Cut and haul costs. Elemental time study. Machine production hour. Ask USDA Forest Service scientist Dana Mitchell about any of these forest engineering terms, and you’re in for a treat.
As a research forest engineer, Mitchell’s work focuses on improving the technology and business of forest operations – with a broader goal of improving forest management and minimizing environmental impacts.
Mitchell’s SRS Research Work Unit is the last of its kind. Although the agency has other forest engineers, no other units are devoted to forest operations.
Mitchell, along with SRS forest engineer John Klepac, attended the 2017 Council on Forest Engineering meeting. Topics ranged from biomass energy to mechanized harvesting and ‘big data’ in the forest products industry. Mitchell and Klepac shared research updates, and published their findings as a series of proceedings.
Klepac and Mitchell evaluated machine performance of a tracked swing-to-tree feller-buncher equipped with a high-speed shear head. They timed the feller-buncher operating in a clearcut harvest in four different loblolly pine plantations across southern Alabama.
The research was part of a larger project to look at woody biomass products. Southern markets for woody biomass and small-diameter trees are growing and may be able to compete with timber and other wood product markets in the future.
“We considered the whole system design,” says Mitchell. “How would it handle smaller trees coming off of pulpwood first thinnings or shorter-rotation woody biomass?”
Their time study analyzed which elements had the biggest impact on machine performance: moving to the first tree, accumulating harvestable material, moving between trees, moving to dump, and dumping.
More than half of all cycle time, across all sites, was spent on accumulating. The next most time-consuming element was dumping – except at one site that had a lower stand density.
When describing the project, Mitchell says that everyone had the same question: “Tell me more about that head.”
The shear head was built especially to cut smaller diameter trees. “It opens and closes quickly and has a bigger accumulation pocket – so it can hold a greater number of smaller trees.”
“Hauling trees with the tops and limbs intact means that the biomass doesn’t get handled separately,” says Mitchell. “But transportation is expensive – so we need a way to accommodate the extra ‘fins and feathers’ with minimal additional trailer weight and a flexible design. Loggers need to stay versatile in the types of products that they’re able to haul.”
Biomass feedstock quality could also be improved with less handling — less dirt and fewer impurities.
The study examined two design modifications. One added a swinging metal guide to the rear bunk of a trailer. It could pivot 180 degrees to open and close, and can be lifted off the pivot when not needed.
The other design added an extendable bolster to the trailer. It has chain nets that move along a sliding rail to contain a tree crown or collapse when not in use.
Mitchell and her colleagues hypothesized that the trailer modifications would allow for hauling untrimmed loads of wood to a central depot where whole stems could be processed for the highest-value products.
Tree lengths can be a limiting factor for hauling chip-and-saw, but the modified trailers suitably contained whole pulpwood stems.
They cut a stand of loblolly pine and let it dry onsite for six weeks. “We could create a higher-value product by not hauling water weight,” says Mitchell.
The trees were then skidded, chipped, and blown into two different chip trailers. One trailer was a standard size and the other was lengthened to haul a full, legal payload of lightweight chips.
The larger trailer was able to handle more wood chips, but its load density was lower. It was difficult to blow the lighter-weight, dried chips all the way to the front of the trailer. Top loading or a shorter, taller trailer are two possible solutions for increasing payload when hauling lighter, drier chips.
Mitchell and Klepac conducted a hardwood harvesting study in partnership with the George Washington and Jefferson National Forests in Virginia. They collected harvesting production data for a shelterwood cut on the Warm Springs Ranger District.
This baseline study could inform biomass removal projects in the future to achieve the goals of a restoration project while potentially providing raw materials to a local biomass boiler.
They observed 82 felling cycles and timed the different elements, including moving to trees, cutting, moving to dump, delimbing, and trimming stumps. Skidder cycle times were also recorded, including positioning, grappling, piling trees, and traveling empty and loaded.
“Feller-buncher harvesting isn’t as common in the mountains as it is on flatter ground, so documenting the production time by cycle element is somewhat novel,” says Mitchell.
Future studies may evaluate the impacts of removing small, unmerchantable stems for biomass.
Mitchell co-authored another study to assess costs for harvesting eastern redcedar (Juniperus virginiana) for biomass. The project was led by David Baker, at the time an undergraduate research assistant, and biosystems engineering professor Mathew Smidt, both with Auburn University.
“In parts of Oklahoma, Texas, and other Plains states, redcedar has encroached upon rangeland and impacted water supply for pasture livestock and wildlife,” says Mitchell.
Redcedar harvesting poses some harvesting challenges. It has low stand density, making it harder to minimize travel time and increase fuel efficiency. It also has a high proportion of branch volume compared to stem volume. Often large branches are located near the base of the stem, which makes it difficult to get a clean cut.
Mitchell and colleagues designed a time study to compare two methods of moving felled material to a landing. One used a skidsteer with a grapple, while the other used an excavator to shovel the stems. The shovel system moved bunches bit by bit rather than dragging the stems.
“It’s common for landowners to pay for felling and leave the stems in piles or burn them,” adds Mitchell. “It seems unlikely that a viable biomass industry could depend on the landowner absorbing the felling costs, so researchers are developing costs for complete harvesting systems.”
Mitchell’s forest operations group continues to study new technologies, applications, and markets. “We strive to develop basic knowledge that better matches tools with resource management jobs,” says Mitchell.
“Forest managers need efficient, sustainable practices and high-value forest products. That’s what forest engineering is all about – that’s our role in improving the conditions of our nation’s forests and grasslands.”
The Department of Energy provided funding for some of these studies.
Read the full text of these proceedings papers:
- Evaluation of a tracked feller-buncher harvesting plantation loblolly pine
- Whole tree transportation system for timber processing depots
- Alternative trailer configurations for maximizing payloads
- Hardwood harvesting (biomass removal study) on the George Washington and Jefferson National Forests
- Biomass chipping for eastern redcedar
For more information, contact Dana Mitchell at firstname.lastname@example.org.