Open Journal of Forestry
2013. Vol.3, No.4, 122-128
Published Online October 2013 in SciRes (
Copyright © 2013 SciRes.
An Evaluation Model for Improving Biodiversity in Artificial
Coniferous Forests Invaded by Broadleaf Trees
Yozo Yamada, Sayumi Kosaka
Graduate School of Bio-Agricultural Sciences, Nagoya University, Nagoya, Japan
Received July 16th, 2013; revised August 17th, 2013; accepted August 29th, 2013
Copyright © 2013 Yozo Yamada, Sayumi Kosaka. This is an open access article distributed under the Creative
Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited.
Increasing attention is being paid to the various functions of forests, especially the conservation of biodi-
versity. In Japan, 67% of national land is covered by forest, 41% of which is artificial forest (i.e., planta-
tions). Therefore, efforts to conserve forest biodiversity should also target artificial forests. In this study,
we investigated the increase in biodiversity resulting from broadleaf tree invasion of artificial coniferous
forests. We examined diversity indices and combinations of indices to identify which ones can aid forest
managers in evaluating forest diversity. We also studied classification according to the richness of diver-
sity, which corresponded to the growth stages of Chamaecyparisobtusa and Cryptomeria japonica planta-
tion forests. Moreover, we developed a model that will contribute to sustainable forest management and
biodiversity over an entire area. The model, based on a specific rotation scenario in a geographic informa-
tion system, is easy to use and presents spatial and temporal changes at sites visually.
Keywords: Artificial Forest; Invading Broadleaf Trees; Broadleaf Tree Diversity; Sustainable Forestry
Management; Basin Scale
The importance of forest biodiversity has been discussed
since the United Nations Conference on Environment and De-
velopment (UNCED) in 1992. The UNCED adopted the Dec-
laration on Forest Principles in Agenda 21, which is a series of
principles for sustainable forest use (Lund et al., 2004). In 1994,
the Montréal Process Working Group agreed on seven criteria
of sustainable forest management and 54 indicators of the crite-
ria. Criterion 1 stipulates the conservation of biological diver-
sity and states that conserving the diversity of organisms and
their habitats supports forest ecosystems and their ability to
function, reproduce, and remain productive (Montreal Process,
2009). Many studies have confirmed the importance of forest
diversity, although most have focused on natural forests (Bur-
slem, 2004; Ehrlich, 1996; Kondoh, 2002; Noss, 1990).
Artificial forests account for approximately 6.5% of the
global forested area. The percentage of artificial forest varies by
country. In Japan, 68.5% of the land area is covered by forest,
41.3% of which is artificial forest (FAO, 2012). By age, artifi-
cial forests in Japan form a nearly perfect normal distribution,
with a top age of 50 years. Thus, vast areas of forest require
immediate thinning (Forestry Agency, 2012). However, many
of the artificial forests that require thinning have been deterio-
rating, mainly because of reductions in forestry surplus. In
countries such as Japan that have vast areas of artificial forest,
biodiversity loss has become a problem, even in the artificial
forests since last few decades. Many artificial forests are de-
signed as monoculture plantations with the aim of growing
high-quality timber. Thus, these forests are generally dark with
sparse understory vegetation. Brockerhoff et al. (2008) noted
that artificial forests usually have less habitat diversity and
complexity. Yamaura (2007) also suggested that artificial for-
ests have been treated as a homogeneous non-habitat (matrix)
and ignored in biodiversity conservation.
However, the environment in an artificial forest can be al-
tered to become more favorable to biodiversity. Hartley (2002)
indicated that earlier thinning schedules or longer rotations can
strongly affect biodiversity, as can reserve trees that are left
after plantation harvest and remain through a second rotation.
Busing and Garman (2002) argued that proportional thinning
retains understory stems, thereby expediting the recruitment of
shade-tolerant trees. El-Keblawy (2005) suggested the impor-
tance of reducing forest crowns to promote species growth.
Yamaura (2007) proposed that the negative effects of artificial
forest could be mitigated by increasing the complexity of the
structure and composition of a plantation through extended
rotation, strong thinning, wider tree spacing, and the retention
of broadleaf trees and coarse woody debris in clear-cuts. Broc-
kerhoff et al. (2008) also stressed that to sustain native biodi-
versity within an artificial forest, managers should consider
using a greater diversity of planted species, extending rotation
lengths in some stands, and adopting a variety of harvesting
The overall aim of the present study was to promote the di-
versity of invading broadleaf trees in artificial coniferous for-
ests. We examined effective diversity indices and combinations
of these indices that can be used by forest managers to evaluate
forest diversity. In addition, we studied classification according
to the richness of diversity, which was found to correspond to
growth stages in Chamaecyparisobtusa and Cryptomeria ja-
ponica artificial forests. Moreover, to help forest managers
maintain sustainable forests with rich biodiversity over the
entire area, we developed an easy-to-use model that shows spa-
tial and temporal changes at forest sites visually. The model,
implemented in a geographic information system (GIS), is
based on a specific rotation scenario.
Survey Location
Our survey was conducted at Danto National Forest (35˚6'N,
137˚28'E), located in Shitara-cho, Aichi Prefecture, Japan. The
forest covers 5303 ha, 93% of which is artificial forest domi-
nated by hinoki (C. obtusa). It ranges from 400 to 1150 m
above sea level, with an average annual temperature of 11.7˚C
and annual precipitation of 2036 mm.
The Chubu Regional Forest Office of Forestry Agency has
performed systematic thinning and promoted understory vege-
tation in their forests, with the farsighted (100-year) goal of
creating diversified, vital, and sound forests for the coexistence
of log production and various public benefit functions (Chubu
Regional Forest Office, 2011).
Danto National Forest is divided into about 1300 sub-com-
partments, and the Forest Office has created specific manage-
ment plans and concrete management schedules for each sub-
compartment. As part of this management, the sub-compart-
ments are arranged to achieve a proper balance of vegetation
types and different stages of succession to aid in the conserva-
tion of biodiversity in the entire forest.
Vegetation Survey
We chose six hinoki (C. obtusa) sub-compartments (aged 13,
36, 44, 56, 76, and 118 years) and eight sugi (C. japonica) sub-
compartments (aged 26, 45, 55, 60, 72, 79, 98, and 116 years).
We also selected four additional plots: two hinoki (aged 34 and
55 years) and two sugi (aged 34 and 55 years) sub-compart-
ments in the Inabu Experimental Forest of Nagoya University,
located near the Danto National Forest. These plots served as
controls, representing forests in which management has been
As in our previous study (Kosaka & Yamada, 2013), we es-
tablished 10 m × 10 m plots within areas representative of each
sub-compartment. We examined all broadleaf and planted trees
that were taller than 1 m. For the broadleaf trees, we deter-
mined the number of species and population size. We also
measured the tree height and diameter at a height of 50 cm from
the ground surface. For planted trees, we measured the height
and diameter and counted the number of planted trees in 20 m ×
20 m areas to calculate the stand density.
Analysis at the Sub-Compartment Scale
For the analysis of broadleaf tree diversity within sub-com-
partments, we used the number of species, population size,
proportion of basal area, and two species diversity indices: the
Shannon-Wiener index and inverse Simpson index. We clari-
fied the characteristics and diversity of invading broadleaf trees
during each growth stage in artificial forests to discuss practical
ways of retaining broadleaf trees to encourage biodiversity
Analysis at the Basin Scale
Using a GIS and the forest age data from the forest register,
we determined the distribution of broadleaf tree diversity across
the entire Danto forest by sub-compartment. Broadleaf tree
diversity was classified into three levels based on forest age,
which was estimated from our analysis at the sub-compartment
We set up a rotation scenario in which all forests more than
80 years old are entirely cut and the plots are then replanted
with the same tree species. That is, clear cutting and reforesta-
tion are repeated every 80 years in each sub-compartment. We
simulated the change in broadleaf tree diversity in Danto forest
from the present to 40 years later and discussed the spatiotem-
poral evaluation of broadleaf tree diversity shown by the model.
Results and Discussion
Analysis at the Sub-Compartment Scale
The invading broadleaf trees were divided into the following
three categories by height:
Lower layer (Shrub): tree height is more than 1 m and less
than 4 m.
Middle layer (Sub-tree): tree height is more than 4 m and less
than 8 m.
Upper layer (Tree): tree height is more than 8 m.
We analyzed every diversity index and its layer composition
by forest age. All diversity indices indicated a similar trend of
change in forest age. For hinoki forests, Figure 1 shows the
relation between forest age and number of species, while Fig-
ure 2 shows the relation between forest age and proportion of
basal area. Both indices increased significantly after an age of
76 years. After that, in the mature stage, the number of species
decreased slightly at 118 years old, but the proportion of basal
area continued to increase. The young stage represented by the
13-year-old forest also had abundant species and was jungle-
like in its appearance, but its proportion of basal area was not
very large. The middle stage, from 36 to 56 years old, was the
stage of height growth, when the crowns closed entirely. Thus,
the lack of sunlight in this stage led to a decrease in invading
broadleaf trees.
For the sugi forests, Figure 3 shows the relation between
forest age and number of species, while Figure 4 shows the
relation between forest age and proportion of basal area. The
sugi forests exhibited a slightly different tendency from hinoki
forests. As shown in the figures, both indices increased remark-
13 36 44 56 76118OG*
Number of species
Forest age
*OG: Old growth
Upper layer
Middle layer
Lower layer
Figure 1.
Forest age and number of species in hinoki (C. obtusa) forests.
Copyright © 2013 SciRes. 123
13 36 44 56 76118OG*
roportion of basa
Forest age
*OG: Old growth
Upper layer
Middle layer
Lower layer
Figure 2.
Forest age and proportion of basal area in hinoki (C. obtusa)
26 45 55 60 72 79 98116
Number of species
Forest age
Upper layer
Middle layer
Lower layer
Figure 3.
Forest age and number of species in sugi (C. japonica) forests.
26 45 55 60 72 79 98116
roportion o
basal area
Forest age
Upper layer
Middle layer
Lower layer
Figure 4.
Forest age and proportion of basal area in sugi (C. japonica)
ably from 60 years of age and kept increasing until 79 years.
The proportion of basal area also reached its maximum at 79
years. The young stage, represented by the 26-year-old forest,
had fewer species compared to the young-stage hinoki forest,
whereas the middle stage (from 45 to 55 years) retained a rela-
tively large number of invading broadleaf species.
The layer composition developed toward the upper layer as
both hinoki and sugi forests reached the mature stage. In the
Danto forest, invading broadleaf trees remained, because they
were not cut as long as they impeded thinning or threatened
From these results, we can define three stages on the basis of
the diversity of invading broadleaf trees: young, middle, and
mature. The young stage includes forests less than 25 years old
that have relatively rich diversity of invading broadleaf trees,
unless those trees were cut by intense weeding and cleaning
The middle stage includes forests more than 26 and less than
60 years old. These forests have poor diversity of invading
broadleaf trees because of a lack of sunlight. Although this
stage does not have good conditions for understory vegetation,
it is indispensable for the production of high-quality logs.
These low-light conditions will not be improved until comer-
cial thinning begins. Commercial thinning usually starts at age
40, but its effect on improving the light conditions does not
appear until a few decades later.
The mature stage includes forests more than 61 years old and
has rich diversity and a developed layer of invading broadleaf
trees. Commercial thinning is performed repeatedly at 15- to
20-year intervals; thus, favorable light conditions for invading
broadleaf trees are kept and improved unless broadleaf trees are
cut during thinning.
In comparison, the four control plots in the Inabu Experi-
mental Forest of Nagoya University were typical examples of
poorly tended forests. The understory was very dark, with in-
sufficient light for the invasion of understory vegetation. Con-
sequently, we found no invading broadleaf trees in these plots.
To promote understory vegetation, management measures
such as thinning must be implemented at appropriate times
based on a specific density control plan. Figure 5 shows the
relation between forest age and stand density in Danto forest,
Inabu Experimental Forest, and Hayami forest, which we ex-
amined in our previous study (Kosaka & Yamada, 2013). In
forests less than 40 years old, before launching commercial
thinning, we found no remarkable differences in stand density
among the plots, including the two plots at Inabu. However, the
stand densities differed after 50 years. As shown in Figure 5,
Hayami forest has maintained the lowest density and is consid-
ered to have ideal density control. Danto forest has also main-
tained a low density, showing good performance and careful
tending. In Inabu, 55-year-old plots have higher densities than
y = 3175e-0.02x
R² = 0.8639
050100 150
Stand density (trees/ha)
For est age
Figure 5.
Forest age and stand density.
Copyright © 2013 SciRes.
Copyright © 2013 SciRes. 125
plots of that age in the other two forests and are clearly too late
for thinning.
We should also consider the traditional practice of removing
all understory vegetation to provide safe and convenient work-
ing conditions for thinning. In this case, invading broadleaf
trees will be cut as often as thinning takes place and will not be
able to grow to create a layered forest structure. Both Hayami
forest and Danto forest have their own guidelines, which say
that invading broadleaf trees should remain as long as they do
not impede thinning or threaten safety.
Selection of Diversity Indices for Broadleaf Trees
The five indices recorded in this research showed a similar
trend of change in forest age, but we found that each index had
certain advantages and disadvantages. For example, although
the number of species is easy to survey, this measure cannot be
used to estimate the degree of uniformity. Moreover, the num-
ber of species may increase in proportion to plot size. Popula-
tion size can be used to estimate the degree of uniformity, but if
one species dominates a plot, uniformity may be overestimated.
Otherwise, this measure may tend to show a trend similar to
that of the number of species. The proportion of basal area can
indicate the species dominance in each layer, but the difference
between dominance and diversity requires careful consideration.
The Shannon-Wiener index is popular and easy to understand
because it includes both the number of species and the popula-
tion size.
The inverse Simpson index has the same characteristics and
can indicate species uniformity. However both indices are in-
fluenced largely by the observed parameter.
Each index has own characteristics, making it difficult to
choose a single index as the best for estimating the diversity of
invading broadleaf trees. By using a few indices in combina-
tion, we can evaluate the different perspectives each provides
and they can compensate for deficits in each other.
To determine the most effective combination of indices, we
created a precedence matrix for each index and calculated the
rank difference of each sub-compartment between pairs of in-
dices. Table 1 shows the average value. Smaller values indicate
that both indices of a combination estimate diversity similarly,
so we could use either one of the two. Meanwhile, a larger
value means that the indices estimate diversity from different
perspectives, so it is effective to use both indices. The inverse
Shannon index is different from the other indices, with the ex-
ception of the Shannon-Wiener index.
Moreover, we cannot determine the best indices based on sta-
tistical results alone; other features should also be considered.
Table 2 shows the advantages, disadvantages, and applications
of each index. On the basis of our results, to evaluate broadleaf
tree diversity at the sub-compartment level, we recommend
using the proportion of basal area to evaluate the species domi-
nance in each layer and the inverse Simpson index to evaluate
diversity from a different perspective than the proportion of
basal area.
However, the total number of species did not largely differ
among forest ages. On the other hand, when considering layer
structure, the number of trees located lower in the canopy de-
creased with age, whereas the number of trees at intermediate
levels increased with age. After age 67, an upper layer began to
form, indicating that layer composition becomes more complex
as the forest matures. In particular, layer composition within
forests at age 99 approaches that of natural forests.
The number of species is an effective index for forest man-
agement, as species richness is a good indicator of current for-
est conditions. However, the evenness of species must also be
considered. Furthermore, the number of species tends to in-
Table 1.
Average rank difference between pairs of indices.
Number of species Population sizeProportio n o f basal areaShannon-Wiener index I nv erse Simpson inde x
Number of species - 1.068 2.006 1.961 4.187
Population size - - 2.442 2.637 4.679
Proportio n o f basal area - - - 2.004 3.510
Shannon-Wiener index - - - - 2.270
Inverse Simpson index - - - - -
Table 2.
Merits, defects, and uses of each index.
Indexes Merits Defects Applications
Number of species High versatility Ambiguous uniformity, Increasing in association with
expansion of sample size
Provides a tentative evaluation
of diversity
Population size Degree of uniformity Apt to show a similar trend to number of species Can be substituted by number of species
Proportion of basal a re a Dominancy, insusceptible to population sizeDominancy diverseness For evaluating dominance
Shannon-Wiener index Popular and easy to follow index Large influence from parameter For evaluating both the number of species
and population size
Inverse Simpson index Evaluation including uniformity of speciesLarge influence from parameter For equivalent evaluations of several
invading species
crease as plot size increases.
Analysis at the Basin Scale
To evaluate diversity of invading broadleaf trees at the basin
level, it is important to consider fragmentation and networks of
sub-compartments having rich diversity within the whole sub-
ject area or forest. Fragmentation is important for avoiding
overly large expanses of even-aged plantations, as large mono-
culture areas may not have good effects in terms of biodiversity
and ecosystem functioning. Meanwhile, the network should be
maintained for all living organisms to move around and live in.
Every management plan must start with an understanding of
the present condition of the entire subject forest. We propose
the following steps to estimate invading broadleaf diversity in a
whole forest. Here, “site” refers to the minimum scale of man-
agement, which in the Danto forest equals the sub-compartment
Step 1 (GIS mapping): Specify the location of each site using
a GIS. Next, add site-specific data obtained from the forest
register, including the main planted species, its percentage of
dominance, and forest age.
Step 2 (Categorization by growth stage): Classify all sites
into three stages (determined previously): young stage (less
than 25 years), middle stage (more than 26 and less than 60
years), and mature stage (more than 61 years).
Step 3 (Visual display): In the GIS, color-code each site ac-
cording to its classification: young stage as green, middle stage
as light green, and mature stage as dark green. That is, a darker
color means richer broadleaf tree diversity.
Figure 6 shows the present condition of broadleaf tree di-
versity in Danto forest. Most of the area is light in color, indi-
cating large homogeneous groups, with dark areas scattered
throughout the forest. This situation is considered insufficient
to maintain the diversity of invading broadleaf trees. To evalu-
ate the potential for improving the diversity within Danto forest,
we simulated changes in conditions up to 40 years in the future.
Figure 6.
Diversity of invading broadleaf trees in Danto forest (present).
Step 4 (Rotation scenario): All sites more than 80 years old
are entirely cut and then replanted with the same tree species
the following year.
Step 5 (Simulation): The change in broadleaf tree diversity
from the present to 40 years later is simulated and the spatio-
temporal changes shown by the diversity model are discussed.
Figures 7 and 8 show the simulation results after 20 and 40
years, respectively. After 20 years, the overall size of the dark
area increased and the different dark areas began to coalesce,
while the light areas became more fragmented. After 40 years,
the dark areas increased in size further and were observed
Figure 7.
Diversity of invading broadleaf trees in Danto forest (after 20 years).
Figure 8.
Diversity of invading broadleaf trees in Danto forest (after 40 years).
Copyright © 2013 SciRes.
throughout the entire forest, while the fragmentation of the light
areas continued. The network of dark areas at 40 years ap-
peared to be strengthened and expanded compared to the pre-
sent and after 20 years.
Figure 9 shows the change in proportion of the sub-com-
partments by stage. The proportion of mature stage sub-com-
partments decreased to 22% after 10 years under the 80-year
rotation scenario and then recovered gradually to 42% after 40
years. Meanwhile, the proportion of young stage sub-compart-
ments increased gradually to 35% after 30 years and then de-
creased. Middle stage sub-compartments occupied 44% of the
area at present, increased to 48% after 10 years, and thereafter
showed a downward trend. The overall percentages of high-
diversity stages (i.e., the sum of the young and mature stages)
were 56% at present, 51% after 10 years, 57% after 20 years,
72% after 30 years, and 65% after 40 years.
In this study, we created a simple scenario in which all sites
more than 80 years old were cut completely; thus, the first dec-
ade was on the overcutting side, and, after 80 years, the sites re-
turned to the starting point. Therefore, this scenario is strongly
affected by the present site age and location, and we cannot
change the arrangement of sites to improve the spatiotemporal
diversity of invading broadleaf trees. Clearly, if we set a dif-
ferent rotation scenario in Step 4, the results may change.
Our sub-compartment-scale analysis suggested that we can
classify each site into one of three stages according to the rich-
ness of diversity of the invading broadleaf trees: young stage,
less than 25 years with rich diversity but small height; middle
age, more than 26 years and less than 60 years with poor diver-
sity; and mature stage, more than 61 years with rich diversity
and a developed layer. For a Scots pine (Pinussylvestris) stand
in northern Scotland, Mason (2004) recognized four phases of
stand development: 1) stand initiation, from 0 to 20 years; 2)
stem exclusion, from 20 to 80 years; 3) understory re-initiation,
from 80 to 150 years; and (4) old growth, from 150 to 350 years.
Busing and Garman (2002) concluded that most wood quantity,
wood quality, and ecological objectives can be met with long
rotations of approximately 260 years. The ranges of our pro-
posed classification are shorter than those in previous studies.
Invading broadleaf trees may still be in the process of growing
even in our mature stage. In Japan, there are artificial forests
29% 31% 35%
48% 43% 28%
22% 26%
37% 42%
presentafter 10after 20after 30after 40
Pr oportion of subcom partm ents
Elapsed year s
Mature stage
Middle stage
Young stage
Figure 9.
Change in the proportion of sub-compartments by stage.
older than 200 years, but the conventional long-rotation system
in Japan is more than 70 and less than 100 years, considering
economic efficiency and management conditions. Busing and
Garman (2002) noted that certain objectives could be met with
shorter rotations of 80 - 150 years, when treatments of thinning
and canopy tree retention were applied. Hartley (2002) argued
that during establishment, forest managers should consider in-
novations in snag and reserve tree management, in which ma-
ture native trees and/or understory vegetation were left unhar-
vested or allowed to regenerate. The same idea applies to our
study area: invading broadleaf trees should be retained as long
as they do not impede thinning or risk safety.
In this study, we examined five diversity indices for broad-
leaf trees and studied the features of each index. We then ex-
amined combinations of the diversity indices. The proportion of
basal area and inverse Simpson index were identified as the
index pair for evaluating the diversity of invading broadleaf
trees at a single site. In future, we plan to test additional diver-
sity indices to identify and confirm the most suitable indices for
evaluating broadleaf tree diversity in artificial forests.
From the results of our basin-scale analysis, we proposed a
model for evaluating the diversity richness of invading broad-
leaf trees in whole forests. The simulation showed a spatio-
temporal change in richness. Regarding the importance of a
basin-scale perspective, Kupfer et al. (2006) noted that the
study of forest fragmentation effects was shifting away from a
patch-based perspective focused on factors to a landscape-mo-
saic perspective that recognizes the importance of gradients in
habitat conditions. Fischer et al. (2006) suggested that a land-
scape should include structurally characteristic patches of na-
tive vegetation, corridors, and stepping stones between them, a
structurally complex matrix, and buffers around sensitive areas.
Although our GIS-based model can show a basin-scale per-
spective, we could not quantitatively evaluate the fragmentation
of sites as a structurally complex matrix and network of rich
diversity sites as corridors in this study. In future, we plan to
improve the rotation scenario of our proposed model by incor-
porating a road-construction plan and felling plan that specifies
when and which sites to fell and plant.
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