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91.
David Fowler J. Neil Cape Mhairi Coyle Chris Flechard Johan Kuylenstierna Kevin Hicks Dick Derwent Colin Johnson David Stevenson 《Water, air, and soil pollution》1999,116(1-2):5-32
The tall, aerodynamically rough surfaces of forests provide for the efficient exchange of heat and momentum between terrestrial surfaces and the atmosphere. The same properties of forests also provide for large potential rates of deposition of pollutant gases, aerosols and cloud droplets. For some reactive pollutant gases, including SO2, HNO3 and NH3, rates of deposition may be large and substantially larger than onto shorter vegetation and is the cause of the so called "filtering effect" of forest canopies. Pollutant inputs to moorland and forest have been compared using measured ambient concentrations from an unpolluted site in southern Scotland and a more polluted site in south eastern Germany. The inputs of S and N to forest at the Scottish site exceed moorland by 16% and 31% respectively with inputs of 7.3 kg S ha-1 y and 10.6 kg N ha-1 y-1. At the continental site inputs to the forest were 43% and 48% larger than over moorland for S and N deposition with totals of 53.6 kg S ha-1 y-1 and 69.5 kg N ha-1 y-1 respectively. The inputs of acidity to global forests show that in 1985 most of the areas receiving > 1 kg H+ ha-1 y-1 as S are in the temperate latitudes, with 8% of total global forest exceeding this threshold. By 2050, 17% of global forest will be receiving > 1 kg H-1 ha-1 as S and most of the increase is in tropical and sub-tropical countries. Forests throughout the world are also exposed to elevated concentrations of ozone. Taking 60 ppb O3 as a concentration likely to be phytotoxic to sensitive forest species, a global model has been used to simulate the global exposure of forests to potentially phytotoxic O3 concentrations for the years 1860, 1950, 1970, 1990 and 2100. The model shows no exposure to concentrations in excess of 60 ppb in 1860, and of the 6% of global forest exposed to concentrations > 60 ppb in 1950, 75% were in temperate latitudes and 25% in the tropics. By 1990 24% of global forest is exposed to O3 concentrates > 60 ppb, and this increases to almost 50% of global forest by 2100. While the uncertainty in the future pollution climate of global forest is considerable, the likely impact of O3 and acid deposition is even more difficult to assess because of interactions between these pollutants and substantial changes in ambient CO2 concentration, N deposition and climate over the same period, but the effects are unlikely to be beneficial overall. 相似文献
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The ultrastructure of porcine circovirus was examined in persistently infected porcine kidney (PK)-15 cells. Virus-infected PK-15 cells had large numbers of intracytoplasmic inclusions, and a few cells had intranuclear inclusions. Intracytoplasmic inclusions were dispersed throughout the cytoplasm but were most numerous in the perinuclear cytoplasm. Inclusion were of various sizes, round to oval, and electron dense and were of two general types. Inclusions of the first type were small (0.1-0.5 microm diameter), not surrounded by trilaminar membranes, and granular with indistinct margins that blended with surrounding cytoplasm. Some contained 12+/-2-nm-diameter icosahedral virions in loose aggregates or rarely forming paracrystalline arrays. Small inclusions could be sites of viral assembly or maturation. Intracytoplasmic inclusions of the second type were larger (0.5-5.0 microm diameter) and more numerous and had abrupt margins surrounded by trilaminar membranes. They were more electron dense than small inclusions and were heterogeneous, containing various proportions of aggregated virions, electron-dense crystalline lamellae of 5 nm periodicity, and/or whorls of myelinoid membranes. Virions usually formed paracrystalline arrays and occasionally were loosely aggregated. Larger inclusions were typical of autophagolysosomes. Intranuclear inclusions were not membrane bound and were often associated with reticulated nucleoli or aggregates of heterochromatin. Some inclusions were irregularly shaped aggregates of indistinct, circular 10-12-nm-diameter viruslike particles. Others were 0.1-1.0 microm in diameter, round or ring shaped, dense, and finely granular, with sharply demarcated margins. 相似文献
94.
G C Lamb B L Miller J M Lynch K E Thompson J S Heldt C A L?est D M Grieger J S Stevenson 《Journal of animal science》1999,77(8):2207-2218
Two experiments were conducted to determine whether milking beef cows two or five times daily in the presence or absence of their own nonsuckling calves would alter postpartum interval to first ovulation. Multiparous Angus x Hereford cow-calf pairs were assigned randomly between 13 and 18 d postpartum to treatments for 4 wk. In Exp. 1, pairs were assigned to six treatments: 1) calf was weaned permanently from its dam (CW; n = 9); 2) same as CW, but dam was milked twice daily (CW+2xM; n = 9); 3) calf was present continuously with its dam but restricted from contact with the udder (CR; n = 9); 4) same as CR, but dam was milked twice daily (CR+2xM; n = 9); 5) same as CR, but calf was allowed to suckle twice daily (CR+2xS; n = 8); and 6) calf was present continuously with its dam and suckled ad libitum (CP; n = 9). The interval from onset of treatments to first postpartum ovulation was shorter (P<.05) in the CW (14.1+/-3.1 d), CR (14.2+/-3.1 d), CW+2xM (13.0+/-3.1 d), and CR+2xM (17.2+/-3.1 d) than in the CP (34.7+/-3.1 d) and CR+2xS (33.9+/-3.3 d) treatments. Daily milk yield during treatment was greater (P<.01) for CR+2xM cows (7.1+/-.6 kg) than for CW+2xM cows (3.5+/-.6 kg). In Exp. 2, cow-calf pairs were assigned to three treatments: 1) CR+2xM (n = 10); 2) same as CR+2xM but cows were milked five times daily (CR+5xM; n = 10); or 3) CP (n = 10). The interval to first postpartum ovulation was shorter (P<.05) in the CR+2xM (23.6+/-3.5 d) and CR+5xM (26.1+/-3.7 d) treatments than in the CP (37.7+/-3.7 d) treatment. Daily milk yield during treatment was greater (P<.05) for CR+5xM cows (7.7+/-.6 kg) than for CR+2xM cows (6.4+/-.6 kg) by 17%. We conclude that suckling twice daily was sufficient to prolong postpartum anestrus as much as suckling ad libitum. Furthermore, milk removal by suckling, but not by milking two or five times daily, even in the presence of the cow's own nonsuckling calf, is essential to prolong postpartum anovulation. 相似文献
95.
Stevenson MA Wilesmith JW Ryan JB Morris RS Lockhart JW Lin D Jackson R 《The Veterinary record》2000,147(13):349-354
The objectives of this study were first to determine the cumulative incidence of bovine spongiform encephalopathy (BSE) in the British cattle population from July 1986 to June 1997, secondly, to identify individual animal-associated risk factors that influenced the age of onset of clinical signs in confirmed BSE cases, and, thirdly, to assess the effectiveness of the measures introduced to control BSE during the epidemic. The analyses were based on the population of British cattle at risk, derived from agricultural census data collected between 1986 and 1996, and BSE case data collected up to June 30, 1997. The unit of interest was individual adult cattle recorded on annual agricultural censuses between June 1986 and June 1996. Univariate and multivariate survival analysis techniques were used to characterise the age of onset of clinical signs. In total 167,366 cases of BSE were diagnosed in Great Britain up to June 30, 1997. The cumulative incidence of BSE between July 1986 and June 1997 was 1.10 (95 per cent confidence interval [CI] 1.09 to 1.10) cases per 100 adult cattle at risk. Cattle from the South east, South west and Eastern regions of England had 4.26 to 5.96 (95 per cent CI 4.15 to 6.14) times as great a monthly hazard of being confirmed with BSE as cattle from Scotland. Compared with cattle born before June 1985, those born between July 1987 and June 1988 had 22.5 (95 per cent CI 22.1 to 22.8) times the monthly hazard of being confirmed with BSE, whereas those born in the 12 months after July 1988 had only 7.39 (95 per cent CI 7.24 to 7.54) times the monthly hazard of being confirmed with BSE. This reduction in hazard was directly attributable to the ban on the use of ruminant protein as a feed instituted in July 1988. Successive cohorts from 1989 to 1991 experienced further reductions in the hazard of experiencing BSE. The additional decrease in hazard observed for the 1990 cohort may be attributed to the effect of the Specified Bovine Offal ban instituted in September 1990. 相似文献
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