Scents on the slope: declining volatile emissions but stable predator-associated cues across elevation in alpine...

Volatile organic compounds (VOCs) can play important roles as indirect defenses and attract natural enemies of herbivores to induced plants. The relative importance of VOCs as indirect defense can be altered by abiotic conditions, plant identity, and the composition of local natural enemies. Elevational gradients provide an effective framework for investigating how plants allocate resources to VOC production and how these compounds influence the attraction of natural enemies of invertebrate herbivores under varying biotic and abiotic conditions. We studied induced VOCs and predation on four willow species along an elevational gradient in the Austrian Alps. We grouped the species into two pairs based on their ploidy (Salix appendiculata-caprea and Salix mielichhoferi-myrsinifolia), each pair collectively spanning the whole elevational gradient (1,000 to 2,230 m a.s.l.). We induced the plants with methyl jasmonate and quantified their induced VOC profiles. We measured vertebrate and invertebrate predation on yellow, green and brown colored dummy caterpillars, representing the diversity of caterpillars that typically feed on willows. Induced VOC emissions tended to decrease with elevation, with a clear decline in the S. appendiculata-caprea pair and a weaker, marginal tendency in the S. mielichhoferi-myrsinifolia pair. Overall predation rates on artificial caterpillars did not vary consistently with elevation. They were, however, influenced by caterpillar color and willow species identity, particularly in the Salix mielichhoferi-myrsinifolia pair, where yellow caterpillars experienced significantly lower predation rates at higher elevations. Predation rates correlated with both overall and individual VOC emissions. These relationships were partly explained by willow species and predation types, but largely independent of elevation. Emissions of individual VOCs associated with predation did not decrease with elevation. Overall, our results thus show that although total induced VOC emissions declined with elevation, predator-associated VOCs remained relatively stable and predicted predation independent of elevation. This suggests that willows may maintain key components of their induced VOC blends even where overall emissions are reduced. Our results indicate that VOC-mediated attraction of natural enemies can persist across heterogeneous mountain environments, supporting the role of induced VOCs as indirect defenses along elevational gradients.