WILDLAND FIRE PATCH DYNAMICS IN THE CHAPARRAL OF SOUTHERN CALIFORNIA AND NORTHERN BAJA-CALIFORNIA

In ecosystems where fire occurrence has significant time-dependence, f ire sequences should exhibit system-regulation that is distinguished b y nonrandom (nonstationary), self-organizing patch dynamics related to spatially constrained fire probabilities. Exogenous factors such as f ire weather, precipitation variability, and terrain alter the flammabi lity of vegetation and encourage randomness in fire occurrence within pre-existing patch structure. In Californian chaparral, the roles of s uccession/fuel build-up and exogenous factors is examined by taking ad vantage of a 100 yr ''natural experiment'' in southern California (SCA ) and northern Baja California, Mexico (BCA), where factors influencin g fire occurrence have been systematically altered by divergent manage ment systems. In SCA, suppression has been practiced since 1900. In BC A, fire control was not official policy until the 1960s and has not be en effectively practiced. Fire perimeter histories for 1920-1971 in SC A and BCA, reconstructed from fire history records and repeat aerial p hotographs, are compared for fire frequency (events/area), size, rotat ion periods, stand age structure, ignition rates, weather, burning sea son, and drought. Landscape-scale fire rotation periods are long (appr oximate to 70 yr) regardless of management policies because fire occur rence is driven by the gradual development of fire hazard during succe ssion, produced by small annual increments of growth and litterfall, a s well as by high fuel moisture in evergreen shrubs. Without fire cont rol frequent fires establish fine-grained mosaics. Fire control reduce s fire frequencies, increases fire size, and encourages coarse-scale p atch structure. Patch dynamics exhibit evidences of nonrandom turnover . Fire size distributions reflect the nearest-neighbor distances betwe en patches below some age-dependent combustion threshold (CT) in the p atch mosaic that resist the spread of fires in stands older than CT. R egional burn rates are poorly related to fire frequency, ignition rate s, drought, and terrain. The small size of fires in BCA may be reinfor ced by interactions between fire and pre-existing, fine-grained patch structure, and by random fire occurrence in the probability distributi ons of fire weather and climate. In SCA, fires are nonrandomly restric ted by fire control to extreme weather.