Post-burning germination responses of woody invaders in a fire-prone ecosystem.
It has been frequently recognised that there is a positive feedback between plant invasion and fire underlying invasion success in fire-prone ecosystems. Accordingly, the response of woody alien species germination to fire may have direct implications on their invasiveness in those ecosystems, particularly when fruit ripening occurs in the fire season. Here, we experimentally evaluated the germination response of some of the main woody invaders of the Chaco Serrano dry woodlands (Gleditsia triacanthos, Cotoneaster glaucophyllus, Ligustrum lucidum, Pyracantha angustifolia and Melia azedarach), which fruit in the seasons of highest fire frequency. Seeds were subjected to heat-shock treatments that simulated a range of heat intensities, and the species were classified according to their germination response as heat sensitive, tolerant or stimulated. Since Gleditsia triacanthos has indehiscent fruits that fall from the plant and might be exposed to flames, its germination response was also assessed of seeds exposed to fruit burning. Germination responses to heat varied among the invasive species. G. triacanthos seeds experienced increased germination under very low and low heat indexes; it was therefore classified as heat stimulated. The other four species showed no change in germination under very low heat indexes and were therefore considered heat tolerant. However, all species were sensitive to high heat as indicated by their significant decline in germination. G. triacanthos would have limited capacity to recruit from seeds following flaming combustion of its fruits. The prevalence of heat-tolerant rather than heat-stimulated germination responses suggests that the occurrence of frequent and seasonal fires in this subtropical savanna system might delay rather than boost the expansion of these invasive species in the system. Yet, the presence of heat-stimulated germination in one of the studied species warns against generalisation, even within the same ecosystem, and further supports the idiosyncratic nature of invasion success.