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

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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.
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Patterns of Longleaf Pine (Pinus palustris)
Establishment in Wiregrass (Aristida beyrichiana)
Understories
Authors: Hope M. Miller, Jennifer M. Fill, and Raelene M.
Crandall
Source: The American Midland Naturalist, 182(2) : 276-280
Published By: University of Notre Dame
URL: https://doi.org/10.1674/0003-0031-182.2.276
Downloaded From: https://bioone.org/journals/The-American-Midland-Naturalist on 17 Oct 2019
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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)
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.
INTRODUCTION
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.
METHODS
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,
respectively.
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).
RESULTS
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).
DISCUSSION
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.
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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
1
Corresponding author: E-mail: jenna999@gmail.com
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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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