JXB Advance Access originally published online on October 3, 2005
Journal of Experimental Botany 2005 56(421):2867-2876; doi:10.1093/jxb/eri281
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RESEARCH PAPER |
Photosynthetic responses of C3 and C4 species to seasonal water variability and competition
1Laboratory of Quantitative Vegetation Ecology, Institute of Botany, the Chinese Academy of Sciences, Xiangshan, Beijing 100093, China
2Institute of Microbiology, the Chinese Academy of Sciences, Zhongguancun, Beijing 100080, China
* To whom correspondence should be addressed. Fax: +86 10 8259 3840. E-mail: swan{at}ibcas.ac.cn
Received 13 May 2005; Accepted 5 August 2005
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Abstract |
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This study examined the impacts of seasonal water variability and interspecific competition on the photosynthetic characteristics of a C3 (Leymus chinensis) and a C4 (Chloris virgata) grass species. Plants received the same amount of water but in three seasonal patterns, i.e. the one-peak model (more water in the summer than in the spring and autumn), the two-peak model (more water in the spring and autumn than in the summer), and the average model (water evenly distributed over the growing season). The effects of water variability on the photosynthetic characteristics of the C3 and C4 species were dependent on season. There were significant differences in the photosynthetic characteristics of the C4 species in the summer and the C3 species in the autumn among the three water treatments. Interspecific competition exerted negative impacts on the C3 species in August and September but had no effects on the C4 species in any of the four measuring dates. The relative competitive capability of the two species was not altered by water availability. The assimilation rate, the maximum quantum yield of net CO2 assimilation, and the maximum rate of carboxylation of the C3 species were 13–56%, 5–11%, and 11–48% greater, respectively, in a monoculture than in a mixture in August and September. The results demonstrated that the photosynthetic characteristics of the C3 and C4 species were affected by water availability, but the effects varied considerably with season.
Key words: C3, C4, interspecific competition, photosynthesis, seasonal water variability
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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It is well known that there is a temporal differentiation between the growth of C3 and C4 species during the growing season in terms of biomass (Ode et al., 1980
; Riesterer et al., 2000; Winslow and Hunt, 2004), root growth (Nobel, 1997), photosynthetic activity (Tieszen, 1970; Williams and Markley, 1973; Monson et al., 1983; Maragni et al., 2000; Kubien and Sage, 2004), and normalized difference vegetation index (NDVI) data (Tieszen et al., 1997). C3 species are more active in the cool spring and autumn periods whereas C4 species are more active in the hot summer (Ode et al., 1980). Different optimum temperatures (Tieszen, 1970; Williams, 1974; Monson et al., 1983) between C3 and C4 species have been proposed to be responsible for their temporal niche differentiation (Kemp and Williams, 1980). Other studies suggested that precipitation/water regimes (amount and temporal variation) may play important roles in regulating the growth dynamics of C3 and C4 plants (Paruelo and Lauenroth, 1996; Martin et al., 1991; Paruelo et al., 1998; Winslow et al., 2003). Dry winters and wet summers promote C4 expansion, while wet winters and dry summers increase the abundance of C3 plants (Monson et al., 1983). At the global scale, increasing variability of seasonal rainfall accelerated the expansion of C4 grassland in Northern America, China, and Africa (Pagani et al., 1999). However, most of the previous conclusions are drawn from field observations where there are concomitant changes in the amount and seasonality of precipitation/water. Therefore, it is difficult to distinguish between the roles of amount and seasonal variability of precipitation/water in affecting the growth dynamics of C3 and C4 species.
Where C3 and C4 co-occur, the temporal separation of these two photosynthetic types could minimize their competition for resources (Kemp and Williams, 1980; Maragni et al., 2000). Great effort has been devoted to the competition between C3 and C4 species (Owensby et al., 1999; Ziska, 2000; Morgan et al., 2001; Derner et al., 2003). However, knowledge of the effect of seasonal water variation on the competition between C3 and C4 species is limited. Better understanding of the photosynthetic responses of C3 and C4 species to seasonal water variability will help in the search for the underlying mechanisms of temporal niche separation between C3 and C4 species and their competition for water resources.
This study was conducted to examine the effects of seasonal water variability and competition on the photosynthetic traits of C3 and C4 species. One C3 grass (Leymus chinensis) and one C4 grass (Chloris virgata) that co-occur in the typical grasslands of North China were planted in monoculture or as a mixture. The two species were treated with the same amount of water but with three different seasonal patterns, i.e. the one-peak model (more water in the summer than in the spring and autumn), the two-peak model (more water in the spring and autumn than in the summer), and the average model (water distributed evenly over the growing season). The following questions will be addressed: (i) how will the seasonality of water availability affect the photosynthetic traits of C3 and C4 species? (ii) how do the photosynthetic traits of C3 and C4 species respond to the interspecific competition? and (iii) does the water seasonality affect the intensity of interspecific competition in terms of photosynthetic traits of C3 and C4 species?
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Materials and methods |
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Experimental design and treatments
The experiment was conducted in the greenhouse of the Institute of Botany, the Chinese Academy of Sciences, in Beijing, China (39° 9' N, 116° 4' E). The mean annual temperature is 13 °C, with a maximum mean monthly temperature of 27.3 °C in July and a minimum temperature of 3.7 °C in January. Daily mean air temperature during the experiment period is presented in Fig. 1. Mean annual precipitation is 507.7 mm. The meteorological data were provided by the Chinese Meteorological Administration.
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One C3 grass species (Leymus chinensis) and one C4 grass species (Chloris virgata) that co-occur in the grasslands of North China were selected. Plant seeds were collected in the field in the autumn of 2003 and germinated in March 2004 on a wet substrate in a glasshouse. Seedlings with one pair of true leaves were transplanted to plastic pots (21 cm in diameter and 21 cm in height) filled with sand on 21 April 2004.
A complete random factorial design was used in the experiment with two factors (water seasonality and species composition). There were three water treatments, including the two-peak model (TPM) with more water in the spring and autumn than in the summer, the one-peak model (OPM) with more water in summer than in the spring and autumn, and the average model (AM) with water evenly distributed over the growing season (Fig. 1). The total amount of water applied was 500 mm for all the three water treatments. Water was supplied at 5 d intervals from 1 May to 28 October.
The C3 and C4 grass species were planted either in a monoculture (two C3 or C4 individuals per pot) or as a mixture (two individuals including one C3 grass and one C4 grass per pot). There were nine treatments in total (three water treatmentsxthree species compositions) in this study. Each of the nine treatments with six replicates was arranged in one block (consisting of 54 pots). There were 12 blocks in total. One of the 12 blocks was randomly selected for biomass measurement by harvesting at 15 d intervals from 1 May to 30 October.
Nitrogen was supplied in the form of NH4NO3 and P in the form of KH2PO3. Each pot received 1.8 mg N and 0.12 mg P every 5 d. All other nutrients were supplied at the beginning of the experiment.
Leaf water content
Leaf water content was measured at the same time as photosynthesis was measured. Six leaves were selected for each treatment. After determining the fresh mass, leaves were oven-dried at 65 °C for about 48 h and weighed. Leaf water content (%) was calculated as: [(fresh mass–dry mass)/fresh mass)x100].
Photosynthesis measurement
An open gas-exchange system (Li-6400; Li-Cor, Inc., Lincoln, NE, USA) with a 6 cm2 clamp-on leaf cuvette was used to measure gas exchange. Three pots in each of the nine treatments were selected for photosynthesis measurement. In each pot, two fully expanded leaves for each species were measured and the two values were averaged as one replicate. Therefore, each data point in the figures represents the mean values of three replicates per treatment.
Diurnal measurements of gas exchange were taken from 06.00 h to 18.00 h with 2 h intervals on clear days: 2 July, 28 July, 27 August, and 22 September. The amount of water applied at these four measuring dates were 0.68 l, 0.52 l, and 0.5 l on 2 July, 0.75 l, 0.14 l, and 0.5 l on 28 July, 0.68 l, 0.63 l, and 0.5 l on 27 August, and 0.49 l, 0.83 l, and 0.5 l on 22 September for the OPM, TPM, and AM treatments, respectively. Air temperature (Tair), photosynthetic photo flux density (PPFD), carbon assimilation rate (A), and stomata conductance (gs) were recorded.
The maximum quantum yield of net CO2 assimilation (
CO2) and the maximum rate of ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco) carboxylation (Vc,max)
Light response curves were taken by fitting and attaching the light-emitting diode array after removing the chamber window. Artificial illumination was applied to leaves from a red-blue LED light source attached to the sensor head. A range of light intensities between 0 and 2000 µmol m–2 s–1 were provided, starting at 2000 µmol m–2 s–1 and ending at 0 µmol m–2 s–1 at 2 min intervals. Measurements were made at 350 µmol mol–1 CO2 concentration. CO2 was calculated as the slope of the linear portion in the photosynthetic light response curve at a PPFD below 100 µmol m–2 s–1 (Long and Bernacchi, 2003).
The relationship between net assimilation and the CO2 partial pressure (A–Ci curve) was examined over a range of nine external CO2 partial pressures (Ca) from approximately 50 ppm to 1500 ppm. Measurements were taken under saturating light of 1500 µmol m–2 s–1 and ambient relative humidity. Cuvette temperatures were maintained at the ambient levels when the measurement was taken on the first leaf, i.e. 28 °C on 2 July and 27 August, 30 °C on 28 July, and 25 °C on 22 September. The maximum rate of carboxylation (Vc,max) was modelled from each A–Ci curve with a modified Farquhar biochemical model of photosynthesis (Farquhar et al., 1980; Collatz et al., 1991, 1992).
Statistical analysis
Four-way ANOVA (Procedure in SPSS 11.0, USA) was used to examine the main effects and interactions of species, competition, water treatments, and dates on A, gs, CO2, and Vc,max (SPSS 11.0 for windows, USA). Values of A and gs at 10.00 h on each measuring date were used for the statistical analyses. One-way ANOVA (Duncan test) was used continually if there were interactions between treatments. Treatment means were compared by least significant difference to determine whether they were significantly different at the 0.05 probability level.
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Results |
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Diurnal changes in photosynthetic photo flux density (PPFD), air temperature (Tair), and assimilation rate (A)
After sunrise around 07.00 h local time, PPFD increased rapidly, peaked between 10.00 h and 12.00 h, and decreased thereafter. Daily maximum PPFD reached 1400 µmol m–2 s–1 on 28 July and 800 µmol m–2 s–1 on 22 September (Fig. 2). Air temperature (Tair) showed similar diurnal patterns with that of PPFD (Fig. 2).
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Assimilation rate (A) increased rapidly in the morning, maximized at 10.00 h, gradually declined till 14.00 h and then decreased rapidly to zero at 18.00 h on 28 July (Fig. 2, left panels). On 22 September, A of the C3 and C4 species peaked from 10.00 h to 12.00 h and then decreased to zero at 16.00 h (Fig. 2, right panels). A of the C3 and C4 species in monoculture and mixture had similar diurnal patterns irrespective of water treatments. However, A of the C4 species was greater (P <0.001) than that of the C3 species during most measuring times (Fig. 2). The differences in A between the C3 and C4 species were greater at noon, when the temperature and PPFD were higher than in the early morning and later afternoon, when the temperature and PPFD were lower. In addition, the differences in A between the C3 and C4 species declined with the season. On 28 July when air temperature was high, A of the C4 species in monoculture at 10.00 h was 28, 107, and 82% higher than the C3 species under TPM, OPM, and AM treatments, respectively (P <0.001). On 22 September when the air temperature was lower than on 28 July, A of the C4 species in monoculture at 10.00 h was 16, 41 and 41% higher than the C3 species under TPM, OPM, and AM treatments, respectively (P <0.001, Fig. 2).
Seasonal dynamics of leaf water content (LWC) and daily maximum A
Leaf water contents of the C3 and C4 species changed consistently with the amount of water applied under different water-treatment models. LWC was similar among the three water treatments on 2 July and 27 August when there was not much difference in the amount of water applied. On 28 July when 0.75, 0.14, and 0.5 l of water were applied for the OPM, TPM, and AM treatments, LWC of the C3 and C4 species was approximately 30% and 13% lower (P <0.001) under the TPM than the OPM and AM, respectively. On 22 September when 0.49, 0.83, and 0.5 l of water were applied for the OPM, TPM, and AM treatments, the C3 and C4 species had approximately 7–13% (P <0.001) higher LWC under TPM than OPM and AM (Fig. 3).
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The main effect on daily maximum A of date and its interaction with species were statistically significant (Table 1). Daily maximum A of the C4 species in monoculture was greatest on 28 July and lowest in September under the OPM treatment. Under the TPM treatment, daily maximum A of the C4 species was greatest on 2 July and lowest on 28 July. Daily maximum A of the C4 species in a mixture showed similar seasonal dynamics to those in monoculture over the growing season (Fig. 4). Daily maximum A of the C3 species in monoculture had similar seasonal changes under the OPM and AM treatments (e.g. highest on 28 July and lowest on 22 September). However, under the TPM treatments, daily maximum A of the C3 species in monoculture had the highest value on 22 September (Fig. 4).
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Although water treatment did not affect photosynthesis, the interactive effects of waterxdate and waterxspeciesxdate were statistically significant (Table 1). On 2 July and 27 August, there were no significant differences in A of the C3 or C4 species among the three water treatments (P >0.05, Fig. 4). On 28 July, A of the C4 species was 91% and 65% (P <0.001) higher under the OPM and AM than the TPM treatments, respectively. Water treatments did not affect A of the C3 species on 28 July (P >0.05). In September when there was no difference (P >0.05) in A of the C4 species among the three water treatments, A of the C3 species was 48% (P <0.05) and 31% (P <0.05) greater under the TPM than the OPM and AM treatments, respectively (Fig. 4).
Competition and its interaction with species significantly impacted photosynthesis of the C3 and C4 species (P <0.01; Table 1). For example, when water was applied evenly over the growing season (AM), A of the C3 species in monoculture was approximately 32, 43, and 13% higher (P <0.05) than in mixture on 28 July, 27 August, and 22 September, respectively, whereas A of the C4 species was not different between the monoculture and the mixture (P >0.05) on any of the four measuring dates.
Stomatal conductance (gs)
Species, date, and their interactions significantly affected gs (Table 1). Stomatal conductance of the C3 species was greater than that of the C4 species during the whole growing season (P <0.001), but with similar seasonal dynamics (i.e. greatest on 28 July and lower in other three measuring dates). Stomatal conductance of the C3 and C4 species was 14–67% higher on 28 July than on the other three measuring dates in both monoculture and mixture (P <0.001) (Fig. 5).
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The interactive effects of waterxdate and waterxspeciesxdate were statistically significant on gs of the C3 and C4 species, irrespective of the insignificant main effect of water treatment (Table 1). On 28 July, gs of the C4 species in monoculture was 39% and 24% higher (P <0.05) under the OPM and AM than the TPM treatments, respectively (Fig. 5). However, gs of the C3 species at this time showed no difference among the three water treatments (P >0.05). On 22 September, gs of the C3 species in monoculture was 32% and 26% (P <0.001) higher under the TPM than the OPM and AM treatments, respectively. There was no difference in gs of the C4 species among the three water treatments on 22 September. gs of the C3 and C4 species in the mixture showed similar responses to water treatments with those in monoculture (Fig. 5).
Quantum yield of net CO2 assimilation (CO2)
Species, date, and their interactions had significant effects on CO2 (Table 1). Overall, CO2 of the C4 species was 10–93% higher (P <0.001) than that of the C3 species on the first three measuring dates (Fig. 6). The greatest CO2 of the C4 species in monoculture (0.084–0.098 mol mol–1) appeared on 28 July while the lowest was on 22 September (0.061–0.064 mol mol–1). CO2 of the C3 species in monoculture was greatest (0.066–0.070 mol mol–1) in September and lowest (0.053–0.055 mol mol–1) on 28 July.
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There was no effect of water treatment on
CO2. However, the interactive effects of waterxdate and waterxspeciesxdate were statistically significant on CO2 (Table 1). On 2 July and 27 August, there was no difference (P >0.05) in CO2 of either the C3 or C4 species among the three water treatments (Fig. 6). CO2 of the C4 species in monoculture was 12–16% (P <0.05) lower under the TPM than the OPM and AM on 28 July when there were no difference (P >0.05) in CO2 of the C3 species among the three water treatments. On 22 September, CO2 of the C3 species was higher under the TPM than the OPM and AM, whereas there was no difference in CO2 of the C4 species among the three water treatments at this time. The responses of CO2 of the C3 and C4 species in mixture were similar to those in monoculture (Fig. 6).
Competition and the interactions of competitionxspeciesxdate had significant impacts on CO2 (Table 1). CO2 of the C4 species in the mixture was similar to that in monoculture at all measuring dates (Fig. 6). However, CO2 of the C3 species was 5–11% greater in monoculture than in the mixture on the last three measuring dates (Fig. 6).
Maximum carboxylation rate (Vc,max)
The effects on Vc,max of species, competition, water, date, and their interactions among each other were all statistically significant (Table 1). Vc,max of the C3 species increased from July to August and decreased in September in monoculture (Fig. 7). Vc,max of the C3 species in the mixture were similar on 28 July and 27 August, which were significantly greater than those on 2 July and 22 September. Across the three water treatments, Vc,max of the C3 species was, on average, 130% (P <0.05) and 84% higher on 27 August than 2 July in the monoculture and the mixture, respectively. Vc,max of the C4 species were greatest on 28 July under the OPM and AM treatments, but relatively constant among the first three measuring dates under the TPM treatment (Fig. 7).
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The interactive effects of speciesxwater on Vc,max were statistically significant on 28 July and 22 September (P <0.001). On 28 July, Vc,max of the C4 species in monoculture was 85% (P <0.001) and 61% (P <0.001) greater under the OPM and AM than under the TPM treatments, respectively, but there was no difference in Vc,max of the C3 species among the three water treatments. On 22 September, Vc,max of the C3 species in monoculture was approximately 50% (P <0.001) higher under the TPM than the OPM and AM treatments (Fig. 7). However, Vc,max of the C4 species were not significantly different among the three water treatments on 22 September. Water treatment affected Vc,max of the C3 and C4 species in mixture in a similar way to that in monoculture (Fig. 7).
Interspecific competition negatively impacted Vc,max of the C3 species (P <0.05), but had no effect on Vc,max of the C4 species (P >0.05). In the C3 species, Vc,max was significantly greater (11–49%, P <0.001) in monoculture than in the mixture in August and September (Fig. 7). Vc,max of the C4 species was similar between the monoculture and the mixture on all the measuring dates. Water treatments influenced the magnitude of competitive response of the C3 species. For example, on 22 September, Vc,max of the C3 species was 48% higher (P <0.05) in monoculture than the mixture under the TPM treatment, but was only 11% higher under the AM treatment (Fig. 7).
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Discussion |
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Seasonal dynamics of photosynthetic characteristics
Seasonal variations in the photosynthetic characteristics of C3 and C4 species are primarily responsible for the temporal differentiation between the growth of C3 and C4 species (Ode et al., 1980
; Monson et al., 1983; Martin et al., 1991; Paruelo and Lauenroth, 1996; Winslow et al., 2003; Kubien and Sage, 2004). Seasonal differentiation in photosynthetic characteristics between C3 and C4 species have previously been attributed to the different temperature responses of net CO2 assimilation of C3 and C4 species (Kemp and Williams, 1980). C3 species generally have higher rates of photosynthesis at lower temperatures (Black, 1971), whereas C4 species have greater A at higher temperatures (Sage and Kubien, 2003). The lower temperature optima in C3 species might result from lower photorespiration which is caused by the higher CO2 solubility and Rubisco affinity at low temperature (Ku and Edwards, 1977; Sage and Sage, 2002). Photosynthetic depression in C3 species in the hot summers could be caused mainly by the high temperature rather than the water stress (Roessler and Monson, 1985; Maragni et al., 2000). However, the temporal separation of photosynthetic activities between C3 and C4 plants could not be completely explained by temperature. Water availability might also have contributed to the seasonal dynamics of photosynthetic traits in C3 and C4 species, which will be discussed in detail later.
Effects of seasonal water variability
Seasonal water availability was observed to influence the seasonal dynamics of photosynthesis of the C3 and C4 species substantially. The seasonal dynamics of photosynthesis of both the C3 and C4 species followed that of water availability under the OPM treatment (i.e. greatest on 28 July and lowest in September). When water was applied evenly over the growing season (AM treatment), the C3 and C4 species showed similar seasonal dynamics of photosynthesis (greater in July than in August and September). However, under the TPM treatment, the greatest photosynthesis of the C3 species occurred in September and the lowest in July. By contrast, the C4 species showed the highest photosynthesis on 2 July and the lowest on 28 July.
The effects of water treatment on the photosynthetic activities of the C3 and C4 species also changed with season. Water treatment exerted little effect on A of the C3 species in the summer, but significantly impacted A of the C3 species in the autumn. By contrast, water availability substantially affected photosynthesis of the C4 species on 28 July but had no effect on the other three measuring dates (Fig. 4). Therefore, seasonal water variability contributed, at least partially, to the seasonal dynamics of the photosynthetic characteristics of the C3 and C4 species.
Changes in the effect of water availability on the photosynthetic traits with season could be explained by the shifts of limitation between water availability and temperature on C3 and C4 species during the different seasons. In this study, daily maximum/minimum air temperature was 35/23 °C and 25/17 °C on 28 July and 22 September, close to the temperature optima for photosynthesis of C4 and C3 species, respectively. Heat stress is the primary limiting factor for C3 species in the summer and overshadows the role of water availability (Roessler and Monson, 1985; Maragni et al., 2000), leading to the insignificance of the water treatment on the photosynthesis of the C3 species observed in this study. When temperature was close to the growth optima of C3 species in the autumn, water availability significantly impacted the photosynthesis of the C3 species. Contrarily, water availability greatly impacted photosynthesis of the C4 species in the hot summer when the air temperature was optimal for C4 species growth and caused no difference in the photosynthesis of the C4 species on the other three measuring dates. These results suggest that both water and temperature are important in the temporal niche separation, but the role of water depends on season. Changes in the photosynthetic characteristics of the C3 and C4 species with the seasonal dynamics of water availability observed in this study may facilitate an explanation of the temporal differentiation of the growth activities in C3 and C4 species (Ode et al., 1980; Martin et al., 1991; Paruelo and Lauenroth, 1996; Winslow et al., 2003; Stock et al., 2004; Winslow and Hunt, 2004).
Effects of competition
The effects of interspecific competition on the photosynthetic traits of the C3 and C4 species changed with season and were asymmetric. Interspecific competition negatively affected the C3 species, as shown by the decreased photosynthetic performance (A, CO2, and Vc,max) of the C3 species in a mixture compared with a monoculture (Figs 4, 6, 7). However, the photosynthetic characteristics of the C4 species were not impacted by interspecific competition. The asymmetric competition between the C3 and C4 species observed in this study could primarily be caused by the difference in water use efficiency between these two photosynthetic types. The greater water use efficiency of C4 species (Knapp and Medina, 1999; Sage, 2003) put it at an advantageous position in competing for water resource compared with C3 species.
The relative competitive capability of the C3 and C4 species was not affected by water seasonality. These results disagree with the previous studies conducted in the field (Monson et al., 1983; Amundson et al., 1994; Winslow et al., 2003), which reported that in years with moist springs and dry summers, C3 grasses become more competitive, whereas C4 species become more competitive in years with dry springs and wet summers. However, water treatments affected the magnitude of competitive response of the C3 species. For example, the competitive response [(mixture–monoculture)/monoculture] of daily maximum A of the C3 species was significantly different (P <0.05) under the OPM (–47%) from that under the AM treatment (–13%) on 22 September.
The photosynthetic responses of the C3 and C4 species to seasonal water variability observed in this study may help to explain the temporal niche differentiation between C3 and C4 species. Temperature interacted with water availability to influence the photosynthetic characteristics of the C3 and C4 species over the growing season, suggesting shifts of limitation between water availability and temperature on C3 and C4 species during the different seasons. The interactions between water seasonality and that of temperature on the temporal niche differentiation between C3 and C4 species need further study. Information about the temporal niche differentiation between C3 and C4 species may help improve our understanding of the shift of C3 and C4 vegetation under global change. The projected changes in precipitation regime (including amount, frequency, and seasonality) under global environmental change (IPCC, 2001) may have profound influences on the photosynthetic characteristics, growth, and competition of C3 and C4 species. C4 species should be favoured where precipitation increases in hot summers, while C3 species should be favoured if precipitation increases during the cool seasons (Monson et al., 1983; Sage et al., 1999). The differential responses of C3 and C4 species to the precipitation regime can potentially lead to changes in the ecosystem structure and function in the C3/C4 mixed communities.
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Acknowledgements |
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The authors thank Xianzhong Wang and three anonymous reviewers for their critical comments on earlier versions of the manuscript. This study was financially supported by the National Natural Science Foundation of China (30470273) and the National Key Basic Research Program of China (2004CB41850x).
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