Recurrent grass fires maintain savannas by favoring grasses over trees. As grass biomass feeds fire, fire can create either a positive or a negative feedback leading to the transformation of savannas into grasslands or woodlands. Heterogeneity in grass flammability can weaken such feedback effect, and help explain the long-term persistence of savannas. Specifically, if tree-associated grasses are less flammable than are tree-avoiding grasses, biased heterogeneity in fire behavior might lead to less damage to trees than would be expected from the average fire severity and thus help maintain the tree and grass mixture. In this work, we examined the relationship between shade tolerance and flammability by determining individual-level flammability and species shade tolerance of 17 grass species. We conducted a greenhouse shading experiment and indoor flammability trials. In addition, we examined plant traits that influenced flammability and post-fire response of grasses.
Results/Conclusions
Grass species mainly varied in heat release. As species shade tolerance increased, plants produced less heat at 50 cm height. Biomass increased heat release and live fuel moisture decreased heat release. However, the negative effect of live fuel moisture on heat release at the soil surface was weakened in plants with high specific leaf area. Heat release at the soil surface negatively influenced post-fire survival rate and resprouting tiller number while pre-fire tiller number had positive effects on the two measures. Moreover, plants with increased pre-fire tiller number were less affected by heat release at soil surface in terms of survival rate and resprouting strength. The observed difference in flammability between shade tolerant and intolerant grasses can potentially contribute to dynamic fire responses in local plants at small spatial scale. However, further field-scale experiments are necessary to determine the biological significance of altered fire behavior due to differences in grass flammability.