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Paddy (Oryza sativa L. var. Taiching-127) fields of Kirtipur, central Nepal, were studied to evaluate the effects of weeds on crop yield during June to October 2003. Weed community was recorded in permanently marked plots (2 x 2 m 2 each), and grain and straw yields were measured. The effect of aqueous extracts (2 to 10%) of leaf, stem and root of dominant weeds (Echinochloa colona, E. crusgalli, Cyperus iria and Ageratum conyzoides) on germination and seedling growth of the paddy (Taiching-127) was examined in the laboratory. Altogether 52 weed species (27 dicot and 25 monocot) belonging to 32 genera and 15 families infested the paddy. The maximum weed density was observed in September (105 pl/m 2 ). There was significant reduction in straw and grain yield of paddy in weedy plots and negative correlation between weed species richness and straw yield. Crop weed competition reduced straw yield by 13% to 38% and grain yield by 25% to 47%. Weed growth changed pH and reduced soil nitrogen and phosphorus content. Aqueous extracts of root, stem and leaf of dominant weeds (Echinochloa colona, E. crus-galli, Cyperus iria and Ageratum conyzoides) showed a weak effect on seeds germination however most of them had inhibitory effect on root and shoot elongation of paddy seedlings. Root elongation was more sensitive to allelopathic inhibition than shoot elongation. Stem extract of these weeds showed higher inhibition on seedling growth of paddy than leaf and root extract except in Ageratum conyzoides in which root extract showed highest inhibition.

We studied the elevational pattern of forest composition and regeneration of the subalpine conifer tree species Pinus wallichiana in Manang, a trans-Himalayan dry valley in north-central Nepal. Thirty-five quadrats (10 m × 10 m) were laid between 3300 and 4000 masl on both north-and south-facing slopes. We measured diameter at breast height (DBH) of each mature individual of all tree species (DBH ≥10 cm), and recorded the number of seedlings (DBH <10 cm, height <30 cm) and saplings (DBH <10 cm, height >30 cm). We also measured soil moisture and soil pH, estimated canopy cover, and recorded slope and altitude in each quadrat. For all species together and for several species individually, tree density, seedling density, sapling density and tree basal area were found to decrease with elevation on both north and south aspects. This trend is largely explained by the progressively harsher environment at higher elevations. The north-facing slopes in our study area have denser forests than the south-facing slopes, the density of all size classes (seedling, sapling and mature plants) and basal area being greater on the northern aspects. These aspect-wide differences are attributable to the stark difference in soil moisture between northern and southern aspects, which is in turn due to the difference in insolation. Irrespective of elevation and aspect, all the forests studied are regenerating, as indicated by inverse J-shaped density-diameter curves. The elevational pattern of seedling and sapling abundance is explained only by elevation. Whereas other variables (e.g., canopy) are considered to have an important influence on seed germination and seedling establishment, they turn out not to be significant predictors of density of seedlings and saplings. This failure to identify a relationship is probably due to our use of non-parametric test (tree regression analysis) that we used to establish the relationship between density and its potential explanatory variables or due to our selection of 1 standard error rule yielding sub-optimal models for regression trees.

Physico-chemical (pH, conductivity, dissolved oxygen, alkalinity, hardness, nitrogen, heavy metal, etc) and bacteriological (coliform bacteria) characteristics of water were studied in samples collected from Sundarijal reservoir and its main feeding streams: Bagmati, Nagmati and Shyalmati in 2003 and 2004 AD. Human activities near the water bodies and sanitary condition of the residential areas of watershed were also examined. Although public awareness and proper management of watershed and reservoir premises were lacking, yet the physico-chemical characters were within the standard of World Health Organization (WHO) and European Commission (EC) for drinking water. However, the coliform bacteria were high and water was not safe to consume without intense treatments with disinfectants.

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Species composition and richness of grassland vegetation were studied in trans-Himalayan region of Manang. Two south-facing sites, both at the similar altitude (3800-4200 m asl), were selected and a total of 40 plots (10 x 10 m) with 200 quadrates (1 m x 1 m) were sampled. Altogether, 97 plant species were recorded. The similarity index between two sites was 12.37%. There was significant negative correlation between species richness and altitude in site I. Species richness did not show significant relationship with altitude in site II. Various physical and biological factors interact differently in different sites to create habitat heterogeneity which determine the distribution pattern of plant species and influence variations in species composition and diversity. Key-words: alpine region; quantitative analysis; species diversity; vegetation.DOI: 10.3126/botor.v6i0.2910 Botanica Orientalis - Journal of Plant Science (2009) 6: 47-51