…The largest mills, concentrated in the southeastern U.S., claim to sustainably harvest timber, from both hardwood and softwood forests. But a new mill, Highland Pellets in Pine Bluff, which harvests fast-growing Southern softwood pine claims to be among the greenest.  Still, the calculated ecological costs and benefits of forest biomass remain hazy. Highland Pellets, Arkansas’s first wood biomass factory, steams and rumbles in the autumn sunlight. A private railroad encircles the new 209-acre facility, packed with tilting aerial conveyors and curving metal chutes. Every 10 minutes a logging truck rolls into the plant, hauling a 28-ton load of tree trunks harvested from Southern pine stands within an 80-mile radius. At full capacity, the new mill will process more than a half-million metric tons of wood pellets a year.