William J. Platt

The Population Dynamics of a Long-Lived Conifer (Pinus palustris)

We investigated the demography and spatial pattern of an old-growth longleaf pine population using a large plot in which all trees of at least 2 cm in dbh were mapped and tagged for individual recognition. The population was of uneven age and size; tree size correlated positively with tree age. Large or old trees were only loosely aggregated, forming a background matrix that filled the forest. In contrast, juvenile trees were highly aggregated and were located in areas of low adult densities. Recruitment within this population thus appears to occur primarily within open spaces created by the deaths of large trees. Variable time lags may occur before the colonization of open spaces, however, because of temporal variation in seed production and the occurrence of summer ground fires. The absence of large gaps within the age-class distribution indicated that recruitment within the mapped plot has occurred frequently for at least the past 250 years. Neither age- nor size-class distributions, however, were stable. Instead, temporal variation in adult mortality and recruitment into open spaces, coupled with strong negative interactions between cohorts of different ages, appears likely to produce alternating phases of population growth and decline that are highly variable in length and magnitude. An upper bound to population size occurs when all available space is filled with trees; but no lower bound exists, and extinction probabilities may be increased at very low densities. The population is buffered from declines to very low densities, however, by the consistent tendency for small trees to recruit into openings created by the deaths of adults. This regeneration pattern, which also occurs in other long-lived conifers, represents a spatial analogue to stochastic boundedness over time, and it may enhance the local persistence of longleaf pine populations. Efficient spatial buffering requires that open spaces not be colonized by other tree species before the establishment of longleaf recruits. We suggest that longleaf pine maintains the environment in an open state suitable for its own regeneration by transmuting a localized disturbance (lightning) into a widespread disturbance (ground fires). Fire facilitation results in an extended, but indefinite, increase in the persistence of environmental conditions in which longleaf pine, but not other tree species, can survive and reproduce. Since longleaf pine influences the fire regime, it is a key species that constitutes environmental conditions and, hence, the composition and relative abundances of other species within the habitat.

EFFECTS OF FIRE REGIME AND HABITAT ON TREE DYNAMICS IN NORTH FLORIDA LONGLEAF PINE SAVANNAS

Frequent, low intensity fire was an important component of the natural dis- turbance regime of presettlement savannas and woodlands in the southeastern USA dom- inated by longleaf pine (Pinus palustris), and prescribed burning is now a critical part of the management of these endangered habitats. Fire season, fire frequency, and fire intensity are three potentially important, though still little understood, components of both natural and managed fire regimes. In this long-term (8-yr) study, we experimentally (through the use of prescribed burning) tested for effects of fire season (eight different times throughout the year) and fire frequency (annual vs. biennial burning), on population dynamics (re- cruitment, growth, mortality, change in density, and change in basal area (the total basal area of all stems in a plot)) and species composition of trees in two quite different types of longleaf-pine-dominated habitats (north Florida sandhills and flatwoods). Limited fire temperature and intensity data were also collected during one year to examine the rela- tionship between fire behavior (temperature and intensity) and tree mortality. Contrary to prior hypotheses, our results showed few systematic or predictable effects of season or frequency of burning on dynamics of longleaf pine. Instead, variability in the population dynamics of this species appeared to be related largely to variation in fire behavior, regardless of the season of burning. Consistent with prior hypotheses, we found that deciduous oak species (Quercus laevis, Q. margaretta, and Q. incana) were least vulnerable to dormant-season burning and most vulnerable to burning early in the growing season. This was shown particularly by seasonal trends in the effect of burning on oak mortality (both topkill and complete kill) and, to a lesser extent, on oak recruitment. Oak densities and basal areas also declined in the spring- burned plots, resulting in a shift away from oaks and towards increased dominance by longleaf pine. Detrimental effects of spring burning on oaks were partly explained by fire behavior, but there appeared also to be an important residual effect of burning season, particularly on complete kill. Though longleaf pine population dynamics did not differ markedly as a result of burning season and frequency, we did find important differences in pine dynamics between the two habitats (i.e., sandhills and flatwoods). In general, populations of longleaf pines in the sandhills appeared to be density regulated, while flatwoods pine populations were declining regardless of the level of intraspecific competition. This suggests that long-term persistence of longleaf pine, and perhaps other fire-adapted species in frequently burned longleaf-pine- dominated communities, may be determined by complex interactions between habitat factors and fire regimes.

Resource Partitioning and Competition within a Guild of Fugitive Prairie Plants

Badger disturbances on tall-grass prairies constitute a limiting resource for a guild of fugitive plants. These plants partition the resource along several dimensions. Divergent centers of resource utilization result from different suites of adaptive life-history characteristics important during colonization. While overlap of utilization functions is considerable along single dimensions of the resource, complementary adaptations of species result in reduced overlap along a complex gradient integrating the separate dimensions. We predict that the number, packing, and organization of species into a guild will change as the nature of the limiting resource changes. Specifically, changes in the abundance of the resource alter the heterogeneity of the resource over space, and as a result relationships among the adaptive centers of resource utilization for species also change. Predictions of changes in resource partitioning and guild organization, based upon assumptions concerning competition among species populations, correspond very closely to empirical field observations. In nature, composition of the fugitive species guild is expected to vary locally with changes in the nature and spatial heterogeneity of badger disturbances, and this variation over space also will depend upon the colonization capabilities of the species. Field investigation of competition provided insight into relationships among species comprising the fugitive guild. Competitive interactions are closely related to the adaptive life-history characteristics involved in the colonization of badger disturbances. The type of interaction occurring among individuals on a site depends upon events occurring during the colonization phase of the life cycle, and different types of competitive interactions have different effects upon the likelihood of subsequent site colonization (i.e., reproductive success of interacting individuals). Both the frequency and types of competitive interactions occurring among individuals in fugitive populations are predicted to depend upon the abundance of the limiting resource; the likelihood of colonization will change as the spatial distribution of colonization sites changes. Thus, the effects of changes in the distribution of the resource over space upon the relative colonization rates of species must be known to predict accurately the impact of competition upon resource partitioning and organization of fugitive plant populations into a guild.

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

SummaryEffects 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.

NATURAL DISTURBANCES AND DIRECTIONAL REPLACEMENT OF SPECIES

The original concept of succession emphasized directional changes in species composition occurring over time on “blank slates” (new substrates or those that followed catastrophic disturbances). In this paper, we explore relationships between different effects of disturbances on residents and the initiation of directional species replacement on sites in a landscape. We present a conceptual model involving recurrent natural disturbances of sites in a landscape containing two species, one of which arrives at sites early and the other arrives late following disturbance. We predict effects of recurrent catastrophic disturbances (no survivors) and non-catastrophic disturbances (some survivors) on the initiation of directional species replacement, assuming temporal gradients in environmental conditions on sites after disturbance. We predict that directional species replacement will be initiated by catastrophic disturbances, and by non-catastrophic disturbances when the early species does not survive, but reinvad...

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.

Replacement patterns of beech and sugar maple in Warren Woods, Michigan

Factors responsible for patterns of canopy tree replacement were studied for 22 yr in Warren Woods, Michigan, USA, an old-growth forest codominated by American beech (Fagus grandifolia) and sugar maple (Acer saccharum). Our goal was to distinguish among four hypotheses: autogenic succession, allogenic succession, autogenic coexistence, and allogenic coexistence. We could discern neither successional change toward increasing dominance by sugar maple or beech nor beech self-replacement by root sprouts. In the forest as a whole, from 1933 to 1980, sugar maple remained dominant in small understory size classes, and beech remained dominant in larger understory size classes and in the canopy. We could identify no plausible species-specific canopy influences or consistent responses of understory individuals that could be the basis for autogenic succession or autogenic coexistence. No significant differences existed below canopy beech or below canopy maple in: (1) beech root influence as measured by Epifagus root parasites; (2) light intensity as measured by extension growth of 0.2-0.8 m tall sugar maple; (3) leaf litter as measured by densities of beech and sugar maple leaves just after autumn leaf fall; or (4) inhibition as measured by death of sapling beech and sugar maple. Comparisons under single canopy trees and under monospecific patches showed that understory beech were larger than maple, and this pattern was accentuated below monospecific canopy patches both of maple and beech. This suggested autogenic succession, but coring data were not consistent with this hypothesis, because the largest subcanopy stems of beech and maple were established less often after conspecific or after heterospecific canopy individuals existed above them than before or at the same time. We could not falsify an allogenic coexistence hypothesis that beech or maple advantage changes as light levels fluctuate with frequencies and sizes of treefall gaps. Maples that had recently reached canopy height had been suppressed for an average of only 20 yr, but most had undergone multiple cycles of suppression and release associated with multiple treefall gap events. In contrast beech that had recently reached canopy height had been suppressed an average of 121 yr despite having a similar number of suppression-release cycles as maple. Differences between paired individuals, matched for light microenvironments and height, confirmed our hypothesis that the strong apical dominance of maple led to an advantage in fast upward growth in vertical light gradients of gaps and the long lateral branches of beech led to an advantage in an understory with light flecks and in horizontal light gradients from nearby gaps. Beechs better performance in the understory and maples better performance in gaps led us to predict that beech would decrease in dominance in the canopy if treefall rate increased. From 1949-1974 treefall gaps averaged 0.16 trees per hectare per year and consisted of mostly single treefalls; we projected that in these gaps beech to maple ratios in the canopy would become 1 to 1. From 1975 to 1994 treefall gaps averaged 1.64 trees per hectare per year and consisted of mostly multiple treefalls; we projected that in these gaps the ratio of beech to maple winners will become 1 to 2.5. We conclude that species-specific differences in response to light level, with allogenic spatial and temporal fluctuation in frequency and area of treefall gaps, are sufficient to explain patterns of beech and sugar maple replacement in the canopy in the old-growth forest at Warren Woods.

FIRE EFFECTS ON RESPROUTING OF SHRUBS IN HEADWATERS OF SOUTHEASTERN LONGLEAF PINE SAVANNAS

Woody plants in fire-frequented ecosystems commonly resprout from underground organs after fires. Responses to variation in characteristics of fire regimes may be a function of plant physiological status or fire intensity. Although these hypotheses have been explored for trees in southeastern longleaf pine (Pinus palustris) savannas, responses of other life forms and stages have not been studied. We examined effects of fire season and frequency, geography, habitat, and underground organ morphology on resprouting of shrubs. In 1994, we located replicated sites, each containing two habitats, upslope savannas and downslope seepages, in Louisiana and Florida. Each site, which contained quadrats located along transects within a 30 × 60 m plot, was burned either during the dormant or growing season and then reburned similarly two years later. Maximum fire temperatures were measured, and densities of shrub stems were censused in quadrats before and after fires. Shrubs collectively resprouted more following dorma...

INFLUENCE OF THE EL NIÑO SOUTHERN OSCILLATION ON FIRE REGIMES IN THE FLORIDA EVERGLADES

Disturbances that are strongly linked to global climatic cycles may occur in a regular, predictable manner that affects composition and distribution of ecological com- munities. The El Nino Southern Oscillation (ENSO) influences worldwide precipitation patterns and has occurred with regular periodicity over the last 130 000 years. We hypoth- esized that ENSO, through effects on local weather conditions, has influenced frequency and extent of fires within Everglades National Park (Florida, USA). Using data from 1948 to 1999, we found that the La Nina phase of ENSO was associated with decreased dry- season rainfall, lowered surface water levels, increased lightning strikes, more fires, and larger areas burned. In contrast, the El Nino phase was associated with increased dry-season rainfall, raised surface water levels, decreased lightning strikes, fewer fires, and smaller areas burned. Shifts between ENSO phases every few years have likely influenced vegetation through periodic large-scale fires, resulting in a prevalence of fire-influenced communities in the Everglades landscape.

SMALL‐SCALE FUEL VARIATION ALTERS FIRE INTENSITY AND SHRUB ABUNDANCE IN A PINE SAVANNA

Small-scale variation in fire intensity and effects may be an important source of environmental heterogeneity in frequently burned plant communities. We hypothesized that variation in fire intensity resulting from local differences in fuel loads produces heterogeneity in pine savanna ground cover by altering shrub abundance. To test this hypothesis, we experimentally manipulated prefire fuel loads to mimic naturally occurring fuel-load heterogeneity associated with branch falls, needle fall near large pines, and animal disturbances in a frequently burned longleaf pine (Pinus palustris) savanna in Louisiana, USA. We applied one of four fuel treatments (unaltered control, fine-fuel removal, fine-fuel addition, wood addition) to each of 540 (1-m2) quadrats prior to growing-season prescribed fires in each of two years (1999 and 2001). In both years fuel addition increased (and fuel removal decreased) fuel consumption and maximum fire temperatures relative to unaltered controls. Fuel addition, particularly wood, increased damage to shrubs, increased shrub mortality, and decreased resprout density relative to controls. We propose that local variation in fire intensity may contribute to maintenance of high species diversity in pine savannas by reducing shrub abundance and creating openings in an otherwise continuous ground cover.