Quantifying Genotype x Silviculture Interaction on Productivity and Carbon Allocation By Manipulating Soil Organic Matter, N Supply and Demand.

Improved genetics and intensive silviculture have the potential to increase forest carbon sequestration.  We examined differences in growth efficiency and stand carbon cycling for two contrasting Pinus taeda ideotypes: a ‘broad-crown’ (BC) and a ‘narrow-crown’ (NC) clone, which partition more growth to leaves and wood, respectively. Tree growth, carbon allocation, fine roots, and soil carbon were measured over eight years. Silviculture treatments were a control consisting of shearing and bedding (-LR) and a mulch treatment where 25 Mg ha^-1 of chipped logging residue (C/N≈856:1) was incorporated into the soil during bedding (+LR). We hypothesized that: 1) +LR would increase productivity and mineral soil carbon; 2) the NC and BC clone would display similar aboveground productivity in -LR, but because of lower leaf area and thus lower nitrogen demand, the NC would display greater productivity in +LR, and 3) because of greater foliage biomass, the BC clone would allocate more carbon belowground.

 At age nine, soil C was 25% greater in the +LR treatment. There were no LR, clone, or interaction effects on stem production.  Stem biomass was 110.4 and 110.2 Mg ha^-1 (p=0.99), for the NC and BC, respectively.  There were significant clone effects on allocation.  The BC clone had 13% greater foliage and 15% greater branch biomass, while the NC clone had 14% greater coarse root (>2mm) biomass.  In addition, the NC clone averaged monthly 30% more fine root biomass (<2mm). Growth efficiency, defined as stem growth per unit leaf area was 5.4 and 4.7 Mg ha^-1 yr^-1 LAI^-1 in the NC and BC, respectively (p<0.0001).  In contrast, BC had greater soil macro-organic matter C (150-2000μm) than NC under +LR treatment (clone x LR, p<0.05). The results show that while ideotypes had similar stem growth rates, clone differences in carbon allocation could impact soil carbon storage and decomposition.  In addition, incorporation of logging residues during site preparation could have positive benefits on productivity and building soil C.