Belowground processes for two loblolly pine (Pinus taeda L.) families respond differently to the intensity of plantation management

Abstract

Intensive forest management (fertilization, weed control) and the planting of fast-growing families of loblolly pine (Pinus taeda L.) can dramatically increase the rate of tree biomass accumulation, but it is unclear how tree genetics and management intensity interact to affect belowground processes. For 2.5 years in a 10–12-year-old plantation in north central Florida, we examined ecosystem carbon (C) accumulation, soil respiration (SR), total belowground C flux (TBCF), and litterfall in forests receiving different levels and types of fertilizer and weed control treatments that effectively reflected a contrast in ‘high’ vs. ‘operational’ management intensity. A fast-growing family was compared with a slower-growing family using single-family block plots. Applying high intensity silviculture treatments significantly (p < 0.05) increased C accumulation in aboveground biomass on average by 55% (20.9 Mg C ha−1) relative to less intensive silviculture, and the fast growing family accumulated 14% (6.3 Mg C ha−1) more C than the slower growing family at the end of 12 years. For the organic layer C, the high intensity silvicultural treatments significantly (p = 0.02) increased C accumulation (9.0 Mg C ha−1) and biomass increment (p = 0.04, 0.7 Mg C ha−1 y−1); however, the family treatment was not significant (p > 0.05) for either annual increment or organic horizon C. In contrast, the response of belowground C dynamics to silvicultural intensity were family specific, with the fast-growing family having significantly (p < 0.001) greater SR and TBCF under the operational treatment, while the slow-growing family showed no change in allocation with silvicultural intensity. The faster growing family also concentrated SR on its mounded planting bed position under the low silvicultural intensity, potentially making it better adapted to receiving silvicultural treatments concentrated near the tree base. These results suggest that loblolly pine’s C allocation belowground could be a characteristic to use for selecting pine families for greater growth potential, compatibility with silvicultural practices, or as a means to affect ecosystem C accumulation.