Abstract As renewable energy initiatives intensify, the benefits of optimizing recycled wood waste and forestry residues for bioenergy generation are clear. Self-heating fires and degradation during outdoor pile storage however continue to result in safety risks and economic losses. Furthermore, the storage dynamics of older wood waste material remain understudied. The following presents the longest continuous North American forestry residue storage trial, comparing the storage characteristics of woodchip piles built with fresh and aged material (previously stored for 20 months). Temperature sensors were placed within industrial woodchip piles (consisting primarily of poplar) at Pineland Nurseries (Manitoba, Canada). Monitoring occurred for 459 days (August 2017–November 2018) where samples were periodically characterized for dry-matter loss, pH, bulk density, particle size, moisture, ash and free sugar content. Significantly higher temperatures were sustained in the aged pile throughout the storage trial compared to the fresh pile where temperatures decreased within a month after an initial thermal spike. Decreased particle size and higher moisture content in the aged material appeared to have the most impact on thermal retention. Microorganism reintegration theory supported our findings where initial sugar content did not correlate with higher initial self-heating in the aged pile. This study highlights several critical considerations for forestry residue pile management.