Effects of seasonal timing and intensity of fire on competition between the invasive and co-occurring native species

Collaborator: Tiffany M. Knight

Exotic species can often invade and subsequently dominate communities in which they are not native, particularly when the disturbance regime of the native habitat has been significantly altered. For example, in many areas throughout the world, humans have altered the fire regime in natural communities (typically by fire suppression), and this often leads to dominance by non-native species. Although fire is frequently reintroduced to these systems under the guise of restoring communities to their native state, often little is known about the effects of fire on invasive species, making it difficult to predict how communities will respond. The success of an invasive plant following fire is predicted to depend on the invasive plant's response to fire, aspects of the fire regime, and presence of co-occurring fire-adapted plants, such as native perennial grasses. To test this prediction, Dr. Tiffany Knight and I are examining how Lespedeza cuneata, an invasive legume, and the native plant community responded to prescribed fires during different phases of L. cuneata and the native community's development (early June, mid-July, October, January).

Before After

Interaction between fire and the presence of invasive species

Collaborator: Tiffany M. Knight

Fire and the presence of invasive species should interact to affect native plant populations and community composition. In oak-hickory forests, recurrent fires benefit populations of native species and increase species richness. The presence of exotic plants has the opposite effect; many native species are out-competed by exotic species and species richness decreases. We are investigating how these contrasting disturbances interact in habitats with an invasive woody shrub (Lonicera maackii) or herbaceous biennial (Alliaria petiolata) to control native plant populations and community composition.

Positive density dependence

Collaborator: Tiffany M. Knight

Changes in the historic disturbance regime often results in communities with some pre-existing, native species in addition to newly-established, exotic species. Over time, the community can become dominated by these nonnative species, particularly if they have a negative interspecific and positive intraspecific feedbacks. We used a spatially explicit model to predict the effects of recurrent disturbances on stability of habitats invaded by exotic species. Our models incorporated the frequency of natives to exotics, feedbacks, life history traits, and competitive ability of the exotic species to examine whether the reintroduction of disturbances can be effectively used to reduce or eliminate populations of exotic species. We are now testing the results of these simulations using field and greenhouse experiments.

Crandall, R.M. and T.M. Knight. 2015. Positive frequency dependence undermines the success of restoration using historical disturbance regimes. Ecology Letters 18:883-891.

Relationship between fire and phenology of pollinators in prairies and glades

Collaborator: Alex Harmon-Threatt

Fire alters the flowering phenology of many fire-adapted plants. It is unknown whether pollinator service might change as a result of these shifts. Pollinators, such as bees, may also shift phenology resulting in no loss of pollinator service. In contrast, flowering earlier might reduce overall pollinator service if bees are not abundant earlier in the growing season. To examine these relationships, we are measuring flowering phenology, pollinator service, and seed set over time in burned and unburned glades and prairies of Shaw Nature Preserve.

Population dynamics of Hypericum reseeders and resprouters

Collaborator: William J. Platt

Woody resprouters and reseeders require different fire-free periods to reach reproductive maturity. Past research indicates that resprouters can persist in areas with frequent fires because they regenerate from underground storage structures and quickly reach reproductive maturity. In contrast, reseeders will go locally extinct if fires occur repeatedly at a frequency that is less than that required for them to become reproductive adults and contribute to the seed bank. Contrary to these predictions, reseeders and resprouters (Hypericum spp.) occur together along Gulf of Mexico coastal ecoclines where natural lightning-ignited fires were as frequent as three to five times per decade (as determined by past dendrochronology studies in the area). In these habitats, even a small change in elevation from a few centimeters to a few meters causes changes in plant community composition. Therefore, this system is ideal for asking questions about the persistence and co-occurrence of functionally different plants in the same habitat.

Effects of timber and fire management on community composition of understory plants

Collaborators: Ronald E. Masters and Ronald J. Tyrl

Low intensity fires were historically recurrent in mixed-pine hardwood habitats in North America. In the absence of fire, open savannas and prairies were converted to closed-canopy forests with low understory plant diversity. The Pushmataha Forest Habitat Research Area was established in the Ouachita Mountains of Oklahoma to test an array of forest management and prescribed fire regimes for large-scale application in restoration of fire-suppressed habitats. We examined the effectiveness of these restoration efforts 17 and 18 years post-treatment by measuring understory plant community composition and species richness. The treatments were: (1) control; (2) four-year interval, late dormant season, rough-reduction burn; (3) harvest pine only and annual burn; and (4) five harvest pine and thin hardwood treatments with no burn, or four-, three-, two-, or one-year burn intervals.

Other interests and research projects

   * Undergraduate projects
  • Heterogeneity of fire along ecoclines and persistence of plants with different life histories
  • Historical fire frequencies determined from scars in tree rings
  • Effects of heat and smoke on germination of seeds*
  • Changes in leaf traits associated with fire and the removal of invasive species*
  • Relationship between monoterpene concentrations and flammability
  • Importance of native, perennial grasses for stabilizing communities and facilitating recruitment of rare species*
  • Relationship between fire history and beta diversity of forest trees
  • Restoration of degraded habitats using prescribed fire
  • Response of arbuscular mycorhizal fungi to fire