Abstract
Summary Insect herbivore damage and abundance are often reduced in diverse plant stands. However, few studies have explored whether this phenomenon is a result of plant diversity effects on host plant traits.We explored indirect effects of tree species diversity on herbivory via changes in leaf traits in a long‐term forest diversity experiment in Finland. We measured 16 leaf traits and leaf damage by four insect guilds (chewers, gall formers, leaf miners and rollers) on silver birch (Betula pendula) trees growing in one‐, two‐, three‐ and five‐species mixtures.A decline in the frequency of birch in mixed stands resulted in reduced leaf area. This, in turn, mediated the reduction in chewing damage in mixed stands. In contrast, associational resistance of birch to leaf miners was not trait‐mediated but driven directly by concurrent declines in birch frequency as tree species richness increased.Our results show that leaf trait variation across the diversity gradient might promote associational resistance, but these patterns are driven by an increase in the relative abundance of heterospecifics rather than by tree species richness per se. Therefore, accounting for concurrent changes in stand structure and key foliar traits is important for the interpretation of plant diversity effects and predictions of associational patterns.
Highlights
Plant diversity has long been known to impact numerous ecosystem processes and the structure of associated communities of consumers
Further tests with host dilution instead of tree species richness demonstrated no significant effect on most leaf traits (Table S5)
Structural traits predict herbivore resistance better than chemical traits we examined a wide range of leaf traits, including the less explored oxidative capacity of tannins (Salminen & Karonen, 2011), we observed that traits related to the structure and morphology of birch leaves were more often retained in least absolute shrinkage and selection operator (Lasso) models over defensive traits (Fig. 3; Table 1)
Summary
Plant diversity has long been known to impact numerous ecosystem processes and the structure of associated communities of consumers. The presence of heterospecific neighbouring plants has frequently been observed to reduce the vulnerability of a focal plant to herbivore attack (Jactel & Brockerhoff, 2007; Barbosa et al, 2009). This phenomenon of associational resistance has often been attributed to a reduced proportion of host plants as diversity increases (resource concentration hypothesis; Root, 1973; Otway et al, 2005; Heiermann & Schu€tz, 2008; Sholes, 2008; Bj€orkman et al, 2010; Plath et al, 2012) and to physical and chemical traits of neighbouring plant species (Atsatt & O’Dowd, 1976; Ruttan & Lortie, 2014). Host traits may vary across gradients of tree species diversity as a result of both inter- and intraspecific interactions
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