Contrasting effects of elevated CO2 on the root ... - Wiley Online Library

New Phytol. (1993). 125, 855-866

Contrasting effects of elevated COg on the root and shoot growth of four native herbs commonly found in chalk grassland BY RACHEL FERRIS AND GAIL TAYLOR School of Biological Sciences^ University of Sussex, Falmer, Brighton, Sussex, BNl 9QG, UK {Received 4 May 1993; accepted 18 August 1993)

SUMMARY The aim of this study was to investigate the impact of ambient (345 ^1 1') and elevated (590/^11"') CO^ on the root and shoot growth of four native chalk grassland herbs: Satiguisorba minor Scop, (salad bumet), Lotus corniculatus L. (birdsfoot trefoii), Anthyllis vulneraria L. (kidney vetch) and Plantago media L. (hoary plantain). Elevated CO^ had contrasting effects on both shoot and root growth of the four species studied. Both leaf expansion and production were stimulated by elevated CO^ for S. minor, L. corniculatus and P. media, whilst for A. vulneraria, only leaflet shape appeared to be altered by elevated CO^, with the production of broader leaflets, cotnpared with those produced in ambient COj. After 100 d shoot biomass was enhanced in elevated CO^ for S. minor and L. corniculatus, whilst there was no effect of elevated CO^ on shoot biomass for A. vulneraria or P. media. Contrasting effects of CO^ were also apparent for measurements of specific leaf area (SLA), which increased for L. corniculatus, decreased for A. vulneraria and remained unaltered for S. minor and P. media in elevated compared with ambient CO^. Elevated COj also had contrasting effects on both the growth and morphology of roots. The accumulation of root biomass was stimulated following exposure to elevated CO^ for S. minor and L. corniculatus whilst there was no effect on root biomass for A. vulneraria or P. media. Root length was measured on three occasions during the 100 d and revealed that exposure to elevated CO^ promoted root extension in S. minor, L. corniculatus and P. media, but not in A. vulneraria. Specific root length (SRL, length per unit dry weight) was increased in elevated CO., for one species, P. media, whilst the root to shoot ratio of all four species remained unchanged by CO^. These results show that four native herbs differ in their response to CO^, suggesting that the structure of this plant community may be altered in the future. Key words: Sanguisorba minor (salad burnet), Lotus corniculatus (birdsfoot trefoil), Anthyllis vulneraria (kidney vetch), Plantago media (hoary plantain), chalk grassland, elevated CO^.

phosphorus status of the soil, the grazing regime, INTRODUCTION

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1

plant competition and the irequent occurrence or soil Chalk grassland is an important plant community in moisture deficits. All these factors act to check the south eastern England and also over areas of Europe growth of potentially dominant species, allowing (Bobbink, 1991) where it may dominate parts of the slower growing herbs to compete (Silvertown, 1979; landscape (Mitchley, 1983; Wells, 1989). It is not Smith, 1980). Any environmental change which unusual to find thirty or more species co-existing in alters the morphology, function and growth of any an area of only 025 m^ (Mitchley & Grubb, 1986). component species in such a community may Chalk grassland has a high conservation value, partly influence the long-term structure of the vegetation due to the high biodiversity and occurrence of many (see Woodward, 1992, 1993; Johnson et al., 1992) rare species, but also because over 8 0 % of such and also the ability to support both invertebrate and grassland has been lost since the second world war vertebrate species. (Wells, 1989, 1991). Competition for above- and There is little information on the responses of below-ground resources is often intense, with the chalk grassland plants to the greenhouse gases which balance of the community maintained by a com- are at present increasing in concentration (IPCC, bination of factors including the low nitrogen and 1990) including COg and trophospheric O3 (in

8S6

R. Ferris and G. Taylor

contrast to stratospheric ozone which is declining), both of which are known to influence plant growth and physiology (Cannel! & Hooper, 1990). Considerable work has looked at the effects of elevated CO2 on crop plants (Warrick, Gifford & Parry, 1986; Kimhall et al., 1993; Rogers & Dahlman, 1993) and trees (Jarvis, 1989) with the few studies on native herbaceous vegetation providing contradictory or complex responses to elevated CO^ making generalizations on the effects of elevated CO^ at the community level extremely difficult. For example, Hunt et ah, (1991) exposed 25 native British herbaceous species to elevated COg for several weeks and concluded that large positive increases in growth were only observed in the plants displaying the 'competitive' strategy (Grime, Hodgeson & Hunt, 1988), suggesting that chalk grasslands may be largely unresponsive to CO3, since they are composed of species displaying the ' stress tolerator' strategy. More recently, Poorter (1993) used data from 156 plant species and found that for C, plants, herbaceous crop plants showed a greater positive response to CO.^. Increases in growth are thought to reflect physiological changes including increased photosynthesis (Cure & Acock, 1986; Bazzaz, 1990; Sastek & Strain, 1991), although the long-term maintenance of increased photosynthetic efficiency may depend on the nutrient status of the plants (Tissue & Oechel, 1987; Drake & Leadley, 1991; Long, 1991, 1993). Elevated CO.^ may also stimulate flowering, dr>' matter production and leaf area development, whilst decreased stomatal conductance and transpiration may lead to an increase in the instantaneous value of water use efficiency (Kramer, 1981; Enoch & Kimball, 1988; Eamus & Jarvis, 1989). Changes in the partitioning of photosynthate between roots and shoots may also be altered following exposure to elevated CO.^, often resulting in a stimulation of root growth (Curtis et al., 1990; Idso & Kimball, 1991 ; Rogers et al., 1992; Stulen & den Hertog, 1993) and alterations in the way in which root systems forage within the soil, including changes in the patterns of root branching (see Berntson & Woodward, 1992). The aim of this research was to provide detailed information on the effects of elevated CO^ on the root and shoot growth of component species of chalk grassland. Four herbs commonly found in chalk grassland and present at a local site. Castle Hill National Nature Reserve were exposed to elevated CO2 for 100 d whilst grown in 'mini-rhizotrons' placed within controlled environment chambers. This study suggests that contrasting effects of COj on component species may lead to alterations in the structure of this plant community.

MATERIALS AND METHODS

Selection and germination of species Four perennial species occurring commonly in a number of chalk grassland types were chosen which had contrasting morphologies (see Salisbury, 1952), particularly rooting depth and/or leaf shape. These were Sanguisorba minor Scop, (salad Burnet, deep rooting); Lotus corniculatus L. (birds-foot trefoil, shallow rooting); Plantago media L. (hoary plantain, medium depth rooting) and Anthyllis vulneraria L. (kidney vetch, shallow to medium depth rooting). Seeds were germinated in vermicuHte moistened with de-ionized water and placed in an illuininated plant growth cabinet (217 + 3-7/imol m~^ s""^); temperature, 22 °C day, 15''C night with a 1 4 h photoperiod. Seeds were germinated in ambient CO^ as the effect of COg on germination rate was not relevant to this study. Seeds were either collected from Castle Hill National Nature Reserve, East Sussex {L. corniculatus and A. vulneraria) or purchased from Emorsgate Seeds, Norfolk (S. minor and P. media). The two legumes were scarified prior to germination and germination was staggered to ensure seedlings emerged at approximately tbe same time.

Growth conditions and exposure to elevated During January, 1992, prior to COn exposure, 10 plants (10-15 d old), of each species were potted into

(a)

(c)

Figure 1. Leaves or leaflets of 4 chalk grassland species showing dimensions measured: {a) Sanguisorha minor; {b) Lotus corniculatus; {c) Anthyllis vulneraria; (d) Plantago media.

Effects of elevated CO^ on chalk grassland herbs plastic tubes (diameter, 69 mm, length 0-5 m) containing vermiculite saturated with water. These were chosen to allow nriore realistic rooting patterns, since they were longer and of a greater volume than the traditional pots which could be accommodated within the growth chambers. This is important since Arp (199]) has shown that responses to CO,^ may be related to the pot size used for plant growth. Each tube could be split vertically into two halves which allowed non-destructive observations of root growth to be made throughout the experiment. Root tubes were opened and resealed on three occasions and this was considered to cause the minimum disruption to the column, as observed previously (Taylor, Dohson & Freer-Smith, 1989; Taylor & Davies, 1990). The two halves were secured with tape and the base of each tube was covered with a double layer of cotton muslin. Two Fisons Fi-totrons were adapted to receive COg via PTFE tubing. One received ambient air, taken from outside of the building, at a CO^ concentration of 345 //.I T ' ; regular checks of ambient CO^ within the chambers were made during the photoperiod and were consistent + 5//I r \ The secotid chamber received CO^ at a target concentration of 590 fi\ r \ chosen to represent a realistic mcrease, likely to occur towards the middle of the next century (IPCC, 1990). Pure CO^ was bled into either of the cabinets from a cylinder (CP grade, BOG special gases, Surrey, UK), with the fiow^ regulated by a gap-flow meter. The COj concentration was continuously monitored throughout the experiment using a bench-top infra-red gas analyser 225 (ADC Ltd., Hoddesdon, Herts), with a chart recorder attached. The mean CO^ concentration (n = 100) throughout the experiment was 5 9 0 ± 4 / d r ^ during the day and 600±5//,ll"' at night. In general CO.^ regulation within the experiment was very reliable due to the 'open' system employed and frequency of air changes within the chambers. The plants in their rooting columns were randomly placed within the chambers and watered daily (approximately 50 ml per plant) with a weak nutrient solution (Liquinure N : P : K , 8 : 4 : 4 - 1 5 % of full strength, giving a total N concentration in the solution of approximately 1 mmol). This nutrient solution was chosen to mimick the low concentrations of N and P characteristic of the rendzina soils beneath chalk grasslands (Smith, 1980). Replication of chambers was achieved by alternating the plants and chambers receiving the COg once a week and the plants were rotated around the cabinet. Mean values of PAR during a 16 h photoperiod in each cabinet were 296-9+14-8 and 306 5±]0-6//mol m " S " \ similar to the PAR recorded in the field at ground level beneath the grass sward where the herbs in this study predominate. During a 100 d period mean relative humidity of both cabinets was 5 4 + 2 % . The mean maximum

857

temperature for each cabinet was 20'6 + 0'18 ^C and 21-2 +0-24 °C, whilst the mean minimum temperature was 16-8 + 0 23 °C and 164 + 0-18 °C. There was no indication that temperature varied between the two cabinets.

Measurement of leaf and shoot growth and development Non-destructive measurements of leaf growth were made using a paper ruler, minimizing damage to the delicate structure of young leaves. Leaf length and breadth was measured in P. media. Length and breadth of the imparipinnate leaflets (terminal leaflet of the pinnate leaves) were measured in S. minor and A. vulneraria and the terminal leaflet of the trifoliolate leaf in L. corniculatus (Fig. 1). Each leaf measured was tagged with coloured cotton and three leaves per plant were followed over several weeks. Mean leaf or leaflet number was recorded at intervals for each species in the early part of the experiment. After a destructive harvest five plants of each species from each treatment were used to measure total shoot dry weight (g) following oven drying at 85 °C for 48 h. With the exception of P. media leaflet dry weight (g) and stem dry weight (g) were separately obtained. Mean total leaf or leaflet area (cm^) was measured using an Image Analyzer (Delta-T Devices Ltd.) and specific leaf or leaflet area (cm^g"^), (SLA) was caJcuiated.

Measurement of root growth Xon-destructive measurements of 'apparent root length' and root distribution at 10cm intervals (0-10, 10-20, 20-^30, 30-40 and 40-50 cm) were made after 35, 70 and 100 d on 10 plants of each species from each treatment. 'Apparent' values of root length refer to those measured from the surface roots visible during the non-destructive measurements on roots. For these estimates of root length, a transparent plastic sheet, engraved with a 2-5 cm grid was sealed inside half a 0-5 m clear, perspex tube. This perspex tube was placed around an opened soil column (after removing half of the original tube) and the numbers of horizontal and vertical intersections of roots with the grid were counted according to the method of Newman (1966) and Tennant (1975), and modified by Taylor & Davies (1990). Roots running parallel and touching any line were counted twice. Separate counts were made for fine roots (diameter < 1-0 mm); medium roots (diameter ^ 1-0 mm and < 1-5 mm); and large roots (semi-suberized, diameter ^ 1-5 mm). Complete counts were converted to length using (/?) = l l / 1 4 x ( A ' ' ) x g n d unit, where R is the Root length, N is the number of intercepts and the grid unit was 2-5 cm (Newman, 1966).

858

R. Ferris and G. Taylor S. minor

18 r

0

1

5

9

13 Time (d)

30

1

7

11

15 19 Time (d)

23

35

Figure 2. Variation in leaflet (or leaf) length (mm) with time for growing leaves of (a) Sanguisorba minor; (6) Lotus corniculatus\ {c) AnthylUs vulneraria and (t/) Plantagn media exposed to either 345 /fl 1"' CO^ (open circles) or 590 fi\ r^ CO.^ (closed circles). Inset in (c) shows variation in leaflet breadth (mm) for this species. Each point represents the mean value of 10 measurements +SE. Asterisks indicate results of a one-way ANOVA using treatment means. • F ^ 0 0 5 ; * * P ^ 0 - 0 1 ; ***P

For the destructive harvest after 100 d, soil profiles vulneraria (A) = 0 - 8 7 ( E ) - F 4 251, (r^ = 0-17) and P. were segmented at 10 cm intervals and the roots media (A) = M6(E) + 2 73, (r= = 0-15). Root dr>removed from the mini-rhizotron and carefully weights (R d. wt) were obtained and specific root washed to remove all vermiculite. Assessments from length (SRL), and root:shoot ratios (RS) were also five plants per species per treatment of 'actual total calculated. One-way ANOVA was used for analysis root length' (m) in the profile and each segment of of the data, assuming no chamber effects, given that the profile was measured (data not shown) on an values of PAR, temperature and humidity were Image Analyzer (Delta-T Devices Ltd.). These comparable throughout the experiment. measurements were used to calibrate the nondestructive estimates for each treatment, in order to estimate the proportion of root visible during RESULTS assessments of 'apparent' root length. The regres- Leaves and shoots sion lines (actual root length {A) versus estimated Exposure to elevated CO^ significantly increased the root length {E) in metres) were significant at P ^ 005 terminal leafiet length of S minor, and L. corniculatus for each species in each treatment. For ambient CO^ and leaf length of P. media (Fig. 2a, b, d). Measurethese were; 5. mmor A = 2-03(E)-0-420, ( r ' = 046); ments for each species from the third successive L. cormculatus (A) = 4-51(E) + 0 2 0 3 , (r^ = 058); A. leaflet or leaf are presented here but similar trends

Effects of elevated CO^ on chalk grassland herbs

S59

S. minor

L. corniculatus

30r

(b)

100 r

25 01

I 70

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CO 60 o

(0

•I 15

1 50 r 40

10

0

30

1 20 10 10

19

28

37

55

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0 10

Time (d)

A. vulneraria

45

R. media

20

L_

37

19 28 Time (d}

46

(d)

40 w 35

15

*

I 30 I 25 10

^ 20

*

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5

10

0

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46 19 28 37 10 1 37 46 19 28 Time (d) Time (d) Figure 3. Number of leaves produced with time for (a) Sanguisorba minor; (b) Lotus corniculatus; (c) Anthyllis vulneraria and {d) Plantago media exposed to either 345 /d 1"^ CO^ (open circles) or 590//I t"^ CO^ (closed circles). Statistical analysis and replication as for Figure 2. 1

10

were observed for leaves 1 and 2 (data not shown). Leaflet length of S. minor was significantly ( P ^ 0-023) greater after 9, 13, 17 and 37 d from emergence in elevated COj (Fig. la). Figure Ib illustrates the increase in terminal leaflet length of leaves of L. corniculatus grow^n in elevated COj- The difference was significant between 17 and 37 d ( P ^ 0-018). There was no significant treatment difference in the terminal leaflet length of leaves of A. vulneraria (Fig. 2c). Mean leaf length of P. media plants grown in elevated CO^ increased (Fig. Id); the difference was significant from 11 to 35 d (P ^ 0-040). Measurements of leaf or leaflet breadth showed significant increases over time in all 4 species. Data for one species, A. vulneraria, is shown (Fig. 2c, inset). S. minor plants grown in elevated CO2 produced a significantly (P = 0'005) greater number of pinnate

leaves after only 10 d of growth (Fig. 3a); L. corniculatus (Fig. 36) showed a similar significant (P = 0-001) increase in trifoliolate leaves after 10 d. Leaf number in these two species was significantly (P < 0-040) greater at each subsequent observation for plants exposed to elevated CO^. For A. vulneraria (Fig. 3i-) a significantly (P = 0-001) greater number of pinnate leaves was observed in control plants after 28 d, whereas at 37 d the reverse was found. P. media, a slower growing species, produced significantly (P = 0-022) more leaves in elevated CO2 after 28 d and this was found at subsequent observations (Fig. 3id). Total shoot dry weight (Fig. 4) increased significantly (P ^ 0-024) for S. minor and L. corniculatus grown in elevated COj- There was no significant effect for A. vulneraria or P. media. For S. minor, both a significant (P ^ 0022) increase in leaflet and stem dry weight was observed (Fig. 4).

860

R. Ferris and G. Taylor 5-5

root length for A. vulneraria and P. media (Fig. 5 c, d). A significantly (P^O-01) greater fine root 4-5 length was found at the top of the soil profile in L. 1 corniculatus (Fig. 56) and a significantly greater 4-0 (P ^ 0-05) medium diameter root length of S. minor 3-5 (Fig. 5 a). 3-0 1 T After 70 d, S. minor, L. corniculatus and P. media 2-5 had significantly (P ^ 0-05) greater fine root length 20 when exposed to elevated COj (Fig. 5e,f, h). In — 1-5 contrast, in elevated CO2 root growth of A. vulner10 T aria was significantly (P = 0005) reduced at this vVv^ ?xys 0-5 stage, particularly at the top of the soil profile (Fig. XXX 5g). 0 345 590 345 590 345 590 345 590 After 100 d root length down the profile had Concentration of carbon dioxide (ul 1"^) increased considerably in plants of S. minor, L. corniculatus and P. media exposed to elevated CO3 (Fig. 5i,j, I). S. minor showed a significant (P ^ 0-05) S. minot i. corniculatus A. vulnerans P. madia increase in root length at all depths measured (Fig. shoot D Total dry weight 5i). L. corniculatus growing in elevated CO^ had a significantly (P :^ 0-05) longer root length at four of leaflet B Total dry weight the five depths measured (Fig. 5^). P. media showed Total stem dry weight similar significant (P ^ 001) increases in root length Figure 4. Dry shoot weight (g) after 100 d of growth of (Fig. 5/). The response of A. vulneraria was Sanguisorba minor, Lotus corniculatus, AnthylUs vulneraria negligible: a significant (P ^ 0-05) increase in root and Ptantago media exposed to either 2iAS fi\\~^ QO^ or length of both fine and medium diameter roots was 590/tl i"' COn. Each point represents the mean value of recorded on one occasion only; at the top of the soil five plants. The -f-SE are for total shoot dry weight. profile (Fig. 5 k). Total apparent root length (m) Statistical analysis as for Figure 2. increased significantly (P ^ 0001) in S. minor, L. corniculatus and P. media after 100 d of growth in For L. corniculatus only leaflet dry weight was elevated CO2. After 100 d A, vulneraria also insignificantly (P — 0*025) increased for plants exposed creased its apparent root length in elevated COj, but to elevated COg (Fig. 4). this was not significant (Table 1). After lOOd in elevated COg, S. minor showed a After 100 d a destructive harvest of roots was also 40 °o increase in total leaflet area but this was not statistically significant (P > 005). For L. corniculatus conducted. Actual root length (m), (data not total leaflet area was significantly (P = 0-023) in- shown), root dry w e i g h t - R d . w t (g) and specific creased in elevated CO^. Total leaf area of P. media root length - SRL (m g'^) were obtained for roots and total leaflet area of A. vulneraria showed no occupying the whole column and each segment of significant difference at final harvest between the two the column. Calculations of the regression betw-een apparent root length (as measured non-destructively) treatments (Table 1). SLA of A. vulneraria plants grown in elevated and actual root length were made which confirmed COg decreased significantly (P = 0-011); whilst that the non-destructive technique was suitable for plants of S. minor had a 38 °o decrease in SLA the assessment of changes in root length in S. minor although this was not significant (P > 0-05). In and L. corniculatus whilst low regression coefficients contrast, L. corniculatus significantly (P = 0-002) for A. vulneraria and P. media emphasized the increased its SLA at elevated CO^. P. media showed importance of measuring ' actual total root' length no difference in SLA between treatments (Table 1) for some species. The percentage estimated root at final harvest. Only one species flowered during the length (m) of the actual root length (m) varied experiment. Table 2 shows that in elevated CO2 between 20-50 % depending on the species (Table 1) more P. media plants fiowered and the mean number and a significant (P ^ 0-01) difference between of flowers per plant was greater. After about 74 d treatments was observed for 5. minor and L. flower death was observed in the elevated CO^ corniculatus. Total R d. wt of 5. minor and L. corniculatus also significantly increased in elevated treatment. CO2 (P ^ 0'05), but there was no significant effect on R d.wt for A. vulneraria and P. media (Table 1). Roots Total SRL was significantly increased (P ^ 0-01) in During the course of the experiment root length was one species, P. media (Table 1). At the end of the assessed non-destructively after 35, 70 and 100 d of experiment, no significant treatment differences were exposure to either ambient or elevated CO^ (Fig. 5). found in the RS ratios (Table 1). Significant differences (P ^ 0-01) in R d.wt were After 35 d there was no effect of elevated CO^ on 50

S. minor A. vulneraria L corniculatus P. media

Effects of elevated CO^ on chalk grassland herbs

861

Table 1. Growth parameters of four chalk downland herbs following exposure to either ambient (345 fil T or elevated {590 /il T^) CO^ for 100 d Species... CO, concentration

Sanguisorba minor

Lotus corniculatus

Anthyllis vulneraria Plantago media

345

590

345

345

590

345

590

Total leaf or leaflet area (cm^)

444-30 (71-70)

622-10 (56-50)

346-10 (30-90)

294-00 (24-80)

205-80 (17-90)

151-50 (24-10)

Specific leaf or leaflet area (cm^ g ')

307-50 (53-00)

(ji]

1"^)

22-01 (3-93)

ns

191-90 (17-10)

62-60 (10-00)

ns

Total apparent root length (m)

Total specific root length (mg-')

18-73 (0-68) *** 1-05 1-74 (0-14) (0-19) * 25'23 28-44 (3-05) (3-23)

Total estimate/total actual root length (%)*

53-51 (2-26)

Root/shoot ratios^

0-51 (0-11)

Total root dry weight (g)

11-18 (0-86)

2-54 (0-39) 0-17 (0-04) 72-77 (6-95)

ns

43-02 (2-04)

19-86 (2-15)

0-41 (0-07)

0-51 (0-12)

#

ns

590 39-82 (5-29) * 114-93 (6-55) ** 5-27 (0-46) *** 0-31 (0-04) # 58-18 (5-01) ns 32-99 (3-00) ** 0-54 (0-13) ns

ns

191-49 (5-28) 9'21 (078) 0-83 (0-13) 33-57 (5-75)

ns

170-68 (3-47) * 11-30 (1-27) ns 0-72 (0-06) ns 44-77 (3-49) ns

32-31 (2-94) 0-33 (0-07)

26-27 (2-41) ns ns

0-29 (0-04)

176-87 (7-37)

178-70 (16-10) ns 5-51 13-07 (0-65) (1-21) **# 0-52 0-60 (0-08) (0-06) ns 40-78 56-82 (313) (3-48) ** 28-67 40-12 (5-29) (5-37) ns 0-54 0-82 (0-07) (0-24) ns

Data are the means of five plants except for total apparent root length (a mean of 10 plants). The SE is given in parentheses. Asterisks indicate results of a one-way ANOVA or Mann-Whitney U Test using treatment means. *, P ^ 0-05 ; **, P < 0-01; ***, P < 0-001; ns, not significant. " Mann-Whitney U test.

Table 2. The effects of elevated CO.;, on flowering in DISCUSSION Plantago media This study has shown that exposure of chalk grasslands herbs to elevated atmospheric COg results Number of plants Mean number of in altered patterns of plant growth and development. flowering flowers per plant C Contrasting responses were often observed in both 590 345 590 o f C O j {/i\ ]-') 345 magnitude and direction for the four herbs studied here suggesting that the fine mat of vegetation Flowering characteristic of the plant community where S. (days from minor, L. corniculatus, P. media and A. vulneraria are sowing) 47 0 1 0 0-10 important component species, could be altered as the (0-10) (0) concentration of atmospheric CO^ continues to rise. 56 1 4 0-20 0-60 Quantifying the efFect of elevated COj on patterns (0-20) (0-31) of leaf area development is necessary since research 67 2 5 0-50 1-00 on a wide range of plants with contrasting growth (0-40) (0-34) rates has indicated that leaf area is an important 74 2 5 0-50 1-40 (0-34) (0-50) determinant of plant productivity (Monteith, 1977) 83 2 4 0-50 1-10 and as such, may represent an important component (0-34) (0-55) of the increased productivity which may occur 2 3 0-60 0-80 92 during the coming decades (Long & Hutchin, 1991). (0-51) (0-43) Previous work has shown that leaf growth is often stimulated by exposure to elevated CO^ (see Eamus Numbers in parentheses are SE. n = 10 plants. & Jarvis, 1989), although there is some uncertainty detected in two species (5. minor and L. corniculatus) as to whether this is due to changes in leaf size, at the top of the root profiles (Figs ba,b)\ but no number or both. Radoglou & Jarvis (1990) suggested difference was found in A. vulneraria (Fig. 6c). A changes in leaf production and not expansion, significant (P ^ 0-01) increase in R d. wt of P. media following exposure to CO^, were responsible for was seen at the bottom of the soil profile (Fig. bd). increased tree leaf areas observed for poplar; whilst

862

R. Ferris and G. Taylor 35 d (a) S. minor

0 F 0-10 M

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70 d

Root length (m) 0-1 02 03

(e) 0-4

0

0-10

>PCOOOO CCOOOOOCOOf

10-20

100 d

Root length (m) 0-4

0-8

^•2

5

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0-10

j

X'OOOoyo^p5o^XOOC