Greenstrips in Hawaii - Can they protect high-value ecosystems from wildfire?

By: Susan Cordell, Research Ecologist with the US Forest Service

Susan Cordell works on the Big Island of Hawaii at the USFS’s Pacific Southwest Research Station and currently serves as the PFX Steering Committee Chair. Her research interests include ecosystem-level impacts of invasive grasses on Hawaiian dry forest ecosystems and investigating the role that native plant reforestation plays in reducing fine fuel load hazards.

Greenstrips in Hawaii – Can they be used to protect high value ecosystems from wildfire?

Dryland Ecosystems & Invasive Grasses

A shady spot in the dry forest. Photo: Susan Cordell

A shady spot in the dry forest. Photo: Susan Cordell

Hawaii’s dry forests once housed a large fraction of Hawaii’s unique biodiversity, especially endemic plant and bird species. In addition, they provided numerous products and services useful to people. Today these dryland ecosystems are severely threatened, existing only in small pockets that have escaped excessive grazing and severe, repeated fires. None have avoided invasion by flammable alien grasses, particularly fountain grass (Pennisetum setaceum).

At the transition from invasive-dominated grassland in the foreground to high-value dry forest in the background. Photo: Susan Cordell

At the transition from invasive-dominated grassland in the foreground to high-value dry forest in the background. Photo: Susan Cordell

Throughout leeward Hawaii extensive areas of former dry forest have been completely converted to fire-prone non-native grasslands. I focus part of my work on designing fire reduction measures to protect dry forest fragments within those grass-dominated landscapes.

Greenstrips – A wildfire management tool

To reduce the spread of large fires across dryland landscapes, we need approaches that can enhance other fire reduction measures – like traditional fuel breaks – that are already in place. One strategy is greenstrips, also referred to as “greenbreaks” or “vegetated fuel breaks”.

This practice uses species with fire-resistant characteristics to create areas of non-flammable vegetation that 1) disrupt fuel continuity and 2) limit the growth of fire-promoting grasses.

Fire-Resistant Plant Characteristics: High moisture content, Low levels of volatile compounds, Large leaves

Theoretically the plants of a greenstrip discourage the growth of non-desirable, fire promoting vegetation by pre-empting space, sunlight, water, and nutrients. Greenstrips are typically placed between fire breaks and protected areas. Like other wildfire reduction measures, they are not bullet-proof. They are not completely immune to burning, and the plants composing the greenstrip need water, which could limit their establishment.

In addition to their fire reduction properties, greenstrips that grade into native-dominated protected areas can also serve as restoration sites through:

  • passive restoration (letting an ecosystem restore itself after threats have been removed, e.g. simply fencing an area and removing invasive grasses),
  • restoration of key ecosystem services (i.e. carbon storage, nutrient and water cycling), and by
  • providing habitat corridors between forest fragments.

Greenstrip techniques have been used widely in arid landscapes of the Intermountain West to limit the spread of large fires in areas invaded by fire-promoting alien grasses. Despite successes with greenstrips in the continental United States, this novel approach to fuels management has not been attempted in Hawaii or the tropical Pacific.

Researchers record observations of the experimental greenstrip plots. Photo: Susan Cordell

Researchers record observations of the experimental greenstrip plots. Photo: Susan Cordell

Using Native Hawaiian Plants in greenstrips

My team and I wanted to know:

  • Can greenstrips with established native species help protect dryland ecosystems by resisting fire-prone invasive grasses, and reduce the maintenance required to remove grass fuels?
  • Which species of native Hawaiian plants used in a greenstrip better resist fountain grass?
Aweoweo (Chenopodium oahuense). Photo: Susan Cordell

Aweoweo (Chenopodium oahuense). Photo: Susan Cordell

To test the concept of native species greenstrips we 1) established plots of varied native plant species that differed in their fuel traits, 2) seeded fountain grass into those plots and then 3) observed the differences in fountain grass growth. Our study revealed that the native species plots significantly reduced fountain grass invasion compared to the controls without native species plantings.

Plots containing the native species aweoweo (Chenopodium oahuense) had the lowest levels of invasion. Chenopodium limits reinvasion of fountain grass by growing quickly, large, and occupying space. In addition, it had the highest moisture content and lowest heating value of the native species tested, which are both important traits in reducing the spread of fire. Chenopodium’s fuel traits and growth patterns make it an excellent species to use for greenbreaks in Hawaii. Overall, greenstrips in general appear be a favorable technique to reduce fuel loads in degraded habitats.

If you would like more information about this study and my other work, please contact me at scordell01@fs.fed.us. 

Susan and John Hall, Director for the Joint Fire Science Program, discuss the experimental fuel break plots. Fountain grass is visible in the foreground, native plantings in the background. Photo: Susan Cordell

Susan and John Hall, Director for the Joint Fire Science Program, discuss the experimental fuel break plots. Fountain grass is visible in the foreground, native plantings in the background. Photo: Susan Cordell