Fertilization effects on forest carbon storage and exchange, and net primary production: a new hybrid process model for stand management
A critical ecological question in plantation management is whether fertilization, which generally increases yield, results in enhanced C sequestration over short rotations. We present a rotation-length hybrid process model (SECRETS-3PG) that was calibrated (using control treatments; CW) and verified (using fertilized treatments; FW) using daily estimates of H2O and CO2 fluxes, canopy leaf area index (L), and annual estimates of tree growth and dimension. Herein, we focus on two decades of loblolly pine (Pinus taeda L.) growth and establishment for stands growing on a nutrient poor, droughty soil (SETRES; Southeast Tree Research and Education Site) in North Carolina, USA, on a site previously occupied by a 30-year-old natural long-leaf pine (P. palustris Mill.) stand. The SECRETS-3PG model combines: (1) a detailed canopy process model with hourly and daily resolution, (2) a biometrically accurate tree and stand growth module for monthly allocation, 3-PG, and (3) empirical models of soil CO2 efflux (RS). Simulated L, quadratic mean tree diameter, and total standing biomass all tracked field measurements over a 10-year period. Simulated maintenance respiration, canopy transpiration, and RS mirrored, with minor exceptions, short-term independently acquired data. Model correspondence with the independent measurements provided a basis for making short-term estimates of net ecosystem productivity (NEP) and longer-term estimates of net primary production (NPP) over the 20-year period from planting. Simulations suggest that optimum fertilization amendments; (1) increased NEP by more than 10-fold over control – FW(952 g C m-2 a-1) and CW(71 g C m-2 a-1) – at maximum NPP and (2) increased NPP two-fold (1334 and 669 g C m-2 a-1 for FWand CW, respectively) at maximum L. Seasonal patterns in NEP suggest that autumn and winter may be critical periods for C uptake in nutrient-limited loblolly pine stands. We conclude that increased L in response to improved nutrition may enable loblolly pine to achieve positive annual NEP earlier in rotation.