Slash is logging debris left in the forest after a harvest. Sometimes it is called logging residue. Slash can serve a range of purposes.
Nutrients in slash become available to plants as they decompose and also to animals that feed on the leaves, buds and twigs. The physical structure of slash can protect new seedlings from excessive browsing by herbivores such as deer. Slash piles serve as effective refuge from predators for many small animals. Forest owners are often concerned about how slash impacts the visual quality of a post-harvest woodland. For loggers, slash is often laid on trails to better support equipment and avoid soil damage. Slash might also become a source of energy as woody biomass.
With increasing interest in using slash as an energy feedstock, greater concern has been placed on the effects on nutrient cycles and how to economically gather woody biomass. The volume of slash is not the largest pool of potential wood-based energy in the Lake States and is not a particularly good one. Still, it has attracted a fair amount of attention, probably because it seems more obvious to many people.
Forest scientists have provided decades of research examining nutrient cycles of forest ecosystems, mostly from the Pacific Northwest and the South in the United States, as well as much from Canada and Sweden. Cycles have many variables, pathways, imports and exports. Most of our north temperate nutrient cycles are “open,” meaning a portion is lost and then replaced from outside the cycle.
Typically, northern forests absorb only a portion of available nutrients which move among various ecosystem pools. Forest management and removal of forest products affect nutrient cycles, but research has seldom reported losses of productivity.
The most commonly evaluated nutrients are nitrogen, potassium, phosphorous, calcium and magnesium. These vital nutrients exist in various chemical forms in both the soil and in living things. Different parts of trees contain different amounts of nutrients and vary among the seasons.
Research suggests that nutrient impacts of slash removal are temporary, with full recovery within about five years in most soils. Recovery is more rapid on sites with more organic material incorporated into mineral soil. On sites already nutrient poor, greater caution needs to be exercised.
The amount of slash removed for energy purposes depends upon the harvest technology used. Whole-tree chipping operations remove the highest percentage of nutrients held by trees, especially when harvest occurs with leaves on, but these systems are not common in our region and harvests often occur during the winter.
In the Lake States, cut-to-length systems manufacture most of our forest products. These systems involve large, complex machines that produce logs and pulpwood in the forest, which are later carried out of the forest by another large machine. All the biomass in limbs and branches is left scattered throughout the forest. Technologies to collect a portion of the slash present economic challenges.
Moving woody biomass to the gate of an energy facility involves a string of operations. This supply chain begins in the forest. As higher value sawlogs and pulpwood are extracted, then only slash, non-merchantable wood or desirable crop trees are left behind.
The physical distribution and composition of the slash has a great impact on how economically it can be gathered, concentrated, moved to a landing and subsequently sold to a mill. From the landing, a range of transportation issues exist, including the cost of fuel, season, regulations, road conditions and equipment. High inefficiencies lead to lower profit margins, which depresses the merchantability of slash.
As the wood-based energy industry emerges, a number of challenges need to be addressed before assurance can be provided that logging slash will be harvested in an environmentally and economically sound manner. However, more than likely, slash will not be a major source of energy products.