Smoke-induced seed germination in California chaparral

Ecology, Oct, 1998 by Jon E. Keeley, C.J. Foteringham

INTRODUCTION

Wildfires are a natural and widespread feature of temperate ecosystems, and many plant species have seedling recruitment restricted to habitats created by such disturbances (Keeley 1994, Bond and van Wilgen 1996). Life-history approaches to timing of recruitment to postfire conditions include postfire dispersal to the site (e.g., many pines), fire-stimulated flowering leading to second-year recruitment (e.g., most geophytes), or maintenance of seed banks cued by fire. Seed banks accumulate either in serotinous cones and fruits, where seeds are maintained in a quiescent state within the canopy, or in the soil, where deep dormancy delays germination until fire. Many species with soil-stored seeds have evolved barriers to germination that are normally overcome only by fire-related cues.

Fire-triggered germination is the result of either heat shock or chemical products of combustion, and species appear to utilize one or the other of these modes. Heat-shock-stimulated germination is widespread in the Fabaceae, Rhamnaceae, Convolvulaceae, Malvaceae, Cistaceae, and Sterculiaceae, and is found in many ecosystems (Ballard 1973, Christensen and Muller 1975, Bewley and Black 1982, Egley 1989, Keeley 1992, Kelly et al, 1992, Thanos et al. 1992, Bell et al. 1993). While an exhaustive study of germination characteristics for these taxa is lacking, those that have been studied are described as "hard seeded," with a prominent waxy cuticle and dense palisade layer of sclerids that enforces dormancy by forming a water-impermeable barrier. Brief heat shock between 80 [degrees] to 120 [degrees] C is sufficient to induce imbibition by loosening cells in localized regions such as the hilum, chalazal cap, or strophiolar plug, or possibly denaturing inhibitors (e.g., Bell et al. 1993). This alone is sufficient to overcome dormancy in many species, although in some species heat shock must be coupled with light and/or cold stratification (Keeley 1987). This heat cue is not specific to fire, and soil heating on exposed sites, created by disturbances other than fire, can also induce germination.

For a substantial number of species with fire-triggered germination, heat shock has no effect on germination, rather germination is induced by chemicals from combustion products (Keeley 1991). Charred wood was first shown to stimulate germination in the postfire annual Emmenanthe penduliflora (Wicklow 1977, Jones and Schlesinger 1980) and also reported for many other species in western North America (Keeley et al. 1985, Keeley 1987, Keeley and Keeley 1987) and South Africa (Keeley 1992). Smoke also is an important chemical stimulant for germination of many "fire type" species, being demonstrated first by de Lange and Boucher (1990) for a South African fynbos shrub, and later for many other fynbos species (Brown 1993), a savannah grass (Baxter and van Staden 1994), a Great Basin annual (Baldwin et al. 1994), and a large number of Australian heath shrubs (Dixon et al. 1995). Smoke-stimulated germination has recently been reported for the California chaparral annual, Emmenanthe penduliflora (Keeley and Fotheringham 1997).

It is unclear whether or not the chemicals in charred wood that are responsible for triggering germination are the same as those responsible for smoke-induced germination. Several studies have attempted to determine the components responsible for charred wood (Keeley and Pizzorno 1986) and smoke-stimulated germination (Baldwin et al. 1994, van Staden et al. 1995), but have not identified the active component(s). based on the observation that neither wood ash nor concentrated Hoagland's solution stimulated germination of several California chaparral species, it was suggested that dormancy is not broken by elevated levels of inorganic nutrients (Keeley 1991). On the other hand, Thanos and Rundel (1995), reported germination of Emmenanthe penduliflora in response to 10 mol/[m.sup.3] nitrate and concluded that this and other nitrogenous ions were responsible for fire-stimulated germination. This hypothesis is attractive since nitrate-stimulated germination has been demonstrated for many weedy species (Karssen and Hilhorst 1992) and has been identified as a potential gap-detection mechanism (Pons 1989). However, further studies with Emmenanthe penduliflora found that the nitrate ion alone failed to induce germination (Keeley and Fotheringham 1998), rather nitrogen oxides were the compounds responsible for smoke-stimulated germination (Keeley and Fotheringham 1997).

Relative to heat-shock-stimulated germination, little is known of the mechanism behind how fire-produced chemicals stimulate germination. Two broad categories of mechanisms are that these chemicals either (1) cause changes in the seed coat or other external layers, which overcome water-impermeability barriers, as is the case with heat-shock-stimulated seeds, or (2) act as internal signals and mediate germination by induction of enzymes or production of growth regulators. Studies of the Great Basin annual Nicotiana attenuata (Baldwin et al. 1994) and of Emmenanthe penduliflora (Keeley and Fotheringham 1997a) support the second hypothesis.