Fuel Treatments by Mulching - A Synthesis of the Ecological Impacts

Concern over severe fire hazards has led to a novel, widespread management treatment in forests and woodlands in the American West. Tens of thousands of hectares have been treated by chipping or masticating (mulching) live trees, with the woody material deposited on the soil surface. This woody material differs from that added through natural processes: pieces are smaller, all of it is deposited at once, and the material forms a compact layer. Because of these differences, the state-of-knowledge on woody debris in forests provides little insight on the potential ecological effects of these mulching treatments for reducing fire hazard. We reviewed the literature to address the impacts of mulching treatments. Mulching fuel treatments tended to: (1) reduce the modeled risk of crown fire, (2) increase soil net nitrogen mineralization, soil carbon, and soil moisture; (3) decrease maximum soil temperature, herbaceous cover/biomass, and tree regeneration; and (4) not change plant nitrogen concentrations and soil compaction in the long-term. Mulching fuels treatments do not appear to decrease soil nitrogen availability, total soil nitrogen, or plant phosphorus concentrations. Few studies addressed questions important to the managers implementing these treatments. What is the optimal distribution and depth of the woody material? How long will it take to decompose? Will the treatments change species diversity, or impact rare or endangered species? What will happen to water yield? Will the treatments change the risk of insect outbreaks or disease? How long will the fuels treatments last? What are the effects of thinning compared to the effects of the added wood? How will ecosystem water and carbon balance change? The widespread application of these mulching treatments together with the lack of knowledge about ecological effects suggests research in this area should be a priority