AOBPreview originally published online on May 10, 2006
Annals of Botany 2006 98(1):187-191; doi:10.1093/aob/mcl086
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A Trade-off Between Guerrilla and Phalanx Growth Forms in Leymus secalinus Under Different Nutrient Supplies
1 Laboratory of Quantitative Vegetation Ecology, Institute of Botany, The Chinese Academy of Sciences, 20 Nanxincun, Fragrant Hills, Haidian District, Beijing 100093, China and 2 The Graduate School of Chinese Academy of Sciences, Beijing 100039, China
* For correspondence. E-mail dongming{at}ibcas.ac.cn
Received: 21 November 2005 Returned for revision: 24 January 2006 Accepted: 8 March 2006 Published electronically: 10 May 2006
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSLITERATURE CITED |
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• Background and Aims A phalanx growth form enables clonal plants to make better use of resource-rich patches, whereas a guerrilla growth form provides them with opportunities to escape from resource-poor sites. Leymus secalinus produces both spreading (guerrilla form) and clumping ramets (phalanx form). Here, the hypothesis that a trade-off between the two growth forms in L. secalinus exists under different resource levels is tested.
• Methods Ramets of L. secalinus were grown under three levels of nutrient supply.
• Key Results With increasing nutrient supply, the proportion of clumping ramets (in total number of ramets) increased, whereas that of spreading ramets decreased. With increasing nutrient supply, the number of buds increased, whereas biomass per bud decreased. A trade-off between bud number and size further supports the above hypothesis because larger buds were more likely to develop into spreading ramets, and smaller buds into clumping ramets. Mean spacer length between spreading ramets was significantly smaller under the high than under the medium nutrient supply.
• Conclusions The results suggest that a trade-off between the two growth forms in L. secalinus exists under different nutrient supplies. Such a trade-off together with plasticity in spacer morphology may enable L. secalinus to make better use of small-scale heterogeneity in resource supply.
Key words: Architectural plasticity, bud size–number trade-off, clumping ramets, growth form trade-off, Leymus secalinus, morphological plasticity, spreading ramets
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSLITERATURE CITED |
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Clonal plants are very common and diverse in the plant kingdom (Callaghan et al., 1992
; Klime
et al., 1997; Song et al., 2001). Based on the spatial arrangement of ramets, clonal plants can be classified into different clonal growth forms (Lovett-Doust, 1981; White, 1984). Clegg (1978; cited by Oborny and Cain, 1997) and Lovett-Doust (1981) distinguished two classes of clonal growth form: phalanx and guerrilla. In the phalanx growth form, connections between ramets (i.e. spacers sensu Bell, 1984) have few and/or short internodes, resulting in closely packed ramets termed ‘clumping ramets’ (Shaver and Billings, 1975; Lovett-Doust, 1981; Bernard, 1990; Navas and Garnier, 1990). In the guerrilla growth form, by contrast, connections between ramets have many and/or long internodes, resulting in widely spaced ramets termed ‘spreading ramets’ (Shaver and Billings, 1975; Lovett-Doust, 1981; Bernard, 1990; Navas and Garnier, 1990).
The ecological significance of these two clonal growth forms has long been recognized. The guerrilla growth form is very common in early successional stages, as well as in resource-heterogeneous and/or disturbed habitats, whereas the phalanx form is more favoured in late successional stages and in relatively homogeneous and/or less disturbed habitats (Schmid and Bazzaz, 1987; Navas and Garnier, 1990; Krystyna, 1995; Scott, 1995; Adachi et al., 1996). The guerrilla growth form enables clonal plants to spread quickly in horizontal space so that, in a spatially heterogeneous habitat, clonal plants can more readily escape from stressful microsites and find favourable ones (Lovett-Doust, 1981, 1987; Sutherland and Stillman, 1988; de Kroon and Hutchings, 1995; Humphrey and Pyke, 1998; Oborny et al., 2001). The phalanx growth form, by contrast, may enable clonal plants to tolerate more stressful conditions, make better use of locally abundant resources (monopolization strategy) and out-compete other species in a favourable microsite (Lovett-Doust, 1981, 1987; Schmid and Happer, 1985; Humphrey and Pyke, 1998).
The perennial grass Leymus secalinus, like many sedge species (Bernard, 1990), produces both spreading and clumping ramets, resulting in a combined growth form. In this growth form, spreading ramets are first produced at the end of long rhizomes and clumping ramets are then developed from the short rhizomes of the spreading ramets (Shaver and Billings, 1975; Bernard, 1990). A previous study has shown that severing rhizomes had little effect on the growth of the newly produced L. secalinus ramets in a spatially relatively homogeneous habitat (Dong, 1999), suggesting that the rhizomes of L. secalinus may be more important in foraging activities. Because spreading ramets have an advantage in exploiting open resources and clumping ramets may be more suited to monopolizing locally abundant resources, it is hypothesized here that a trade-off between the production of clumping and spreading ramets in L. secalinus exists under different resource supplies.
To test this hypothesis L. secalinus were grown under three levels of nutrient supply. With increasing nutrient supply it is predicted that the proportion of clumping ramets in L. secalinus will increase, whereas that of spreading ramets will decrease.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSLITERATURE CITED |
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The species
Leymus secalinus (Georgi) Tzvel. is a perennial grass approx. 45–90 cm high, distributed in the Autonomous Regions of Inner Mongolia and Tibet, and the provinces of Hebei, Shaanxi, Gansu, Shanxi and Qinhai of China, as well as in Mongolia, Japan and the Far East (Yang, 1994
). It occurs in grasslands, sandy grasslands, mountain slopes, farmlands and roadsides (Dong, 1999).
Experimental design
In May, 2004, ten clones of L. secalinus were randomly collected in a sandy grassland near Ordos Sandland Ecological Station (OSES; 39 °02'N, 109 °51'E; Institute of Botany, the Chinese Academy of Sciences), located in the south-east of the Mu Us sandy grassland of China. Three similar-sized ramets of each clone were selected and planted into 30 x 20 x 20-cm plastic containers at OSES. On 15 June, 2004, after 2 weeks of recovery, the three ramets of each clone were randomly subjected to three nutrient treatments. For nutrient treatments, a commonly available granular lawn fertilizer was used (Osmocote 301, Scotts, Marysville, OH, USA) with 15N : 11P : 13K : 2 Mg (elemental ratio) as the basic fertilizer and the commercial nutrient solution (Peters1, Scotts) with 20N : 20P : 20K (elemental ratio) as the supplemental fertilizer. Under the high-nutrient treatment each container received 13·6 g Osmocote 301 once and 100 mL Peters1 nutrient solution (0·417 g L–1) once a week (Dong and Alaten, 1999). Both fertilizers (Osmocote 301 and Peters1) were reduced to 50 % in the medium-nutrient treatment and to 10 % in the low-nutrient treatment, respectively.
Measurement and analysis
The experiment was ended on 14 September, 2004. The number of clumping ramets, number of spreading ramets and number of buds (potential ramets) were counted. Spreading ramets are ramets produced by long rhizomes that eventually grow upright to form new ramets, whereas clumping ramets are ramets produced by tillering or very short rhizomes of spreading ramets. Each plant was then separated into roots, shoots, rhizomes and buds. Rhizome length and biomass of each plant part were measured after drying them at 80 °C for 48 h. Mean rhizome spacer length was calculated as the mean distance between two adjacent spreading ramets and specific spacer dry mass as the dry mass per unit spacer length. The data were analysed by using two-way ANOVA (without replication) followed by least-significant difference (LSD) tests to investigate the differences among the nutrient treatments.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSLITERATURE CITED |
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Biomass
With increasing nutrient availability, total plant dry mass and root, shoot, bud and rhizome dry mass all increased considerably (Table 1, Fig. 1A). With increasing nutrient availability, biomass allocation to root, bud and rhizome decreased greatly, whereas that to shoot increased (Table 1, Fig. 1B).
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Ramet number and size
Both number of clumping ramets and number of spreading ramets were higher under higher nutrient supply (Table 1, Fig. 2A). The proportion of clumping ramets in total ramets increased significantly with increasing nutrient supply, whereas that of spreading ramets decreased (Table 1, Fig. 2B). Dry mass per ramet and number of leaves per ramet did not differ among the three nutrient treatments (Table 1).
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Bud number and size
With increasing nutrient supply, the number of buds increased considerably (Table 1, Fig. 3A), whereas dry mass per bud decreased (Table 1, Fig. 3B).
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Spacer morphology
Mean spacer length was the greatest under the medium level of nutrient supply and did not differ between the high- and low-nutrient treatments (Table 1, Fig. 4A). Specific spacer dry mass was statistically similar under the three nutrient treatments (Table 1, Fig. 4B).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSLITERATURE CITED |
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Higher nutrient availability generally enhances biomass and module production, and decreases root to shoot ratio (Aung, 1974
; Hunt and Nicholls, 1986; Sutherland and Stillman, 1988; Sakai, 1995; Stuefer et al., 1996; Verburg et al., 1996; Gardner and Mangel, 1999). This holds also for L. secalinus (Figs 1–3).
Trade-offs between growth forms
Within increasing nutrient supply, the proportion of clumping ramets in L. secalinus increased, whereas that of spreading ramets decreased (Fig. 2B). These results support our hypothesis that a trade-off between guerrilla and phalanx growth forms exists in the clonal grass L. secalinus, in response to resource availability.
Moreover, the number of buds of L. secalinus increased significantly with increasing nutrient supply (Fig. 3A), whereas dry mass per bud decreased (Fig. 3B), indicating a trade-off between bud number and size. The buds will develop into either spreading or clumping ramets. However, it is most likely that larger buds will develop into spreading ramets and smaller buds into clumping ramets, because developing into spreading ramets will involve more energy costs than developing into clumping ramets, and larger buds have a greater carbohydrate storage potential than smaller buds (McGinley et al., 1987; Dong et al., 1997). Producing more small buds under the high-nutrient supply enhanced production of clumping ramets. The number–size trade-off of L. secalinus buds therefore further supports the above hypothesis.
Mean spacer length between two adjacent spreading ramets was lower under the high- than under the medium-nutrient supply (Fig. 4A), whereas mean spacer thickness and potentially also mechanical strength, as indicated by specific spacer dry mass (Fig. 4B), were not affected by nutrient supply. Therefore, the architecture of L. secalinus was more aggregated under high- than under medium-nutrient supply, suggesting a plastic modification from a guerrilla form towards a phalanx form (de Kroon and Knops, 1990; Evans, 1992; Dong and Pierdominici, 1995; de Kroon and Hutchings, 1995). However, mean spacer length was lower under the low- than under the medium-nutrient supply and did not differ between the low- and the high-nutrient supply (Fig. 4A). It is possible that under the low-nutrient supply there were insufficient resources available to be allocated to rhizomes. Therefore, plasticity in spacer length of L. secalinus in response to nutrient supply only partly supports the hypothesis.
Ecological significance in heterogeneous environments
In a spatially heterogeneous habitat, shortening spacers under resource-rich patches and elongating spacers in resource-poor patches may potentially position more newly produced ramets (feeding sites sensu Bell, 1984) in resource-rich patches to acquire resources efficiently, and thereby to increase survival (de Kroon and Knops, 1990; Evans, 1992; Hutchings and de Kroon, 1994; Dong and Pierdominici, 1995; de Kroon and Hutchings, 1995; de Kroon et al., 2005). However, the extent of plasticity in spacer length is genetically determined and is also affected by the phylogeny of the species (de Kroon et al., 1994). Therefore, if the resource-rich patches are too small (e.g. patch size close to the canopy size of an individual ramet or to spacer length), even shortening spacers may not guarantee that newly produced ramets are positioned in the same resource-rich patches, and thus may not enable clonal species to forage for resources efficiently and, consequently, will fail to increase the fitness of the whole genet (Briske and Derner, 1998).
In resource-poor nutrient patches, L. secalinus will not only increase spacer length between spreading ramets, but also increase allocation to buds and biomass to spreading ramets. These combined responses may enable L. secalinus to search for resource-rich patches more efficiently. Once a spreading ramet is positioned in a resource-rich patch, a decrease in spacer length together with an increase in the proportion of clumping ramets could enable L. secalinus to monopolize these locally abundant resources and to compete efficiently with other species (de Kroon and Schieving, 1990; Derner, 1999). Even when the favourable patches are very small, clumping ramets may still be readily positioned in the resource-rich patches, because there are either no spacers between the clumping ramets of L. secalinus or the spacer is very short (Shaver and Billings, 1975; Bernard, 1990). A previous study has shown that clonal integration was of little importance in L. secalinus in relatively homogeneous habitats (Dong, 1999). Therefore, trade-offs between the two growth forms together with plasticity in spacer length may have been selected for in L. secalinus to make good use of spatial heterogeneity in resource supply, even where this occurs at very small scales.
Although decreasing nutrient availability greatly reduced biomass and ramet production in L. secalinus, overall ramet size was not affected by nutrient supply (Table 1). Similar results were found by Stuefer et al. (2002), in which low-light treatment reduced plant biomass and number of ramets, but did not affect average ramet weight. This may be interpreted as a strategy that enables L. secalinus to cope with spatial and temporal heterogeneity in resource supply, and to ensure its survival in different habitats.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSLITERATURE CITED |
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It was predicted that there would be greater architectural plasticity in clonal species with a mixture of phalanx and guerrilla forms (e.g. L. secalinus) than in those with either a distinctly phalanx or a distinctly guerrilla growth form. A trade-off between the two growth forms together with plasticity in spacer length enables L. secalinus, and possibly other species with a similar architecture, to make better use of spatial heterogeneity in resource supply, even where this occurs at very small scales.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSLITERATURE CITED |
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We thank Dr David R. Causton and two anonymous reviewers for their valuable comments on an earlier version of the manuscript. This research was supported by Key Project of the National Natural Science Foundation of China (30330130) and by the National Natural Science Foundation of China (30500070).
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSLITERATURE CITED |
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