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Crop Science 41:810-817 (2001)
© 2001 Crop Science Society of America

CROP QUALITY & UTILIZATION

Response of Perennial Ryegrass to Free-Air CO2 Enrichment (FACE) Is Related to the Dynamics of Sward Structure during Regrowth

Daniel Suter, Josef Nösberger and Andreas Lüscher*

Institute of Plant Sciences, Swiss Federal Institute of Technology (ETH), 8092 Zurich, Switzerland

* Corresponding author (andreas.luescher{at}ipw.agrl.ethz.ch)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIAL AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
At elevated partial pressure of CO2 (60 Pa pCO2), field-grown perennial ryegrass (Lolium perenne L.) swards generally show a strong increase in photosynthesis of the youngest fully expanded leaves but a weak yield response. We investigated this discrepancy by determining the effects of elevated pCO2 on the dynamics of regrowth. During the whole regrowth period, swards at elevated pCO2 exhibited a greater dry mass of roots (68%), of pseudostems (34%), and of shoot necromass below cutting height (45%). During the first 4 wk of regrowth, when dry mass (DM) and the leaf area index increased strongly, elevated pCO2 stimulated the yield, total DM, and leaf area by up to 46, 63, and 30%, respectively. Elevated pCO2 also resulted in an increase of almost 60% in the number of tillers during this period when the transmission of photosynthetically active radiation to the tiller bases was high. Towards the end of regrowth, however, the leaf area, total DM, and yield showed no net increase; under elevated pCO2, the number of tillers decreased strongly. These results indicate that there was not a strong sink for additionally fixed C during the second half of regrowth. The gap between leaf photosynthesis and yield at elevated pCO2 is suggested to be related to a loss of tillers when shading of tiller bases was strong and to a change in dry mass allocation.

Abbreviations: DM, dry mass • FACE, Free-Air CO2 Enrichment • LAI, leaf area index • PAR, photosynthetically active radiation • pCO2, partial pressure of CO2 • R/FR ratio, red/far-red ratio


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIAL AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
BURNING FOSSIL FUELS and changes in land management have caused levels of partial pressure of CO2 in the atmosphere to increase from 28 Pa before industrialization to about 36 Pa today. Atmospheric pCO2 is rising at a rate of about 0.15 Pa yr-1, and there is no doubt that it will continue to increase (Houghton et al., 1996). It is widely accepted that elevated pCO2 stimulates photosynthetic C fixation and biomass production (Newton, 1991; Drake et al., 1997). Thus, terrestrial ecosystems are considered to play an important role in the global C budget (Houghton et al., 1998).

Grasslands make up a substantial part of these terrestrial ecosystems, covering a considerable area world wide (FAO, 1995). Furthermore, the role of grasslands in the global C budget is important because of their high capacity to sequester C (Parton et al., 1995). Perennial ryegrass is an important species in intensively managed grasslands in temperate climates. A number of investigations reported weak or even negative yield responses (-13% to +19%) for perennial ryegrass swards to elevated pCO2 under field conditions (Hebeisen et al., 1997a,b) or field-like conditions (Sæbø and Mortensen, 1995; Casella et al., 1996; Schapendonk et al., 1997). These studies concentrated on the effect of elevated pCO2 on the final yield of dry mass (DM) at the end of regrowth. Therefore, no information exists about the effect of the dynamics of regrowth of established swards on the yield response of perennial ryegrass to elevated pCO2.

In contrast to the weak yield response to elevated pCO2, the rate of photosynthesis increased strongly in the youngest fully expanded leaves of perennial ryegrass swards in the same FACE array. This increase in leaf photosynthesis at elevated pCO2 occurred not only at the beginning of fumigation, but was still detectable after 2 (Rogers et al., 1998) and 5 yr (Isopp et al., 2000) of continuous fumigation. Elevated pCO2 also increased the rate of photosynthesis at the beginning (44 and 28%) and towards the end (50 and 39%) of individual regrowth periods (Rogers et al., 1998; Isopp et al., 2000) when the swards had very different leaf area indices (LAI). At the end of regrowth, the response of perennial ryegrass swards to elevated pCO2 under field conditions was probably dominated by a sink limitation for additionally fixed C. This led to a decreased specific leaf area (Hebeisen et al., 1997a), to an increased concentration of nonsturctural carbohydrates in source leaves (Fischer et al., 1997; Rogers et al., 1998; Isopp et al., 2000), and to a reduced apparent night export of nonstructural carbohydrates (Fischer et al., 1997).

Light conditions at the tiller base change significantly during the regrowth of a sward, including transmission of photosynthetically active radiation (PAR) and light quality (e.g., red/far-red [R/FR] ratio or blue light) (Gautier et al., 1999). The R/FR ratio has an impact on tiller dynamics and, consequently, on growth (Schapendonk et al., 1990; Van Loo et al., 1992). Dense swards, in which the tiller bases are strongly shaded, show a weaker response to elevated pCO2 than young plants in gaps, which show a strong increase in tiller number and, thus, represent an expanding system that produces new tillers (Lüscher et al., 1996, 1998). Shading has a negative effect on the production of new tillers and on the survival of tillers in perennial ryegrass swards (Gautier et al., 1999). Shading in a dense canopy towards the end of regrowth may limit the initiation and growth of tillers, a basis for new sinks for C. Thus, the dynamics of tiller number may help to explain the sink limitation, which was measured in perennial ryegrass swards at elevated pCO2 towards the end of regrowth (Fischer et al., 1997; Hebeisen et al., 1997a,b; Rogers et al., 1998). However, the dynamics of tiller number during the regrowth of perennial ryegrass swards at elevated pCO2 has never been studied under field conditions.

An important component of C fluxes in grassland ecosystems is the production of litter. Both the quality and quantity of litter are important factors in the sequestration of C into the soil. They affect the rates of C input into the soil and of C release through mineralization. Numerous investigations demonstrated that, in grassland ecosystems with no legumes, the quality of litter under elevated pCO2 was reduced, mainly because of a higher C/N ratio (e.g., Ball, 1997; Ball and Drake, 1997; Cotrufo et al., 1998; Hartwig et al., 2000). However, little is known about the effects of elevated pCO2 on the quantity of litter produced by a grass sward (Blum et al., 1997; Norby and Cotrufo, 1998; Hartwig et al., 2000). Tiller turnover and the decay of the attached roots may play an important role in the production of litter.

The objective of this investigation was to study the effects of elevated pCO2 on the dynamics of the transmission of radiation, the tiller number, the leaf area, and the necromass during regrowth of perennial ryegrass swards under field conditions. These results should help to explain the weak yield response to elevated pCO2 and the C-sink limitation of perennial ryegrass swards at the end of regrowth. The effects of the observed dynamics on litter production are discussed.


   MATERIAL AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIAL AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Site
The experimental site was located in the Swiss FACE (Free-Air CO2 Enrichment) array (Hebeisen et al., 1997b) at Eschikon (8°41'E, 47°27'N) near Zurich, 550 m above sea level. The soil was an eutric cambisol and consisted of about 360 g kg-1 sand, 330 g kg-1 silt, and 280 g kg-1 clay and was designated as a clay loam according to the U.S. classification. The organic matter content varied from 29 g kg-1 to 51 g kg-1. The pH (water extracted) ranged from 6.5 to 7.6. The available phosphorus and potassium (CO2-saturated water extraction) ranged from 1.2 to 6.0 mg P kg-1 soil and from 18 to 47 mg K kg-1 soil (Lüscher et al., 1998). To maintain phosphorus and potassium levels that were non limiting for intensive grassland, 4.3 g P m-2 yr-1 and 24.6 g K m-2 yr-1 were applied. The annual amount of N fertilizer was 56 g N m-2. This amount was split into eight applications, with the doses being adjusted to the expected DM production per regrowth period, as described by Zanetti et al. (1996). Dry mass production was expected to be highest in spring and lowest towards the end of the growing season. Nitrogen was applied as NH4NO3 every 4 wk, beginning at the start of the growing season. Table 1 lists the meteorological data.


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  		Table 1. Weekly average of daily mean temperature and sum of precipitation in the FACE experiment at Eschikon immediately before and during the examined regrowth period in summer 1997.

 
Experimental Preparation and Treatments
An existing stand of perennial ryegrass, which was grown in the Swiss FACE array since 1992, was removed by applying glyphosate [N-(phosphonomethyl)glycine] followed by shallow tillage in mid-July in 1996. Individual plants of ‘Bastion’ perennial ryegrass were sown in 15-mL pots in soil taken from the FACE area at the beginning of the last week of July and grown in a greenhouse for 6 wk. In the first week of September, the perennial ryegrass plants were transplanted to the main-plots (9.24 by 3.15 m) of the experiment at intervals of 7 by 7 cm (204 plants m-2). The swards were cut at 5 cm above ground in the first week of November. In the spring of 1997, all the plots were weeded by hand before the onset of the growing season.

The experiment was designed as a split-splitplot with three blocks. The blocks were set up according to the crops grown in the year preceding the installation of the FACE facility. These crops were winter wheat (Blocks 1 and 3) and grass/white clover mixture (Block 2). The main-plot treatment was pCO2 (ambient [36 Pa] and elevated [60 Pa] pCO2). Fumigation was done by the FACE technique as described by Lewin et al. (1992). Three fumigated circular FACE plots (18-m diameter) and three control plots (ambient pCO2) of the same size and shape were installed in the spring of 1993. There was a distance of at least 100 m between the plots in order to eliminate the possibility of unwanted effects among the plots, e.g., ambient plots being affected by elevated pCO2. Each FACE plot and each control plot contained one main-plot. Fumigation with CO2 was continuous during the day from spring until November when the air temperature was at least 5°C (2 m above ground), taking into account the fact that plant growth is marginal and that pCO2-induced effects on photosynthesis are weaker at low temperature (Long, 1994). Pretreatments were applied during the establishment of the swards to produce plants of different morphology at the beginning of the investigated regrowth period. The pretreatments were combinations of cutting height (4 and 8 cm) and cutting frequency (intervals of 4 and 8 wk = 8 and 4 cuts yr-1, respectively) and were randomly assigned to the four subplots (2.31 by 3.15 m) within each main-plot.

Data Collection and Measurements
One year after sowing (establishment period with pretreatments), measurements were made for 8 wk from 21 July until 15 Sept. 1997 after 0, 1, 2, 4, 6, and 8 wk of regrowth (sampling date). On each of the six sampling dates, one of six subsubplots (0.21 by 0.21 m, surrounded by a border of 0.21 m), selected randomly within each subplot, was harvested destructively. Strata of 0 cm above ground to cutting height, and above cutting height were separated by using a metal frame to ensure exact cutting height. In each subsubplot, the soil was removed to a depth of 10 cm and the roots were washed. Harvested subsubplots were replaced immediately by turves from the same pretreatment to avoid gaps within the subplots. Green and necrotic plant material from each stratum were separated. Green plant material was divided into leaf laminae and pseudostems (leaf sheaths and enclosed parts of the growing leaf). Tillers were considered to be small tillers when the leaf sheath of the youngest fully expanded leaf was shorter than 4 cm or when the tiller had not produced a fully expanded leaf yet. Leaf area was measured with a photoelectronic leaf area meter (LI 3000, LI-COR, Lincoln, NE), and the number of tillers was determined. All the fractions were oven-dried at 65°C for 48 h. The transmission of PAR was measured with a ceptometer (Decagon Devices Inc., Pullman, WA) by measuring alternately above the canopy and 4 cm above the ground in the respective subplot. Five sequential measurements were made in each subplot just before each sampling date. The measurements were made between 1100 and 1300 h.

Statistical Analysis
The statistical analysis was carried out using the GLM procedure of the SAS package (SAS Institute, 1996). The model was a split-splitplot with pCO2 as the main-plot factor, cutting frequency and cutting height the subplot factors, and sampling date the subsubplot factor. The entire experiment consisted of 144 experimental units (2 pCO2 x 2 cutting heights x 2 cutting frequencies x 6 sampling dates x 3 blocks). Because of the split-splitplot design, the significance of the pCO2 effect was tested with the pCO2 x block interaction mean square. This is a weak test, and pCO2 effects are more readily detected by the different dynamics of the pCO2 response of the different pretreatments (pCO2 x sampling date and pCO2 x cutting frequency x cutting height x sampling date). Values for LAI and DM were ln transformed before the analysis of variance to normalize the distribution and stabilize the variances.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIAL AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Although the cutting height had direct effects on the examined traits, it did not influence the effect of elevated pCO2 or cutting frequency on the swards. Thus, the data of cutting height were pooled. When regrowth started, the swards that had been cut frequently during pretreatment (frequently pretreated) had greater (P < 0.0001) residual leaf area (LAI 0.11 vs. LAI 0.47 [Fig. 2a]). In all treatments the tiller number was about 5200 tillers m-2 (Fig. 3a) except in the frequently pretreated swards at elevated pCO2, which showed a trend (P < 0.1) towards a higher number of tillers (6400 tillers m-2).



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  		Fig. 2. Leaf area index (a) and transmission (%) of photosynthetically active radiation (PAR) to 4 cm above the soil surface (b) during regrowth of perennial ryegrass swards from 21 July to 15 Sept. 1997 at ambient (36 Pa) and elevated (60 Pa) pCO2 and after an infrequent (4 cuts yr-1) and frequent (8 cuts yr-1) cutting treatment before the beginning of the experiment (Error bars = SE, n = 6).

 


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  		Fig. 3. Total tiller number (a) and number of tillers shorter than 4 cm (b) during regrowth of perennial ryegrass swards from 21 July to 15 Sept. 1997 at ambient (36 Pa) and elevated (60 Pa) pCO2 and after an infrequent (4 cuts yr-1) and frequent (8 cuts yr-1) cutting treatment before the beginning of the experiment (Error bars = SE, n = 6).

 
Dynamics of Yield and Total Dry Mass
At the beginning of regrowth, the swards under elevated pCO2 had 125 g m-2 more total DM (P < 0.001) than swards grown at ambient pCO2 (Fig. 1a). During the first 4 wk, the total DM of the elevated pCO2 treatment increased strongly (P < 0.0001) but did not increase further during the second half of regrowth (NS). The greatest CO2-induced difference in total DM (334 g m-2 or 63%; P < 0.0001) was measured at the end of Week 4. At ambient pCO2, total DM continued to increase until the end of Week 6 leading to a smaller CO2-induced difference after Week 6 than after Week 4.



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  		Fig. 1. Total DM (a) and yield (b) during regrowth of perennial ryegrass swards from 21 July to 15 Sept. 1997 at ambient (36 Pa) and elevated (60 Pa) pCO2 and after an infrequent (4 cuts yr-1) and frequent (8 cuts yr-1) cutting treatment before the beginning of the experiment. Necromass in (a) and (b) not included (Error bars = SE, n = 6).

 
During the first 4 wk of regrowth, yield (above cutting height) increased (P < 0.0001) to approximately 210 g m-2 (Fig. 1b). Thereafter yield showed only a slight increase (NS) to about 270 g m-2. After 2 wk of regrowth, swards grown at elevated pCO2 had a 46% (30 g m-2) higher yield than swards grown at ambient pCO2 (P < 0.01). This relative advantage decreased to 20% (38 g m-2) by the end of Week 4. At the end of the eighth week of regrowth all the treatments had reached a yield of about 300 g m-2 with the exception of the frequently pretreated swards at ambient pCO2, which yielded about 190 g m-2.

Dynamics of Leaf Area Index and Transmission of Photosynthetically Active Radiation
Leaf area index was initially very low in all treatments (Fig. 2a). During the first 4 wk of regrowth, LAI increased strongly (P < 0.0001) but showed no further increase beyond 4-wk regrowth. After 2 wk of regrowth, swards grown at elevated pCO2 exhibited a 30% greater LAI than swards grown at ambient pCO2 (P < 0.05). This advantage lasted until the end of Week 4 but disappeared during the last 4 wk of regrowth (NS). At the end of the 8-wk regrowth period, all treatments reached a comparable LAI of approximately 5, except for the frequently pretreated swards at ambient pCO2 which reached a value of only about 3.

Transmission of PAR to the tiller bases, which was very high immediately after the cut, decreased (P < 0.0001) rapidly during the first 4 wk of regrowth (Fig. 2b). After 2 wk, transmission of PAR was below 35% and decreased to less than 6% by the end of Week 4. Compared with swards grown at ambient pCO2, those at elevated pCO2 had 16% (P < 0.01) and 44% (P < 0.0001) higher transmission of PAR after 1 wk and 2 wk of regrowth, respectively.

Dynamics of Tiller Number
Tiller number (Fig. 3a) was affected differently by ambient and elevated pCO2 during regrowth (P < 0.0001). During the second week of regrowth, the swards at elevated pCO2 showed a 55% (P < 0.0001) increase in the number of tillers and reached a maximum of 10100 tillers m-2 after 4 wk. Subsequently, at elevated pCO2, the number of tillers decreased again strongly (P < 0.001) to 7700 tillers m-2. In contrast, at ambient pCO2, the number of tillers was more stable during regrowth. The number of tillers was constant over the whole regrowth period in the frequently pretreated swards. In the infrequently pretreated swards it increased to a lesser extent than at elevated pCO2, reaching about 7000 tillers m-2 towards the end of regrowth.

The fluctuation in total tiller number at elevated pCO2 corresponded with a strong fluctuation (P < 0.0001) in the number of small tillers (Fig. 3b), which reached a maximum density of about 7700 tillers m-2 by the end of Week 2 but decreased (P < 0.0001) to 1600 tillers m-2 by Week 8. The dynamics of the number of small tillers under ambient pCO2 differed significantly (P < 0.0001) from that under elevated pCO2; it started at 3200 tillers m-2 and decreased (P < 0.01) steadily to a final value of 1300 tillers m-2.

Dynamics of Pseudostem, Root Mass, and Necromass
From the end of Week 1 to the end of Week 4, the pseudostem DM increased (P < 0.0001) by 180 g m-2 under elevated pCO2 but did not increase thereafter. At ambient pCO2, the pseudostem DM increased until the end of Week 6. Pseudostem DM was 34% (P < 0.05) greater at elevated pCO2 than at ambient pCO2, averaged over the whole regrowth period (Fig. 4a). At the end of Week 1, the elevated pCO2 treatment had produced 30 g m-2 more pseudostem DM than the ambient treatment (P < 0.05). This advantage increased to 125 g m-2 (P < 0.0001) by the end of Week 4.



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  		Fig. 4. Pseudostem DM (a) and root DM between 0 and 10 cm soil depth (b) during regrowth of perennial ryegrass swards from 21 July to 15 Sept. 1997 at ambient (36 Pa) and elevated (60 Pa) pCO2 and after an infrequent (4 cuts yr-1) and frequent (8 cuts yr-1) cutting treatment before the beginning of the experiment (Error bars = SE, n = 6).

 
There was 68% more (P < 0.01) root DM at elevated pCO2 than at ambient pCO2 throughout the whole regrowth period (Fig. 4b), with only a slight fluctuation (NS) during the regrowth period. Necromass above cutting height was very low during the first 4 wk of regrowth, but showed a strong increase (P < 0.0001) thereafter (Fig. 5a). The necromass above cutting height was not affected (NS) by elevated pCO2. In contrast, there was on average 45% more shoot necromass below cutting height (P < 0.001) at elevated pCO2 over the entire regrowth period (Fig. 5b).



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  		Fig. 5. Shoot necromass above cutting height (a) and below cutting height (b) during regrowth of perennial ryegrass swards from 21 July to 15 Sept. 1997 at ambient (36 Pa) and elevated (60 Pa) pCO2 and after an infrequent (4 cuts yr-1) and frequent (8 cuts yr-1) cutting treatment before the beginning of the experiment (Error bars = SE, n = 6).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIAL AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Carbon Source and Sink Limitations to Regrowth
During the first 4 wk of the regrowth period, perennial ryegrass swards grown at elevated pCO2 developed more new tillers than swards at ambient pCO2. This growth strategy was successful and led to more total DM, a higher yield, and higher LAI in swards grown under elevated pCO2 (Fig. 1, 2a) compared with swards grown under ambient pCO2. Due to increased tillering during this period, the sink for additionally fixed C at elevated pCO2 must have been large, enabling the plant to benefit from the increased C supply. Higher rates of photosynthesis were measured without exception in perennial ryegrass swards in the same FACE array under the same management treatments after two (Rogers et al., 1998) and five years (Isopp et al., 2000) of fumigation. These findings are in agreement with results obtained with other species (Drake et al., 1997). A higher rate of photosynthesis at elevated pCO2 during the experimental regrowth period is also consistent with the observed 17% increase (P < 0.05) in the C/N ratio of leaf blades (data not shown). The importance of new tillers for the C sink in the plant system is supported by other reports (Bos and Neuteboom, 1998; Marshall, 1990). At elevated pCO2, increased tillering at the beginning of regrowth may be related to two processes: First, a greater transmission of PAR (Fig. 2b) most probably increased the R/FR ratio at the tiller bases, which in turn may have increased the activation of tiller buds (Casal et al., 1987; Davis and Simmons, 1994; Gautier et al., 1999). This difference in transmission of PAR was unexpected, because the LAI increased by 18% under elevated pCO2 during the same period. According to the model of Monsi and Saeki (1953), more erect leaves would explain the higher transmission of PAR at the same or slightly increased LAI at elevated pCO2 compared with ambient pCO2. The extinction coefficient was about 30% lower at elevated pCO2 (data not shown). The steeper mean leaf angle in the elevated pCO2 swards during the first 4 wk of regrowth may have been due to the greater number of young tillers, which may have more erect leaves than older tillers. Second, an increased availability of carbohydrates due to a higher rate of photosynthesis (Rogers et al., 1998; Isopp et al., 2000) a higher concentration of carbohydrates in pseudostems (Fischer et al., 1997) combined with a larger amount of storage tissue (pseudostems and roots [Fig. 4a,b]), may have stimulated the growth of newly activated buds in the very beginning of regrowth when light interception was low.

The stimulation of photosynthesis at elevated pCO2 continued until the end of regrowth (Rogers et al., 1998; Isopp et al., 2000) when perennial ryegrass swards had high LAI. However, our data indicate a lack of C sinks; the biomass and yield of the whole plant did not increase further and tiller number decreased strongly. This source to sink imbalance can explain the accumulation of water soluble carbohydrates in the leaves (Rogers et al., 1998; Isopp et al., 2000) and the reduced apparent night time export of carbohydrates (Fischer et al., 1997) at elevated pCO2. The decrease in tiller number in the second half of regrowth may be related to a reduction in light quality, because R/FR decreases much faster than transmitted PAR with increasing leaf area (Ballaré et al., 1987). A second reason may be that transmission of PAR to the layer below 4 cm was less than 4% (Fig. 2b). Thus, small tillers may not have been able to fix a sufficient amount of C for survival. Large tillers represent a stronger C sink (Davies, 1988) and can compete for C more efficiently than developing tillers (Ryle and Powell, 1976; Powell and Ryle, 1978; Donaghy and Fulkerson, 1998). Small tillers tend to die off in gramineous plant stands under stress conditions such as shading (Alberda and Sibma, 1982), drought (McMaster et al., 1994), or low content of water soluble carbohydrates (Donaghy and Fulkerson, 1998) due to the competitive advantage of already existing tillers (Davies, 1988; Davies et al., 1983). During the examined period of regrowth, small tillers died off when source activity was large due to the interception of nearly all the incident PAR by a large leaf area.

Temperature and rainfall can be considered as non-limiting to the tiller number dynamics during regrowth (Table 1). Thus, we suggest that severely shaded tiller bases limited the dynamics of tiller number, resulting in a limited sink for additionally fixed C (Fischer et al., 1997; Rogers et al., 1998) in swards that had already developed a high LAI. This may be an important cause for the different responses to pCO2 in established swards compared with spaced plants, which grow exponentially at early developmental stages. Shortening the cutting interval and, consequently, the duration of shading may not result in growth conditions comparable to exponentially growing spaced plants. This is because the tiller number per ground area cannot increase infinitely. In addition, at high cutting frequency too little time may remain to replenish C reserves, especially at ambient pCO2. This was shown by the yield (Fig. 1a), the LAI (Fig. 2a), and the tiller number (Fig. 3a) in the swards at ambient pCO2 that had been cut frequently during the pretreatment. The much more pronounced relative response to elevated pCO2 of the frequently pretreated swards compared with the infrequently pretreated swards was based on the slow development of the ambient swards rather than on exceptionally high productivity at elevated pCO2.

Change in Dry Mass Allocation at Elevated pCO2
Compared with the pCO2-induced stimulation of leaf photosynthesis of perennial ryegrass at the same site and under the same management treatments (Rogers et al., 1998; Isopp et al., 2000), the average increase in yield above cutting height of field-grown perennial ryegrass at the end of regrowth was only moderate in this experiment and in experiments conducted by Hebeisen et al. (1997a)(b), and Daepp et al. (2000). In addition to a lack of sinks related to tiller dynamics, our data reveal three other possible reasons for this discrepancy. First, the increase in the amount of respiratory tissue (pseudostems, roots) at elevated pCO2 may have led to a greater C loss as respiration partially offsets the greater C fixation at elevated pCO2 (Fischer et al., 1997; Rogers et al., 1998). Van Ginkel et al. (1997) reported an increase of 111% in the respiration of roots and soil and a greater loss (50%) of C per unit root mass at elevated pCO2 in a perennial ryegrass stand after 79 days of growth. In a two-year experiment, Schapendonk et al. (1997) found a 39% increase in root and soil respiration in perennial ryegrass swards at 70 Pa pCO2. Second, the increased root DM at elevated pCO2 may lead to a greater C loss through rhizodeposits (Paterson et al., 1996). Third, despite an increase of 50% (Rogers et al., 1998) and 39% (Isopp et al. 2000) in the rate of mid-day photosynthesis of young source leaves at elevated pCO2, the rate of photosynthesis of the whole canopy may be stimulated to a lesser extent, because a large part of the leaf area in the swards is shaded during the second half of regrowth. Thus, it is necessary to study the dynamics of loss and gain of C of the canopy during regrowth at different pCO2. This may help to clarify the extent of the increase in the rate of photosynthesis and respiration not only at the leaf level but of the whole sward.

Necromass Production
The development of yield (Fig. 1b) and LAI (Fig. 2a) demonstrates a strong net production during the first 4 wk of regrowth. However, there was no net increase in yield, total DM, and LAI from Week 4 to the end of regrowth. The most probable explanation for this is that the loss of senescent leaves counterbalanced the production of new leaves, resulting in a dynamic equilibrium (Davies, 1988). This is strongly supported by the number of leaves per tiller, which reached a ceiling of 2.4 (data not shown). In addition, the amount of necromass above the cutting height increased strongly after Week 4 (Fig. 5a). Even though this is only an estimate of litter production through leaf turnover, it demonstrates the importance of this mechanism.

In addition to the leaf turnover, the turnover of tillers may also be an important source of litter. At elevated pCO2, the net decrease in tiller number from Week 4 to 8 was about 23% (Fig. 3a). If it is assumed that the loss of tillers affects the loss of shoot DM below cutting height and the loss of attached roots, then the amount of litter at elevated pCO2 is probably considerably greater than at ambient pCO2. This estimate of tiller turnover is conservative because only the net loss of tillers was taken into consideration. Gross losses may be much bigger but may be masked by the simultaneous production of new tillers. The marked increase in the amount of necromass below cutting height strongly supports the importance of elevated pCO2 for litter quantity in perennial ryegrass swards.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIAL AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Perennial ryegrass swards showed only a moderate yield response to elevated CO2. Elevated CO2 stimulated tiller number in the early phase of regrowth, enhanced biomass below cutting height and increased shoot necromass. The results indicate that the gap between leaf photosynthesis and yield at elevated CO2 is due to a sink limitation, caused by a loss of tillers in later growth stages, and a change in dry mass allocation.


   ACKNOWLEDGMENTS
 
We thank K. Rüegg and W. Wild for technical assistance and H. Blum, R. Bossi, J. Nagy, and G. Hendrey for maintaining the FACE. M. Schoenberg checked the English. The Swiss FACE was supported by the Swiss Federal Institute of Technology, the Swiss National Energy Research Fund, the Swiss National Science Foundation, the Swiss Department for Energy, the Swiss Department for Agriculture and the Brookhaven National Laboratory (NY).

Received for publication December 27, 1999.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIAL AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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