Positive interactions between great longhorn beetles and forest structure

Great longhorn beetles (GLBs) may affect forest structure, because their larvae colonize trees and create networks of cavities further exploited by other species, resulting in dead or broken branches and tree mortality. At the same time, tree size, condition and location are known to affect the likelihood of tree colonization by GLBs. Here, we study the relationship between GLBs and the structure of old-growth holm oak Mediterranean forests in Sardinia, Italy. Generalized linear models were used to relate presence of GLBs with structural parameters that can be affected by or can affect these ecosystem engineers. We found a strong positive relationship between GLBs and the number of trees with cavities, while the relationships between GLBs and the number of dead wood decaying classes, amount of coarse woody debris, stand basal area, forest canopy layer complexity were positive but weak. Overall, our results suggest that larval activity of GLBs could be responsible for increased tree cavity and dead wood formation, enhancing forest structure. On the other hand, forests with larger and older trees could be more likely to be colonized by GLBs. While it is not possible with observational studies to disentangle to what extent GLBs affect forest structure or vice versa, it seems likely that reciprocal interactions may enhance the structure of holm oak forests. Therefore, parasitism from native GLBs should be seen and understood more as a key ecological process that enhances the structure of Mediterranean holm oak forest habitats.