Engineering of fire by savanna trees can facilitate high plant species biodiversity

Background/Question/Methods

Savannas are characterized by a discontinuous overstory of trees and a continuous groundcover dominated by graminoids, usually warm-season grasses. Models for savanna dynamics most often have been based on relationships between trees and graminoids with respect to utilization of limiting resources (water) or effects of disturbances (fires, herbivory, windstorms) on dynamics of tree populations. These models typically reduce an often complex groundcover to essentially one species (a generic grass), ignoring biodiversity that resides in the groundcover and associated fauna. We hypothesize that if savanna trees produce flammable fuels, they could facilitate high plant species diversity in the groundcover via fires that negatively affect graminoids, reducing sizes of bunchgrasses and the extent of clonal growth by rhizomatous graminoids. The occurrence and persistence of such openings should depend on the frequency and coverage of local high intensity fires, as well as rates of regrowth of dominant graminoids. Occurrence of fire-generated openings in the groundcover matrix of grasses should enable potential colonists to immigrate, become established, and colonize new openings (over space or time). Fire-generated openings should be suitable, but not necessarily result in colonization by woody species (shrubs and trees), as well as herbaceous plants, before recolonization by graminoids.

Results/Conclusions

If no dominant graminoid or woody plant rapidly colonizes openings, opportunities should exist for other species. Such colonists might include herbaceous species capable of rapid growth and maturation, followed by dispersal of seeds over space or time. The length of time such species might be expected to persist aboveground in groundcover openings (and hence, sizes and fecundity of colonists) should depend on rates of growth of dominant graminoids or woody species, as well as their depressant effects on smaller-statured species.  Local site conditions (e.g., moisture, soils) also should influence sizes and species of colonists. Diversity of colonists in groundcover openings should be enhanced by large pools of species capable of tolerating local conditions. Restricted dispersal of these species in space and time should promote heterogeneity in species composition among openings, generating variation in local suites of colonists. Small sizes of plants colonizing fire-generated openings should further enhance local richness of the species composition of savanna groundcover.  Flammable fuels produced by savanna trees that open space in the groundcover thus can facilitate biodiversity  supported by large pools of species that tolerate post-fire conditions and that are capable of sufficient dispersal to be present, yet incapable of reaching all available sites.