Choosing a fieldwork location
A suitable site must be found that is easy to access and safe, with no issues regarding access rights. Ideally displaying all the seral stages of salt marsh succession within a relatively small area. Many salt marshes are fragile habitats, please be mindful of this when visiting the sites.The Gann salt marsh near Dale in Pembrokeshire is an excellent location for studying salt marsh succession.
|Type of data||Data collection technique||Equipment needed|
|Vegetation characteristics (distribution, abundance and diversity)||
Frame quadrat or point quadrat
Care should be taken when working near the seashore.
- Wear long sleeves and trousers, as some vegetationcan be prickly or sharp.
- Beware of strong winds
- Wear sturdy footwear and beware of deep mud.
- Do no enter the sea.
- Always check tide times - the tide can come in quickly in esturaies.
- Carry out a pilot survey to identify a section of estuary where the salt marsh is running at least 300 metres.
- Carry out an interrupted belt transect from as close to the waters edge as possible. You can use a frame quadrat or a point quadrat. This should be done at regular intervals, perhaps every 30 metres. Or alternatively when you notice a difference in abiotic factors such as wind speed, humidity or temperature, giving an opportunistic or stratified sample, alternatively these may be in the ten locations highlighted under the dune profile below. At each site vegetation data should be collected to allow for the calculation of percentage cover.
- Use ranging poles and a clinometer to collect slope profile data. Take readings for every significant break of slope.
- Collect abiotic measurements such as soil and micro climate data at regular intervals. This could be the same time and locations.
Random sampling is used to select a sample that is unbiased. Within each area, every part of the surface must have an equal chance of being chosen. Random sampling with quadrats is used to examine differences between contrasting habitats within the sand dune system. There are many possible questions that could be investigated in this way.
In each habitat you are investigating, mark out a 10m x 10m square on the ground by laying one tape measure 10m lengthways and a second tape measure 10m at right angles to the first. Use a random number table (or random number generator on a calculator) to select numbers from 1-10. Each pair of random numbers can be used as x and y co-ordinates, using the metre interval markings oneach tape measure. Use these to locate the lower left hand corner of a frame quadrat. Take at least 10 quadrat samples in each area.
Systematic sampling is used where the study area includes an environmental gradient. A transect is used to sample systematically along the environmental gradient.
(a) Continuous transect
In a continuous transect, you record vegetation along the whole length of the transect line. In practice, this can be achieved by turning over a frame quadrat on the side perpendicular to the transect line as soon as you have finished recording at each point. For a shorter distance, such as an investigation into how vegetation changes across a singe creek, a continuous transect could be useful. But a typical 300m transect, perhaps sampled with a 1m2 quadrat, would require many more samples than you could realistically take.
|Part of an interrupted belt transect. Samples were taken at points a, b, c and d.|
In an interrupted transect you record data from quadrats placed at intervals along the line. Quadrat sampling points (or 'stations') can be regularly spaced along the line (perhaps every 30m). Or you can choose to place stations at specific points of interest along the transect line. One quadrat per station is not enough. Instead take at least 3-5 quadrat readings at each station. You could do this by placing a frame quadrats nex to the transect line, then turning the quadrat over on its side to take the next quadrat reading. You will collect data across a broader belt of the saltmarsh; a technique known as an interrupted belt transect.
For a comparison of ground vegetation in two areas, construct a random sampling strategy. You need to justify your choice of sampling locations. Use quadrats (either frame or point quadrats) in each sampling location. There is always a minimum number of samples that must be taken if you are performing statistical tests on your results.
Recording vegetation data
(a) Species lists
The simplest data that you can collect is a species list. This is a record of the names of each plant species that are present. lt is also known as presence /absence data. A species list provides no information about how abundant each species is.
(b) ACFOR scale
The ACFOR scale can be used to collect data on the abundance of each species
If you are comparing more than one site, some plants could be totally absent in some samples, so can be recorded as N for NONE. Data collected on the ACFOR scale is often biased. Many investigators over-estimate conspicuous plants (especially those in flower) and under-estimate inconspicuous plants. More reliable data will come from quantitative methods.
(a) Frame quadrats
This is a square frame which is used to outline a sample area. The simplest way to collect data using a frame quadrat is to make a species list for each quadrat. This allows you to calculate the percentage frequency of each species for the whole sample area. Percentage frequency is the probability that a species will be found within a single quadrat.
For example, an investigation was made into the difference in vegetation between a grazed and an ungrazed section of the fixed dunes. A random sample using 30 frame quadrats is taken in each area. In the grazed area, bird's-foot trefoil was present in 18 of the 30 quadrats; in the ungrazed area, bird's-foot trefoil was present in 6 of the 30 quadrats.
`"% frequency in ungrazed area" = 18 / 30 xx 100 = 60%`
`"% frequency in grazed area" = 6 / 30 xx 100 = 20%`
The number of quadrat samples taken affects the reliability of the results. The number must be large enough to minimise the effect of anomalies, but not too large that it cannot realistically be carried out in the time available. lt must also be sufficient for any statistical tests you are using. In practice, 10 quadrat samples is the absolute minimum for each sample area.
lt is often useful to know what amount of each species is present. There are two ways of describing the amount of each species within a frame quadrat: percentage cover and local frequency. Percentage cover is faster but is more subjective. Simply estimate what percentage of the area enclosed by the quadrat is occupied by each species. One drawback of this method is that plants in flower tend to be over-estimated while low-growing plants are under-estimated. Local frequency is easiest to measure if you have a gridded quadrat of 10 x 10 squares. For each species, count the number of squares which are at least half-occupied. The final figure (which will vary between 1 and 100) is the local frequency. This method is more time consuming than estimating percentage cover, but the results should be more accurate, as there is less risk of over-estimating some plants and under-estimating others.
(b) Point quadrats
A point quadrat is a frame shaped like a T. The bar of the T has ten holes in it, and to sample vegetation a long pin much like a knitting needle is stuck through each hole. The different plants that the pin 'hits' as it is pushed towards the ground are identified and counted.
To collect data on abundance, count the first hit made by the pin on each different plant. Count the first hit only.
Local frequency can be calculated for each species.
% freq = (total number of hits of a species x 100) / total number of pin drops
For example, if at a single station sea holly is hit 2 times out of 10 pin drops
`"% freq" = 2/10 xx 100 = 20`
The % frequency of sea holly is 20%
If you have classified each species into communities (e.g. pioneers, mobile sand dunes, dune slack plants, etc.) you can use point quadrat data to calculate percentage composition. This shows what percentage of plants at each station are from each community.
`"% composition" = ("% frequency of a community" xx 100) / "% frequency of all communities"`
For example, if at a single station, the % frequency for all pioneers is 40 and the % frequency for all communities is 80.
`"% composition" = 40/80 xx 100 = 50`
Pioneers make up 50% of all plants found at that station.
Some of the most useful abiotic factors to record are soil characteristics.
Remove vegetation with a trowel. Take a small sample of soil (a quarter of a trowel full is plenty) from 15cm below the surface of the vegetation and seal it in a polythene bag straight away to prevent it from drying out. Put a label marked in pencil inside the bag to show the location and time. Plastic 35mm film pots (becoming much rarer now that digital cameras are widely used) are a useful alternative to polythene bags.
Collecting soil samples by digging holes is destructive and you must have permission from the landowner before you start. Only remove a small amount from each place - not a trowel-load. Replace any vegetation you have removed. Wash your hands thoroughly after carrying out soils work.
|Taking a soil sample in a dune slack at Harlech sand dunes, near Rhyd-y-creuau Field Centre||Measuring soil infiltration rate in semi-fixed dunes at Mullaghmore sand dunes, County Sligo, near Derrygonnelly Field Centre|
Here are some of the most useful soil factors that you can measure:
1. Soil pH
A chemical pH test can be carried out in the field, or on the soil samples in the lab.
- Put 1 cm of soil into a test tube
- Add 1 cm of barium sulphate to the test tube (this attaches to fine particles and makes them sink leaving a clear layer above it)
- Add enough distilled water to bring the level half way to the top of the test tube
- Add two pipettes full of indicator solution
- Seal the test tube with a rubber bung and shake well (make sure that the contents are mixed thoroughly)
- Allow the test tube to stand until the soil has settled out leaving a coloured solution
- Hold the tube next to the colour chart and decide which pH colour it most closely matches.
- Dispose of the soil in the bin.
2. Soil texture
The soil texture is determined by the proportion of the soil made up of sand, silt and clay. A quick way of judging soil texture in the field is outlined in the OPAL Soil Survey (pdf).
3. Soil infiltration rate
You will need
- container of water (keep this constant between experiments - 4 litres is enough)
- 30 cm ruler
- metal / plastic tube (to be the 'infitration tube')
- block of wood
- at least 2 people - one to pour in water, the other to operate the stopwatch and take readings
Bang the infiltration tube into the ground until it forms a seal (using the wood and mallet to bang it in evenly and avoid affecting the results by stamping on the area of land being tested).
One person fills the infiltration tube to a standardised level (e.g. 15cm). The other starts the stopwatch, and then record the water level every 30 seconds. To maintain water pressure, person 1 should top up the infiltration tube if their water level falls below a certain point (e.g. 10cm). Person 2 should make a note of when this is done, so that it can be taken it into account when they are calculating their infiltration rate.
4. Soil moisture content
Soil water content is measured by weighing the fresh soil sample, drying the sample, then weighing the dry soil. The difference between the two figures is the soil moisture content. It can be expressed as a percentage of the mass of the fresh soil sample.
The most effective way to dry the soil sample is to use an oven (a microwave oven will do), but if this is not available then you could leave the soil sample to dry overnight.
- Find the mass of a heat resistant crucible.
- Half fill the crucible with soil from the soil sample and then weigh it again.
- Place the crucible in an oven at a temperature just above the boiling point of water (ideally around 105°C) until it is dry.
You can tell when it is completely dry by weighing the crucible and soil, returning it to the oven for 10 minutes and weighing again and so on, until there is no change in weight after 10 minutes. Don't be tempted to turn the oven up to make the drying take place more quickly. You will only begin to burn off the humus in the soil and won't then be able to find out either the weight or humus content accurately. Now record the weight of the crucible and dry soil. Find the difference between the dried soil and the fresh soil, and use this to calculate the % moisture content of the fresh soil.
5. Humus content
Soil humus content is measured by weighing a dry soil sample, burning the humus in the soil, then weighing the soil left. The difference between the two figures is the humus content, which can be expressed as a percentage of the mass of fresh soil sample.
It is usual to measure for soil humus content after you have measured for moisture content, so that you have figures for the mass of fresh soil and also samples of dry soil.
The most effective way to burn off the humus is to use a furnace that is capable of reaching a temperature of 550°C. If this is not available, a Bunsen burner can be used instead.
If using a furnace, place the crucible and dry soil into a furnace at 550°C for 15-30 minutes, until all the humus has been burnt off.
Or if using a Bunsen burner Using a tripod and gauze sheet, heat the soil sample in a crucible above a full-flame Bunsen burner for 30 minutes to 2 hours, until all the humus has been burnt off.
Record the weight of the crucible and soil and calculate the percentage of humus in the soil. The mass of dry soil, rather than the mass of wet soil, is used to calculate the percentage humus content. Using dry soil mass allows you to compare soil samples taken on different days.
Microclimate data (including soil temperature, humidity and light levels) can be used to support investigations into how vegetation changes across the saltmarsh. If carried out carefully, your results will provide a snapshot of the environmental stresses experienced by plants. Ideally all measurements should be made at the same time of day, with the same technique at each site.
A simple mercury thermometer can be used. Air and soil temperature can all be measured.
It is sometimes useful to find the temperature range over a particular time period, such as 24 hours. Maximum/minimum thermometers are widely available. Try as much as you can to standardise the temperature reading between sample sites.
An anemometer will measure wind speed directly. Take readings at the same height above the ground each time.
The most practical way of measuring humidity in woodland is a hand-held whirling hygrometer. This piece of equipment compares the temperatures recorded by wet and dry thermometer bulbs. The bulb surrounded by a wet fabric sleeve usually shows a lower temperature than the other because of the cooling effect of evaporation. The bigger the difference recorded by the two bulbs, the lower the relative humidity. There are standard tables for the conversion.
- Light levels at ground level often vary throughout the wood so a large number of readings need to be taken at fixed points.
- Light conditions constantly change e.g. with seasons, the time of day and changes in cloud cover. If two areas being compared readings must be taken simultaneously.
- Different light meters vary in what they measure and their accuracy. Matched calibrated meters should always be used.
Because of these difficulties it is often simpler and more meaningful to obtain an indirect measure of how much light might reach the ground community (e.g. by assessing the amount of open sky which can be seen through the canopy at a number of fixed points, or by recording whether the ground at these points is in shade or sun at set time intervals during the day).