High Planting Density Southern Pine Feedstock Production: Above and Belowground Carbon Partitioning.

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.  Early results for installations in the lower coastal plain of Georgia indicate that basal area can continue to increase with planting density through age 15 years from ~37 m²/ha (160 ft²/ac) at initial planting density of 600 trees/ac up to ~44 m²/ha (190 ft²/ac) at a planting density of 4448 trees/ha (1800 trees/ac).  Interestingly these basal area increases were not fully match by volume increases, which were little distinguished between planting densities of 1483 and 4448 trees/ha (600 and 1800 trees/ac) and averaged ~385 m³/ha (5500 ft³/ac).  Most novel was a decline in tree height with planting densities from ~18 m (60 ft) at 1483 trees/ha (600 trees/ac) to <16.5 m (55 ft) at 4448 trees/ha (1800 trees/ac).  This is counter to current knowledge that suggests tree diameters but not heights are impacted by planting density.  Planting density also impacted biomass partitioning aboveground with greater partitioning to stems biomass at higher density.  Partitioning to belowground taproot mass does not appear to be impacted by density, although data on coarse lateral roots mass are not presently available.  Silvicultural manipulation of planting density appears to provide a useful tool for managing product outputs and C resources.