Patterns of Longleaf Pine (Pinus palustris) Establishment in Wiregrass (Aristida beyrichiana) Understories.
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 Can' Forest in Gainesville, Florida, which is owned and managed by the University- of Florida (29.73[degrees] N, 82.22[degrees] W). The area is dominated by mesic flatwoods and is maintained with prescribed fires every 2-4 v. It is characterized by large, widely-spaced pine trees (mostly longleaf pine), with some slash pine (Pinus elliottii in wet depressions) in the overston; and wiregrass, saw palmetto (Serenoa repens), gallberry (Gaylussacia dumosa), and resprouting perennial forbs in the understory.
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 = 0.07) or overstory density (P = 0.59). 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 even 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 al 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 lo 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 = 0.003) but not in the 2 y site (P = 0.311) 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 to 1 m) in the 1 y site (P < 0.001). This relationship was not observed in the 2 y site (P = 0.188), 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 = 0.861) or height (P = 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 systems has also shown positive spatial associations of woody and herbaceous species with bunchgrasses (e.g.. Puhlick et at., 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).
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 lo 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, Schizachyiium 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. Submittal 27 February 2019; Accepted 14 June 2019
(1) Corresponding author: E-mail: firstname.lastname@example.org
Caption: 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
Caption: 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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|Title Annotation:||Notes and Discussion Piece|
|Author:||Miller, Hope M.; Fill, Jennifer M.; Crandall, Raelene M.|
|Publication:||The American Midland Naturalist|
|Date:||Oct 1, 2019|
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