Dogwood canker is a serious production problem of unknown etiology. From May 1985 through April 1989, cankers from 290 flowering dogwood trees in 15 separate nurseries were sampled for nematodes. Seventy-three percent (213) of the cankers contained nematodes. Panagrolaimus rigidus (Schneider) Thorne (115/290) and Aphelenchoides spp. (91/290) were the most frequently collected taxa. Panagrolaimus rigidus was reared on 2% water agar with unidentified bacteria as the food source. Aphelenchoides spp. were reared in antibiotic-amended agar culture with the fungus Glomerella cingulata (Stoneman) Spauld. &Schrenk as a food source. Repeated attempts to culture Aphelenchoides spp. on dogwood callus tissue were unsuccessful. Artificially created stem wounds inoculated with combinations of Aphelenchoides spp. and P. rigidus callused completely in 60 days with no indication of canker development. Very low numbers of nematodes were recovered from inoculated trees, but P. rigidus and one Aphelenchoides sp. were efficient dispersers and occurred in treatments other than those in which they were inoculated. 相似文献
Rangeland condition was assessed in the lowveld of Swaziland to determine the current status with emphasis on contrasting different land management systems and soil classes in two study areas. The assessment incorporated the grass and woody plant layer. The methods employed to evaluate the grass layer were ecological condition index (ECI) and weighted palatability composition (WPC). The government ranch had significantly higher ECI (mean 714.5) and WPC (mean 61) than the communal land (mean: ECI – 533.5; WPC – 48.7) and the game reserve (mean: ECI – 578.9; WPC – 47.9). The ECI and WPC values did not differ between the soil classes in most cases (range: ECI 551.5–645.9 and WPC – 43.7–57.6). The density of all woody plants and encroaching plants alone were the highest in communal land. Basal cover ranged from poor (2–3%) to good (>5%), while bare ground was rated from low (<1%) to high (>5%). Overall results showed great variability of studied variables at site and landscape levels of resolution. Generally, the grass layer was rated between fair and good when assessed on the basis of ecological and palatability merits. There was a clear indication of advancement of bush encroachment. 相似文献
Australian acacias introduced to the Cape about 145 years ago are replacing indigenous vegetation over extensive lowland areas. This paper describes their growth, litter-fall, reproductive and nectar secretion phenology. Results indicate that growth and litter-fall are seasonal events, but timing varies with species and site. However, reproductive phenology is characteristic of a species, and varies little with time and place. Nectar secretion appears to be associated with both growth and flowering. Comments are made on the use of these phenological data in distribution mapping, control and utilization. 相似文献
This paper concerns the morphology of hemp woody core cells, investigated by optical and scanning electron microscopy, and the chemical analysis of the hemp cells. Steam explosion was investigated as a pre-treatment step for woody hemp ‘chènevotte’, with the aim of optimizing the separation and delignification of woody fibres.
In this study, we report the results of five experiments performed on ‘chènevotte’ samples impregnated in acid solution (0.1% w/w H2SO4) and steamed at 200, 210, 220, 230 and 240°C for 180 s. The effect of process temperatures on the woody hemp core after acidic impregnation was followed by optical and scanning electron microscopy, by assessment of the chemical composition, and by evolution of the average degree of polymerization (DPv) values of the purified wood fibres.
We found that treatment at 200 and 210°C led to samples that were difficult to delignify because the destructuring and disintegration of lignocellulosic materials were insufficient. A temperature of the order of 220–230°C is required to obtain well-separated fibres. However, at a temperature of 240°C, degradation and fibre damage were noted. 相似文献
Problems of distinction among succulent Euphorbia species from eastern tropical Africa. Identification of herbarium specimens of succulent Euphorbia species is especially difficult due to distortion of important characters upon drying. Problems encountered in collecting and preserving material result in poor representation in the herbarium, and true variation and distribution of closely related taxa are thus often established only from field studies. These problems are exposed and some solutions given in four groups of species from east and northeast Africa: the non-spiny E. schimperi-nubica group, the single-spined E. triaculeata group, the shrubby E. heterochroma-stapfii group and the tree E. nyikae-kibwezensis group. 相似文献
The objective of the present study was to develop an empirical cold hardiness model applicable to several taxa of deciduous trees. Cold hardiness expressed as lowest survival temperature of Acer rubrum, Betula nigra, Liquidambar styracifiua, Fraxinus pennsylvanica, Prunus serotina and Quercus alba was evaluated at approximately weekly intervals during the winters of three consecutive years. Plant samples and meteorological data were collected from Georgia Experiment Station, Griffin, Georgia. Maximum, minimum and average temperatures, hourly chill and heat accumulation. day length and time of year were used as input variables for model development. The statistical method of stepwise procedure of regression analysis was employed to select variables for the model. Based on the assumption that model components should be the same for all taxa included in this study and all three winters, the following independent model variables were selected as valid inputs: day length, number of accumulated hours with temperature above 20°C and number of accumulated hours with temperature below 10°C. Equation coefficients of species-specific models were determined for each species. Cold hardiness predictions were compared to actual observations for each species. The model components were interpreted as representing two processes: (1) internally regulated and independent of actual temperature, and (2) externally regulated and dependent on the amount of accumulated chill or heat. The model allowed for comparisons of cold hardening and dehardening between the studied taxa and between years. 相似文献
The macroinvertebrate community inhabiting woody debris in low‐order mountain streams was investigated using hand collections and emergence traps. With respect to dry mass, Amphipoda, Plecoptera and Diptera are the most important taxa. From the gut content analyses it was found that the microhabitat is colonized by xylophagous species as well as taxa of other feeding types. Regarding the whole community, there is no correlation between the abundance of specimens and density of wood, tree species, bark cover, consistency class, and surface structure. However, single species do show preferences; surface structure is the most important factor determining community composition. 相似文献