ABSTRACT Fire-related cues may enhance seed recruitment in flammable ecosystems and differences in fire-germination responses of invasive vs. native species can drive the invasion process. We evaluated the effects of heat shock and smoke water solution treatment on seed germination of invasive (Melinis minutiflora, Urochloa decumbens, U. brizantha) and native grasses (Axonopus pressus, Aristida setifolia, Gymnopogon foliosus) of the Cerrado. The effect of fire temperature was tested by exposing seeds to heat shock treatments of 60, 100, and 200 °C for one minute, comparing them to untreated seeds. The effect of smoke was tested by soaking seeds for 24-h in an aqueous smoke comparing them to seeds soaked in distilled water. Differences among treatments were evaluated by ANOVA and randomization tests. None of the tested species had germination stimulated by the temperature or smoke. Both species of Urochloa experienced decreased viability with increasing temperature, while seeds of native species and M. minutiflora tolerated heat shock up to 200 °C. The invasive grasses usually germinated faster than the native grasses. Germination time is therefore a crucial trait driving the invasion process, irrespective of fire-related cues. Other seed traits (e.g. dormancy) and higher resprouting capacity may help invasive species to persist in frequently burned savannas.

The Cerrado is threatened by wildfires and invasive species. We aimed to evaluate in laboratory conditions whether temperature fluctuation at the soil surface, resulting from the absence of vegetation due to fire, can affect the germination of Urochloa decumbens and U. brizantha, two invasive African grasses. Seeds of both species were submitted to simulations: 1) temperature during fire at 1cm belowground (F); 2) temperature fluctuation at 1cm belowground without vegetation cover for a month (TF); 3) (F) + (TF); 4) control at 25ºC. After treatments, seeds were put to germinate at 25ºC for 40 days. We had four replicates per treatment and three temporal replicates. We compared germination percentage and the mean germination time among treatments using ANOVA. The treatments TF and F+TF had the highest germination values for both species. The results showed that fire per se could not stimulate seed germination, however, they suggest that a disturbance that produces a pattern of temperature fluctuation is able to break dormancy and enhance seed germination and, consequently, increase the invasiveness of the study species. Vegetation gaps resulting from disturbance may become new sites of invasion. This information is important for making management decisions regarding the control of these species.