Bottom-up control of a northern Arizona ponderosa pine forest fire regime in a fragmented landscape

Fire regimes often vary at fine spatial scales in response to factors such as topography or fuels while climate usually synchronizes fires across broader scales. We investigated the relative influence of top-down and bottom-up controls on fire occurrence in ponderosa pine (Pinus ponderosa) forests in a highly fragmented landscape at Mount Dellenbaugh, in northwestern Arizona. Our study area of 4,000 ha was characterized by patches of ponderosa pine forest in drainages that were separated by a matrix of pinyon-juniper woodlands, sagebrush shrublands, and perennial grasslands. We reconstructed fire histories from 135 fire-scarred trees in sixteen 25-ha sample sites placed in patches of mature ponderosa forest. We found that, among patches of ponderosa forest, fires were similar in terms of frequency but highly asynchronous in terms of individual years. Climate synchronized fire but only across broader spatial scales. Fires occurring at broader scales were associated with dry years that were preceded by several wet years. The remarkable level of asynchrony at finer scales suggests that bottom-up factors, such as site productivity and fuel continuity, were important in regulating fire at Mount Dellenbaugh. Understanding where bottom-up controls were historically influential is important for prioritizing areas that may best respond to fuel treatment under a warming climate.