ECOLOGICAL INTERPRETATION OF WEED FLORA DYNAMICS UNDER DIFFERENT TILLAGE SYSTEMS

In northern Italy, on soil managed with three different tillage system s (conventional tillage, ridge tillage, and no-tillage) and submitted to standard cultural practices (crop rotation, and chemical weed contr ol), the weed vegetation was assessed at the beginning of the trial (1 987) and after six, and eight years. The aims were to evaluate (1) the effect of tillage systems on the weeds; and (2) the possibility of li nking the floristic changes under reduced disturbance to the theory of ecological succession. The weeds were categorised according to life-f orms (biological groups), periodicity types (ecophysiological groups), dispersal types and seed longevity. Data were analysed using Sorenson 's Indices of Similarity, the Independence test, and Principal Compone nts Analysis. The tillage systems profoundly altered the weed communit y: in undisturbed soils the importance of the geophyte and hemicryptop hyte species, and among the annuals, Digitaria sanguinalis, Conyza can adensis and Kickxia elatine increased, as well as that of the wind-dis persed weeds. The species linked to disturbance were annuals and in pa rticular Amaranthus spp., Chenopodium album and Echinochloa crus-galli . After eight years the floristic evolution in the reduced tillage sys tem can be interpreted on the basis of ecological succession. The comm unity that has formed assumes, from the quantitative point of view, ch aracteristics of a pioneer community of secondary succession with a pr edominance of annual species and a large number of wind-dispersed plan ts. Qualitatively there is a movement towards a more mature community that could become similar to that of the woodland edge, with more pere nnial species, shrubs, and bird-dispersed plants. The implications of these conclusions are discussed in terms of weed management. (C) 1997 Elsevier Science B.V.