Seasonal Lateral Root Growth of Juvenile Loblolly Pine after Thinning and Fertilization on a Gulf Coastal Plain Site

h 1989, two levels each of stand density and fertilization were factorially established in an 8-year-old loblolly pine plantation on a P-deficient site. Levels of stand density were nonthinned at 2,732 trees per hectare and thinned at 721 trees per hectare. Fertilizer levels were none or application of 150 kilograms P plus 135 kilograms N per hectare. In 1994, stand basal areas of the nonthinned and thinned plots were 42 and 25 square meters per hectare, respectively, and a second thinning on the previously thinned plots left 15.8 square meters per hectare. The previously fertilized plots were refertilized with 200 kilograms N, 50 kilograms R and 50 kilograms K per hectare. In 1994 and 1995, tree growth was quantified at the end of the growing season, and lateral root initiation and elongation, soil temperature, and soil water content were measured throughout the growing season. The maximum rate of loblolly pine root growth occurred in May through July with more root growth in the 0to 5-cm depth than in the 5to 30-cm depth. A positive relationship between soil water content and root growth was observed. Thinning stimulated root growth 5 years after initial thinning and immediately after rethinning. Fertilizition did not affect root growth 5 years after application and refertilization had a limited positive effect on root growth. Although tree growth was not immediately affected by treatment reapplication, a positive relationship was found between current annual tree volume increment and root elongation during peak root growth. We conclude that root system growth is sensitive to environmental variables that affect root metabolism in May through July, and that on the Gulf Coastal Plain, loblolly pine volume gains after silvicultural treatment result, in part, from an increase in soil resource uptake.