PARENT SESSION
Oral Session #54: Fire Ecology -- Trees, forests, woodlands.
Presiding: E. Menges
Wednesday, August 7. 8:00 AM to 11:30 AM. Apache Meeting Room, TCC.

The event-area relationship: spatial and temporal scaling rules for surface fire regimes in southwestern Ponderosa pine forests.

Falk, Donald^*,1, Swetnam, Thomas¹, ¹ University of Arizona, Tucson, AZ

ABSTRACT- In the extensive Ponderosa pine forests of southwestern USA and northwestern Mexico, the dominant natural fire regime consisted of relatively frequent, low-intensity fires. These ecosystems differ significantly from high-intensity, stand-replacing ecosystems in their ecology and scaling properties. Most adult trees survive such events, leaving a richly detailed record of fire events over more than 500 yr. A network of fire history sites in old-growth forests in the Jemez Mountains permits us to examine spatial and temporal scaling relationships in the fire regime. We apply a variety of analytical tests adapted from species biogeography, including collector's curves, rank-abundance functions, and asymptotic estimators of fire-event richness to demonstrate an analytical framework for understanding scaling of disturbance regimes. These analyses allow us to evaluate the extent to which statistical descriptors of the fire regime are spatially scale-dependent. A new spatio-temporal scaling index, the slope of the event-area function, provides a measure of the synchrony of events within watersheds (where fire spread regulates the distribution of events) as well as among mountain ranges. Among-range synchrony, beyond the probable range of spread, indicates independent ignitions in a given year, and thus the extent to which local fire regimes are entrained by synoptic climate conditions. When climate entrainment is weak, fires are locally heterogeneous and asynchronous, and local-scale conditions regulate disturbance. We propose two alternative mathematical models of fire interval distributions based on inherent properties of the fire record and the ecology of frequent-fire disturbance regimes, a discrete probability model and a probabilistic application of the lognormal distribution. Because they involve distribution of energy and matter, these spatial and temporal scaling rules indicate more general disturbance event-area relationships that can facilitate the analysis of disturbance regimes in a broader ecological framework.

KEY WORDS: fire ecology, disturbance, fire-climate relationship, scaling