A vertical shaft masticator, also known as a rotary disc masticator. Credit: Kari Greer

Topic: Fuels treatment / Prescribed fire Preparedness Type: Research Synthesis

Science Tuesday: Mastication: Not a Simple Story

Authors: Lenya Quinn-Davidson

Early last week, I visited a landowner who’s been working on fuels reduction and restoration projects in his oak woodlands. Unlike much of the oak woodland restoration work going on in my region, which relies on hand thinning to remove encroaching conifers and shrubs from the understory, the treatments on this property were almost exclusively mastication-based. The landowner had his own skid steer and excavator, and used a rotary head to masticate small trees and chaparral. He had accomplished an impressive amount of work in a short period—more than 300 acres in the past four years! I could feel the oaks smiling in their new-found light and space.

However, as we tromped across the masticated particles, some questions came up: how would the understory respond under that thick layer of masticated fuels? And how resilient would these stands be if a wildfire burned through them? The primary goals of the project—beyond protecting the oaks—were to reduce wildfire hazard and increase forage for cattle and wildlife.

Masticated fuels in an Oregon white oak (Quercus garryana) woodland. Trinity County, California, March 2016. Credit: Lenya Quinn-Davidson

Masticated fuels in an Oregon white oak (Quercus garryana) woodland. Trinity County, California, March 2016. Credit: Lenya Quinn-Davidson

In revisiting the literature later that week, I was reminded that there’s an illusory quality to mastication treatments. They result in an ostensibly striking reduction in fuels, but they’re really only rearranging them—and this has important implications for fire behavior, fire severity and understory response. Given the increasing popularity of these treatments, I thought it would be useful to revisit some of the important management implications that have emerged from mastication research in the last decade.

  • Mastication doesn’t reduce fuels; it rearranges them. Mastication treatments convert ladder fuels to surface fuels, but they don’t reduce the overall fuel load. By breaking up the vertical continuity of fuels and moderating fire behavior, mastication can greatly improve access for fire suppression efforts. It has been shown to reduce potential flame lengths and rates of spread. However, under severe fire weather conditions, mastication may not be effective at reducing residual tree mortality because of the heavy surface fuel loads that result from treatment.
  • Mastication takes complex natural fuels and turns them into more homogenous fuels that are concentrated in the 1-hr and 10-hr size classes (<1 inch in diameter). These novel fuelbeds do not conform to traditional fuel models, and fire behavior and effects can be difficult to predict. Standard fuel models tend to over-predict rates of spread and under-predict flame lengths. Research has also shown that masticated fuels can flame and smolder for prolonged periods.
  • Mastication can increase understory plant cover by allowing more light in, but follow-up treatments like prescribed fire may be needed to increase plant diversity in treated areas. Mastication alone may not provide the ground disturbance and litter removal necessary for increased species colonization in treated stands. However, understory response will vary according to the composition of the original stand, and local research and observations may be needed to evaluate whether treatments are meeting objectives. In some cases, mastication and follow-up treatments could result in invasion by non-native species, undesired type conversions and/or other issues of concern to landowners.

There is no doubt that mastication is an important tool—the oak woodland restoration projects I visited last week were testament to its efficiency, especially in areas with dense ladder fuels and shrubs. However, because it’s still a relatively new approach, there are many unknowns about the influence of mastication on fire behavior and effects, and these questions are especially pressing in wildland-urban interface and restoration settings that have important social or ecological values. More research is needed to understand the complexities of mastication treatments, and the variability in effects across different ecosystems and fuel types.

Further Reading:

Kreye, Jesse K., et al. 2014. Fire behavior in masticated fuels: A review. Forest Ecology and Management 314: 193-207.

Knapp, Eric E., et al. 2012. Behaviour and effects of prescribed fire in masticated fuelbeds. International Journal of Wildland Fire 20.8: 932-945.

Kane, Jeffrey M., et al. 2010. Understory vegetation response to mechanical mastication and other fuels treatments in a ponderosa pine forest. Applied Vegetation Science 13.2: 207-220.

Kane, Jeffrey M., J. Morgan Varner, and Eric E. Knapp. 2009. Novel fuelbed characteristics associated with mechanical mastication treatments in northern California and south-western Oregon, USA. International Journal of Wildland Fire 18.6: 686-697.


2 thoughts on “Science Tuesday: Mastication: Not a Simple Story”

  1. Wendy Fulks says:

    Also see this compilation by the Southern Rockies Fire Science Network. http://www.southernrockiesfirescience.org/fuel-treatment-test/2015/11/3/lop-and-scatter

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