Effects of tree fall on soil Collembola: Disentangling the role of gap formation and deadwood addition

• Responses of soil Collembola to gap formation and deadwood addition are context dependent. • Gap formation and deadwood addition change the vertical distribution and body size but not reproductive mode of Collembola. • The effects of gap formation and deadwood addition on Collembola communities are independent of each other. Increased frequency of climate extremes causes large scale forest decline associated with gap formation and input of deadwood to the forest floor, which largely changes soil systems. However, for disentangling the effects of gap formation and deadwood addition, experimental manipulations allowing to separate the effects of each are needed. Based on a large-scale full-factorial forest gap and deadwood addition experiment, we analyzed the response of soil invertebrates (Collembola) to gap formation and deadwood addition across three geographical regions in Germany i.e., the Alb, Hainich and Schorfheide. Both gap formation and deadwood addition altered the taxonomic richness, density and traits of Collembola communities. The effects of gap formation and deadwood addition were independent of each other but varied among regions, reflecting the importance of both geographic and historical context, as well as environmental changes such as variations in climate. Gap formation strongly decreased total density of Collembola in the Hainich but increased it in the Schorfheide, indicating that the effect is negative in regions with high precipitation and deep soils but positive in regions with low precipitation and shallow soils. Deadwood addition little affected Collembola density but restructured the community composition and increased overall functional and species taxonomic richness, presumably by expanding niche space by increased habitat heterogeneity. Gap formation filtered for small-sized and soil-living species via decreased soil moisture, but did not affect other traits such as number of ocelli and reproduction mode. The results suggest that gap formation and deadwood addition affect Collembola communities and traits in an independent way. Overall, the results indicate that deadwood is pivotal for soil diversity conservation, and forest gaps detrimentally affect animals deeper in soil being adapted to moist conditions.