The fire-climate relationship changes along the aridity gradient
We recently analyzed the fire-climate relationship in the Iberian Peninsula (western Mediterranean Basin) , and found that climate shapes fire activity on a temporal scale by modifying fuel flammability (i.e., more fire during dry years; left figure below) and on a spatial scale by affecting fuel structure (i.e., more fire in productive Iberian regions). On the temporal scale, fire and climate are not linearly related, but there is a critical aridity level (i.e., the aridity threshold) above which fuels become highly flammable and area burnt increases sharply (left figure below). This aridity threshold is not universal, but rather intrinsic to each ecosystem (i.e., to its landscape structure). The drier the region, the higher the dryness level needed for switching from non-flammable to flammable conditions (right figure below), suggesting that the aridity threshold is mediated by fuel. In productive regions, an ignition may lead to a fire under relatively high moisture conditions (compared to drier regions) due to the high fuel load and connectivity. On the contrary, in dry regions, wildfires are more fuel-limited, so more extreme climatic conditions (higher aridity than in more mesic regions) are needed for fires to successfully spread. The fact that the aridity threshold is intrinsic to the ecosystem emphasizes the importance of landscape structure in determining fire-climate relationship.
Fuel structure does not depend exclusively on environmental conditions (e.g., aridity/productivity); shifts in fire activity have also been related to changes in land-use [2,3] and fire-suppression policies. Gradual historical shifts in land-use may produce abrupt changes in fuel structure across landscapes and thus, in fire activity . Therefore, the fire-climate relationship changes not only with climatic conditions, but also in response to different land uses and management practices (and often in an abrupt way).
Figure: [left:] Relation between area burnt and monthly aridity ( (PET-AET)/PET) in one of the 13 Iberian regions considered (temporal scale); vertical line indicates the location of the aridity threshold. [Right:] Relationship between the the aridity threshold and the aridity of the region, for 13 Iberian regions (Pausas & Paula, 2012 ).
 Pausas J.G. & Fernández-Muñoz S. 2012. Fire regime changes in the Western Mediterranean Basin: from fuel-limited to drought-driven fire regime. Climatic Change 110: 215-226. [doi | springer | pdf | post]