Growth and Nutrient Accumulation of the Biennials Cirsium vulgare and Cynoglossum officinale under Nutrient-Rich Conditions

Abstract

Under nutrient-rich conditions all individuals of the two species behaved as biennials in that they grew vegetatively in the first year and flowered and set seed in the second year, after which the plants died. In Cynoglossum officinale total biomass and accumulated amounts of nutrients in September of the first year were about the same as at the time of flowering. Much of the stored nutrients were lost through die back of leaves during the winter. Uptake of all nutrients in the second year contributed to a great extent to the amounts at the end of the life cycle. Uptake of N, P and K in the second year was mainly in March and April. Uptake of Ca continued throughout the summer of the second year. The pattern of growth and nutrient accumulation in Cirsium vulgare was different. Dry weight in September of the first year was only 2-6% of the final dry weight. Growth during both autumn and the second year was important and at the end of the second year dry weights of plants of the two species were similar (ca 50 g per plant). Uptake of all nutrients in the second year contributed to a great extent to the final amounts. In the second year uptake of N was largely in March, after which the uptake of P, K and Ca continued. Reproductive effort (dry weight seeds/total plant dry weight) was 30% in Cynoglossum officinale and 8-10% in Cirsium vulgare and was independent of total plant dry weight. When compared with a hypothetical annual that synchronizes its germination and flowering with the rest of the population, the biennials had a selective advantage because they were able to grow and accumulate nutrients throughout a larger part of the year and were thus able to produce many more seeds.