Modeling the Effects of Fire on the Long-Term Dynamics and Restoration of Yellow Pine and Oak Forests in the Southern Appalachian Mountains

1Modeling the Effects of Fire on the Long-Term Dynamics and 2Restoration of Yellow Pine and Oak Forests in the Southern 3Appalachian Mountains 5Charles W Lafon1, John D Waldron2, David M Cairns1, Maria D Tchakerian3, 6Robert N Coulson3, Kier D Klepzig4 81 Department of Geography, Texas A&M University, 3147 TAMU, College Station, TX 977843, USA 102 Department of Environmental Studies, University of West Florida, Fort Walton Beach, 11FL 32547, USA 123 Knowledge Engineering Laboratory, Department of Entomology, Texas A&M 13University, 2475 TAMU, College Station, TX 77843, USA 144 USDA Forest Service, Southern Research Station, 2500 Shreveport Hwy., Pineville, LA 1571360, USA 1 1Abstract We use LANDIS, a model of forest disturbance and succession, to simulate 4successional dynamics and restoration of forests in the southern Appalachian Mountains 5In particular, we focus on the consequences of two contrasting disturbance regimes – fire 6exclusion versus frequent burning – for the yellow pine and oak forests that occupy dry 7mountain slopes and ridgetops These ecosystems are a conservation priority, and 8declines in their abundance have stimulated considerable interest in the use of fire for 9ecosystem restoration 10 Under fire exclusion, the abundance of yellow pines is projected to decrease, even 11on the driest sites (ridgetops, south- and west-facing slopes) Hardwoods and white pine 12replace the yellow pines In contrast, frequent burning promotes high levels of Table 13Mountain pine and pitch pine on the driest sites, and reduces the abundance of less fire14tolerant species Our simulations also imply that fire maintains open woodland 15conditions, rather than closed-canopy forest With respect to oaks, fire exclusion is 16beneficial on the driest sites because it permits oaks to replace the pines On moister sites 17(north- and east-facing slopes), however, fire exclusion leads to a diverse mix of oaks and 18other species, whereas frequent burning favors chestnut oak and white oak dominance 19Our results suggest that reintroducing fire may help restore decadent pine and oak stands 20in the southern Appalachian Mountains 21 22Key words: disturbance, fire, forest restoration, simulation, succession 1Introduction Historic changes in the disturbance regimes of eastern North American landscapes 4have greatly modified the composition and structure of forest ecosystems Cultural 5disturbances associated with forestry, agriculture, and urbanization have created forest 6landscapes that differ strongly from conditions prior to European settlement (Foster et al., 71998; Abrams, 2003) At the same time, suppression activities have greatly reduced the 8frequency of fire, which formerly was a pervasive disturbance integral to the functioning 9of many ecosystems (Pyne, 1982; Abrams, 1992) The removal of fire permitted the 10successional replacement of fire-dependent vegetation by species intolerant of fire, and 11also favored the development of dense stands of stressed trees that are vulnerable to 12insect infestation and disease (Schowalter et al., 1981; Coulson and Wunneburger, 2000) 13The impacts (ecological, economic, and social) of these changes have served as the 14impetus for research on forest restoration approaches that foster conditions in which the 15disturbances operate within the historic range of amplitude, frequency, and duration 16(Frelich, 2002; Mitchell et al., 2002; Palik et al., 2002) 17 Of particular interest to many resource managers is the use of fire as a restoration 18tool, especially in forests dominated by Pinus L., subgenus Diploxylon Koehne (yellow 19pine) and Quercus L (oak) (Pyne 1982; Haines and Busby 2001; Palik et al 2002; van 20Lear and Brose 2002) These forests are hypothesized to depend on periodic burning for 21their long-term maintenance (Abrams 1992; Agee 1998; Williams 1998; Wade et al 222000; Abrams 2003) Most pine and oak species are intolerant of shade and appear to 1thrive best in open stands maintained by fire They also are more fire-tolerant than their 2associates, and were favored in the regime of frequent surface fires that historically 3characterized many landscapes in eastern North America Fire exclusion, in concert with 4insects, disease, and other natural disturbances, has contributed to recent, widespread 5declines in the abundance of yellow pine and oak The declines have prompted concern 6about the long-term maintenance of these species, because they are among the most 7valuable trees in North America for wildlife habitat, timber production, and biodiversity 8conservation Reversing these declines may require the reintroduction of frequent 9burning similar to the pre-suppression fire regime (SAMAB 1996; Harrod et al 1998; 10Williams 1998; Dey 2002; Palik et al 2002) 11 In the southern Appalachian Mountains, a considerable proportion of the 12landscape is under federal ownership, and resource managers are using fire to restore 13yellow pine and oak forests on these lands (SAMAB 1996; Elliott et al 1999; Waldrop 14and Brose 1999; Welch et al 2000; Hubbard et al 2004) Oak forests are the 15predominant land cover type, occupying xeric, subxeric, and submesic sites on ridgetops 16and dry slopes (Stephenson et al., 1993; SAMAB, 1996) These are among the most 17extensive oak forests in North America (McWilliams et al., 2002) Yellow pine stands are 18less extensive but nonetheless comprise the second most widely distributed forest type in 19the region (approximately 15% of the forest cover) (SAMAB, 1996) They generally are 20confined to ridgetops and southwest-facing slopes, the driest sites on the landscape 21(Whittaker, 1956; Stephenson et al., 1993) One species, Pinus pungens Lamb (Table 1Mountain pine), is endemic to the Appalachian Mountains and is a species of concern for 2land managers (SAMAB 1996; Williams 1998) In the past, burning by Native Americans, European settlers, and lightning-set 4fires was widespread in the Appalachian Mountains and likely promoted oak and pine 5(Harmon et al 1983; van Lear and Waldrop 1989; Delcourt and Delcourt 1997; Delcourt 6and Delcourt 1998) Paleoecological analyses of sediment charcoal and pollen reveal that 7fires were common on southern Appalachian landscapes during the last 3000–4000 years, 8and that oak, chestnut, and pine were the dominant tree species (Delcourt and Delcourt 91997; Delcourt and Delcourt 1998) Delcourt and Delcourt (1997, 1998, 2000) argued 10that burning, particularly on dry upper slopes and ridgetops, was a major factor 11contributing to the dominance of these species More detailed records of fire history have 12been constructed for the past 150–400 years using dendroecological techniques (Harmon 131982; Sutherland et al 1995; Shumway et al 2001; Armbrister 2002; Shuler and McClain 142003) These studies suggest that surface fires burned at intervals of about 5–15 years in 15pine and oak forests of the southern and central Appalachians Occasionally, more 16intense, stand-replacing fires also occurred (Sutherland et al 1995) The fire history 17analyses also reveal a sharp decline in fire frequency during the mid-1900s This change 18was a consequence of efforts to exclude fire from the forests 19 Recent work demonstrates that the abundance of more shade-tolerant, and less 20fire-tolerant, species has increased in xerophytic pine- and oak-dominated stands of the 21Appalachians during the era of fire exclusion, and suggests that successional replacement 22of pine and oak may be occurring (Harmon 1984; Williams and Johnson 1990; Abrams 11992; Harrod et al 1998; Williams 1998; Harrod et al 2000; Shumway et al 2001; Lafon 2and Kutac 2003) Acer rubrum L.(red maple), Nyssa sylvatica Marsh.(black gum), Pinus 3strobus L., (eastern white pine, a subgenus Haploxylon Koehne pine), and Tsuga 4canadensis (L.) Carr.(eastern hemlock) are among the species becoming more abundant 5on xeric sites in the southern Appalachians At the same time, regeneration of yellow 6pine and oak appears to be declining These trends suggest that in the continued absence 7of fire, pine and oak stands will be replaced by more mesophytic vegetation, although the 8rates and specific directions of change will vary spatially and temporally Oaks 9themselves are among the potential replacing species in the more xerophytic yellow pine 10forests (Williams and Johnson 1990; Williams 1998; Welch et al 2000) Storms, 11droughts, and native and exotic insects and diseases likely will accelerate these 12successional trends (Schowalter et al 1981; McGee 1984; Fajvan and Wood 1996; Lafon 13and Kutac 2003; Waldron et al, in press) 14 Assessing the potential consequences of different disturbance regimes, such as 15burning versus fire exclusion, for long-term forest dynamics is difficult because of the 16long lifespan of the trees Simulation modeling provides a useful tool for exploring long17term forest dynamics In this paper, we apply LANDIS, a computer model that simulates 18disturbance and succession on forested landscapes (He et al., 1996; Mladenoff et al., 191996; He and Mladenoff, 1999a, 1999b; He et al., 1999a, 1999b; Mladenoff and He, 201999), to the simulation of forest dynamics in the southern Appalachian Mountains, USA 21LANDIS originally was developed for the Great Lakes region of North America 22(Mladenoff 2004), but has been adapted for use in other locations, including the Ozark 1Plateau (Shifley et al., 1998; Shifley et al., 2000), the southern California foothills 2(Franklin et al, 2001; Franklin, 2002; Syphard and Franklin, 2004), northeastern China 3(He et al., 2002; Xu et al, 2004), Fennoscandia (Pennanen and Kuuluvainen, 2002), 4Quebec (Pennanen et al., 2004), and the Georgia Piedmont (Wimberly, 2004) Our work 5extends the application of LANDIS to the floristically diverse and environmentally 6heterogeneous landscape of the Appalachian Mountains Southern Appalachian forests are affected by various agents of natural and 8anthropogenic disturbance, in addition to fire LANDIS is designed to be able to simulate 9multiple disturbances However, in this study we focus solely on fire because it is 10thought to be the key disturbance process in pine- and oak- dominated forests (SAMAB, 111996; Williams, 1998; Dey, 2002; Lafon and Kutac, 2003), and because of the 12widespread interest in using fire for ecosystem restoration Simulation modeling is 13employed frequently to evaluate the role of a specific disturbance process independent of 14the influences of other disturbances (e.g., Le Guerrier et al., 2003; Hickler et al., 2004; 15Lafon, 2004; Sturtevant et al., 2004) Simulating the role of fire will establish the 16template onto which other disturbances can be imposed The work reported in this paper 17is a step within a larger effort that will use LANDIS to assess the influences of fire, 18Dendroctonus frontalis Zimmermann (southern pine beetle), and other disturbances (e.g., 19Adelges tsugae Annand (hemlock wooly adelgid), Adelges piceae Ratzeburg (balsam 20wooly adelgid), Phytophthora ramorum Werres, de Cock & Man in’t Veld (sudden oak 21death disease)) on the spatial and temporal dynamics of forests on southern Appalachian 22landscapes, and to investigate the implications of restoration efforts The landscape simulated in this study is an idealized landscape that captures the 2predominant physical gradients (elevation and moisture) that influence vegetation 3distribution in the southern Appalachian Mountains (Whittaker, 1956) Such idealized 4landscapes commonly are used in simulation modeling studies to facilitate the 5straightforward interpretation of model projections (e.g., Mladenoff and He, 1999; 6Pennanen et al, 2004; Syphard and Franklin, 2004; Waldron et al., in press) An idealized 7landscape is useful for this initial application of LANDIS to our study area, because we 8seek to elucidate successional dynamics on the individual site types (“landtypes” in 9LANDIS parlance), without the influences of spatial complexities Understanding 10projected successional patterns on this simple landscape will inform our interpretation of 11subsequent modeling investigations using the same landtypes in more complex 12arrangements The subsequent analyses will explore specifically the implications of 13landscape structure for vegetation patterns and for disturbance dynamics such as southern 14pine beetle infestations and the spread of fires 15 16Methods 17Study area 18 19 The southern Appalachians region is a mountainous area with a humid, 20continental climate (Bailey 1978) Temperature and precipitation exhibit pronounced 21fine-scale spatial patterns because of the mountainous terrain Oak forests are the 22predominant land cover type, occupying xeric, subxeric, and submesic sites (Stephenson 1et al 1993; SAMAB 1996) Because of their topographic complexity, however, 2Appalachian landscapes contain a variety of community types These range from 3mesophytic hemlock-hardwood forests on the moist valley floors, to yellow pine 4woodlands on ridgetops; and from temperate deciduous forests in the low elevations to 5Picea Dietr.-Abies Mill.( spruce-fir) stands on the high summits (Whittaker 1956; 6Stephenson et al 1993) The landscape we simulate is based on Great Smoky Mountains 7National Park (35°35' N, 83°25' W), in which most major ecosystems of the southern 8Appalachians are represented, and for which the general topographic distribution of 9communities and tree species has been described (Whittaker 1956) For this paper, we 10focus our discussion on the dry, pine- and oak-covered sites only 11 12Model description 13 14 LANDIS 4.0 operates on a raster-based landscape in which the presence or 15absence of 10-year age classes of each tree species is simulated for each cell Succession 16on each cell is influenced by dispersal, shade-tolerance, and the suitability of the habitat 17for each tree species With respect to habitat suitability, the landscape can be divided into 18a series of “landtypes,” each of which represents different conditions of topography, 19elevation, soil, and/or climate For each landtype, an establishment coefficient between 20and is assigned to each species to govern the relative growth capability of the species 21on that site (He and Mladenoff, 1999b) LANDIS 4.0 permits the simulation of disturbance by fire, wind, harvesting, and 2biological agents such as insects and disease (Sturtevant et al., 2004) Fire ignition, 3initiation, and spread are stochastic processes (Yang et al., 2004) The probability that a 4fire will initiate and spread becomes higher as time since last fire increases Fire spreads 5until it reaches a pre-defined maximum possible size or encounters a fire break (e.g., a 6recently burned patch) (Yang et al., 2004) Different fire regimes can be defined within a 7single landscape by assigning different fire parameters (e.g., ignition density, frequency, 8intensity) to different landtypes Low-intensity fires kill only the most fire-sensitive trees 9(young trees and/or fire-intolerant species), while fires of higher intensity kill larger trees 10and more fire-tolerant species (He and Mladenoff, 1999b) Because burning is simulated 11as a stochastic process, fire interval varies temporally, fluctuating around the mean for 12each landtype These variations in fire interval also lead to temporal variability in fire 13intensity, which is greater after a long fire-free interval than after a shorter interval with 14minimal time for fuel to accumulate In the absence of disturbance, mortality occurs only 15when a tree cohort approaches the maximum age for the species 16 Detailed sensitivity analyses of the LANDIS model have been conducted 17(Mladenoff and He, 1999; Syphard and Franklin, 2004; Wimberly, 2004; Xu et al., 2004), 18and indicate that model 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Sweet cagl 300 75 2 0.3 Carya tomentosa (Poir.) Nutt cato 200 40 2 0.4 Fagus grandifolia Ehrh fagr 300 60 0.3 Fraxinus americana L fram 200 55 0.3 Halesia carolina L haca 100 60 0.2 Liriodendron tulipifera L litu 300 45 0.3 Magnolia acuminata L maac 150 55 0.4 Magnolia fraseri Walt mafr 70 55 0.2 Nyssa sylvatica Marsh nysy 200 55 3 0.3 Oxydendrum arboreum (L.) DC oxar 100 55 3 0.4 Picea rubens Sarg piru 400 70 Pinus pungens Lamb pipu 250 35 Pinus rigida Mill piri 200 35 0.2 Pinus strobus L pist 400 30 Pinus virginiana Mill pivi 100 35 Prunus serotina Ehrh prse 200 30 1 0.4 Quercus alba L qual 450 50 0.3 Species Abies fraseri (Pursh) Poir 34 Quercus coccinea Muenchh quco 130 50 0.4 Quercus prinus L qupr 350 55 0.4 Quercus rubra L quru 300 50 0.4 Quercus velutina Lam quve 150 40 0.3 Robinia pseudoacacia L rops 100 15 1 0.4 Tilia heterophylla Vent tihe 250 60 0.4 Tsuga canadensis (L.) Carr tsca 450 70 1Maturity: age of sexual maturity; Shade tolerance: between 1-5 (intolerant to tolerant); 2Fire tolerance: between 1-5 (intolerant to tolerant); Vegetative reproduction: probability 3of vegetative reproduction following mortality of a parent cohort on a cell 35 1Figure Captions 3Figure Number of cells initially occupied by each species as a percentage of the six 4land types A) Middle elevation, north- and east-facing slopes, B) Middle elevation, 5south- and west-facing slopes, C) Middle elevation ridgetops, D) Low elevation, north6and east-facing slopes, E) Low elevation, south- and west-facing slopes, F) Low 7elevation ridgetops Species abbreviations are provided in Table 9Figure LANDIS simulation results for ridgetop sites Results are presented for the 10middle elevation range (A & B) and the low elevation sites (C & D) Fire exclusion 11conditions are shown on the left (A & C) and results from simulations with fire are shown 12on the right (B & D) Only species that occur on more than 10% of the landscape at any 13time during the simulation are shown Species abbreviations are provided in Table 14 15Figure Proportion of empty cells over time for each simulated landscape type for 16simulations with (solid lines) and without (dotted lines) fire A) Middle elevation 17ridgetops, B) Middle elevation south- and west-facing slopes , C) Middle elevation north18and east-facing slopes, D) Low elevation ridgetops, E) Low elevation south- and west19facing slopes, F) Low elevation north- and east-facing slopes 20 21Figure LANDIS simulation results for south- and west-facing slopes Results are 22presented for the middle elevation range (A & B) and the low elevation sites (C & D) 36 1Fire exclusion conditions are shown on the left (A & C) and results from simulations with 2fire are shown on the right (B & D) Only species that occur on more than 10% of the 3landscape at any time during the simulation are shown Species abbreviations are 4provided in Table 6Figure LANDIS simulation results for north- and east-facing slopes Results are 7presented for the middle elevation range (A & B) and the low elevation sites (C & D) 8Fire exclusion conditions are shown on the left (A & C) and results from simulations with 9fire are shown on the right (B & D) Only species that occur on more than 10% of the 10landscape at any time during the simulation are shown Species abbreviations are 11provided in Table 1 37 1Figure 38 1Figure 2 39 1Figure 40 1Figure 41 1Figure 42 ... vegetation change in the southern Appalachian Mountains The simulations imply 20 that ongoing vegetation changes linked to fire exclusion will contribute to long- 21 term declines in pine and oak abundance... In the southern Appalachian Mountains, a considerable proportion of the 12landscape is under federal ownership, and resource managers are using fire to restore 1 3yellow pine and oak forests on. .. in? ??t Veld (sudden oak 21death disease)) on the spatial and temporal dynamics of forests on southern Appalachian 22landscapes, and to investigate the implications of restoration efforts The landscape