Underestimated spider diversity in a temperate beech forest

We used canopy fogging to study the high (20–26 m), intermediate (13–19 m) and low (5–6 m) strata in three European beech patches (Fagus sylvatica) in nine months (2005–2007) and estimate species richness and diversity of arboreal spiders. Eight species (10%) were previously unseen in European beech trees, and one of these is likely a new species. Moreover, two species are on the Bavarian red list. Our results revealed that the high stratum of the old-growth trees provided unique resources and possessed the greatest diversity and evenness, whereas intermediate and low strata had high similarity in respect to diversity, dominance, species, and family composition. Since the majority of beech forests consists of mature and young trees in Central Europe, and old-growth forests are rarely preserved, we recommend young beech be used in a sampling protocol for rapid biodiversity assessment. However, adding samples from the two higher strata to the lowest stratum (55 species), almost doubled the estimated species richness (102 species). This suggests that the lower stratum alone does not represent a true image of the total canopy fauna inventory in this, and likely other, beech stands. To complete this comprehensive inventory in European beeches, the Chao1 predicted that additional sampling would be needed in the highest stratum, where there is a high probability to find previously undetected species in a next survey. Our study clearly shows that neglecting the crowns of the largest, tallest trees risks underestimating the overall spider diversity in Central European forests.