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Patterns of Longleaf Pine (Pinus palustris) Establishment in Wiregrass (Aristida beyrichiana) Understories
Abstract
Ecosystem community structure and function is shaped in part by intra- and inter-specific interactions among plants. Facilitative interactions, wherein one plant benefits another's fitness, can strongly influence plant community dynamics. We investigated the potential of an endemic, perennial bunchgrass, wiregrass (Aristida beyrichiana), to function as a nurse plant for longleaf pine (Pinus palustris) seedlings in fire-maintained pine savannas of the southeastern U.S.A. We documented significantly more pine seedlings growing close to established wiregrass bunchgrasses in a site burned one year prior to sampling. Pine seedlings growing close to wiregrass were also significantly taller than those growing further away. This positive spatial association between wiregrass and pine seedlings suggests that wiregrass facilitates early longleaf pine establishment in flatwoods environments, at least within the first year after fire.
... Initial assessments of longleaf pine seedling performance across shrub and herbaceous vegetation indicated that neither vegetation type was exerting a strong competitive or facilitative effect. This result was somewhat surprising considering that understory vegetation can either facilitate or compete with longleaf pine seedlings depending on life history stage and stand conditions (McGuire et al., 2001;Pecot et al., 2007;Iacona et al., 2012;Miller et al., 2019). We suspect the presence of a relatively dense overstory across most of our blocks contributed to this result, as many previous studies reporting facilitative or competitive effects were conducted in harvest gaps or in savannas where the influence of overstory trees is reduced (Brockway and Outcalt, 1998;McGuire et al., 2001;Pecot et al., 2007;Miller et al., 2019). ...
... This result was somewhat surprising considering that understory vegetation can either facilitate or compete with longleaf pine seedlings depending on life history stage and stand conditions (McGuire et al., 2001;Pecot et al., 2007;Iacona et al., 2012;Miller et al., 2019). We suspect the presence of a relatively dense overstory across most of our blocks contributed to this result, as many previous studies reporting facilitative or competitive effects were conducted in harvest gaps or in savannas where the influence of overstory trees is reduced (Brockway and Outcalt, 1998;McGuire et al., 2001;Pecot et al., 2007;Miller et al., 2019). Consequently, our results suggest that interactions between longleaf pine seedlings and shrub and herbaceous vegetation are overwhelmed by the influence of the overstory in stands managed at or above a woodland density. ...
Recent reports of xerophytic hardwood facilitation of longleaf pine seedlings challenge the traditional paradigm that co-occurring hardwood tree species constrain longleaf pine (Pinus palustris Mill.) regeneration. To better understand this dynamic, we established an experiment examining the effects of hardwood retention duration (no retention, one-year retention, four-year retention), overstory basal area (2-to-41 m 2 ha − 1), and understory vegetation cover on microclimatic conditions and longleaf pine seedling survival, development, physiological performance, and brownspot needle blight (Mycosphaerella dearnessii) infection in the Sandhills Ecoregion of North Carolina, USA. Light availability (μmol m − 2 s − 1) at the forest floor (~20 cm) was reduced by 27% in the four-year retention treatment which significantly exceeded reductions found in the one-year (<1%) and no retention (<1%) treatments, respectively, post midstory removal. However, midstory treatment had little effect on soil moisture and temperature at 15 and 30 cm depth. Longleaf pine seedling survival (85%), aboveground biomass (7.51 g dry weight), and root collar size (1.53 cm) were significantly highest in the no retention treatment compared to the four-year retention treatment. Between treatments with midstory retention (one-year vs. four-year retention), longleaf pine seedlings were statistically larger in the one-year retention treatment (5.37 g dry weight) compared to the four-year retention treatment (4.84 g dry weight). Seedling brownspot infection did not statistically vary among midstory retention treatments. Across all midstory treatments, overstory basal area had a stronger negative effect on longleaf pine seedling aboveground biomass development, root collar development and brownspot infection compared to midstory hardwood stem density or understory vegetation cover. Moreover, for most measured metrics, overstory basal area had a stronger negative effect on longleaf pine physiological performance compared to midstory hardwood stem density. Collectively, these results indicate that hardwood retention did not facilitate longleaf pine survival and development in the grass stage and that asymmetric competition from the overstory represents a stronger impediment to longleaf pine sapling recruitment than midstory hardwoods.
... This species is widespread in pine savannas along edaphic gradients (Wells & Shunk 1931), important for vertebrate diversity (Means 2006), able to establish in degraded sites (Walker & Silletti 2006), resilient to climate extremes once established (Young et al. 2021), and a key driver of the grass-fire feedback (Fill et al. 2016). Wiregrass-dominated understories also support many endemic and rare species (Hardin & White 1989;Walker 1993), and wiregrass itself might facilitate pine seedling establishment (Miller et al. 2019). ...
... For example, Myers and Harms (2009) found limited evidence that grass competition reduces understory plant species richness. On the other hand, facilitative interactions play an essential role in shaping plant communities in savannas (Loudermilk et al. 2016;Miller et al. 2019;Pel aez et al. 2019), some of which could be related to unique microclimatic conditions generated by the architecture of different grass species (Vinton & Burke 1995;Iacona et al. 2012) or to varying degrees of flammability (Simpson et al. 2016). For example, those species that are more resistant to differences in grass flammability would be more likely to persist and maintain an association with that grass species. ...
Grass species are often included during restoration to reinstate feedbacks with fire. In biodiverse pine savannas of the southeastern U.S., wiregrass (Aristida beyrichiana) is the preferred candidate for restoring low intensity fires through the understory. Less common but locally co‐dominant in dry pine savannas, pineywoods dropseed (Sporobolus junceus) is another bunchgrass candidate for restoration. If two bunchgrass species associate with different suites of plants in the understory, restoring both could support a two‐fold goal of restoring fire regimes and plant biodiversity. To guide restoration goals, we tested the hypothesis that in natural (i.e., not recently restored) dry pine savannas in North Florida where wiregrass and dropseed are co‐dominant, they are associated with different understory plant species richness, occurrence, and abundance under or adjacent to their canopies. Species richness of associated plants did not differ between bunchgrass species or with distance from bunchgrass (i.e., under vs. adjacent). Some associated plant species, however, were more likely to occur and had greater abundance with one or the other bunchgrass species irrespective of distance. Our results suggest that including more than one bunchgrass species in restoration plantings could increase dry pine savanna biodiversity while also reinstating plant‐fire feedbacks. This article is protected by copyright. All rights reserved.
... In areas where encroachment has occurred, the existence of a dense forest floor could constrain the establishment of smaller-seeded species (Westoby et al., 2002;Varner et al., 2005). Increases in midstorey and canopy density could also exclude Aristida stricta from the understorey, which has been negatively associated with Pinus taeda invasion (Fill et al., 2017), but positively associated with Pinus palustris seedling establishment (Willis et al., 2019;Miller et al., 2019). Currently, it is unknown whether Aristida stricta inhibits Pinus taeda seedling establishment. ...
... Also, our results indicate that any potential facilitative effects on seedling establishment created by midstorey retention are not affecting germinant density for either species (Wahlenberg, 1946;Louise Loudermilk et al., 2016;Prévosto et al., 2020). Similarly, neither species' germinant density was statistically improved by proximity to Aristida stricta cover, as has been noted in previous studies (Miller et al., 2019;Willis et al., 2019). However, Aristida stricta cover had a biologically relevant positive effect on germination for both species, indicating that Aristida stricta was not impeding germination. ...
Questions
While much is known about the impact of tree encroachment on flammability in degraded pine woodlands, little is known about how encroachment is impacting other important ecosystem functions. We investigated how the availability of seed from four encroaching tree species and the presence of a midstorey and litter layer affect seed predator selection. Additionally, we investigated how seed predators, the midstorey, overstorey basal area, substrate availability, and vegetation cover affect germination for a foundational species (Pinus palustris) compared to an encroaching species (Pinus taeda).
Location
Sandhills Ecoregion, NC, USA (35°3′34.6932″ N, 79°22′22.0872″ W).
Methods
We measured seed depredation of Pinus palustris, Pinus taeda, Liquidambar styraciflua, Acer rubrum, and Quercus nigra in cafeteria trials. Each trial was held within a 2 × 2 factorial involving vertebrate seed predator exclusion and midstorey and litter layer removal across a gradient of overstorey basal area (6–25 m²). Additionally, we measured Pinus palustris and Pinus taeda germination within each treatment and correlated germinant density to substrate and understorey vegetation cover.
Results
Granivory generally varied inversely with seed size, with small‐seeded Liquidambar styraciflua experiencing the highest (27%) and large‐seeded Quercus nigra (7%) and Acer rubrum (6%) the lowest depredation pressure. Pinus palustris and Pinus taeda germinant density was significantly highest where vertebrate seed predators were excluded and the midstorey and litter layer were removed. For both pine species, this result corresponded with a significant positive association with mineral soil and negative associations with hardwood and pine litter where vertebrate predators were excluded. Basal area did not affect granivory or germination for any species.
Conclusions
Our results demonstrate that granivores did not select Pinus palustris, and that large‐seeded species encroachment was less inhibited by seed predators. Pinus palustris and Pinus taeda are depredated at comparable rates and germinate best under similar understorey conditions.
... Bunchgrasses are a prominent fuel source for spreading fires during prescribed burns [13,14]. Wiregrass, in conjunction with other understory bunchgrasses such as dropseed (Sporobolus junceus) and little bluestem (Schizachyrium scoparium), is a highly flammable species that plays a crucial role in maintaining sustainable ecosystems and providing important ecosystem services in the eastern portion of the southeastern US Coastal Plain [13][14][15]. Prescribed burns help stimulate seed production and plant regeneration in savanna understories while also topkilling hardwood trees and maintaining open-canopy conditions [e.g., 16]. Prior to addressing goals for overstory conditions, restoration practitioners strive to establish a self-sustaining grass cover through strategic seeding or planting methods [17]. ...
Uncertainty in ecosystem restoration can be mitigated by information on drivers of variability in restoration outcomes, especially through experimental study. In southeastern USA pine savannas, efforts to restore the perennial bunchgrass wiregrass (Aristida beyrichiana) often achieve variable outcomes in the first year. Although ecotypic differentiation and competition with other native vegetation are known to influence wiregrass seedling establishment and growth, to our knowledge, no studies have examined interactions between these drivers. We experimentally quantified individual and interactive effects of competition, seed source, and soil type on wiregrass density, size, and flowering culm production in the field. We sowed seeds from dry and wet sites reciprocally into dry and wet soils and weeded half of the plots. We found that competition removal resulted in significantly larger plants and a greater proportion of flowering plants with more culms on average, regardless of seed source or soil type. Seeds sourced from a wet site resulted in more plants per plot than seeds from a dry site, which might have been influenced by the greater number of filled seeds from the wet site. After seedlings become established, competition contributes to variation in growth and reproduction. Although competition removal could help start wiregrass populations, the necessity of mitigation depends on fire management needs.
... & Rupr.) is considered an essential component of many longleaf pine savannas of the North American Coastal Plain. This highly flammable bunchgrass facilitates the spread of low-intensity ground layer fires, which maintain the open-canopy pine savanna habitat structure (Drewa et al. 2002;Brockway et al. 2007;Fill et al. 2016;Miller et al. 2019). Some studies have found that wiregrass reproduction is fire stimulated with the greatest number of seeds produced when plants are burned during the lightning fire season (e.g., Abrahamson 1984;Platt et al. 1991;Streng et al. 1993). ...
Environmental heterogeneity can interact with ecosystem processes to alter individual plant reproduction. A better understanding of the factors that contribute to variation in plant reproduction between and within populations will increase our ability to predict larger-scale plant persistence. Longleaf pine (Pinus palustris) savannas are mostly open, heterogeneous landscapes characterized by occasional patches of trees that create partially closed canopies. This creates a mosaic of microsite conditions that can alter plant reproduction. Frequent, low-intensity fires that occur during different seasons are also considered fundamental drivers of plant reproduction in this system. The effects of varying fire season on plant reproduction have produced mixed results, likely because fire season interacts with microsite conditions, such as canopy cover. We investigated fire season and canopy cover effects on reproduction of wiregrass (Aristida beyrichiana), a species whose reproduction can be fire stimulated. We established plots under open and partially closed canopies in three pine savanna management units burned during different seasons (i.e., early dry, mid-dry, and early wet). We recorded reproductive state and number of inflorescences produced by individuals occurring within plots and germinated seeds from reproducing individuals. We found that wiregrass burned during the early dry season had the lowest reproduction with few individuals flowering. When burned during the mid-dry season, probability of reproduction was the highest, but seed germination was low. Plants burned during the early wet season produced seeds with the highest probability of germination, especially under partial canopy. Our results indicate that wiregrass reproduction is affected by both small-scale environmental variation and large-scale ecosystem processes, with fires during the early wet season most likely to promote the production of viable seeds.
... Restoration efforts often target grasses in the understory to support fire spread and reinstate vegetation-fire feedbacks. Aristida beyrichiana (wiregrass) is a perennial, endemic C4 bunchgrass that commonly dominates pine savanna understories, contributing to vegetation-fire feedbacks and biodiversity dynamics [16][17][18]. In savannas where fire has been excluded however, the size and number of wiregrass individuals are greatly reduced [19,20], which could be attributed to competition, lower light levels from overstory shading, or heavy litter [21][22][23]. ...
Restoring fire regimes is a major goal of biodiversity conservation efforts in fire-prone ecosystems from which fire has been excluded. In the southeastern U.S.A., nearly a century of fire exclusion in pine savannas has led to significant biodiversity declines in one of the most species-rich ecosystems of North America. In these savannas, frequent fires that support biodiversity are driven by vegetation-fire feedbacks. Understory grasses are key components of these feedbacks, fueling the spread of fires that keep tree density low and maintain a high-light environment. When fire is reintroduced to long-unburned sites, however, remnant populations of bunchgrasses might experience high mortality from fuel accumulation during periods of fire exclusion. Our objective was to quantify fire effects on wiregrass (Aristida beyrichiana), a key component of vegetation-fire feedbacks, following 16 years without fire in a dry pine savanna typically considered to burn every 1–3 years. We examined how wiregrass size and fuel (duff depth and presence of pinecones) affected post-fire survival, inflorescence and seed production, and seed germination. Wiregrass exhibited high survival regardless of size or fuels. Probability of flowering and inflorescence number per plant were unaffected by fuel treatments but increased significantly with plant size (p = 0.016). Germination of filled seeds was consistent (29–43%) regardless of fuels, although plants in low duff produced the greatest proportion of filled seeds. The ability of bunchgrasses to persist and reproduce following fire exclusion could jumpstart efforts to reinstate frequent-fire regimes and facilitate biodiversity restoration where remnant bunchgrass populations remain.
Globally, savanna trees experience bottlenecks to recruitment. Likelihoods are low that juveniles, especially of nonclonal, reseeder species, will survive and reach sizes that survive recurrent fires. We hypothesized if ground layer vegetation within savannas contained patches with reduced fire effects, likelihoods of juvenile trees surviving fires would be increased. We refined our general hypothesis based on a field study in an old-growth southeastern pine savanna of North America, in which longleaf pine (Pinus palustris Mill.) is the most abundant tree. We hypothesized that recruitment of pines into the fire-resistant ‘grass stage’ may be more likely in three ground layer microhabitats (inside crowns of fallen pines, around pine tree stumps, and inside patches of oak/hardwood stems) than in surrounding groundcover located away from overstory pines. We measured the composition and abundance of ground layer vegetation and censused juvenile grass stages (< 1.5 m height) of longleaf pine in plots in replicated patches of these three microhabitats and in the surrounding ground layer matrix, all located away from large trees. Ground layer vegetation was less abundant inside than outside the three microhabitats and abundances of grasses and shrubs differed among microhabitats. A zero-inflated Poisson model indicated that occurrence of grass stage longleaf pines was >5 times more likely inside the three microhabitats than in the surrounding ground layer matrix. Recruitment was also more likely in pine than oak/hardwood microhabitats. We propose that altered microhabitats, especially those generated by death of large longleaf pines, likely facilitate recruitment into populations of this reseeding savanna tree.
Plant‐soil feedback (PSF) theory provides a powerful framework for understanding plant dynamics by integrating growth assays into predictions of whether soil communities stabilise plant–plant interactions. However, we lack a comprehensive view of the likelihood of feedback‐driven coexistence, partly because of a failure to analyse pairwise PSF, the metric directly linked to plant species coexistence. Here, we determine the relative importance of plant evolutionary history, traits, and environmental factors for coexistence through PSF using a meta‐analysis of 1038 pairwise PSF measures. Consistent with eco‐evolutionary predictions, feedback is more likely to mediate coexistence for pairs of plant species (1) associating with similar guilds of mycorrhizal fungi, (2) of increasing phylogenetic distance, and (3) interacting with native microbes. We also found evidence for a primary role of pathogens in feedback‐mediated coexistence. By combining results over several independent studies, our results confirm that PSF may play a key role in plant species coexistence, species invasion, and the phylogenetic diversification of plant communities.
Old-growth longleaf pine savannas are characterized by diverse ground-layer plant communities comprised of graminoids, forbs, and woody plants. These communities co-exist with variable-aged patches containing similar-aged trees of longleaf pine (Pinus palustris Mill.). We tested the conceptual model that physical conditions related to the cycle of longleaf pine regeneration (stand structure, soil attributes, fire effects, and light) influence plant species’ composition and spatial heterogeneity of ground-layer vegetation. We used a chrono-sequence approach in which local patches represented six stages of the regeneration cycle, from open areas without trees (gaps) to trees several centuries old, based on a 40-year population study and increment cores of trees. We measured soil characteristics, patch stand structure, fuel loads and consumption during fires, plant productivity, and ground-layer plant species composition. Patch characteristics (e.g., tree density, basal diameter, soil carbon, and fire heat release) indicated a cyclical pattern that corresponded to the establishment, growth, and mortality of trees over a period of approximately three centuries. We found that plants in the families Fabaceae and Asteraceae and certain genera were significantly associated with a particular patch stage or ranges of patch stages, presumably responding to changes in physical conditions of patches over time. However, whole-community-level analyses did not indicate associations between the patch stage and distinct plant communities. Our study indicates that changes in composition and the structure of pine patches contribute to patterns in spatial and temporal heterogeneity in physical characteristics, fire regimes, and species composition of the ground-layer vegetation in old-growth pine savanna.
Research Highlights: Spatial patterns of fire spread and severity influence survival of juvenile pines in longleaf pine savannas. Small areas that do not burn during frequent fires facilitate formation of patches of even-aged longleaf pine juveniles. These regeneration patches are especially associated with inner portions of openings (gaps) and where canopy trees have died in recent decades. Patterns of prescribed fire can thus have an important influence on stand dynamics of the dominant tree in pine savannas. Background and Objectives: Savannas are characterized by bottlenecks to tree regeneration. In pine savannas, longleaf pine is noted for recruitment in discrete clusters located within gaps away from canopy trees. Various mechanisms promoting this pattern have been hypothesized: light limitations, soil moisture, soil nutrients, pine needle mulching, competition with canopy tree roots, and fire severity associated with pine needle litter. We tested the hypothesis that regeneration patches are associated with areas that remain unburned during some prescribed fires, as mediated by gaps in the canopy, especially inner portions of gaps, and areas re-opened by death of canopy trees. Materials and Methods: We mapped areas that were unburned during prescribed fires applied at 1–2 year intervals from 2005–2018 in an old-growth pine savanna in Georgia, USA. We compared the maps to locations of longleaf pine juveniles (<1.5 m height) measured in 2018 and canopy cover and canopy tree deaths using a long-term (40 year) tree census. Results: Logistic regression analysis showed juveniles to be associated with unburned areas, gaps, inner gaps, and areas where canopy trees died. Conclusions: Patterns of fire spread and severity limit survival of longleaf pine juveniles to patches away from canopy trees, especially where canopy trees have died in recent decades. These processes contribute to a buffering mechanism that maintains the savanna structure and prevents transition to closed canopy forest or open grassland communities.
The presence of native grasses in communities can suppress native forbs through competition and indirectly benefit these forbs by suppressing the invasion of highly competitive exotic species. We conducted a greenhouse experiment to examine the potential of direct and indirect interactions to influence the aboveground biomass of four native forb species in the presence of the native perennial grass Schizachyrium scoparium and exotic invasive Lespedeza cuneata. We examined patterns of growth for the invasive legume, the perennial grass, and four native species in four scenarios: 1) native species grown with the grass, 2) native species grown with the legume, 3) native species grown with both the grass and legume together, and 4) native species grown alone. Schizachyrium scoparium significantly decreased biomass of all forb species (p<0.05). In contrast, L. cuneata alone only significantly affected biomass of Asclepias tuberosa; L. cuneata increased the biomass of A. tuberosa only when the grass was present. When S. scoparium and L. cuneata were grown together, L. cuneata had significantly lower biomass (p = 0.007) and S. scoparium had significantly greater biomass (p = 0.002) than when each grew alone. These reciprocal effects suggest a potential pathway by which L. cuneata could alter forb diversity in grassland communities In this scenario, L. cuneata facilitates grass growth and competition with other natives. Our results emphasize the importance of monitoring interactions between exotic invasive plant species and dominant native species in grassland communities to understand pathways of plant community change.
Understanding the mechanisms that allow exotic species to have rapid population growth is an important step in the process of controlling existing invasions and preventing future invasions. Several hypotheses have been proposed to explain why some exotic species become invasive, the most prominent of which focus on the roles of habitat disturbance, competitors and consumers. The magnitude and direction of each of these mechanisms on population dynamics observed in previous studies is quite variable. It is possible that some of this variation results from interactions between mechanisms.
We examined all of these mechanisms and their interactions on the population dynamics of the Asian exotic tree Ailanthus altissima (Simaroubaceae) in fire‐suppressed oak‐hickory forests in Missouri, USA . We experimentally reduced herbivory (using insecticide), reduced interspecific competition (plant removals), and manipulated disturbance with prescribed fire. We projected the effects of these treatments and their interactions on population dynamics by parameterizing an integral projection model.
The lowest population growth rate is found where fire is absent and biotic interactions are present. Fire increased population growth rate, likely through the suppression of interspecific competitors, since competitor removal treatments increased population growth rate in the absence but not presence of fire.
These results indicate that biotic resistance from interspecific competitors, more so than consumers, is important for slowing the invasion of A. altissima . Furthermore, disturbances that weaken biotic interactions, such as fire, should be used with caution when restoring habitats invaded by A. altissima .
Synthesis and applications . Examining the main and interactive effects of disturbance, competition and herbivory on the population dynamics of exotic species provides a comprehensive understanding of the role of these factors in the invasion process and provides guidance for exotic species management.
Understanding the flammability of species in fire-prone or fire-dependent ecosystems is necessary for modeling and predicting ecosystem dynamics. Wiregrass (Aristida stricta syn. A. beyrichiana), a keystone perennial bunchgrass, is a dominant groundcover species in southeastern United States pine savannas. Although wiregrass flammability as a driver of pine savanna fire regimes is a fundamental paradigm in pine savanna dynamics, no studies have quantified its fuel structure and flammability at the individual bunchgrass level. We studied wiregrass flammability at the Aiken Gopher Tortoise Heritage Preserve in Aiken County, South Carolina, USA. We linked tussock fuel structure characteristics (total biomass, live:dead biomass, mass of perched litter and pine needles, moisture content, and bulk density) to flammability (flaming duration, smoldering duration, and flame length). Flame length was strongly and positively related to wiregrass biomass. Pine needles and other litter fuels perched on wiregrass tussocks were not related to flame length, but increased the duration of flaming and smoldering. Within the ranges evaluated, neither fire weather (relative humidity, wind speed, and air temperature) nor fuel moisture significantly affected tussock flammability. Our results indicate that different fuel structural properties drive separate aspects of wiregrass flammability. Together with litter from pines and other groundcover shrubs and trees, wiregrass modifies fire behavior locally, potentially influencing ecosystem dynamics at larger scales. These results have strong implications for southeastern pine savannas and more broadly where grass-dominated vegetation influences fire regimes.
Scientific models that guide restoration/management protocols should be reviewed periodically as new data become available. We examine ecological concepts used to guide restoration of pine savannas and woodlands, historically prominent but now rare habitats in the southern North American Coastal Plain. For many decades, pine savanna management has been guided predominantly by a biome-centric succession model. Pine savannas have been considered early-successional communities that, in the absence of fire, transition rapidly toward closed-canopy hardwood forests. Recurrent fires have been viewed as exogenous disturbances that maintain savanna ecosystems as a sub-climax, blocking succession to an equilibrium steady state (closed-canopy forests). Over recent decades, a vegetation-fire feedback model has emerged in which pine savannas are conceptualized as persistent, non-equilibrium communities maintained by endogenous, co-evolutionary vegetation-fire feedbacks. Endemic plant species are resistant to fires and specialized for post-fire conditions generated by frequent lightning fires, primarily within a distinct fire season. These species produce pyrogenic fine fuels that are easily ignited. The resulting fire regimes, entrained by these vegetation-fire feedbacks, are predicted to result in persistent pine savannas. Local variation over space and time in evolutionary feedback mechanisms between pyrogenic vegetation and fire regimes produces heterogeneous landscapes. Disturbances of these feedbacks, such as human fire suppression, are postulated to result in rapid transition to communities lacking feedback elements, such as closed-canopy forest and those without pyrogenic species. Succession-based management focuses on reversing the transition to forest, primarily by removing hardwoods and reintroducing fire as a disturbance. However, we advocate restoration and management approaches that target reinstitution of functional vegetation-fire feedbacks. Such approaches should favor native pyrogenic plant species and reinstitute fire regimes that mimic historical, evolutionarily derived fire regimes. Vegetation-fire feedback concepts should be useful in addressing resistance and resilience of fiery ecosystems worldwide to inherent changes in feedback mechanisms, constituting a framework useful in addressing global management challenges.
Almost all natural plant communities contain arbuscular mycorrhizal fungi (AMF). We hypothesized that the species composition of AMF communities could have the potential to determine plant community structure if the growth response to different AMF species or to communities of AMF species varies among plant species. To test the existence of such a differential response we conducted a pot experiment where each of three plant species, Hieracium pilosella, Bromus erectus,and Festuca ovinawere inoculated with each of four AMF species, or with a mixture of these four AMF species, or were uninoculated. The AMF species originated from a calcareous grassland in which the three plant species also coexisted. We obtained three pieces of evidence suggesting that AMF have the potential to de- termine plant community structure. First, plant species differed in their dependency on AMF, thus varying in degree of benefit received. Second, specific AMF species and a mixture of these AMF species had significantly different effects on several plant growth variables, and these effects were not the same on each plant species. Third, the amount of variation in the growth response of a plant species to four AMF species and to the mixture of AMF species differed among the plant species. Hieracium differed greatly in its growth response to several AMF species while Bromus did not exhibit much variation in its response to different AMF species. The varying mycorrhizal dependency of different plant species has previously been proposed as a mechanism determining plant community structure. However, we found that the mycorrhizal dependency of a plant species can vary greatly because of differential growth responses to specific AMF species compared to the growth of the uninoculated plants. Consequently mycorrhizal dependency, as a measure indicating how much a plant depends on AMF for its growth, is not necessarily a fixed value and therefore cannot be used as a definitive term. In addition, those plant species with highly variable responses to single AMF species or to combinations of AMF species (AMF communities) will be strongly affected by the specific species of AMF that occupy their roots, in contrast to plant species that do not respond differently to different AMF species. We conclude that, through their differential effects on plant growth, AMF species that co-occur as natural AMF communities have the potential to determine plant community structure, and that future studies on plant population and community structure need to consider the strength of their role as a determinant.
a b s t r a c t Improving predictions of ponderosa pine (Pinus ponderosa var. scopulorum) regeneration is critical as landscape-scale restoration efforts are being implemented across the western United States. The estab-lishment of new ponderosa pine cohorts following restoration treatments will have long-term impacts on the future development of forest stands. At a regional scale, we used data associated with 364 Forest Inventory and Analysis (FIA) plots within the ponderosa pine forest type on National Forests in Arizona and New Mexico. At a local scale, we sampled 28 stands on basalt and sedimentary parent materials on the Coconino National Forest in northern Arizona. Structural equation models were developed to deter-mine the relative importance of direct and indirect factors affecting ponderosa pine seedling densities at both regional and local scales. The regional model only explained 13% of the variation in pine seedling density. The model indicated that ponderosa pine seedling density was highest where (1) mean annual precipitation was highest, (2) average May minimum temperature was highest, (3) overstory stand basal area was lowest and (4) Muhlenbergia spp. were dominant herbaceous plants. The local model explained 76% of the variation in pine seedling density. Densities were highest where (1) mean annual precipitation was highest, (2) soil clay content was lowest, (3) soil pH was lowest, (4) Muhlenbergia virescens frequency was highest and (5) seed tree frequency was highest. If additional factors such as soil texture, soil pH, screwleaf muhly frequency and seed tree frequency were measured on FIA plots, then perhaps more var-iation in pine seedling density could be explained at the regional scale.
▪ Abstract Savannas occur where trees and grasses interact to create a biome that is neither grassland nor forest. Woody and gramineous plants interact by many mechanisms, some negative (competition) and some positive (facilitation). The strength and sign of the interaction varies in both time and space, allowing a rich array of possible outcomes but no universal predictive model. Simple models of coexistence of trees and grasses, based on separation in rooting depth, are theoretically and experimentally inadequate. Explanation of the widely observed increase in tree biomass following introduction of commercial ranching into savannas requires inclusion of interactions among browsers, grazers, and fires, and their effects on tree recruitment. Prediction of the consequences of manipulating tree biomass through clearing further requires an understanding of how trees modify light, water, and nutrient environments of grasses. Understanding the nature of coexistence between trees and grass, which under other ci...
Facilitative interactions between neighboring plants can influence community composition, especially in locations where environmental stress is a factor limiting competitive effects. The longleaf pine savanna of the southeastern United States is a threatened and diverse system where seedling recruitment success and understory species richness levels are regulated by the availability of moist microsites. We hypothesized that the dominant bunch grass species (Aristida stricta Michx.) would facilitate moist seedling microsites through shading, but that the effect would depend on stress gradients. Here, we examined the environmental properties modified by the presence of wiregrass and tested the importance of increased shade as a potential facilitative mechanism promoting seedling recruitment across spatial and temporal stress gradients. We showed that environmental gradients, season, and experimental water manipulation influence seedling success. Environmental properties were modified by wiregrass proximity in a manner that could facilitate seedling success, but we showed that shade alone does not provide a facilitative benefit to seedlings in this system.
The genus Pinus extends into tropical environments in the neotropics of Mesoamerica, the Caribbean islands, and the southern tip of the Florida peninsula. In Old World tropics, pines occur from southeast China to the Philippines and Southeast Asia. Only one species, Pinus merkusii De Vries in Sumatra, occurs south of the equator. Pines in the tropics, as elsewhere, are associated with stressed and/or disturbed environments. In seasonal environments, they are frequently linked to fire, with some species, and the ecosystems they characterize, dependent on recurring fire. This chapter focuses on the role of fire in tropical pine ecosystems. We rely on a limited literature base; a wealth of information from analogous ecosystems in warm temperate environments, particularly those found on the southeastern coastal plain of North America; observations made by the authors in the neotropics; and a growing body of information coming from recent and ongoing studies in the highlands of Mexico.
Current concepts of the role of interspecific interactions in communities have been shaped by a profusion of experimental studies of interspecific competition over the past few decades. Evidence for the importance of positive interactions — facilitations — in community organization and dynamics has accrued to the point where it warrants formal inclusion into community ecology theory, as it has been in evolutionary biology.
In the 6 yr following the 1980 eruption of Mount St. Helens in Washington state, vascular plant invasion of barren substrates in subalpine habitats has been limited despite the proximity of seed sources from resprouted vegetation. Seedling recruitment declined from 1983 to 1985, apparently in response to drought; most seedlings occurred within 3 m of a conspecific adult; and plots with an intermediate vegetation cover (5-72%) contained a significantly higher fraction of seedlings than did unvegetated plots. Nurse plants may play a key role in trapping seeds and promoting seedling establishment. Viable seeds from 22 subalpine species were sown into plots in barren substrates at 2 sites on the volcano. Survivorship through 1985 varied from 0-12%, with Sitanion jubatum, Stipa occidentalis, Polygonum newberryi, Eriogonum pyrolifolium and Spraguea umbellata attaining the highest values. Survivorship was correlated with seed mass, and was higher at the site where the pre-eruption surface was exposed. Fertilizer increased size of most seedlings but had only a marginal effect on survivorship. Species with high environmental tolerance generally dispersed short distances; species that dispersed farther generally had low tolerances and apparently require site amelioration prior to establishment. -from Authors
If plants cannot simultaneously acclimate to shade and drought because of physiological trade-offs, then plants are expected to be less tolerant to shading under drier conditions. One observation that, at first sight, seems incompatible with this idea is the fact that the establishment of new plants in dry areas is often restricted to shady sites under the canopy of other plants, called "nurse plants". We use a graphical model to resolve this paradox. The model visualises how facilitative patterns can be understood from the simultaneous effects of plant canopies on microsite light and moisture, and the growth responses of seedlings to those factors. The approach emphasizes the fact that positive and negative effects of plant canopies always occur simultaneously. In the presented light-water model, facilitation only occurs when the improvement in plant water relations under the canopy exceeds the costs caused by the lower light levels. This may be true under dry conditions, whereas in less dry situations, competition rather than facilitation is observed. The model shows how changes in water availability may shift interactions from competitive to faciltative and vice versa, as observed in some field patterns. It is argued that other environmental factors explaining facilitative patterns can be understood in the same context.
Worldwide, grassland ecosystems have experienced a major shift in growth-form dominance as woody plant species have expanded and replaced native grasses. In the C(4)-dominated grasslands of central North America, a reduction in fire frequency is the most cited cause of this shift in growth forms as fire both enhances grass productivity and constrains the establishment and expansion of native woody vegetation. Using an 18-yr plant species composition data set, we quantified patterns of change in shrub cover, frequency, and species richness associated with three distinct fire regimes. During the study period (1983-2000), shrub cover increased most dramatically in sites in which the frequency of fire was once every 4 yr (intermediate frequency; 28.6%) followed by sites in which fire occurred only once during the 18-yr period (low frequency; 23.7%). Annual fire effectively prevented the recruitment of new woody species, but even with this high fire frequency, shrub cover increased slightly (3.7%). Comparatively, shrub species richness increased by three and six, respectively, in the intermediate- and low-frequency fire sites. These data indicate that within this grassland, periods without fire are necessary for recruitment of both new individuals and additional shrub species; however, once established, shrub cover will increase regardless of fire frequency and even annual fire will not reduce shrub abundance.
Early growth and physiology of longleaf pine (Pinus palustris Mill.) seedlings were studied in response to light, water and nitrogen under greenhouse conditions. The experiment was conducted with 1-year-old seedlings grown in 11.3 l pots. The experimental design was a split-plot factorial with two levels (low and high) of each of the factors, replicated in three blocks. The four factorial combinations of water and nitrogen were randomly applied to 15 pots (sub-plots) in each of the light treatment (main plot). Data were collected on survival, root collar diameter (RCD), and height on a monthly basis. Biomass (shoot, root and needle), leaf area index, specific needle area, and needle nutrient (N, P, K, Ca, and Mg) concentrations were determined following final harvest after 16 months. Physiological data (net photosynthesis and transpiration) were collected monthly from March to July during the second growing season.
1. Once neglected, the role of facilitative interactions in plant communities has received considerable attention in the last two decades, and is now widely recognized. It is timely to consider the progress made by research in this field. 2. We review the development of plant facilitation research, focusing on the history of the field, the relationship between plant-plant interactions and environmental severity gradients, and attempts to integrate facilitation into mainstream ecological theory. We then consider future directions for facilitation research. 3. With respect to our fundamental understanding of plant facilitation, clarification of the relationship between interactions and environmental gradients is central for further progress, and necessitates the design and implementation of experiments that move beyond the clear limitations of previous studies. 4. There is substantial scope for exploring indirect facilitative effects in plant communities, including their impacts on diversity and evolution, and future studies should connect the degree of non-transitivity in plant competitive networks to community diversity and facilitative promotion of species coexistence, and explore how the role of indirect facilitation varies with environmental severity. 5. Certain ecological modelling approaches (e.g. individual-based modelling), although thus far largely neglected, provide highly useful tools for exploring these fundamental processes. 6. Evolutionary responses might result from facilitative interactions, and consideration of facilitation might lead to re-assessment of the evolution of plant growth forms. 7. Improved understanding of facilitation processes has direct relevance for the development of tools for ecosystem restoration, and for improving our understanding of the response of plant species and communities to environmental change drivers. 8. Attempts to apply our developing ecological knowledge would benefit from explicit recognition of the potential role of facilitative plant-plant interactions in the design and interpretation of studies from the fields of restoration and global change ecology. 9. Synthesis: Plant facilitation research provides new insights into classic ecological theory and pressing environmental issues. Awareness and understanding of facilitation should be part of the basic ecological knowledge of all plant ecologists.
A nucleated pattern of establishment by pine seedlings of Pinus strobus and P. resinosa around pre-established oak trees of Quercus rubra in a sand dune succession in Ontario, Canada was examined using field observations, seed planting and habitat manipulation. Densities of young pines beneath oak canopies were approximately six times greater than in treeless areas, and densities on the north sides of the trees were significantly greater than on the south sides. However, oaks younger than 35 yr showed no preferential establishment beneath them, while the pine population structures beneath older oaks indicated single periods of successful recruitment. Pine seed planted beneath and beyond oak canopies of three sizes germinated primarily beneath the canopies of medium and large-sized oaks, but subsequent survivorship of seedlings over two growing seasons was poor. Several micro-environmental conditions were changed by oak canopies, but only shade showed a pattern closely corresponding to that of seedling establishment. A habitat manipulation experiment confirmed the primary role of shade in facilitating pine seedling establishment beneath oaks. Failure of pine to continue recruiting successfully beneath facilitating trees is tentatively attributed to intraspecific competition among pine individuals.
Many of the stand structural characteristics of longleaf pine (Pinus palustris Mill.) forests that existed prior to European colonization have been altered or lost from past disturbance histories (Frost
this volume).
Early ecologists emphasised the role of positive interactions in plant communities occurring in habitats characterised intense abiotic stress (Clements et.al. 1926; Allee et.al. 1949). Until recently, however, positive interactionshave been largely overlooked as important factors in community structure. In the last decade, renewed interest in facilitation has resulted in a number of studies that document the importance of this interaction in communities. Positive interactions have been described for sessile animals in intertidal zones(Witman 1985; Lively & Raimondi 1987) and for plants in variety of environments such as deserts (e.g. McAuliffe 1988; Franco & Nobel 1989), salt marshes (Bertness 1991; Bertness & Shumway 1993; Callaway 1992), and Graet Lakes sand-dunes(Kellman & Kading 1992; see review in Callaway 1985). Plants acting as facilitators may lower the soil surface temperatures (Franco & Nobel 1989; Evans & Cabin 1995), decrease evaporation and hence soil salinity beneath their canopies(Bertness & Shumway 1993), offer protection from herbivores(McAuliffe 1984), or increase soil nutrients beneath their canopies(Callaway et.al. 1991).
Recent research presuming multiple, interacting causal mechanisms for community structure has demonstrated that facilitation and interference may operate similtaneously and that the overall effect of one species on another may vary among different microhabitats as mechanisms shift in relative importance (Callaway et.al. 1991; Bertness & Hacker 1994; Bertness & Yeh 1994; Chapin et.al. 1994). Complex combinations of negative and positive interactions operating similtaneously suggest that current conceptual models of interplant interactions based on resource competition are limited in their potential for accurately depicting processes in natural plant communities. In an attempt to integrate biotic and abiotic factors into one model, researchers have hypothesised (Bertness & Callaway 1994) that the relative importance of of interference and facilitation may vary inversely along gradients of abiotic stress.; the importance of facilitation is predicted to increase, and the importance of interference is predicted to decrease along a gradient of increasing abiotic stress. Hee we provide the first test of this hypothesis.
Controversy over the effect of Quercus douglasii on the productivity of California grassland has been fueled by conflicting reports. In some studies, understory grassland productivity was <25% of open grassland, whereas in other studies understory productivity exceeded 200% of that of surrounding grassland. We examined light, temperature, soil nutrients, soil moisture, & fine tree root distribution under selected Q.douglasii trees (12 that appeared have suppessive effect on on understory productivity, & 12 that appeared to enhance understory productivity) in order to determine how variations in these factors were associated with the difference in understory grassland productivities. We found that grassland productivity is likely to be facilitated by nutrient inputs via litterfall & throughfall under all the trees, but that trees with low understory productivities had substantially higher amounts of fine roots in the upper 50cm of soil & much lower pre-dawn xylem pressures potentials than trees with high understory productivities & presumably deeper root systems. Root exclosures reduced the negative effects of trees on the dominant understory grass species , & further experiments indicated that the negative effects of oaks may partially result from allelopathic oak root exudates. Thus, shallow fine roots may inhibit understory productivity, & variations in Q.douglasii root morphology may explain the intertree variations in facilitating/ interfering effects on understory species
Interactions among organisms take place within a complex milieu of abiotic and biotic processes, but we generally study them as solitary phenomena. Complex combinations of negative and positive interactions have been identified in a number of plant communities. The importance of these two processes in structuring plant communities can best be understood by comparing them along gradients of abiotic stress, consumer pressure, and among different life stages, sizes, and densities of the interacting species. Here, we discuss the roles of life stage, physiology, indirect interactions, and the physical environment on the balance of competition and facilitation.
1. Once neglected, the role of facilitative interactions in plant communities has received considerable attention in the last two decades, and is now widely recognized. It is timely to consider the progress made by research in this field.
2. We review the development of plant facilitation research, focusing on the history of the field, the relationship between plant–plant interactions and environmental severity gradients, and attempts to integrate facilitation into mainstream ecological theory. We then consider future directions for facilitation research.
3. With respect to our fundamental understanding of plant facilitation, clarification of the relationship between interactions and environmental gradients is central for further progress, and necessitates the design and implementation of experiments that move beyond the clear limitations
of previous studies.
4. There is substantial scope for exploring indirect facilitative effects in plant communities, including their impacts on diversity and evolution, and future studies should connect the degree of non-transitivity in plant competitive networks to community diversity and facilitative promotion of species coexistence, and explore how the role of indirect facilitation varies with environmental severity.
5. Certain ecological modelling approaches (e.g. individual-based modelling), although thus far largely neglected, provide highly useful tools for exploring these fundamental processes.
6. Evolutionary responses might result from facilitative interactions, and consideration of facilitation might lead to re-assessment of the evolution of plant growth forms.
In high-diversity communities, rare species encounter one another infrequently and therefore may compete more intensely with common species or guilds for limiting space and resources. In addition, rare species may be strongly recruitment limited because of their low abundances. Under these conditions, stochastic dispersal and immigration history can have an important influence on community structure. We tested the hypothesis that local immigration and competition from common, large-stature guilds interact to structure local biodiversity in high-diversity longleaf pine savanna groundcover assemblages (>30 species/m2). In two factorial field experiments, we increased local immigration by adding seeds of 38 mostly rare, small-stature forbs and sedges to plots physically dominated by either a common, large-stature bunchgrass or shrub species and to plots in which competition from these dominant guilds was reduced. We measured species richness and abundance at two spatial scales (0.01 and 0.25 m2) over two years. Immigration increased total species richness and richness of focal seed addition species regardless of levels of competition with bunchgrasses and shrubs, indicating that many rare, small-stature species can recruit in the face of potential competition from dominant guilds. Removal of dominant guilds increased total and focal species richness in shrub-dominated but not bunchgrass-dominated plots. In addition, competition from both dominant guilds had no clear effect on rank-abundance distributions of focal species. Our results suggest a key role for dispersal assembly in structuring local biodiversity in this high-diversity plant community, but the importance of this mechanism depends on the strength of local niche assembly involving competition from some, but not all, dominant guilds.
There has been considerable recent interest in understanding the role of positive inter-specific interactions within ecology, and significant progress has been made both empirically and theoretically. Similarly, considerable progress has been made in improving our understanding of the mechanisms that limit species' ranges. In this contribution, we seek to understand the setting of species' borders when some species within the assemblage exhibit positive inter-specific interactions. We use a spatially explicit dual-lattice simulation model to explore the distribution of different interactions across environmental gradients. We first simulate community dynamics when there is either a gradient in reproductive rate or in mortality. We then consider what happens when gradients in reproduction and mortality run in parallel or perpendicular to one another. If the stress gradient impacts on reproductive potential, positive interactions are found where there is high abiotic stress. In this instance, the mutualists are able to tolerate an environment that the cheaters cannot. However, when the stress gradient influences mortality, we find that the mutualists occur as a stripe surrounded by cheaters both towards the better and the harsher ends of the gradient. Previous theory and most empirical evidence tend to indicate that net positive interactions are likely to occur in environments characterized by high abiotic stress. However, evidence from some stress gradients suggests that the distribution of positive and negative interactions can be more complex, with the most stressful environments being occupied by individuals engaging in negative rather than positive interactions. Our results provide a potential theoretical explanation for these recent field observation, and highlight the need for further theoretical and empirical work to better our understanding of how positive and negative interactions act to determine the limits to species' ranges.
Reproduction and survival are more affected by man's intrusion than by environmental extremes.
The effect of vegetation on the growth of longleaf pine seedlings
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School of Forest Resources and Conservation
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HOPE M. MILLER, JENNIFER M. FILL 1 AND RAELENE M. CRANDALL, School of Forest Resources and
Conservation, University of Florida, Gainesville 32611. Submitted 27 February 2019; Accepted 14 June 2019
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