Modelling responses of pine savannas to climate change and large‐scale disturbance

Global warming can potentially influence ecological communities through altered disturbance regimes in addition to increased temperatures. We investigate the response of pine savannas in the southeastern United States to global warming using a simple Lotka‐Volterra competition model together with predicted changes to fire and hurricane disturbance regimes with global climate change. In the southeastern United States, decreased frequency of both fires and hurricanes with global warming will shift pine savannas toward a forested state. A CO₂ fertilization effect that increases the growth rate of tree populations will also push southeastern landscapes from open savannas towards closed forests. Transient dynamics associated with climate driven changes in vegetation will last on the order of decades to a century. In our model, the sensitivity of savannas to relative changes in the frequency of fire versus hurricanes is linearly dependent on the growth rate and mortality of trees in fire and hurricane disturbances.     

Response of Six‐Lined Racerunner (Aspidoscelis sexlineata) to Habitat Restoration in Fire‐Suppressed Longleaf Pine (Pinus palustris) Sandhills

Six‐lined racerunner (Aspidoscelis sexlineata) is an indicator species of frequently burned Longleaf pine (Pinus palustris) forests. To evaluate how the species responded to forest restoration, we conducted a mark‐recapture study in formerly fire‐suppressed Longleaf pine forests exposed to prescribed fire or fire surrogates (i.e. mechanical or herbicide‐facilitated hardwood removal) as well as in fire‐suppressed control sites and reference sites, which represented the historic condition. After initial treatment, all sites were exposed to over a decade of prescribed burning with an average return interval of approximately 2 years. We used population‐level response of A. sexlineata as an indicator of the effectiveness of the different treatments in restoring habitat. Specifically, we compared mean numbers of marked adults and juveniles at treatment sites to that of reference sites. After 4 years, restoration objectives were met at sites treated with burning alone and at sites treated with mechanical removal of hardwoods followed by fire. After over 10 years of prescribed burning, restoration objectives were met at all treatments. We conclude that prescribed burning alone was sufficient to restore fire‐suppressed Longleaf pine sandhills for A. sexlineata populations.     

Effects of Chronic Human Activities on Invasion of Longleaf Pine Forests by Sand Pine

Chronic human activities may result in new and permanent successional trajectories in certain ecosystems. The invasion of longleaf pine ecosystems by sand pine in the Florida Panhandle is one such change in the landscape. This study examined the spatial pattern of sand pine expansion and explored the natural and anthropogenic disturbances that fostered this invasion. Aerial photographs (1949, 1994) and Geographic Information Systems analyses confirmed sand pine expansion at Eglin Air Force Base. In 1949, there were 8,982 ha of sand pine in the southern portion of the study area near riparian and coastal lowland forests. By 1994, sand pine had expanded further upland and inland, for a total of 17,147 ha in the study area. Sand pine age data showed that this expansion had started by 1920 but increased rapidly in the 1940s. Historical accounts and structural data from stands suggest that land-use activities associated with the extraction of turpentine promoted the invasion by sand pine. Fires were suppressed in longleaf pine forests to protect turpentine trees, resulting in increased vegetation cover and decreased regeneration of longleaf pine. In addition, stands were typically harvested after turpentining, and there was little or no advanced regeneration of longleaf pine. Sand pine age histograms showed that the onset of high establishment rates (1940s) coincided with changes in land ownership and widespread fire suppression. Sand pine is likely to persist in these ecosystems due to its abundant regeneration. Received 17 March 1999; accepted 28 January 2000.     

Disturbance,seeds, restoration,and the importance of experiments and long‐term observations: the Editors' Award for 2005

Question: Can the geographic patterning of endemic plant species inform reserve selection in a region of high endemism? Location: The Southeastern Coastal Plain of North America, focusing primarily on the imperiled longleaf pine (Pinus palustris P. Miller) ecosystem. Methods: We documented the high level of plant endemism in the region, and characterized the endemic taxa into distributional subregions. Results: A total of 1630 plant taxa are endemic to the Coastal Plain, a large proportion of which are endemic to phytogeographical subregions within the Coastal Plain, with particularly large numbers of narrow endemics occurring in the East Gulf Coastal Plain and Florida Peninsula. Conclusions: This pattern of local endemism presents challenges in conserving the full biota of the region: a reserve system focusing on few and large conservation areas has theoretical benefits for long‐term management and viability, but will fail to capture many local endemics. We propose that the dispersed distribution of endemic species will require a mixture of large core reserves and smaller satellite reserves.     

Seed Bank Viability in Disturbed Longleaf Pine Sites

Some of the most species‐rich areas and highest concentrations of threatened and endangered species in the southeastern United States are found in wet savanna and flatwood longleaf pine (Pinus palustris Mill.) communities. Where intensive forestry practices have eliminated much of the natural understory of the longleaf ecosystem, the potential for reestablishment through a seed bank may present a valuable restoration opportunity. Longleaf pine sites converted to loblolly pine plantations and non‐disturbed longleaf sites on the Coastal Plain of North Carolina were examined for seed bank presence and diversity. Conducting vegetation surveys and examining the seed bank using the seedling emergence technique allowed for verification of the seed bank presence, as well as evaluation of the quality of the seed bank on disturbed longleaf pine sites. Forty‐three species and over 1,000 individuals germinated, and the seed banks of both the disturbed and non‐disturbed stand types contained species not noted in the vegetation survey. Although many of these species were considered weedy and typical of disturbance, numerous taxa were indicative of stable longleaf pine communities. This study confirms both the presence and quality of seed banks in highly disturbed former longleaf pine sites, suggesting that the seed bank may be an important tool in restoration efforts.     

Can Managers Bank on Seed Banks When Restoring Pinus taeda L. Plantations in Southwest Georgia?

In the southeastern United States, private forestland managers are under increased pressure to provide wildlife habitat and biodiversity in addition to commercial products such as timber. This study used a stand classification scheme based on vegetation biodiversity from Hedman et al. to compare seed bank composition of benchmark (BM) and nonbenchmark (NBM) Loblolly pine ( Pinus taeda ) stands. In the Hedman et al. study, BM stands contained species associated with Longleaf pine ( P. palustris )/Wiregrass ( Aristida stricta ) communities, whereas NBM stands contained species associated with disturbed sites. The current vegetation of the BM and NBM stands had an average cover of 7.9%/m² and an average richness of 11 species/m². The intent for this study was to assist in understanding the potential role of the seed bank during stand development and restoration. We collected seed bank samples from six pine plantations in the winter of 2006. Seed bank samples yielded 2,885 germinants representing 56 unique species but only 4 were found in both current herbaceous vegetation plots and seed bank. The seed bank was dominated by native dicots. In BM stands, 76% of species were native, whereas in NBM stands, 69% were native. Seed bank samples from NBM stands had greater species richness ( p = 0.03) and total germinants ( p = 0.03) than BM stands. Although the seed bank in all stands was dominated by native species, our data suggest that the seed bank under P. taeda stands should not be viewed as the sole source of native species for most restoration goals.     

Conservation of genetic diversity in old‐growth forest communities of the southeastern United States

Question: How do studies of the distribution of genetic diversity of species with different life forms contribute to the development of conservation strategies? Location: Old‐growth forests of the southeastern United States. Methods: Reviews of the plant allozyme literature are used to identify differences in genetic diversity and structure among species with different life forms, distributions and breeding systems. The general results are illustrated by case studies of four plant species characteristic of two widespread old‐growth forest communities of the southeastern United States: the Pinus palustris – Aristida stricta (Longleaf pine – wiregrass) savanna of the Coastal Plain and the Quercus – Carya – Pinus (Oak‐hickory‐pine) forest of the Piedmont. Genetic variation patterns of single‐gene and quantitative traits are also reviewed. Results: Dominant forest trees, represented by Pinus palustris(longleaf pine) and Quercus rubra (Northern red oak), maintain most of their genetic diversity within their populations whereas a higher proportion of the genetic diversity of herbaceous understorey species such as Sarracenia leucophylla and Trillium reliquum is distributed among their populations. The herbaceous species also tend to have more population‐to‐population variation in genetic diversity. Higher genetic differentiation among populations is seen for quantitative traits than for allozyme traits, indicating that interpopulation variation in quantitative traits is influenced by natural selection. Conclusion: Developing effective conservation strategies for one or a few species may not prove adequate for species with other combinations of traits. Given suitable empirical studies, it should be possible to design efficient conservation programs that maintain natural levels of genetic diversity within species of conservation interest.     

Patterns of herbaceous plant diversity in southeastern Louisiana pine savannas

Question: How diverse are Louisiana pine savanna plant communities and how is diversity affected by time since burn and removal of a competitively dominant species? Location: Lake Ramsay, southeastern Louisiana, USA. Methods: Species‐area curves were constructed from nine nested quadrats in open savanna differing in time since burn (6, 18 and 30 months). Species frequency was determined for 100 1 ‐m² quadrats. The dominant gras s, Andropogon virginicus, was removed with herbicide from moist and dry sites to test for possible effects of competition. Results: Slopes of log‐log species‐area relationships were steep (0.195 to 0.379). Time since burn did not affect the richness of herbaceous plants, only woody species. More than half of all species recorded (43/79, 54%) were infrequent (in < 10% of quadrats). After two years, there were no differences in species richness and composition for plots with and without A. virginicus. Conclusions: The high species diversity is typical of other savannas across the coastal plain. The large number of infrequent species indicates that the core‐satellite pattern of species occurrence found in temperate grasslands does not apply to southern pine savannas. The absence of effects due to removal of a dominant may be due to insufficient observation time, or low competition. Most species have traits, such as diminutive life forms, that suggest they are weak competitors for light in the presence of robust matrix grasses and in the absence of fire. Many species in Pinuspalustris savannas are likely either fugitive or peripheral species.     

Northern Bobwhite Non-Breeding Habitat Selection in a Longleaf Pine Woodland

Efforts to halt the decline of the northern bobwhite (Colinus virginianus; bobwhite) across its distribution have had limited success. Understanding bobwhite habitat requirements across the annual cycle and at varying scales is essential to aid efforts to conserve bobwhites. We monitored radio-tagged bobwhites from 2016 to 2018 on a 165-km² portion of Fort Bragg Military Installation in the Sandhills physiographic region of North Carolina, USA, to determine factors influencing non-breeding bobwhite habitat selection at multiple scales. We used generalized linear models (GLM) and generalized linear mixed models to assess bobwhite habitat selection at the microsite scale (the immediate vicinity of an animal) and the macrosite scale (across the study area), respectively, by comparing used points to available random points. At the microsite scale, bobwhites strongly selected areas with greater woody understory cover. Also, bobwhite selection increased with greater forb and switchcane (Arundinaria tecta) cover, but this effect plateaued at 65% forb cover and 50% switchcane cover. At the macrosite scale, bobwhites generally selected areas with greater understory cover within a 200-m radius but avoided areas with >55% understory cover; these areas primarily were located in the core areas of drainages with extensive ericaceous vegetation. Bobwhites selected areas with 3–6 m²/ha hardwood basal area in uplands, potentially because of the availability of mast, but avoided uplands when pine (Pinus spp.) or hardwood basal area exceeded 20 m²/ha or 12 m²/ha, respectively, likely because high basal area is associated with increased shading and subsequent loss of understory cover. In addition, bobwhites selected uplands 1 growing season (≥2-month period falling entirely between 1 Apr and 1 Oct) post-fire regardless of burn season. Overall, managers seeking to improve habitat quality for bobwhites in longleaf pine (Pinus palustris) woodlands should employ management practices that maintain available woody understory across the landscape to provide cover during the non-breeding season. © 2020 The Wildlife Society.     

Natural disturbances and the physiognomy of pine savannas: A phenomenological model

Abstract. Question: The decline of the Pinus palustris ecosystems has resulted from anthropogenic influences, such as conversion to pine plantation forestry, agriculture and land development, all of which are closely related to increases in human populations. Other effects, however, have arisen from alterations in disturbance regimes that maintain the structure and function of these ecosystems. How have alterations of the disturbance regime altered the physiognomy of ‘old‐growth’ stands, and what are the implications for ecosystem conservation and restoration? Methods: In contrast to models that emphasize close interactions among the vertically complex strata, we develop a conceptual phenomenological model for the physiognomic structure of Pinus palustris stands. We relate two natural disturbances (tropical storms and fire) that affect different stages of the life cycle to different aspects of the physiognomic structure. We then compare overstorey stand structure and ground cover composition of two old‐growth longleaf stands near the extremes of different composite disturbance regimes: the Wade Tract (frequent hurricanes and fire) and the Boyd Tract (infrequent hurricanes and long‐term fire exclusion). Results: We predict that tropical storms and fires have different effects on stand physiognomy. Tropical storms are periodic, and sometimes intense, whereas fires are more frequent and less intense. Hurricanes directly influence the overstorey via wind‐caused damage and mortality, and indirectly influence the herb layer by altering the spatial distribution of shading and litter accumulation. Fire exerts direct effects on juvenile stages and indirect effects on the herb layer via fine fuel consumption and selective mortality of potential competitors of P. palustris juveniles. These differences in effects of disturbances can result in widely different physiognomies for P. palustris stands. Finally, some global climate change scenarios have suggested that changes may occur in tropical storm and fire regimes, altering frequency and severity. Such changes may greatly affect pine stands, and ultimately entire pine savanna ecosystems. Conclusions: Our phenomenological model of disturbance regimes in Pinus palustris old‐growth produces very different physiognomies for different disturbances regimes that reflect natural process and human management actions. This model can be used to derive restoration strategies for pine savannas that are linked to reinstitution of important ecological processes rather than specific physiognomic states.     

Countering the Broadleaf Invasion: Financial and Carbon Consequences of Removing Hardwoods during Longleaf Pine Savanna Restoration

The once widespread Longleaf pine (Pinus palustris)–dominated ecosystems of the southeastern coastal plain of the United States have been greatly reduced in extent, and many of the remaining stands are being degraded by hardwood invasion due to fire suppression. The first step toward pine savanna restoration is often hardwood removal, a costly process due to their large volumes and low market values. Despite these problems, by marketing a wide range of hard‐ and softwood products, the costs of 13 restoration projects in northern Florida were substantially reduced. Ten different products were sold to 19 different buyers. Fuel chips represented 71–100% of all biomass removed (8.2–81.1 Mg/ha). Although landowners were charged modest amounts for removing biomass harvested as fuel chips, other marketed products yielded revenues. Overall, four projects earned net profits of $29–$383/ha, and four projects generated sufficient revenue to pay 17–99% of the cost of hardwood removal as fuel chips. A carbon accounting of a second set of projects demonstrated that carbon harvested as fuel chips far exceeded that consumed in harvest and transport, yielding net carbon offsets of 451–1,320 Mg C/project (3.3–13.9 Mg C/ha). Using fuel consumption results of this second set of projects, carbon offsets for the 13 restoration projects were estimated as 89–1,524 Mg (3.8–37.9 Mg C/ha). Restorationists should monitor traditional forest product markets as well as developing carbon markets for price fluctuations that could provide significant revenues to restoration projects.     

Conservation and restoration of the Pinus palustris ecosystem

Abstract. The well‐documented decline of the Pinus palustris ecosystem has resulted from several anthropogenic influences, such as forest clearing (e.g. pine plantation forestry, agriculture) and urban development, both of which are closely related to increases in human populations. Other impacts have arisen from alterations in disturbance regimes responsible for maintaining the structure and function of these ecosystems. Restoration and management of degraded pine savanna ecosystems is critical. Identification of ecological processes that determine the structure and function of the intact system are important because successful restoration efforts should be based on sound scientific understanding. In this paper, we introduce this special issue on the ecology, conservation, and restoration of the Pinus palustris ecosystem. Some global climate change scenarios have suggested that future changes may occur that alter frequency and severity of disturbances such as fires and hurricanes. Such changes may have large effects on pine stands, and ultimately entire Pinus palustris savanna ecosystems, thus presenting further challenges to their sustainable management.     

Influence of Temperature on Development of Pine Wilt in Scots Pine

The effect of temperature on pine wilt development in Scots pine (Pinus sylvestris) was examined in three experiments. Container-grown pines (4-6 years old) inoculated with 1,500 Bursaphelenchus xylophilus were incubated at constant temperatures in growth chamber for 8 weeks, then at a temperature range of 15-30 C in a greenhouse for 10-12 weeks. Nematode infection was greater, tree mortality was higher, and disease incubation was shorter at 32 and 30 C than at 25, 23, 18, and 11 C. Foliar symptoms developed more rapidly and uniformly at higher temperatures. Ninety-five percent of tree deaths at 32 and 30 C and 88% at 25 and 23 C occurred within the 8-week exposure to constant temperatures. Mortality at 18, 16, and 11 C occurred only after transfer to the greenhouse. Results indicate that pine wilt incidence is directly related and disease incubation period is inversely related to temperature and that high-temperature stress predisposes Scots pine to lethal infection by B. xylophilus.     

Effects of fire season on flowering of forbs and shrubs in longleaf pine forests

Summary Effects of variation in fire season on flowering of forbs and shrubs were studied experimentally in two longleaf pine forest habitats in northern Florida, USA. Large, replicated plots were burned at different times of the year, and flowering on each plot was measured over the twelve months following fire. While fire season had little effect on the number of species flowering during the year following fire, fires during the growing season decreased average flowering duration per species and increased synchronization of peak flowering times within species relative to fires between growing seasons. Fires during the growing season also increased the dominance of fall flowering forbs and delayed peak fall flowering. Differences in flowering resulting from variation in fire season were related to seasonal changes in the morphology of clonal forbs, especially fall-flowering composites. Community level differences in flowering phenologies indicated that timing of fire relative to environmental cues that induced flowering was important in determining flowering synchrony among species within the ground cover of longleaf pine forests. Differences in fire season produced qualitatively similar effects on flowering phenologies in both habitats, indicating plant responses to variation in the timing of fires were not habitat specific.     

Effects of Hardwood Reduction Techniques on Longleaf Pine Sandhill Vegetation in Northwest Florida

We tested whether the intensity of hardwood midstory reduction causes commensurate improvements of herbaceous groundcover in fire‐suppressed Pinus palustris (longleaf pine) sandhills. Using a complete randomized block design, we compared the effects of three hardwood reduction techniques (spring burning, application of the ULW^® form of the herbicide hexazinone, chainsaw felling/girdling) and a no‐treatment control on plant species richness, and on life form and common species densities at Eglin Air Force Base, Florida, U.S.A., from 1995 to 1998. ULW^® and felling/girdling plots were burned for fuel reduction two years after initial treatment application. We also sampled the same variables in frequently‐burned reference sandhills to establish targets for restoration. Spring burns achieved partial topkill of oaks (17.6–41.1% from 1995 to 1998) compared to reductions of 69.1–94% accomplished by ULW^® and of 93.2–67.8% by felling/girdling treatments. We predicted that plant species richness and densities of herbaceous groundcover life forms would increase according to the percent hardwood reductions. Predictions were not supported by treatment effects for species richness because positive responses to fire best explained increases in plant richness, whereas ULW^® effects accounted for the largest initial decreases. Legumes, non‐legume forbs, and graminoids did not respond to treatments as predicted by the hypothesis. Again, positive responses to fire dominated the results, which was supported by greater herbaceous densities observed in reference plots. Overall, we found that the least effective and least expensive hardwood midstory reduction method, fire, resulted in the greatest groundcover improvements as measured by species richness and herbaceous groundcover plant densities.     

Three centuries of fire in montane pine‐oak stands on a temperate forest landscape

Question: What was the role of fire in montane pine‐oak (Pinus‐Quercus) stands under changing human land uses on a temperate forest landscape in eastern North America? Location: Mill Mountain in the central Appalachian Mountains, Virginia, US. Methods: A dendroecological reconstruction of fire history was generated for four stands dominated by xerophytic pine and oak species. The fire chronology began under presettlement conditions following aboriginal depopulation. Subsequent land uses included European settlement, iron mining, logging, and US Forest Service acquisition and fire protection. Results: Fires occurred approximately every 5 years until 1930 without any evidence of a temporal trend in fire frequency. Burning ceased after 1930. Area‐wide fires affecting multiple pine stands were common, occurring at intervals of approximately 16 years. Most living pines became established during the late 1800s and early 1900s. Dead pines indicated that an older cohort established ca. 1730. Most hardwoods were established between the 1920s and 1940s. Conclusions: Except for fire protection, changes in land use had no discernible influence on fire frequency. Lightning ignitions and/or large fire extent may have been important for maintaining frequent burning in the 1700s, while fuel recovery may have constrained fire frequency during later periods. The disturbance regime appears to be characterized by frequent surface fires and occasional severe fires, insect outbreaks or other disturbances followed by pine recruitment episodes. Industrial disturbances appear to have had little influence on the pine stands. The greatest impact of industrial society is fire exclusion, which permitted hardwood establishment.     

Responses of root‐crown bearing shrubs to differences in fire regimes in Pinus palustris (longleaf pine) savannas: Exploring old‐growth questions in second‐growth systems

Question: What are the effects of fire season and intensity on resprouting of different root‐crown bearing shrub species in second‐growth Pinus palustris (longleaf pine) savannas? Location: northern Florida and eastern Louisiana, USA. Methods: In Florida, quadrats were burned biennially either during the dormant season or the growing season. In Louisiana, we applied intensity treatments to quadrats by manipulating ground‐cover fuels, just prior to biennial growing season fires. Maximum fire temperatures were measured, and stem densities were censused before and after fires in both regions. Results: After dormant season fires in Florida, stem densities were seven times greater than initial levels for Hypericum spp. In contrast, growing season fires reduced densities of H. brachyphyllum by 65%, but did not change densities of H. microsepalum. Only resprouting of H. microsepalum decreased with increased fire intensity. In Louisiana, fire intensity influenced Ilex vomitoria, but not Quercus spp. Following fires, stem densities oil. vomitoria were five times greater in fuel removal than fuel addition areas. Conclusions: Past use of dormant season fires likely contributed to increased abundances of some species of root‐crown bearing shrubs observed today in old‐growth savannas. Reintroduction of growing season fires will be effective in maintaining or decreasing stem densities, depending on species and fuel type. Genet mortality and stem density reductions appear most likely in areas at localized scales where tree falls and needle coverage create hotspots in Pinus palustris savannas.     

Effects of long-term fire exclusion on tree species composition and stand structure in an old-growth Pinus palustris (Longleaf pine) forest

Frequent fire is an integral component of longleaf pine ecosystems, creating environmental conditions favoring survival and growth of juvenile pines. This study examined stand structure, species composition, and longleaf pine regeneration in an old-growth tract of longleaf pine forest (Boyd Tract) experiencing long-term (>80 yr) fire exclusion in the Sandhills of North Carolina. Sampling of woody stems (i.e., 2.5 cm diameter at breast height) and tallies of longleaf pine seedlings were carried out in plots established randomly on upland, mesic areas and lowland, xeric areas within the Boyd Tract. Dominant woody species in mesic plots were black oak, hickories, and large, sparse longleaf pines. Xeric plots had high densities of turkey oak with the large longleaf pines, as well as higher frequencies of smaller longleaf stems. These differences between areas were associated with higher clay content of upland soils and higher sand content of lowland soils. Age-class frequency distributions for fire-suppressed longleaf pine following the last wildfire at the Boyd Tract approximately 80 yr ago contrasted sharply with data from an old-growth longleaf tract in southern Georgia (Wade Tract) that has been under a long-term frequent fire regime. Post-burn recruitment for the Boyd Tract wildfire appears to have been initially high on both site types. Longleaf pine recruitment diminished sharply on the mesic site, but remained high for 60 yr on the xeric site. Currently, longleaf pine regeneration is minimal on both site types; several plots contained no seedlings. Sharp contrasts in longleaf pine dominance and stand structure between the Boyd and Wade Tracts demonstrate the importance of large-scale disturbance, especially hurricanes and fire, in shaping the structure and function of longleaf pine ecosystems of the southeastern United States. In particular, long-term exclusion of fire on the Boyd Tract has altered stand structure dramatically by permitting hardwoods to occupy at high densities the characteristically large gaps between longleaf stems that are maintained by fire and other disturbances.