Interference interactions in experimental pine-hardwood stands

A replacement series field experiment was established in 1989 in the Piedmont physiographic province of Virginia with loblolly pine (Pinus taeda L.), red maple (Acer rubrum L.), and black locust (Robinia pseudoacacia L.) to quantify the effects of interference and elucidate its mechanisms. Stands were planted with and without herbaceous vegetation. After three growing seasons, interference among pine, hardwood species, and herbaceous vegetation significantly affected growth and yield. While herbaceous vegetation significantly affected all stands, it reduced the yield of hardwood species more than of loblolly pine. Loblolly pine appeared to ameliorate the effect of herbaceous vegetation on hardwoods in some stands. Based on relative yield totals, interference relationships between pine and hardwood species were neutral in all replacement series except for the loblolly pine--black locust series with herbaceous vegetation, where a mutualistic relationship was evident on poorer sites of the study. Pine appeared to compete effectively with hardwoods through efficient use of soil moisture and nitrogen, although pines had lower root/shoot ratios. Pine also had the greatest amount of leaf area of all species. Black locust reduced light availability to pine in some stands, but may have provided some nutritional benefits. Black locust and red maple appeared to increase allocation to roots in herbaceous vegetation, especially in pure stands. Root/shoot ratio, water-use efficiency, and leaf area were the measured variables most closely correlated with tree yield