THE BIOLOGY OF AMBROSIA TRIFIDA L.

Abstract

Ambrosia trifida plants were grown in monocultures in the field at five densities ranging from 4 to 500 plants m −2 . Plant growth, productivity, biomass allocation, plant architecture, and seed predation were studied in repeated harvests. Plant growth was very plastic and the weight of individual plants was greatly influenced by density. Mean dry weight per plant varied from 11 g in the highest density to 320 g in the lowest density. At the final harvest in September the plants had lost up to 34% of their biomass through senescence of lower leaves. Above ground biomass production was extremely high and averaged from 1590 g m −2 in the lowest density to 3058 g m −2 in the highest density. Leaf area index was very high especially at high densities. The relationship between plant height and density was complex and some individuals in the intermediate density were more than 3 m tall. Reproductive ratios, the weight of reproductive materials/shoot biomass, generally declined with increasing density and were significantly higher when the weight of the male flowers was added to the weight of seed. With time a pronounced decline in leaf area ratio occurred in all densities. There was no significant effect of density on unit leaf rate. The plants were able to discard their lower unproductive leaves in dense plots and effectively raised their canopy further up the stem. But this left much unproductive stem structure below and leaf area ratio and relative growth rate declined. Plants in high densities were single stemmed while those in the lowest density were much branched.The presence in dense stands of tall and short plants created a bimodal distribution of leaf area and seed weight. Most seed biomass was located in the upper canopy of low density plants. The percentage of plants producing male flowers decreased with increasing density and, in the highest density, with decreasing height in the canopy. The shorter plants, without male flowers, had a higher reproductive ratio. They invested their reproductive energy into seed. Insects damaged many seeds and the percentage of damaged seeds per plant declined with increasing density. Furthermore in the population of highest density seed predation was higher in the ‘apparent’ tall plants than in less apparent short plants.