Sawfly (Hym., Diprionidae) outbreaks on Scots pine: effect of stand structure, site quality and relative tree position on defoliation intensity

From 1997 to 2001, Finland has experienced its largest-ever documented outbreak of Diprion pini L. (500 000 ha). We investigated—in four affected areas—the effects of site quality, stand structure and relative tree position on outbreak dynamics during two consecutive years (1999–2000). Most of the variation in outbreak intensity, as measured by defoliation intensity, was accounted for by variables pertaining to tree size, indicating a preference for more mature trees. However, we also found elevated and systematic defoliation of younger stands. Tree species composition, age difference with surrounding stands, stand area, stand openness and soil characteristics are secondary to the tree size variables in explaining the defoliation intensity, and often had ambiguous effects. As for the common assumption linking outbreaks to shallow, well-drained and unproductive soils, our data only suggest such a link on the level of pine forest vulnerability on a landscape level. As for the various tree canopy strata, we found only in young stands significant differences in defoliation intensity between them, with the dominant trees most affected. This has implications for forest management, because these trees, now undergoing the highest growth losses due to defoliation, normally benefit as trees of the future from thinning activities.