An Experimental Test of Insect-Mediated Colonisation of Damaged Pinus radiata Trees by Sapstain Fungi

Vector-pathogen dynamics play a central role in understanding tree health and forest dynamics. There is substantial evidence that bark beetles act as spore vectors for many species of fungi that cause 'sapstain' discolouration of damaged trees and timber. However, the direct quantitative link between vector-mediated spore dispersal and subsequent sapstain colonisation of wood is not fully understood. Here, we used caged versus uncaged experimental logs to test whether the exclusion of bark beetles quantitatively alters the distribution and intensity of sapstain fungal spread within damaged trees. Using generalised linear mixed models, we tested the effect of bark beetle exclusion on sapstain intensity within and among cut logs at two plantation forest sites. Overall, sapstain was found on all logs regardless of caging treatment, indicating that sapstain colonisation can occur (to some degree) without arthropod vectors, probably via wind, rain-splash and, potentially, latent endophytic development. This was supported by the dominance of Diplodia pinea in fungal isolations taken from trees felled at the site, as this fungal species is known to disperse independently of bark beetles. However, the intensity of sapstain within and among experimental logs was significantly greater in uncaged than in caged logs, where beetle colonisation was significantly greater. This appeared to be driven by a significant within-log association between the intensity of staining and the intensity of beetle, and other arthropod, tunnelling and feeding activities. Taken together, these results strongly suggest that the dominant mechanism underlying the role of bark beetles in sapstain development in this study system is not vector-mediated spore dispersal, per se, but rather the facilitation of spore entry and hyphal development through tunnelling and feeding activities. We discuss the implications of these findings for forest management and the effective salvage-harvest of trees damaged by stochastic climate events such as storm and fire damage.