ENVIRONMENTAL AND GENETIC INFLUENCES ON SHORT-ROTATION BIOMASS PRODUCTION OF BLACK LOCUST (ROBINIA-PSEUDOACACIA L) IN THE GEORGIA PIEDMONT

A test plantation of black locust (Robinia pseudoacacia L.) was established in the Piedmont region of Georgia (USA) for the purpose of determining productivity under short-rotation, intensive culture management. The influence of irrigation (at the rate of 1.1 cm week-1 during the growing season), nitrogen fertilization (at the rate of 113 kg ha-1 year-1, applied as NH4NO3 in two applications per year), site conditions (four blocks) and genetic stock (24 open-pollinated families) on productivity was assessed using a split-split plot test design with irrigation treatments as main plots, nitrogen treatments as sub-plots, and families as sub-sub-plots. After 3 years, the following observations were made. Growth of black locust was rapid during the study period, comparing favorably with hybrid poplars grown in other parts of the world. But, by the third year current annual increment was already declining in plots with the largest trees. Irrigation increased biomass production by about 80% in the first year, but the advantage dropped to 50% by the end of the third year. Nitrogen fertilization also increased biomass production, although by modest amounts in this nitrogen-fixing species - 30% in the first year, but only 12% by the end of the third year. The effects of irrigation and nitrogen fertilization on biomass production were additive and resulted from increases in individual tree biomass; survival was unaffected by these treatments. Family differences in biomass production were large and uninfluenced by irrigation, nitrogen or block effects. From family selection alone, biomass productivity can be increased by more than 50%. Block differences in biomass yield were also large and appear to confirm numerous anecdotal reports that black locust growth is adversely affected where soil moisture, aeration and rootable depth are limited. Through 3 years of growth, biomass production of selected black locust families in the Piedmont of the southeastern USA is projected to range from about 8 to 3 Mg ha-1 year-1, depending on site and cultural conditions.