A11. Vegetation Survey - Quadrat

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1. Objectives

The quadrat method has been the most commonly employed sampling technique for vegetation survey and monitoring in habitats across the UK. It is a standard and effective sampling technique and is employed in the National Vegetation Classification. Two types of quadrat survey can be applied in the monitoring of an area affected by river restoration:

  • Quadrat in homogeneous habitats to assess plant assemblage and processes in relation to phytosociology.
  • Permanent quadrat, placed at a defined location to perform a multi-year study and monitor temporal change in plant community composition and richness.

The permanent quadrat method can be very useful in order to monitor the response of macrophyte communities following river restoration, conducted over a period of months or years, to highlight the effects of modification. This technique is not often used but could be considered more often, to target very localised areas of restoration. Both techniques can be used on the same site, it is up to the programme manager to choose the best technique according to the type of restoration work being assessed.

The method described in the NVC is totally suitable for wetland and riparian habitats associated with rivers. This sampling methodology can be applied in both habitats with quadrat location and size adapted according to the specific habitat characteristics. Considering the wide effects river restoration can cause on floodplain features it is necessary to extend the vegetation monitoring to the floodplain areas that can be affected by restoration measures. Avoiding this step would reduce the ability to assess the wider benefits of any restoration works and restrict evaluation of the improvements (or not) to habitats occurring outside of the riverbed.

Ensuring the monitoring programme includes all habitats which can be affected by river restoration is crucial. Often, a BACI (Before-After-Control-Impact) process should be followed and mapping of the habitat before restoration measures are started is highly recommended.

Mettre ici un schéma explicatif !!!!!!!

2. Method summary

Figure 1 : Comparative sizes of NVC samples. The diagram shows different quadrat sizes as used for different types of vegetation. The rectangular shapes apply to hedges. Source : National Vegetation Classification

The method consists of an assessment on the vegetation inside a frame, which can be of a different size and shape Figure 1. The National Vegetation Classification: Users’ handbook, provides examples of the size and shape of the quadrat suitable for monitoring different vegetation types. Examples of quadrat size are:

  • 1x1m or 2x2 m short herbaceous vegetation, dwarf-shrub heaths
  • 4x4 m short woodland field layers, tall herbaceous vegetation, heaths, open vegetation
  • 10x10 m dense scrub, tall woodland field layers, species-poor herbaceous vegetation
  • 50x50 m woodland canopy and shrub layers sparse scrub

Quadrat shape for macrophyte

The quadrat shape used should be adapted to the habitat morphology. A rectangular quadrat is applied to edges. It is a typical shape that could be applied on river banks as well. To sample macrophytes, diving is the better monitoring technique to get a good image of the plant community without destroying the habitat by removing the macrophytes and performing identification on the bank. With this method it is often possible to record species in the usual NVC fashion, sampling homogeneous stands using quadrats of appropriate size (Spence 1964) or alternatively, using permanent quadrats

Quadrat size for macrophyte

If the plant community is visible by boat or by wading, the method can be applied this way. However, diving is much more consistent as some small plants are often not visible or identifiable from the surface. Quadrats normally consist of a square frame usually 1 m or 4 m square in size. This size should be carefully chosen according to the river width. The 4m square quadrat is used for sampling in large rivers, but is not suitable in rivers under 10m width for which the surveyor should select the 1m square quadrat.

Sampling method

Figure 2 : Left: The Braun-Blanquet scale. Right: The domin scale of cover/abundance. Source: National Vegetation Classification Users Handbook.
Figure 3 : Example of a completed NVC sample card for mire vegetation (redrawn from Rodwell, 1991b). Source: National Vegetation Classification Users Handbook.

The aim of monitoring the macrophyte community after river restoration is to detect and assess changes in the community.

The first step of the NVC survey methodology is to delimit by the eye the location of homogeneous stands of vegetation, also considering the species composition and its physiognomy (structure, pattern of ground covering and vertical layering). Stands generally have a uniformity of color and texture in the vegetation, with repetition of any patterning over the ground and consistency of vertical layering. This step is not applicable to permanent quadrats unless the homogenous quadrat is also permanent. In this case the exact location is chosen subjectively.

Choosing the location for homogeneous quadrat sampling of river macrophytes is not a simple task for two reasons. On the first hand, a stream or a river can contain various habitats within a small area due to river current, gravels or sand shoals etc… which creates a lot of ecotones.

On the other hand, the assessment of the vertical layering has always been difficult because of the multiple growth forms and structural overlap of the plants. For aquatic vegetation, the common practice is to sample separately for each different vegetation type (submerged, floating-leaved, free-floating, helophytes). The separation of plants according to their vegetation structure is often used in traditional phytosociological approaches on the basis that these plants are related to distinct communities and to distinct environmental conditions. They also play different roles in succession during river colonisation (National Vegetation Classification: Users’ handbook). Sampling these vegetation communities can be tricky and a standardised datasheet should be used to reduce complexity in the field and provide a consistent approach to plant recording.

The estimation of plant cover is achieved using either the Domin scale of cover/abundance or the Braun-blanquet coefficient (less used in UK) Figure 2. The cover/abundance is a measure of the vertical projection over the ground of the extent of the living part of a species. In the NVC, this is estimated using the Domin scale (sensu Dahl and Hadac 1941). For the majority of quadrats, the vegetation cover can exceed 100% due to the structural overlap between species, which can be very common for macrophyte communities. In this case, it is necessary to separate the plant records according to their structure in the water column, as mentioned above. Domin values can be converted to the Braun-Blanquet scale (which is less precise) but the reverse is not true.

Survey data should be recorded on the NVC record sheet Figure 3, or on a bespoke record sheet and then entered into a database. Data on the record sheet must include the following criteria:

  • Site: site name, date of sampling, latitude and longitude, measures of river restoration undertaken or that will be undertaken, year and the unique reference number of the record (if done during several years)
  • Landscape: general aspect, geology, soil, activities in the catchment
  • River: altitude of the source, slope, riparian vegetation
  • Reach: altitude of the sample, distance to the source, sample size and context, riparian vegetation, channel width, depth, water flow, substrate size and shape, shade, turbidity and whether the channel is perennial or otherwise
  • Surveyor biotic and abiotic impact appraisal: sewage, cows, pollution, invasive species, weir or obstacle impacts, poor floodplain management (coniferous plantation, over-grazing, urbanisation)
  • Others: landmarks to find the quadrat next year
  • Photographs (identification number and comments)

It is important to keep in mind that everything has to be done underwater in the case of a survey requiring diving, and vegetation cover will be assessed while gently swimming against the current, or by standing on the river bed. It is necessary to make preparations before the survey and check that essential items like underwater writing sheets are included. Beside it is necessary to have two people present for underwater survey for health & safety.

3. Advantages

  • Cost effective method (more than 30 quadrat samples per day on a site)
  • Suitable for observing and recording community evolution at a defined location
  • The method can be applied on adjacent habitats to monitor the wider effects of river restoration on the floodplain
  • Two people required – Health & Safety

4. Disadvantages

  • Need to have wetsuit (£50 - 100) and underwater writing device (£30)

5. Recommendation for method application

  • Select the quadrat location randomly on the river restoration area or subjectively but to monitor the several habitats (riffles, banks, bar features…) created by river restoration measures
  • Complete at least 3 quadrats per habitat to have enough statistical replicates, and keep the same quadrat size when sampling a homogeneous habitat
  • Conduct the methodology by two persons for convenience and safety reasons
  • Keep a watch on recent weather conditions to avoid potential turbid periods
  • It is sometimes useful to record species that are absent from a sample but present in the adjacent homogeneous stand of vegetation where the sample has been located. In such cases, a ‘Domin’ score of 11 can be used for recording and computer coding. Signs such as “+” or “x“ should be avoided. Another solution is to have another table to write these species (keep the quadrat number)
  • Using prepared species lists for recording can be useful
  • Complete the field sheet on site to accurately record the location context, and fill in all the required entries to assist in data interpretation
  • Take at least two pictures, one of the quadrat on the ground and one of the quadrat inside the habitat
  • A year to year mapping of the predominant habitats makes it possible to quantify and visualise changes in response to modifications in the restored area. This is most effectively achieved by mapping habitats in the floodplain (using a tablet/GPS or by drawing on a detailed paper map) and storing the data in a GIS for analysis.
  • Aerial photography -suitable for monitoring habitat change and riparian vegetation.

6. Cost

Cost will vary and depends on the type of vegetation habitat sampled as this will be a key factor in the time spent per quadrat. Also, site access, remoteness and travel distance will affect costs for fieldwork.

7. Data analysis

  • Creating floristic tables and deduce phytosociological association and plant communities
  • Characterizing specific vegetation types, and habitats
  • Find Preferential and differential species
  • Character species and fidelity
  • Species frequency and abundance
  • Dominant species

8. Protocol

National Vegetation Classification: Users’ handbook

9. References

  • National Vegetation Classification: Users’ handbook
  • The British Standards Institution 2014. Water quality — Guidance for the surveying of aquatic macrophytes in running waters. 24p.
  • DAVEY, A., GARROW, D., 2009. Variability components for macrophyte communities in rivers: summary report. Final report. Environment Agency Report SC070051/R4.34p.
  • Rodwell, J. S. (2006) National Vegetation Classification : Users ’ handbook.
  • WILLBY, N.J., PITT, J.A. AND PHILLIPS, G. 2009. Development of a system for the classification of lakes and rivers in the UK using aquatic macrophytes. Part II. Rivers. Environment Agency Science Report. Collaborators: Countryside Council for Wales, Natural England, Scottish Environment Protection Agency
  • https://jncc.gov.uk/our-work/nvc/