Progress 09/15/05 to 09/14/09
Outputs
OUTPUTS: Interactions between disturbance and demography of invasive species can contribute to our understanding of the differential vulnerability of ecological communities to invasion. In southern Florida, pine savannas vary in the extent of invasion by the exotic Brazilian pepper (Schinus terebinthifolius). Pine savannas that have been heavily invaded typically contain a canopy of pine and a dense mid-story of Brazilian pepper. This generates low light conditions that prevent pine regeneration, permanently transforming open pine savannas to closed stands of Brazilian pepper. Other pinelands, in marked contrast, contain only occasional seedlings of Brazilian pepper. Differential invasion may result from a nonlinear interaction between fire and Brazilian pepper. Frequent, low-intensity fires, typical of pine savannas, prevent establishment of Brazilian pepper seedlings. As fire frequency decreases, Brazilian pepper may become increasingly likely to establish in inter-fire intervals and survive subsequent fires. Once established, Brazilian pepper potentially can alter fire regimes. Brazilian pepper has low flammability and fuel characteristics that decrease fire frequency and intensity. We are investigating whether the establishment of Brazilian pepper above a critical threshold reduces the likelihood of fire, generating a positive feedback that results in conversion of savannas to Brazilian pepper stands. The identification of nonlinear processes in ecological systems is important since slight differences in initial conditions can result in large changes in final community structure. We have completed the collection of field data exploring fire-invasion feedbacks and the development of a simple computer model that simulates the process of invasion in southeastern pine savannas. We have analyzed our field data and explored the behavior of our simulation model with regards to invasion and fire frequency. PARTICIPANTS: This project partially supported a graduate student and formed the basis of their thesis project. TARGET AUDIENCES: Not relevant to this project. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
We have shown that the invasion of southeastern pine savannas is sensitive to slight variations in initial conditions or invasion pressure. This sensitivity results from feedbacks between Brazilian pepper and fire that can lead to nonlinear thresholds: Increasing abundance of Brazilian pepper leads to a reduction in fire frequency that promotes continued population growth of Brazilian pepper. Land managers can protect pinelands from invasion by Brazilian pepper by maintaining high frequencies of fire that prevent establishment of Brazilian pepper above a threshold density.
Publications
- Beckage B., and C. Ellingwood. 2008. Fire feedbacks with vegetation and alternative stable states. Complex Systems 18: 159-173.
- Stevens, J., and B. Beckage. 2009. Fire feedbacks facilitate invasion of pine savannas by Brazilian pepper (Schinus terebinthifolius). New Phytologist 184: 365-375.
- Stevens, J., and B. Beckage. 2010. Fire effects on demography of the invasive shrub Brazilian pepper (Schinus terebinthifolius) in Florida pine savannas. In press, Natural Areas Journal.
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Progress 09/15/07 to 09/14/08
Outputs
OUTPUTS: We are investigating the potential for vegetation-fire feedbacks to result in nonlinear transitions from pine savannas to invaded stands of Brazilian pepper using both field studies and simulation modeling. Our objectives are to determine 1) if Brazilian pepper mortality and fire intensity decrease with stem density of Brazilian pepper and 2) if the observed coupled fire behavior and demographic processes lead to nonlinear threshold densities for Brazilian pepper invasion. We have completed field studies that have demonstrated that high densities of Brazilian pepper reduce fire temperature up to 200 degrees C, reducing mortality of Brazilian pepper by more than 80%. We have used a simple cellular automaton model of savanna dynamics to show that frequent fire may control low-density populations of Brazilian pepper, but that Brazilian pepper can reach a density threshold during fire-free periods, initiating a positive feedback and converting the savanna to a Brazilian pepper stand. PARTICIPANTS: A graduate student that was partially supported by this project completed their graduate thesis. TARGET AUDIENCES: Nothing significant to report during this reporting period. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
Our results suggest that fire-return intervals of 4 years or less may protect pine savannas from invasion, but our simulation model does not yet include empirically estimated demographic parameters. We are currently working to fully incorporate our field studies into our simulation model in order to better predict the threshold of Brazilian pepper invasion of pinelands. Identifying these ecological thresholds is an important step in preventing the invasion of native communities.
Publications
- No publications reported this period
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Progress 09/15/06 to 09/15/07
Outputs
Interactions between disturbance and demography of invasive species can contribute to our understanding of the differential vulnerability of ecological communities to invasion. In southern Florida, pine savannas vary in the extent of invasion by the exotic Schinus terebinthifolius (Brazilian pepper). Pine savannas that have been heavily invaded typically contain a canopy of pine and a dense mid-story of Schinus. This generates low light conditions that prevent pine regeneration, permanently transforming open pine savannas to closed stands of Schinus. Other pinelands, in marked contrast, contain only occasional seedlings of Schinus. Differential invasion may result from a nonlinear interaction between fire and Schinus. Frequent, low-intensity fires, typical of pine savannas, prevent establishment of Schinus seedlings. As fire frequency decreases, Schinus may become increasingly likely to establish in inter-fire intervals and survive subsequent fires. Once established,
Schinus potentially can alter fire regimes. Schinus has low flammability and fuel characteristics that decrease fire frequency and intensity. We are investigating whether the establishment of Schinus above a critical threshold reduces the likelihood of fire, generating a positive feedback that results in conversion of savannas to Schinus stands. The identification of nonlinear processes in ecological systems is critical since slight differences in initial conditions can result in large changes in final community structure. In the last year, we have initiated field studies of Schinus growth and mortality in prescribed fires in Everglades National Park. We have located, marked, and measured individual stems of Schinus in landscape units with different frequencies of prescribed fire. We will be relocating and recensusing these stems over the winter to assess mortality rates in fires this last summer. In addition, we will begin measurements of fruit production as a function of stem size.
We have also begun construction of a computer simulation model to investigate the potential for nonlinear transitions from pine savannas to Schinus stands. We are initially modeling this process at a stand scale using a cellular automata model. As we collect more demographic data, we will explicitly model the dispersal and growth of individual stems of Schinus as a function of fire frequency, and predict consequent effects of fire on Schinus invasion.
Impacts
Invasive species displace native species, transforming ecological communities and resulting in the loss of the services provided by native communities. Understanding the mechanisms that lead to invasion of native communities will better enable us to protect ecological communities from invasion by exotic species. My project will investigate the mechanisms by which the invasive tree Brazilian pepper transforms fire frequented pinelands in the southeastern U.S into Brazilian pepper thickets, and our results will help identify management strategies to protect native pinelands. This study will benefit land managers by providing them guidance in contending with Brazilian pepper, and will benefit the public through continued enjoyment of the economic and recreational benefits of pinelands.
Publications
- No publications reported this period
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Progress 09/15/05 to 09/15/06
Outputs
This project just began in September. We are currently in the process of hiring a technician to begin our field season. The active Atlantic hurricane season this fall has delayed the beginning of our field season and may have resulted in the loss of some of our seed traps and seed dispersal data.
Impacts
The completion of this project will provide a better theoretical understanding of the processes and feedback loops that facilitate invasion of fire maintained ecosystems.
Publications
- No publications reported this period
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