Fire mediated tree-grass interaction in the southwestern US: Grass flammability traits and fire effects on woody plants

Persistence of grass species in natural plant communities often depends on recurrent disturbances such as fires to maintain an open tree canopy, especially in regions where climate allows formation of closed canopy forest. In savannas and open woodlands, accumulation of senescent aboveground biomass by grasses forms a continuous, well-aerated fuel bed that burns frequently and can prevent development of tree saplings into mature trees. In contrast, most grass meristems can escape severe fire damage and enable postfire recovery. The asymmetric effects of grass fire on trees and grasses are the key to balancing these two life forms in fire-dependent, open ecosystems. Variation in species flammability traits can influence fire behavior, which in turn can alter fire effects and post-fire plant responses. Although it is well known that plant traits determine fire behavior, there are few studies that identify grass flammability traits. Past work on grass flammability and fire effects often assumed biomass is the main driver of grass fire, or only focused on single species fuels. Both lab- and field-based observations indicate that species specific traits, especially canopy architecture and plant phenology, can influence grass fire behavior in addition to the biomass effects. However, studies examining canopy architecture effects on multiple species are rare, and no attention has been paid to whether canopy architecture effects on fire behavior can influence post-fire plant mortality. Fire has played important roles in the evolution of plant traits in fire-prone ecosystems. Selection on flammability traits can occur if post-fire habitats are more favorable for seed or propagule development. For example, recurrent fires can benefit grasses preferring open habitats by reducing light competition through fire-induced tree mortality. Grass species, however, vary in both flammability and shade tolerance. It is possible that flammability traits are selected against in shade tolerant grasses given the high energy cost to resprout and compete with shade intolerant grasses in post-fire habitats. The potential negative correlation between grass flammability and shade tolerance has not been examined yet. I set out to examine canopy architecture effects on grass fire behavior and fire effects, and to determine the correlation between flammability and shade tolerance in grasses by completing the following objectives: 1) I determined how the proportion of aboveground biomass allocated above 10 cm relative to the ground influenced heat release at different locations during indoor burning of individual grasses; 2) I examined if shade tolerant grasses are less flammable than shade intolerant grasses, and determined the effects of plant traits on flammability and post-fire mortality in grasses by conducting greenhouse shading experiments and measuring plant traits and flammability; 3) I tested if grass canopy architecture, namely biomass allocation in vertical space, can influence post-fire mortality of a tree species by doing field burning experiment with manipulated fuel load and fuel structure. In summary, I found that grasses allocating more biomass above 10 cm height relative to the ground released less heat at the soil surface. However, such canopy architecture effect was not important for determining post-fire tree mortality in eastern redcedar, for which the tree flammability itself might be a more important factor influencing its crown damage and mortality rate. Moreover, shade tolerant grasses were less flammable than shade intolerant grasses in terms of heat released at 50 cm height. Heat release at 50 cm height is an important fire behavior metric predicting fire-induced mortality in tree saplings. Such heterogeneity in grass flammability, specifically that tree-associated grasses are less detrimental to trees than tree-avoiding grasses, can help weaken the positive grass-fire feedback effect and help better understand the long-term persistence of the tree-grass mixture in natural plant communities.

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