Growth dynamics of Ambrosia artemisiifolia L. in response to plant density and mowing

Ambrosia artemisiifolia L. (common ragweed) is an annual species of the Asteraceae family originating from southern North America. This plant, which was intro- duced in Europe in the early 1940s, is nowadays one of the major causes of summer pollinosis in several European countries. In Italy, A. artemisiifolia distribution includes mainly the northern regions. Since the mid-1990s, the species has been increasingly reported as a weed of annual summer crops, even in regions where it has been present for decades only in field edges or in non-agricultural areas. This trend suggests the need to obtain more information on the growing behaviour of A. artemisiifolia from a weed ecology perspective. The objectives of this research were to assess the effect of plant density on plant growth dynamics and to study the response of the plant to mowing. Two field experiments were carried out in 2006-2007, starting from a natural infestation of more than 300 seedlings/m2. In the first experiment, seedlings were thinned to densities of 4, 12.5 or 25 plants/m2, arranged according to a RCB design with 3 replicates. Plant height, biomass, number of leaves and leaf area were measured weekly on 3 plants per plot, starting from the 2-leaf stage until seed production. In the second experiment, a single starting density of 12.5 plants/m2 plants was considered. The plants were mowed, cutting them at 3 cm from the soil at different growth stages: 20 cm (3 cuttings in total per season), 50 cm (3 cuttings) or 80 cm (1 cutting) plant height. Treatments were arranged according to a CR design with 3 replicates. The percentage of surviving plants and the number of flowered plants per unit area were assessed. Thanks to more favorable meteorological conditions, plant growth was significantly higher in 2007. In both years, plant height was not influenced by density during the entire considered period. Final height ranged from 105 to 110 cm. The number of leaves per plant and the leaf area per plant were inversely related to density. In particular, 2,100 (in 2006) and 4,400 (2007) leaves per plant and 5,600 (2006) and 12,900 (2007) cm2/plant were recorded at 4 plants/m2. The number of leaves per unit area peaked 12,400 (2006) and 44,300 (2007) at 25 plants/m2 and was directly related to density. Similar behaviour was observed for LAI, that reached 4.5 (2006) and 12.8 (2007) at 25 plants/m2. The biomass per plant was similar between densities until about 90 days after emergence and then significantly higher at 4 plants/m2. Mowing resulted only in a partial reduction of the surviving plants. Even in the most disturbing conditions (mowing at plant height of 20 cm), total plant density was lowered only by about 30%. The number of flowered plants ranged from 60% to 78% of surviving plants and was not related to the treatment. A. artemisiifolia showed a high plasticity in the response of the main growth parameters to different pure-stand densities. This plasticity was consistently recorded, despite a remarkable difference between years, in total plant growth. Mowing resulted in reduction of plant density but did not prevent flowering.