Defensible space is the zone around a structure from which it can be safely and effectively defended from a wildfire in “normal” burning conditions.
With over 8.5 million acres burned across the U.S. in 2017 and the prominent loss of Sperry Chalet in Glacier National Park to fire, NPS staff will be looking harder at creating defensible space around historic buildings. Defensible space is the zone around a structure from which it can be safely and effectively defended from a wildfire in “normal” burning conditions.
Some defensible space guidance calls for the reduction of vegetation within a 30-200 foot swath around a building. This practice, however, can be detrimental in cultural landscapes where the historic building is only one component of the historic property. Ornamental vegetation, native vegetation, and small scale features also should be considered when creating or improving defensible space in order to reduce the impact on the overall historic character.
There are three components of creating defensible space in a cultural landscape:
1.) Identify and map contributing historic features including small scale features (such as fences and corrals), archeology, and vegetation. Consult your regional Cultural Landscapes Program staff or Park Cultural Resources Manager for existing Cultural Landscape Inventories (CLI) and maps.
2.) Incorporate wildland fire issues into new Cultural Landscape Reports.
3.) Consider Historic American Landscapes Survey documentation for sites at high risk.
4.) Utilize NPS Wildland Fire Risk Assessment criteria to assess site. Understand the local fire regime and structural vulnerability. Not all buildings within a forested landscape are highly susceptible to fire. Create a treatment plan commensurate to risk. Include fire managers when planning and designing treatment projects.
5.) Follow Section 106 of the National Historic Preservation Act compliance when conducting any work within a cultural landscape/historic property, including emergency stabilization.
6. Do not store flammable materials such as firewood near historic buildings.
7. In vulnerable cultural landscapes, install a concealed metal barrier between wooden fences and historic structures.
8. Avoid chemical fire retardants on historic structures. Instead, utilize water and Class A foams. During a fire, utilize specialists to install a temporary exterior sprinkler system.
9. Purchase fire resistant building wraps for at risk buildings. Utilize experts to install if necessary. (Wraps are more effective on smaller buildings and for vulnerable elements on larger ones.)
10. Avoid clearing a preset perimeter around a historic building. Where vegetation removal is necessary due to vegetation density, work to increase spacing rather than clearing (i.e., 10’ between tree canopies and 4’ between shrubs).
11. Allow low-growing, high moisture, drought and fire-resistant plants that are compatible with the historic character to remain.
12. Low-mow turf to avoid grassland fuel build-up.
13. Prune trees and shrubs to remove deadwood, and shorten branches that overhang chimneys and roofs.
14. Remove woody debris, leaf litter, and dried needles from roofs, gutters and the ground around historic features.
15. In highly fire prone areas, prune tree canopies 6-10’ from the ground.
16. When removing select trees, fell away from the historic property, flush cut stumps, roughen ends to encourage decay, and conceal with soil and duff.
17. In highly fire prone areas, install and maintain irrigation system to keep vegetation hydrated. Otherwise, water manually in the driest season.
18. Mulch around cultural landscape vegetation to preserve soil moisture.
19. During a fire, install a temporary irrigation system and a fire resistant wrap around historic vegetation (e.g., an orchard, specimen trees).
An Example from the Field
Historical Landscape Architects can recommend more nuanced approaches to managing the building ignition zone in order to preserve vegetative cultural resources and the valued forested setting at historic sites. One such approach to managing fire risk is utilizing fuel reduction and thinning of canopies. At North Cascades National Park, for example, an interdisciplinary team of staff from the fire, natural resources, and cultural resources divisions worked on site to ensure a fuel reduction plan was implemented with limited impacts to both cultural and natural resources within the Buckner Homestead.
Buckner Historic District is located on the eastern side of the North Cascades, within dry fire-prone Douglas-fir/Ponderosa pine forest. While the forest provides habitat for specific endangered species and conveys the historic character of the homestead, the forest has become dense over time. The team surveyed the Buckner Homestead to ensure that the fuel reduction plan (thinning stands of conifers to minimize the risk of fire spreading through the crowns of trees) avoided removal of specimen trees that were important for cultural resource or natural resource values.
Instead of following a uniform prescription of ignition zone management, the team developed a site specific defensible space plan. They instigated a selective thinning approach in the adjacent forest, flagging specific trees to create a more open canopy, while preserving the open forest and shade. After the trees are removed the forest will have a character more similar to historic, pre-fire-suppression era conditions.