Agro-Ecosystems最新文献

This study examined the possibility that great tits (Parus major), blue tits (Parus caeruleus), and tree sparrows (Passer montanus) might be poisoned by the application of diflubenzuron (Dimilin) used to control insects in woodland and roadside trees. Breeding results were compared in nestboxes in sprayed and unsprayed orchards, and in ash coppices. Diflubenzuron was found to have no significant effect on the breeding results or on the growth of nestlings. The composition of the nestlings' food and the quantity of diflubenzuron found in leaf-feeding insects in the food was measured. On this basis the maximum possible daily intake of diflubenzuron by the nestlings was estimated. These amounts were compared with the daily toxic intake by captive birds, on which dietary feeding tests were carried out.

The vertical distribution of leaf area by species; transmission of photosynthetically active radiation; root biomass and fine-root surface area; and leaf damage were measured in nine tropical ecosystems: six in Costa Rica and three in Mexico. Ecosystems studied included monocultures of maize (young and mature) and sweet potato; year-old natural succession and vegetation designed to mimic succession; a 2.5-year-old mixture of three arborescent perennials (cacao, plantain, Cordia alliodora); 2.7-year-old plantation of Gmelina arborea; coffee shaded by Erythrina poeppigiana; and an old, diverse wooded garden.

Leaf area index ranged from 1.0 in young maize to 5.1 in natural succession and the gmelina plantation. The vertical distribution of leaves was most uniform in diverse ecosystems, and most clumped in species-poor ecosystems. Light transmission was inversely proportional to leaf area, and two dense-canopied monocultures (sweet potato and gmelina) were nearly as effective at light capture as were some of the more diverse ecosystems. Optical density of the canopy ranged from < 0.5 (35% transmission) in the young maize to > 2.0 (< 1% transmission) in the natural succession.

Large roots (> 5 mm diameter) accounted for most root biomass in the older ecosystems at a soil dept of 5–25 cm, and fine roots (< 5 mm diameter) were most important in the surface 5 cm in all ecosystems. The range of values for root biomass (39 to 422 g m⁻² to a depth of 25 cm) were similar to the range of values for leaf biomass (33 to 345 g m⁻², and, with the exception of two monocultures, ecosystems with high leaf biomass also had high root biomass. The surface area of the fine roots was lower than leaf area, and ranged from 0.5 to > 2.0 m² m⁻² of ground. Total root surface area increased with age and diversity, and the monocultures — even those effective at light capture — had low root surface area.

Herbivore damage on leaves of 35 species ranged from < 2 to > 16% of leaf area. Heavily damaged species contributed less to total ecosystem leaf area than did species damaged less than average. Ecosystem-level damage was not well correlated with age or diversity. Leaf damage in all ecosystems ranged from about 2 to 10% of leaf area, or < 2 to > 25 g m⁻² of ecosystem.

Young monocultures do not necessarily capture less light, provide less soil cover, and experience more herbivory than older, more diverse ecosystems. However, root surface area (and therefore possible nutrient-capture ability) is high only in ecosystems that are diverse or old, and this is an important design consideration for agroecosystems appropriate for the humid tropical lowlands.

Arrhenatherum elatius (Oatgrass) and Bromus erectus (Bromegrass) were grown for two years in pots in pure and mixed stands (total competition) with three levels of nitrogen supply. There was one harvest in the first year and one final harvest of shoots and roots in the second year. Plant height, proportion of flowering stands, and fresh and dry weight were measured. The plant material was analysed for protein and soluble nitrogen, for total ash, total lipids and carbon.

In both grasses, additional nitrogen supply resulted in greater biomass, shoot length, proportion of flowering stands and nitrogen content, but Oatgrass was able to use the nitrogen more effectively than Bromegrass.

In mixed stands both shoot biomass and the shoot/root ratio of Oatgrass were increased compared with pure stands. This increase was greater in the second than in the first year. In the first year Oatgrass showed a higher nitrogen content under competition, but this was true for Bromegrass in the second year. The differences between the years appear to be mostly due to the different developmental stages of the plants.

It was concluded that nitrogen supply is effective in regulating the distribution of Oatgrass and Bromegrass. Nitrogen seems to be of greater importance than water for which experimental results were reported in an earlier paper (Salinger and Bornkamm, 1982).