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Vegetation Analysis

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					                 North York Moors National Park Authority


Vegetation Analysis
What do we mean by vegetation analysis?

In most ecological projects, vegetation analysis is used to record and interpret the
vegetation within a habitat. At a basic level we may be interested in identifying the
species present within a site. Therefore our analysis is no more than recording the
presence or absence of species to create a list of plant species present. This may
not be as simple as it first seems. The identification of plant species requires some
knowledge of plant families and groups, an ability to use a plant taxonomic key (ID
key) and a familiarity with botanical terms and the parts of a plant. The collection of
data on plant species present within a field site is an excellent opportunity to develop
an understanding of botany and botany related skills.

Beyond listing species present or absent within a site we can also carry out a range
of additional vegetation analysis techniques, including abundance, biomass and
succession.

Abundance
As well as measuring the presence of species within a site we can also measure their
abundance. This can be carried out in two ways depending on the nature of the
habitat and the species present. You may be able to count the number of plants of a
particular species present within a particular habitat or sample site, for example, you
may be able to count the number of oak trees in a wood. The second approach does
not attempt to count individual plants but requires the surveyor to estimate the
percentage cover occupied by a single species of plant within a habitat or sample
area (e.g. within a quadrat). This second method works well with plants that live in
colonies with many hundreds of individual plants, e.g. heathers, mosses, woodland
flowers. Both methods allow statements to be made regarding relative dominance of
species.

Remember that counting gives a quantitative measure of plants whilst percentage
cover estimation gives a qualitative measure.




Figure 1. Species rich grassland. Presented with this view it initially appears difficult
to see how this environment can be assessed botanically. However, a range of
techniques are available and some are relatively straight forward.
                  North York Moors National Park Authority


Biomass
The collection of biomass data needs to be considered carefully as it normally
involves the collection of vegetation from within a sample area. This means that the
vegetation is removed (destroyed). Ethically we should try to minimise destructive
sampling within projects. However there are projects in which this type of sampling
will be acceptable, e.g. sampling crops that will be harvested and some grassland
studies. In order to measure biomass all the plant(s) within a sample area or quadrat
are collected, including the roots and all the plant above ground. The plant material
is then weighed. This can be done either by sample area or can be sub-divided by
species if appropriate for a study. After weighing, the material is then dried to
remove all water from the sample and reweighed. The new weight is the biomass.
Subtracting biomass weight from original weight will give you water content.

In addition, by burning the dry biomass material and reweighing the ash remaining
and then subtracting this weight from the dry biomass weight we can calculate
carbon content.

Succession
We can explore succession within a habitat over time by returning periodically to a
site and re-recording species presence and/or abundance. It is important to note that
in order not to influence natural succession our data collection method must be as
non destructive as possible. The return period of this type of study should perhaps be
annually to avoid confusion with seasonal changes.

Seasonal Variation
If we sample more frequently, our results may record seasonal variations rather than
successional change. The ground flora in a deciduous woodland over a year is an
example of seasonal change, with spring flowers giving way to brambles and ferns as
the year progresses.




Figure 2: Succession on moorland edge; note that deciduous trees, including birch, are creeping
into the moorland edge.
                     North York Moors National Park Authority


             Further Techniques for Vegetation Analysis

Measuring Biodiversity - Species Diversity Index
It is not always the case, but generally the greater the number of species present
within a habitat the “better” that habitat is. This often means the more stable or robust
an environment is likely to be. It is assumed therefore, that an environment with
many species in relatively equally numbers without a single dominant species is
stable and is high in biodiversity, whereas an environment with few species and a
single dominant species is viewed as being in a state of stress and has low
biodiversity.

To measure biodiversity we are interested in the relationship between the total
number of organisms present and the total number of species.

SIMPSON’S DIVERSITY INDEX
Simpson’s diversity index is an accepted and often used calculation of plant diversity
within a habitat. Within a sample area all plants of all species are counted. The
diversity is then calculated using the following equation where:-
D=     Simpson’s Index
N=     total number of plants
n=     number of individuals per species
=     sum of
With this index, 0 represents infinite diversity and 1, no diversity. That is, the bigger
the value of D, the lower the diversity. This is somewhat counter intuitive therefore
Simpson’s Diversity Index = (1- D) that is, the bigger the value of D, the higher the
diversity.

          species              number (n)      n(n-1)

 Barnacles                               82     6642
 Blenny                                   1        0
 Limpets
 sand hopper
                                         12
                                          4
                                                 132
                                                  12
                                                                        n(n-1)
 sea slater                               3        6          D=
 sea urchin                               4       12
                                                                         N(N-1)
 shore crab                               2        2
 Shrimp                                  10       90     Simpson’s Diversity Index = (1 – D)
 velvet swimming crab                     3        6

 total (N)                              121     6902
 N(N-1)                              14520
 n(n-1) =                            6902
 Simpson’s index (D) =                  0.47
 Simpson’s diversity index    (1-0.45)=0.55

Figure 3. Example data for calculating Simpson’s Diversity Index
            North York Moors National Park Authority



Some Project Suggestions


     Compare and contrast species assemblages within heather moorland
      after burning and non burnt mature heather.

     Explore vegetation change along an environmental gradient such as
      distance from the sea, with altitude or distance from a hedge or wood.

     Compare and contrast changes in vegetation assemblage under
      different soil types.

     Compare and contrast vegetation assemblages in managed and non-
      managed woodland.

     Compare and contrast species diversity and abundance in grazed and
      non grazed habitats.

				
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posted:11/21/2011
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