EFFECT OF 2 FOREST GRASSES DIFFERING IN THEIR GROWTH DYNAMICS ON THE WATER RELATIONS AND THE GROWTH OF QUERCUS-PETRAEA SEEDLINGS

Interference between 1-year-old sessile oak (Quercus petraea (Mattus.) Liebl.) seedlings and two grass species (Agrostis stolonifera L. and Deschampsia cespitosa (L.) Beauv.) was evaluated during 3 years after seedling transplantation. The seedlings were grown in large containers under crossed levels of watering regimes (regular irrigation that mai ntained the soil at field capacity or seasonal water stress) and grass competition (with Agrostis, Deschampsia, or bare soil). After 3 years , seedling basal diameter was 2.5 times greater in the absence of comp eting vegetation than with one of the two grasses. Within the grass-co mpetition treatments, diameter was 1.3 times as great for the seedling s grown in the irrigated treatments than for the seedlings grown in th e water-stressed treatments and was 1.3 times greater for the seedling s grown with Agrostis than for the seedlings grown with Deschampsia. D uring the first 2 years, the seedlings grew much faster in the bare so il than in the weedy treatments, whatever the watering level. During t he 3rd year, the seedlings in all the irrigated treatments showed simi lar relative growth rate values, which were much higher than those in the water-stressed treatments. Each year, soil water potential reached -2.5 MPa under the two grasses and -0.7 MPa under bare soil by the en d of summer. On the contrary, competition for nutrients seemed to be l ow. Assessments of plant carbon isotope composition showed that the se edlings in the weedy water-stressed treatments had higher intrinsic wa ter-use efficiency, and also suggested that the competition exerted by the two grasses occurred mainly for water. We conclude that during th e first 2 years, the negative effects of the two grass species on seed ling growth seemed to be largely independent of any competition for wa ter. Conversely, during the 3rd year, the influence of the grasses on the seedlings seemed to be related only to competition for water.