A survey of environmental stresses on crop fields as shown by changes in the structure of weed communities.
Changes in the weed communities of crops in Taiwan were investigated as part of a study on the impact of environmental stresses on agriculture. The weed population was assessed using soil samples collected from various sites including the coastal area of Luchu Hsiang, abandoned fields on a hill near a copper smelter near Keelung, and in fields under different cropping systems around Matou Town, Tainan County. Reduced weed diversity was noted at the coastal site subject to saline winds following the destruction of a windbreak forest. Cyperus difformis became the dominant species (77% rel. frequency); other species benefiting from the environmental stresses were Ludwigia epilobioides, Echinochloa crus-galli, Eclipta prostrata, Fimbristylis miliacea and Wahlenbergia gracilis. Weed growth at the site near the copper smelter was poor, with no plants emerging from soil collected from one of the abandoned fields studied. A comparison of the effects of different cropping systems on weed development indicated that a rotation with 2 rice crops/year resulted in higher weed densities in the winter and summer than a continuous sugarcane system or the 3-year Chianan rotation consisting of an upland crop, rice and sugarcane. C. difformis was common in all fields but other weeds displayed rotation-specific preferences. Reductions in rice yields were observed in the coastal area of Taoyuan County, especially during the winter season.