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Patterns of Longleaf Pine (Pinus palustris) Establishment in Wiregrass (Aristida beyrichiana) Understories



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.
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Patterns of Longleaf Pine (Pinus palustris)
Establishment in Wiregrass (Aristida beyrichiana)
Authors: Hope M. Miller, Jennifer M. Fill, and Raelene M.
Source: The American Midland Naturalist, 182(2) : 276-280
Published By: University of Notre Dame
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Am. Midl. Nat. (2019) 182:276–280
Notes and Discussion Piece
Patterns of Longleaf Pine (Pinus palustris) Establishment in Wiregrass (Aristida beyrichiana)
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.
Plant community structure is influenced by interactions among plants of different species, life stages,
and growth forms (Bertness and Callaway, 1994; Harrington, 2006). Over time and across space, plants
can affect one another via aboveground (e.g., shading, fire behavior) or belowground mechanisms (e.g.,
allelopathic chemicals released from roots, competition for water or nutrients) (Brooker et al., 2008).
These interactions have the potential to affect changes in population and community characteristics,
such as plant growth rates, survival, fecundity, and horizontal and spatial distribution (Travis et al., 2006;
Crandall and Knight, 2018; Fill et al., 2019).
Despite a historical emphasis on competition as a driving mechanism of plant community dynamics,
the role of facilitative interactions has gained increasing recognition (Bertness and Callaway, 1994;
Brooker et al., 2008). Through facilitation, one plant can reduce physical stress or consumer pressure on
another, thereby increasing the other’s survival, growth, or fitness. One common facilitative interaction
involves positive spatial associations between adults of one species and seedlings of another, termed the
‘‘nurse plant syndrome’’ (Niering et al., 1963). Nurse plants, such as perennial grasses or shrubs, can
facilitate the survival or growth of seedlings of another species through different processes, such as
shading and nutritive litter deposition below the canopy (Callaway et al., 1991; Kellman, 1984).
Facilitation among plants appears common in harsh or stressful environments, such as deserts (Bertness
and Callaway, 1994; Greenlee and Callaway, 1996), although it has been shown to occur in many
different biomes worldwide, including tropical forests (Holmgren et al., 1997).
We investigated the potential of an endemic, perennial bunchgrass to function as a nurse plant in
pine savannas of the southeastern U.S.A. In these fire-maintained ecosystems, wiregrass (Aristida stricta/
beyrichiana) is a perennial bunchgrass that dominates many groundcover plant communities across a
broad gradient of xeric to mesic habitats. There is evidence that microclimates near wiregrass individuals
are more favorable to herbaceous species establishment (Iacona et al., 2012). We hypothesized that
wiregrass facilitates seedling establishment of the dominant canopy tree, longleaf pine (Pinus palustris).
A positive spatial association of pine seedlings with wiregrass plants would indicate that wiregrass could
be functioning as a nurse plant for longleaf pine seedlings in pine savanna communities.
We conducted this study at the 2040-acre Austin Cary Forest in Gainesville, Florida, which is owned
and managed by the University of Florida (29.738N, 82.228W). The area is dominated by mesic
flatwoods and is maintained with prescribed fires every 2–4 y. It is characterized by large, widely-spaced
pine trees (mostly longleaf pine), with some slash pine (Pinus elliottii in wet depressions) in the overstory,
and wiregrass, saw palmetto (Serenoa repens), gallberry (Gaylussacia dumosa), and resprouting perennial
forbs in the understory.
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Wiregrass individuals were sampled in two management sites on different prescribed fire schedules.
Both sites are mesic flatwoods pine savannas dominated by longleaf pine, and do not differ in size (P ¼
0.07) or overstory density (P ¼0.59). One site sampled has been burned annually during the wet (i.e.,
mid-growing) season since 1978. The second site has been similarly burned during the wet season, but
only every 2 to 3 y since at least the early 1980s. We refer to these as the 1 y site and 2 y site, respectively.
Prior to sampling, the 1 y and 2 y sites were last burned in prescribed fires in July 2017 and June 2016,
To quantify spatial patterns of longleaf pine seedling establishment relative to wiregrass individuals at
different times since fire, we counted and measured pine seedlings present at increasing distances away
from 60 haphazardly-chosen wiregrass bunches in each management site. Wiregrass individuals were
selected from those tagged in an ongoing demographic study that includes plants across a range of sizes
with no minimum or maximum. The size of each wiregrass individual was determined by measuring the
horizontal length and width of each bunch and calculating its basal area as the area of an ellipse, which
is correlated with number of leaves.
We used a 0.33 m x 1 m PVC frame that was divided into three connected 0.33 m x 0.33 m squares.
The first 0.33 m x 0.33 m square of the frame was centered over a focal wiregrass individual, which
included the bunch itself and the area immediately around the bunch. The second square was
immediately adjacent to the first with the two squares sharing a PVC boundary. This second square often
included shade from drooping wiregrass leaves and other young grasses or forbs but was not directly
under a wiregrass plant. The third square immediately followed the second and, unless the focal
wiregrass individual was very large, this square had no direct contact with the grass’s leaves or base. This
square was also not directly adjacent to other wiregrass plants, although they could be nearby.
Care was taken to avoid sampling the effects of more than one wiregrass individual. The direction in
which the frame was extended away from each wiregrass individual was randomized each time by
choosing a random degree on a compass. We selected the first random direction to ensure the frame
would not directly contact another wiregrass bunch. To avoid simultaneously observing the effects of
more than one wiregrass individual, neighboring wiregrass was avoided as much as possible, leaving a
minimum of 10 cm between the edge of the frame and a neighboring bunch. We tallied the number of
pine seedlings present within each square and recorded their heights. We only observed and counted
seedlings (i.e., individuals) that had not yet reached the grass stage. Pines were identified as longleaf
seedlings based on the species’ distinctive cotyledon stage.
Because management sites were not replicated, each management site was analyzed separately with
individual bunchgrasses as the units of replication. The number and height of seedlings were compared
within sites across distances from wiregrass bunches using ANOVAs and Tukey pairwise comparisons.
The relationship between wiregrass size and pine seedling number and size in or under wiregrass (i.e., 0
to 0.33m) was determined in each site separately using a linear regression. All analyses were conducted
using R Statistics (R Core Development Team, 2018).
We observed a significant effect of proximity to wiregrass in the 1 y site but not in the 2 y site. Pine
seedlings were significantly more numerous in and under wiregrass individuals (0 to 0.33 m) as
compared to farther away in the 1 y site (P ¼0.003) but not in the 2 y site (P ¼0.311) for which pine
seedlings were rare regardless of distance from wiregrass (Fig. 1). Furthermore, pine seedlings were
significantly taller in and under wiregrass (0 to 0.33 m) as compared to further away (0.34 to 0.66 m and
0.67 to1 m) in the 1 y site (P ,0.001). This relationship was not observed in the 2 y site (P ¼0.188), once
again likely because of the low number of pine seedlings at this site (Fig. 2). Wiregrass size (i.e., basal
area) was not associated with differences in either pine seedling number (P ¼0.861) or height (P ¼
0.930) in or under the wiregrass individuals (0 to 0.33 m).
Despite the potential for competition between wiregrass and pine seedlings, our findings suggest
wiregrass might function as a nurse plant for pine seedlings during fire-free periods. Research in other
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systems has also shown positive spatial associations of woody and herbaceous species with bunchgrasses
(e.g., Puhlick et al., 2012, Greenlee and Callaway, 1996). Iacona et al. (2012) documented microsite
conditions near wiregrass could potentially promote pine seedling establishment and growth in that xeric
sites exhibited higher relative humidity below wiregrass clumps (suggesting greater moisture availability)
than away from wiregrass, and mesic sites exhibited lower soil temperature under wiregrass tussocks (but
no differences in relative humidity). Although light levels were lower under wiregrass clumps (Iacona et al.,
2012), light availability might be less important for pine seedling growth than other seedling
requirements, such as nitrogen and water (Jose et al., 2003). Over time, the patterns we documented
could change, if bunchgrasses compete with pine seedlings during later life stages as the pine seedling
and/or grass grows (Wood and del Moral, 1987; Kellman and Kading, 1992; Callaway and Walker, 1997).
FIG. 1.—Average number of pine seedlings immediately in or under wiregrass individuals (0 to 0.33 m)
and at increasing distances where the influence of wiregrass likely decreases (0.34 to 0.66 m and 0.67 to
1 m) in two management sites with different times-since-fire. One site is burned annually (1 y) and the
other is burned every 2–3 y. These sites were last burned in July 2017 and June 2016, respectively
FIG. 2.—Average height (cm) of pine seedlings immediately in or under wiregrass individuals (0 to
0.33 m) and at increasing distances where the influence of wiregrass likely decreases (0.34 to 0.66 m and
0.67 to 1 m) in two management sites with different times-since-fire. One site is burned annually (1 y)
and the other is burned every 2–3 y. These sites were last burned in July 2017 and June 2016, respectively
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The spatial associations we documented could result from other mechanisms as well, such as the time
since the last fire or interactions with other plant species in the community (Watson and Wardell-
Johnson, 2008; Myers and Harms, 2009). We only found evidence for facilitation one year after fire,
which suggests this relationship could diminish as time since the last fire increases and woody species
become larger, generating shadier and possibly more competitive conditions (Heisler et al., 2003). This
relationship might also be detrimental to pines when fires are very frequent given longleaf pine
seedlings (i.e., before grass stage begins) are sensitive to fire, and wiregrass is known to be highly
flammable and promote fire spread (Fill et al., 2015, 2016). Therefore, we predict this spatial
relationship will not continue over the long-term, resulting from fire-caused mortality or competition
with other species or growth forms, such as woody shrubs or overstory trees (Mugnani et al., 2019;
Robertson et al., 2019). Pessin (1938) and Pessin and Chapman (1944) suggested competition between
longleaf pine seedlings and other grass species (e.g., little bluestem, Schizachyrium scoparium) has a strong
effect on early seedling growth. In addition, belowground interactions, including mycorrhizal symbionts
and rooting patterns (van der Heijden et al., 1998; Crawford et al., 2019), could also influence pine and
wiregrass dynamics over time, but such possible mechanisms have not been studied in this context.
Bunchgrasses are a dominant component of the groundcover community in pine savannas around the
world (Myers and Rodriguez-Trejo, 2009). We have documented the potential for bunchgrasses to
facilitate the establishment of pines, which should shed light on tree-grass coexistence in fire-frequented
pine savannas (Scholes and Archer, 1997). Understanding the role of wiregrass in the longleaf pine
ecosystem, including species-specific interactions at the microhabitat scale, should benefit the
restoration and management of these ecosystems as a whole.
Acknowledgments.—We appreciate funding to R. M. Crandall from the University of Florida
Foundation, Inc. Gage LaPierre and Javier Salazar Castro helped collect data in the field.
BERTNESS,M.D.AND R. CALLAWAY. 1994. Positive interactions in communities. Trends Ecol. Evol., 9:191–193.
¨RGER,J.M.TRAVIS,AND C. ARMAS. 2008. Facilitation in plant communities: the past, the
present, and the future. J. Ecol., 96:18–34.
CALLAWAY,R.M.AND L. R. WALKER. 1997. Competition and facilitation: a synthetic approach to
interactions in plant communities. Ecology,78:1958–1965.
———, N. M. NADKARNI,AND B. E. MAHALL. 1991. Facilitation and interference of Quercus douglasii on
understory productivity in central California. Ecology,72:1484–1499.
CRANDALL,R.M.AND T. M. KNIGHT. 2018. Role of multiple invasion mechanisms and their interaction in
regulating the population dynamics of the exotic tree Ailanthus altissima. J. Appl. Ecol., 55:885–
where plant-soil feedback may promote plant coexistence: a meta-analysis. Ecol. Lett., doi:10.
FILL, J. M., W. J. PLATT,S.M.WELCH,J.L.WALDRON,AND T. A. MOUSSEAU. 2015. Updating models for
restoration and management of fiery ecosystems. For.Ecol. Manage., 356:54–63.
———, B. M. MOULE,J.M.VARNER,AND T. A. MOUSSEAU. 2016. Flammability of the keystone savanna
bunchgrass Aristida stricta.Plant Ecol., 217:331–342.
———, E. PEARSON,T.M.KNIGHT,AND R. M. CRANDALL. 2019. An invasive legume increases perennial grass
biomass: An indirect pathway for plant community change. PloS one, 14:e0211295.
GREENLEE,J.T.AND R. M. CALLAWAY. 1996. Abiotic stress and the relative importance of interference and
facilitation in montane bunchgrass communities in western Montana. The Am. Nat., 148:386–
HARRINGTON, T. B. 2006. Plant competition, facilitation, and other overstory-understory interactions in
longleaf pine ecosystems, p.135–156. In: Jose, S., Jokela, E. J., and D. L. Miller, editors. The
Longleaf Pine Ecosystem: Ecology, Silviculture, and Restoration. Springer, Berlin, Heidelberg.
Downloaded From: on 17 Oct 2019
Terms of Use: Access provided by University of Florida
HEISLER, J. L., J. M. BRIGGS,AND A. K. KNAPP. 2003. Long-term patterns of shrub expansion in a C4-
dominated grassland: fire frequency and the dynamics of shrub cover and abundance. Am. J.
Bot., 90:423–428.
HOLMGREN, M., M. SCHEFFER,AND M. A. HUSTON. 1997. The interplay of facilitation and competition in
plant communities. Ecology,78:1966–1975.
IACONA, G. D., L. K. KIRKMAN,AND E. M. BRUNA. 2012. Experimental test for facilitation of seedling
recruitment by the dominant bunchgrass in a fire-maintained savanna. PloS one,7:e39108.
JOSE, S., S. MERRITT,AND C. L. RAMSEY. 2003. Growth, nutrition, photosynthesis and transpiration
responses of longleaf pine seedlings to light, water and nitrogen. For.Ecol. Manage., 180:335–
KELLMAN, M. 1984. Synergistic relationships between fire and low soil fertility in neotropical savannas: a
hypothesis. Biotropica,16:158–160.
——— AND M. KADING. 1992. Facilitation of tree seedling establishment in a sand dune succession. J. Veg.
Sci., 3:679–688.
MUGNANI, M. P., K. M. ROBERTSON,D.L.MILLER,AND W. J. PLATT, 2019. Longleaf pine patch dynamics
influence ground-layer vegetation in old-growth pine savanna. Forests,10:389.
MYERS,J.A.AND K. E. HARMS. 2009. Local immigration, competition from dominant guilds, and the
ecological assembly of high-diversity pine savannas. Ecology,90:2745–2754.
´GUEZ-TREJO. 2009. Fire in tropical pine ecosystems, pp. 557–605. In:M.A.
Cochrane, editor. Tropical Fire Ecology. Springer, Berlin, Heidelberg.
NIERING W. A., R. H. WHITTAKER,AND C. H. LOWE. 1963. The saguaro: a population in relation to
environment. Science,142:15–23.
PESSIN, L. J. 1938. The effect of vegetation on the growth of longleaf pine seedlings. Ecol. Mono., 8:115–
PESSIN,L.J.AND R. A. CHAPMAN. 1944. The effect of living grass on the growth of longleaf pine seedlings in
pots. Ecology,25:85–90.
PUHLICK, J. J., D. C. LAUGHLIN,AND M. M. MOORE. 2012. Factors influencing ponderosa pine regeneration
in the southwestern USA. For.Ecol. Manage., 264:10–19.
ORE DEVELOPMENT TEAM. 2018. R: A Language and Environment for Statistical Computing. Software
version 3.5.0. R Foundation for Statistical Computing, Vienna.
ROBERTSON, K. M., W. J. PLATT,AND C. E. FAIRES, 2019. Patchy Fires Promote Regeneration of Longleaf
Pine (Pinus palustris Mill.) in Pine Savannas. Forests,10:367.
SCHOLES,R.J.AND S. R. ARCHER. 1997. Tree-grass interactions in savannas. Annu. Rev. Ecol. Syst., 28:517–
TRAVIS, J. M. J., R. W. BROOKER,E.J.CLARK,AND C. DYTHAM . 2006. The distribution of positive and negative
species interactions across environmental gradients on a dual-lattice model. J. Theor. Biol.,
VAN DER HEIJDEN, M. G. A., T. BOLLER,A.WIEMKEN,AND I. R. SANDERS . 1998. Different arbuscular
mycorrhizal fungal species are potential determinants of plant community structure. Ecology,
WATSON,P.AND G. WARDELL-JOHNSON. 2008. Fire frequency and time-since-fire effects on the open-forest
and woodland flora of Girraween National Park, south-east Queensland, Australia. Austral Ecol.,
WOOD, D.M. AND R. DEL MORAL. 1987. Mechanisms of early primary succession in subalpine habitats on
Mount St. Helens. Ecology,68:780–790.
AND RAELENE M. CRANDALL, School of Forest Resources and
Conservation, University of Florida, Gainesville 32611. Submitted 27 February 2019; Accepted 14 June 2019
Corresponding author: E-mail:
Downloaded From: on 17 Oct 2019
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... 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. ...
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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.
... & 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). ...
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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]. ...
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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.