Physical geometry of enhanced fire risk in creosote shrublands invaded by exotic annuals

Background/Question/Methods

Fire risk is increasing in ecoregions historically not prone to fire, such as creosote shrublands of the desert Southwest.  It is hypothesized that one of the primary drivers producing novel fire regimes in these shrublands is the increasing abundance of exotic annuals.  Schismus sp. is particularly important as it can spread between shrubs, whereas native annuals, at least in the Mojave, tend to be restricted to growing under shrubs.  The increase in exotic annuals potentially provides a fuel bed for fire spread that did not previously exist.  We studied these issues utilizing an experimental approach in two sites, one in the Mojave and one in the Sonora.  This included an exploration of changing water availability, soil disturbance and seed input on recruitment and growth of annuals at different distances from shrubs.  Using the results, we explore the implications for fire spread using a modeling approach.  The model has three vegetation layers: (i) intershrub occupied mostly by annuals, (ii) annuals growing under the protective influence of creosote canopies, and (iii) the shrubs themselves.  We model the impact of disturbance, precipitation and seed availability on fire risk to explore the role that exotic annuals play in enhancing that risk.  Our approach is to first explore fire behavior in a more abstract, theoretical mode and then build in more ecological reality.  In this way, we can fully explore the implications of the physical geometry of the system in allowing fire spread. 

Results/Conclusions

We find that the impact of exotic annuals on fire risk is greater in the Mojave than in the Sonora, as more widespread native annuals in the Sonora can act as a bridge for fire, even without exotic annuals.  This difference has been under-appreciated as little work on fire risk has been conducted in the Sonora.  In the simplest model versions, we examine fire spread for annuals growing only in the open, with no shrub and under-canopy layers.  The results link directly to percolation theory.  However, fire spread can be altered significantly when we take into account more detail.  For example, fire spreads more readily if annuals in the open, which tend to be drier, are able to easily ignite annuals under shrubs under dry conditions, whereas spread is inhibited if conditions under shrubs are wetter.  Our results are being used to develop strategies for mitigating fire risk under different environmental conditions.