Understanding the nuances of fuels: Balancing forest structural complexity and crown fire hazard

A conceptual diagram visualizing different characteristics of vertical and horizontal tree groupings and sizes. Wildfire can spread through Vertical Fuel Continuity (e.g. from the surface up through ‘ladder fuels’ to the tree crown). Wildfire can also spread from Horizontal Fuel Continuity (e.g. from tree crown to tree crown). 

Managing forests to create stands with a mix of tree sizes and groupings, or heterogeneity, is often desirable for improving wildlife habitat and forest resilience. However, concerns over ladder fuels associated with heterogeneous tree groupings can result in forest managers focusing on treatments that simplify stand structure to reduce fuel continuity and fire hazard risk. A more complete picture of how vertical and horizontal forest structure and distribution of fuels influences fire behavior will aid forest and fire managers in meeting dual goals of reducing crown fire hazard while simultaneously increasing forest structural complexity. To address this need, researchers used a fire model to simulate wildfire moving through different tree group structures. They tested a range of tree size mixtures and group density designed to tease out the relative importance of vertical and horizontal fuel continuity on fire behavior.

Overall, the researchers found that horizontal connectivity of tree crowns influences fire hazard in structurally complex forest stands and that this effect can somewhat override the influence of vertical ladder fuels. For example, tree groups with vertically continuous ladder fuels and limited horizontal connectivity, or limited overstory trees touching, sustained less large-tree consumption than tree groups with a significant vertical gap between the surface and canopy but high tree crown horizontal connectivity. These results add nuance to our understanding of fuel connectivity in fire behavior and suggest that stands with a variety of tree sizes and groupings can be made more fire-resistant by limiting the horizontal connectivity of the overstory trees.

This work has direct implications on managing forest stands to:

1. Reduce crown fire hazards
2. Reduce torching and mortality of old and large trees of ecological and cultural significance
3. Restore heterogeneous forest structure for improved ecosystem services 

Ultimately, this research can help inform forest treatment designs that can meet multiple management objectives.