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Ecology |
Division of Biology, Kansas State University, Manhattan, Kansas 66506 USA
Received for publication November 7, 2000. Accepted for publication February 27, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: clonal propagation • community assembly dynamics • functional types • pocket gopher mounds • regeneration strategies; • seed rain recruitment • soil seed bank • succession
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Hobbs and Hobbs, 1987
; Goldberg and Gross, 1988
). By disrupting the established plant canopy and altering resource availability, soil disturbances create "regeneration niches" which provide opportunities for less competitive species to become established and coexist (e.g., Grubb, 1977
; Denslow, 1980
; Pickett, 1980
; Shmida and Ellner, 1984
). Consequently, pocket gopher (Geomyidae) mounds often undergo successional dynamics that differ from the undisturbed plant community, and thereby increase community diversity (Inouye et al., 1987a
; Martinsen, Cushman, and Whitham, 1990
; Stromberg and Griffin, 1996
). Plant densities are often decreased on pocket gopher mounds because many individuals are unable to survive being buried (Laycock, 1958
; Hobbs and Mooney, 1985
; Umbanhowar, 1995
), although one might expect that the extensive belowground mass of most tallgrass prairie plant species would facilitate resprouting after burial (Weaver, 1968
; Stanton, 1988
). When burial does decrease ramet densities and cause plant mortality, soil disturbances can provide "safe sites" (sensu Harper, 1977
; Fowler, 1988
) for seedling colonization by creating opportunities for propagules dispersed in the seed rain to germinate. Reduced competition and increased resource availability allow species adapted to ephemeral, unpredictable environments, mostly forbs and annuals, to become established (Grubb, 1977
; Schaal and Leverich, 1982
; Tilman, 1983
; Schmida and Ellner, 1984). The presence of buried, viable seeds brought to the soil surface by burrow excavation and mound deposition is another mechanism by which pocket gopher mounds may be revegetated (Rice, 1989
; Chambers, 1995
). Long-term seed dormancy is a plant life-history strategy that results in the accumulation of viable seeds in a soil seed bank allowing dispersal in time rather than in space (Fenner, 1985
; Baskin and Baskin, 1998
). This can serve as a bet-hedge against local extinction in years of poor seed production or environmental stochasticity (Silvertown, 1988
; Levin, 1990
). Altered resource conditions on a mound disturbance could promote the germination of dormant seeds in the soil seed bank (Rice, 1989
; Chambers, 1995
; Baskin and Baskin, 1998
). Lateral spread through clonal propagation of neighboring plants can be the primary mechanism responsible for the revegetation of soil disturbances in some grassland habitats (Collins, 1989
; Hartnett and Fay, 1998
). Vegetative regrowth from roots and rhizomes of buried plants or nearby undisturbed plants can rapidly refill space created by small mounds, thus preventing the less competitive species from establishing on mound disturbances (Laycock, 1958
; Foster and Stubbendieck, 1980
; Goldberg and Gross, 1988
; Gibson, 1989
). Revegetation of soil disturbances by a combination of sexual and asexual reproduction strategies is likely to have profound effects on the maintenance of species and genetic diversity within plant populations (e.g., Harper, 1977
; Silvertown, Franco, and Harper, 1997
) and influence the manner and rate of plant community assembly (e.g., Connell and Slayter, 1977
; Huston, 1994
).
Patch size is another important characteristic affecting the recolonization and successional dynamics of a soil disturbance (Denslow, 1980
; Miller, 1982
; McConnaughay and Bazzaz, 1987
; Coffin and Laurenroth, 1988
). Pocket gopher mounds tend to exhibit a clumped spatial distribution (Andersen, 1987
; Moloney et al., 1992
; Benedix, 1993
; Klaas, Moloney, and Danielson, 2000
) where numerous small mounds that are highly aggregated function equivalently to a larger disturbance (Loucks, Plumb-Mentjes, and Rogers, 1985
). Individual plants establishing on large soil mounds often experience a higher degree of mortality from desiccation (Schaal and Leverich, 1982
; Hobbs and Mooney, 1985
) and herbivory (Huntly and Inouye, 1988
; Reichman, 1988
), but less competition and greater resource availability allow those that survive to grow faster and achieve a larger size than plants growing in undisturbed areas (Reichman, 1988
; Peart, 1989
; Davis et al., 1991
).
While many studies have focused on the patterns of local colonization and successional recovery of vegetation on soil disturbances (e.g., McConnaughay and Bazzaz, 1987
; Coffin and Laurenroth, 1988
; Huntly and Inouye, 1988
; Gibson, 1989
; Carson and Pickett, 1990
; Rogers, Hartnett, and Elder, 2001
), few have sought to understand the mechanisms and demographic processes responsible for recolonization. We conducted an experiment designed to simulate pocket gopher disturbances present in a Kansas tallgrass prairie in order to evaluate the potential contribution of various plant recolonization mechanisms. Specifically, we wanted to assess the potential contributions of seed rain recruitment (SDRAIN), soil seed bank (SDBANK), vegetative spread via clonal propagation (CLONAL), and regrowth of buried plants (REGROW) in revegetating different-sized soil mounds. Another goal of the study was to understand the temporal dynamics of the various colonists (classified according to both species and functional types) by isolating the processes responsible for the successional patterns observed on disturbances. We expected large SDRAIN and SDBANK mounds to provide greater opportunities for successful germination and establishment of a high diversity of annual and subdominant forbs, while small CLONAL and REGROW mounds were more likely to be rapidly revegetated by a lower diversity of nearby undisturbed perennial plant species. Because SDRAIN and SDBANK are forms of sexual reproduction while CLONAL and REGROW are asexual mechanisms, the manner in which disturbances are revegetated is likely to have important implications for maintaining genetic diversity in addition to species diversity.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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). A highly diverse mixture of other species includes warm-season and cool-season graminoids, composites, legumes, and numerous other forbs and woody shrubs (Freeman, 1998
). The residual soils are Chase silt loams and silty clay loams derived from Permian limestones, shales, and cherty limestones. Soil depth and plant community composition are important determinants of pocket gopher distribution on the Konza Prairie, but immigration constraints appear to prohibit their occupation of all suitable sites (Benedix, 1993
). This experiment was conducted on a deep soil bench in a Konza Prairie watershed that is edaphically and floristically similar to areas where pocket gophers are found, but none have been present in the area for the past decade (J. Reichman, personal communication, University of California-Santa Barbara). The study site was burned annually in the spring during this experiment.
Experimental design
Artificial mounds were created in May 1996, following burning of the study site, when levels of pocket gopher activity have been shown to naturally increase (Cameron et al., 1988
; Reichman and Smith, 1990
; Benedix, 1993
). A 0.2-ha area was divided into 3 x 3 m grids and every other grid cell was randomly assigned a mound size with one of four experimental treatments (Table 1). The treatments were designed to ascertain potential contribution of the following mechanisms of recolonization: seed rain recruitment (SDRAIN), vegetative spread via clonal propagation (CLONAL), soil seed bank (SDBANK), and survival and regrowth of buried plants (REGROW). Soil cores with 20 cm (small mound) or 60 cm (large mound) diameters were excavated to a depth of 25 cm. These disturbance sizes were chosen to represent a single gopher mound and a cluster of mounds, respectively (Benedix, 1993
; Rogers, 1998
). Completely intact soil cores were removed using appropriate-sized cylindrical coring devices fashioned out of sharpened steel. Steel flashing was used around the outside edges of SDRAIN, SDBANK, and REGROW mounds to prevent lateral clonal propagation of tillers from entering the soil mounds. Although still within the rooting zone of many tallgrass prairie plants, these excavations were sufficiently deep to exclude clonal propagation from nearby roots or rhizomes (Weaver, 1968
; Stanton, 1988
). For the REGROW mounds, excavated soil cores with aboveground vegetation intact were replaced after the steel flashing was installed. Excavated soil cores from the other experimental mounds were discarded, and soil for refilling the excavations and creating the mounds was collected from excavated burrows of a nearby Konza Prairie pocket gopher population. These burrows were 
10–20 cm below the soil surface. The collected soil was passed through a 1-cm2 steel-wire mesh sieve that was small enough to exclude roots and rhizomes, but large enough to allow passage of seeds. Sieving produced a soil texture that was very similar to that of naturally created pocket gopher mounds (W. E. Rogers, personal observation). Soil for the SDRAIN, CLONAL, and REGROW mounds was steam sterilized (90°C) to eliminate potentially viable seeds. The steam equipment used has proven effective in completely sterilizing soil for other experiments (G. Wilson, personal communication, Kansas State University). Unsterilized soil was used for creating SDBANK mounds in order to preserve the viability of the soil seed bank. The SDRAIN mounds included only plants that established via aerial seed dispersal (wind or animal), however, no means of effectively excluding the seed rain from the other experimental treatments was logistically feasible. Consequently, successional dynamics on all treatments were likely affected by the seed rain, and the SDRAIN mounds served as an experimental disturbance control for the other recolonization mechanisms. Although the inability to exclude the seed rain from the other mounds hinders our interpretation of the data, any instance where the SDBANK, CLONAL, or REGROW mounds statistically differ from the SDRAIN mounds implies an explicit contribution from that treatment. Soil was added until a mound was 10 cm above ground level. This is the depth of a typical pocket gopher mound found on Konza Prairie (Benedix, 1993
; W. E. Rogers, personal observation). During the subsequent week, additional soil was added to the mounds, as settling occurred, in order to maintain the 10 cm height. All experimental treatments were completed by the end of May. Each recolonization treatment was replicated nine times for both small (N = 36) and large (N = 36) mounds. Nine undisturbed 20 cm diameter and nine 60 cm diameter control plots (CONTRL) were also randomly assigned to the remaining grids. Because we were already monitoring many natural and simulated pocket gopher mounds for other experiments (Rogers, 1998
; Rogers, Hartnett, and Elder, 2001
; Rogers and Hartnett, 2001
), we did not create an additional disturbance control subject to all recolonization mechanisms explicitly for this study.
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). The functional types were C3 graminoids, C4 midgrasses, C4 tallgrasses, C4 annual grasses, annual forbs, perennial forbs, and woody plants. Additionally, inclusive monocot and dicot categories were used to account for newly emerged seedlings that could not be assigned to a more specific classification. Only one species of forb in our study was monocotyledonous, Sisyrinchium campestre; all other forbs recorded were dicotyledonous. All other monocots were graminoids. At the end of the second growing season, but before senescence had begun, aboveground vegetation was clipped on all small and large mounds and in control plots. Clipped vegetation was separated into graminoids and forbs. There were no woody plants present on the mounds at the end of the experiment. The vegetation was dried for 72 h at 60°C and weighed.
Data analysis
The data were examined for normality, and statistical analyses for the stem and species density data were performed using a repeated-measures analysis of variance (SAS, 1998
). The stem density, species density, and vegetation mass measured in September 1997 were analyzed with a standard ANOVA, and pairwise comparisons of the treatments were calculated within each mound size using Fisher's protected least significant difference test (SAS, 1998
). Values are reported as means ± 1 SE, and significance levels for all statistical tests is 
< 0.05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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Although the total number of species observed on large mounds (49 species) was greater than the total number of species found on small mounds (36 species), species density, like stem density, was significantly greater on small than large mounds (Table 2). Plant species densities were also significantly different among the recolonization treatments, but again there was no statistical interaction between mound size and recolonization treatment (Table 2). Species density was higher on REGROW mounds compared to other recolonization treatments early in the experiment, but by September 1996 species density on CLONAL and SDBANK mounds began increasing markedly (Fig. 2). While species density remained high on CLONAL mounds throughout the following growing season, species density on small SDBANK mounds decreased in June 1997 (Fig. 2A). Species density on SDRAIN mounds tended to be lower than the other recolonization treatments early in the experiment, but species density on small SDRAIN mounds was not significantly different than small SDBANK and REGROW mounds during much of 1997 (Fig. 2A). Species density on large SDRAIN mounds was lower than other large recolonization treatments for much of the first two growing seasons (Fig. 2B). Small CLONAL mounds had the highest species density of the recolonization treatments throughout much of 1997 (Fig. 2A). At the end of the experiment, species density on CLONAL mounds was higher than other recolonization treatments within their respective mound sizes (Table 3). Species density was not measured on CONTRL plots until the final data collection. After two growing seasons, only species densities on CLONAL mounds were similar to undisturbed vegetation (Table 3).
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Temporal dynamics of plant functional types
The percentage change of functional types on small and large mounds with time was striking among the different recolonization treatments (Figs. 3, 4). Small SDRAIN mounds contained mostly dicot seedlings during the first growing season, the majority of which died. Surprisingly, by the end of the experiment in September 1997, C3 graminoids and C4 midgrasses represented >80% of the functional types present on small SDRAIN mounds, while perennial and annual forbs accounted for <20% of the final composition (Fig. 3A). Small CLONAL mounds were bare during June and July 1996, but perennial C3 graminoids and forbs established by August 1996. By the end of the experiment, small CLONAL mounds were dominated by perennial C3 graminoids, C4 mid- and tallgrasses, and perennial forbs (Fig. 3B). Small SDBANK mounds contained mainly C3 graminoids at the beginning of the experiment (Fig. 3C). Dicot seedling germination was considerable in September and October 1996, though these suffered high winter mortality and decreased further in response to spring burning and low rainfall during May and June 1997 (50 and 85 mm below average, respectively). Many of these dicot seedlings were recruited into the annual and perennial forb categories and the contribution of perennial C3 graminoids and C4 midgrasses remained high throughout 1997 (Fig. 3C). SDBANK mounds had more functional types appear and disappear than any other recolonization treatment. Small REGROW mounds had the most consistent presence of particular functional types throughout the experiment (Fig. 3D). Perennial C3 graminoids, C4 mid- and tallgrasses, and perennial forbs were all present throughout 1996. A flush of dicot seedlings occurred in September and October 1996, but declined and disappeared in May and June 1997. Although most of these dicot seedlings died, several were recruited into the annual and perennial forb categories during May, June, and July 1997. By the end of the experiment, graminoids contributed >80% of the functional types on small REGROW mounds (Fig. 3D).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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). This has important effects on multiple stages of plant growth, development, and competitive interactions and supports other theoretical and empirical studies suggesting that large disturbances fill more slowly than small disturbances and provide increased time for colonization by a variety of species (e.g., Grubb, 1977
; Denslow, 1980
; Miller, 1982
; McConnaughay and Bazzaz, 1987
). Although this was confirmed in our study by higher seed germination, particularly of forbs, on many large mounds, the increase appears to be temporary. Because high seed germination was offset by high seedling mortality, vegetative regrowth strategies were the dominant recolonization mechanisms on both large and small disturbances. Nevertheless, as evidenced by the greater vegetation mass, particularly of forbs, on large mounds many of the surviving seedlings grew vigorously and significantly influenced plant community structure. Our results indicate that multiple vegetative and sexual reproductive mechanisms are potentially important for recolonizing different-sized soil disturbances and likely concurrently, albeit differentially, contribute to the maintenance of diversity in tallgrass prairies.
Despite many individuals establishing via seed rain recruitment, our expectation that SDRAIN mounds would support a high diversity of annuals and subdominant forbs was not supported. The low diversity and similarity of plant species present on the soil mounds to the species in the undisturbed plant community was surprising. None of the nonnative weeds commonly found in disturbed areas on Konza Prairie (Freeman, 1998
) were observed on the soil mounds. The scarcity of invasive, weedy species on these soil mounds is likely due to a combination of low survivorship and lack of propagules in this relatively large, intact tallgrass prairie landscape. Recruitment limitation can play an important role in influencing plant population dynamics and community structure in a variety of ecosystems (e.g., Inouye et al., 1987b
; Glenn and Collins, 1992
; Hurtt and Pacala, 1995
; Tilman, 1997
). Moreover, the invasion, establishment, and continued presence of many ruderal plant species often require recurring soil disturbances characteristic of an area with an active pocket gopher population (Hobbs and Hobbs, 1987
; Huntly and Inouye, 1988
; Carson and Pickett, 1990
; Reader and Buck, 1991
).
Similar to large mounds, there was a decrease in plant stem density on many small mounds in early 1997. This was likely due to the combined effects of phenological shifts, negative effects of fire on cool-season species, drought events, and competitive exclusion (Rabinowitz and Rapp, 1985a, b
; Abrams, 1988
; Reader and Buck, 1991
; Huntly and Reichman, 1994
). However, species density was not as greatly affected as stem density by these mortality events, and many new species established on both large and small mounds. This was particularly evident by the recruitment of numerous dicot seedlings on SDBANK mounds. Buried, viable seeds of some plant species can be triggered to germinate by altered microclimatic conditions and increased resource availability on soil disturbances (Rice, 1989
; Baskin and Baskin, 1998
). These altered conditions often increase opportunities for species that are unable to germinate and establish beneath the undisturbed plant canopy (Platt, 1975
; Grubb, 1977
; Inouye et al.,1987a
; Carson and Pickett, 1990
). Increases in stem and species densities on SDBANK mounds were greater than increases on SDRAIN mounds, thereby indicating that many seeds of a variety of species were being released from the soil seed bank, but not colonizing via the seed rain.
Total-plant mass on SDRAIN and SDBANK mounds tended to be lower than on CLONAL and REGROW mounds and CONTRL plots because new seedlings often have lower initial growth rates relative to vegetative recruits that are provisioned with metabolic reserves via the parental rhizome system (Harper, 1977
; Bazzaz, 1996
). Total-plant mass per unit area was significantly greater on large compared to small mounds. This was primarily due to the large size of individual forbs (W. E. Rogers, personal observation), a frequently observed result of reduced competition and increased resource availability on large disturbances (Reichman, 1988
; Peart, 1989
; Davis et al., 1991
). Additionally, the percentage of total mass that was forbs was significantly greater on large mounds than small mounds. This effect did not occur in large control plots, however.
Some recolonization treatments were surprisingly less affected by mound size than we expected. The dominant C4 grasses did not refill space on small CLONAL mounds rapidly enough to prevent the establishment of other functional types. Mound size also influenced vegetation responses to fire. Many of the individuals growing on large mounds were protected from the negative effects of spring burning. The seedlings centered on large mounds had less dead mass from the previous growing season in close proximity, which reduced the fire intensity at the center of the mound (W. E. Rogers, personal observation). Small mounds were not protected in the same manner and suffered greater seedling, and likely seed (Abrams, 1988
), mortality due to burning.
Soil deposited on intact vegetation decreased stem and species densities compared to undisturbed plots, but, contrary to other findings (Laycock, 1958
; Hobbs and Mooney, 1985
; Umbanhowar, 1995
), many individuals were resilient enough to survive burial. The substantial belowground mass, root plasticity, and interconnected ramets of many tallgrass prairie plant species, particularly the C4 tallgrasses, allowed individuals to regrow through the soil mounds (Weaver, 1968
; Stanton, 1988
). As observed from REGROW mounds and other field experiments (Rogers, Hartnett, and Elder, 2001
; Rogers and Hartnett, 2001
), the regrowth of buried vegetation appears to be the dominant mechanism contributing to recolonization of naturally created pocket gopher disturbances in tallgrass prairie. Vegetative regrowth from the roots and rhizomes of buried plants can rapidly refill space created by small soil disturbances (Laycock, 1958
; Foster and Stubbendieck, 1980
; Goldberg and Gross, 1988
; Gibson, 1989
). Plants on the large REGROW mounds initially suffered greater mortality due to burial, but plants that survived grew vigorously, likely as a result of increased resources and decreased competition (Reichman, 1988
; Peart, 1989
; Martinsen, Cushman, and Whitham, 1990
; Davis et al., 1991
). Clonal propagation of plants adjacent to disturbances will further accelerate recolonization as evidenced by the CLONAL mounds. Since the dominant grasses in this system reproduce primarily via vegetative means (Hartnett and Fay, 1998
), a significant proportion of the species recolonizing disturbances in the tallgrass prairie are likely to be those that are currently the most abundant. The paucity of ruderal species is further reinforced by the surrounding regional landscape being almost entirely intact tallgrass prairie. Although important to the recolonization of these soil mounds, the soil seed bank and seed rain would be expected to exert a greater influence in a more fragmented or heavily disturbed ecosystem.
Species density and vegetation mass on natural gopher mounds are statistically indistinguishable from undisturbed areas after 2 yr (Rogers, Hartnett, and Elder, 2001
). In contrast, our experimental disturbances remained distinct from each other and the adjacent undisturbed vegetation after two growing seasons. While this may be an artifact of our experimental design, it seems equally plausible that multiple mechanisms of recolonization are concurrently contributing to the revegetation of naturally created mounds. This increases the rate at which disturbances undergo successional transitions and recovery in tallgrass prairies. Fast rates of competitive displacement, in part due to multiple mechanisms of recolonization and establishment, may decrease coexistence among plant species and thus diminish overall community diversity (e.g., Tilman, 1988
; Huston, 1994
). The resilience of tallgrass prairie plant communities to disturbance limits the persistence of less competitive species (e.g., Gibson, 1989
; Collins and Wallace, 1990
; Knapp et al., 1998
). Nevertheless, the combined effects of sexual (SDRAIN and SDBANK) and asexual (CLONAL and REGROW) reproductive mechanisms are likely to significantly contribute to the genetic diversity of grassland vegetation (Harper, 1977
), despite the relatively transient effects of soil disturbances on plant species diversity (Gibson, 1989
; Carson and Pickett, 1990
; Umbanhowar, 1995
; Rogers, Hartnett, and Elder, 2001
).
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FOOTNOTES |
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2 Author for reprint requests, current address: Department of Ecology and Evolutionary Biology, Rice University, 6100 Main Street, Houston, Texas 77005 USA (wer{at}rice.edu
).
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