Tree diversity promotes generalist herbivore community patterns in a young subtropical forest experiment

Abstract

Stand diversification is considered a promising management approach to increasing the multifunctionality and ecological stability of forests. However, how tree diversity affects higher trophic levels and their role in regulating forest functioning is not well explored particularly for (sub)tropical regions. We analyzed the effects of tree species richness, community composition, and functional diversity on the abundance, species richness, and beta diversity of important functional groups of herbivores and predators in a large-scale forest biodiversity experiment in south-east China. Tree species richness promoted the abundance, but not the species richness, of the dominant, generalist herbivores (especially, adult leaf chewers), probably through diet mixing effects. In contrast, tree richness did not affect the abundance of more specialized herbivores (larval leaf chewers, sap suckers) or predators (web and hunting spiders), and only increased the species richness of larval chewers. Leaf chemical diversity was unrelated to the arthropod data, and leaf morphological diversity only positively affected oligophagous herbivore and hunting spider abundance. However, richness and abundance of all arthropods showed relationships with community-weighted leaf trait means (CWM). The effects of trait diversity and CWMs probably reflect specific nutritional or habitat requirements. This is supported by the strong effects of tree species composition and CWMs on herbivore and spider beta diversity. Although specialized herbivores are generally assumed to determine herbivore effects in species-rich forests, our study suggests that generalist herbivores can be crucial for trophic interactions. Our results indicate that promoting pest control through stand diversification might require a stronger focus on identifying the best-performing tree species mixtures.

Herbivore and pathogen effects on tree growth are additive, but mediated by tree diversity and plant traits

“…Total herbivore and pathogen damages in our study were positively related, which might be indicative of an important role of facilitative direct effects (herbivores as vectors) or indirect effects via plants. Leaf chewers were the main herbivores at our study site (Zhang et al., 2017) and responsible for the main damage on most trees. Leaf chewing herbivores and pathogens such as biotrophic fungi can trigger separate defense pathways that are most effective against only one of the two damage types or that even inhibit alternative defense pathways via signaling cross talk (Thaler, Humphrey, & Whiteman, 2012).…”

“…This can be explained by the opposite effects of tree richness on herbivory and pathogen damage (see also Hantsch et al., 2014; Schuldt et al., 2010, 2015). Herbivore assemblages in our study system are dominated by generalists (Zhang et al., 2017), which can benefit from the diversity of resources available in more diverse tree communities (Lefcheck, Whalen, Davenport, Stone, & Duffy, 2013; Zhang et al., 2017). In contrast, most foliar fungal pathogens are highly host‐specific and their passive mode of dispersal makes them dependent on the density of suitable hosts in their surroundings (Hantsch et al., 2014), which is highest in monocultures and decreases with increasing tree species richness.…”

“…Our results indicate that direct or indirect facilitation of herbivory by pathogens could be most effective in monocultures (i.e., in the absence of opportunities for diet mixing), for example because it might help generalist herbivores to overcome tree species‐specific defense mechanisms or alter host traits (e.g., Cardoza, Lait, Schmelz, Huang, & Tumlinson, 2003). The dominant herbivorous arthropods at our study site included many adult leaf chewers, such as weevils (Zhang et al., 2017). These herbivores are highly mobile and can easily move between trees to select suitable host plants.…”

“…These herbivores are highly mobile and can easily move between trees to select suitable host plants. The fact that mobile, generalist herbivores play an important role in determining overall herbivory levels is further supported by the finding that the abundance of adult leaf chewers with a generalized host use increased with increasing tree species richness at our study site, whereas more specialized and less mobile herbivore abundances were unaffected by changes in tree species richness (Zhang et al., 2017). A dependence of facilitative effects on host plant quality would fit the observation that herbivory was positively related to pathogen damage particularly on trees with high tannin contents.…”

“…However, opposite patterns of increasing damage with increasing tree diversity have also been reported (Kambach, Kühn, Castagneyrol, & Bruelheide, 2016;Nguyen et al, 2016;Plath, Dorn, Riedel, Barrios, & Mody, 2012;Schuldt et al, 2010). Discrepancies among studies could, for example, depend on the degree of host specialization of dominant consumer species (Castagneyrol, Jactel, Vacher, & Brockerhoff, 2014;Zhang et al, 2017). Most of the studies analyzing herbivore or pathogen damage in a biodiversity context have not, however, explicitly tested whether changes in the relationship between damage and diversity have consequences for tree performance (but see e.g.…”