Dead-wood addition promotes non-saproxylic epigeal arthropods but effects are mediated by canopy openness

Restoring dead-wood amounts in forests is an increasingly and successfully applied conservation measure to counteract negative effects of intensive logging on biodiversity of saproxylic taxa. By contrast, if and how dead-wood addition benefits the vast number of non-saproxylic forest taxa, and how this varies with contextual factors like canopy openness, remains poorly understood. To enhance dead-wood addition strategies, it is thus important to understand how dead wood affects entire forests communities, not just saproxylic taxa. To untangle effects of dead-wood addition and canopy openness on non-saproxylic epigeal arthropods, we exposed different amounts of logs and branches on 190 0.1-ha plots located in sunny or shady mixed montane forests and sampled epigeal arthropods over three years. Canopy openness was a major driver of species assemblage composition and clearly mediated the effects of dead wood on epigeal beetles, spiders/harvestmen and springtails. Most species groups responded positively to the addition of dead wood. All groups decreased in number with increasing distance to dead wood. Dead wood affected taxa of both lower and higher trophic levels directly and taxa of higher trophic levels benefitted also indirectly owing to bottom-up effects. Our results indicate that increasing the amount of dead wood for conservation of saproxylic taxa benefits also non-saproxylic epigeal arthropods and thus, a larger number of forest species than commonly assumed. Because of the strong effects of canopy openness, similar to those found for saproxylic taxa, dead wood in both sunny and shady forest stands is needed.