Bacterioplankton in the acidified Lake Gårdsjön

Total bacterial numbers in different strata of lake water and in inlet and outlet streams have been recorded during a yearly cycle. A calculated mean cell volume of 0.342 µm² has then been used to estimate bacterial biomass in the lake. Change of biomass during the year was substantial and the range was from about 0.1 g · m^–3 to about 1.0–1.2 g · m^–3. The seasonal development included a spring-early summer increase followed by a decrease to the minimum in July–August. Correlation between epi- and hypolimnion was high and in both strata two dominant autumn peaks in biomass appeared. With the exception of the last autumn peak the development of bacterial biomass was closely related to development of phytoplankton biomass and production.     

Competition between laboratory populations of green leafhoppers,Nephotettix spp. (Homoptera: Cicadellidae)

Intra- and interspecific competition between laboratory populations of four green leafhoppers, Nephotettix spp. was studied in the laboratory under three different temperature regimes of 24°C, 27°C and 30°C. For the single-species population of the three tropical species, the equilibrium density increased as the temperature increased. On the other hand, for the temperature species N. cincticeps, the highest equilibrium density was at the intermediate temperature and the lowest at high temperature. Interspecific interactions between two tropical (N. virescens vs. N. nigropictus), a tropical and a temperature (N. virescens vs. N. cincticeps) and a rice-feeding and a grass-inhabiting (N. virescens vs. N. malayanus) Nephotettix species were also studied in the laboratory at the three temperature regimes. Temperature differentially affected the outcome of competition between two Nephotettix species. Between N. virescens and N. nigropictus, the latter was more successful over the former at low and intermediate temperatures, while the former was more successful at high temperature. Between N. virescens and N. cincticeps, the temperate species inhibited the growth of the tropical species at low temperature while the tropical species inhibited the growth of the temperate species at high temperature. At intermediate temperature, the population of N. virescens persisted at a slightly higher density over the population of N. cincticeps. Between the rice-feeding N. virescens and the grass-inhabiting N. malayanus, regardless of temperature the population density of the latter was greatly reduced and later became extinct while the population of the former continued its growth. These consequences of competition between two Nephotettix species conformed fairly well to those predicted by theLotka-Volterra model using demographic parameters specified for each species.     

阔叶红松林合理经营与土壤微生物活性

森林是可再生的自然资源,研究证明,阔叶红松林具有很高的生产力,因为它具有最优化的林分结构,这个结构特点,使它的复层异龄和针阔混交的群落结构影响下的土壤肥力较高,土壤微生物类群复杂,活动活跃,森林有机残体的分解与转化较快。系统内的氮、碳及无机养分的循环,能满足林木生长对养分不断增长的需要。系统内各个生态因子处于动态平衡状态。因此,为了不断提     

Response of pondcypress growth rates to sewage effluent application

Cores were collected from dominant pondcypress trees growing in a swamp that had received sewage effluent for 7 yr and a nearby control swamp to determine the combined effects of changes in nutrient supply and hydrologic regime on tree growth. The cores were used to measure two indices of tree growth: basal area increment (BAI) and relative basal area increment (RBAI, which accounts for differences in growth due to the size of teh tree) between 1970–1983 while one swamp remained untreated and the other received weekly additions of sewage effluent from 1974–1981. Throughout the whole period, the mean BAI and RBAI of pond-cypress trees in the untreated swamp remained unchanged, ranging between 5.55–6.38 cm² yr^–1 and 1.09–1.27% yr^–1, respectively. In contrast, trees in the treated swamp increased their BAI approximately two-fold from 7.40 cm² yr^–1 prior to treatment to 14.83 cm² yr^–1 after the onset of treatment and maintained this rate of growth in the 2 yr period after cessation of treatment. Relative basal area increment showed a similar response, but the proportional increase due to treatment was less (1.5-fold factor) than for BAI. The response of pondcypress trees to the sewage effluent differed depending upon whether the trees were located in the deep or shallow water zones. Trees in the deep zone of the treated swamp had lower BAIs and RBAIs than those in the shallow zone during the treatment period, whereas in pre- and post-treatment periods growth indices were equal in both zones. No significant differences in growth between deep and shallow zones were observed during all three time periods in the control swamp.     

Breeding phenology,variation in reproductive effort and offspring size in a tropical population of the woodlouse Porcellionides pruinosus

Summary Most species of woodlice in temperate habitats have discrete breeding seasons. It is hypothesised that breeding synchronises with favourable environmental conditions to maximise offspring growth and survivorship (Willows 1984). We measured the breeding phenology of a species introduced to a tropical environment, primarily to consider the assumption that life histories in the tropics will differ fundamentally from those in temperate habitats. In addition to breeding phenology we considered variation in reproductive effort between individual females and the division of this effort between the size and number of young.A continuous breeding phenology was observed in a synanthropic population of Porcellionides pruinosus within the tropics. Reproductive effort varied between months, showed a weak relationship with female size and was independent of female fecundity. Female sizefecundity relationships varied between samples and when the proportion of reproductive females was high size-fecundity slopes were steeper than at other times. Mean offspring size varied between months and there was a wide range in offspring size within broods. Offspring size was not related to female body mass, reproductive effort or fecundity; consequently brood mass increased linearly with an increase in fecundity. Increased reproductive effort goes into more rather than larger offspring.We propose that the continuous breeding in this population was the result of the constant presence of an environmental cue to reproduction evolved in temperate habitats. Continuous breeding is not necessarily equivocal to high individual reproductive success even though overall population growth may be rapid. However, variation in reproductive effort suggests that individuals respond to current environmental conditions on short time scales.     

Infidelity of leafcutting ants to host plants: resource heterogeneity or defense induction?

Summary Leafcutting ants have strong among- and within-plant preferences, and generally abandon plants long before they are completely defoliated. Two tropical deciduous forest tree species preferred by the leafcutting ant Atta colombica were studied to determine how variation in resource quality affects ant selectivity and partial defoliation of plants. Significant differences in palatability and leaf characteristics of Spondias mombin and Bursera simaruba were found among trees and among leaf types within trees, but not among branches within trees. No short-term responses to experimental defoliation of up to 50% of total canopy were found in either species. Leaf nutrient and poisture content were positively correlated, and phenolic content negatively correlated, with the palatability of Spondias mombin, a species containing hydrolyzable tannins. Leaf moisture and phenolic content were both positively correlated with the palatability of Bursera simaruba, which contains predominantly condensed tannins. The results suggest that variation in leaf quality among and within plants is at least a partial explanation for ant selectivity and partial defoliation of preferred species. There is no evidence that rapidly induced changes in plant chemistry affect ant decisions to abandon these plants. Instead, it appears likely that ants abandon plants once high-quality leaf patches are exhausted. Quantitative variation in leaf nutrients, moisture, and secondary chemicals all appear to contribute to ant preferences for individuals and tissues of highly palatable plants.     

Sexual and site differences in calcium consumption by the Malabar Giant Squirrel Ratufa indica

Summary The calcium ingestion of Malabar Giant Squirrels Ratufa indica (Sciuridae) was examined at two sites, Magod and Bhimashankar, in western India. In females at Magod a positive correlation was found between rates of calcium ingestion from food resources and the contribution of those resources to the daily diet. This relationship was true for all females at Magod irre-spective of their reproductive condition. This relationship was neither significant for males at the same site nor for both sexes at Bhimashankar. A differential requirement for calcium between the sexes and the occurrence of higher rates of calcium ingestion or the probable presence of more easily digestible calcium at Bhimashankar are postulated to explain the observed phenomenon. Mature leaves and bark appear to be reliable sources of calcium at these sites.     

Nitrification and nitrate uptake: Leaching balance in a declined forest ecosystem in eastern France

Nitrate uptake and leaching were measured during one year in a declined fir forest on the Vosges highlands (eastern France), in order to investigate whether excess nitrification could be responsible for a deleterious acidification of the ecosystem. Nitrate uptake by the vegetation was active mainly from spring to early fall, and then reached about 66 kg N ha^-1. No significant leaching loss occurred during the growth period of the vegetation. Significant nitrate leaching occurred in winter (about 17 kg N ha^-1). During fall and winter the nitrification rate was of the same magnitude as values reported for other ecosystems, and, thus, was not considered to be abnormaly strong. No abnormal temporal discoupling of nitrate production and nitrate uptake occurred in the ecosystem, and forest decline must therefore have some other cause.     

Effect of harvest intensity and ground flora establishment on inorganic-N leaching from a Sitka spruce plantation in north Wales,UK

Inorganic-N concentrations in soil solution of whole tree harvest (WTH) and conventional fell (CF) plots were monitored for two years before felling and four years after felling. Concentrations in the mineral soil after felling were higher than in standing forest for up to 14 months in both felling treatments. In the WTH plots inorganic-N concentrations then dropped steadily until four years after felling they approached zero. In contrast, inorganic-N concentrations of the CF plots remained comparatively large. Inorganic-N was dominated by nitrate throughout the period of the study, and especially in the mineral horizons.Felling debris was not a source of inorganic-N, unless indirectly through release and mineralisation of soluble organic-N. Vegetation cover, biomass and N content were substantially greater in the WTH plots two to three years after felling, compared with the CF. Vegetation cover and brash cover (slash cover in N. America) were negatively correlated. There was also a negative correlation between inorganic-N concentration in soil water samplers and the vegetation cover within the collection area of, or a 1 m square surrounding, these samplers.Two factors are probably responsible for the reduction in inorganic-N concentrations after felling in the WTH — the rapid re-establishment of vegetation and the lack of a N source in felling debris. In the CF plots, brash prevents re-establishment of vegetation over wide areas for at least four years. However, brash is not directly a source of inorganic-N at this stage.