An Integrated Model of Two Fire Regimes

Abstract

Casual observers of chaparral in southern California(SCA) quickly learn two key facts about fires: they maybe started in a variety of weather conditions—windy orcalm, dry or humid—and the most spectacular modernburns occur during Santa Ana winds. Serious research-ers who seek to understand ecosystems and guide landmanagers must ask whether this has always been true.Readers should be surprised that Keeley and Fothering-ham (2001 [this issue]) answer yes. First, the chaparralecosystem has certainly not been static: it has experi-enced major changes in fire management as southernCalifornia’s population has increased. Second, observa-tions of the chaparral in nearby Baja California (BCA)immediately reveal a very different dynamic that surelydeserves serious consideration as a model for SCA, onethat precedes the establishment of modern suppressionpractices and dense population. The chaparral of BCA ischaracterized by smaller stands and a propensity forlow-intensity fires in relatively calm, humid weather.The sharp transition between the two regimes cannotbe explained by natural gradients in flora or weather; itfollows the international border, an artificial line drawnby human beings. Vegetation maps show that Californiaecosystems extend 200 km into BCA (Minnich & Franco-Vizcaino 1998).Climatic gradients including temperature and meanannual precipitation cross the border at right angles, andprevailing winds are everywhere westerly. Open-rangecattle grazing is practiced in BCA, but chaparral is unpal-atable to livestock (Minnich & Bahre 1995). Without dis-tinctive suppression systems, changes in fire regime shouldbe expressed in a continuum along environmental gradi-ents, not the discontinuity seen along the border. Theprimary difference is that BCA has not experienced thesame protectionist management policies as SCA, and thedivergence in chaparral fire ecology may provide insightinto the nature of historical vegetation change in SCA.The seminal question is how fires shaped ecosystemswithout management interference. Suppression is un-precedented in ecological history.Keeley and Fotheringham are obviously aware of theneed to balance the BCA and SCA models in any plausi-ble account of chaparral history. Yet their analysis fo-cuses on a 1983 account of the cross-border contrastand ignores a substantial body of more recent publica-tions, including a 52-year transborder fire history (1920–1971; Minnich & Chou 1997), a study of transborder post-fire chronosequences (Minnich & Bahre 1995), and stud-ies of presuppression fire regimes in SCA (Minnich 1987,1988). To evaluate their case for dismissing the BCAmodel, I must review the key findings they ignore. Thesefindings concern the following sequence of topics: (1)the nonrandom turnover of fire patches, (2) the over-abundance of natural ignitions compared with fuel pro-duction; (3) the dominance of landscape burning by rel-atively few fires (most fire starts fail), and (4) therandom phasing of fires with normal weather in the ab-sence of fire control. This is not just an intellectual exer-cise: these are serious implications for fire management.Both the original and subsequent studies (Minnich1983; Minnich & Chou 1997) show that, in the absenceof suppression, the chaparral of BCA is a diverse, fine-grained patch mosaic. With fire suppression in SCA, thechaparral comprises unbroken carpets of mature vegeta-tion interspersed with a few extensively denuded water-sheds (Fig. 1). There are ample data to suggest differ-ences in fire management, including the number of firesof 15 ha (BCA, 2000 events; SCA 350 events), maxi-mum fire size (BCA, 3,000 ha; SCA 59,000 ha), and dif-ferences in fire weather (BCA, onshore flows; SCA, off-shore flows). The only similarity is that fire-return intervalsare 50–70 years in both countries. Our integrated modelexplains the disparate fire outcomes that have existed inthe two countries since the 1920s, Keeley and Fother-ingham do not.An important aspect in developing fire-disturbancetheory is choosing an incontrovertible starting point thatleads in directions productive for research. The modelby Minnich and Chou (1997) is based on the fact that