Smoke-cued emergence in plant species of ponderosa pine forests: Contrasting greenhouse and field results

Abstract

Smoke-cued seed germination and emergence is common in some frequent-fire ecosystems, but this process is little studied in frequent-fire conifer forests of the southwestern United States. To assess whether aqueous smoke promotes plant emergence in frequent-fire ponderosa pine (Pinus ponderosa) forests in northern Arizona, I conducted three experiments at different scales (seed, seed bank, and plant community) in both greenhouse and field settings. In the first greenhouse experiment, aqueous smoke significantly increased (P < 0.05) emergence of seeds of 13% of 61 assayed species. Five of eight (63%) Penstemon species exhibited significant increases ranging from two- to ten-fold. In the second greenhouse experiment, aqueous smoke increased emergent density by 67% and species richness by 60% in soil seed bank samples collected from nine sites that had undergone tree thinning. Contrary to these greenhouse experiments, spraying aqueous smoke on 0.05 ha field plots at nine sites in the third experiment had no effect on plant species cover, richness, or composition relative to control plots 15 mo after treatment. Many factors, such as time since fire or interactions with other cues, could have contributed to this lack of response. While aqueous smoke appears promising for some applications such as enhancing seed germination for plant production, its potential for promoting emergence in field settings is currently uncertain.