Forest type interacts with milkweed invasion to affect spider communities.
Non-native tree plantations constitute a large part of forestation worldwide. Plantations are prone to invasion by exotic herbaceous plant species due to habitat properties, including understory vegetation structure. We established 40 sampling sites in 10 plantation forests. Sites were selected according to tree species (native poplar forests and exotic pine plantations) and common milkweed (Asclepias syriaca) density (invaded and non-invaded sites) in a full factorial design. We collected spiders with pitfall traps. We found a significant effect of A. syriaca invasion on spider functional diversity (Rao's quadratic entropy), with invaded sites having a lower functional diversity than non-invaded sites. A larger effect of invasion with A. syriaca on the RaoQ of spiders was observed in pine compared to poplar plantations. Spider species were larger, and web-building spiders were more frequent in poplar forests than in pine plantations. We found no effect of A. syriaca invasion on species richness or abundance of spiders. Species composition of spider assemblages in the two forest types was clearly separated according to non-metric multidimensional scaling. We identified seven species associated with pine plantations and six species associated with poplar plantations. The similar species richness and the higher functional diversity of non-invaded sites suggested that these trait states were less similar than invaded sites and that functionally different species were present. In contrast, the invaded sites had lower functional diversities and thus more uniform trait state compositions, suggesting that environmental filtering played an important role in species sorting, making invaded plantations low-quality secondary habitats for the original spider fauna.