Biotic disturbances in Northern Hemisphere forests - a synthesis of recent data, uncertainties and implications for forest monitoring and modelling

AimBiotic disturbances (BD, including insects, pathogens and wildlife herbivory) can alter forest structure and the capability of forests to deliver ecosystem services. Impact assessments, however, are limited by the lack of reliable and timely disturbance data at large spatial scales. This review synthesizes empirical data on the magnitude and distribution of spatio-temporal impacts of BD. LocationNorthern Hemisphere. MethodsBased on large-scale, multi-year BD data sets, covering c. 46% of the global forest, we calculated annual disturbance fractions D-f (percentage of forest area affected) and their inter-annual variability at a grid cell resolution of 1 degrees. The impact of BD on forest carbon pools was determined by overlaying D-f with data on forest cover and carbon density. ResultsOverall, 43.9 million hectares (Mha) (D-f=2.6%) of forests were affected annually by BD, particularly by insects (36.5 Mha, D-f=2.2%). Our synthesis demonstrates that fractions affected by BD (1) vary greatly over space and time, mainly in response to ephemeral bark beetle and defoliator outbreaks, (2) show temporal trends that are inconsistent across regions, yet are largely increasing over recent decades, and (3) are substantially higher than D-f caused by fire and other abiotic disturbances. Tree mortality was estimated over an area of 3.3 Mha year(-1) (medium estimate which assumed mortality at 7.5% of the affected area), with associated committed carbon fluxes from living biomass to litter and the atmosphere at 129.9 Mt C year(-1). Main conclusionsBD are key drivers of forest dynamics, making a contribution to tree mortality of a similar magnitude to fire. Despite inherent uncertainties, the data reported can be used to improve the representation of BD in global ecosystem models. Our findings call for future forest monitoring approaches to provide accessible, precise and consistent data on the occurrence and severity of BD which are harmonized across jurisdictions.