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cover measurement methods
using points and lines
as we learned in previous lesson there are a variety of
different approaches that we can use to measure plant
cover and in this lesson
we'll be focusing on methods that use points and lines. Point intercept methods
are one of the most commonly used methods to
estimate cover.
we use single points, points that are
arranged on transects, point frames,
or on point
quadrats
We record the number hits on
canopy, foliage, basal vegetation,
or soil surface depending on the type
of cover that
we're trying to estimate
and the calculations are quite straight forward. the percent cover
equals the number of hits on a particular species
divided by the total number of points
that were measured
we multiply by 100 to convert this
to percent
cover
in the example that we see in front of us there are
eight different points that were measured along a transect
and this is a ver simple system
in the image there is a grass and a forb
if we look at how cover is estimated
from this example
you see that they're 5 of those points
that hit
the grass
so 5 out of 8, multiplied by 100
equals 62.5%
for the forb
there was only one example where
the
point hit the
forb species and even though
it hit different leaves on that forb it only counts as one
which equals 12.5%
let's look at a slightly more complicated example which
shows a stand of vegetation with different kinds of ground cover
and we're going to run a transect with 10 points
we're recording
multiple types of cover and so that includes
ground cover, basal cover, foliar cover,
and canopy cover.
we can see the
data that was recorded for each one of the points
the first point did not hit anything
in the second point
there were no hits on any plant materials
so basal cover, foliar and canopy cover do not have any hits
but there was one hit on the rock for the ground cover category so that is recorded as
rock
for the next point
in this case
the point hits a leaf
and so it's recorded as foliar hit
and also because
foliar hits are included in the canopy
of the plant, it's also recorded as a canopy
hit on grass. In this case there was no
ground cover recorded because it hit bare soil
We can see by examining
all these points
and then totaling them up that we had four hits
on ground cover for 40%,
one hit
on basal cover here on point five
for 10%,
and that the foliar hit, or foliar cover, at 20% is
less than canopy cover at
50% and this makes sense because foliar
cover is always less than the canopy cover because
foliar is
included in canopy
so let's look at another approach which is a step
point transect. this is also called
a pace transect. in this case
we determine to walk a specified
direction and take
measurements at regular intervals where
we record the hit. the way this is actually done is that
someone's walking along at the specified time and pace
that they are supposed to take a point,
they will usually raise their foot to a 30 degree
angle and then drop a pin flag
at the tip of their shoe or boot
and then record what that hits
This approach is
very good for monitoring because it is rapid.
it does require a large number
of points, usually between three to five hundred points
per area,
and one of the problems with step-point transects is that they are subject
to bias because of where the pin is
lowered and also a
tendency
of someone walking to try to avoid walking
through plants
particular nasty
do to
being spiny or being in
some other way
undesirable.
so let's look at how we can take multiple measurements remember we have
canopy cover
and also
ground cover
and so we measure often times
multiple types of cover
at the same time
also we could get
more than one species and we want to take measurements
there. so in this case we're lowering a pin flag
and I would like to mention at this point that we do
want, usually, to use a pin flag or something
with a very small diameter because a point, in theory, doesn't have any
dimension at all and
with this first flag, it
hits the beargrass twice, it hits
blue grama
once, and it hits a globemallow before it lands
on a rock.
with the second pin flag
this point hits
the beargrass and then hits
soil or bare ground. so let's look at this
a little more closely on how we would record this. for point number one,
because the top canopy hit was on
bear grass, that is recorded here. now
the lower canopy layers, if there are
hits on other species in lower layers
there's room for that to be recorded as well and we record
the blue grama
and globemallow
because those were hit subsequently to
the top canopy hit.
note that we don't write beargrass twice even though it
was hit twice because you only record
one species once for each
hit
and then finally at the soil surface
the rock is recorded here.
for the second point
the hit for top canopy layer was on
beargrass and then there were no lower
canopy hits and the soil surface
hit was on bareground or soil.
if we look at a slightly more complicated system
again this has just two species,
we're going to look at a series of hits
first one was a miss, the second one there is a top and sub-canopy hit so
two species,
third one there is a top canopy hit,
the fourth one has a top canopy and sub-canopy hit,
and the fifth one there's a top canopy hit
and also a basal hit, or hit on the base of
the plant and this table shows how
these data
would be
recorded.
let's take a look at how we would then analyze this for
ground cover and for
canopy cover.
for ground cover, there were 2 hits out of 5 on
rock for 40% cover of rock.
there was 1 hit on a basal vegetation
out of 5 for 20% and the total
ground cover
is the recordings of all the ground cover hits
divided by the number of
points taken, or 60%.
now you may wonder why soil doesn't show up
in our ground cover classifications and that's because
remember that bare ground, or
bare soil, is not a ground cover
category. Groundcover is defined
as
those features that cover
soil.
now we look at canopy cover,
we sum for each individual species the
number of hits, so for blue grama there were 4 hits out of 5
for 80% cover
so we sum the top layer and lower layers
for our canopy cover categories.
similarly with globemallow
there was one top canopy hit and
one lower canopy, or sub-canopy, hit
for 2 hits, or 40%.
now you may note that the total canopy cover
equals 80%, not the sum of these two plants,
which would be 120%, and the reason for that
is that when we're recording, or calculating,
total canopy cover
we only use top layer hits. If we
were to include lower canopy hits,
then that would be artificially
increasing the
number
of hits on the top canopy
and we would get
a value that is an inflated value
for the total canopy cover.
so let's discuss how we place our points
throughout the landscape.
well one of the intuitive ways to do it is
to randomly place single points throughout the
study area and while random placement is
intuitively appealing
it's not very practical because in order for
canopy estimations, or cover estimations,
to work with points we usually need a large number of
points. if we went to each one of these points, it would take time
to go each point and also
we would only get one measurement per point, so in general
this is not an approach that we take for placing points
in the environment. we have seen that it is very common
to place points along transect lines. we also can
place points on quadrats so if we're doing
measurements that use quadrats, we can measure
cover at various locations
not necessarily all these locations
sometimes just one or two
on a quadrat frame. another approach is to use
grid-point frames.
in this case we have a frame
that is fitted with cross-hairs,
usually made of some sort of filamentous material such as
string, wire,
or fish line,
and there's usually a pair of the frames that are
held together in one solid configuration. we see
a picture here of someone using a pin flag
so the way that grid-point frames
work is that we measure,
we take a measurement of, cover at each one of the intersections
of these various points
in the grid
and so by having them, two
frames superimposed over each other, then
we have very specific control
on the angle of the placement of those
pins
in this case, the grid-point frame is the sample
unit and each one point measurement
is a subsample.
another option is to use a pin frame.
In this diagram we see a 5 pin frame
and the way that this works
is that the pin frame is the sampling unit and
each pin is
lowered and it's recorded what kind of canopy
and also what the ground surface
cover is
where that pin is lowered.
here we see a picture of a pin frame.
in this case this has 10 points, or 10
pins.
each one is lowered into the vegetation. we can see from these diagrams
that pinpoint pin frames are generally
best used in relatively dense and
somewhat homogeneous vegetation. pin frames do not work very well
in sparsely
distributed vegetation.
another advantage of pin frames is that they can be made
so that you can angle the points, rather than lower the points
vertically, we can actually lower the
point at a specific angle and sometimes
this is
a desirable trait.
so you may be wondering, what is the appropriate number
of points to be taken? so far we
we've looked at examples that have all been based on very few points
and that
by just adding one or two points
you can have a very remarkable
affect on
the estimate cover. the
number of points that we collect depends on
our management and sampling objectives,
why we're collecting the data, and how we plan to use the data.
It also depends on the system that we're working in,
the site characteristics, as well as the characteristics of the
species we're working with. In
very sparse vegetation
you could imagine
if you had a plant species that had
only 1% cover on average we would have to
measure 100 points just
to detect that plant once,
and so
the number of points used is
definitely affected by the kind of vegetation.
the type of point method we're
using is also very important
as well as the kind of sampling unit.
are we measuring points along transects? if so,
how many points per transect and how many transects?
same thing with using point-frames.
how many points per frame and how many
frames? as we've discussed
in previous lessons
about sampling design, determining the number of
sampling units, as well as the number of points
is something that needs to be determined.
most often this is
best determined by pilot sampling. in general
when we are doing sampling with
points, we usually need to
have several hundred points combined
in order to be able to characterize
the vegetation of the site.
obviously determining the number of
points depends on a large variety of factors.
now let's move on to
line intercept methods. in this case we're
looking at the interception of canopy
or basal vegetation along
transects.
line intercept is best used when we have
compact and very distinct plant canopies
or plant bases
and similarly to points
the way that we calculate
canopy cover
using line intercept methods is by
dividing the length
of the canopy intercepted
by the transect length and
multiplying that by 100. as we see in this diagram
that was originally published in
Elzinga et al, essentially if you have a
transect, what we look at is the distance
of the intercept of a plant canopy
along the transect. so let's look at a very
simple
example where we have a plant canopy
with a transect going through it
this is a 10 meter transect, we can see
in the orange lines there, the length of the canopy where it's intercepted
by the transect tape, is in
orange. if we take those
individual lengths of intercept and add
them together and then divide
that by the length of the total transect,
we find that cover in this case,
there was 6.1 meters of
intercept,
canopy that was intercepted,
and divide that by total length of the transect
for 61% canopy cover.
now let's look at how this applies to basal
cover. in
this case again we have the orange intercept.
we can see that in the case of this second plant here
even though the canopy is intercepted the base
of the plant is not and so it's not counted. we
only estimate the length of
the intercept of the basal cover.
again,
we combine the length of the intercepted
plant base, divide that by the length of the
transect, and that equals 10%
basal cover.
now let's quickly look at what happens if we
want to estimate cover
from photographs rather than from field-based
methods and we can do this using
aerial images or ground-based
photographs. we're going to
focus on a program that was developed by the USDA
Agricultural Research Service called
sample-point and this is
a very useful tool because the way that it works
is you have an image, your image can be either
ground-based image as the one we see here or an aerial-based
image and you can set the
program to display a certain number of points
usually as a grid. we have a grid here
of 5 by 5 (points) in yellow
and the active point, or the one that is being measured at
this point, is
red. we can also specify a
certain number of categories whether they
be functional groups, such as grass,
forb, shrub, cactus,
et cetera; ground cover: litter, soil,
rock or unknown
and these categories can be set depending on
what your sampling objectives are.
if we take a close-up on that active point we can see that
it is on the canopy but the thing that is really
valuable with sample-point
is that you really are interested in the
intersection of these two cross-hairs and
we can continue to zoom
until we feel very confident what is at the intersection
of those two cross-hairs. in this case,
this point would be categorized as
shrub because it is on foliage
of the snakeweed plant.
so in conclusion
when we're using points and lines to estimate cover
there are a variety of approaches that are available.
the decision about the approach
that we use should always be guided by our management and
sampling objectives and by a
consideration of the site and species
characteristics that we're working with