Showing papers on "Growing season published in 1999"

Growing season extended in Europe

Abstract: Changes in phenology (seasonal plant and animal activity driven by environmental factors) from year to year may be a sensitive and easily observable indicator of changes in the biosphere. We have analysed data from more than 30 years of observation in Europe, and found that spring events, such as leaf unfolding, have advanced by 6 days, whereas autumn events, such as leaf colouring, have been delayed by 4.8 days. This means that the average annual growing season has lengthened by 10.8 days since the early 1960s. These shifts can be attributed to changes in air temperature.

Bidirectional facilitation and interference between shrubs and annuals in the mojave desert

Abstract: Composition and structure of plant communities can be strongly influenced by plant interactions. Interactions among plants commonly comprise positive and negative effects operating simultaneously and bidirectionally. Thus, a thorough understanding of plant interactions requires experimental separation and quantitative assessment of the bidirectional positive and negative effects that add up to the net effects of plant interactions. Using the close spatial association of annual plants with a desert shrub (Ambrosia dumosa) in a sandy area of the Mojave Desert of California as a test system, we separated and quantified negative and positive effects of annuals on shrubs and of shrubs on annuals. We achieved the separation of negative and positive effects with an experimental design that included reciprocal removals of neighbors and simulations of physical effects of neighbors using artificial structures. All experimental manipulations were conducted on space originally occupied by Ambrosia shrubs to focus on immediate effects of neighbors on water availability rather than on long-term microenvironmental effects (e.g., nutrient accumulation). We quantified positive effects by calculating the difference between performance parameters of neighbors growing with artificial structures (thatch to mimic the physical effects of the presence of annuals and artificial canopies to mimic the physical effects of shrubs) and those of neighbors growing alone (removals). We estimated negative effects by calculating the difference between plant performance on control plots (shrubs and annuals growing together) and performances of plants growing with the artificial structures. Annuals had simultaneously strong negative and weak positive effects on shrub water status, growth, and reproductive output. Annuals also had an impact on the sex expression of shrubs by inducing shifts toward a higher male proportion in inflorescences of monoecious Ambrosia. In contrast, we found strong positive and weak or no negative effects of the shrubs on survival, biomass production, and seed production of the entire annual community and of selected annual species (the abundant native Chaenactis fremontii and the two dominant introduced annual species Bromus madritensis ssp. rubens and Schismus barbatus). Overall, in net effect, the interaction between shrubs and annuals can be described as facilitation or positive net effects of shrubs on annuals, and interference or negative net effects of annuals on shrubs. However, during the growing season, the ratios between positive and negative effects shifted. Annual plants benefited from the presence of shrubs to the greatest extent early in the growing season, and initial negative effects of annuals on shrubs declined as annuals senesced later in the season. Results of this study support the view that an experimental resolution of bidirectional positive and negative effects is necessary to achieve an accurate, mechanistic understanding of species interactions.

Links between microbial population dynamics and nitrogen availability in an alpine ecosystem

Abstract: Past studies of plant-microbe interactions in the alpine nitrogen cycle have revealed a seasonal separation of N use, with plants absorbing N primarily during the summer months and microbes immobilizing N primarily during the autumn months. On the basis of these studies, it has been concluded that competition for N between plants and microbes is minimized along this seasonal gradient. In this study, we examined more deeply the links between microbial population dynamics and plant N availability in an alpine dry meadow. We conducted a year-round field study and performed experiments on isolated soil microorganisms. Based on previous work in this ecosystem, we hypothesized that microbial biomass would decline before the plant growing season and would release N that would become available to plants. Microbial biomass was highest when soils were cold, in autumn, winter, and early spring. During this time, N was immobilized in microbial biomass. After snow melt in spring, microbial biomass decreased. A peak in the soil protein concentration was seen at this time, followed by peaks in soil amino acid and ammonium concentrations in late June. Soil protease rates were initially high after snow melt, decreased to below detection limits by midsummer, and partially recovered by late summer. Proteolytic activity in soil was saturated early in the growing season and became protein limited later in the summer. We concluded that the key event controlling N availability to alpine plants occurs after snow melt, when protein is released from the winter microbial biomass. This protein pulse provides substrate for soil proteases, which supply plants with amino acids during the growing season. On average, microbial biomass was lower in the summer than at other times, although the biomass fluctuated widely during the summer. Within the summer months, maximum numbers of amino-acid-degrading microorganisms and the max- imum amount of microbial biomass coincided with the peak in soil amino acids, when plants are most active. All bacterial strains isolated from this summer community had the ability to grow rapidly on low concentrations of amino acids and to degrade protein. This explains the previously observed result that the soil microbial biomass can compete strongly with plants for organic N, despite the seasonal offset of maximum plant and microbial N uptake.

Increases in early season ecosystem uptake explain recent changes in the seasonal cycle of atmospheric CO2 at high northern latitudes

Abstract: We report changes in the seasonal cycle of atmospheric CO2 at high northern latitudes from 1980 to 1997 based on NOAA/CMDL observation stations. Using a combination of biogeochemical and atmospheric modeling approaches, we show that increases in early season net ecosystem uptake explain the recent trends in the seasonal cycle. A strong year-to-year correlation between spring temperatures and early season uptake further suggests that increased photosynthetic activity is the primary mechanism. At the end of the growing season, a strong correlation between fall temperatures and late season releases provides evidence for a large active pool of decomposing soil carbon. Taken together, our results suggest that the seasonal timing of temperature anomalies may have important consequences for the interannual carbon balance of northern ecosystems.

Seasonal partitioning of nitrogen by plants and soil microorganisms in an alpine ecosystem

Abstract: The seasonal dynamics of plant N assimilation and microbial N immobili- zation were studied in an alpine ecosystem to evaluate temporal patterns of plant and microbial N partitioning and the potential for plant vs. microbial competition for N. Plant N uptake was higher in the first half of the growing season than later in the season, as indicated by changes in biomass N and by 15 N uptake. Microbial N pools were low during the first half of the growing season (9.5 g N/m 2 on 1 June) and increased late in the season, from 11.4 g N/m 2 on 1 August 1991 to 38.6 g N/m 2 on 14 October 1991. Two different measures of N availability were highest in the midseason. Ion exchange resin bag N uptake was greatest in July (86.0 m gN · g

Restored cut-away peatland as a sink for atmospheric CO2.

Abstract: In a field study, we examined the relationship between vegetation, abiotic factors and the CO2 exchange dynamics of a cut-away peatland 20 years after production had ended. The main objective was to determine the effect of rewetting on the CO2 exchange dynamics, measured separately in Eriophorum vaginatum tussocks and intertussocks (almost non-vegetated surfaces) using closed-chamber techniques, one growing season before and three growing seasons after the rewetting treatment. Rewetting lowered total respiration (RTOT) and increased gross photosynthesis (PG), which resulted in a higher incorporation of CO2 into the system. The seasonal CO2 balance for the almost continuously submerged section of the rewetted site became positive 2 years after rewetting (9.1 g CO2-C m−2), and it was still higher in the 3rd year (64.5 g CO2-C m−2), i.e. the system accumulated carbon. In the driest section of the rewetted site the seasonal balance increased strongly, but the balance was still negative during the 3 years following rewetting with losses from the system of 44.1, 26.1, 38.3 g CO2-C m−2 in 1995, 1996 and 1997 respectively. At the control site seasonal balance during 1995–1997 varied between ecosystem C losses of 41.8 and 95.3 in an area with high Eriophorum cover and between 52.1 and 109.9 g CO2-C m−2 with lower cover. Simulation of a cut-away peatland with dense Eriophorum vegetation (Eriophorum cover 70%) showed that if the water level (WT) is low, the seasonal CO2 balance of the ecosystem can reach the compensation point of no net C change (PG = RTOT) only if weather conditions are favourable, but with a high WT the seasonal CO2 balance would be positive even under varying weather conditions. It can be concluded that with dense Eriophorum vegetation a restored cut-away peatland acts as a functional mire and becomes a sink for atmospheric CO2.

Wintertime CO2 efflux from Arctic soils: Implications for annual carbon budgets

Abstract: Estimates of annual carbon loss from arctic tundra ecosystems are based nearly entirely on measurements taken during the growing season in part because of methodological limitations but also reflecting the assumption that respiration during winter is near zero. Measurements of CO2 flux during winter, however, indicate significant amounts of carbon loss from tundra ecosystems throughout the 240-day nongrowing season. In our study during the 1996 and 1997 nongrowing seasons, winter carbon losses ranged from 2.0 g CO2 m−2 season−1 in moist dwarf shrub communities to 97 g CO2 m−2 season−1 in natural snowdrift communities, with an average wintertime CO2 efflux of 45 g CO2 m−2 for all Low Arctic tundra communities (0.14 Pg CO2 yr−1 worldwide). These measurements indicate that current estimates of annual carbon loss from tundra ecosystems are low. Inclusion of wintertime losses of CO2 into annual carbon budgets increases the annual carbon efflux of arctic tundra ecosystems by 17% and changes some ecosystems from net annual sinks to net sources of CO2 to the atmosphere.

Factors controlling the establishment of fremont cottonwood seedlings on the upper Green River, USA

Abstract: Declines in cottonwood (Populus spp.) recruitment along alluvial reaches of large rivers in arid regions of the western United States have been attributed to modified flow regimes, lack of suitable substrate, insufficient seed rain, and increased interspecific competition. We evaluated whether and how these factors were operating during 1993–1996 to influence demographics of Fremont cottonwood (P. deltoides Marshall subsp. wislizenii (Watson) Eckenwalder) along reaches of the Green and Yampa Rivers near their confluence in northwestern Colorado. We examined seedling establishment, defined as survival through three growing seasons, at three alluvial reaches that differed primarily in the level of flow regulation: a site on the unregulated Yampa, an upper Green River site regulated by Flaming Gorge Dam, and a lower Green River site below the Green–Yampa confluence. Seed rain was abundant in all sites, and led to large numbers of germinants (first-year seedlings) appearing each year at all sites. The regulated flow in the upper Green River reach restricted germination to islands and cut banks that were later inundated or eroded; no seedlings survived there. Mortality at the lower Green River site was due largely to desiccation or substrate erosion; 23% of 1993 germinants survived their first growing season, but at most 2% survived through their second. At the Yampa River site, germinants appeared on vegetated and unvegetated surfaces up to 2.5 m above base flow stage, but survived to autumn only on bare surfaces at least 1.25 m above base flow stage, and where at least 10 of the upper 40 cm of the alluvium was fine-textured. Our studies of rooting depths and the stable isotopic composition of xylem water showed that seedlings in the most favorable locations for establishment at the Yampa site do not become phreatophytic until their third or fourth growing season. Further, the results of experimental field studies examining effects of shade and competition supported the hypothesis that insufficient soil moisture, possibly in combination with insufficient light, restricts establishment to unvegetated sites. Collectively, the demographic and experimental studies suggest that, in arid regions, soil water availability is at least as important as light level in limiting establishment of Fremont cottonwood seedlings. We hypothesize that in cases where arid land rivers experience large spring stage changes, recruitment is further constrained within bare areas to those sites that contain sufficient fine-textured alluvium, saturated during the spring flood, to provide the flood-derived soil moisture normally necessary for late-summer seedling survival. Copyright © 1999 John Wiley & Sons, Ltd.

Effects of water status and soil fertility on the C-isotope signature in Pinus radiata.

Abstract: The efficiency with which trees use water is a major determinant of growth under water-limited conditions. We investigated whether increased access to water and nutrients alters water-use efficiency in Pinus radiata D. Don. Intrinsic transpiration efficiency, defined here as the ratio of CO(2) assimilated and water transpired at a given vapor pressure deficit, is determined by the difference between ambient atmospheric CO(2) concentration (c(a)) and leaf intercellular CO(2) concentration (c(i)). The mean value of c(i)/c(a) can be inferred from an analysis of carbon isotope discrimination (Delta) in wood samples. A total of 117 trees, growing at sites with widely varying soil and climatic conditions in Australia and New Zealand, were cored and distinct annual rings were analyzed for their carbon isotope ratio, and correlated with rainfall during the July-June growing season in the year in which the wood was grown. Where possible, carbon isotope ratios were compared for different years within the same trees. The c(i)/c(a) ratio decreased with decreasing water availability, suggesting that intrinsic transpiration efficiency increased with decreasing water availability. An increase in growing season rainfall of 900 mm resulted in an increase in Delta of about 2.0 per thousand, corresponding to a decrease in intrinsic transpiration efficiency of approximately 24%. A stronger relationship was obtained when carbon isotope discrimination was expressed as a function of the ratio of rainfall to potential transpiration. Carbon isotope discrimination was also negatively correlated with mean annual vapor pressure deficit at different sites. In contrast, nutrient availability had no significant effect on carbon isotope discrimination.

Plant performance and soil nitrogen mineralization in response to simulated climate change in subarctic dwarf shrub heath

Abstract: To simulate a future, warmer climate, we subjected subarctic dwarf shrub heath to 5°C direct soil warming for five consecutive growing seasons (1993-1997). Supplemental air warming treatments were imposed on warmed soil by plastic tents in 1994 and open-top chambers in 1995. Plant responses to warming were assessed by changes in: 1) shrub phenology, 2) current-year aboveground biomass in the dominant shrubs (Empetrum hermaphroditum. Vaccinium myrtillus, V. uliginosum and V. ritis-idaca), and 3) vascular and nonvascular plant cover. We estimated warming effects on soil nitrogen (N) availability by in situ buried bag incubation of soils. Soil warming stimulated soil N cycling and shrub growth and development in the short term (2.3 yr). In the second year, net N mineralization rates doubled in warmed soil (4.3 kg N ha -1 season -1 in untreated soil vs 9.2 kg ha -1 scason -1 ). Greater N availability likely contributed to the observed 62% increase in current-year growth of V. myrtillus. the dominant deciduous shrub. In the third year, soil and air warming increased shoot production by > 80% in the evergreen shrubs V. vitis-idaea and E. hermaphroditum. Soil warming had no detectable effects on plant growth or soil N cycling in the fifth year, suggesting that the long-term response may be less dramatic than short-term changes Past fertilization studies in arctic and subarctic tundra reported an increase in the abundance of graminoids. Despite enhanced soil N mineralization in the second year, we found that warming had little effect on plant community composition after five years Even in an extreme climate warming scenario, it appears that subarctic soils mineralize an order of magnitude less N than was applied in fertilization experiments. High-dose fertilization studies provide insight into controls on plant communities but do not accurately simulate increases in N availability predicted for a warmer climate.

Long‐term observation of the atmospheric exchange of CO2 with a temperate deciduous forest in southern Ontario, Canada

Abstract: This paper reports the results of the analysis of eddy covariance CO2 data obtained at a successional forest of maple and aspen at Camp Borden in southern Ontario, Canada, between July 1995 and December 1997. Main findings are (1) The Michaelis-Menton model explains .50 - 65% of the observed variance of the daytime net ecosystem carbon exchange (NEE) during the growing season; leaf wetness appears to be an important variable contributing to the remaining variance. (2) The whole-ecosystem respiration rate as a function of the 5-cm soil temperature shows a seasonal "hysteresis" (higher rate in the later part of the year), suggesting a nonnegligible contribution by deep soil/roots and the influence of litter age. (3) There is evidence of photosynthetic activities immediately after the spring snowmelt/soil warming, but the daily NEE did not switch sign till about 40 days later; our best estimates of the annual net carbon uptake by the ecosystem (net ecosystem production (NEP)) are 21.0, 21.2, and 2 2.8 tCh a 21 yr 21 for the periods July 19, 1995, to July 18, 1996, January 1 to December 31, 1996, and January 1 to December 31, 1997, respectively, with an uncertainty of 6 0.4 tCh a 21 yr 21 . (4) The higher NEP value in 1997 than in 1996 was caused by lower growing season soil temperature, cooler spring and fall transitional periods, and higher photon flux in 1997; possible enhancement in canopy photosynthetic capacity may also have played a role. In addition, three main sources of uncertainties, data gap, fetch, and mass flow, are discussed. It is suggested that collective use of the methods available for assessing the whole-ecosystem respiration (friction velocity threshold, mass flow theory, and dark respiration from the forest light response) may increase the confidence level of NEP estimates.

Soil Management and Supplemental Irrigation Effects on Potato: II. Root Growth

Abstract: This study was conducted to determine if supplemental irrigation and/or application of soil amendments rich in organic matter can improve root growth of potato (Solanum tuberosum L.). Root length density (RLD) and root dry matter production of potato were studied under two levels of supplemental irrigation (nonirrigated check vs. moderate irrigation) and two levels of soil amendments [none vs. amended with compost and manure from beef cattle (Bos taurus L.)] during 1993 and 1994. Ameudments significantly increased RLD during both growing seasons and these increases occurred consistently throughout each growing season. Irrigation effects were not observed in 1993, while irrigation significantly increased RLD during 1994. Approximately 85% of the root length was concentrated in the upper 30-cm layer of the soil. The supplemental irrigation and soil amendment treatments did not affect the proportional distribution of roots among the soil layers. Root-to-shoot ratio was not affected by irrigation or amendment during either growing season. The RLD and leaf area index (LAI) at all four sample dates during 1994 were significantly correlated with final tuber yields, indicating that growth patterns early in the season were important in establishing the productivity of the potato crop. Root length density tended to have a higher correlation with yields than did LAI (r = 0.58 * to 0.80 ** vs. 0.51 * to 0.68 ** ).

Control of plant species richness and zonation of functional groups along a freshwater flooding gradient

Abstract: We investigated the role of plant interactions in producing the zonation of strong competitors, i.e. clonal dominants, and weak competitors, i.e. interstitials. In addition, it was tested whether the effect of plant interactions on species richness depends on the position on the flooding gradient. During one growing season vegetation canopy was removed at a low elevation, dominated by Phragmites australis and at a high one, dominated by tall forbs, mainly Urtica dioica. The seed bank was examined and in half of the plots seeds of clonal dominants from high elevations and interstitials from low elevations were sown to ensure that seeds of both groups were present at both zones. At both elevations, removal of vegetation canopy resulted in a strong increase of interstitial species, but interstitials from low elevations failed to establish in cleared plots at the tall forb zone. This could not be attributed to the absence of seeds and we concluded that conditions unfavourable for germination, rather than plant interactions determine the zonation of interstitials from the P. australis zone. Many seedlings of tall forb dominants emerged in cleared plots at the low elevation. However, number of seedlings rapidly declined during the first year. Hence, abiotic conditions, most probably flooding, prevented seedling establishment of tall forb dominants at zones dominated by P. australis. Canopy removal increased species richness at the low elevation in the first year only, whereas at the higher elevation species richness in cleared plots remained higher throughout the second year when the canopy was no longer removed. We therefore concluded that species richness of freshwater shorelines is controlled by abiotic factors in the frequently flooded zone and by plant interactions at higher elevations. [KEYWORDS: Water tidal wetland; seed-bank; emergent macrophytes; fen vegetation; salt-marsh; communities; england; growth; level; competition]

Eddy covariance measurements of co2 and energy fluxes of an alaskan tussock tundra ecosystem

Abstract: Eddy covariance was used to measure the net CO2 exchange and energy balance of a moist-tussock tundra ecosystem at Happy Valley, Alaska (69°08.54′ N, 148°50.47′ W), during the 1994–1995 growing seasons (June–August). The system operated for 75–95% of the time, and energy balance closure was within 5%, indicating good system performance. Daily rates of evapotranspiration (ET) were on average 1.5 mm/d, while seasonal ET ranged between 100 and 150 mm. Daily ET was strongly correlated with daily fluctuations in net radiation. However, the “omega factor” (an index of the relative importance of meteorological and physiological limitations to evapotranspiration) was generally <0.5 throughout June and early July, indicating that biological limitations to ET were relatively more important than meteorological limitations during the first half of the growing season. The biological limitation to ET was presumably due to bryophyte desiccation and subsequent reductions in canopy water-vapor conductance, especially under conditions of high evaporative demand. The moist-tussock tundra ecosystem was a net sink for atmospheric CO2 of −3.3 and −4.6 mol/m2 during the 1994 and 1995 growing seasons, respectively (negative flux depicts net CO2 accumulation). Over diel (24-h) periods, 60–90% of the variation in net CO2 exchange was explained as a hyperbolic function of photosynthetic photon flux density (PPFD), while over seasonal time scales, model estimates of the estimated quantum yield and maximum gross assimilation indicate that daily variations in net CO2 uptake were driven more by the seasonal trend in ecosystem phenology than by meteorology. Approximately 70% of the variation in nighttime net CO2 exchange, an estimate of the whole-ecosystem respiration rate, was explained by variations in water-table depth and temperature. Although other environmental factors may be important, interannual differences in observed net CO2 exchange were almost completely explained by the interannual differences in estimated whole-ecosystem respiration.

Effects of agricultural intensification on soil-associated arthropod population dynamics, community structure, diversity and temporal variability over a seven-year period

Abstract: While most studies focusing on the effects of agricultural intensification on soil biota are inherently short-term in nature, long-term (multiyear) studies are essential in assessing long-term temporal responses of soil biota to agronomic practices. We investigated the effects of three components of agricultural intensification, i.e. cultivation (disturbance), herbicide addition (modification of floristic composition) and mulching (resource addition) on soil-associated arthropods in an annual (maize) and a perennial (asparagus) cropping system over a 7 yr period. An additional treatment (hand-hoeing of weeds during the crop growing season) was used to represent minimal intensification. Many taxa of arthropods responded positively to mulching and to treatments which allowed high weed biomass in the non crop-production period, e.g. the hand-hoeing and cultivation treatments in the perennial crop. Herbicide treatments also facilitated high numbers of many taxa in the annual crop when this coincided with plot invasion by herbicide-tolerant weeds. Generally, arthropod taxa were positively correlated with weed biomass and negatively with crop plant biomass, probably because of the superior resource (litter) quality produced by the former. Ordination analyses indicated that arthropod community structure was often correlated with weed community structure. Mulching and allowing high weed biomass also promoted a high species richness of soil-associated Coleoptera, but coleopteran diversity was not related to weed species diversity. Analyses of temporal variability (inversely related to stability) of arthropod taxa across years revealed few treatment effects in the annual crop, but showed destabilising effects of weed reduction in the perennial crop. In the perennial crop, temporal variability was also positively correlated with crop biomass and negatively with weed biomass across plots. Our study shows that agricultural intensification is not consistently harmful to the soil fauna, that soil-associated arthropods are most responsive to management practices which affect the nature and quality of resource input, and that long-term experiments are essential for answering questions about how agricultural practices affect soil organisms against the natural backdrop of temporal variation.

Effects of elevated atmospheric CO2 on phenology, growth and crown structure of Scots pine (Pinus sylvestris) seedlings after two years of exposure in the field

Abstract: Three-year-old Scots pine (Pinus sylvestris L.) seedlings were grown for two years in the ground in open-top chambers supplied with either an ambient or elevated (ambient + 400 mmol mol(-1)) CO(2) concentration. Phenological observations and measurements of height and stem diameter growth, absolute and relative growth rates, starch and soluble carbohydrate concentrations of the needles, and crown structure and needle properties were made at frequent intervals throughout the two growing seasons. Elevated CO(2) significantly advanced the date of bud burst in both years. The increase in total needle area in response to elevated CO(2) was accounted for by longer shoots and an increase in individual needle area in the first year, and by an increase in the number and length of shoots in the second year. Stem diameter and tree height were enhanced more by the elevated CO(2) treatment in the first year than in the second, indicating a decreased effect of CO(2) on growth over time. This was confirmed by a study of absolute and relative growth rates of leader shoots. During the first growing season of CO(2) enrichment, mean weekly relative growth rates over the growing season (RGR(m)) were significantly enhanced. During the second year, RGR(m) in ambient CO(2) closely matched that in elevated CO(2).

Maize residue decomposition measurement using soil surface carbon dioxide fluxes and natural abundance of carbon-13

Abstract: The decomposition rate of crop residues in soils directly impacts organic matter content and nutrient cycling. We hypothesized that natural abundance 13 C analyses could be used with soil CO 2 flux measurements to quantify the short-term decomposition rates of maize (Zea mays L.) residues under undisturbed field conditions. For this purpose, maize was grown in a sandy loam (Umbric Dystrochrept) that developed under C3 vegetation. Residues were returned to the field at the end of the growing season. During the following snowfree period (May to November), the maize residue decomposition rate was calculated for plots that were either under no-till or moldboard plowed, using the C isotope ratio ( 13 C/ 12 C) of the soil CO 2 , the C isotope ratio of the plant and soil substrates, and the soil respiration rate. The incorporation of residue-derived C into the soil microbial biomass was also evaluated. Maize residue decomposition increased the C isotope ratio of the soil CO 2 by 2 to 7‰ relative to unamended control plots. Decomposition rates peaked in June (2-3 g C m -2 d -1 ) and were low at both the beginning and end of the growing season (<0.5 g C m -2 d -1 ). For a given soil temperature, the decomposition was more active early than late in the season because of decreased substrate availability as decomposition proceeded. The decomposition rate of maize-derived C correlated with the fraction of the microbial biomass derived from maize residues. This active pool represented 9% of microbial biomass and showed a high level of specific activity. The total maize residue-C losses during the study corresponded with 35% of the added residue C under no-till plots and 40% with moldboard plowing. Natural abundance 13 C analyses may be successfully used with respiration measurements to quantify crop residue decomposition rates under undisturbed field conditions.

Switchgrass biomass and chemical composition for biofuel in Eastern Canada.

Abstract: Switchgrass (Panicum virgatum L.) is one of several warm-season grasses that have been identified as potential biomass crops in North America. A two-year field study was conducted, on a free-draining sandy clay loam (St. Bernard, Typic Hapludalf), to characterize the growth and evaluate changes in biomass accumulation and composition of switchgrass at Montreal, QC. Three cultivars, Cave-in-Rock, Pathfinder, and Sunburst, were grown in solid stands in a randomized complete block design. Canopy height, dry matter (DM) accumulation and chemical composition were monitored biweekly throughout the growing season. Average maximum canopy heights were 192.5 cm for Cave-in-Rock, 169.9 for Pathfinder, and 177.8 for Sunburst. The respective end-of-season DM yields were 12.2, 11.5, and 10.6 Mg/ha. Biomass production among cultivars appeared to be related to time of maturation. Nitrogen concentration of DM decreased curvilinearly from 25 g/kg at the beginning of the season to 5 g/kg DM at season's end. Both acid-detergent fiber (ADF) and neutral-detergent fiber (NDF) concentrations increased to a maximum early in the season, after which no changes were detected. The average maximum values of ADF and NDF were, respectively, 647.6 and 849.0 g/kg DM for Cave-in-Rock, 669.1 and 865.2 for Pathfinder, and 661.8 and 860.9 for Sunburst. Changesmore » in canopy height, DM accumulation, and chemical composition could all be described by predictive regression equations. These results indicate that switchgrass has potential as a biomass crop in a short-season environment.« less

Photosynthesis and stomatal conductance of mature canopy Oak (Quercus robur) and Sycamore (Acer pseudoplatanus) trees throughout the growing season

Abstract: 1. Gas exchange is strongly seasonal in temperate deciduous forests; however, virtually no studies have described measurements made at the leaf scale through the whole growing season. This level of detail is important to understanding processes as substantial differences may occur within the canopy between species and between sun and shade leaves. 2. Gas exchange was measured weekly throughout the growing season in leaves of Oak (Quercus robur) and Sycamore (Acer pseudoplatanus) trees at Wytham Woods, UK. In Q. robur sun, shade and intermediate leaves were sampled on each occasion. 3.Acer pseudoplatanus came into leaf before Q. robur and commenced net assimilation (A) earlier. Maximum irradiance saturated photosynthetic rates (Amax) in Q. robur were reached approximately 50 days after budburst and were maintained until approximately 20 days before abscission. Acer pseudoplatanus lost its leaves before Q. robur in the autumn and showed no seasonal trend in Amax. The lag between budburst and attainment of maximum photosynthetic rate in Q. robur was substantial and represented a quarter of the potential growing season. 4.Amax of A. pseudoplatanus (3·5 μmol m–2 s–1) sun leaves was substantially lower than that of Q. robur (10·4 μmol m–2 s–1) despite A. pseudoplatanus being a fast-growing species. The earlier flushing of A. pseudoplatanus leaves in the spring could not completely compensate for this. It is likely that other variables, such as total leaf area, are responsible for the high growth rate of A. pseudoplatanus, rather than high photosynthetic rates. 5. Shade leaves of Q. robur had lower Amax and mean A than sun leaves; their quantum efficiency was also lower. Consistent with this, sun leaves had a higher nitrogen content per unit area, resulting from a lower specific leaf area (SLA). 6. Week to week variations in solar radiation influenced A; this could be described by a non-rectangular hyperbola. Stomatal conductance (gs) decreased in conditions of high vapour pressure deficit (VPD). However, this did not decrease A as high VPD was associated with high solar radiation and the stimulation of photosynthesis caused by high photosynthetic photon flux density (PPFD) more than compensated for the reduction by VPD.

Interactive effects of elevated [CO2] and drought on cherry (Prunus avium) seedlings II. Photosynthetic capacity and water relations

Abstract: Cherry seedlings (Prunus avium) were grown from seed for two growing seasons in three ambient [CO2] (∼350 μmol mol−1) and three elevated [CO2] (ambient+∼350 μmol mol−1) open-top chambers, and in three outside blocks. A drying cycle was imposed in both the growing seasons to half the seedlings: days 69–115 in the first growing season, and in the second growing season days 212–251 on the same seedlings which had already experienced drought. Stomatal conductance was significantly reduced in elevated [CO2]-grown, unstressed seedlings in both the first and second growing seasons, but was not caused by a decrease in stomatal density. Droughted seedlings showed little or no reduction in stomatal conductance in response to elevated [CO2]. However, stomatal conductance was highly correlated with soil water status. Photosynthetic rate increased significantly in response to elevated [CO2] in both water regimes, leading to improvement in instantaneous transpiration efficiency over the whole duration of the experiment, but there was no relationship between instantaneous transpiration efficiency and long-term water use efficiency. The Amax was strongly reduced in the second growing season, but unaffected by [CO2] treatment. Although photosynthetic rate was not down-regulated, Rubisco activity was decreased by elevated [CO2], possibly because of the increased leaf carbon: nitrogen ratio which had occurred by the ends of the two growing seasons. Elevated [CO2] did not improve plant water relations (for example, bulk leaf – water potential, osmotic potentials at full and zero turgor, relative water content at zero turgor, bulk modulus of elasticity of the cell) and thus did not increase water-stress tolerance of cherry seedlings.

Seasonal trends in soil biochemical attributes: Effects of crop management on a Black Chernozem

Abstract: Knowledge of the response of soil biochemical attributes to crop management and growing season weather is important for assessing soil quality and fertility Long-term (38–39 yr) crop rotations on a Black Chernozem at Indian Head, Saskatchewan, were sampled (0- to 75-cm depth) between early May and mid-October, 11 times in 1995 and 9 times in 1996 We assessed the effect of cropping frequency [fallow–wheat (Triticum aestivum L) (F–W) vs F–W–W, vs Continuous (Cont) W], fertilizers (unfertilized vs N + P applied), straw harvesting, legume green-manure (GM) in GM–W–W (unfertilized), and legume-grass hay (H) in F–W–W–H–H–H (unfertilized) systems Changes in organic C and total N (OC, TN), microbial biomass C (MBC), light fraction C and N (LFC and LFN), mineralizable C and N (Cmin and Nmin), and water-soluble organic C (WSOC) were monitored Organic C and TN were constant and unaffected by rotation phase during the season, but most of the other more labile soil biochemical attributes varied during the sea

Mid-Holocene climate in Europe: what can we infer from PMIP model-data comparisons?

Abstract: Within the framework of the PMIP (Paleoclimate Modelling Intercomparison Project), we have compared mid-Holocene climate simulations from 16 atmospheric general circulation models (AGCMs) with new pollen-based reconstructions of the European bioclimatic variables for winter and growing season temperatures as well as annual water budget changes. In winter, some models are able to simulate the reconstructed northeastern warming, due to an increased heat transport from the ocean, associated with a larger north-south pressure gradient over the northern Atlantic. Whereas most models are only able to simulate a strong summer warming, data indicate a shorter and/ or colder growing season in southern Europe and a longer and/or warmer growing season in northwestern Europe. The reconstructed change in annual water budget indicates drier conditions in northwestern Europe and wetter conditions in southern Europe. Some models simulate such moisture changes, due to more summer evaporation over Scandinavia during summer, and more autumn-winter-spring precipitation over southern Europe. To address the PMIP approximation of no change in ocean and land boundary conditions, we have performed short sensitivity experiments to surface boundary conditions (sea-surface-temperatures, vegetation) using one single model. The model-data disagreements over Europe are probably due to the local influence of the surrounding oceans which are not taken into account in the first PMIP simulations. We therefore stress the need for more mid-Holocene SST reconstructions and further analysis of pollen data in the Mediterranean region.

Ozone exposure results in various carry‐over effects and prolonged reduction in biomass in birch (Betula pendula Roth)

Abstract: In the first experiment, saplings of ozone-sensitive and a more tolerant clone of Betula pendula Roth were exposed to ambient ozone (control treatment, accumulated exposure over a threshold 40 nmol mol -1 (AOT40) exposure of 10 μmol mol -1 h) and 15 X ambient ozone (elevated-ozone treatment, AOT40 of 173 μmol mol -1 h) over one growing season, 1996 After over-wintering, the dormant elevated-ozone saplings were transferred to the control blocks and assessed for short-term carry-over effects during the following growing season In the second experiment, three sensitive, four intermediate and three tolerant clones were grown under ambient ozone (control treatment, AOT40 of 05-08 μmol mol -1 h per growing season) and 16-17 X ambient ozone (elevated-ozone treatment, AOT40 of 183-186 μmol mol -1 h per growing season) from May 1994 until May 1996, and were assessed for long-term carry-over effects during growing season 1997, after a 12-16 months recovery period Deleterious short-term carry-over effects of ozone exposure included reduced contents of Rubisco, chlorophyll, carotenoids, starch and nutrients in leaves, lower stomatal conductance, and decreased new shoot growth and net assimilation rate, followed by a 75% (shoot dry weight (DW)), 152% (root DW) and 232% (foliage area) decreased biomass accumulation and yield over the long term, including a reduced root:shoot ratio However, a slow recovery of relative growth rates during the following two seasons without elevated ozone was apparent Several long-lasting structural, biochemical and stomatal acclimation, stress-defence and compensation reactions were observed in the ozone-tolerant clone, whereas in the sensitive clone allocation shifted from growth towards defensive phenolics such as chlorogenic acid The results provide evidence of persistent deleterious effects of ozone which remain long after the ozone episode

Deciduous conifers : High N deposition and O3 exposure effects on growth and biomass allocation in ponderosa pine

Abstract: Ponderosa pines (Pinus ponderosa Dougl. ex. Laws) 21 to 60 yr old were used to assess the relative importance of environmental stressors (O3, drought) versus an enhancer (N deposition) on foliar retention, components of aboveground growth, and whole tree biomass allocation. Sites were chosen across a well-described gradient in ozone exposure (40 to 80 ppb per h, 24 h basis, 6 month growing season) and nitrogen deposition (5 to 40 kg ha-1 yr-1) in the San Bernardino Mountains east of Los Angeles, California. A high level of chlorotic mottle indicated high O3 injury at sites closest to the pollution source, despite potential for the mitigating effects of N deposition. At the least polluted site, foliar biomass was evenly distributed across three of the five needle-age classes retained. At the most polluted site, 95% of the foliar biomass was found in the current year's growth. High N deposition and O3 exposure combined to shift biomass allocation in pine to that of a deciduous tree with one overwintering needle age class. Based on whole tree harvests, root biomass was lowest at sites with the highest pollution exposure, confirming previous chamber exposure and field studies. Aboveground growth responses in the high-pollution sites were opposite to those expected for O3 injury. Needle and lateral branch elongation growth, and measures of wood production increased with increasing proximity to the pollution source. An enhancement of these growth attributes suggested that N deposition dominated the ponderosa pine response despite high O3 exposure.

Biomass production of Salix viminalis in southern Finland and the effect of soil properties and climate conditions on its production and survival

Abstract: Ten Salix viminalis clones were cultivated in 35 experimental plantations situated on privately-owned farms in southern and central Finland. Management instructions were given to the farmers who established and tended the study plots. The biomass production and survival of the clones were recorded by inventory every year during the first rotation period, which consisted of the establishment year and three following growing seasons. The greatest amount of biomass production exceeded 30 above-ground tonne of dry matter/3 years, but amounts varied greatly between the experimental plantations. The main reasons causing this variation were the low frost hardiness of most of the S. viminalis clones and unsuccessful weed control during the year the plantations were established. Early autumn frost in particular caused severe damage. S. viminalis grew rather well on a wide variety of soil types. Sufficient amounts of nitrogen, potassium and organic matter seem to be the properties which most clearly have an effect on the biomass production and survival of S. viminalis in mineral soils. A relatively small amount of manganese in wet and low oxygenic soils is recommended, because a high content may cause toxic effects. In this study S. viminalis clones only succeeded well in southernmost Finland, where properly managed plantations produced rather high biomass yields. In other parts of southern and central Finland more research work on frost resistance and the breeding of new clones is needed. To conclude, in the climatic conditions of Finland energy willow husbandry is a compromise between high biomass yield and frost hardiness.

Impacts of enhanced ultraviolet‐b radiation on mosses in a subarctic heath ecosystem

Abstract: A subarctic heath where Hylocomium splendens and Polytrichum commune dominate the bryophyte layer was irradiated with enhanced UV-B radiation, simulating 15% ozone depletion. The extra UV-B radiation was supplied by fluorescent tubes during three growing seasons. H. splendens reacted quicker and to a greater extent to enhanced UV-B radiation than did P. commune. Annual length increment in H. splendens was unaffected in the first year but was suppressed in the subsequent years. In P. commune, annual length increment did not decrease until the third growing season. Dry mass production of H. splendens decreased in segments from all three years. Annual dry mass production in P. commune did not change due to enhanced UV-B. Shoot morphology was measured after the third growing season. In H. splendens, the segment area decreased. Leaf density along P. commune shoots increased, and the shoots became stunted. The UV-B enhancement induced a higher variance in the concentration of UV-B absorbing compounds in H. splendens after the first and the third growing season. In P. commune, the variance remained unaffected, but the concentration was significantly decreased after the third year. Indirect UV-B impacts on intra- and interspecific relations, which may be mediated by changes in shoot growth and morphology, are discussed.