Data from: Anti-herbivore defences and insect herbivory: interactive effects of drought and tree neighbours

1. How much a plant is attacked by insect herbivores likely depends on its
apparency and ability to produce defensive traits, which may be modified
by neighbouring plants and abiotic conditions. Yet, how much the direct
and trait-mediated effects of neighbours on herbivory is modified by
abiotic factors is still unknown. 2. By using a tree diversity experiment
in SW France, we measured leaf insect herbivory (chewers and miners),
nutritional quality (water content, C/N ratio, sugar and starch content)
and chemical defences (total polyphenolics and condensed tannins) on birch
(Betula pendula) trees growing in monocultures and mixtures with oak, pine
or both species. We alleviated water stress by irrigating trees in half of
the plots while trees remained unwatered (i.e., drought-stressed) in the
other half. 3. Overall, insect herbivory was higher among heterospecific
neighbours, which corresponds to associational susceptibility. Consistent
with this finding, leaves had lower amount of anti-herbivore defences
among heterospecific neighbours. In turn, insects caused more damage in
drought-stressed conditions, but such effect was independent of leaf
chemistry. We also found that the effect of tree species diversity on
herbivory was contingent of drought conditions as associational
susceptibility only occurred in drought-stressed trees. The independent
and interactive effects of neighbour diversity and irrigation on leaf
herbivory remained significant after accounting for birch apparency and
leaf chemistry, suggesting that unmeasured plant traits or some other
mechanisms not associated with plant trait variation and apparency might
be involved in the observed herbivory patterns. 4. Synthesis. By
demonstrating that associational effects are contingent upon abiotic
constraints, we bring new insights into our understanding of the
mechanisms driving diversity - resistance relationships across climatic
gradients