An Evaluation Model for Improving Biodiversity in Artificial Coniferous Forests Invaded by Broadleaf Trees

doi:10.4236/ojf.2013.34020

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Open Journal of Forestry

2013. Vol.3, No.4, 122-128

Published Online October 2013 in SciRes (http://www.scirp.org/journal/ojf) http://dx.doi.org/10.4236/ojf.2013.34020

122

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

Email: [email protected]

Received July 16th, 2013; revised August 17th, 2013; accepted August 29th, 2013

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

Introduction

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

approaches.

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

Y. YAMADA, S. KOSAKA

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.

Methods

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

insufficient.

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

scale.

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.

Y. YAMADA, S. KOSAKA

10%

15%

20%

25%

13 36 44 56 76118OG*

roportion of basa

area

Forest age

*OG: Old growth

Upper layer

Middle layer

Lower layer

Figure 2.

Forest age and proportion of basal area in hinoki (C. obtusa)

forests.

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.

10%

15%

20%

25%

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)

forests.

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

safety.

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

operations.

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

500

1000

1500

2000

2500

3000

3500

4000

4500

050100 150

Stand density (trees/ha)

For est age

Danto

Hayami

Inabu

Figure 5.

Forest age and stand density.

124

Y. YAMADA, S. KOSAKA

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

Y. YAMADA, S. KOSAKA

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

scale.

Step 1 (GIS mapping): Specify the location of each site using

a GIS. Next, add site-specific data obtained from the forest

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).

126

Y. YAMADA, S. KOSAKA

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.

Conclusion

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

21%

29% 31% 35%

23%

44%

48% 43% 28%

35%

22% 26%

37% 42%

20%

40%

60%

80%

100%

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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