Forest habitat complexity is crucial for biodiversity, although fine-scale complexity remains understudied. On three trunks, arthropods assemblages are shaped by bark structure and seasonality, but epiphytic lichens add structural complexity to trunks, contributing to retain moisture and microclimate stability. This study examines how microhabitat complexity provided by Lobaria pulmonaria, a foliose cyanolichens, interacts with bark structure and seasonal variation to shape arthropod communities in montane Mediterranean forests (Fagus sylvatica and Quercus pyrenaica). Seasonal comparisons among lichen thalli and basal/high traps, analyzed with two-way ANOVA, NMDS, PERMANOVA, SIMPER, Bray-Curtis, and bootstrap ratio-of-means (ROM), revealed microhabitat as the dominant driver. L. pulmonaria consistently supported much higher arthropod abundances than bark, with ROM reaching around 20–32x on Quercus in autumn and winter relative to traps. Taxon richness did not differ significantly among microhabitats, but decomposers and detritivores (Acarina, Collembola, Psocoptera) concentrated on Lobaria pulmonaria. Tree identity modulated effect size, with stronger responses on Quercus pyrenaica, while Acarina Oribatidae responded more on Fagus sylvatica. These results indicate that L. pulmonaria acts as a refugial microhabitat buffering environmental stress, especially during leaf-off, cold seasons. From a conservation perspective, maintaining large cyanolichen thalli on host trees is essential to sustain arthropod abundance and functional diversity in Mediterranean forests, where cyanolichens are vulnerable to fragmentation and management. Their decline could lead to cascading effects on arthropod communities and ecosystem processes such as decomposition and nutrient cycling. Keywords: Forests; Interactions; Lichens; Mediterranean Spain.