Long-term dynamics of aboveground carbon stocks in managed loblolly pine plantations in the southeast United States

Intensively managed pine plantations in the southeastern US could not only meet the increasing demand for traditional wood products and biomass for energy, but also play a critical role in meeting US greenhouse gas emission reduction goals through additional carbon (C) sequestration. Adapting forest management to help mitigate climate change requires better understanding of long-term effects of silvicultural treatments on forest production and C sequestration. With two long-term culture-by-density research experiments for loblolly pine plantations across the southern US, we assessed the impacts of site quality, cultural intensity (operational and intensive), and planting density (741, 1483, 2224, 2965, 3706 and 4448 trees ha-1) on dynamics of aboveground net primary production in C (ANPPC), aboveground live tree C stocks (AGC), and cumulative C lost to tree mortality and other litter production (CLC), and total gross C capture (TGC). Cultural intensity significantly affected ANPPC and AGC before age 15 years, and CLC and TGC during the whole study period; while planting density significantly affected ANPPC before age 15 years, CLC after age 6 years, and AGC and TGC over time. Site quality significantly affected AGC at early and late ages, CLC at mid-rotation, and TGC at early ages. After age 20 years, higher quality sites still resulted in higher AGC than lower quality sites, while higher-planting densities might no longer promote aboveground live tree C stock. Intensive management practices that promote growth along with C accumulation in young stands do not necessarily result in higher C stocks in older stands. There exist a maximum C sequestration rate and a maximum C storage in loblolly pine plantations. Continuing carbon capture in the extended rotation or deferred harvest forests is in no way guaranteed to continuously increase carbon stocks. To develop effective climate-smart forest management recommendations, long-term dynamics of carbon capture, release, and storage, especially in managed forest plantations, need more research.