Mastication is a silvicultural technique that grinds, shreds, or chops trees or shrubs into pieces and redistributes the biomass onto the forest floor to form a layer of woody debris. Unlike other fuel treatments that remove this biomass, masticated biomass often remains on site, which increases total fuel loading and causes concern over how the masticated particles may burn if exposed to prescribed fire or wildfire. Central to the question of how these particles may burn is how the time since mastication affects the decomposition of the wood particles comprising the fuels. We conducted controlled laboratory experiments to investigate how the particles changed chemically over the time since they were masticated and how those chemical changes affected fire behavior characteristics. The objectives were (1) to quantify the chemical differences of masticated materials from different climates and different decomposition stages, (2) determine whether chemical changes occurred similarly in all fuel particles, and (3) describe the fire behavior characteristics exhibited by these fuels. Masticated materials came from mixed-conifer forests at fifteen different sites throughout the Rocky Mountains. Paired stands from these sites were of similar vegetation and forest stage. They represented wet and dry climates, different stages of wood decomposition, and variable piece sizes based on the type of machine used to masticate the biomass.
Time since mastication and piece size affected the rate of chemical changes in the masticated particles. Fragmented particles had less heat value, N, and C than intact particles from the same site. C decreased and N increased with time since treatment. In most cases, cellulose decreased as decomposition occurred. Age of the particles, tree species, climate, and quantity of fuel load were all important factors influencing chemical change and burn characteristics. In the smoldering experiments, age was not a significant factor but soil substrate was. Soil surface temperatures in the smoldering tests differed significantly between dry sand and dry duff, and most of the smoldering burns in dry duff easily reached temperatures and durations at the surface between the fuel and the soil that would kill soil plants, microbes, and fauna and severely affect soil ecology. When planning prescribed burns in these treatments, managers need to consider not only the moisture of the fuels, air temperature, and wind, but also the dryness and type of soil, the amount of decomposition (time since mastication) of the fuel particles, fuel depths, fuel loads, and the spatial distribution of the fuel loads left by the masticator.