Management Effects on Soil Respiration in North Carolina Coastal Plain Loblolly Pine Plantations

Loblolly pine is the most widely planted tree for plantation management in the southern US. In the southern coastal plain, where much of the original longleaf pine and bottomland hardwood forests have been converted to loblolly pine plantations, inland areas are commonly characterized by deep organic soils that can store up to 80 kg C m^-2. Intensive management activities on these sites disturb the forest floor and soil and their impact on soil respiration rates and long term soil storage capabilities is unclear. We measured soil respiration rates in three loblolly pine plantations being managed with a combination of ditching, bedding, clearcutting, thinning and fertilization. Sites and management regimes represented a wide range of real world conditions found in managed southern US forestry plantations. Soil efflux rates along with soil temperature and moisture were measured throughout the year at four to six plots on each site and best fit relationships were developed. Annual soil respiration rates where modeled using 30-minute soil temperature and moisture measurements recorded at a centralized meteorological station on each site. Soil efflux rates were highly correlated with soil temperature and moisture, but interaction between the two effects was uncommon. Soil temperature was the primary driver of soil respiration rates, but rates were suppressed under high soil moisture content. Modeled annual soil efflux rates were higher the first two years following clearcut harvest and thinning operations, but lower two years following fertilization. Rates were lower in the gaps, where entire tree rows were removed, compared to thinned areas, especially on the unfertilized site. Results indicate that soil respiration rates can be strongly impacted by forest management practices; however, the period of increased soil CO₂ efflux due to site disturbance may last only a few years.