In northern Patagonia, Argentina we quantify changes in fire frequency along a gradient from mesic Nothofagus dombeyi forest to xeric woodlands of Austrocedrus chilensis at the steppe ecotone, and we examine patterns of vegetation change coincident with the changes in fire regimes across a range of spatial scales. At a regional scale changes in land cover types are documented by comparing 1:250000 scale cover type maps from 1913 and 1985. Changes in landscape structure are analyzed by comparing vegetation patterns on 1:24000 scale aerial photographs taken in 1940 and 1970. Fire frequency peaked in the late nineteenth-century due to widespread burning and clearing of forests by European settlers late in the century. Subsequently, fire frequency declined dramatically about 1910 due to the cessation of intentional fires and has remained low due to increasingly effective fire exclusion. At a regional scale there has been a dramatic increase during the twentieth century in the proportion of forest cover relative to areas mapped as recent burns or shrublands in 1913. Remnant forest patches that survived the widespread late-nineteenth century burning have coalesced to form more continuous forest covers, and formerly continuous areas of shrublands have become dissected by forest. Under reduced fire frequency there has been a shift in dominance from short-lived resprouting species (mostly shrubs) towards longer-lived species and obligate seed-dispersers such as Austrocedrus chilensis and Nothofagus dombeyi. Due to limited seed dispersal of these tree species, the spatial configuration of remnant forest patches plays a key role in subsequent changes in landscape pattern. 相似文献
The aim of this analysis was to characterise the temporal pattern of infection during the 1997/98 classical swine fever (CSF) epidemic in The Netherlands and hence identify and quantify risk factors for infection in different enterprise types and areas. Survival analysis and Cox proportional hazards regression were used to describe the epidemic. Substantial differences in temporal survival patterns (herd breakdown rate) were found between areas where different control policies operated. Factors with a significant influence on the infection hazard of individual herds included: sow numbers as a percentage of total sows and fatteners (HR = 3.38 for mixed herds (0.1–60% sows) vs. fattening herds (0% sows) and HR = 2.74 for breeding herds (60–100% sows) vs. fattening herds), the number of ‘transport contacts per month’ (>0.3 vs. <0.3; HR = 4.11), pig density (pigs/km2) in the area (HR1000 pigs 1.48) and herd size (HR100 pigs = 1.01).
Pre-emptive slaughter in an area appeared to be associated with lower subsequent disease levels. Higher frequency of transport contacts for welfare slaughter during the epidemic, however, well regulated and controlled, was associated with a substantially higher risk of becoming infected. The positive association of a higher pig density with CSF indicates the potential importance of local spread as a factor in disease transmission and emphasizes that dilution of the pig population can contribute to reduction in CSF occurrence. This analysis suggests however, that if pre-emptive slaughter can promptly be applied effectively in an area after initial diagnosis, pig density is then not a significant factor. Mixed and breeding herds had a higher probability of becoming infected than fattening herds, possibly due to different types and frequencies of inter-herd contacts. These contacts continue to some extent during the epidemic, despite the standstill of animal movements. 相似文献
The effects of intercrop spacing patterns on the silage yields of both maize (lea mays L.) and soybean (Glycine max [L.] Merr.) were examined from 1985 to 1987. Dwarf maize was intercropped with nonnodulatmg or nodulating soybean in the spacing patterns, S40same (two crops in the same row, 40 cm row width) and S20ait or S40ak (two crops in alternate rows, 20 cm or 40 cm row width, respectively). Tall maize was intercropped with nodulating soybean in S40sames S40alt and S40pair (maize in 40 cm paired rows, soybean rows 20 cm outside each maize row and 80 cm from the next set of four rows) at 0 or 60 kg N ha−1 and at population densities of 67% maize: 67% soybean or 50 % maize: 50% soybean. Maize and soybean were also intercropped and stripcropped on a farm-scale. The only difference between intercrops arranged in the same rows versus those in alternate rows was that the average soybean protein yields were higher in S40same than in S40alt. In 1986, the S40alt maize-soybean intercrops produced higher maize yields, total biomass yields and Land Equivalent Ratios (LERs) than in S40pirs, and in 1987, these responses were higher in intercrops than in stripcrops. In 1986, at 0 kg N ha−1, the soybean biomass and protein yields were lower in S40alt, than in S40pairs and in 1987, these responses were lower in intercrops than in stripcrops. 相似文献
This study compared the sensitivity of ECG and cardiac troponins to predict cardiac histopathological changes, clinical severity, and survival in canine babesiosis. One control group (n = 9) and 4 groups of dogs with mild uncomplicated babesiosis (n = 8), severe uncomplicated babesiosis (n = 9), complicated babesiosis (n = 8), and babesiosis and concurrent immune-mediated hemolytic anemia (IMHA) (n = 9) were studied. A 1-minute lead II ECG was recorded, and cardiac troponin I (cTnI) and T (cTnT) concentrations in plasma were measured. cTnI concentrations were significantly higher in the complicated (mean, 9.9; SE, +/-5.76) and concurrent IMHA (mean, 6.53; SE, +/-4.32) groups and in the 3 dogs that died of the disease (mean, 22.17; SE, +/-12.85) than in the control dogs (concentration below detection limit of test, -0.3 ng/mL). The 3 nonsurvivors had the most severe cardiac histopathological changes, but no arrhythmia and minimal other ECG changes. Dogs with babesiosis developed a variety of ECG abnormalities, but the abnormalities were not associated with disease severity, outcome, or plasma cardiac troponin concentrations. The exception was the presence of ventricular premature complexes (VPCs), which were associated with high cardiac troponin concentrations. This study showed an association between cTnI concentration and histological changes, clinical severity, and survival and no correlation between ECG abnormalities and histological changes or biochemical evidence of myocardial damage as reflected by cTnI concentrations. From this study, it was concluded that the analysis of plasma cTnI is a feasible and sensitive test and is superior to cTnT in diagnosing cardiac involvement in dogs with babesiosis. 相似文献