Climate, topography, or fuels? Top-down versus bottom-up controls on fire refugia across British Columbia, Canada

Surviving pockets of vegetation within fire perimeters, termed fire refugia, are an important component of ecological recovery following disturbance. Understanding the relative influence of the drivers of fire refugia throughout diverse landscapes and climate conditions can help identify areas that are conducive to their formation. We investigated the role of various top-down (climate) and bottom-up (fuels, physical setting) controls on fire refugia creation throughout 21 unique fire regime units in the forests of British Columbia, Canada, over a 20-year (2000–2019) period. Boosted regression tree models were used to determine the relative influence of each of these controls and their associated variables on fire refugia, as well as to create predictive maps of fire refugia probabilities over a range of annual climate conditions. We found that the bottom-up controls, particularly variables relating to physical setting, generally held the greatest influence on fire refugia creation, though those relating to fuels were of higher importance in the more disturbance-prone forests of the boreal and central interior regions. These bottom-up controls, however, can be overwhelmed by extreme climate conditions, which have variable effects on refugia depending on the region. There was an overall positive correspondence between locations of persistent (long-term) fire refugia and mapped old-growth, suggesting that strong, static terrain features may shelter some forests over the course of multiple fire events, allowing for the development of old-growth stands. We concluded that, while strong topographic features confer the strongest measure of protection in some regions of the province, there are many areas in which fuel mitigation tactics (e.g., fuel thinning, prescribed and cultural burning) may be particularly useful for protecting areas of high human or ecological value in the face of increasingly extreme climate conditions. Although our maps can help predict where and when fire refugia may form under provided climatic and environmental conditions, they do not reflect real-time conditions and are therefore not intended for risk assessment or for operational management.