Modeling Long-Term Effects of Altered Fire Regimes following Southern Pine Beetle (Dendroctonus frontalis) Outbreaks

Weimin Xi1, John D. Waldron2, Charles W. Lafon3,  David M. Cairns3, Andrew G. Birt1, Maria D. Tchakerian1, Robert N. Coulson1, and Kier D. Klepzig4

1Knowledge Engineering Laboratory, Texas A&M University, College Station, TX 77843
2Department of Environmental Studies, University of West Florida, Ft. Walton Beach, FL 32547
3Department of Geography, Texas A&M University, College Station, TX 77843
4USDA Forest Service Southern Research Station, 2500 Shreveport Hwy, Pineville, LA 71360

 
  INTRODUCTION

Periodic fires are an important factor shaping the species-rich southern Appalachian forest landscape, and fire regimes in this region have changed significantly over time. In addition to fire, southern pine beetle (Dendroctonus frontalis, SPB) is a major biological disturbance agent affecting pines in this region. While prescribed fire is increasingly utilized as a means to restore decadent pine forests, the long-term effects of fire following SPB outbreaks are still unclear.

To investigate the synergistic effects of fire and SPB, we used LANDIS-II, a spatially explicit landscape simulation model of forest succession and disturbance. Specifically, we simulated changes in the abundance of pines under SPB disturbance and two fire scenarios: 1) fire suppression, and 2) historic fire regimes. Our goal is to understand the long-term effects of altered fire regimes in pine forest recovery, and to provide insights into the effectiveness of post-SPB restoration strategies.
 
 
  STUDY AREA


Study Area The Grandfather Ranger District, the Pisgah National Forest of western of North Carolina, USA. The landscape is divided into 11 ecozones, including three pine-oak forest dominated ecozones: I) shortleaf pine-oak forest; II) xeric pine-oak and oak forests; and III) white pine–oak forests.



 

  •  This mountainous region consists of diverse landtypes and high plant diversity.
     
  • The region is predominately hardwood forests, however, pine-oak and pine forests cover about 14.2% of federally managed lands.
     
  • The area experienced substantial SPB outbreaks between 1999-2003.
 
  MODELING ENVIRONMENT MODEL PARAMETERIZATION
Table 1. Characteristics of three pine-oak ecozones and the parameters for historic vs. current fire regime. FRI is fire return interval in yr. and size is in ha

  •  The landscape is represented as a two-dimensional grid of equal-sized (30×30 m) cells (100×100 cells) . We created a simplified landscape of four existing major forest types as a starting point for the simulations. We parameterized 36 most dominant trees and using the double exponential algorithm for seed dispersal.
     

  • We used a finer scale ecosystem process model (LINKAGES) to calculate the establishment coefficients based on the growth and competitive ability of species during first 10-year simulations. The Biological Disturbance Agent module was parameterized to represent the temporal and spatial pattern of SPB outbreaks.
     

  • Fire regimes (fire sizes, ignition probabilities, and spread ages) for different ecozones, were parameterized based on the literature and communications with fire experts.
     

  •  Succession in the model is based on life history of each species, the composition of different species within a cell, and the composition of species in surrounding cells.
     

  • As a base scenario, we ran simulations with SPB as the only disturbance. This baseline was thene compared to: 1) historic fire; 2) current fire suppression regime.
     


 
 
  SIMULATION RESULTS 
 


 

Figure 1. Changes in species abundance for four pine species in three pine-oak ecozones over 500 years under two fire regimes.

Our results indicate that SPB outbreaks alone (i.e., without fire) lead to the disappearance of all pine species from the landscape. Fire suppression promotes the increase of white pine within the landscape, but leads to the reduction of all other pine species. In contrast, historic fire regime favors the natural restoration of shortleaf pine, Table Mountain pine, and pitch pine and reduces the frequency of white pines in the landscape. 		 
 
 
  DISCUSSION
  • Our findings are consistent with the hypothesis that SPB and fire disturbance have historically driven succession of pitch pine and Table Mt. pine forests in a drought-beetle-fire-growth cycle, and wildfires are an integral part of the long disturbance regime that forms and maintains pine woodlands.
     
  • The simulation results under altered fire regimes also help explain recent (ca. 50-100 years) increases in the abundance of white pines which likely benefit from modern fire suppression policies in the southern Appalachian region.

  • The modeling projections suggest that the regime of multiple interacting disturbances have important implications for the successional dynamics and vegetation characteristics in yellow pine woodlands of the southern Appalachian Mountains.
 
  CONCLUSIONS & FUTURE WORK
  • Although SPB damage is largely a natural, uncontrollable phenomenon, we have shown that historical fire and fire suppression lead to very different forest compositions.
     
  • Frequent fires may assist restoration of pine forests damaged by SPB outbreaks, especially species such as shortleaf pine, Table Mountain pine, and pitch pine, thought to be underrepresented in the present day southern Appalachian landscape.
     
  • Moreover, fire-based restoration efforts should focus on the shortleaf pine-oak forest, and xeric pine-oak forest, and oak forest ecozones.
     
  • Our studies may help forest managers and landowners better understand the effects of multiple disturbances on the composition and structure of forests and the potential problems caused by long-term fire suppression policies.
 
  REFERENCES

Delcourt, P.A. and H.R. Delcourt. 1998. The influence of prehistoric human-set fires on oak-chestnut forests in the Southern Appalachians. Castanea 63:337-345.

Lafon, C.W., J.D. Waldron, D. M. Cairns, M.D. Tchakerian, R.N. Coulson, and K.D. Klepzig. 2007. Modeling the Effects of Fire on the Long-term Dynamics and Restoration of Yellow Pine and Oak Forests in the Southern Appalachian Mountains. Restoration Ecology 15(3): 400-411. 

Scheller, R. M., J. B. Domingo, B. R. Sturtevant, J. S. Williams, A. Rudy, D. J. Mladenoff, and E. J. Gustafson. 2007. Introducing LANDIS-II: design and development of a collaborative landscape simulation model with flexible spatial and temporal scales. Ecological Modelling 20I: 409-419.

Simon, S.A., T.K. Collins, G.L. Kauffman, W.H. McNab and C.J. Ulrey. 2005. Ecological zones in the Southern Appalachians: First approximation. USDA Forest Service Research Paper SRS-41.

Waldron, J.D., C.W. Lafon, R.N. Coulson, D.M. Cairns, M.D. Tchakerian, A.G. Birt, and K.D. Klepzig. 2007. Simulating the Impacts of Southern Pine Beetle and Fire on Pine Dynamics on Xeric Southern Appalachian Landscapes. Applied Vegetation Science 10:53-64.

Xi, W., R.N. Coulson, J.D. Waldron, M.D. Tchakerian, C.W. Lafon, D.M. Cairns, A.G. Birt and K.D. Klepzig. 2008. Landscape modeling for forest restoration planning and assessment: Lessons from the southern Appalachian Mountains. Journal of Forestry 106:191-197.