The dynamic carbon balance of New Zealand’s natural forests: Evidence from two decades of live above-ground biomass monitoring

For understanding the role of natural forests in the global carbon cycle it is essential to accurately quantify their carbon dynamics. New Zealand’s National Natural Forest Inventory established 940 permanent plots, which have been measured every five years over the last two decades. We used these observations to assess changes in live above-ground biomass between 2002 and 2022. Tree diameters were converted to biomass using species-specific allometric equations, and net change was partitioned into productivity and mortality. Mean net biomass change across all plots was small (−0.2 tDM ha⁻¹ yr⁻¹ in the North Island and −0.5 tDM ha⁻¹ yr⁻¹ in the South Island). However, that small net change resulted from most plots having positive biomass gains, offset by a small number of plots experiencing large mortality losses. This indicates that even New Zealand’s mature natural forests are predominantly continuing to grow, but this is counterbalanced by disturbance-driven mortality. On the small number of plots subject to mortality, losses can be very large and balance overall the ongoing productivity on many growing plots. Mean productivity was 2.7 tDM ha⁻¹ yr⁻¹ in the North Island and 2.5 tDM ha⁻¹ yr⁻¹ in the South Island, with corresponding mean mortality losses of 2.9 and 3.0 tDM ha⁻¹ yr⁻¹ . A national map of natural forest productivity was derived using process-based modelling, revealing strong spatial dependence on climatic and edaphic factors. Overall, while New Zealand’s natural forests are dynamic systems, with total biomass remaining stable, management interventions could shift the national carbon balance towards a net sink. • Two decades of monitoring temperate forest plots in New Zealand. • Mature natural forests are predominantly growing. • Some natural forests experience large mortality losses which counter-balance mean growth. • National map of natural forest growth has been inferred from plot data.