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

Abstract

Aim Biotic 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 spatiotemporal impacts of BD.Location Northern Hemisphere.Methods Based 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 18. The impact of BD on forest carbon pools was determined by overlaying D f with data on forest cover and carbon density.Results Overall, 43.9 million hectares (Mha) (D f 5 2.6%) of forests were affected annually by BD, particularly by insects (36.5 Mha, D f 5 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 21 (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 21.Main conclusions BD 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.

Decadal-Scale Reduction in Forest Net Ecosystem Production Following Insect Defoliation Contrasts with Short-Term Impacts of Prescribed Fires

“…During the course of our study (2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016), moderate-to-severe defoliation and tree mortality caused by gypsy moth totaled 503,000 ha, and damage by southern pine beetle resulting in >90% pine tree mortality totaled 19,550 ha in New Jersey alone, equivalent to approximately 65% of the forested area in the state [46,47,76]. The proportion of forest area impacted by insects in New Jersey during some years of our study exceeded 12% year −1 , one of the highest rates of forest disturbance in the mid-Atlantic or northeastern USA, which averaged 2 to 4% year −1 over the same period [15,16,76].…”

“…Divergent outcomes were related to the fate of detritus following each disturbance and its effect on R h and R e . Over the course of our study, the area of forest impacted by insect damage greatly exceeded that affected by wildland fires at landscape and regional scales [15,16,[46][47][48]. Explicitly simulating C release from detritus following tree and sapling mortality in process-based models will lead to more accurate estimates of the magnitude of the long-term C sink in mid-Atlantic forests.…”

“…Aerial surveys and remotely sensed information derived from Landsat, MODIS, and other satellite products are highly effective at detecting changes in leaf area resulting from insect defoliation or wildland fire [8,15,16,45,76]. However, changes in tree and sapling biomass, and the increase in standing dead and CWD that we quantified using a much denser network of FIA-type forest inventory plots that was sampled more frequently than standard FIA plots distributed across the region are more difficult to detect remotely.…”

“…[13][14][15][16]47,48,81]. If our results can be extended to intermediate-age forests across these regions that will be impacted by insect damage in the future, the potential for long-term reductions in NEP dwarfs C losses from all other disturbances other than forest harvesting [15,16,81]. Accurately detecting and accounting for the impacts of invasive and native insects on rates of tree mortality and enhanced losses of C via respiratory pathways in process-based simulation models will greatly improve our estimates of the magnitude of the long-term CO 2 sink in forests in the mid-Atlantic and northeastern USA.…”

“…Overall NEP has been projected to decline slowly as forests age across the region [1, 9,11,12], with productivity declining more slowly for hardwood-dominated forests than conifer-dominated forests [5,9,10]. These intermediate-age forests now experience disturbance regimes dominated by wind and ice storms, insect damage, and managed wildland fire that differ in temporal and spatial scales and intensity compared to past stand-replacing disturbance such as intensive timber management or large wildfires [6,[13][14][15][16]. Following disturbance, they are also likely to have different long-term trajectories of NEP compared to patterns characterizing recovery following stand-replacing disturbances [17,18].…”