Flammability, physiology and coexistence in fire prone environments
Approximately 40% of the earth's terrestrial surface is covered in fire-prone vegetation. Many of the plants in these habitats have evolved characteristics to avoid damage during wildfires, or to take advantage of the post-fire environment. As a consequence of being the fuel for wildfires, plants also possess characteristics that contribute to fire frequency and intensity. This dissertation considers both sides of the plant--fire interaction. The first chapter provides background information on our current understanding of plant trait and fire relationships and discusses the evolution of plant flammability.
The second chapter presents a model exploring coexistence among plant post-fire persistence strategies in fire-prone environments. In this model coexistence of more than two post-fire strategies is not stable on long time scales. The model also suggests that species with the ability to postpone seed germination until post-fire are favored at shorter fire intervals as long as variation in fire intervals is moderate.
The third chapter describes a non-destructive method for rapidly assessing fuel structure and load in shrubland systems like California chaparral. Results obtained by using this method correlate well with more time consuming destructive harvesting. In the fourth chapter this method is applied to four chaparral species that differ in their capacity for post-fire seed germination. The species differed in the amount of fine fuels, but not by post-fire strategy. Species with post-fire seeding did have significantly more dead branches than obligate post-fire resprouters.
The final chapter explores the effect on photosynthesis to plants which are not themselves burned by a wildfire, but exposed to its smoke. After smoke exposures as short as 15 minutes, tobacco plants show significantly reduced photosynthesis, stomatal conductance, and chlorophyll fluorescence. These reductions are short lived, after one day even plants exposed for 45 minutes no longer differ from controls. An appendix describes a method for assessing whole plant flammability characteristics, presents preliminary results, and discusses shortcomings of the method.
This dissertation adds to a growing body of research demonstrating the interdependence of plant flammability traits and fire response characteristics.