Context-dependent effects of woody layer complexity on arthropod biomass and abundance in deciduous forests

Forest structural complexity influences arthropod communities by shaping habitat availability, microclimatic conditions, and resource distribution. However, the extent to which structural complexity and specific structural components drive arthropod abundance and biomass remains poorly understood in temperate forests. This study examined how local and landscape-scale forest characteristics influence arthropod communities across vertical strata (forest floor (FF), herb layer (HL), and shrub layer (SL)) in 19 temperate deciduous forests in Belgium, dominated by pedunculate oak, European beech, or Canadian poplar. At the local scale, we assessed dominant tree species identity, overall forest structural complexity, and its components (vertical and horizontal structure, woody layer, herbal layer, and deadwood). At the landscape scale, we evaluated forest area, edge length, forest cover, and vegetation greenness (normalized difference vegetation index (NDVI)). Contrary to expectation, arthropod biomass and abundance did not consistently increase with higher structural complexity. Instead, woody layer complexity, dominant tree species, and NDVI emerged as key drivers, with effects varying by context and stratum. Arthropod abundance and biomass were the highest in oak- and poplar-dominated forests and the lowest in beech forests, likely due to differences in litter quality, microhabitat availability, and understory development. Woody layer complexity positively influenced forest floor arthropods in poplar forests but had a negative effect in oak forests. At the landscape scale, NDVI unexpectedly showed negative relationships with arthropod abundance across strata and with arthropod biomass in the herb layer, likely reflecting dense canopy suppression of understory productivity. Arthropod biomass on the forest floor increased with forest cover, while abundance in the shrub layer decreased with forest cover but increased with forest area. These findings highlight the complex interplay between forest structural attributes, dominant tree species, and landscape factors in shaping arthropod communities. By identifying the key drivers of arthropod abundance and biomass, this study contributes to a better understanding of biodiversity patterns in temperate forests and their ecological dynamics.