Estimates of New Zealand forest and scrub biomass from the 3-PG model

We present the application of a simple physiological model (3-PG) for estimating biomass accumulation in New Zealand vegetation. The simulation was performed utilising monthly surfaces of temperature, precipitation, and radiation coupled with digital soil maps classified into fertility classes at a 1-km2 resolution. From 3-PG simulations, we investigated trends in long-term biomass accumulation by simulating vegetation growth over the entire country for 100 years. The predictions were compared with data collected at a number of spatial scales including: (1) individual plot measurements of forest and scrub stem biomass; (2) average stem biomass for South Island forest types; and (3) total vegetation biomass for forest and scrub vegetation types for North and South Islands. The model was calibrated by comparing simulated stem biomass data to literature values for individual plots. Simulated and plot based estimates of stem biomass were highly correlated (r2=0.98) once key parameters were calibrated for New Zealand vegetation. 3-PG predictions correlated well with regional estimates of aboveground stem biomass for the South Island of New Zealand (r2=0.82) and also with total vegetation biomass for the entire country (r2=0.72). However, both were slightly underestimated due to factors associated with assumptions about allocation of biomass to roots, soil characteristics, and stand age. The results indicate that climate and soil fertility exert considerable control on biomass accumulation at the broad scale. Application of the 3-PG model can provide accurate national estimates of biomass that supplement field-based programs by improving large-scale field sampling efficiency and highlight research related to physiology, biomass allocation patterns, and environmental mapping.