Commercial Tree Crops for Phosphate Mined Lands


To further commercial forestry on underutilized phosphate mined lands in central Florida, our five-year Short Rotation Woody Crops (SRWC) project initially assessed genotypes, management practices, productivities, and/or economic potential for cypress (Taxodium distichum), cottonwood (Populus deltoides), and slash pine (Pinus elliottii)on clay settling areas (CSA) and overburden sites and then expanded these assessments to include the non-invasive eucalypts Eucalyptus grandisand E. amplifolia. In 15 field studies and/or commercial-scale plantings, cypress showed promise with intensive culture, slash pine performance was less than expected, and cottonwood grew well in the absence of cogongrass (Imperata cylindrica) but was not as productive as E. grandis and E. amplifolia. SRWCs on CSAs require herbiciding/disking, bedding, watering/packing seedlings, fertilization, high planting density, superior trees, and winter harvesting so that coppice regeneration suppresses weeds. Assuming high site preparation and planting costs ($728 and $486 acre-1, respectively), double row growth (14.2 dry tons acre-1 yr-1,a reasonable stumpage price ($18.20 dry ton-1), and discount rates of 10% and 4%, production of E. amplifolia on CSAs is profitable, with land expectation values (LEV) of $308 and $2,633 acre -1, respectively. With a carbon sequestration benefit of $4.55 ton-1, LEVs increased to $383 and $2,718 acre-1, respectively, while the CO2 mitigation benefits associated with biofuels increased LEVs to $532 and $3,185 acre-1, respectively. In addition, below-ground carbon sequestration (roots + soil organic carbon) is likely to be $437-475 acre -1 or $330-364 acre-1, assuming discount rates of 4% and 7%, respectively. When land is already owned and SRWC production involves only establishment and management costs, the internal rates of return, the actual return on invested capital, are estimated to range from 4 to 32%; at a likely stumpage price of $10/green ton or higher, rates of return on investment could exceed 12%. E. grandisSRWCS on CSAs are at risk of blowdown near harvest age of three to four years. SRWC cost competitiveness depends on establishment success, yield improvements, harvesting costs, markets, and incentives; public and private partnering is necessary for commercializing SRWCs on ~123,000 acres of reclaimed phosphate mined lands. E. grandis and E. amplifolia SRWCs appear to improve CSA soil characteristics, exclude cogongrass, and facilitate the establishment of native plants, and E. grandis– and cottonwood-dominated portions of one CSA had much more favorable soil than cogongrass-dominated areas, as, e.g., ten- and four-fold increases in soil nitrogen and carbon, respectively, were noted in the E. grandis commercial planting. SRWCs may serve as “bridge crops” to restore cogongrass infested CSAs to native forest or productive agricultural lands while maintaining or even augmenting forestry’s current $685 million annual value and 6,800 jobs in Polk County. These results also emphasize both the potential for SRWCs on CSAs to mitigate atmospheric CO2 and for CO2 mitigation incentives to contribute to SRWC profitability.

D.L. Rockwood, D.R. Carter, and J.A. Stricker, University of Florida. May 2008.