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Ecology |
2Department of Plant Biology, Arizona State University, Tempe, Arizona, 85287-1601 USA; 3Division of Biology, Kansas State University, Manhattan, Kansas, 66506-4901 USA
Received for publication August 6, 2002. Accepted for publication October 31, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: C4 grassland • fire frequency • growth form substitution • shrubs • woody encroachment
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Knight et al., 1994
; Archer, 1995
; Hoch and Briggs, 1999
; Roques et al., 2001
; Briggs et al., 2002
; Hoch et al., 2002
). In North American grasslands, this substitution in growth forms is driven largely by native, rather than introduced, woody species whose expansion has paralleled a reduction in cover by native grasses (Scholes and Archer, 1997
; Van Auken, 2000
). The search for causal factors of this shift has often included disturbance and its influence on plant community structure and ecosystem function (Schlesinger et al., 1990
). While changes in land-use practices such as grazing and reductions in fire frequency are considered proximate causes, atmospheric CO2 enrichment, climate change, and N deposition are potentially important contributing factors as well (Archer et al., 2001
). In contrast to the disturbance regime under which these ecosystems evolved, these perturbations are chronic and may lead to long-term vegetation changes (Schlesinger et al., 1990
).
The C4-dominated grasslands of central North America are characterized as a tension zone between dominance by grass vs. woody growth forms (Axelrod, 1985
). In these grasslands, moisture availability, on average, is considered sufficient to support both growth forms, and fire frequency is instrumental in determining the relative abundance of each (Gibson and Hulbert, 1987
; Anderson, 1990
; Hartnett and Fay, 1998
; Knapp and Seasedt, 1998
). Historically, the estimated fire return interval in the tallgrass prairie was approximately once every 3–5 yr (Wright and Bailey, 1982
; Knapp and Seastedt, 1998
), but little is known of the dynamics of native grass and woody species prior to European settlement. While frequent fire enhances the productivity of the warm-season C4 grasses, in its absence, litter accumulates, grass production declines, and woody plants (all of which are C3 species) increase in both distribution and abundance (Bragg and Hulbert, 1976
; Abrams et al., 1986
; Knapp and Seastedt, 1986
; Knapp et al., 1998
; Briggs and Knapp, 2001
). As such, fire suppression has been implicated as a key factor responsible for the expansion of woody vegetation. An extreme example of this shift from graminoid to woody dominance has occurred in northeastern Kansas, where changes in land management have resulted in the conversion of grasslands to juniper forest (Norris et al., 2001
; Hoch et al., 2002
). Long-term change associated with woody plant expansion (primarily tree species) into grasslands (Briggs and Gibson, 1992
; Knight et al., 1994
; Hoch and Briggs, 1999
) as well as the immediate responses of shrub species to isolated fire events are well documented (Knapp, 1986
; McCarron and Knapp, 2001
); however, those factors that influence the trajectories of the increase in shrub cover and richness remain to be evaluated. Given that woody species are native to this biome, the initial dynamics of woody encroachment in grasslands will be influenced by (1) the establishment of new individuals and species (recruitment) and (2) the expansion of existing shrubs. Fire can influence both of these processes and an understanding of the consequences of both fire events and long-term fire regimes is critical in assessing the key mechanisms driving this change.
The general objectives of this study were to quantify patterns of change in shrub cover, frequency, and species richness associated with long-term fire regimes in a native C4 grassland. We used data from an 18-yr experiment within the tallgrass prairie landscape that included three distinct fire frequency treatments (annual fire, fire once every 4 yr, and one fire in 18 yr) to characterize the interannual dynamics and long-term changes in shrub cover. A recent study (Briggs et al., 2002
) focused on the long-term trends in expansion of trees and a single shrub species. This data set, however, included all shrub species found within this grassland (which typically precede tree establishment). Specifically, we hypothesized that annual fire would reduce cover and frequency and potentially eliminate any shrubs that were present when the treatment was initiated. Additionally, we expected to detect both the expansion of existing shrubs as well as the establishment of new species within sites in which fire was limited to a single event. Thus, we hypothesized that the shrub cover would be inversely related to the number of fires that occurred within the study period. Given that the intermediate fire treatment approximated pre-settlement burn frequencies (Wright and Bailey, 1982
; Knapp and Seastedt, 1998
), we expected no net increase in shrub cover to take place in this treatment. We further hypothesized that this treatment was likely to be the most dynamic, with episodes of shrub establishment and expansion in the absence of fire followed by reductions in shrub cover and richness in the growing season immediately after fire. In this way, shrub cover would be maintained between upper and lower bounds throughout fire cycles, with no significant increase over the long-term.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). The plant community is dominated by a few native perennial C4 grasses, specifically Andropogon gerardii Vitman, Sorghastrum nutans (L.) Nash, and A. scoparius Michx., but is floristically diverse, containing more than 600 higher plant species (Great Plains Flora Association, 1986
; Freeman, 1998
). Shrubs (C3) such as Cornus drummondii C. A. Mey and Rhus glabra L. are considerably more abundant in areas in which fire has occurred less frequently (Bragg and Hulbert, 1976
). Mean annual precipitation (a 100-yr average) is 835 mm with 75% falling during the growing season (April–September), but is highly variable from year to year (Hayden, 1998
). Similarly, mean annual maximum and minimum temperatures are 43°C and –37°C, respectively, and occur during the months of July and August (Reichman, 1987
).
Konza Prairie is divided into replicate watershed units (about 60 ha) that have been subjected to spring (April ± 20 d) fires at annual, 2-, 4-, 10-, and 20-yr intervals since 1981. Prior to the implementation of this experimental design, the area was grazed by domestic ungulates (cattle) and burned every 2–3 yr, a management practice typical of tallgrass prairie sites within the Flint Hills (Knapp and Seastedt, 1998
).
Beginning in 1983, cover and frequency of plant species have been measured twice annually during the growing season (in May and August) along permanent transects stratified across topographic gradients. In each watershed, 4 transects (50 m in length, each with 5 circular 10-m2 plots) are located in both upland and lowland sites (N = 4 transects/topographic position per watershed). During each sampling period, aerial cover for each plant species within a plot is estimated using a modified Daubenmire scale (Daubenmire, 1968
), and maximum cover values for the growing season were used in the subsequent analyses. Species that were classified as woody (Great Plains Flora Association, 1986
) were subsetted from the data set for use in this analysis. Because trees were infrequently encountered and strictly limited to lowland transects, they were excluded from this study.
Analysis of change in shrub cover, frequency, and species richness
To determine the role of fire frequency on shrub species composition, we focused our analyses on three distinct treatments: high-frequency fire (annual burning), intermediate-frequency fire (a 4-yr fire return interval), and low-frequency fire (burned only once during the study period during a wildfire in 1991). This allowed us to capture both treatment extremes as well as a fire regime that is thought to most closely approximate the fire frequency prior to European settlement. To maintain continuity of sampling and site history throughout the 18-yr period, we used data from three watersheds that provided the longest continuous species composition data available for this grassland. Because of the large size of these watersheds (>40 ha), individual transects were typically >1 km apart and thus, we considered transects (N = 8 per fire history treatment) as independent observational units.
To quantify the impact of a particular fire regime on woody vegetation, both shrub frequency and cover were utilized as metrics. Shrub frequency was calculated as the proportion of plots in which at least one shrub species was present (40 plots per fire regime). Because of different watershed management histories, initial frequency in 1983 varied among treatments; thus, we report change in frequency relative to 1983 levels so that treatments are comparable. Additionally, to reduce interannual variability and better depict the trends associated with a particular treatment, a smoothing technique was used in which frequency for a given year was a 3-yr mean that included adjacent years. Next, shrub cover was calculated by species for all shrubs found within transects, and linear regression analysis was subsequently used to characterize change in shrub cover during the 18-yr period. From 1983 to 2000, four complete cycles of the intermediate frequency fire treatment (or 4-yr burn) occurred and to assess the dynamics of shrub cover in transects exposed to this fire regime, we characterized a cycle as consisting of the following 4 growing seasons: (0) pre-fire, (1) fire, (2) fire +1 yr, and (3) fire +2 yr. Finally, we calculated shrub species richness throughout this study to assess absolute relative change through time.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Dynamics of cover
From 1983 to 2000, an increase in total shrub cover was observed in all sites, regardless of fire frequency (Fig. 2). Despite annual fire, total shrub cover increased approximately 3.7% (P < 0.001) across the 18-yr period, a result that was contrary to predictions that high-frequency fire would reduce, or effectively maintain, existing shrub cover. For both the intermediate- and low-frequency fire treatments, a linear relationship explained the change in shrub cover over most of the study period, with the exception of the rapid increase in cover in the most recent years. Total change in cover across the 18-yr period was much more dramatic (five-fold) along transects exposed to reduced fire frequencies relative to annual fire, but surprisingly, increases were similar under both of these low-frequency fire regimes despite a four-fold difference in the number of fires. A 1991 wildfire that burned the low-frequency fire transects did reduce shrub cover for the subsequent growing season; however, this was followed by a twofold increase in cover by the third growing season post-fire (1993), suggesting a stimulatory effect of an isolated fire event.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Our data demonstrate that within the highly fragmented grasslands that exist today, annual fire can still constrain, though not eliminate, shrub expansion, a finding that is consistent with results of other studies (Hartnett and Fay, 1998
; Briggs et al., 2002
). Annual fire may effectively prevent new species recruitment of native shrub species. Indeed, throughout this 18-yr study, not a single individual from a shrub species different than the four initially present in the eight transects became established in this 400-m2 sampling area. This absence of recruitment may be a consequence of biotic interactions (such as competition), abiotic conditions, scarcity of seed, or a combination of these factors (Gibson and Hulbert, 1987
; Kochy and Wilson, 2000
). In contrast to our hypothesis that a high-frequency fire regime would eliminate shrubs entirely, a group of four species persisted amidst this extreme level of fire. While annual burning limits the total biomass and cover of these shrubs to a single season's growth, they are capable of coexisting with the highly competitive C4 grasses by resprouting annually. Given that only a subset of shrub species was present in this site when the experiment commenced in 1983, we cannot determine whether the other species that colonized in the less frequently burned sites would also persist if subjected to annual fire.
Our analysis of both year-to-year shrub dynamics and long-term directional trends reinforces the value of understanding the mechanisms that drive long-term patterns. Woody encroachment is a process that includes recruitment of new species (an increase in richness) as well as the expansion in cover of existing shrubs. The results of this study indicate that fire frequency influences the relative contribution of each of these processes. Despite 18 fires, shrub frequency in the annually burned watershed was 52.5% in 2000, suggesting that woody vegetation is indeed a common component of this grassland, as it exists today. When the fire return interval was extended to 4 yr or greater, these individuals are able to rapidly expand in cover. In both the intermediate and low fire frequency watersheds, however, recruitment of new species accompanied the expansion in cover of extant species, as evidenced by the increase in richness that occurred in the initial years of the study. Given the greater number of new shrub species in the low fire frequency watershed, it appears that recruitment was enhanced as the time period between fires became longer. In contrast, the expansion of existing species, rather than the establishment of new species, accounted for a greater proportion of the increase in cover in the intermediate-frequency fire sites. In both of these fire treatments, a dramatic increase (2–3 fold) in shrub cover was observed in the final years of this study. The expansion of existing species accounts for this increase, as no new shrub species were recorded in the last 2 yr.
A highly dynamic pattern of response characterized three of the four fire cycles of the intermediate-frequency fire treatment. As expected, shrub cover was reduced in the growing season immediately after fire. With the exception of one fire cycle, however, shrub cover increased from year 0 (pre-burn) to year 3 (fire +2 yr) across a given 4-yr fire cycle. Our initial hypothesis was that perhaps monthly or annual precipitation would explain this observation; however, this cycle was not atypical with respect to climate. Resprouting, following a severe disturbance, is common in woody plant species in a variety of habitats (Trollope, 1984
; Matlack et al., 1993
; Olson and Platt, 1995
; Bond and van Wilgen, 1996
). Knapp (1986)
reported a significant increase in shoot density in the shrub Rhus glabra immediately following a spring burn, suggesting a positive response to fire, at least from a short-term perspective. It is not surprising that shrubs in tallgrass prairie are capable of resprouting, as shrubs in highly disturbed environments often rely on this strategy for long-term persistence. Rather, it is surprising that they are capable of doing so with such a relatively short fire return interval and that it is a recurrent, rather than an isolated, response.
In summary, this 18-yr data set indicates that the processes and dynamics of shrub expansion in tallgrass prairie are closely tied to fire frequency. We conclude that: (1) shrub species richness is inversely related to fire frequency (sites exposed to high-frequency fire contained the fewest number of species, whereas sites exposed to a low frequency of fire were characterized by the greatest shrub species richness); (2) over an 18-yr period, substantial increases in shrub cover, abundance, and species richness were observed when the fire return interval was equal to or greater than 4 yr, but shrub cover increased even under an annual fire regime; and (3) the interaction of fire and established shrubs is complex (particularly at an intermediate fire frequency) and can be characterized as either positive or negative, depending on the time scale of the response.
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FOOTNOTES |
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4 Author for reprint requests (Jana.Heisler{at}asu.edu
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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