Forest Thinning Yields Highly Variable Impacts Across Global Soil Fauna Communities

Forest thinning is widely deployed across global silviculture to reduce tree stand density, lower vulnerability to wildfires, insect infestations, and droughts, and promote tree vigor while supporting timber extraction. However, while the structural consequences of tree harvesting on aboveground vegetation and soil microbes have received extensive scientific scrutiny, the responses of soil fauna remain poorly understood despite their essential contributions to ecosystem functioning. Soil invertebrates drive crucial belowground processes including litter fragmentation, nutrient mineralization, soil porosity generation, and carbon sequestration. A comprehensive systematic review published in Annals of Forest Science by Charlotte Biryol and colleagues evaluates how forest thinning influences these cryptic communities. Synthesizing data from forty-one field experiments conducted between 1975 and 2024, the investigation reveals that soil fauna responses are highly variable, displaying positive, neutral, and negative outcomes shaped by silvicultural methods, microclimatic shifts, and taxonomic identities across the soil food web.

To systematically assess the evidence, the researchers structured their investigation around the Population, Exposure, Comparator, Outcome, and Study design framework, searching the ISI Web of Science database for peer-reviewed literature published over five decades. The literature query combined terms encompassing soil invertebrates across multiple taxonomic ranks with forest thinning, stand density reduction, and tree retention management. The initial search yielded 824 publications, which were rigorously evaluated against explicit inclusion criteria: studies had to represent field forest experiments where canopy cover was altered through thinning or retention treatments, examine thinning as a disturbance, and quantitatively evaluate changes in soil fauna abundance or species richness. A total of 783 publications were excluded due to confounding agronomic settings, pest issues, or missing quantitative fauna metrics. The final systematic dataset comprised 41 studies containing 279 biological observations, partitioned into 27 articles investigating macrofauna across 170 observations, 20 analyzing mesofauna across 96 observations, and six examining microfauna across 13 observations.

Geographic analysis revealed pronounced regional and ecological imbalances in the global research landscape concerning thinning impacts on subterranean fauna. The forty-one identified investigations were distributed across only 15 countries, with an overwhelming concentration located in the temperate and continental regions of the Northern Hemisphere between 30 and 35 degrees North latitude. European and North American sites dominated the evidence base, accounting for 20 and 15 experiments respectively. Within this geographic distribution, macrofauna and mesofauna were predominantly investigated within the United States, which contributed 10 of the 27 macrofaunal studies, whereas Asian research concentrated heavily on microfauna, supplying three of the six microfaunal papers. Only a single published field experiment was conducted in the Southern Hemisphere, situated in an Indonesian tree plantation. Stand composition was heavily skewed toward coniferous evergreen forests, representing 31 of the 41 studies. Deciduous forests were evaluated in only eight experiments dominated by oaks, maples, or poplars, while mixed forests were examined in just two studies.

The review documented extensive disparities in silvicultural interventions, experimental designs, and post-harvest monitoring durations that hinder direct comparisons across studies. Thinning treatments reduced stand density across a spectrum ranging from 14 percent to 81 percent, utilizing diverse silvicultural techniques including thinning from below, selective thinning, canopy opening, line thinning, mechanical harvesting, and crop-tree release. Control stands varied substantially, with baseline tree densities ranging from 200 to 2500 stems per hectare and basal areas spanning from 4.8 to 57.2 square meters per hectare. Despite these contrasting stand structures, only five of the forty-one studies simultaneously measured both tree density and basal area per hectare, severely limiting standardized evaluations. Furthermore, thinning treatments were frequently confounded by secondary interventions, such as prescribed burning, understory clearing, soil harrowing, or logging residue mastication and removal. Monitoring intervals exhibited severe temporal limitations: 38 of the 41 investigations were conducted within one to seven years post-thinning, only three assessed outcomes exceeding seven years, and just fourteen utilized before-after sampling designs.

Macrofauna—defined as invertebrates exceeding two millimeters in body length, such as spiders, beetles, ants, millipedes, centipedes, and earthworms—exhibited divergent responses across twenty-seven investigations conducted predominantly in Douglas-fir, pine, and Norway spruce stands. Researchers collected organisms using active trapping across 13 studies, forest floor material collection across 11 studies, and flight-interception traps targeting floor-associated beetles across four studies, with specimens extracted via hand-sorting or funnel sieving. Neutral responses were common: fourteen experiments reported that thinning caused no significant changes in macrofauna abundance across groups including Coleoptera, Hymenoptera, Chilopoda, Araneae, and Hemiptera, while eight recorded neutral effects on species richness. Conversely, ten studies documented significant positive responses in the abundance or richness of spiders, beetles, ants, earthworms, and true bugs. Authors attributed population increases to enhanced understory vegetation biomass and structural heterogeneity, which generated expanded food resources, varied microhabitats, and sheltered environmental refuges that facilitated macrofaunal proliferation beneath open forest canopies.

Despite these positive observations, five experiments detected negative consequences of thinning on macrofauna, including reductions in the abundance or richness of ground-dwelling spiders, beetles, millipedes, and harvestmen. These declines were consistently traced to detrimental microclimatic shifts, as canopy removal increased solar radiation and wind speeds at the forest floor, driving desiccation of the protective leaf litter layer. Crucially, macrofaunal responses were moderated by seasonal variations, post-treatment intervals, and secondary disturbances. In an Oregon Douglas-fir forest, thinning elevated the abundance of spiders, grasshoppers, and polydesmid millipedes during the wet season, yet produced zero detectable differences during the dry season. In a Taiwanese Cryptomeria plantation, hunting spiders and web-building spiders increased during the initial post-harvest year, but after three years only ground hunters maintained elevated populations under 50 percent thinning. Furthermore, in California conifer forests, thinning combined with prescribed fire suppressed predatory ground beetles and wolf spiders while promoting wood-boring beetle guilds, demonstrating that disturbance interactions reshape community trajectories.

Soil mesofauna—invertebrates measuring between 0.2 and 2 millimeters, primarily composed of mites, springtails, and pseudoscorpions—exhibited starkly constrained responses across twenty reviewed experiments utilizing Berlese-Tullgren funnels, MacFadyen extractors, wet funnels, and pitfall traps across twelve evergreen and eight deciduous forests. Notably, not a single reviewed study reported a positive effect of forest thinning on mesofauna species richness. Thirteen investigations reported neutral responses to thinning across mite, springtail, thrips, and pseudoscorpion populations. Six studies detected direct negative impacts, recording significant decreases in potworm abundance and richness, mite abundance and diversity, springtail richness, and thrips densities. Additionally, six investigations identified interactive effects where mesofaunal outcomes varied across soil layers, forest stands, and sampling dates. For instance, in an American conifer forest, thinning sharply diminished mite abundance within the organic litter layer while leaving mineral topsoil communities unaffected. Within that litter horizon, detritivorous oribatid and predatory prostigmatid mites suffered substantial population declines, whereas opportunistic astigmatid mites exhibited no response, underscoring guild-specific microhabitat vulnerability.

Only a single investigation reported an unqualified positive effect of thinning on overall mesofauna abundance, with positive shifts elsewhere remaining highly conditional upon sampling season, vertical soil stratification, and species life-history strategies. In Italian black pine stands, mite and proturan abundances increased in thinned plots relative to unthinned controls exclusively during summer sampling periods. In ponderosa pine stands, thinning elevated the abundance of deeper soil-dwelling springtails without altering litter-dwelling assemblages, with soil springtail abundance correlating positively with expanding grass cover. In temperate French oak forests younger than 90 years, thinning prompted distinct functional trait turnover among Collembola communities. Opportunistic epiedaphic springtails—surface-dwelling r-strategists that rapidly colonize new environments—experienced notable increases in both abundance and species richness, stimulated by increased understory plant richness and organic resources. In contrast, hemiedaphic and deeper-dwelling euedaphic springtails—specialized K-strategists inhabiting deeper soil profiles—suffered marked declines in species richness, demonstrating that canopy opening selectively disadvantages deeper, moisture-dependent mesofauna while favoring resilient surface colonists.

Microfauna—invertebrates measuring under 0.2 millimeters, represented solely by six studies evaluating soil nematodes extracted with wet funnels in coniferous stands—demonstrated distinctly positive overall responses compared to larger fauna. Remarkably, none of the six investigations recorded a negative overall effect of thinning on total nematode populations. Five experiments reported that forest thinning boosted total nematode abundance and taxonomic richness, though outcomes varied depending on tree species and stand composition. In Chinese pine and cypress plantations, thinning elevated nematode taxonomic richness, whereas adjacent fir plantations showed no response due to impoverished understory development and scarce trophic resources. When researchers evaluated three thinning intensities in spruce and pine stands, total nematode abundance peaked under intermediate thinning, corroborating the intermediate disturbance hypothesis. However, disaggregating nematodes into trophic guilds revealed stark trade-offs: herbivorous and plant-parasitic nematodes increased in thinned plots, whereas omnivorous, predatory, and fungivorous nematodes experienced declines, with omnivore and predator numbers increasing in upper soil layers while diminishing at depth.

Synthesizing their findings, the authors emphasize that current understanding of soil fauna responses to forest thinning remains constrained by methodological fragmentation, inconsistent metrics, and severe biogeographical biases. With only a single study conducted in tropical forests, existing silvicultural paradigms cannot be reliably generalized across vulnerable, biodiversity-rich tropical biomes. To resolve these discrepancies, future research must establish standardized reporting standards that simultaneously quantify tree density, basal area, and understory plant cover before and after thinning interventions. Standardized extraction protocols—such as manual monolith hand-sorting using the Tropical Soil Biology and Fertility methodology for macrofauna and laboratory funnels for mesofauna and microfauna—must be uniformly deployed alongside detailed records of soil sampling depth, litter thickness, and seasonal meteorological conditions. Furthermore, integrating environmental DNA metabarcoding and high-throughput sequencing with functional trait frameworks will allow researchers to transcend traditional taxonomic barriers, connecting subterranean faunal turnover with crucial biogeochemical processes to guide climate-resilient forest management strategies worldwide.

Subject of Research: Systematic review of forest thinning impacts on soil fauna communities and research gaps

Article Title: Influence of forest thinning on the soil fauna: a systematic review of current knowledge and research gaps

Article References: BIRYOL, C., BALDY, V., PREVOSTO, B., TRAP, J., FOREY, E., PEREZ-IZQUIERDO, L., BALLINI, C., GAUQUELIN, T., & SANTONJA, M. (2026). Influence of forest thinning on the soil fauna: a systematic review of current knowledge and research gaps. Annals of Forest Science, 83(1), Article 6. https://doi.org/10.1186/s13595-025-01320-8

Image Credits: AI Generated

DOI: 10.1186/s13595-025-01320-8

Keywords: soil fauna, forest thinning, macrofauna, mesofauna, microfauna, Collembola, Acari, nematodes, canopy cover, understory vegetation, microclimate, stand density, basal area, litter decomposition, Berlese-Tullgren funnel, systematic review, soil biodiversity, tree retention