Plant ontogeny as a driver of elevational patterns of entomopathogenic nematode activity in Mediterranean oaks

Abstract

Background and aims

A long-standing ecological assumption posits that insect herbivory increases in warmer, more stable climates at lower elevations. However, this paradigm has been called into question in recent decades. Some studies suggest that differences in tri-trophic interactions, particularly the diversity, abundance, and activity of herbivore natural enemies, may explain inconsistent patterns in herbivory. Additionally, plant ontogeny significantly influences herbivore susceptibility, with adult plants being more apparent and thus more susceptible to herbivore attacks than saplings. These ontogenetic differences in herbivory might, in turn, determine changes in herbivore predation across elevations. Unfortunately, most research addressing these ecological assumptions has focused on aboveground tri-trophic interactions.

Methods

Here, we investigated elevational differences in the activity of entomopathogenic nematodes (EPNs), known killers of soil-dwelling insects, and compared these patterns between young and adult oak (Quercus, Fagaceae) trees. We collected soil samples from rhizospheres of adult trees and saplings throughout the optimal elevational range (low, mid, and high) of 10 Mediterranean oak species in the Iberian Peninsula, estimating EPN activity through insect baiting with wax moth larvae.

Results

Our results showed higher larval mortality and EPN activity at lower elevations, with this effect being influenced by plant ontogeny; therefore, elevation-related variations were observed only in rhizospheres of mature trees. Additionally, we found that soil characteristics did not significantly affect these outcomes.

Conclusions

Our study provides evidence that plant ontogeny influences belowground tri-trophic interactions along elevational gradients in oak species, emphasizing the minimal impact of abiotic soil factors on these processes.