Seed germination of bromeliad species from the campo rupestre: thermal time requirements and response under predicted climate-change scenarios

The characterization of thermal thresholds, base (Tb) and ceiling (Tc) temperatures and thermal-times for germination (θg) has gained importance for predicting the persistence and distribution of species in the face of climate change. This study aimed to determine the thermal thresholds for germination of bromeliads from campos rupestres in order to verify if sympatric species have similar thermal requirements. We also examined whether inter-annual variations in climatic conditions drive differences on thermal requirements; and how the predicted climate change scenarios would influence the thermal time for germination. Seeds of Vriesea friburgensis, V. bituminosa, V. pardalina, Tillandsia gardneri, and Racinaea aerisincola were collected in 2000 and 2013 and sown to germinate in a temperature gradient (15-35 °C) under light and dark conditions. A thermal-time approach was then applied to the germination results, and the thermal-times needed to reach 50% germination (θ50) were modeled according to the climatic conditions of both years, as well as two simulated scenarios climate change. The bromeliads exhibited differences in their thermal requirements for germination. Tb's of from 6.2 to 10 °C and Tc's of from 31.6 to 41.7 °C were identified, depending on the species. The extent and severity of the dry season during fruiting appeared to have determined differences in θ50 values. All species, except V. pardalina, which have fruiting during the rainy season, presented higher θ50 values in the year with a more intense drought (2013). Under the warming scenarios, a reduction in the time required to sum heat units to germinate was found, but a risk for persistence was identified for R. aerisincola in the campos rupestres due to its narrow seed germination window. The determination of thermal thresholds for germination can be useful to identify species vulnerability to climate change and to promote strategies for conservation of bromeliads species.