Post-fire vegetation regeneration data
In order to illustrate the method we will use a stratified vegetation
data set containing data from 96 stands. The data was obtained to
investigate patterns vegetation regeneration three years after the
impact of a wildfire. Data were collected in 2012 by Miquel De Caceres
and Albert Petit in Horta de Sant Joan (Catalonia, Spain). The R object
is of class stratifiedvegdata (actually a list).
data(medreg)
class(medreg)
## [1] "stratifiedvegdata" "list"
## [1] 96
The dataset contains 96 stands (list elements), each of them a
data.frame where rows correspond to broad plant functional groups (Pine
trees, Oak trees, Tall shrubs and small trees, Scrubs and small shrubs
and Grass) and columns correspond to vegetation strata (1 to 7). The
upper heights of the vegetation strata are the following (in cm.)
vector:
strataUp = c(20,50,100,300,600,1200,2400)
And the width (range of heights) of each stratum is:
strataWidths = c(20,30,50,200,300,600,1200)
Species abundance values are percentage cover values estimated using
cover classes:
## 1 2 3 4 5 6 7
## Pine trees 0.0 0.0 0 0 0 0 0
## Quercus trees 12.5 25.0 0 0 0 0 0
## Tall shrubs and small trees 0.0 62.5 25 0 0 0 0
## Scrubs and small shrubs 12.5 37.5 0 0 0 0 0
## Grass 50.0 0.0 0 0 0 0 0
The data is read as follows. Shrubs reaching stratum 3 (50 - 100 cm)
had a cumulative cover of 25%, while shrubs reaching only stratum 2 (20
- 50 cm) had a cumulative cover of 62.5%. Thus, the observers grouped
plants according to their height and functional group, and estimated the
cover for those groups of plants.
Cumulative abundance profiles
The cumulative abundance profile (CAP) is a function
that takes a value of size as input (here the size is
a vegetation stratum) and returns the cumulative abundance of organisms
(here the cumulative cover value) whose size is equal to or
larger than the input value. In our case, the CAP function is
the cumulative cover of plants reaching the current stratum or higher
strata. Calculations are made using function CAP() of
vegclust:
medreg.CAP <- CAP(medreg)
Note that a different CAP is calculated for each functional group and
plot. The structure of the resulting R object is very similar to the
stratified data:
## [1] "CAP" "list"
## [1] 96
If we inspect the first element of the list, we can see the
difference between the original data and the cumulative abundance
profile.
## 1 2 3 4 5 6 7
## Pine trees 0.0 0.0 0 0 0 0 0
## Quercus trees 37.5 25.0 0 0 0 0 0
## Tall shrubs and small trees 87.5 87.5 25 0 0 0 0
## Scrubs and small shrubs 50.0 37.5 0 0 0 0 0
## Grass 50.0 0.0 0 0 0 0 0
Moreover, it is possible to graphically display the CAP of a given
stand (it may become difficult to interpret when the number of species
is large). For example, we can display the CAP for each functional group
of the first stand (plots="1"):
plot(medreg.CAP, plots="1", sizes=strataUp, xlab="Height (cm)",
ylab="Cumulative percent cover")
legend("topright", col=1:5, lty=1,
legend=c("Pines","Oaks","Tall shrubs","Scrubs","Grass"),
bty="n")
In this case the vegetation is a short but dense shrubland. Note that
in the plot we used strataUp to set the x-axis, so that
real heights are adequately represented.
Cumulative abundance surfaces
The concept of cumulative abundance profile can be extended to two
structural variables, which leads to the concept of cumulative
abundance surface (CAS). The CAS is a function that takes a the
values of two structural variables (size1 and size2)
as input and returns the cumulative abundance of organisms whose size is
equal to or larger than the input values in one of the
structural variables or in both. In the case of forests, natural choices
for structural variables are tree diameter and tree height. Since our
post-fire regeneration dataset only includes one structural variable, we
will illustrate the concept of CASs using a synthetic data set
consisting in a single plot where the species identity, diameter and
height of a hundred trees has been measured. We start by building a
tree-based data set:
pl = rep(1,100) # All trees in the same plot
sp = ifelse(runif(100)>0.5,1,2) # Random species identity (species 1 or 2)
h = pmin(100,rgamma(100,10,2)) # Heights (m)
d = pmin(150,rpois(100, lambda=h^2)) # Diameters (cm)
m = data.frame(plot=pl,species=sp, height=h,diameter=d)
In this example, we will use basal area (m2) as measure of abundance.
We calculate the area (in square meters) of each tree:
m$ba = pi*(m$diameter/200)^2
This specific data looks as follows:
## plot species height diameter ba
## 1 1 1 6.374945 49 0.188574099
## 2 1 2 4.527298 26 0.053092916
## 3 1 1 5.227101 23 0.041547563
## 4 1 2 6.010618 44 0.152053084
## 5 1 1 2.917073 8 0.005026548
## 6 1 2 3.039010 12 0.011309734
We start our analysis by defining two sets of size classes, one for
height and the other for diameter:
heights = seq(0,5, by=.25)^2 # Quadratic classes
diams = seq(0,150, by=5) # Linear classes
We are ready to stratify the data set:
tree.S<-stratifyvegdata(m, sizes1=heights, sizes2=diams,
plotColumn = "plot", speciesColumn = "species",
size1Column = "height", size2Column = "diameter",
abundanceColumn = "ba")
Function stratifyvegdata() is used to reshape data sets
so that they are suitable for calculating CAPs or CASs. In the case of
one structural variable, the function returns a list of matrices, one
for each plot. The post-fire vegetation data presented in the previous
section is an example of this. In the case of two structural variables,
stratifyvegdata() returns a list of three-dimensional
arrays, one for each plot. The cumulative abundance surface is then
calculated using function CAS:
We can plot the surface corresponding to each species as follows:
par(mfrow=c(2,1), mar=c(4,5,2,1))
plot(tree.CAS, species=1, sizes1=heights[-1], xlab="height (m)",
ylab="diameter (cm)", sizes2=diams[-1], zlab="Basal area (m2)",
zlim = c(0,6), main="Species 1")
plot(tree.CAS, species=2, sizes1=heights[-1], xlab="height (m)",
ylab="diameter (cm)", sizes2=diams[-1], zlab="Basal area (m2)",
zlim = c(0,6), main = "Species 2")
One nice property of CAS is that its marginal distributions are CAPs.
This can be easily shown if we compare the marginal CAP for height:
print(CASmargin(tree.CAS, margin=1))
## $`1`
## [0,0.0625] (0.0625,0.25] (0.25,0.562] (0.562,1] (1,1.56] (1.56,2.25]
## 1 5.358143 5.358143 5.358143 5.358143 5.358143 5.358143
## 2 4.233767 4.233767 4.233767 4.233767 4.233767 4.233767
## (2.25,3.06] (3.06,4] (4,5.06] (5.06,6.25] (6.25,7.56] (7.56,9] (9,10.6]
## 1 5.356180 5.328691 5.204519 4.694482 3.501698 2.298232 0.9959634
## 2 4.229919 4.204472 4.025323 3.460621 2.485078 1.130424 0.6647610
## (10.6,12.2] (12.2,14.1] (14.1,16] (16,18.1] (18.1,20.2] (20.2,22.6] (22.6,25]
## 1 0 0 0 0 0 0 0
## 2 0 0 0 0 0 0 0
##
## attr(,"class")
## [1] "CAP" "list"
with the CAP directly build using heights:
tree.S2<-stratifyvegdata(m, sizes1=heights, plotColumn = "plot",
speciesColumn = "species", size1Column = "height",
abundanceColumn = "ba")
print(CAP(tree.S2))
## $`1`
## [0,0.0625] (0.0625,0.25] (0.25,0.562] (0.562,1] (1,1.56] (1.56,2.25]
## 1 5.358143 5.358143 5.358143 5.358143 5.358143 5.358143
## 2 4.233767 4.233767 4.233767 4.233767 4.233767 4.233767
## (2.25,3.06] (3.06,4] (4,5.06] (5.06,6.25] (6.25,7.56] (7.56,9] (9,10.6]
## 1 5.356180 5.328691 5.204519 4.694482 3.501698 2.298232 0.9959634
## 2 4.229919 4.204472 4.025323 3.460621 2.485078 1.130424 0.6647610
## (10.6,12.2] (12.2,14.1] (14.1,16] (16,18.1] (18.1,20.2] (20.2,22.6] (22.6,25]
## 1 0 0 0 0 0 0 0
## 2 0 0 0 0 0 0 0
##
## attr(,"class")
## [1] "CAP" "list"
Finally, compare the previous three-dimensional figures, with the
marginal CAP plots for diameters and heights:
par(mfrow=c(2,1), mar=c(4,5,2,1))
plot(CASmargin(tree.CAS,margin=1), plots=1, sizes=heights[-1],
xlab="height (m)", ylab="Basal area (m2)", ylim = c(0,7))
plot(CASmargin(tree.CAS,margin=2), plots=1, sizes=diams[-1],
xlab="diameter (cm)", ylab="Basal area (m2)", ylim = c(0,7))
Dissimilarities in structure and composition
Although CAPs can be used to graphically display the structure and
composition of vegetation stands, the whole point of defining the CAP
function was to allow comparisons between stands. Returning to the
post-fire vegetation regeneration data, we can calculate dissimilarities
for all pairs of stands, thus obtaining a square and symmetric matrix
with dissimilarity values:
medreg.D = vegdiststruct(medreg.CAP, method="bray",
classWeights=strataWidths)
In the above sentence we forced strata to have different weight,
according to the range of heights that each stratum occupies. There are
different alternatives with respect to the dissimilarity index. In our
case we chose the CAP generalization of Bray-Curtis (De Caceres et al
2013). If we want to know, for example, the dissimilarity between stands
1' and2’ we simply write:
## [1] 0.2713178
When calculating dissimilarities it is possible to transform the CAP
values in order to prevent large abundance values to have an undue
influence in the analysis. In our case we choose to take the square root
of cumulative cover values:
medreg.Dsqrt = vegdiststruct(medreg.CAP, method="bray",
classWeights=strataWidths, transform="sqrt")
We can use metric multidimensional scaling to represent the distances
between stands obtained in both cases:
par(mfrow=c(2,1), mar=c(4,5,2,1))
X<-cmdscale(medreg.D, k=2)
plot(X, xlab="MDS 1", ylab="MDS 2", asp=1,
main="Cover untransformed", cex=0.5)
Xsqrt<-cmdscale(medreg.Dsqrt, k=2)
plot(Xsqrt, xlab="MDS 1", ylab="MDS 2", asp=1,
main="Cover sqrt-transformed", cex=0.5)
Note that the differences between the two ordination plots are
remarkable.
Classification of vegetation stands
In this section we use the square-root transformed dissimilarities
between vegetation stands to obtain a classification of the stands in
terms of their structure and composition. If you are not familiarized
with non-hierarchical clustering, you can read the tutorial about
vegclust package. We start by setting the number of
clusters to be found (nclusters) and the size of clusters
(dnoise, a parameter used to leave stands that are far from
all group prototypes unclassified):
nclusters = 6
dnoise = 0.40
We call function vegclust() using the clustering method
"HNCdd", which indicates (a) hard clustering, (b) medoids
as prototypes, and (c) noise clustering (i.e. excluding outliers in a
special class called noise class):
vc<-vegclustdist(medreg.Dsqrt, mobileMemb = nclusters,
method="HNCdd", dnoise=dnoise, nstart=100)
With nstart=100 we indicate that the algorithm should be
run 100 times starting from random seeds. This is advisable in order to
maximize the chance of having suboptimal solutions. The prototypes
identified by the algorithm are the following medoids (these are indices
of stands in medreg):
medoids<-vc$mobileCenters
print(medoids)
## [1] 20 92 24 4 38 7
The number of stands belonging to each cluster can be found
using:
cluster<-defuzzify(vc)$cluster
table(cluster)
## cluster
## M1 M2 M3 M4 M5 M6 N
## 20 9 24 9 13 11 10
Note that, because of the model chosen (and with the parameter
dnoise), there are a number of stands that are left
unclassified (i.e. those assigned to class “N”). A useful way to display
the results of the cluster analysis is by showing the stand memberships
to clusters in the ordination:
clNum = as.numeric(as.factor(cluster))
plot(Xsqrt, xlab="MDS 1", ylab="MDS 2",
pch=clNum, col=clNum)
legend("topleft", col=1:(nclusters+1), pch=1:(nclusters+1),
legend=levels(as.factor(cluster)), bty="n")
While the stands belonging to true clusters are more or less
close, those that are assigned to the noise can be far appart,
because the only fact that makes them be in the same class is their lack
of membership for true clusters.
To facilitate the interpretation of the clusters we can extract the
cumulative abundance profiles of the cluster medoids:
CAPm = CAPcenters(medreg.CAP, vc)
n = names(CAPm)
For example, we can inspect the structure and composition of the
fourth group:
round(CAPm[[n[4]]], dig=1)
## 1 2 3 4 5 6 7
## Pine trees 12.5 0.0 0.0 0.0 0 0 0
## Quercus trees 25.0 25.0 25.0 12.5 0 0 0
## Tall shrubs and small trees 62.5 62.5 37.5 0.0 0 0 0
## Scrubs and small shrubs 37.5 25.0 0.0 0.0 0 0 0
## Grass 50.0 0.0 0.0 0.0 0 0 0
The following displays graphically the CAPs of all six groups of
vegetation stands (code not shown):
