Consequences of a dieback disease on carbon stocks and fluxes in forests dominated by a susceptible foundation species

Abstract Disease-causing plant pathogens are an emerging global threat to forests, compromising carbon sequestration and shifting forests from carbon sinks to sources. Phytophthora agathidicida , the causal agent of a host-specific lethal root rot disease, kauri dieback, infects Agathis australis (New Zealand kauri). The disease causes progressive canopy decline and mortality of kauri, threatening forests where kauri dominate. We hypothesised that increasing kauri dieback severity would reduce growth-driven carbon fluxes while increasing mortality-driven carbon fluxes. Live vegetation carbon stocks were quantified in permanent plots across two censuses, ten years apart. Carbon sequestration was estimated as the difference between carbon growth and losses, which were modelled independently against gradients of kauri dieback severity and initial carbon stocks. We further examined how species-specific proportional contributions to stand carbon varied with total stand carbon and disease severity. Mean carbon stocks across plots were 169.9 ± 17.5 Mg ha⁻¹. Carbon sequestration declined with increasing disease severity due to both reduced productivity and greater carbon loss from mortality, with largest reductions occurring in initially carbon-dense stands. Kauri contributed the largest share of stand carbon, with its dominance increasing in carbon-rich stands owing to the concentration of biomass in large individuals. These findings show that kauri dieback threatens both the carbon sequestration and long-term carbon storage capacity of kauri forests, particularly in mature stands. Conservation efforts that prevent the spread of pathogens and reduce disease symptoms, especially in mature carbon-dense stands, are critical for maintaining the carbon storage capacity of forests.