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Ecology |
Division of Biology, Kansas State University, Manhattan, Kansas 66506 USA
Received for publication October 22, 2002. Accepted for publication May 20, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDiscussionLITERATURE CITED |
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Key Words: C3 • C4 • Cornus drummondii • fire • shrub • tallgrass prairie • transient maxima hypothesis • woody expansion
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDiscussionLITERATURE CITED |
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). In particular, a dramatic expansion of woody vegetation has occurred at the prairie-forest ecotone in North America since European settlement in the mid-1800s (Abrams, 1986
; Knight et al., 1994
). Fire suppression has been considered a primary factor responsible for the increase in abundance of C3 woody species in C4-dominated grasslands (Daubenmire, 1968
; Bragg and Hulbert, 1976
; Abrams, 1986
; Hulbert, 1986
). Although many woody species are native to mesic grasslands and are present even in frequently burned grasslands, their historical abundance was low and often limited to stream banks and areas sheltered from fire (Hulbert, 1986
). In the absence of fire, shrub invasion and expansion typically precedes forest development and may facilitate conversion of grasslands to forest (Weaver, 1968
; Petranka and McPherson, 1979
).
Although many woody species are intolerant of fire those native to mesic grasslands can, once established, persist even with occasional fires and may respond vigorously after fire by producing numerous vegetative resprouts (Adams et al., 1982
; Knapp, 1986
). In clonal shrubs, for example, the impact of fire may be limited to temporary changes in the density and age structure of aboveground shoots, and in many cases stem density may increase in the first growing season after fire (Abrahamson, 1984
; Knapp, 1986
; Matlack et al., 1993
). The historical fire regime in the tallgrass prairie is not known, but even a fire frequency of one in 4 yr has not been successful at halting woody encroachment in this grassland (Knight et al., 1994
), and there is anecdotal evidence to suggest that fire may even stimulate shrub encroachment within infrequently burned grasslands (Briggs et al., 2002
).
High growth rates in plants after fire are often associated with the release of nutrients from burned biomass (Rundel and Parsons, 1980
; DeSouza et al., 1986
; Fleck et al., 1995
) and the use of energy reserves stored underground (Chapin et al., 1990
; Hodgkinson, 1998
). In infrequently burned grasslands, the C4 grasses respond positively to spring fires with an increase in aboveground net primary productivity (ANPP; Hulbert, 1969
, 1988
). However, this increase in ANPP has been attributed to a temporary release from multiple resource constraints rather than just from nutrients released after fire (Blair, 1997
). This phenomenon, described by the "transient maxima hypothesis" (Seastedt and Knapp, 1993
), predicts that post-fire increases in ANPP in sites previously unburned for several years are due to a transient period of concurrently high availability of light and N, as the system undergoes transition from one limited by light (unburned grassland; Knapp and Seastedt, 1986
) to one limited more by N availability (burned grasslands; Seastedt et al., 1991
; Turner et al., 1997
).
The primary goal of this study was to assess the mechanistic value of the transient maxima hypothesis, originally developed to explain responses of grasses, for predicting responses of grassland shrubs to fire. We hypothesized that although fire in tallgrass prairie acts as a pruning mechanism for aboveground shoots of shrubs, post-fire resprouts can take advantage of the transient period of greater resource availability (light and nitrogen) and respond positively compared to those in unburned grasslands. However, fire may also increase interspecific competition for light, water, and nutrients between resprouting shrubs and grasses (Knapp, 1986
). Therefore, a second objective was to assess the effect of the size of shrub patches (islands) on the abundance and vigor of resprouts and new ramets produced in burned and unburned tallgrass prairie. We predicted that small shrub islands, which have grasses codominant with the shrubs, would be more negatively affected by fire than large islands, which exclude grasses from the understory, because small shrub islands have (1) greater relative fine-fuel loads, (2) lower belowground reserves, and (3) potentially stronger competitive interactions with grasses.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDiscussionLITERATURE CITED |
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). The KPBS has in place an experimental design that includes manipulations of fire frequencies of 1-, 4-, 10- and 20-yr (unburned) intervals (Knapp and Seastedt, 1998). Plant communities are dominated by warm season C4 grasses Andropogon gerardii Vitman, A. scoparius Michx., and Sorghastrum nutans (L.) Nash (Freeman, 1998
). Shrubs (C3), such as Cornus drummondii C. A. Mey, Rhus glabra L., and to a lesser extent Prunus americana Marsh., can be found as monospecific "islands" within the matrix of grass or as large multispecies communities. Shrubs are more numerous along seeps and intermittent lowland streams and become less abundant with distance from streams; shrub cover and island sizes tend to increase with a decrease in fire frequency (Briggs et al., 2002
). For this study, the clonal shrub species C. drummondii was selected due to the large number of monospecific, distinct islands of this species found in most fire treatments on the KPBS.
Experimental design
Three fire treatments were used in this study: annually burned (burned annually for >18 yr), infrequently burned (4-yr intervals), and unburned (last burned in 1991). The annual and infrequently burned plots were burned in late April of 2000. Ten small (mean area <5 m2) and five large (>140 m2) monospecific C. drummondii islands, embedded in a matrix of prairie grasses, were selected for study in each of the infrequently burned and unburned treatments. Care was taken in selection of islands so that they were all located in similar soils types and topographic positions, and were at least 5 m distant from the nearest neighbor. For the annually burned treatments, no distinct shrub islands were present; however, three diffuse clusters of C. drummondii stems were found associated with dry ephemeral streambeds. These streams beds remained dry during this study.
The experimental unit in this study was the shrub island, and only the current year's shoots (i.e., new sprouts in unburned sites and resprouts in burned sites) were measured. At the conclusion of the field season, we determined that shoots were physically connected to the larger root systems of the islands by excavation to insure that we included only vegetatively produced stems. No individual seedlings were found for C. drummondii in this study.
Physiological measurements
Gas exchange, xylem pressure potential, and percentage of full (above canopy) photosynthetic photon flux density measurements were made once every two weeks (and within the same day) during the growing season on three leaves from each of five randomly selected small shrub islands and five leaves from each of three randomly selected large shrub islands within each of the unburned and infrequently burned treatments. For the annually burned plants, five leaves were sampled from each of the three groups of stems. Xylem pressure potential (XPP) was measured in the field on fully expanded leaves at predawn (about 0530 CDT) using a Scholander-type pressure chamber (PMS, Corvallis, Oregon, USA). Gas exchange (net photosynthesis and stomatal conductance to water vapor) was measured at midday (between 1000 and 1400 CDT) for all species under high light conditions (1500 µmol · m–2 · s–1) with an LI-6400 portable photosynthetic system (LI-COR, Lincoln, Nebraska, USA). Percentage of full photosynthetic photon flux density (PPFD) was calculated from measurements made at mid-stem height of each new shoot and in ambient sunlight above the canopy using a 0.5 m ceptometer (Decagon, Pullman, Washington, USA).
The response of plants to PPFD was determined in the field with an LI-6400 portable photosynthetic system on eight leaves form each burn and size category in June and early July of 2000. Attached leaves were selected from the upper canopy and placed within a leaf chamber equipped with a red-blue diode light source. Initial measurements were made under saturating PPFD conditions (2000 µmol · m–2 · s–1) with humidity, leaf temperature, and CO2 levels held constant (typically 30°C, 50% relative humidity, 360 µL/L CO2). Photon flux density levels were incrementally decreased until the leaf was in complete darkness. Measurements at a specific PPFD were only recorded after the system had reached equilibrium (typically 10 min). We estimated maximum photosynthesis (Amax) by averaging all asymptotic values above 1000 µmol · m–2 · s–1. The light (PPFD) saturation point (LSP) was defined as the PPFD required to produce 90% of Amax. Subsequently, parameters such as light compensation point (LCP), apparent quantum use efficiency (QE; estimated from PPFD 0–150 µmol · m–2 · s–1), and dark respiration (Rd) were estimated from individual photosynthetic light response curves using a nonrectangular hyperbola following models developed by Prioul and Chartier (1977)
.
Leaf N content and plant biomass
Once every two weeks during the growing season, five leaves from each of five randomly selected shrub islands were collected within the unburned and infrequently burned treatments, for each size class. For the annually burned treatment, five leaves were randomly collected from each of the three sites. Within each island, leaves were combined, dried (60°C), and ground. Leaf N was determined by combustion/reduction and gas chromatography (model NA-1500 C/N analyzer; Carlo-Erba, Milan, Italy).
Aboveground biomass of current year shoots was determined by randomly harvesting stems (N = 10 stems for large islands and N = 3–5 for small islands) within each island during mid-September prior to leaf senescence for all burn treatments and size islands. For each stem, height and basal diameter were measured and the leaves and stems were dried and weighed. To estimate the stem density of new stems produced in the current year large islands, five 1 x 1 m plots were randomly located along the inside circumference of each island, and within these plots all new shoots were counted and height and basal diameter of each was measured. Within the small islands, all current year's shoots were tallied. For the annually burned treatment, the stems were diffusely located and only individual shoot parameters could be measured. Height and basal diameter were used to calculate stem volume, which was regressed with leaf and stem mass to determine a size/mass relationship for each burn treatment and island size (significant at P < 0.0001 and an average r2 = 0.91).
Statistical analysis
For ecophysiological data, biweekly values were combined into monthly averages. An analysis of variance (ANOVA) with repeated measures was used to assess burn treatment, island size, and date as main effects for each response variable using SAS (SAS, 1989
). Within each month, means were separated using the least significant difference means comparison.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDiscussionLITERATURE CITED |
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Foliar N concentrations of shoots in the infrequently burned treatments were greater than those in unburned and annually burned treatments in June (Fig. 1B). Except for shoots from large unburned islands, foliar N concentrations were highest in June and decreased by July, with no differences among islands by August. Within a treatment, leaves from small islands had greater foliar N concentrations than those from large islands (P < 0.0001).
Leaf level gas exchange and xylem pressure potentials
There were no differences in photosynthetic rates and stomatal conductance between large and small islands, and thus these data were pooled within a treatment (Fig. 2A and B). Photosynthetic rates were greater for shoots in infrequently burned islands compared to those in unburned treatments in both June and July, but differences diminished at the onset of the summer drought in August. Photosynthetic rates for shoots in the annually burned treatment were also greater than in unburned shoots during July and were greater than shoots in both unburned and infrequently burned treatments during July and August. Seasonal mean A for shoots in burned treatments was 20% higher than shoots in unburned sites (P = 0.0162). Stomatal conductance in both burned treatments was greater than for unburned shoots in June and July, with the infrequently burned resprouts having greatest stomatal conductance during June. By August, stomatal conductance for all treatments was reduced considerably compared to earlier in the season.
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Under the controlled conditions in which photosynthetic responses to changes in PPFD were measured, there were no differences in large and small islands in the unburned treatments so data were pooled (Fig. 3 and Table 1). Shoots in burned treatments had greater Amax and LSP than the unburned shoots (P < 0.0001 and P = 0.002, respectively), with large islands having the highest Amax of the burned treatments. Large and small islands in the infrequently burned treatments had similar QE, which was greater than in either the unburned and annually burned treatments (P = 0.016). Finally, there were no differences among treatments for LCP and Rd.
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Discussion |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDiscussionLITERATURE CITED |
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). Even though shrub encroachment into these grasslands is increasing due to a lack of fire, the same accumulation of detritus can limit the expansion of established shrub clones, since new shoots of shrubs must also emerge through the litter layer. Brown and Archer (1989)
have shown that herbaceous litter greater than 100 g/m2 can have negative impacts on emergence and establishment of seedlings of woody species. In the absence of fire and grazers, detritus can accumulate to an average greater than 500 g/m2 (Abrams et al., 1986
), with a maximum as much as 1500 g/m2 in tallgrass prairies (Weaver and Rowland, 1952
). When detritus is removed through burning, there is always an increase in light, and in sites that have not been burned for several years, N availability remains higher relative to annually burned sites (Blair, 1997
). The fire that produces these transient conditions stimulates grass productivity and might be expected to negatively affect shrub growth due to the loss of aboveground stems. However, shoots of C. drummondii responded to the higher light levels in the burned sites with increased A, Amax, gs, LSP, and increased shoot density compared to shoots from unburned sites. Similar results have been reported for coppiced shoots (produced both by harvesting and fire) and have been attributed to abundant resources afforded shoots by a relatively large surviving root system (Radosevich and Conard, 1980
; Tschaplinski and Blake, 1989
).
Higher leaf N concentrations early in the growing season for new shoots from burned islands suggests that resource supply was increased to leaves either from soil or root/rhizome reserves. In unburned prairie, reduced uptake of N by grasses resulting from light limitations should contribute to greater availability of N to shrubs. Whereas in burned sites, grasses respond positively to fire by increasing biomass and uptake of N by as much as 53% compared to unburned sites (Knapp, 1985
, 1986
), and thus, the availability of N to shrubs would be expected to be reduced. Knapp (1986)
found that Rhus glabra had a lower maximum leaf N for shoots in an infrequently burned site (0.94%) compared to an unburned site (1.4%). However in this study, maximum leaf N concentrations for shoots of C. drummondii were greater than R. glabra and more similar to the maximum N concentrations of the dominant C4 grass Andropogon gerardii from burned grasslands (Knapp, 1985
; Turner and Knapp, 1996
). High N in post-fire shrubs may reflect the increase in the root : shoot ratio after fire, which would provide more N for surviving tissues and newly developing shoots. Additionally, high leaf N (2.6%) was measured in surviving adults from the center of large burned islands, where fire did not penetrate (J. K. McCarron and A. K. Knapp, unpublished data). This indicates that more N was available to the entire clone and not just new shoots.
Generally, C3 plants have higher N and water requirements than C4 grasses (Pearcy and Ehleringer, 1984
), and there is a strong positive relationship between N content and maximum photosynthesis within and across species (Field and Mooney, 1986). This relationship has been inferred for post-fire shoots and resprouts of other species of shrubs (Oechel and Hastings, 1983
; Fleck et al., 1995
). In C. drummondii, higher Amax was measured in leaves from burned islands at low and high light levels compared to those in unburned islands. Given that grasses are also responding positively to fire and growing rapidly and that much of the new shoot canopy is exposed to reduced light, higher A may be the result of greater N availability in burned sites and may compensate partially for unfavorable light conditions. In the annually burned site where grasses quickly overtopped shoots, light levels decreased rapidly in the early summer, but new shoots were also able to maintain relatively high A.
In this study, fire caused considerable damage to many preexisting stems in large islands and all stems in small islands. Nonetheless, all the islands studied survived the fires and the current year's shoot density was increased, with greater biomass, compared to new shoots in unburned sites. Increased stem densities after fire have been reported as a positive response for many woody species to fire (Adams et al., 1982
; Knapp, 1986
; Cirne and Scarano, 2001
). The shoots that were produced in unburned islands tended to be as tall as those in burned sites, although less biomass was allocated to these shoots, which was due primarily to lower leaf mass. Remarkably, the biomass and height of shoots from the annually burned sites were similar to those in the infrequently burned sites, even though the site had been burned for 18 yr. By the end of the growing season, most shoots had reached heights that placed them well above the herbaceous canopy, minimizing potential light limitations during the next growing season.
Large vs. small islands
Resprouting vigor after fire has been shown to be proportional to plant size, suggesting that larger clones accumulate more reserves and have more active underground buds for recovery (Malanson and Trabaud, 1988
; Cirne and Scarano, 2001
). Indeed, burning caused vigorous resprouting in both small and large islands; however, small burned islands produced more shoots that were more productive on an area basis. Additionally, leaf N content was higher in small islands (independent of fire frequency), and this may reflect a greater root : shoot ratio in smaller islands or a greater proportion of roots in upper soil depths, where N is more abundant. This shallow rooting pattern may also explain why smaller islands in burned sites had severely reduced predawn XXP. Nevertheless, A and gs did not differ between island sizes. Similarly, small islands produced more shoots per unit area and had greater leaf N compared to large islands in unburned sites. Conversely, large islands produced flowers and fruits while small islands did not (J. K. McCarron, unpublished data), perhaps reflecting a shift in resource allocation to sexual reproduction in large islands, whereas more resources were allocated to island expansion in small islands. Overall, these results suggest that small islands were not at a disadvantage compared to large islands during recovery after fire.
In summary, fire is a natural component of most mesic grasslands, and in tallgrass prairie infrequent fire leads to woody plant increases and a transient period of high resource availability (light and nitrogen). These short-term pulses in resources provide a mechanism for the post-fire pulse in aboveground net primary productivity of grasses in tallgrass prairie (i.e., transient maxima hypothesis). Our data suggest that this same mechanism is responsible for the post-fire increase in growth and recovery of C. drummondii in these grasslands. Indeed, infrequent fire may actually accelerate the encroachment of some shrub species, once established, in grasslands by evoking vigorous resprouting and the use of increased resources.
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FOOTNOTES |
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2 Current address: Department of Plant Pathology, Kansas State University, Manhattan, Kansas 66506 USA (mccarro{at}ksu.edu
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDiscussionLITERATURE CITED |
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