Why was historical ponderosa pine forest structure spatially complex?
Ponderosa pines on the Black Hills Experimental Forest
Restoration treatments in ponderosa pine forests that increase the stand-level proportion of isolated trees and small tree groups will have the greatest benefits for forest resistance to crown fire. Historical ponderosa pine forest structure that was maintained by frequent fire resulted in structurally variable forests comprised of scattered, large individual trees, variously sized groups of trees, and non-treed openings.
As land managers implement ponderosa pine restoration treatments, they desire to limit crown fire spread while also maintaining spatial structural elements that promote ecological integrity. Having a mechanistic understanding of how fine-scale forest structure and resistance to crown fire are linked through the effects of the local arrangement of crown fuels on convective and radiative net heat transfer is important for the design of fuel hazard reduction and restoration treatments.
To improve understanding of the linkage between fine-scale forest pattern and self-regulation in frequent-fire forests of the western United States, researchers used a physics-based fire model, the Wildland-Urban Interface Fire Dynamics Simulator, to address two interrelated questions:
- How does group size influence the surface fire-line intensity required for tree torching (or torching threshold) and crown fuel consumption?
- What effect does crown separation distance have on torching thresholds and crown fuel consumption?
Isolated trees and, to a lesser extent, small groups of trees (3-tree groups) have higher torching thresholds and reduced consumption compared to trees within larger groups (7 or 19 trees). However, increased resistance associated with these forest structures is conditional upon the surface fireline intensity. Enhanced resistance for isolated trees and small groups of trees have implications for reinforcing spatial patterns and self-regulation in frequent-fire ecosystems. Fires burning under low to moderate conditions can reinforce and maintain spatial patterns by favoring the survival of individuals and small clumps and converting larger tree groups to isolated individuals and small groups.