Site preparation (site prep) tractors are large-wheeled machines with mulcher head attachments. These machines can turn shrubs and small trees into wood chips and chunks. They are often used to control vegetation before a prescribed fire, improve wildlife habitat, or as a replacement for fire where the risk is too high.
If site prep tractors could also be used to install fire breaks, they could provide managers a more versatile and responsive tool to manage risks associated with prescribed burning.
As USDA Forest Service research engineers, we develop and test new management options that are economically and ecologically viable. The SRS Forest Operations Research group integrates ecology and engineering to identify, develop, and test new tools and machines for sustainable and socially acceptable forest resource management.
Southern forests cover 215 million acres, about 40 percent of the land area in 13 states. Over the past decades, some forests have become overstocked, some homes are nearer to the wildland-urban interface, and fire behavior has become more extreme.
Using site prep tractors to install fire breaks, among their other uses, would help managers reduce wildfire risk and build community capacity to suppress wildfires.
We tested a site prep tractor on young stands in the southern Piedmont and upper Coastal Plain in Alabama. The fire lines we created were intended for the application of prescribed fire.
Time studies showed that the machines could produce three quarters of a miles of fire break in an hour, exposing more than 60 percent mineral soil. If the machine made two passes, the speed dropped to about a third of a mile per hour but exposed 80 percent mineral soil.
However, the forest must be open enough to navigate – most of the path should be 12 feet wide or more for a fire break. And if the vegetation is too dense, mulching would just create a mulched mat of fuels, rather than the bare mineral soil needed to stop fire.
We found that up to a quarter of the time could be spent on productive delays – the time it took to maneuver or re-treat areas that the mulching head skipped over due to sudden changes in the terrain or other obstacles. In stands that had been regeneration harvested, productive delays took nine percent of the total time, while in stands that had been thinned, these delays took 27 percent of the time due to maneuvering around standing trees.
Even with productive delays, most experimental design speeds could be achieved. At the higher speeds, the cost of creating a fire break with a site prep tractor was near the current costs for creating fire breaks with a bulldozer. In open woodlands or young stands, where the mulcher can easily navigate, its productivity could be two or three times greater than that of dozers.
Utilization rates refer to the total productive time of equipment. These rates frequently have an inverse relationship with costs. High utilization rates are preferred because they will decrease the cost of a forest operation. Site prep tractors might be cost-competitive with dozers for fire break establishment if the utilization rate is more than 60 percent. This utilization rate could be realized if these machines are used both for vegetation control and fire break establishment.
If site prep tractors can find balanced opportunities for vegetation control in southern forests, they may become a valuable tool for prescribed fire. Unlike dozers, site prep tractors have tires so they can travel on surfaced roads. They could be particularly useful in high-risk locations — such as near homes in the wildland urban interface or in areas that can take advantage of on-road travel between sites.