The role of terrain and forest structure on forest fire dynamics

Wildfires are a major disturbance within many forest ecosystems impacting both biodiversity and economy. During times of global change in some regions also the fire dynamics are expected to change leading to more frequent and severe wildfires. Especially high severity running crown fires can impact the stability and character of ecosystems. To lower this risk, forest management often clears the understory to prevent a fire going up to the canopy. However, it remains unclear which role fuel ladders really play in the dynamic of a large-scale wildfire. We used the large forest fire in 2023 on La Palma, Canary Islands, Spain where 100s of hectare canary pine forest burned down as a model for understanding how canopy fires are influenced by understory and terrain metrics like aspect, slope and elevation. We found out that the ALS derived pre-fire leaf area density (LAD) in the understory differs on sites with mid severity surface fire compared to crown fire sites. Also, the gap found in the verticals structure plays a critical role indicating a lower risk of crown fire with a larger gap. However, slope and elevation are also key drivers of crown fire occurrence and may in some cases even exceed the influence of understory.