Productivity, Biomass Partitioning, and Energy Yield of Low-Input Short-Rotation American Sycamore (Platanus occidentalis L.) Grown on Marginal Land: Effects of Planting Density and Simulated Drought

Short-rotation woody crops (SRWC) grown for bioenergy production are considered a more sustainable feedstock than food crops such as corn and soybean. However, to be sustainable SRWC should be deployed on land not suitable for agriculture (e.g., marginal lands). Here we quantified productivity and energy yield of four SRWC candidate species grown at different planting densities (1250, 2500, 5000, and 10,000 trees ha−1) under a low-input regime on a marginal site in the Piedmont of North Carolina and responses to reduced water availability. By the end of the first growing season, 75 to 100% tree mortality occurred in all tested species (Liquidambar styraciflua, Liriodendron tulipifera, and Populus nigra) except American sycamore (Platanus occidentalis), the productivity of which was positively affected by planting density, but unaffected by the throughfall reduction treatment. After 4 years of growth, the 10,000 trees ha−1 sycamore treatment produced smaller individual trees but the largest amount of total tree biomass (23.2 ± 0.9 Mg ha−1), which, although greater, was not significantly different from the 5000 trees ha−1 treatment (19.6 ± 1.5 Mg ha−1). The two highest planting density treatments had similar aboveground net primary productivity (ANPPwood) of 7.2 Mg ha−1 year−1. By contrast, in the 1250 and 2500 trees ha−1 treatments, ANPPwood was significantly lower, ranging from 3.4 to 5.4 Mg ha−1 year−1. Stem wood made up a majority of the biomass produced regardless of spacing density, but live branch biomass weight increased with decreasing planting density, comprising up to 31% of total aboveground biomass in the 1250 trees ha−1 treatment. Gross energy yield reached 140 GJ ha−1 year−1 for the 10,000 trees ha−1 treatment. Given this productivity, American sycamore could potentially yield 2400 (±380) L ethanol ha−1 year−1 over the first 4-year rotation. This study demonstrated that of the four species tested, only American sycamore grown on marginal land under low inputs (no fertilizer, no irrigation, limited weed control) had the capacity to successfully establish and maintain SRWC productivity, which might compare favorably with other fast-growing tree and grass species that typically require high inputs.

Duke Scholars

Author Jean Christophe Domec Environmental Sciences and Policy