Regional Assessment of Carbon Pool Response to Intensive Silvicultural Practices in Loblolly Pine Plantations

Tree plantations represent an important component of the global carbon (C) cycle and are expected to increase in prevalence during the 21st century. We examined how silvicultural approaches that optimize economic returns in loblolly pine (Pinus taeda L.) plantations affected the accumulation of C in pools of vegetation, detritus, and mineral soil up to 100 cm across the loblolly pine's natural range in the southeastern United States. Comparisons of silvicultural treatments included competing vegetation or 'weed' control, fertilization, thinning, and varying intensities of silvicultural treatment for 106 experimental plantations and 322 plots. The average age of the sampled plantations was 17 years, and the C stored in vegetation (pine and understory) averaged 82.1 +/- 3.0 (+/- std. error) Mg C ha(-1), and 14.3 +/- 0.6 Mg C ha(-1) in detrital pools (soil organic layers, coarse-woody debris, and soil detritus). Mineral soil C (0-100 cm) averaged 79.8 +/- 4.6 Mg C ha(-1) across sites. For management effects, thinning reduced vegetation by 35.5 +/- 1.2 Mg C ha(-1) for all treatment combinations. Weed control and fertilization increased vegetation between 2.3 and 5.7 Mg C ha(-1) across treatment combinations, with high intensity silvicultural applications producing greater vegetation C than low intensity (increase of 21.4 +/- 1.7 Mg C ha(-1)). Detrital C pools were negatively affected by thinning where either fertilization or weed control were also applied, and were increased with management intensity. Mineral soil C did not respond to any silvicultural treatments. From these data, we constructed regression models that summarized the C accumulation in detritus and detritus + vegetation in response to independent variables commonly monitored by plantation managers (site index (SI), trees per hectare (TPH) and plantation age (AGE)). The C stored in detritus and vegetation increased on average with AGE and both models included SI and TPH. The detritus model explained less variance (adj. R-2 = 0.29) than the detritus + vegetation model (adj. R-2 = 0.87). A general recommendation for managers looking to maximize C storage would be to maintain a high TPH and increase SI, with SI manipulation having a greater relative effect. From the model, we predict that a plantation managed to achieve the average upper third SI (26.8) within our observations, and planted at 1500 TPH, could accumulate ~85 Mg C ha(-1) by 12 years of age in detritus and vegetation, an amount greater than the region's average mineral soil C pool. Notably, SI can be increased using both genetic and silviculture technologies.