Spatial distribution of two invasive alien species,Podocarpus nagi

and Sapium sebiferum, spreading in a warm-temperate evergreen

forest of the Kasugayama Forest Reserve,Japan

Yuri MAESAKO ,Satoshi NANAMI

and Mamoru KANZAKI

Graduate School of Human Environment,Osaka Sangyo UniversityGraduate School of Science,Osaka City UniversityGraduate School of Agriculture,Kyoto University

We studied the spatial distribution of two alien species, Podocarpus nagi (Thunb.)Zoll. et Moritz.(Podocarpaceae)and Sapium sebiferum (L.)Roxb.(Euphorbiaceae), in Kasugayama Forest Reserve(KFR),a warm-temperate evergreen forest that is designated a Special Natural Monument of Japan and a

UNESCO World Heritage site. The deer population,which was designated a natural monument in 1957,recently attained a size of approximately 1200-1300 head in Nara Park near KFR. Local increases in

mammalian herbivores might contribute to the establishment or spread of alien species in the forest. Thus,we focused on interactions between invasive trees and mammalian herbivores and their effects on the original

floral composition of KFR. We conducted field surveys and distribution mapping of the two alien species

with the aid of geographic information system (GIS) technology from July to October 2002 to determine

their distributions and characterize their invasion patterns. We recorded the locations of 6300 P.nagi

individuals and 4543 S.sebiferum individuals using a global positioning system (GPS) in a 45 ha sampling

area. Most S.sebiferum individuals were aggregated in canopy gaps (54.4%),whereas P.nagi tended to

occur in closed canopies (57.9%). The density of P.nagi tended to decrease along a west to east gradient;however,that of S.sebiferum indicated no clear directional gradient. These two alien species,which first

established in Mt.Mikasa and Nara Park adjacent to KFR, have expanded markedly because of their

unpalatability to sika deer,although they differ greatly in invasion history and spreading behavior.

Key words:alien species,Kasugayama Forest Reserve,Podocarpus nagi,Sapium sebiferum,warm-temperate

evergreen forest

INTRODUCTION

In 1924, Kasugayama Forest Reserve(KFR)was

designated a Special Natural Monument of Japan on Mt.

Kasuga because of its rich flora and fauna. In 1998,

UNESCO recognized KFR as a World Heritage site

because of its harmonized landscape consisting of tradi-

tional buildings such as temples and shrines and various

types of vegetation such as protected natural forests,

managed secondary forests, and grasslands(Kimura &

Motonaka 1999). In the warm-temperate zone of Japan,

the amount of primary evergreen forest has decreased

considerably with increasing urbanization and human

activity. The forest on Mt.Kasuga is one of the remnant

patches of lowland primary evergreen forest in western

Japan. The forest is isolated and is surrounded by an

urban area, Cryptomeria japonica and Chamaecyparis

obtusa plantations,and secondary deciduous oak forest.

Sika deer(Cervus nippon Temminck)have tradition-

ally been protected as sacred messengers of the gods at the

Kasuga Taisha (Kasuga Shinto Shrine) in Nara,which

was established in the early eighth century. In 1957,the

population of sika deer in Nara Park,including the KFR,

was designated a natural monument. The deer popula-

tion consisted of about 1000 individuals from the 1960s

until the early 1990s,and it attained a size of approximately

1200-1300 in Nara Park,except for the forested area of Mt.

Kasuga (Society of Deer Conservation Nara 2003).

Local increases in mammalian herbivores such as sika deer

affect vegetation through browsing,grazing,and trampling(Prior 1983;Gill 1992;Shimoda et al. 1994;Akashi &

Nakashizuka 1999;Maesako 2004). Deer in KFR feed

freely in the grasslands and forests,affecting the structure

and species composition of vegetation in and around KFR(Maesako & Torii 2000;Maesako 2001a,b;Maesako et

al.2003).

Two alien tree species, Podocarpus nagi (Thunb.)

Zoll.et Moritz.(Podocarpaceae)and Sapium sebiferum

Original article

Vegetation Science 24:103-112,2007

Graduate School of Human Environment,Osaka Sangyo University,Nakagaito,Daito,Osaka 574-8530,Japan E-mail:maesako＠due.osaka-sandai.ac.jp

The Society of Vegetation Science◆ Received September 4,2006／Accepted November 9,2007

(L.)Roxb.(Euphorbiaceae), occur in KFR. Both

species are unpalatable to sika deer. The abundance of

these two alien species is thought to be increasing in the

forest and affecting the original floral composition of

KFR,but a quantitative analysis of the spread and effect of

these alien tree species has not yet been conducted. An

increase in the abundance of these invasive species may

demonstrate a need for forest conservation via the regula-

tion of mammalian herbivores(Suganuma&Asai 1969;

Maesako 2001b,2002,2005;Yamakura et al.2001a),but

no data are available on the distribution of these invasive

trees in KFR. Thus,we attempted to clarify the spatial

distribution and relevant characteristics of these alien tree

species with respect to the forest ecology of KFR.

MATERIALS AND METHODS

Study site

The evergreen broadleaf forest on Mt.Kasuga (34°41′N,

135°51′E) is located east of Nara City(Fig.1),and the

area is managed by the prefectural government as part of

Nara Park. The highest peak in KFR is Mt.Hana (498

m a.s.l.). The forest has been preserved as a holy forest of

the Kasuga Taisha. For this reason,hunting and logging

in the forest have been prohibited since 841 AD. The

KFR,an area of ca. 300 ha covering two-thirds of Mt.

Kasuga,is dominated by the evergreen oak species Cas-

tanopsis cuspidata, Quercus salicina, Q.acuta, Q. ses-

silifolia, and Q.glauca (Koshimizu et al. 1971;Nakane

1975;Suganuma & Kawai 1978;Naka 1982;Naka &

Yoda 1984;Mizuno et al.1999). Other evergreen broad-

leaf trees(e.g.,Neolitsea aciculata,Symplocos prunifolia,

and Litsea coreana),deciduous broadleaf trees(e.g.,Acer

rufinerve,Prunus jamasakura,and Sapindus mukorossi),

and coniferous trees(e.g.,Tsuga sieboldii,Chamaecyparis

obtusa, and Abies firma)are codominant in this forest(Suganuma & Kawai 1978;Naka 1982;Mizuno et al.

1999).

A P.nagi stand on Mt. Mikasa lies adjacent to the

evergreen broadleaf forest;part of this stand is contained

within in the natural monument. Although the natural

vegetation on Mt.Mikasa is evergreen broadleaf forest,the

vegetation has been altered by browsing pressure from

abundant deer and is characterized by unpalatable plant

species such as P.nagi,N.aciculata,Pieris japonica,and

Illicium religiosum (Koshimizu 1943;Koshimizu et al.

1971;Suganuma 1975;Suganuma & Kawai 1978;

Shimoda et al.1994;Ohmae et al.1996,1999;Yamakura

et al. 2000, 2001b;Nanami et al. 2002),which seem to

escape browsing pressure via chemical defense mechanisms(Takatsuki 1989).

Focal species

Podocarpus nagi (Thunb.)Zoll.et Moritz.

P.nagi (Podocarpaceae)originally distributed in the

Chugoku, Shikoku, and Kyushu districts of Japan, Ta-

iwan, and southern China is a coniferous tree(Ohwi

1992). The species was not originally distributed in Nara;

it is an alien species from other regions of Japan. The

population of P. nagi at Nara City is believed to be

derived from trees planted around Kasuga Taisha,located

at the foot of Mt.Mikasa,approximately 1200 years ago(Suganuma 1975;Suganuma&Kawai 1978).

Nanami et al.(1999, 2000) reported that the narrow

range of seed dispersal by gravidity and dioecy induced the

spatial heterogeneity of individuals in the P.nagi stand.

In contrast,Yamakura et al.(2003)examined meteoro-

logical data and suggested that wind has promoted P.nagi

seed dispersal in this area.

Vegetation Science Vol.24, No.2, 2007

Fig.1. Study area and location of the sampling area(45 ha)

in Kasugayama Forest Reserve(34°41′N,135°51′E).

Sapium sebiferum (L.)Roxb.

S.sebiferum(Euphorbiaceae),an alien species originat-

ed from China (Satake et al.1989),is a deciduous broad-

leaf tree that was introduced as an ornamental roadside tree

in Nara Park in the 1930s. The species then invaded

forest canopy gaps in KFR that were caused by the second

Muroto Typhoon in September 1961 (Suganuma&Asai

1969).

Because S.sebiferum is a highly light demanding pio-

neer species(Shimoda et al.1994),its seedlings appear to

be confined to open sites. Seeds of S.sebiferum are

thought to be effectively dispersed by birds(Hashiguchi&

Ueda 1990;Hukui & Ueda 1999), and the species can

probably expand its area of distribution quickly.The two

alien species possess contrasting ecological characteristics,

although they are both unpalatable to deer.

Field survey

We focused on interactions between invasive trees and

mammalian herbivores and their effects on the original

floral composition of KFR. For this purpose,we selected

a 45 ha area of KFR to carry out intensive investigations

for the following reasons. First,the 45 ha area has been

exposed to strict browsing pressure by deer because the area

is adjacent to Nara Park, where sika deer have been

protected by the Kasuga Taisha. Second,sufficient time

has elapsed in this area since the invasion of alien trees.

In areas distant from the seed sources of the alien species,

i.e.,Mt.Mikasa for P.nagi and Nara Park for S.sebifer-

um, the two alien trees have only invaded recently,and

their effects on the flora may still be progressing. Thus,

we decided that the 45 ha area adjacent to Mt.Mikasa and

Nara Park was ideal to detect the eventual results of plant-herbivore interactions and their effects on the flora in

KFR. Furthermore,a 45 ha area is large enough to study

spatial patterns of trees,and the observed patterns would

be highly representative for comparisons of ecological

characteristics between the two alien species.

The survey was conducted from the western edge of

KFR toward the eastern part from July to October 2002.

In this area,the locations of all individuals of P.nagi and

S.sebiferum,including current-year seedlings,were record-

ed using global positioning system (GPS) receivers, in-

cluding a Pathfinder PRO/XR (Trimble)and a GPS III

PLUS (Garmin). The altitude of the sampling area

ranged from 250 to 380 m a.s.l. We tracked and recorded

the survey routes with GPS receivers to ensure that we

covered the entire sampling area.

When the focal species formed a clump,the position of

the center of the clump and the number of individuals in

the clump was recorded. The number of individuals＜

1.3 m in height was counted separately. Stem diameter at

breast height (DBH)was recorded for individuals 1.3 m

in height. Individuals were separated into three size

classes:saplings(height＜1.3 m, including seedlings),

small trees(height 1.3 m,DBH＜10 cm),and large trees(height 1.3 m,DBH 10 cm).

The canopy condition was recorded at all GPS points.

It was classified into four types:“closed canopy”for forest

canopy covering 90%, “unclosed canopy”for forest

canopy covering ＜90%,“edge of gap,”and“gap.”

GIS map

GPS data were imported into a geographic information

system (GIS),and the distributions of the two invasive

species were mapped (in UTM zone 53N and JGD 2000).

The GIS map of the distributions was drawn using

ArcView ver. 8.2 software(ESRI). A topographical

map of Nara at a scale of 10000 (Geographical Survey

Institute)was used as a base map for the GIS map.

The sampling area was divided into 25×25 m cells,and

the raw spatial distribution data were converted to a

population density mesh map using the cells. In the mesh

map,the spatial distributions of P.nagi and S.sebiferum

individuals were mapped by size class,i.e.,saplings,small

trees, and large trees, in the 45 ha area of KFR. The

maximum diameter at breast height (DBH 10 cm)and

canopy condition (closed canopy,unclosed canopy,edge

of gap,and gap) for each clump were also indicated on

GIS maps.

Spatial distribution of two invasive alien species in a warm temperate evergreen forest

Table 1. The number and proportion in each size class of Podocarpus nagi and Sapium sebiferum

captured in the sampling area (ca.45 ha)of Kasugayama Forest Reserve.

Size class

Podocarpus nagi

n %

Sapium sebiferum

n %

Saplings (height＜1.3 m) 3308 52.5 4171 91.8

Small trees (height 1.3 m and DBH＜10 cm) 2533 40.2 299 6.6

Large trees (height 1.3 m and DBH 10 cm) 459 7.3 73 1.6

Total 6300 100.0 4543 100.0

Vegetation Science Vol.24, No.2, 2007

Fig.2. Spatial distribution of Podocarpus nagi individuals in 25×25 m cells for (a) saplings (height＜1.3 m,including seedlings),(b) small trees (height 1.3 m,DBH＜10 cm), and (c) large trees (height 1.3 m,DBH 10 cm) in a 45 ha area of Kasugayama Forest Reserve.(d)Distribution of clumps of P.nagi.Circles

indicate the number of individuals per clump. See Fig.1 for the location of the sampling area.

Spatial distribution of two invasive alien species in a warm temperate evergreen forest

Fig.3. Spatial distribution of Sapium sebiferum individuals in 25×25 m cells for (a) saplings(height＜1.3 m,including seedlings),(b) small trees (height 1.3 m, DBH＜10 cm), and (c) large trees (height 1.3 m,DBH 10 cm) in a 45 ha sampling area of Kasugayama Forest Reserve.(d)Distribution of clumps of S.sebiferum. Circles indicate the number of individuals per clump.See Fig.1 for the location of the sampling

area.

RESULTS

Abundance and spatial distribution

In total,6300 P.nagi and 4543 S.sebiferum individuals

including current-year seedlings were recorded at 501 and

237 points,respectively,in the 45 ha sampling area. The

individual size-class distributions of the two species were

quite different. Saplings accounted for 52.5%of P.nagi

and 91.8%of S.sebiferum individuals. In contrast,large

trees accounted for 7.3%of P.nagi and 1.6%of S.sebifer-

um individuals(Table 1).

Cells with high densities of P.nagi saplings and small

trees were located at the western and southern corners of

the sampling area (Fig.2a,b),and large trees were abun-

dant in the sampling area adjacent to the northern part of

Mt.Mikasa (Fig.2c). On the other hands, cells with

high densities of S. sebiferum saplings, small trees, and

large trees were scattered throughout the entire sampling

area (Fig. 3a-c). Clumps with high densities of S.

sebiferum were scattered throughout the entire sampling

area. In contrast,clumps with high densities of P.nagi

were concentrated in the west and south areas,but not in

the northeastern corner(Figs.2d&3d). These distribu-

tion indicate that the density of P.nagi tended to decrease

along a west to east gradient, and that of S.sebiferum

indicated no clear directional gradient.

For large trees(DBH 10 cm),we examined the spatial

distribution of maximum DBH in each locality for the two

species(Figs.4a,b). The maximum DBH was 52.5 cm

in P.nagi and 51.2 cm in S.sebiferum.Trees that attained

maximum DBH occurred in the western part of the sam-

pling area.

Canopy types and alien species

P.nagi tended to occur under closed canopies,whereas

S.sebiferum mainly occurred in gaps(Fig.5a,b).For P.

nagi,6.4,8.6,27.1 and 57.9%of individuals were found in

gaps,at the edge of gaps,under unclosed canopy and under

closed canopy,respectively. For S.sebiferum,54.4,10.5,

29.1 and 5.9%of individuals were found in gaps,at the

edge of gaps, under unclosed canopy and under closed

canopy,respectively. We also classified the canopy situa-

tion by size class.P.nagi saplings and small trees occurred

under closed canopies,whereas large trees occurred under

unclosed canopies.S.sebiferum saplings and small trees

occurred in gaps,whereas large trees occurred under un-

closed canopies(Fig.6).

DISCUSSION

Suganuma & Asai (1969) reported the invasion of

alien tree species in KFR and emphasized the necessity of

future studies concerning the distributional pattern and

density of these alien species. We investigated quantita-

tively the invasion and spread of alien species in KFR.

We elucidated the invasion,establishment,and spread of P.

nagi and S.sebiferum in the climax forest of KFR. The

spatial distribution of P.nagi showed a gradient in the

forest,whereas that of S.sebiferum had no clear gradient.

The species’preferences for canopy conditions differed.

The process of the territorial expansion of P.nagi and S.

sebiferum and their impact on the KFR can be deduced.

Patterns of invasion

The two alien species’populations had quite different

size structures and spatial distribution patterns,reflecting

differences in their patterns of invasion (Figs.2&3). In

P.nagi,the spatial distribution of saplings depended on

the occurrence of larger trees,and high sapling density was

observed in the western and southern parts of the sampling

area,which were located near Mt.Mikasa. This spatial

pattern probably reflected the low mobility of P.nagi

seeds. The pattern also suggests that the invasion of P.

nagi to KFR began in these two areas and that the species

is expanding northeastward along the valley.

P.nagi individuals of small size isolated from large trees

were observed in KFR (Fig.2). For P.nagi,most trees

with DBH＞30 cm reproduce(Nanami et al. 2005).

Because of the short dispersal distances of seeds,young P.

nagi individuals are clumped in the vicinity of mother trees(Nanami et al.1999). However,this regeneration process

cannot explain the small P.nagi individuals that were

found isolated from large trees. Such small P.nagi indi-

viduals might be ascribed to occasional wind dispersal of

P.nagi seeds,as suggested by Yamakura et al.(2003).

P.nagi saplings mainly occurred under closed canopy,

indicating their high shade tolerance and low dependence

on canopy gaps(Fig.5). Saplings of S.sebiferum occur-

red throughout the entire sampling area,but most saplings

were found in gaps(Fig.5). S.sebiferum seeds are

dispersed by birds and thus are highly mobile,but success-

ful germination is limited to gaps in which a high amount

of solar radiation is available.

Suganuma＆ Asai (1968)confirmed the presence of

clumps of S.sebiferum in canopy gaps near this sampling

area after typhoon disturbance in 1961. Their report and

our present survey suggest that S.sebiferum has expanded

widely,mainly under gap conditions in KFR.

Thus, the two alien species have different ecological

characteristics with respect to shade tolerance and seedling

establishment. Uninvaded forest may be invaded by one

of these two alien species,depending on the canopy condi-

Vegetation Science Vol.24, No.2, 2007

Spatial distribution of two invasive alien species in a warm temperate evergreen forest

Fig.4. Spatial distribution of the maximum diameter at breast height (DBH 10 cm)of(a) P.nagi and(b)S.sebiferum for each clump. Circles indicate the range of maximum DBH.

Fig.5. Spatial distribution of canopy types in which clumps of(a)P.nagi and (b)S.sebiferum were found.The canopy was classified into four types:gap,edge of gap,unclosed canopy and closed canopy.

tions.

The invasion of alien plants into forests had been report-

ed in various types of forest;in most cases,open sites in the

forest were occupied by the alien species(e.g., Shimizu

1988,Kowarik 1995,Peters 2001). In the KFR,the inva-

sion of two alien trees with different site preferences has

occurred. This situation indicates that the forest is

exposed to the invasion of alien species,not only in canopy

gaps,but also under closed canopy.

Historical time scale

The population of P.nagi,which is believed to have

been planted at the foot of Mt.Mikasa approximately 1200

years ago (Suganuma 1975;Suganuma & Kawai 1978)

occupies Mt.Mikasa with a high density(Nanami et al.

1999, 2000). In contrast, S.sebiferum, which was

introduced to Nara Park as an ornamental tree in the 1930s

within 1 km from the sampling area,was found in gaps(300-350 m a.s.l.) in the KFR that were caused by the

second Muroto Typhoon in September 1961 (Suganuma

&Asai 1969).

P.nagi and S.sebiferum have invaded the KFR and

spread throughout the whole 45 ha sampling area, even

though S.sebiferum has a very short history of introduc-

tion to the study area compared to P.nagi. Considering

the length of time since their first introduction to the

Kasugayama area,S.sebiferum has expanded quite rapid-

ly. This difference is clearly attributable to the seed

dispersal system of S.sebiferum because its seeds are effec-

tively dispersed by birds, especially Sturnus cineraceus(Hashiguchi & Ueda 1990;Hukui & Ueda 1999).

Although the density of large S.sebiferum trees is still low

compared to that of P.nagi (Figs. 2-4), this species

should be carefully monitored to prevent outbreaks after

large-scale canopy disturbances.

These two alien species are both unpalatable to deer,

and biological disturbance such as browsing by sika deer(herbivores)may facilitate the expansion of alien species

in the Kasugayama area,where over 1000 sika deer current-

ly reside. In the KFR,shade-intolerant pioneer species

can exist in the forest by depending on canopy gaps caused

by typhoons(Naka 1982). However,the regeneration of

native pioneer species that are palatable to deer, e.g.,

Zanthoxylum ailanthoides, Mallotus japnonicus, and

Aralia elata, is strongly inhibited by deer browsing(Shimoda et al.1994). The occurrence of canopy gaps in

the forest allows S.sebiferum to regenerate,and browsing

by deer on native pioneer species would enhance the

dominance of S.sebiferum in the gaps.

P.nagi and S.sebiferum were intentionally introduced

to the area nearby KFR by humans 1200 and 70 years ago,

respectively. Since then,they have spread considerably in

the evergreen broadleaf forest as a consequence of their

unpalatability to sika deer and, in part, natural distur-

bance. KFR was designated a Special Natural Monu-

ment of Japan because of its rich flora and a World

Heritage site because of its cultural landscape consisting of

traditional buildings and protected natural forests, how-

ever,our results show that the spread of these alien canopy

trees may cause dramatic changes in the vegetation and

landscape in the forest (KFR).

ACKNOWLEDGEMENTS

We are grateful to the Nara Park Management Office for

permission to work in Kasugayama Forest Reserve. We

thank Mr.T.Morita of Regional Environmental Planning

Inc. for technical assistance with GIS. This study was

Vegetation Science Vol.24, No.2, 2007

Fig.6. The proportion of canopy types in which individuals of P.nagi and S.sebiferum were

found for each size class:height＜1.3 m,height 1.3 m and DBH＜10 cm,and height 1.3 m and

DBH 10 cm.

supported by grants from the Sumitomo Foundation,Pro-

Natura Foundation,and the Ministry of Education,Cul-

ture, Sports Science and Technology of Japan (No.

15570024).

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要約

春日山照葉樹林に侵入した外来種ナギとナンキンハゼの空間分布. 前迫ゆり（大阪産業大学大学院人間環境学研究科) 名波 哲（大阪市立大学大学院理学研究

科) 神崎 護（京都大学大学院農学研究科）奈良市に位置する春日山原始林特別天然記念物指定

域（34°41′N,135°51′E;298.6 ha）に発達する照葉樹林は,奈良公園一帯に生息するニホンジカの局所個体群の増加を背景に, さまざまな影響を受けている. 本研究は

700年代に春日大社に献木されたのが起源とされる中国地方以南分布種である国内外来種ナギ（Podocarpus

nagi）および 1930年代に奈良公園に街路樹として植栽された中国原産の国外外来種ナンキンハゼ（Sapium

sebiferum）が, 春日山原始林の照葉樹林域に侵入していることから, これら 2種の分布を定量的に把握し, 外来種の空間分布を示すGIS図を作成することによって,照葉樹林への外来種の侵入を明らかにしたものである.現地調査は 2002年 7月から 10月までの期間に, 奈良公園側の春日山原始林域西端から東側に約 45 haの範囲で調査を行った. 当年生実生を含む全個体または個体パッチの位置をGPSを用いて記録するとともに, 高さ

1.3 m以上の個体の胸高直径（DBH）を測定し, 両種の分布をサイズ別にGISマップに示した. 両種が生育していた林冠タイプをギャップ, ギャップ辺縁, 疎開林冠および閉鎖林冠に区分し, 個体数比率を算出した結果,ナンキンハゼ（n＝4543）はギャップ下において出現頻度が高く（54.4%), ナンキンハゼの侵入はギャップ形成に依存する傾向を示した. 一方, ナギ（n＝6300）は閉鎖林冠下で出現頻度が高く（57.9%), それぞれの種は異なる光環境の立地に侵入していた. 両種をサイズクラスに分けて空間分布を把握した結果, ナギは照葉樹林の西端（天然記念物ナギ群落が成立している御蓋山の北側の調査地域に相当する）に多く分布し, 調査地域内において西側から東側への密度勾配を示した. 一方, ナンキンハゼは顕著な密度勾配はみられなかった. 生態的特性の異なる 2種の外来種は侵入時期が異なるものの,照葉樹林に侵入後, 広域的に拡大していることが明らかになった. 春日山照葉樹林は文化的景観によって世界文化遺産に登録されているが, 外来種の拡大によって, 今後, 組成的にも景観的にも大きく変化する可能性が示唆された.

Vegetation Science Vol.24, No.2, 2007