Flammable deserts: Understanding the impacts of fire on southwestern desert ecosystems of USA
Ramirez, A. H. F. 2015. Iowa State University
Abstract
Traditionally, fire has been seen as an important modeler of ecological systems,
especially through the history of fire-adapted systems (e.g., savannas and Mediterranean
ecosystems). Nevertheless, hot deserts in the southwestern US (the Sonoran and Mojave) are
thought to be non-fire adapted systems due to the scarce woody plants and insufficient
vegetation cover to carry fire. Over the last few decades, however, fire have become more
prevalent within the mentioned deserts because of the recent invasions of exotics grasses that
now sufficiently provide a continuous plant coverage that is able to spread fire through the
landscape. To date, however, fire effects on desert ecosystems remain unclear. I this
dissertation I examined the impact of fire in changing the spatial distribution of creosote bush (Larrea tridentata) in burned areas of the Sonoran Desert (Chapter 2). In Chapter 3, I assessed
the effects of fire on changing the availability of soil nutrients, comparing burned and
unburned areas of the Mojave Desert. In Chapter 4, I moved forward and investigated
flammability characteristics of desert plant species, and how these properties were related to
the spread of fire. Overall, I found that (i) fire changed the spatial pattern of L. tridentata
shrubs that survived the fire, compared to areas not affected by fire; (ii) over the short term
(i.e., 6 months after fire) there was an increment of soil nutrients (N and K) within burned
areas, compared to unburned areas, but over the long-term (i.e., 7 years after fire) N and K decreased, especially under the canopy of L. tridentata shrubs; and (iii) native plant species
exhibited flammability characteristics that confer them properties of "igniters" of thicker
fuels, whereas exotic invasive species presented flammable properties of "spreaders"of fire.
In summary, the interplay of the three major ideas described above provided insights about the potential novel plant-fire dynamics in the southwestern US deserts. Given the flammable
characteristics of "spreaders" and "igniters", the prevalence of wildfires may be continued
over time. While fire can kill large sections of L. tridentata shrubs, this phenomenon would
provide more openness in the landscape to be colonized by exotic invasive species. Over the
long-term, and as soil nutrients tend to decrease within burned areas, it would be difficult for
native species, including L. tridentata shrub, to recover after fire. Thus, heterogeneous
fertility islands once associated with Larrea shrubs may disappear and become replaced by
more uniform nutrient landscapes, dominated by exotic invasive grasses.
