Quantification and physiology of carbon dynamics in intensively managed loblolly pine (Pinus taeda L.)

Loblolly pine (Pines taeda L.) occupies over 13 million hectares in the United States and represents a critical component of the global carbon (C) cycle. Forest management alters above- and belowground C dynamics, which may affect the C sequestration capacity of a site. Identifying drivers that influence C cycling, quantifying C fluxes, and determining how management alters processes involved in C cycling will allow for a comprehensive understanding of C sequestration capacity in managed forests.; C dynamics in loblolly pine were examined in two studies. Objectives of the first study included (1) investigating environmental, soil C, root, and stand influences on soil CO2 efflux on the South Carolina coastal plain and (2) quantifying soil CO2 efflux over a rotation in loblolly pine stands located on the South Carolina coastal plain and the Virginia piedmont. In relation to the first objective, temporal variation in soil CO2 efflux was most highly related to soil temperature. Spatial and temporal variability in soil CO2 efflux was weakly related to soil C and root biomass and not at all related to coarse woody debris, stand age, stand volume, or site index [Chapter 2]. Soil CO2 efflux was not significantly related to stand age on the South Carolina plain sites and strongly positively related to age on the Virginia piedmont. Cumulative soil C efflux on the South Carolina coastal plain over 20 years is an estimated 278.6 Mg C/ha compared with an estimated 210.9 Mg C/ha for the same time period on the Virginia piedmont [Chapter 3].; Objectives of the second study were (1) to investigate short-term effects of fertilization on physiological processes permitting enhanced growth in loblolly pine and (2) to determine the short-term effects of fertilization on autotrophic, heterotrophic, and total soil respiration. Major results from the second study included the finding that fertilization caused a transient rise in photosynthetic capacity, which paralleled changes in foliar nitrogen (N). Leaf area accumulation and subsequent growth following fertilization was partly due to enhanced foliar C fixation capacity [Chapter 4]. Also, fertilization altered the contribution of autotrophic and heterotrophic respiration to total soil CO2 efflux over the short-term. Enhanced specific root respiration was short-lived while suppressed microbial respiration following fertilization was maintained over the course of the nearly 200-day study. Growth of respiring root biomass over time increased total soil respiration [Chapter 5].