Experimental growth responses to groundwater level variation and competition in five British wetland plant species

  • Published source details Kennedy M.P., Milne J.M. & Murphy K.J. (2003) Experimental growth responses to groundwater level variation and competition in five British wetland plant species. Wetlands Ecology and Management, 11, 383-396.


The relationships between the survival of wetland plant species and the underlying hydrological and hydrochemical conditions of wetland habitats are far from fully known. This is despite the critical nature of these relationships for the continued existence of wetland plant communities, which are among the most threatened of vegetation types throughout the world. There are strong practical reasons for improving understanding of the eco-hydrology of individual wetland plant species and in particular the groundwater dynamics which affect plant survival. For example, better knowledge of the relationship between plant success and hydrology may help assess potential threats to rare species in wetlands subject to altered groundwater regime. In general there has been relatively little experimental testing of the tolerance of wetland plants to water level variation, as hypothesised from field investigations. The target species (creeping bent Agrostis stolonifera, beaked sedge Carex rostrata, meadowsweet Filipendula ulmaria, reed canary-grass Phalaris arundinacea and tufted hair-grass Deschampsia cespitosa) represent a reasonable cross-section of the species present in northern poor-fen vegatation of the UK, and show a range of defined established-phase survival strategies. Reed sweet-grass Glyceria maxima was additionally used as a competititor species only.

Study site: This glasshouse study was undertaken at the Brian Research Laboratory, Garscube Campus of the University of Glasgow in Glasgow, Scotland.

Experimental design: Plants were grown, in five separate experiments in a heated glasshouse with supplementary lighting, in 36-litre plastic tanks with either fixed or fluctuating water level treatments. Competitive interactions (utilising a De Wit replacement design) were imposed in three experiments, between Agrostis v. Deschampsia, Filipendula v. Glyceria, or Carex v. Glyceria. In the other two experiments fluctuating water level treatments (of differing magnitude and duration) were imposed on individual populations of Deschampsia and Phalaris. Biomass components and specific leaf area were the primary response variables measured.

Influence of water level and competition by Glyceria maxima on Filipendula ulmaria and Carex rostrata: Water level treatments had a significant effect on above-ground biomass, below-ground biomass and average weight per leaf of F.ulmaria plants. The presence or absence of G.maxima explained none of the variation in any of the variables measured. Water levels below (–8 cm) or at the soil surface (0 cm) resulted in significantly greater height, above-ground biomass, below-ground biomass and average weight per leaf than when the water level was above the soil surface (+18 cm). There were no significant effects on any of the variables due to the water level being below (–8 cm), compared to at the soil surface level (0 cm). C.rostrata plants had significantly lower belowground biomass in the presence of G.maxima than in its absence. Specific leaf area was significantly different between water level treatments, the mean value being greater when the water level was below the soil surface than when it was at or above soil surface level.

Influence of water level and competition between Agrostis stolonifera and Deschampsia cespitosa: Above-ground biomass differed significantly in relation to water level (-7 cm, 0 cm, +7 cm) for both species, while competitive interaction had a significant effect only upon D.cespitosa. For both species, above-ground biomass was generally higher for the treatments with water levels at soil surface (0 cm). Average above-ground biomass for A.stolonifera grown in single species stands with a 0 cm water level treatment did not differ significantly from treatments with a higher water level (+7 cm). However, values were significantly higher for individuals grown in replacement series with D.cespitosa, where the water level was at the surface (0 cm). The opposite was observed for D.cespitosa, suggesting a possible competitive advantage of A.stolonifera over D.cespitosa at this treatment level. Total root biomass differed significantly between single D.cespitosa stands and both the mixed species and single A.stolonifera stands, with a general decrease in total biomass as water level increased. Plant height was smallest where water level was below soil surface (–7 cm) for both A.stolonifera and D.cespitosa.

Influence of fluctuating water level on Phalaris arundinacea and Deschampsia cespitosa: For P.arundinacea, measured attributes exhibiting significant differences in means in relation to water table fluctuation were: above-ground biomass, total plant biomass, stem biomass, adventitious root length, and rhizome length. Root length was the only attribute found to differ significantly between treatments for D. cespitosa.

To summarise, lowest biomass production was for those treatments were:

•water levels were constantly below soil level (–12 cm and –6 cm)

• water levels fluctuated between the surface (0 cm) and below the surface (–12 cm) on a fortnightly basis

• there were periods of inundation (+6 cm and +12 cm), interspersed with periods at soil surface over shorter (weekly) periods

Higher biomass production was seen for those treatments where:

• water levels were fixed at surface level (0 cm), or above the surface (0 cm and +12cm)

• water level fluctuated between surface level (0 cm) and –6 cm on a fortnightly basis

• there were periods of inundation (+6 cm and +12 cm), interspersed with periods where water levels were at soil surface over longer (fortnightly) periods

Conclusions: The presence of competitor species was generally less important for target plant response than the impacts of stress produced by variation in water level – either drought or flooding. These responses provide evidence for potential advantages in survival and ability to spread vegetatively in wetland plant species. The data also provide new insight into factors controlling the distribution of wetland plants along gradients of water table level, in the presence or absence of competing species.

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