Burn hot or tolerate trees: flammability decreases with shade tolerance in grasses

In ecosystems where trees and grasses coexist, some grass species are found only in open habitats and others persist under trees. The persistence of shade intolerant grasses in ecosystems such as open woodlands and savannas depends on recurrent fires to open the tree canopy. Therefore, grasses that depend on open sites might benefit from high flammability. We tested if shade intolerant grasses are more flammable than shade tolerant grasses and if flammability differences affected post-fire grass growth. We examined the relationship between shade tolerance and flammability by determining individual-level flammability and species shade tolerance of 17 grass species. We also measured grass traits to determine trait effects on flammability and the post-fire response. Grass species varied in flammability, mainly in the amount of heat produced during burning. Shade tolerant species produced less heat at 50 cm above the ground. Biomass and live fuel moisture had the greatest effects on 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. In addition, grass bulk density negatively influenced heat release at 50 cm height. Heat release at the soil surface negatively influenced post-fire growth. However, the influences of soil heating and species-specific traits on individual survival were more complex with 2- and 3-way interactions. Shade tolerance was negatively correlated with a major axis of flammability variation: shade tolerant grasses produced less heat where that heat could damage tree boles. Such heterogeneity in grass flammability may help maintain the tree-grass mixture in natural plant communities. If shade tolerant grasses near trees cause less fire damage to woody plants, especially tree saplings, this may weaken positive grass-fire feedbacks and thus aid the long-term coexistence of trees and grasses.

Automated summary

In ecosystems where trees and grasses coexist, the flammability of shade intolerant grasses is found to be higher than that of shade tolerant grasses, affecting post-fire grass growth. Additionally, grass traits such as biomass and live fuel moisture have significant effects on heat release during burning. Furthermore, a negative correlation between shade tolerance and flammability variation suggests that grasses near trees may cause less fire damage to woody plants, thus impacting the long-term coexistence of trees and grasses.