Suppression of smoke-stimulated seed germination by moist soil conditions

Background/Question/Methods

Smoke stimulates seed germination in over 1200 plant species (>80 genera) and is partially responsible for the abundant germination observed after fire. Previous work has suggested that seed dormancy alters the effectiveness of smoke in stimulating germination. Beyond that, few studies have identified factors that modify the effect of smoke on germination. Because stimulatory compounds in smoke are generally water soluble, we hypothesized that moist surfaces (seed or soil) would absorb more of the stimulatory fraction, rendering smoke more effective. To assess effects of moisture on seeds directly exposed to smoke (such as those on the soil surface), we applied aerosol smoke to seeds of two smoke-responsive species (Emmenanthe penduliflora and Brassica tournefortii) then monitored germination in Petri dishes. To assess effects of moisture on a buried seed bank, we applied aerosol smoke to pots containing buried seeds where soil was either moistened or remained dry prior to smoke exposure (exposure duration: 0 – 3 hours). To assess effects of a seed's hydration state on its response to smoke compounds in solution (such as a post-fire soil solution), we exposed dry and imbibed seeds to aqueous solutions of smoke compounds. Results/Conclusions As expected, moist seeds that were exposed directly to smoke were more responsive than dry seeds. Pre-imbibed seeds germinated better with short exposure times and appeared to reach toxic levels faster than seeds smoked dry. In contrast, seedling emergence from buried seed was lower when soil was moist prior to smoke exposure for most exposure times. Longer exposure times (e.g., three hours), however, could eliminate or reverse this pattern. Investigations into underlying mechanisms suggest that differences in responses of pre-imbibed and dry seeds to smoke compounds in solution may largely account for the suppression of the germination response of the buried seed bank under moist conditions. These results suggest that the hydration status of the soil seed bank may be a significant driver of smoke sensitivity. Where the soil seed bank is the main source of post-fire recruitment, these results constitute one mechanism by which the timing of fire in fire-prone landscapes may lead to marked recruitment differences. For land restoration programs that incorporate the artificial application of smoke products as a tool, these results suggest that application may be more effective when applied to dry soils.