CO2 release and dry matter loss of Scots pine forest chips stockpiled from late summer to winter

We studied CO2 release dynamics between a stockpile and the atmosphere and estimated dry matter losses of forest chips made from small-diameter Scots pine, either whole trees or stemwood. A stockpile established in early August in northern Finland was monitored for six months. Based on changes in the basic density of the chip samples, a dry matter loss of 2.6–2.8% occurred during the storage period, but there were no statistically significant differences between the two chip assortments. The dry matter loss derived from the CO2 effluxes was 1.3–1.5%. Even when capturing only about half of the carbon loss deduced from dry mass measurements, the CO2 measurements provided valuable information about decomposition dynamics in stockpiles. The CO2 flux rates were not static, but could differ by an order of magnitude, depending on the spatial location of flux measurement and the evolution of temperature conditions within the pile. Whole-tree chips generated more heat during the first two months after pile construction. The highest emissions and largest mass flows were measured at that time near the top of the pile, where also the humidity had condensed, suggesting that the stockpile structure and temperature gradient probably channeled the gas flows.