Grass Fire/Altered Disturbance Regime

Grass Fire/Altered Disturbance Regime

The most significant ecosystem level effect of invasive grasses is that caused by fire.  Fire is a type of disturbance, and ecosystems are partly defined on the basis of disturbance regimes.  In the case of fire, we describe the regime by frequency, intensity, extent, type (ground, surface, crown), and seasonality (Brooks 2004). Disturbance patterns affect ecosystem properties such as the rates of soil erosion or formation and the pathways and temporal patterns of nutrient cycling and energy flow. Disturbances act as a selective force affecting the life history traits of individual species and the composition, structure, and properties of entire groups of organisms.  In the past, the fire regime promoted coexistence of plant species with different life forms dominating at different stages of post-fire succession (Brooks 2004).

Grasses, in general, are more flammable because of their supply of standing dead material, their large surface to volume ratio that easily desiccates, their tissue chemistry, low moisture content, and fine size (Mooney 1981).  Alien grasses tend to also replace the discontinuous woody shrubs with a horizontaly continuous fuel and create a fuel packing ratio that facilitates ignition (Brooks 2004).  As a result of the thin, uniform canopy, the grassland microclimate tends to be hotter and less humid than forests and woodlands.  A grass’s life cycle allows quick recovery following a fire because there is little above ground structural tissue, so all new tissue fixes carbon and grows.  Following grass-fueled fires, invasion and extension of alien grasses leads to greater susceptibility to fire. Grassland fires are a part of their natural history, and to prevent their occurrence serves to increase their severity later (D’Antonio 1992).  However, invasive grasses have increased the frequency of fires from one in 60-100 years to 3 in 3-5 year return cycles converting native shrub lands to alien dominated grasslands (Foxcroft 2013).

                Brush fire due to cheatgrass                                 Altered disturbance regime - cheatgrass                                                                                                     replacing native sagebrush following fire  

Ecosystem nutrient stores are altered by fire.  Carbon and nitrogen are volatilized but others become biologically available.  Atmospheric nitrogen loss leads to nitrogen limitation further exacerbated by losses to erosion, ground water, and streams (D’Antonio 1992).  The large African grass, Andropogon gayanus, or gamba grass, was introduced into Australia as pasture grass but later spread out into the native vegetation.  It also has altered the fire regime in the ecosystems it has invaded.  It has a number of advantages over native grasses including higher rates of photosynthesis, transpiration, and soil water uptake plus drought resistance, and a longer growing period.  In the nitrogen-depleted soils of Australia, its rapid growth seems paradoxical.  Gamba grass has developed a mechanism for conserving soil nitrogen by inhibiting nitrifying soil microorganisms with secondary allelopathic compounds.   Nitrate, which would have been used by native plants or leached away, is not produced.  Nitrogen is maintained as the relatively immobile ammonium ion in soil.  Gamba grass roots can take up ammonium six times faster than native grasses and prefer it as a source of nitrogen, giving the plant an additional advantage over native grasses (Rossiter-Rachor 2009).

                                                            Andropogon gayanus

Globally, the effects of alien grasses on fire and ecosystems are compounded by anthropomorphic land use changes.  Humans clear wooded lands to create grassland for domestic animals, often using fire to clear and maintain the land as grassland.  Even by selective logging, the amount of combustible material increases.  The probability of fires increases, and the fires increase the rate of conversion of wooded areas and forests to grassland.  Fire and grasses are increased separately, but both are increased synergistically by the grass-fire positive feedback cycle.  Land use change in the Americas and Australia has increased fire and grazing to the highest levels ever; the population of selected grasses, often the Eurasian and African varieties that can tolerated fire and grazing best, has also increased.  Fire frequency has delayed or prevented the succession to woody plants (D’Antonio 1992).  Invasive grasses cause permanent degradation of ecosystems and prevent successional changes.

References

1. D’Antonio, Carla M., Vitousek, Peter M., (1992). Biological Invasions by Exotic Grasses, the Grass/Fire Cycle, and Global Change. Annual Review of Ecology and Systematics.

 2. Brooks, Matthew L., et al. (2004). Effects of Invasive Alien Plants on Fire Regimes. BioScience. 54:677-688.

3. Foxcroft, L. C., et al., (2013). The Bottom Line: Impacts of Alien Plant Invasions in Protected Areas. Plant Invasions in Protected Areas: Patterns, Problems and Challenges. (Springer)

4. Mooney, H. A., Bonnicksen, T. M., Christensen, N. L., Lotan, J. E., Reiners, W. A., (1981). Fire regimes and ecosystem properties. USDA Forest Service General Technical Report. WO-28 Washington, DC.

5. Rossiter-Rachor, N. A., et al. (2009). Invasive Andropogon gayanus (gamba grass) is an ecosystem transformer of nitrogen relations in Australian savanna. Ecological Applications. 19:1546-1560.