Leaf Area and Biomass in Mixed and Pure Plantations of Sycamore and Black Locust in the Georgia Piedmont

Abstract Sycamore ( Platanus occidentalis ) and black locust ( Robinia pseudoacacia ) trees were harvested for determination of leaf area distribution among four crown strata, leaf area index (LAI), and aboveground leafless biomass at three times during their fourth growing season. The two species were growing in pure and mixed plantings at a 1.2 x 2.4 m spacing on an eroded upland site in the Georgia Piedmont. LAI and biomass were estimated based on linear regressions on D²H established from the harvested trees. Both species carried 75 percent or more of their leaf area in the middle two crown strata. In the pure plantings, leaf area per tree was higher for sycamore than for black locust, whereas in the mixed planting the reverse was true. LAIs of black locust and sycamore were 4.3 to 4.9 m²/m² in the pure plantings early in the growing season, but in August LAI of black locust dropped to 1.7 m²/m² because of drought-induced defoliation. LAI in the mixed planting (3.2-3.3 m²/m²) was lower than in either of the pure plantings, with leaf area of black locust comprising 58 to 64 percent of the total; late in the growing season black locust's share of total LAI fell to 39 percent. Though leafless aboveground biomass productivity in the pure black locust planting was 12.6 mt/ha compared to 11.6 mt/ha in the sycamore planting after the first 3 years, the relatively poor growth of black locust during the fourth growing season essentially neutralized its advantage. Four-year mean annual increment in both pure plantings was 4.6 mt/ha/year. Leafless biomass productivity in the mixed planting (8.9 mt/ha after 3 years) was substantially below that in either of the pure plantings, with biomass of locust comprising 66 percent of the total. Forest Sci. 31:509-517.