Abstract
The diversity of plant neighbors commonly results in direct, bottom‐up effects on herbivore ability to locate their host, and in indirect effects on herbivores involving changes in plant traits and a top‐down control by their enemies. Yet, the relative contribution of bottom‐up and top‐down forces remains poorly understood. We also lack knowledge on the effect of abiotic constraints such as summer drought on the strength and direction of these effects. We measured leaf damage on pedunculate oak (Quercus robur), alone or associated with birch, pine or both in a long‐term tree diversity experiment (ORPHEE), where half of the plots were irrigated while the other half remained without irrigation and received only rainfall. We tested three mechanisms likely to explain the effects of oak neighbors on herbivory: (1) Direct bottom‐up effects of heterospecific neighbors on oak accessibility to herbivores, (2) indirect bottom‐up effects of neighbors on the expression of leaf traits, and (3) top‐down control of herbivores by predators. Insect herbivory increased during the growth season but was independent of neighbor identity and irrigation. Specific leaf area, leaf toughness, and thickness varied with neighbor identity while leaf dry matter content or C:N ratio did not. When summarized in a principal component analysis (PCA), neighbor identity explained 87% of variability in leaf traits. PCA axes partially predicted herbivory. Despite greater rates of attack on dummy caterpillars in irrigated plots, avian predation, and insect herbivory remained unrelated. Our study suggests that neighbor identity can indirectly influence insect herbivory in mixed forests by modifying leaf traits. However, we found only partial evidence for these trait‐mediated effects and suggest that more attention should be paid to some unmeasured plant traits such as secondary metabolites, including volatile organic compounds, to better anticipate the effects of climate change on plant‐insect interactions in the future.
Highlights
| INTRODUCTIONPlant diversity is a key driver of insect herbivory in grassland, agricultural, and forest ecosystems (Allan et al, 2013; Broad, Schellhorn, Lisson, & Mendham, 2008; Castagneyrol et al, 2014), because heterospecific neighbors may either decrease (associational resistance) or increase (associational susceptibility) the likelihood of focal plants being attacked by insect herbivores (Barbosa et al, 2009; Root, 1973; Tahvanainen & Root, 1972; White & Whitham, 2000)
Plant diversity is a key driver of insect herbivory in grassland, agricultural, and forest ecosystems (Allan et al, 2013; Broad, Schellhorn, Lisson, & Mendham, 2008; Castagneyrol et al, 2014), because heterospecific neighbors may either decrease or increase the likelihood of focal plants being attacked by insect herbivores (Barbosa et al, 2009; Root, 1973; Tahvanainen & Root, 1972; White & Whitham, 2000)
Because trait values may have changed during the course of the season, and because predation was only assessed in early summer, we tested the effects of specific leaf area (SLA), leaf dry matter content (LDMC), leaf toughness, thickness, and predation on early season herbivory, and the effect of C:N on late season herbivory, separately
Summary
Plant diversity is a key driver of insect herbivory in grassland, agricultural, and forest ecosystems (Allan et al, 2013; Broad, Schellhorn, Lisson, & Mendham, 2008; Castagneyrol et al, 2014), because heterospecific neighbors may either decrease (associational resistance) or increase (associational susceptibility) the likelihood of focal plants being attacked by insect herbivores (Barbosa et al, 2009; Root, 1973; Tahvanainen & Root, 1972; White & Whitham, 2000). Associational resistance has been proposed to result primarily from a reduced ability of herbivores to locate and reach their host plants among heterospecific neighbors This may be due to patches with greater plant diversity being less attractive than monospecific patches (i.e., the resource concentration hypothesis, Root, 1973; Andersson, Löfstedt, & Hambäck, 2013) or from heterospecific neighbors reducing the physical (Castagneyrol et al, 2013; Damien et al, 2016; Floater & Zalucki, 2000) or chemical (Jactel, Birgersson, Andersson, & Schlyter, 2011; Zhang & Schlyter, 2004) apparency of host plants. | 3522 tree neighbors indirectly mediate the expression of traits involved in herbivore-oak interaction; (3) heterospecific neighbors increase top- down predation by natural enemies, and (4) water stress changes the strength of top-down and bottom-up processes. This study addresses how drought indirectly mediates tree neighbor effects on insect herbivory via changes in leaf traits and top-down control by herbivores’ enemies. We provide partial support to the hypothesis of indirect, bottom-up, effect of tree species diversity on herbivory through changes in leaf traits, regardless of drought
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