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(American Journal of Botany. 2001;88:1818-1823.)
© 2001 Botanical Society of America, Inc.

Ecology

C3 woody plant expansion in a C4 grassland: are grasses and shrubs functionally distinct?1

James K. McCarron2 and Alan K. Knapp2,3

2Division of Biology, Kansas State University, Manhattan, Kansas 66506 USA

Received for publication December 21, 2000. Accepted for publication March 27, 2001.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
The expansion of C3 shrubs into C4-dominated tallgrass prairies represents a fundamental shift in growth-form dominance accompanied by changes in resource acquisition and use. We assessed these changes by comparing the ecophysiological traits of the dominant C4 grass Andropogon gerardii, with traits of three C3 invasive shrub species, Cornus drummondii, Prunus americana, and Rhus glabra. We tested the hypothesis that ecophysiological traits of the shrubs would be similar within this growth form but distinct from grasses and that these species would conform to the two-layer soil water model. Photosynthetic rates in R. glabra were similar to A. gerardii and higher than in the other two shrubs, while water use efficiency was markedly greater in A. gerardii. Among all species, midday xylem pressure potentials (XPP) were distinctly lower (70%) for P. americana, but were similar among the other species. Predawn XPP was related to soil water at shallow depths for A. gerardii (r2 = 0.59) and P. americana (r2 = 0.62), and to deeper soil moisture for R. glabra (r2 = 0.63); there was no relationship for C. drummondii at any soil depth. Thus, a simple two-layer soil water model for partitioning shrub/grass resource acquisition was not appropriate for this grassland. We conclude that these shrubs could not be considered functional equivalents from an ecophysiological perspective, nor were they, as a group, distinct from A. gerardii in resource acquisition and use.

Key Words: C3 and C4 photosynthesis • shrub • tallgrass prairie • two-layer soil water model • water use efficiency • woody expansion


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Anthropogenic disturbances and changes in land management have enabled woody species to expand into grasslands worldwide (Archer, Boutton, and Hibbard, in press). In the C4-dominated tallgrass prairies of the central United States, fire suppression has been implicated as a major factor responsible for the increase in abundance of C3 woody species (Daubenmire, 1968 ; Bragg and Hulbert, 1976 ; Towne and Owensby, 1984 ; Abrams, 1986 ). As fire frequency has decreased in the tallgrass prairie, litter has accumulated and grass production declined, followed by an increase in woody vegetation (Abrams, Knapp, and Hulbert, 1986 ; Knapp and Seastedt, 1986 ; Gibson and Hulbert, 1987 ). Patterns and rates of forest expansion into tallgrass prairies have been well documented (Kucera, 1960 ; Abrams, 1986 ; Briggs and Gibson, 1992 ; Knight, Briggs, and Nellis, 1994 ). However, shrub invasion often precedes forest development and may even facilitate forest expansion within grasslands (Weaver, 1968 ; Petranka and McPherson, 1979 ). Yet many of the ecological interactions driving or resulting from this grass–shrub transition phase remain to be evaluated.

Our understanding of the consequences of the expansion of woody species in grasslands is based on the assumption that the obvious and fundamental shift in growth form (grasses to shrubs) is accompanied by a shift in patterns of resource acquisition and use as shrubs first coexists with and then replace grasses. One mechanism proposed to explain the coexistence of woody and grass species in grasslands is that these two growth forms obtain soil water from different depths (Sala et al., 1989 ; Brown and Archer, 1990 ; Weltzin and McPherson, 1997 ; Golluscio, Sala, and Lauenroth, 1998 ). Although originally developed for savanna ecosystems (Walter, 1971 ), the two-layer soil water model predicts that grasses acquire water primarily from shallow soil layers in grassland ecosystems and can take advantage of smaller precipitation amounts, whereas shrubs rely more on deeper soil water. The two-layer model has been used most successfully to describe resource partitioning between shrubs and grasses in more arid environments (Sala et al., 1989 ; Wan, Sosebee, and McMichael, 1995 ; Dodd, Lauenroth, and Welker, 1998 ; Golluscio, Sala, and Lauenroth, 1998 ) and subtropical savannas (Brown and Archer, 1990 ; Weltzin and McPherson, 1997 ). However, in the humid and mesic savannas of Africa, roots of shrubs were found to be less deeply distributed and competed directly with grasses for shallow soil water (Belsky, 1994 ; Le Roux, Bariac, and Mariotti, 1995 ). In North America, the largest remaining tracts of tallgrass prairie are in Kansas and Oklahoma, a region at the driest edge of the original extent of this mesic grassland. With the suppression of fire in this region, there has been a rapid increase in woody vegetation in once open tallgrass prairie (Briggs and Gibson, 1992 ; Hoch, 2000 ). However, the role that soil water partitioning between grasses and shrubs plays in this shrub expansion is unclear.

Our objectives for this study were to (1) to evaluate the assumption that water and carbon acquisition in C3 shrubs (Cornus drummondii, Prunus americana, and Rhus glabra) in tallgrass prairie were similar within this growth form, but distinct from the dominant C4 grass (Andropogon gerardii); and (2) to assess the applicability of the two-layer soil water model for woody and grass species in the tallgrass prairie ecosystem.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 LITERATURE CITED
 
Research was conducted at the Konza Prairie Biological Station (KPBS) in northeast Kansas, USA (39°05' N, 96°35' W) during the 1998 and 1999 growing seasons. The KPBS is a 3487-ha native tallgrass prairie preserve located within the Flint Hills of Kansas. Elevation at KPBS varies from 320 to 444 m above sea level. Average annual rainfall is 835 mm with 75% falling during the growing season (April–September; Hayden, 1998 ). Most watersheds on the KPBS have been exposed to fire frequencies of 1-, 4-, 10-, and 20-yr (unburned) intervals since 1981. 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. Densities of shrub populations are greater along seeps and intermittent lowland streams, becoming less dense with distance from streams, and shrub densities and island sizes tend to increase with a decrease in fire frequency (J. K. McCarron, personal observation.).

To compare ecophysiological traits of these three shrubs with the dominant grass (A. gerardii), shrub islands embedded in a matrix of prairie grasses were studied in an unburned watershed (last burned in 1991). Three distinct monospecific shrub islands with a diameter of at least 5 m were selected for each species. Each island was at least 3 m distant from its nearest neighbor. Within 2 m of each shrub island, a paired plot of A. gerardii dominated grassland was selected for comparative sampling.

Ecophysiological measurements were made biweekly during the growing season on five leaves for each shrub island and adjacent plot of A. gerardii. Gas exchange (net photosynthesis and stomatal conductance to water vapor) was measured at midday for all species under high light conditions (>1000 µmol · m–2 · s–1) with an LI-6200 portable photosynthetic system (Li-Cor, Lincoln, Nebraska, USA) equipped with a 0.25-L chamber. For each shrub species, single attached upper canopy leaves were measured within the island and then detached to determine leaf area. An LI-3100 area meter (Li-Cor) was used to measure leaf area. Gas exchange for A. gerardii was measured by placing at least two upper canopy leaves in the chamber. Xylem pressure potential (XPP) was measured in the field on fully expanded canopy leaves at predawn (at approximately 0530 central daylight savings time [CDT]) and midday (at approximately 1300 CDT) for all species using a Scholander-type pressure chamber (PMS, Corvallis, Oregon, USA).

The response of plants to photosynthetic photon flux density (PPFD) was determined in the field with an LI-6400 portable photosynthetic system (Li-Cor) on eight leaves for each species in July of 1999. 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 minutes). 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 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 non-rectangular hyperbola following models developed by Prioul and Chartier (1977) .

Predawn XPP data were combined with soil moisture data to estimate effective rooting depth of these species. We assumed that predawn XPP would be strongly associated with soil moisture levels at the depth that most water uptake occurred. Soil moisture was measured every 2 wks at 25, 50, 75, 100, 125, and 150 cm below soil surface, from April through November, as part of the Long-Term Ecological Research (LTER) program at KPBS, using neutron probes and thin-walled aluminum access tubes (3330 Series Troxler soil moisture gauge; Troxler Electronic, Research Triangle Park, North Carolina, USA). Only results from two access tubes, located within the same watershed as the shrub islands, were used in this study.

Data for both years were combined to produce average seasonal responses at ~2-wk intervals. An analysis of variance (ANOVA) was used to assess species and date as main effects for each response variable. Within each 2-wk period, means were separated using the least significant different means comparison.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 LITERATURE CITED
 
Seasonal patterns of gas exchange for the three shrubs indicate that there were distinct differences between R. glabra and the other two shrubs, C. drummondii and P. americana. Photosynthetic rates (A) for R. glabra were 30% to 50% greater than in C. drummondii and P. americana (P < 0.001) during most of the growing season (June through late August; Fig. 1). Patterns in stomatal conductance were similar (data not shown). Water use efficiency (WUE) for R. glabra was also greater than for C. drummondii and P. americana (P < 0.001) during the growing season, with no difference between C. drummondii and P. americana (Fig. 1). This overall trend among the three species was also apparent in the seasonal averages for A and WUE, with R. glabra distinctly higher than the other two shrub species (Fig. 1). In the more controlled conditions in which photosynthetic response to changes in PPFD were measured, Amax for R. glabra was 32 % (P < 0.001) greater than in C. drummondii and P. americana (Fig. 2). Both C. drummondii and R. glabra had a similar LSP, which was higher than in P. americana (Table 1; P = 0.003 and P = 0.028, respectively). Apparent quantum use efficiency (QE) was similar in P. americana and R. glabra, but was lower in C. drummondii. Finally, there were no differences among species in light compensation point and dark respiration.



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  		Fig. 1. Net photosynthesis and water use efficiency measured in the field and averaged for ~2-wk periods over two growing seasons (1998–1999) for Andropogon gerardii and three common shrub species (Cornus drummondii, Prunus americana, and Rhus glabra) in tallgrass prairie. Asterisks represent significant differences between closest upper and lower species (P < 0.05, not all statistical differences among species are shown) and vertical bars represent ± 1 SE. Numbers in parentheses represent the seasonal mean for each species ± 1 SE. Means followed by the same letter were not significantly different

 


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  		Fig. 2. Response of net photosynthesis to photon flux density for Andropogon gerardii and three common shrub species (Cornus drummondii, Prunus americana, and Rhus glabra) in tallgrass prairie. Vertical bars represent ± 1 SE

 

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  		Table 1. Mean estimates of the light saturation point (LSP), light compensation point (LCP), maximum net photosynthesis (Amax), dark respiration rates (Rd), apparent quantum use efficiency (QE), and seasonally averaged predawn and midday xylem pressure potentials (XPP) ± 1 SE for the dominant C4 grass (Andropogon gerardii) and three C3 shrub species (Cornus drummondii, Prunus americana, and Rhus glabra) in unburned tallgrass prairie. Means followed by the same superscript letter in a column are not significantly different

 
Seasonal patterns of XPP varied substantially among the three shrub species. Predawn XPP were typically lower in P. americana and R. glabra than in C. drummondii on a seasonal basis (Fig. 3; and Table 1). Midday XPP was also lower for P. americana (70%) relative to the other two shrubs.



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  		Fig. 3. Seasonal course of predawn and midday xylem pressure potential (XPP) for Andropogon gerardii and three common shrub species (Cornus drummondii, Prunus americana, and Rhus glabra) in tallgrass prairie. Data were averaged for 2-wk periods over two growing seasons (1998–1999). Open symbols represent predawn XPP; closed symbols represent midday XPP. Vertical bars represent ± 1 SE. Bottom panel depicts percent of maximum (field capacity) soil moisture at 25-cm soil depth

 
As expected, the C4 grass A. gerardii had consistently higher seasonal A and Amax than C. drummondii and P. americana, but not when compared with R. glabra (Fig. 1). Indeed, during June and July, A for R. glabra equaled that of A. gerardii. As expected for a C4 species, stomatal conductance in A. gerardii (seasonal mean = 0.215 mol · m–2 · s–1) was lower than for any of the shrubs. Water use efficiency for A. gerardii was 30% greater than in R. glabra (P = 0.0001) and 50% higher than in C. drummondii and P. americana (P < 0.0001; Fig. 1). However, R. glabra and A. gerardii had similar Amax, LCP, R, and QE. Photosynthesis in A. gerardii saturated at a lower PPFD than did the shrubs (P < 0.005; Fig. 2); however, differences were minor.

Seasonal patterns in XPP for A. gerardii were more variable than in the three shrubs. Typically, A. gerardii had higher predawn XPP than R. glabra and P. americana, but not compared with C. drummondii (Fig. 3). Only when shallow soils were driest, during late July, were midday XPP similar for A. gerardii and P. americana (about –2.8 MPa). However, midday XPP for A. gerardii increased markedly during a wet period in August, while midday XPP for P. americana did not respond.

For three of the four species studied (R. glabra, P. americana, and A. gerardii), predawn XPP was strongly related to soil moisture levels at 50 cm (Fig. 4). However, XPP for A. gerardii and P. americana were most strongly associated with soil moisture at shallow depths (25 cm). In contrast, R. glabra was most strongly related to soil moisture at deeper depths (75–150 cm). Predawn XPP was not related to soil moisture at any depths for C. drummondii.



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  		Fig. 4. Significant coefficient of determination (r2) values from regressions of predawn xylem pressure potential (XPP) vs. soil moisture at various depths for Andropogon gerardii and three common shrub species (Cornus drummondii, Prunus americana, and Rhus glabra) in tallgrass prairie (nonsignificant r2 values are not shown). Data used in the regression were from two growing seasons (1998–1999). Inserted panels at the right depict regression relationships for the 50-cm soil depth. Predawn xylem pressure potentials were not significantly related to any soil depth for C. drummondii

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
LITERATURE CITED
 
Generally, C4 grasses are expected to exhibit physiological characteristics distinct from those of C3 plants (Pearcy and Ehleringer, 1984 ). The first objective of this study was to test the hypothesis that ecophysiological traits of the three C3 shrubs, Cornus drummondii, Prunus americana, and Rhus glabra, would be similar to one another and distinct from the C4 grass Andropogon gerardii. This was the case for A. gerardii for average seasonal A and WUE, both of which were greater than for the C3 shrubs (Fig. 1). However, during most of the growing season in this unburned grassland, A and Amax for R. glabra were similar to values in A. gerardii and much higher than for the other two shrubs (Figs. 1 and 2). The high maximum photosynthetic rates for R. glabra reported here are not unusual, as similar values were measured by Larson (1998) . Rhus glabra also had greater WUE than the other two shrubs, but this was still lower than in A. gerardii. Among the four species, P. americana was the most distinctive species from a water relations perspective, with midday XPP 70% lower than for the other species during most of the growing season (Fig. 3).

Photosynthetic rates for A. gerardii were lower than might be expected for a C4 grass in a tallgrass prairie (Turner, Kneisler, and Knapp, 1995 ), perhaps because measurements were made in an unburned prairie where the accumulation of standing dead biomass (often >30 cm deep) can adversely affect the photosynthetic development of A. gerardii leaves (Knapp and Seastedt, 1986 ). For example, the LSP values for A. gerardii (Table 1) were low compared with values from a burned prairie (Schimel et al., 1991 ), an indication that leaves developed under low light conditions, which is characteristic of this detrital layer (Knapp, 1985 ). Thus, if the grassland is not burned, the C4 photosynthetic advantage of A. gerardii is reduced relative to the shrubs that grow above the litter layer.

A second objective of this study was to determine whether the shrubs and grasses in tallgrass prairie conformed to the two-layer soil water model of resource partitioning (Walter, 1971 ). Predawn XPP in both A. gerardii and P. americana was strongly related to shallow soil water, while XPP in R. glabra was associated with deeper soil water (Fig. 4), and there was no significant relationship between predawn XPP and soil moisture at any depth for C. drummondii. Andropogon gerardii is known to have a moderately shallow root system along with some deep roots (maximum reported rooting depth of 2.1 m; Weaver, 1958 ), whereas R. glabra has a deeper maximum root depth (6.7 m; Weaver, 1919 ). Less is known of the rooting depth for C. drummondii and P. americana in grasslands. Because of these differences in rooting depth, partitioning of water would be expected for R. glabra and A. gerardii. However, for P. americana and A. gerardii, the similarities in patterns of predawn XPP and relationship with shallow soil water indicate substantial overlap in soil water use. In unburned prairies, the detritus layer reduces evapotranspiration, allowing soil to remain moist longer into the growing season than in burned prairies (Briggs and Knapp, 1995 ). This condition may favor the encroachment of potentially shallow rooted shrubs, such as P. americana into tallgrass prairie, while concurrent light limitations decrease the physiological advantage of C4 grasses. Gradual downward shifts in water uptake by plants may occur as upper soils dry (Taylor and Klepper, 1975 ; Rambal, 1984 ; Sala et al., 1989 ), and this may be important for these shrubs. However, with the exception of R. glabra, there was little support for distinct partitioning of soil water between shrubs and grasses as predicted by the two-layer soil water model.

In summary, with the absence of fire there has been an increase in the abundance of woody species in tallgrass prairie ecosystems. Although, these shrubs share many morphological traits, their patterns of resource acquisition and use are variable and in some cases quite similar to the dominant C4 grass. In this study we found that (1) these shrubs are not functionally similar to one another from a ecophysiological perspective in a tallgrass prairie, (2) although A. gerardii obviously differs from shrubs morphologically, its relative physiological superiority was reduced in an unburned grassland, and (3) consistent soil water partitioning between shrubs and grasses was not evident for these two growth forms. The ability of R. glabra to maintain high photosynthetic rates and greater water use efficiency, in combination with its deep rooting ability, may allow this species to be a more successful invader of grasslands across a greater range on environmental conditions than C. drummondii and P. americana. Indeed, of the three shrubs in this study, R. glabra has the widest geographical distribution in the continental United States (USDA [National Resources Conservation Service], 1999 ). Furthermore, R. glabra is one of the most abundant native shrub species on the Konza Prairie and occupies the greatest topographical and hydrological ranges (J. Briggs, Arizona State University, personal communication). Finally, much recent research has been based on the implicit assumption that growth forms represent assemblages of species that are similar to one another and distinct from other growth forms in how they function and respond in ecological studies (Leishman and Westoby, 1992 ; Aguiar et al., 1996 ; Paruelo and Lauenroth, 1996 ; Tilman et al., 1997 ). Results from this study suggest that caution should be used in such species aggregations.


   FOOTNOTES
 
1 The authors thank the Konza Prairie Long-Term Ecological Research Program for use of long-term soil moisture data. Research was supported by the NSF LTER program, the Konza Prairie Biological Station, The Nature Conservancy, and the Kansas Agricultural Experiment Station (02-58-J). Back

3 Author for reprint requests (e-mail: aknapp{at}ksu.edu ). Back


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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
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