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Research Methods

How many plots and what size of plots should I use for sampling plant communities?

A common plot size for sampling vegetation is 10 x 10 meters, which is large enough for sampling trees and understory plants, but not so large as to be difficult to sample or replicate. There is no magic size that is inherently better than another. You want to select a plot size that is large enough to capture some of the variability in your plant community (usually at least 1m2 for understory plants), but not so large that it becomes impractical. The size and distribution of the particular plants of interest also affect the plot size. For example, if you are sampling trees, you need a much larger plot than if you are only sampling small understory plants. Sometimes it is necessary to use one sized plot for some plants (like trees) and another size plot for others (like understory plants). Perhaps more important than the size of the plots, is the total area sampled. You want to sample enough plots so that the total area sampled accurately captures the composition and diversity of the plant community of interest. Calculating a species-area curve for your study site is a very good way to assure that this goal is met (click here to read more about species-area curves).

Sampling plant species diversity

Plant diversity seems like a simple thing to measure, simply count the number of species, right? Well, yes and no. For example, say there are two sites that each contains 15 plant species. In one site, two of those species account for 90% of the total plant community with the other 13 species being present at very low levels, while in the other site, all 15 species are about equally abundant. Do these sites have the same diversity?

Sometimes, it is desirable to have a measure of diversity that includes both richness (number of species present) and their relative abundance. Several indices of diversity have been developed to help you do just that (see table 3 below). One that is very commonly used in plant communities is the Shannon Diversity Index, indicated by the symbol H’. I can be very interesting to see how the different measures of diversity can sometimes give different impressions of the same plant community!

Index* References Emphasis

Richness

n/a

Number of Species

Shannon's H' =[-Σp)]-[(s-1)/2N]

Shannon and Weaver 1964; Poole 1974

Integrates the number of species and relative abundance; derived from information theory; measure of entropy for the sample; full formula is and expanding series; the first two terms are shown here.

Relative H' = H'/ln(s)

Magurran 1988

Represents the evenness of the relaive abundance of the species present; the percentage of relative entropy for a system.

Simpson's D= Σ²

pi

Magurran 1988

A measure of dominance by one or a few species; the probability that two individuals selected from the population at random will be of the same species.

Margalef: (s-1)/(ln N)

Magurran 1988

Derived by fitting data; no theoretical explanation beyond how author believes species number and relative abundance may be related.

Menhinick: sl(N)^0.5

Magurran 1988

Derived by fitting data; no theoretical explanation beyond how author believes species number and relative abundance may be related.

Hulbert's: [Nl(N-1)]/(1-Σ pi² )

Washington 1984

Related to H' and D above; the probability of interspecific encounters


*pi, proportion of N made up by the ith species; s, number of species recorded; N, total number of individuals.

 

Table 3. Diversity Indices from ‘Hale et al 1999’

Sampling plant abundance

The two most common measures of plant abundance include stem counts and percent cover. Each has their pro’s and con’s. Depending on the questions you are asking, sometimes it is desirable to use both.

Stem counts are accurate when determining the population size of a particular plant species, but can be time consuming in areas with dense plant communities. Plus it is always a challenge to figure out how to count clonal species like grasses that have many stems for each individual. Also, individuals of different species can vary a lot in size! For example, when using stem counts, a small violet is counted the same as a very large fern. When examining plant population dynamics within species, stem counts are the way to go. But when looking at overall plant communities, sometimes a measure of “how much space” a particular species occupies is a valuable thing to know. That’s where percent cover can be helpful.

Percent cover is a method of determining relative abundance based on the amount of space they take up. In this method, rather than counting the number of individuals you assign each species to a “percent cover class” based on a visual estimate of how much of the sample plot they occupy (see figure 1 below). The reason most scientists use cover classes, rather than assigning a species number, like 16% or 48%, is that most people would not select the same number if they were looking at the same sample plot, it’s just too subjective. However, research has shown that most of the time, most people will select the same cover class when looking at the same sample plot, so it is a more reliable measure of plant abundance.

The Braun-Blanquet cover/abundance scale (adapted from “Aims and methods of vegetation ecology” by D. Mueller-Dombois and H. Ellenberg. 1974. Wiley New York) is perhaps the most commonly used in plant ecology:

Cover Class  
Percent Cover  
Description

r

<  5%

Assigned to a species where there is only a single individual of the species (a plant with multiple stems arising from the same root would be classified as a single individual) and covers less than 5% of the sample plot area.

+

<  5%

Assigned to a species where there are only a few (approximately 2-20) individuals of the species and those individuals collectively cover less than 5% of the sample plot area

1

<  5%

Assigned to a species where there are numerous individuals of the species, but those individuals collectively cover less than 5% of the sample plot area

2

5% - 25%

Assigned to a species where that species’ cover is between 5% and 25% of the sample plot area.

3

25% - 50%

Assigned to a species where that species’ cover is between 25% and 50% of the sample plot area.

4

50% - 75%

Assigned to a species where that species’ cover is between 50% and 75% of the sample plot area.

5

75% - 100%

Assigned to a species where that species’ cover is between 75% and 100% of the sample plot area.


Percent Cover

A 1 x 1 meter sample plot for sampling the understory plant community.

Visiually dividing the sample plot into sub-sections can help in estimating the percent of the sample plot occupied by different species.

 

Osmorhiza claytonii (sweet cicely)
Cover class 3 = 25-50%

 

Matteuccia struthiopteris (ostrich fern)
Cover class 2 = 5-25%

 

Maianthemum canadense (Canada May Flower)
Cover class + = few individuals less than 5%

 

Viola pubescens (Canada Violet)
Cover class 2 = 5-25%

 

Acer saccharum (sugar maple)
Cover class 2 = 5-25%

 

Asarum canadense (wild ginger)
Cover class 2 = 5-25%

 

Bare soil
Cover class 2 = 5-25%

Total cover of all plants combined
Cover class 5 = 75-100%

Figure 1. This series of images illustrate how percent cover was assigned to each of the understory plant species a 1 x 1 meter sample plot. When estimating percent cover it is handy to divide the plot visually into subdivisions of known percent and then try to visualize whether the cover of a given species will fit into a given space. For larger plots, marking the sided of the plot with flagging at several locations can help visualize this. Notice that percent cover values were recorded for “bare soil” and “total cover of all plants combined” as well as for each species. These can be very important parameters to measure since they indicate what proportion of the forest floor is shaded by plants and what proportion plants are not growing in. You can also see that the sum of the cover values of the individual plant species can be more than 100% because they may overlap, that’s another reason it’s a good idea to record the total cover for all plant species combined.

 

Sampling different layers of a forest
In addition to plant species, forests also have structural layers, and it is often the case that the plants occupying different structural change as you move from the seedling layer to the herbaceous plant layer to the sapling layer, and on up through the forest. How the species and relative abundance change with structural layer in the forest call provide insight into the past history and the future succession of the forest. For example, many oak dominated forests today have thick sapling layers of sugar maple due to historic fire suppression, indicating that as the mature oak trees begin to die back, they will probably be replaced by sugar maple and this has implications for forest organisms, nutrient cycling and other ecosystem processes.

The structural layers of most interest in hardwood forests of the Great Lakes region include:

  1. Tree layer (canopy) – includes all tree species with diameter at breast height (dbh) greater than or equal to 10 cm.

  2. Large Saplings & Shrubs (sub-canopy layer) – includes all woody species, large tree saplings and shrubs greater than or equal to 2m tall and <10cm diameter at breast height (dbh).

  3. Small Saplings and Shrubs (sapling layer) – includes all woody species, small tree saplings and shrubs greater than 0.5m tall and less than 2m tall.

  4. Seedling layer – includes all woody species, tree seedlings and low shrubs, less than or
    equal to 0.5m tall.

  5. Herbaceous Plant layer – includes all non-woody herbaceous, grass and grass-like plant species, irrespective of how tall they are.

    NOTE: Sometimes the seedling and herbaceous layers are combined since they occupy the same physical space. However, depending on the questions being asked, it is sometimes desirable to keep them separate.

Estimating percent cover of plants by structural layer is done in the same manner as described above, but the cover estimates only include plants in the given structural layer. For example, a tree sapling who’s top reaches the uppermost canopy, but has a dbh of 9cm, would be included in the “Large Sapling & Shrub” structural layer, not the “tree” layer; or if a sapling is tall enough to be considered a “Large Sapling” but has most of it’s foliage below 2 meters, you still record that coverage in the “Large Sapling & Shrub” layer. Similarly, estimates for individual species within a given structural layer only include individuals in that structural layer. So, you may have the same species, especially tree species, present in more than one structural layer.

When estimating the total cover of a layer, you would generally include coverage from plants rooted outside the plot if they extend into the space above the plot. However, when assigning a percent cover value to individual species you would generally only include those plants that are rooted in the sample plot. If there is a large plant rooted outside the plot that contributes a lot of cover, you can record it simply as “OP” for “out of plot” so that information is not lost. In some analysis this data may be included; in others it may be excluded.

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