Non-native species capable of altering disturbance regimes have invaded ecosystems worldwide. Within subtropical and tropical dry forests, this problem is particularly acute due to the widespread invasion of non-native grasses that increase the frequency and intensity of anthropogenic fire. Since many dry forest trees are susceptible to fire, this can result in the decline of native species and loss of forest cover. While increases in flammability associated with non-native grass invasion have been well documented, less attention has been paid to the potential for native grasses to potentially contribute to a grass/fire cycle. We assessed differences in flammability among sites dominated by either native grasses or invasive non-native grasses within a subtropical dry forest preserve in Puerto Rico. We quantified fuel loading, fuel continuity and seasonal changes in live to dead ratios for the following site types: (1) native grass with no history of recent fire, (2) invasive non-native grass with no history of recent fire and (3) invasive non-native grass that burns frequently. Sampling was conducted during the wet season (August-October 2008) and again in the following dry season (February 2009).  

Results/Conclusions

Overall fuel loading was highest in the native grass site type. However, grass fuels made up a larger proportion of total fuels within sites dominated by invasive non-native grasses. Frequently-burned invasive non-native grass sites had the highest proportion of grass fuels and the highest fuel continuity. During the wet season, the live to dead ratio of grass fuels was higher in the invasive non-native grass sites than in the native grass sites. However, this pattern switched in the dry season with invasive non-native grasses sites containing a high proportion of highly flammable dead grasses. These data suggest that frequently-burned non-native sites may be more flammable than sites that have not experienced frequent fire, even though overall fuel loads are lower. Furthermore, although native grass sites contain significant amounts of fine fuels, differences in fuel continuity and phenology may make them less susceptible to fire. These results have implications for the management and restoration of tropical dry forests that have been affected by grass invasion and fire.