Quantification of seasonal bark loss for two Oregon commercial tree species, Douglas-fir (Pseudotsuga menziesii) and ponderosa pine (Pinus ponderosa), was conducted at monthly intervals beginning in late October to early November 2009 and finishing in August 2010. All assessments were carried out on harvesting operations that were using mechanized processor heads with chains over rubber feed wheels. A total of 450 stems were assessed.

There was a substantial (up to five times) increase in areal bark loss, expressed as a percentage of log surface area, during late spring and early summer compared with the loss in winter. Areal bark loss appeared to be species dependent, with Douglas-fir incurring more than twice as much bark loss as ponderosa pine.

Seasonal differences in postharvesting bark volume and weight were determined by accounting for bark thickness and bark density. In winter, bark could be expected to account for 3 to 4 percent more of a stem's weight than in late spring to early summer. There were considerable differences between species in the contribution of bark to total weight, but not in the percent drop in weight between seasons.

It is shown that the seasonal changes in bark loss could be expected to lead to changes in solid wood truck payloads, transport costs, bark, and available energy (from bark) delivered to mills.