THE EFFECTS OF TIMING AND DURATION OF FLOODS ON GROWTH OF YOUNG PLANTS OF PHALARIS-ARUNDINACEA L AND URTICA-DIOICA L - AN EXPERIMENTAL-STUDY

A pot experiment was performed with young plants of Phalaris arundinac ea L. and Urtica dioica L. cultivated from seeds sown in April. Seedli ngs of both species were flooded during the spring (May-June) or summe r (June-August) or autumn (August-October). Flooding in spring did not affect the survival of P. arundinacea seedlings but increased mortali ty of U. dioica seedlings. Summer flooding reduced the growth of rhizo mes and tillering in P. arundinacea, while none of the U. dioica plant s survived more than 4 weeks of summer flooding. Plants of P. arundina cea flooded in autumn had a higher dry mass than those grown in mesic conditions, but their growth terminated earlier. All plants of U. dioi ca survived the whole period of autumn flooding (9 weeks) but their dr y mass decreased as a result of leaf and root mortality. The response of the two species to flooding is discussed in relation to their estab lishment in floodplain habitats.