GROWTH, PHOTOSYNTHESIS, WATER RELATIONS AND COMPETITION OF THE OLD-FIELD SPECIES ABUTILON-THEOPHRASTI, AMBROSIA-TRIFIDA, AND HELIANTHUS-ANNUUS GROWN UNDER CONTROLLED CONDITIONS

The effects of intra- and interspecific competition on growth, net pho tosynthetic rate (P(N)), internal CO2 concentration (c(i)), leaf condu ctance to CO2 (g(l)), and xylem leaf water potentials (PSI) were inves tigated among well-watered seedlings of the old-field species Abutilon theophrasti, Ambrosia trifida, and Helianthus annuus. In all comparis ons, PSI values of the target plants were unaffected by the presence o f neighbors. On the other hand, P(N) revealed inhibitory effects of ne ighbors on A. theophrasti and H. annuus. The above- and belowground bi omass of all species was reduced in the presence of neighbors. Hence b oth inter- and intraspecific competition among seedlings may be import ant in influencing community structure in old-field communities.