• Physiological integration between ramets is beneficial when acquiring heterogeneously distributed resources, and is hypothesized to occur when the benefits of resource sharing outweigh the costs. Our aim was to investigate if resource availability affected physiological integration in Carex arenaria and Carex disticha.
• Ramet systems were grown in high potassium and high water (K+ W+), high K and low water (K+ W–), or low K and high water (K– W+) for 1 month. Thereafter, water and K transport were traced with erythrosin and rubidium, respectively.
• Carex arenaria and C. disticha transported erythrosin over seven ramet generations and rubidium throughout the whole ramet system, but C. arenaria exported 20% more rubidium from the labelled shoot than C. disticha. A detailed analysis of a subset of plants suggested that C. disticha in low K abundance shared more rubidium than in high K abundance, and that C. arenaria ramets in both K+ W– and K–W+ shared more resources than K+ W+ ramets.
• We demonstrated long-distance resource transport for K and water in C. arenaria and C. disticha. The distance of integration was not affected by resource availability in C. arenaria or C. disticha, but local concentrations of K showed marked and contrasting responses to nutrient and water treatment in both species.
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We compared physiological integration in high- and low-resource systems of C. arenaria, which grows in resource-poor sand dunes, and of C. disticha, which grows in more productive habitats (lake shores and productive pastures). Plants were grown at high and low resource levels to study specifically if resource availability affected the pattern and amount of resource sharing. The resources that were manipulated and traced were water and potassium (K) because water can be scarce in the sand dunes and potassium allows us to study integration for a macronutrient.
We hypothesized that plants grown at low resource availability would show higher physiological integration, due to high, acropetal demand by young, unestablished (i.e. nonrooted) ramets, than plants grown at high resource availability. Specifically, we investigated whether a basipetal ramet exported more K towards the apex, in terms of amount and the transport distance, when the plant was previously grown with restricted access to K than when it had received ample K. We also investigated whether water taken up by a below-ground, basipetal ramet of a plant grown at low water availability was transported further towards the apex than when plants were well watered. Finally, we compared the distance and amount of integration for K and water in C. arenaria, from a poor site, with that in C. disticha, from a more productive site, in order to study physiological integration in species from contrasting habitats.
Materials and Methods
Species and field sites
The experiment was performed with the sand sedge, C. arenaria L., and the brown sedge, C. disticha Huds. (Sect. Ammoglochin Dumort.; Tutin et al., 1980). Both species produce large rhizomatous systems by sympodial growth. Inter-ramet connections and ramets are long-lived, often 4 yr or longer in C. arenaria (Tidmarsh, 1939; D’Hertefeldt & Jónsdóttir, 1999) and appears to be similar in C. disticha (T. D’Hertefeldt, pers. obs.). In the field, C. arenaria transported 14C and red dye over up to 40 ramet generations, covering several metres (D’Hertefeldt & Jónsdóttir, 1999). Physiological integration has not previously been studied in C. disticha.
The plant material had been grown for 5 yr in the experi-mental garden at the University of Lund, Sweden, after collection in 1993; this eliminated carry-over of site effects. Rhizomes of the two species were collected from two dune areas in Scania, Sweden. Carex arenaria rhizomes were collected at Sandhammaren, a coastal dune area (55°23′ N, 14°12′ E) and C. disticha rhizomes at the shore of lake Häljasjön (55°40′ N, 13°33′ E), Sweden. The site at Häljasjön is situated in the inland dune area Vombs Fure, and is higher in moisture and nutrient availability than Sandhammaren (D’Hertefeldt, 2000). The vegetation is more patchy at Sandhammaren, with alternating open sand and closed vegetation, than at the Häljasjön site, which is a grassy lake shore with several sedge species forming a closed vegetation.
In September 1998, 30 clonal fragments of each species were prepared for planting. The length at the start of the experiment was 13 ± 3.5 (mean ± SD) ramet generations for C. disticha and 16 ± 3.2 for C. arenaria (n = 30). The distance between two consecutive shoots, measured at harvest, was 6.4 ± 1.1 cm in C. arenaria (n = 30) and 3.4 ± 0.9 cm in C. disticha (n = 30) and therefore, the fragment length was, on average, 102 cm in C. arenaria and 45 cm in C. disticha (Table 1). The clonal fragments consisted of an apex, younger ramets with green shoots and older, below-ground ramets, consisting of a rhizome and, in general, roots (Fig. 1). Carex disticha branches more frequently than C. arenaria, and to obtain fragments with as few branches as possible, both shorter and longer clonal fragments than of C. arenaria were included. The extent of physiological integration was measured by labelling two healthy roots of an old, below-ground ramet, and measuring the presence of tracer in acropetally (towards the apex) and, if present, basipetally (to older ramets) situated ramets (Fig. 1).
Table 1. Morphological traits, concentration of potassium (K) and concentration and total uptake of rubidium (Rb) in Carex disticha and Carex arenaria
Concentrations of K and Rb and the total Rb uptake are means ± SE of three individual plants per species; other characteristics were calculated for all plants (n = 27–30 per species, except biomass of green shoots where n = 12–14 per species); n.a., not analysed. Differences between means were analysed by one- or two-way anovas.
Fragment size (cm)
45 ± 16
102 ± 18
Rhizome growth (cm)
5.0 ± 5.0
18 ± 14
Number of green shoots
7.7 ± 3.4
11.0 ± 3.8
Green biomass (g d. wt)
1.24 ± 0.8
11.36 ± 0.48
Plant biomass (g d. wt)
3.30 ± 1.3
3.90 ± 0.79
K µmol × g d. wt−1 shoot
288 ± 85
365 ± 48
K µmol × g d. wt−1 rhizome
175 ± 43
255 ± 51
Rb µmol × g d. wt−1 shoot
0.07 ± 0.05
0.12 ± 0.03
Rb µmol × g d. wt−1 rhizome
0.18 ± 0.16
0.12 ± 0.04
Rb total uptake per plant (µmol)
0.41 ± 0.30
0.50 ± 0.36
Each clonal fragment was planted in a 35-cm long, 15-cm wide and 9-cm deep box, filled with 5500 g of washed quartz sand with a mean particle size of 1 mm. The sand was covered with 500 g of black plastic pellets to prevent evaporation and algal growth. The plants grew in a greenhouse at 21 ± 2°C/16 ± 2°C day/night temperature (mean of computerized, continuous measurements). Additional light was provided for 12 h d−1 with Osram lights (Powerstar HQI 400 WID HQ I-T, 167.5 µE m−2). Relative air humidity was kept at 70%. The position of the plants in the greenhouse was randomized and shifted twice during the 40-d experiment.
All plants were watered with 0.3 l of a full nutrient solution every other day for 7 d, and received 250 µm of NH4 and NO3, respectively, 200 µm K and 20 µm phosphate with added magnesium (Mg), iron (Fe), manganese (Mn), zinc (Zn), molybdenum (Mo), copper (Cu) and boron (B) (details in Falkengren-Grerup, 1998). A substantial part (0.1 l) of the nutrient solution ran through the sand, thereby renewing the nutrient solution. During the first 10 d, all green leaves wilted and senesced and later, new leaves were produced.
After 7 d of acclimatization, the plants grew in manipulated K or water (W) treatments for 30 days. Plants received high or low levels of water (W+/W–) and potassium (K+/K–), resulting in three treatments, K+ W+, K+ W– and K– W+. The low K/low water treatment was omitted because a pilot study showed low survival for C. disticha in a dry, low-nutrient treatment. Every other day, 0.3 l of nutrient solution (Falkengren-Grerup, 1998) was added to the K+ W+ treatment. The K– W+ treatment received the same amount of nutrient solution but without K. In the W+ treatments, water-holding capacity (WHC) was kept at 18–20% by weighing the boxes each week. In the W– treatment, the sand was first allowed to dry to 5% WHC. Thereafter, 0.1 l of a more concentrated nutrient solution per week was added, providing the same amount of nutrients as in the K+ W+ treatments, while keeping sand moisture at approximately 5% WHC. After 30 d, tracers were added to study physiological integration.
Two roots, one for each type of tracer, were excavated on ramets basipetal to the oldest green shoot, and then labelled with tracers (Fig. 1). We used the K analogue rubidium (Rb) to trace transport of K in the plants (Drobner & Tyler, 1998). The red dye erythrosin was used to trace water movement (Roach, 1952). We generally used two roots on the same ramet, but when there was only one fresh root we used a root on an adjacent ramet. On average, ramet generation 11 ± 3 (mean ± SD) was labelled in C. disticha, while in C. arenaria, ramet generation 15 ± 4 was labelled. Two roots were labelled because Rb is readily taken up by an intact root (Drobner & Tyler, 1998) while erythrosin needs to be fed to a cut root (Price et al., 1992).
In each replicate, one intact root absorbed Rb by being placed in a 15-ml vial filled with a full nutrient solution where the K received during the 30 d of treatment had been replaced with a corresponding amount (200 µm) of Rb tracer. The tip of the other root was cut before it was placed in another 15 ml-vial filled with a 2% erythrosin solution dissolved in nutrient solution. The vials were sealed with parafilm, placed in the original position of the roots and covered with sand. After 48 h, the plants were harvested. All vials contained tracer solution at harvest, showing that no roots dried out during the 48 h of labelling.
At harvest, the labelled roots were wrapped in parafilm to avoid tracer contamination. The plants were spread on paper sheets, deep-frozen to −25°C directly after harvest to prevent further tracer transport, and thereafter dried to constant weight at 70°C. The number of green shoots and increase in length (cm) of the rhizome (length at harvest subtracted from length at planting) were measured to calculate growth during the experiment.
Presence or absence of erythrosin in the xylem was noted in cross-sections (approximately 2 mm thick) of rhizomes and green shoots under a microscope. The cross-sections were taken at the internode immediately preceding each shoot, or at shorter intervals until the maximum distance of erythrosin transport was detected. Both acropetal (towards the apex) and basipetal transport (to older ramets) was determined.
Rubidium and potassium analysis
The transport of Rb was analysed in five plants per treatment, out of which one plant was completely analysed (hereafter, these plants will be called completely analysed plants), while in the remaining four plants, subsets of ramets were analysed for the concentration of Rb in order to reduce the number of Rb analyses. Potassium was also analysed in all the samples of the completely analysed plants.
In the completely analysed plants, the rhizomes were cut into approximately 10-cm sections, which were individually analysed for Rb and K. Green shoots of individual ramets were analysed or, if shoot biomass was < 0.01 g, pooled with shoots of neighbouring ramets (Table 1). This survey of Rb and K transport showed relatively low basipetal transport owing to few basipetal ramets in most plants (Fig. 2, Table 2), and therefore, subsequent analysis focused on acropetal Rb-transport.
Table 2. The extent of acropetal and basipetal translocation of erythrosin in Carex disticha and Carex arenaria
The translocation in the rhizome was measured as absolute length (cm), as the number of dyed ramets, or as transport of erythrosin relative to the total rhizome length (%). The translocation in the shoots was measured as the number of dyed shoots. Basipetal translocation was recorded, but because there were relatively few basipetal ramets no statistical tests were performed on these data. Values are means ± SE (n = 59).
Acropetal erythrosin translocation (cm)
15.4 ± 11.3
36.4 ± 25.8
Percentage acropetal translocation (%)
57.0 ± 29.6
44.8 ± 26.9
Acropetal ramets with erythrosin
6.9 ± 3.8
7.1 ± 4.3
Basipetal erythrosin translocation (cm)
2.8 ± 2.2
6.7 ± 6.9
Percentage basipetal translocation (%)
37.5 ± 32.2
62.0 ± 34.5
Basipetal ramets with erythrosin
0.54 ± 0.96
1.2 ± 1.6
Total number of basipetal ramets
1.9 ± 1.9
1.2 ± 1.4
The completely analysed plants were used to determine which subsets of ramets to sample for Rb analyses in the remaining four plants per treatment. As transport of Rb occurred over long distances in both species, the most basipetal photosynthesizing ramet (‘Oldest green’, Fig. 1), the young, green ramets (‘Young’, Fig. 1) and the apex (‘Apex’, Fig. 1) were sampled. Since old ramets, like the labelled ramet, lack green shoots, the most basipetal photosynthesizing ramet was analysed for Rb rather than the labelled, below-ground ramet itself. The amount of Rb imported into ramets situated between the sampled ramets (such as ramet 5 in Fig. 1) was assessed by multiplying their biomass with the Rb concentration of the young, green ramets. The percentage of acropetally transported Rb was thereafter calculated as the amount of Rb found in all ramets acropetal to the oldest photosynthesizing ramet divided by all acropetal Rb (i.e. from the oldest photosynthesizing ramet to the apex).
For Rb and K analyses, samples were finely ground and 0.02 g d. wt per sample was digested in 15 ml of concentrated HNO3, which equalled 30–100% of the biomass of the analysed ramets. For very small samples, 0.01 g d. wt was analysed. The digest was evaporated and made up to 10 ml with distilled water and analysed using a flame atom absorption spectrometer (Tecator FIAstar analyser; Tecator, Höganäs Sweden) with 10% K as a catalyst for Rb and 10% Cs for K.
Differences among species and treatments were tested with one- or two-way analyses of variance (anova). The treatment effects on Rb concentration in the apex, young ramets and the old green ramets were analysed by a three-way anova with the factors species (C. arenaria and C. disticha), treatment and plant part (apex, young ramets and the old green ramets). The treatments were high K and high water (K+ W+), high K and low water (K+ W–) and low K and high water (K– W+). Correlation tests were used to test if concentration of Rb, distance of Rb transport or distance of erythrosin transport were correlated with the size of the clonal fragment, the position of the labelled ramet, the number of green shoots or the amount of green biomass. Model assumptions were tested using standard methods (Sokal & Rohlf, 1981). Data for ramet fragment size and rhizome growth (length in cm) were log-transformed in order to improve homogeneity of variances, and percentages were arcsin vx-transformed (Sokal & Rohlf, 1981). The three-way anova was analysed using Proc MIXED in SPSS 11.0 (SPSS Inc., 2001), all other analyses were carried out using systat (Wilkinson, 1988).
In the field, C. arenaria produces long rhizomes with numerous, relatively small shoots, while C. disticha produces shorter rhizomes with large shoots. During the experiment, C. arenaria rhizome growth was three times longer than in C. disticha and the number of green shoots produced during the experiment was 40% greater than in C. disticha (Table 1). Carex disticha produced fewer but larger shoots than C. arenaria; therefore, the total green biomass was not significantly different between the species (Table 1). At harvest, the total rhizome length of the C. arenaria clonal fragments was twice that of the C. disticha fragments (Table 1); this corresponded with the initial difference at planting. Within species, there was no difference in shoot production, green biomass or rhizome length between treatments (Table 1).
Neither the position of the labelled ramet (measured as position along the primary rhizome starting numbering at the apex) nor the length of the clonal fragment affected the Rb concentration in the apex (Pearson r = 0.12–0.29, P > 0.05). As water uptake is governed by the transpiration stream, we tested if plants with more green shoots had higher uptake and transport of Rb. The Rb concentration in the apex was neither correlated with total number of green shoots nor with biomass of green shoots (Pearson r = 0.13–0.42, P > 0.05). The plant material varied in size both between and within species, but these differences did not translate into effects of plant size and related characteristics on tracer uptake or distribution.
The K concentration in the rhizome and in the shoots did not differ between species (Table 1). Correspondingly, there was no difference in Rb concentration in C. arenaria and in C. disticha rhizomes or shoots (Table 1). Total Rb uptake (µmol) did not differ significantly between species (Table 1).
In C. arenaria, erythrosin was transported over 36 cm acropetally from the labelled root, which was twice the distance of that of C. disticha (species P < 0.001, two-way anova; Table 2). Both species transported erythrosin over seven ramet generations (species P = 0.67, two-way anova; Table 2) or approximately 50% of the acropetal rhizome distance (species P = 0.54, two-way anova; Table 2). Within species, treatments did not affect the extent of erythrosin transport, either in absolute (cm, P treatment = 0.36; number of ramets P = 0.57) or in relative acropetal rhizome length (%, P = 0.24, Table 2).
The experiment was designed to study acropetal transport, and therefore, C. disticha and C. arenaria clonal fragments had, on average, only 1.9 and 1.2 ramets basipetal to the labelled ramet, respectively (Table 2). Erythrosin was detected basipetally in both species (Table 2), and appeared to be present further basipetally in C. arenaria than in C. disticha. However, the number of basipetal ramets were too few to detect patterns in basipetal transport, and therefore, basipetal transport was not statistically tested.
The completely analysed plants showed that Rb was transported throughout the whole rhizome system in both C. arenaria and C. disticha, with the apex receiving, on average, 6–33% of the total, transported Rb (Fig. 2). The results also appeared to show that Rb transport to the apex was higher in C. disticha plants grown in low K (K– W+) than in plants with high K (K+ W+ , K+ W–). In the K+ W+ treatment, C. disticha appeared to share Rb locally, resulting in a high Rb content in the rhizomes of R8-9 (Fig. 2). In C. arenaria, both low K (K– W+) and low water availability (K+ W–) appeared to result in greater sharing of Rb than in the K+ W+ treatment (Fig. 2).
Three out of the six completely analysed plants had ramets positioned basipetally to the labelled root (Fig. 2). Depending on the number of ramets basipetal to the labelled ramets, basipetal transport could vary substantially (3.3–40%) (Fig. 2). However, as most of the remaining 12 plants per species had only one or two basipetal ramets, only acropetal transport of Rb was studied further.
Patterns of acropetal Rb transport
In the remaining 12 plants per species, Rb concentrations in the apex, young green ramets and the oldest green ramet were compared. The concentration of Rb was significantly higher in the apex, young green ramets and old green ramets of C. arenaria than of C. disticha, with C. arenaria ramets containing up to eight times greater concentrations of imported Rb than C. disticha (Fig. 3, Table 3). The treatments had an overall effect on Rb concentrations, suggesting effects of K and/or water availability on the integration of Rb (treatment P < 0.001; Fig. 3, Table 3). The treatments affected concentrations in C. arenaria and C. disticha differently (species × treatment P = 0.006). In C. arenaria, the Rb concentration in the apex was similar in all three treatments while the concentration in the old green ramets was much lower in the drier plants (K+ W–) than in the watered plants (K+ W+ and K– W+; Fig. 3, Table 3). In C. disticha, the concentration of Rb appeared to be higher in plants grown in low water or low K (K+ W–, K– W+) than in the K+ W+ plants (Table 3, Fig. 3). For example, the apex of C. disticha plants grown without K had twice the Rb concentration of the apex of the K+ W+ plants (Table 3, Fig. 3).
Table 3. Differences in rubidium (Rb) concentration between Carex disticha and Carex arenaria were analysed with three-way anova, with species, treatment and part of the analysed ramets (apex, young, or oldest green ramets) as factors
Species × treatment
Species × part
Treatment × part
Species × treatment × part
There was a significant difference in Rb concentration between the apex, young green and old green ramets in both C. arenaria and C. disticha (part P < 0.001, Fig. 3). In addition, C. arenaria and C. disticha differed significantly in the Rb-concentration of the plant parts (part × species P < 0.001; Fig. 3). In C. arenaria, the apex had a significantly higher Rb concentration than the young green ramets and the old green ramets, demonstrating that despite its small size, the C. arenaria apex constituted a strong sink for Rb (Fig. 3, Table 3). In C. disticha, the apex and the old green ramets had the highest Rb concentrations (Fig. 3).
The percentage of Rb transported acropetally in C. arenaria and C. disticha was not significantly affected by treatment (two-way anova, treatment P = 0.589). Therefore, the per-centage of Rb was assessed on pooled data from all three treatments within one species. In both species, 55–90% of acropetal Rb was found in younger ramets. In C. arenaria, 20% more Rb was transported acropetally than in C. disticha (two-way anova, species P = 0.017).
The results show that C. arenaria and C. disticha transported resources over long distances (i.e. erythrosin over seven ramet generations, and Rb from a labelled basipetal root to the youngest ramets) which identifies them as ‘extensive integrators’ (Jónsdóttir & Watson, 1997). The K and water treatments affected the concentration of Rb in C. arenaria and C. disticha, demonstrating the importance of basipetally acquired mineral nutrients for young ramets. In addition, the detailed complete analyses of a subset of plants suggested that low resource levels did not affect the extent of integration in C. arenaria and C. disticha, but that resource sharing increased.
Transport of erythrosin
For C. arenaria, the distance of erythrosin transport found in the present experiment (50% of the acropetal distance) was shorter than that previously found in the field, where red dye was transported over an average of 25 ramet generations (90% of the acropetal distance) (D’Hertefeldt & Jónsdóttir, 1999). This shorter transport of dye may depend on differences in resource distribution, time of labelling and on the type of dye used, as these factors differed between the two experiments. First, resource distribution was homogeneous in the present study while soil moisture and mineral nutrients vary over relatively short distances in the field, causing steeper transpiration gradients between ramets, which promotes longer transport (Alpert & Mooney, 1996; Cain et al., 1999). Second, the acid fuchsin dye used in the field is rated as a more highly transportable molecule than erythrosin (Roach, 1952). Third, roots absorbed dye for 72 h in the field experiment and for 48 h in the present experiment. Further, K, and therefore also its analogue Rb, is transported both in the xylem and in the phloem, which might affect transport distance (Marschner, 1995). However, this mechanism may be more important for transport to the nonphotosynthesizing apex in the present experiment than for long-distance transport of Rb.
Transport of K
The tracer Rb was detected in the apical ramets in all C. arenaria and C. disticha plants, demonstrating that both C. arenaria and C. disticha are extensively integrated. This is interesting because at the Häljasjön field site (where C. disticha was collected), C. disticha and Carex hirta (a restrictive integrator sensuJónsdóttir & Watson, 1997) grew at the lake shore, while C. arenaria was only found in the adjacent sand dunes, suggesting that C. disticha and C. hirta grew in more similar habitats in terms of productivity than C. arenaria did. Studies of integration in C. disticha growing in heterogeneous resource distribution are needed to confirm the integration patterns (Hutchings et al., 2000).
Although C. arenaria and C. disticha were both extensively integrated, C. arenaria had higher concentrations of imported Rb in apical and young ramets and exported 20% more of its acquired K to younger ramets than C. disticha did. Carex disticha and, to a lesser extent, C. arenaria appeared to transport Rb more locally when grown in K+ W+ than when one resource was low (K– W+ and K+ W–). Experiments in heterogeneous conditions are needed to study whether C. disticha and/or C. arenaria show relatively local transport under high resource conditions and long-distance transport under low resource conditions, and if this could be linked to population differences (Alpert, 1999).
Intraspecific patterns of physiological integration
The 5% soil moisture in the present experiment appeared to reduce the uptake and transport of Rb in C. disticha, whereas increased Rb transport was found in C. arenaria. Reduced transpiration has been demonstrated to reduce nitrogen transport (de Kroon et al., 1998), and phosphorus and calcium transport (Lötscher & Hay, 1996), while a higher percentage of phosphorus was found in unwatered than in watered ramets in Agrostis stolonifera (Marshall, 1990). Therefore, the different patterns at low water availability might reflect physiological differences between C. arenaria (from dry habitats) and C. disticha (from moist habitats) in the uptake of mineral nutrients from relatively dry soil.
The ecological relevance of the extensive physiological integration demonstrated in C. arenaria is beneficial in the resource-poor, heterogeneous sand dunes, where C. arenaria rhizomes invade open sand patches from surrounding, closed vegetation, which has more humus soil and higher soil moisture (D’Hertefeldt & Jónsdóttir, 1999). Below-ground rhizomes are rooted in this richer soil, and transport water and mineral nutrients to young ramets that are produced in the open sand patches. In the present experiment, C. arenaria had high concentrations of imported Rb in the apical and young ramets in all treatments, but at low water availability, this resulted in lower Rb concentrations in the oldest green ramet, showing that export to the apex and young ramets was strongly maintained. The relevance of long-distance physiological integration in C. disticha, which grows in productive, moist pastures, is less well understood. Carex disticha grows mixed with other sedges and grasses and is probably exposed to high competition; it may therefore be beneficial for C. disticha to be able to subsidize its ramets through physiological integration in order to occupy space.
Transport patterns and nutrient availability
Our aim was to investigate if well-fed ramets were less dependent on resource subsidy than ramets in poor or dry soil, and how this affected the extent of physiological integration in large ramet systems. Resource availability did not affect the extent of integration in C. arenaria and C. disticha, but low resource availability appeared to increase the degree of resource sharing. Interestingly, the patterns of Rb transport in the completely analysed plants indicated that although the apex received Rb in all treatments, the bulk of Rb appeared to be more locally distributed at high than at low nutrient availability in both species. This experiment suggests that resource concentration can affect integration patterns, but that future studies would benefit from including resource heterogeneity, as stronger source–sink gradients between ramets increase the probability of detecting differences in the extent of physiological integration or the amount of resource sharing in clonal plants (Alpert, 1999; Hutchings et al., 2000; van Kleunen et al., 2000).
We thank Peter Alpert, Bengt Å. Carlsson, Hans de Kroon, Ingibjörg Jonsdottir, Chris Marshall, Olle Jonsson, Lars Pettersson and anonymous referees for constructive comments on previous drafts of this manuscript. We also thank Anita Balogh for laboratory assistance and Mimmi Varga for drawing Fig. 1. Financial support was received from Lund University.