COMPETITION-INDUCED REDUCTIONS IN SOIL-WATER AVAILABILITY REDUCED PINE ROOT EXTENSION RATES

The relationship between soil water availability, root extension, and shoot growth of loblolly pine seedlings (Pinus taeda L.) was evaluated in a rhizotron sand mixture in the absence and presence of crabgrass (Digitaria spp,) competition, Heights and diameters of seedlings grown with crabgrass were reduced 33 and 50%, respectively, compared with s eedlings grown in pine monoculture. Furthermore, survival of seedlings was reduced by 16% in the mixed species treatments compared with pine monoculture cells, Soil water contents at all depths were at least 50 % lower in mixed species treatments compared with pine monocultures du ring a dry-down cycle. Maximum loblolly pine root extension rate was 0 .10 m wk(-1) in pine monoculture compared with 0.02 m wk(-1) in the pr esence of grass. Pine seedling root extension ceased in the presence o f grass during prolonged periods of induced water stress, Crabgrass ro ots extended at a rate of 0.6 m wk(-1), a rate largely unaffected by w atering regime or differences in water content within the rhizotron, P ine root distributions, at the end of one growing season, were differe nt for seedlings grown with grass than for those in pine monoculture; however, when seedlings of similar size were compared, root distributi ons were not different, Our results indicate that competition-induced reductions in soil water availability reduced the rate of pine root ex tension and thus the volume of soil exploited, thereby reducing resour ce availability and tree growth.