Climate-linked cascading disturbances drive conifer forest decline in a boreal–temperate ecotone: evidence from Kunashir Island (Northwest Pacific)

This study investigates the ecological impacts of a severe winter extratropical cyclone and a subsequent bark-beetle outbreak on the unmanaged hemiboreal (temperate–boreal ecotone) forests of Kunashir Island (Kuril Islands, Northwest Pacific). Using very high-resolution satellite imagery analysed with convolutional neural networks, we demonstrate that the extratropical cyclone acted as a major disturbance agent, disproportionately affecting coniferous trees (Abies sachalinensis (F. Schmidt) Mast., Picea jezoensis (Siebold & Zucc.) Carrière, Picea glehnii (F.Schmidt) Mast.) within dark coniferous and mixedwood forests. Windthrow patches covered 21.73 km2 (1.83% of the island’s total forest area), with 97% occurring in dark coniferous and mixed forests, particularly conifer-dominated stands. The resulting coarse woody debris accumulation created favourable conditions for the Japanese spruce bark beetle (Ips typographus japonicus Niisima, 1909), triggering the first large-scale outbreak recorded for the Kuril Islands. Within the 76 km2 covered by cloud-free imagery snapshotted in four growing seasons after the initial extratropical cyclone disturbance, we detected >350 ha of standing dead trees, while initial windthrow areas accounted for ~820 ha. Based on our estimates, the bark-beetle outbreak led to spruce dieback over ~8.6 km2, or ~40% of the original windthrow area. Univariate Ripley’s K-function revealed significant clustering of dead trees, presumably shaped by the spatial distribution of spruce stands and the localized nature of beetle infestations. Bivariate Ripley’s K-function analysis indicated spatial segregation between windthrow and beetle-induced mortality, confirming that standing dead trees occurred less frequently near windthrow patches than would be expected if the two patterns were spatially independent. This pattern suggested that, while the outbreak was initially triggered by windthrow, it subsequently decoupled from its source and evolved into a self-sustaining phenomenon. Regional warming in recent decades, evidenced by rising mean temperatures and thermal sums, likely extended the beetle flight and oviposition period, supporting multiple generations per season. The cascading disturbances reflect a broader pattern across temperate–boreal ecotones, where climate-driven compound disturbances accelerate conifer decline and promote expansion of temperate broadleaf trees.