Effects of smoke and fire-related cues on Panstemon barbatus seeds

r.-Previous research has found that exposure to fire-related cues enhances germination of some plant species, and such species may exist in frequent-fire southwestern United States Pinus ponderosa forests. I performed four greenhouse experiments with Penstemon barbatus, a perennial forb common in P ponderosa forests, testing seed responses to liquid and air smoke, charred P ponderosa wood and leachate, heat and emergence substrates. Liquid smoke increased P barbatus emergence to as high as 63%, 44% greater than controls, and enhanced emergence in all 4 experiments. Air smoke produced by burning P ponderosa litter for 15 min appeared to increase emergence similar to liquid smoke. In contrast, P ponderosa charred wood and charred wood leachate did not improve emergence, and sometimes inhibited positive effects of smoke. Heating samples at 100 C for 30 min did not affect emergence. Substrate and liquid smoke interacted in one experiment, with smoke increasing emergence more sharply on basalt and potting soil than on limestone soil. These greenhouse findings have practical implications for germinating P barbatus, but need testing under field conditions to evaluate their importance in this species' population biology after fire in P ponderosa forests. INTRODUCTION Seeds of some plant species inhabiting fire-prone ecosystems are thought to have evolved responses to fire cues triggering germination (Keeley, 1991; Dixon et al., 1995; Van Staden et al., 2000). These cues presumably cause germination to coincide with post-fire environments favorable for seedling survival and growth (Baldwin et al., 1994; Baskin and Baskin, 1998; Bell, 1999). In frequent-fire Australian heath ecosystems, for example, Burne et al. (2003) found that spraying soil with liquid smoke induced emergence of the shrub Grevillea rudis Meisn. from the soil seed bank whereas no emergence occurred on untreated plots. In greenhouse experiments with 45 species from frequent-fire California chaparral, Keeley (1987) reported that Adenostomafasciculatum Hook. & Arn. charred wood enhanced germination of 25% of the species, while 25% of the species exhibited heat-stimulated germination. Fire-germination relationships may also be important in the plant ecology of frequent-fire southwestern United States Pinus ponderosa P. & C. Lawson forests, but little is known about these relationships in these forests. Fire-return intervals for presettlement P ponderosa forests before fire exclusion in the late 1800s, for example, averaged 3.7 y near Flagstaff in northern Arizona (Fule et al., 1997). More than 800 plant species occur in northern Arizona P ponderosa forests (McDougall, 1973), and it seems plausible that some species have evolved germination strategies in accordance with frequent surface fires long characteristic of these forests. In preliminary experiments I found that emergence of the perennial forb Penstemon barbatus (Cav.) Roth inhabiting southwestern P ponderosa forests responded strongly to liquid smoke, so I conducted several experiments to more closely examine its responses to fire-related cues. Penstemon barbatus has small seeds (=0.8 mg) thought to be primarily wind dispersed, and as with many Penstemon is believed to be an opportunistic species colonizing disturbed habitat (Fule et al., 2001). This paper reports the results of 4 greenhouse experiments testing emergence responses of P barbatus seeds to liquid and air smoke, charred P ponderosa wood and leachate, heat and emergence substrates.