be larger than during cooler and wetter periods. This results from small stomatal resistance in the sparse canopy of well-rooted sedges, and from the ability of peat soils to supply water under conditions of large atmospheric demand. This demand is expressed in terms of the vapour pressure deficit and it counteracts the large surface resistances which develop during dry periods. In many respects, the energy balance of a subarctic wetland tundra is comparable to observations and models for temperate agricultural and forest lands, in spite of the fact that the soils are organic, the vegetation canopy is sparse and there is continuous permafrost. A dry year promotes deeper thaw depths in the permafrost soils, during the growing season, than does a wet one. This is due to larger ground heat fluxes and larger soil thermal diffusivities. We concluded that maritime, wetland tundra, growing on peat soils, displays feedback mechanisms, which can offset the effects of moisture stress, caused by summer climate warming of a similar magnitude to that simulated by General Circulation Models for a 2 × CO2 scenario.

Modelling the effects of moisture on barley straw and root decomposition in the field

Abstract: In a field experiment with barley on a clay soil, four treatments were compared: the control (C) was not nitrogen fertilized or irrigated; drought treatment (D) was identical to C, except that screens had been installed to divert rainwater; irrigation (I) and irrigation-fertilization (IF) treatments were irrigated using a drip-tube system, with liquid fertilizer (260kg N ha−1yr−1) applied daily in IF according to predicted plant uptake. Litter-bags with barley straw were buried in all treatments at 10 cm depth on 19 May 1988 and sampled five times during the growing season. Decomposition rates were calculated assuming exponential decay. The rates were highly dependent on soil moisture, and the constants ranged from 0.54% day−1 in D to 0.92% day−1 in IF. A simulation model with driving variables based on Q10 temperature dependence and a log-linear relationship between soil water tension and activity was fitted to the same data set. The rate constants became quite similar between the four treatments, ranging from 1.1 to 1.3% optimum day−1. Thus it was concluded that the assumptions and parameters used for temperature and moisture dependence were valid. Roots collected from each of the four treatments on four occasions were put in litter-bags, buried in the soil and taken up after about 20 days. Observed root decomposition rates were lowest in D. The rates for all treatments were highest early in the growing season and decreased thereafter. When temperature and moisture effects were compensated for using the climatic correction factors for barley straw, the decreasing trend remained, which was attributed to reduced decomposability of the roots during the growing season. The results indicate that using a constant value for root decomposability when calculating root turnover can give misleading results.

Interspecific and Intraspecific Interactions Via Plant Responses to Folivory: An Experimental Field Test

Abstract: I tested the hypothesis that phytophagous insect species compete interspe- cifically on shared host plants by inducing chemical, physical, and phonological changes in the plant that either directly reduce insect growth and survival or indirectly alter attack by natural enemies. I manipulated levels of simulated folivory on branches within eight trees of Qiiercus emor vi in a complete randomized block design. Survival and causes of mortality, including attack by natural enemies, of a dominant leafminer, Catneraria sp. nov., were monitored over two growing seasons. Survival varied significantly among trees but not among treatments. Causes of mortality also varied significantly among trees. Death from premature leaf abscission increased significantly with increasing levels of branch damage, while mortality from other causes, including death from bacterial and fungal attack, declined significantly with increasing levels of damage. However, neither of these changes in mortality was sufficient to cause changes in overall survival. Leaf damage did not influence attack by macroscopic natural enemies (hymenopteran parasitoids or vertebrate and invertebrate predators). By far, the most important factor affecting survival and causes of mortality within trees was the presence of co-occurring larvae on leaves (i.e., intraspecific competition). Simulated folivory in the first growing season did not affect folivory by Cameraria or other folivorous arthropods in the next season. Colonization, survival, rate of attack by natural enemies, and amount of herbivory by other arthropods in the second growing season were not affected by levels of leaf damage in the first one. Neither survival nor causes of mortality of Cameraria in the second growing season were associated with folivory by endemic leafchewing arthropods. Simulated folivory in the first growing season did, however, reduce leaf production in the second season. These results do not support the hypothesis that insect species compete interspecifically via induced plant responses and their effects on the third trophic level (natural enemies of the herbivores). The effects of simulated (first growing season) and insect (second growing season) folivory via induced plant responses on the Cameraria population was negligible relative to intraspecific interactions. Further, my results do not support the notion that folivore-induced responses are defensive and that plants have evolved these responses to manipulate the third trophic level. In natural settings, other factors that influence population dynamics of phytophagous insects, such as intraspecific competition and interactions with microparasites, may obscure any negative effects of herbivore-induced plant responses on interactions among herbivore species and their macroscopic natural enemies.

Effects of moisture supply in the dry season and subsequent defoliation on persistence of the savanna grasses Themeda triandra, Heteropogon contortus and Panicum maximum

Abstract: The close correlation between grazing-induced mortality and major climatic patterns in Australian savannas, led us to the hypothesis that moisture conditions during the dry, non-growing season could affect sensitivity to grazing in the subsequent growing season. Using three widespread savanna species (Themeda triandra, Heteropogon contortus and Panicum maximum), this hypothesis was tested experimentally and the mechanisms controlling this response examined and quantified. In T. triandra drought during the dry season led to major mortality in defoliated plants in the next growing season. This mortality was caused by a synchrony of tillering at the commencement of the wet season, leaving few buds for replacement once parent tillers were killed by defoliation. T. triandra was also the most sensitive species to defoliation. This sensitivity was due to the poor ability of the plant to maintain positive carbon gain after defoliation. Several factors contributed to this poor ability, including: low total photosynthetic rate, low specific leaf area, and a large proportion of sheath material with poor photosynthetic capacity remaining after cutting. Both H. contortus and P. maximum growing under irrigated and fertilized conditions did not display any effects of previous moisture treatments when defoliated during the next wet season and were much less sensitive to defoliation than T. triandra.

Following practices, soil water storage, plant-available soil nitrogen accumulation and wheat performance in South West Queensland

Abstract: Radford, B.J., Gibson, G., Nielsen, R.G.H., Butler, D.G., Smith, G.D. and Orange, D.N., 1992. Fallowing practices, soil water storage, plant-available soil nitrogen accumulation and wheat performance in South West Queensland. Soil Tillage Res., 22: 73-93. The effects of tillage frequency (conventional, reduced and zero), primary tillage implement (disc, blade and chisel plough), stubble management (retention and removal), gypsum application and paraplowing were examined with respect to soil water storage, soil nitrate accumulation, seedling establishment, crop growth, yield and grain protein content for four successive years of wheat, grown on a sodic, duplex soil in South West Queensland, Australia. Stubble retention generally resulted in more soil water at sowing but less at harvest, compared with stubble removal. This led to higher grain yields in dry growing seasons and has implications for reducing runoff. Zero tillage with stubble removal had the lowest mean water storage and, in dry growing seasons, the lowest grain yield. This indicates that when stubble is absent or lacking, some tillage is needed, probably to break surface seals and increase surface roughness. Zero tillage with stubble retention accumulated the most soil water but the least soil nitrate. Consequently this treatment outyielded all others in the driest growing season but was outyielded by almost all others in the wettest. Increased frequency and aggressiveness of tillage, and stubble removal, increased the amount of soil NO3-N at 0-60 cm. There were no significant (P< 0.05 ) tillage × stubble interactions. In the wettest growing season, wheat grain yields reflected the different levels of NO3-N in the soil. In the other three years, grain protein contents reflected these levels. Both zero tillage and stubble retention reduced the efficiency of water use, probably because both also reduced nitrogen supply. So despite high yields from zero tillage with stubble retention in the dry growing seasons, the full yield potential of this treatment with respect to its water supply was not realized. Gypsum application at 5 t ha- t had no commercially useful effect on establishment or grain yield and is not recommended. Similarly, paraplowing had no effect on yield. Reduced or zero tillage is recommended, provided that stubble is present and nitrogen supply is adequate.

Seasonal activity and body mass of Columbian ground squirrels

Abstract: Recent research on Columbian ground squirrels (Spermophilus columbianus) has invoked the lengths of the active season and plant growing season to explain differences in life history and social behaviours among populations at different elevations. We evaluated an assumption of these studies that the active season for individual ground squirrels is significantly shorter at high than at low elevation. Adult males and females were active for about 100 days at low elevation and about 86 days at high elevation. Juvenile ground squirrels also had a longer active season at low (50 days) than at high elevation (45 days), but for yearlings the active season was similar (about 87 days). The active season for adults was about 2 weeks shorter than the plant growing season at low elevation, but up to 2 weeks longer than the plant growing season at high elevation. Differences in body mass of adult ground squirrels between low and high elevations at spring emergence from hibernation and at fall immergence into hibernatio...

Tillage and Winter Cover Management Effects on Fruiting and Yield of Cotton

Abstract: A field experiment was conducted from 1986 to 1988 on a highly erodible soil in north Mississippi to study the effects of seven cotton (Gossypium hirsutum L.) cropping systems on soil erosion, stand establishment, fruiting, earliness, and yield. Stand counts for no-till cotton following hairy vetch (Vicia villosa Roth) averaged 30% less than for conventional tillage cotton. No significant difference in yield between cropping systems occurred in 1986. Yields in 1987 decreased as tillage intensity increased, but this trend was reversed in 1988. Rainfall distribution within each growing season contributed to the variability in yield and fruiting response to tillage and cover crops (...)