High Planting Densities For Southern Pine Bioenergy Feedstock Production: Belowground Carbon Partitioning and Storage.

In the Southeast, southern pines are a promising woody biomass feedstock, and are already widely grown throughout the region. Loblolly pine (Pinus taeda L.) is the most commercially important southern pine species making up over 50% of the standing pine volume (1.4 billion cubic meters) and occupying 11.7 million hectares.  This research fills a gap in the region by investigating modifications and optimizations of pine plantations for biofuel feedstock production or for integrated product objectives including traditional resources of timber and fiber along with biofuel feedstock production. Project PIs have been investigating high planting densities (up to 4448 trees/ha (1800 trees/ac) as opposed to the common 1483 trees/ha (600 trees/ac) as a means to augment the product outputs from pine plantations such as thinnings for biofuel feedstocks at relatively young ages.  This component of the research focuses on the potential for belowground accumulation of carbon and the influence of initial planting density.  Early results for installations in the lower coastal plain of Georgia indicate that planting density  impacted biomass partitioning aboveground with greater partitioning to stems biomass at higher density.  In contrast, partitioning to belowground taproot mass does not appear to be impacted by density; data on coarse lateral roots mass are not presently available.  Modeling of taproot decomposition based on size fractionation of taproots suggest smaller diameter material under higher planting densities may reduce belowground carbon storage.