PARENT SESSION
Special Session - Scaling laws in fire regimes: moving landscape fire history into the 21st century Chair(s): Miller, Carol¹, McKenzie, Don¹, ¹ Pacific Wildland Fire Sciences Lab, Seattle, WA
Thursday, April 1, 2004 8:00 AM - 11:00 AM Zeus Room C

Fire regimes are commonly characterized by frequency, or fire return interval, severity, and typical fire size. These fire statistics are highly dependent, however, on the spatial and temporal scale and extent of data collection and analysis. These scale dependencies are often ignored or over-simplified when using fire statistics to guide ecosystem restoration and management. Correct parameterization of landscape fire models and interpretation of model results depend on an explicit understanding of scaling relationships.
It is time to move fire history studies into a landscape framework that explicitly accounts for the scale-dependence in fire-regimes. In this special session we will examine how spatial and temporal patterns in landscape fire regimes change across scales, under a transdisciplinary framework that includes perspectives from stochastic theory, physics, physical geography and terrain analysis, climatology, paleoecology, and social science. We address the following specific topics: 1) the event-area relationship, 2) neutral landscape models, 2) scale-dependence in past and present landscape fire regimes, 3) climatic and topographic constraints on landscape fire regimes at multiple scales, and 4) integration of land-use, vegetation, and fire regimes across scales.

Neutral landscape models, scaling laws, and constraints on fire regimes. ^*MCKENZIE, DON , ¹ Pacific Wildland Fire Sciences Lab, US Forest Service, Seattle, WA, USA

ABSTRACT- Climate, topography, fuel loadings, and human activities all affect spatial and temporal patterns of fire occurrence. Because fire is a stochastic process, for which each fire history is only one realization, a simulation approach is necessary to understand baseline variability, thereby identifying constraints, or forcing functions, that affect fire regimes. With a suitable null, or neutral, model, characteristics of natural fire regimes estimated from fire history data can be compared to a null hypothesis. I generated random landscapes of fire-scarred trees via a point process with sequential spatial inhibition. Random ignition points, fire sizes, and fire years were drawn from uniform and exponential family probability distributions. I compared neutral fire regimes to those from five watersheds in eastern Washington that have experienced low-severity fire, with respect to scale dependence in patterns of fire frequency. Composite fire intervals (CFIs) at multiple spatial scales displayed similar event-area relationships in neutral vs. real landscapes. In contrast, parameters of the Weibull distribution associated with temporal trends in fire hazard exhibited different forms of scale dependence in real vs. simulated data. The presence of clear patterns on neutral landscapes suggests that deviations from them in empirical data represent real constraints on fire regimes (e.g., topography, fuels) rather than sampling artifacts. As with any null model, however, neutral fire-regime models need to be carefully tuned to differentiate these constraints from artifacts of modeling.

KEY WORDS: low-severity fire regimes, neutral landscape models, scale dependence, stochastic process