Fertilization effects on forest carbon storage and exchange, and net primary production: A new hybrid process model for stand management

Abstract

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 H 2O and CO 2 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 CO 2 efflux ( R S). 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 R S 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 FW and 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.