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

The relationship between soil water availability, root extension, and shoot growth of lohlolly pine seedlings (Pinus taedu 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 SO%, 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 cetls. 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-’ in pine monoculture compared with 0.02 m wk-’ 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-‘, 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 tree growth. E VIDENCE FROM FIELD EXPERIMENTS suggests that water stress is directly responsible for much of the shoot growth reduction associated with competitive interference in young pine plantations (Carter et al., 1984; Shribbs and Skroch, 1985; South and Barnett, 1986; Elliott and White, 1987). Water stress resulting from competitor use of available soil water has both immediate effects on gas exchange and C fixation as well as longterm impacts on physiological processes and seedling development. A strong positive correlation between soil moisture and growth of loblolly pine has been demonstrated (Wenger, 1952; Zahner, 1962; Stransky and Wilson, 1964; Kaufman. 1968; Morris et al., 1993). Reductions in nutrient availability can occur coincident with reduced water availability (Elliott and White, 1987; Morris et al., 1993); however, direct competition for limiting nutrients in young plantations appears more important than water limitations only on sites where high water tables are within the rooting zone and water stress is minimal (Neary et al., 1990). Distribution of roots within the solum will affect the water and nutrient pool accessible to the tree. Most evidence suggests that after an initial period of free interpenetrating root growth, alterations in seedling root growth patterns occur due to soil water or nutrient K.H. Ludovici, USDA Forest Service, 3041 Cornwallis Road. Research Triangle Park, NC 27709; and L.A. Morris, Daniel B. Warnell School of Forest Resources, Univ. of Georgia, Athens, GA 30602. Received 15 Dec. 1995. *Corresponding author (KLUDOVICIQ rtpmail.emapfhm.gov). Published in Sod Sci. Sot. Am. J. 61:1196-1202 (1997) depletion by neighboring vegetation (Spurway, 1980). These changes in root distribution may determine the ability of a seedling to compete for spatially stratified water and nutrient resources during subsequent water stress periods. In this regard, both direct effects of reduced soil water potential and indirect effects associated with increased soil resistance to penetration can alter root distribution. Torreano (1992) found that in the absence of growth-limiting physical resistance, controlled reductions in soil water potential affected only root elongation rates and had little effect on pine seedling root distribution. Whether this result can be generalized to include pine root response to patterns of water depletion from competitor withdrawals is unknown. In this study, we had four objectives: (i) to quantify aboveground and belowground growth response of lob1011~ pine and a grass competitor to drought, (ii) to evaluate the relationship between soil water depletion and root elongation rates of loblolly pine seedlings, (iii) to determine if localized use of soil water by a grass competitor would alter pine root distribution compared with that found in a similar monoculture environment, and (iv) to evaluate the effectiveness of mycorrhizal inoculation in reducing drought stress. MATERIALS AND METHODS The influences of grass competition and mycorrhizal inoculation on growth of loblolly pine seedlings were evaluated under controlled rhizotron conditions during the summer of 1991. The work was conducted in the University of Georgia Whitehall Experimental Forest rhizotron described by Torreano (1992). The experimental design was a factorial combination of two competition treatments (pine alone and pine grown in mixture with crabgrass) and two ectomycorrhizal inoculation treatments (noninoculated and inoculated with Pisolithus tinctorius Pers. Coker and Couch) replicated in three complete blocks. The rhizotron consisted of 16 individual 1 by 1 by 2 m (length by width by depth) cells with a tempered-glass plate installed at a 7” angle as the inner wall. Rhizotron cells were filled with an artificial soil medium of washed sand (no. 10 fine masonry sand, Curtis Brothers, Watkinsville, GA) and fritted clay (Terra Green Soil Conditioner, Southern Turf Co., Norcross, GA) in a 5:l ratio by volume. The mixture had a sandy loam texture, provided minimal structural impedance (maximum cone resistance of <OS MPa), excellent aeration. and uniform nutrient and moisture distribution throughout the cell depth. Nutrient-holding capacity of the mixture was relatively low, allowing easy manipulation of labile nutrient concentrations. Soil nutrient analyses were performed using a double-acid extract (0.05 M HCl plus 0.025 M H,SO,). Initial concentrations of NH,-N, P, K, Ca, and Mg were 5. 13, 116, 220. and 65 mg kg-‘, respectively. with measured acidity of pH 5.1 (1:l soil/water mixture). These conditions were within the range of acidic, nutrient-poor conditions found in southern forest soils (Pritchett and Fisher, 1987). Union Camp Corporation (Prattville, AL) provided loblolly pine seeds (Family 022S-lo-0005-AA) that were germinated