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Competition-Induced Reductions in Soil Water Availability Reduced Pine Root Extension Rates

USDA Forest Service, 3041 Cornwallis Road, Research Triangle Park, NC 27709

L. A. Morris

Daniel B. Warnell School of Forest Resources, Univ. of Georgia, Athens, GA 30602

*Corresponding author (KLUDOVICI{at}rtpmail.emapfhm.gov).

ABSTRACT

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 seedlings 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 during 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 presence of grass. Pine seedling root extension ceased in the presence of grass during prolonged periods of induced water stress. Crabgrass roots extended at a rate of 0.6 m wk^–1, a rate largely unaffected by watering regime or differences in water content within the rhizotron. Pine root distributions, at the end of one growing season, were different for seedlings grown with grass than for those in pine monoculture; however, when seedlings of similar size were compared, root distributions were not different. Our results indicate that competition-induced reductions in soil water availability reduced the rate of pine root extension and thus the volume of soil exploited, thereby reducing resource availability and three growth.

Received for publication December 15, 1995.

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