Pacific Science (1992), vol. 46, no. 2: 179-196© 1992 by University of Hawaii Press. All rights reserved

Origin of Distylium Dry Forest and Occurrence ofEndangered Species in the Bonin Islands1

YOSHIKAZU SHIMIZU2

ABSTRACT: The Distylium dry forest is a low-stature forest or scrub, 0.5-8 mhigh, growing in dry habitats with shallow soils in the Bonin Islands. The forest,dominated by Distylium lepidotum, has the highest species diversity andproportion of endemics of all vegetation types in the Bonins, and it includesmany endangered species. Distribution and species composition of the Distyliumdry forest and distribution patterns of 25 endangered species were studied inChichijima-retto, a cluster of islands in the Bonins. The relationship between thedistribution of forest and fog occurrence was investigated. Also analyzed weredamage caused by the severe drought in 1980 and the habitats of congenericspecies of the three islands at different altitudes. A hypothesis is proposed toexplain the origin of the Distylium dry forest and the occurrence of endangeredspecies: Distylium dry forest originated from a mesic forest similar to a cloudforest when the Islands were once higher than they are at present; it has beendeclining in area and species composition with the sinking of the Islands and thetendency toward increasing aridity over time, producing many endangeredspecies. This trend has been accelerated by a rise of sea level of about 100 mafter the last Ice Age, during which the total area of the Islands decreasedto one-third of the former area.

THE BONIN ISLANDS (Ogasawara-gunto inJapanese) are located about 1000 km south ofTokyo in the Pacific Ocean. The islands aredivided into three clusters, Mukojima, Chi­chijima, and Hahajima-rettos (island groups)from north to south (Figure 1). The headadministrative office of Ogasawara Village islocated at Chichijima, the largest of the BoninIslands. This paper is focused on the vegeta­tion of Chichijima-retto.

The basal rocks of the Islands were formedby submarine volcanic activity in the Tertiary.Boninite, a kind of andesite characterized byrich MgO, Cr, and Si02 content, is spreadwidely. Volcanic breccia covers the boninitelava in some places (Kuroda 1979).

The Islands are almost completely sur­rounded by sea cliffs 50-100 m high. Landwith low relief (hilly land) occurs on higher

I Manuscript accepted 2 May 1991.2 Department of Natural Sciences, The Komazawa

University, Komazawa 1-23-1, Setagaya-ku Tokyo 154,Japan.

regions above the sea cliffs (Tamura andImaizumi 1981).

The climate is subtropical and maritime(Figure 2). The annual mean temperature is22.9°C. The annual range of temperature issmall compared with that of Okinawa Island,a continental island located at nearly the samelatitude. It rains throughout the year to someextent, although annual precipitation (about1200 mm) is not high. Rainfall centers aroundthe rainy seasons of May-June and Octo­ber- N ovember. Water deficits occur in placeswith shallow soils in midsummer.

Vegetation

I classified the natural forests (except thecoastal ones) of the Bonin Islands on the basisof forest structure and dominant species(Shimizu 1989). Six vegetation types wererecognized; they are grouped into mesic typesand dry types (Figure 3).

Elaeocarpus-Ardisia mesic forest (E-A m.f.)is the tallest forest in the Bonin Islands.

179

180

12<fE 1300

PACIFIC SCIENCE, Volume 46, April 1992

Chichijima-Retto

°OtohtOjima I

..........1km

Chichijima

FIGURE I. Location of the Bonin Islands with inset showing the three main islands of Chichijima-retto.

FIGURE 2. Climate diagram of Chichijima Island basedon monthly mean temperatures and precipitation re­corded from 1969 to 1989.

30

20

10

JFMAMJJASOND

(mm)

200

100

80

60

40

20

It consists of large trees of Elaeocarpusphotiniaefolius, Pisonia umbellifera, Pouteriaobovata, and others in the crown layer and isdominated by Ardisia sieboldii trees in thelower layers. It once dominated at low altitudeon flat places and gentle slopes with thicksoils, but it was destroyed almost completelywhen these habitats became cultivated fieldsbefore 1945.

Pinus-Schima mesic forest (P-S m.f.), asecondary forest composed of Pinus lutchuen­sis (introduced pine) and Schima mertensiana(native pioneer), is presently replacing theElaeocarpus-Ardisia mesic forest (Figure 4).

Dendrocacalia-Fatsia mesic scrub (D-Fm.s.) occurs only on the mesic ridges above400 m on Hahajima, the main island inHahajima-retto. It consists of an endemicwoody composite, Dendrocacalia crepidifo­lia, and some other shrubs preferring moistconditions.

Distylium-Schima dry forest (D-S d.f.)

Distylium Dry Forest in the Bonin Islands-SHIMIzU 181

2m Jl 15m

f. Distyl iUIl-Pouteria dry scrub

d. Distyl iua-Schilla dry forest

e. Rhaphiolepis-Livistona dry forest

-1m

b. Pinus-Schi.a mesic forest

C. Dendrocacal ia-Fatsia .esic scrub

3m. 20m

o

a. Elaeocarpus-Ardisia .esic forest

FIGURE 3. Profile diagrams of the six vegetation types. Key to abbreviations: Ar, Ardisia sieboldii; Bo, Boninia glabra;Ci, Cinnamomum insularimontanum; Cy, Cyathea mertensiana; De, Dendrocacalia crepidifolia; Dl, Distylium lepidotum;Dr, Drypetes integerrima; EI, Elaeocarpus photiniaefolius; Fo, Fatsia oligocarpela; Ga, Gardenia boninensis; Ge,Geniostoma glabrum; Hg, Hedyotis grayi; Hi, Hibiscus glaber; 1m, Ilex matanoana; 1p, Ilex percoriacea; Li, Ligustrummicranthum; Lv, Livistona chinensis v. boninensis; Mb, Machi/us boninensis; Me, Melia azedarach; MI, Melastomatetramerum v. pentapetalum; Mm, Myrsine maximowiczii; Ne, Neolitsea sericea; Pa, Pandanus boninensis; Pi, Pinuslutchuensis; Po, Pouteria obovata; Pu, Pisonia umbe//ifera; Py, Psychotria homalosperma; Rh, Rhaphiolepis indica v.integerrima; Sb, Syzygium buxifolium; Sk, Symplocos kawakamii; Sm, Schima mertensiana; Sp, Symplocos pergracilis;St, Stachyurus macrocarpus; Ta, Tarenna subsessilis.

characterizes the dry-type vegetation of theBonin Islands. It is distributed mainly inhigher areas on Chichijima-retto (Figure 3).Distylium lepidotum is the most dominantspecies. Schima mertensiana, Pouteria obovata,Syzygium buxifolium, and Pandanus boninensisare subdominant (Table I). The forest has thehighest species diversity of all forest types inthe Bonins. It contains many endemics, mostof which are rare and endangered species in abroad sense.

Distylium-Pouteria dry scrub (D-P d.s.) is

a dwarf type of Distylium-Schima dry forest.Distylium lepidotum, Pouteria obovata, Syzy­gium buxifolium, and Pandanus boninensis arealso common in this forest type. It differs fromthe preceding forest type in that tree speciespreferring mesic habitats disappear and somearid-tolerant species such as Juniperus taxi­folia and Dodonaea viscosa appear in thescrub. It is distributed over shallow soilson main ridges and in peripheral parts ofhigher areas (Figure 5). Forest structure andspecies composition change gradually be-

182

~ Elaeocarpus-Ardisia .esic forest

o Pinus-Schin lesic forest

mIIIIII Oishi iUII·Schiu dry forest

• Distyl iUIl-Pouteria dry scrub

~ Rhaphiolepis-Livistonia dry forest

FIGURE 4. Vegetation map of Chichijima-retto. Blankspaces denote exposed basal rocks, village areas, culti­vated fields, coastal forests, and secondary vegetationsuch as Leucaena leucocephala scrub and Slachylarphelajamaicensis grassland.

PACIFIC SCIENCE, Volume 46, April 1992

tween Distylium-Schima dry forest and Dis­tylium-Pouteria dry scrub, so I call these twotypes Distylium dry forest collectively in thispaper.

Rhaphiolepis-Livistona dry forest (R-Ld.f.) is mainly distributed on dry rocky slopesbelow the higher areas (Figure 5). Rhapiolepisindica v. integerrima, Livistona chinensis v.boninensis, Pandanus boninensis, and Ochrosianakaiana are dominant species, but the domi­nance ofeach species varies greatly from placeto place. I call this forest Rhaphiolepis dryforest to distinguish it from Distylium dryforest in this paper.

Endangered Species

Many plants and animals have becomeextinct on oceanic islands because of direct orindirect human activities (see Elton 1958,Carlquist 1965). Protection of endangeredspecies is a worldwide problem.

Ono et al. (1986) pointed out 85 endangeredplant taxa classified into two categories (groupA: 21 endangered species; group B: 64 rareand/or vulnerable species) in the Bonin Is­lands. The Japanese Red Data Book (NatureConservation Society of Japan and WorldWildlife Fund Japan 1989) recognizes threeextinct, 20 endangered, and 39 vulnerablespecies ofvascular plants in the Bonin Islands.Shimizu (1990) categorized seven causal fac­tors that have made these species endangered,as follows: (1) Felling ofparticular tree speciesin the past (e.g., Morus boninensis, Clinostigmasavoryana); (2) reduction of habitat by defor­estation (e.g., Claoxylon centinarium, Ficusiidaiana); (3) collection for commercial use(e.g., Calanthe hattorii, Cirrhopetalum bon­inense); (4) damage by introduced animals(goats, land snails) (e.g., Cirsium boninense,Lobelia boninensis); (5) competition with in­troduced plants (e.g., species of Elaeocarpus­Ardisia mesic forest); (6) damage by droughtand typhoon (e.g., Symplocos kawakamii,Procris boninensis); (7) fluctuation ofenviron­ments (and a long-term trend of aridity) (e.g.,species of Distylium dry forest).

Figure 6 shows that woody species withsmall numbers of individuals (less than10,000) are concentrated in the Distylium dry

Distylium Dry Forest in the Bonin Islands-SHIMIZU 183

TABLE 1

SPECIES COMPOSITION OF THE Distylium-Schima DRY FOREST

NO. OF INDIVIDUALS (900 m2)

HEIGHT CLASSES I II III IV VSPECIES NAME (4-6 m) (2-4 m) (0.7-2 m) (0.2-0.7 m) «0.2 m)

Bischofia javanicao 1Boninia glabra* 31 69 5 51Boninia grisea* 6 1 1CaUicarpa glabra* 2 9 60 102Cinnamomum insularimontanum+ 1 1Clinostigma savoryana* 3 2 3Distylium lepidotum* 108 83 42 396 639Elaeocarpus photiniaefolius* 5 10 9 20 50Evodia nishimurae* 13 3 12Fagara boninensis* 5Ficus boninsimae* 1 3 9 6Gardenia boninensis* 2 19 9 1 2Geniostoma glabrum* 6 1 1 6 32Hibiscus glaber* 7 16 10 3 9flex matanoana* 33 9 13 32 225Juniperus taxifolia* 1Ligustrum micranthum* 10 91 198 1,626 1,727Machi/us boninensis* 3 36 163 137 78Machilus kobu* 1 1Myrsine maximowiczii* 41 2 4 4 18Neolitsea sericea+ 5 28 3Osmanthus insularis+ 1 7Osteomeles boninensis* 1 8Pandanus boninensis* 37 15 51 70 3Photinia wrightiana+ 1 6Pinus lutchuensiso 2 5 7 16 81Pit tosporum chichisimense* 5 15 37Pouteria obovata+ 82 39 54 114 13,416Psychotria homalosperma* 12 2 3Rhaphiolepis indica v. integerrima+ 14 32 37 319 716Schima mertensiana* 79 18 21 17 430Stachyurus macrocarpus* 1 3 4 4Symplocos pergracilis* 8 58 11Syzygium buxifolium+ 57 68 76 319 579Tarenna subsessilis* 1 4 62 229Vaccinium boninense* 2 3 1Viburnumjaponicum v. boninsimense* 1Wikstroemia pseudoretusa* 6 5 39 86 325Total 520 518 933 3,364 18,766

• Endemics of the Bonin Islands.+ Indigenous species.°Exotics.

forest (D-S d.f. and D-P d.s.) on Chichijima.Most of the rare and endangered species haveonly smaIl numbers of seedlings and saplingsrelative to the number of mature trees of thesame species (Figure 7). This situation is trueall over the Bonin Islands (field observation).

It seems that regeneration ofindividuals is notgoing weIl for these species. Therefore, theywiIl probably become extinct in the future,though the possibility of some unknown re­generation mechanism such as highly episodicreproduction cannot be ruled out. I selected

184 PACIFIC SCIENCE, Volume 46, April 1992

soilgravelcloud

300-

200-

100-

E-Am.f.(P-Sm.f.)

D-Pd.s.

FIGURE 5. Diagramatic presentation ofan east-west transect through the center ofChichijima Island, showing depthof soils, kinds of basal rocks, and vegetation types.

25 woody species (see Figure 14) that havesmall numbers of mature trees and whoseindividuals occur mainly in the Distylium dryforest in Chichijima-retto. I call these "endan­gered species" in a broad sense in this paper.

Elaeocarpus-Ardisia mesic forest also hassome rare and endangered species (Figure 6),but this is because the present distribution ofthis forest is confined to a very narrow area onChichijima-retto (Figure 4) because of nearlycomplete destruction in the past (category 2above). More individuals of species in thistype ofmesic forest may be found in Hahajima,where this type of forest is preserved morewidely.

Because the dry habitat of Distylium dryforest was not suitable for cultivation, therewas less human impact on this forest in thepast, and more of its original distributionremained than that of the mesic forest. Mostof the endangered species in the Distylium dryforest have little value for practical use. Thus,the reduction ofdensity of these species shouldbe attributed first to natural causes. I ascribeit to the fluctuation of the environment duringa long range of time (category 7).

In this paper I analyze the distribution andspecies composition of Distylium dry forestand the distribution patterns of the endan-

gered species on Chichijima-retto. I propose ahypothesis connected with the origin of theDistylium dry forest to explain why manyendangered species occur there.

METHODS

Figure 8 presents the route offield observa­tion and the location of plots surveyed in thisstudy. Plots (10 x 10 m-20 x 20 m) werelocated in dry forests. The number of plotswas as follows: Chichijima, 70; Anijima, 74;Otohtojima, IS. I categorized woody individ­uals as trees in the canopy and subcanopylayers, as saplings in the shrub layer, and asseedlings in the herb layer (less than 0.2 mhigh). The survey of plots included three

. potential methods, as follows: (I) Recordingnames of all vascular plants; (2) counting thenumber of trees for woody species and check­ing names for herbaceous species; or (3) mea­suring dbh (diameter at breast height) of allstems of trees, counting the number of sap­lings and seedlings of woody species, andchecking the names of herbaceous species in20 quadrats (1 x I m) in a plot. For any givenplot I chose one of these methods accordingto the time and assistance available.

No.of individuals

185

• : Survey plot

....... : Survey route

FIGURE 8. Survey route and plots in Chichijima-retto.

~ 30

~g. 20

o5 l

· 0 0.1 1 10 100apmg

only Sapling I Mature Tree Ratio

-o~ 10

FIGURE 7. Ratio of number of saplings to number ofmature individuals in the 95 plots on Chichijima Island.Hatching shows species with a ratio between 0 and I.Calculated from Shimizu (1982a).

Distylium Dry Forest in the Bonin Islands-SHIMIzu

~~1E-A m.f.

w~P-S m.f.

10

0

U)

l~jR-L d. f.

<II

£b-,.-u<IIa.U)- 200

0 10Z

0

D-P d.s.10

0102 103 104 105 106 10710

FIGURE 6. Estimated number of mature individuals ofall woody species of six different vegetation types onChichijima Island. Hatching shows species with less than10,000 individuals. Calculated from Shimizu (1981).

186

RESULTS

Distribution of Distylium Dry Forest

Figure 9 shows the distribution ofDistyliumdry forest on Chichijima, Anijima, andOtohtojima in Chichijima-retto. The con­figuration of these islands and the adjacent seabottom is also presented. The distribution ofDistylium dry forest roughly coincides withareas above 100 m on Chichijima and Anijima,with densest distribution in areas above 200 m.Small patches of this forest are dispersedon ridges and mountaintops and surroundedby Rhaphiolepis dry forest on the southernhalf of Chichijima and on Otohtojima. Flat

FIGURE 9. Distribution of the Disly/ium dry forest(stippled area) and configuration of the three islands andadjacent sea bottom in Chichijima-retto. Contour linesare drawn every 100 m on land and every 20 m at sea.

PACIFIC SCIENCE, Volume 46, April 1992

areas and gentle slopes with deep soils arecovered with mesic forests instead of Dis­tylium dry forest, even though located ataltitudes above 200 m (Figure 4).

Sea level was at least 100 m lower in the lastIce Age (ca. 20,000 yr B.P.) than at present(Ohta and Yonekura 1987). All islands be­longing to Chichijima-retto were united as alarger island (Figure 9). It is estimated thatthis single, large island was about three timesas large in area as the present area of Chi­chijima-retto. In those days, the presentpatches of Distylium dry forest might havebeen connected with each other, resulting in awide continuous distribution on the largeisland.

Relationship with Fog Occurrence

The higher areas of Chichijima are some­times shrouded by fog (Figure 10). The distri­bution of Distylium dry forest seems to berelated to fog occurrence (altitude of cloudbottom). The frequency of fog is greatestduring the rainy season (May-June andOctober-November) (Figure 11), especiallyfor fog occurring at altitudes lower than270m.

The frequency of fog has two peaks: 270­330 m and 450-600 m (Figure 12). Thesealtitudes are nearly equivalent to the highestpoints on Chichijima and Hahajima, respec­tively. The frequency is very low at altitudesless than 210 m (Figure 12). This means thatthe Distylium dry forest at altitudes less than200 m gets scarcely any fog.

When the islands were united into a singlelarger and higher island in the last Ice Age, thewhole area of Distylium dry forest might havebeen shrouded in fog more frequently.

Species Composition

Figure 13 shows the proportion of canopytrees in the three dominant species (Distyliumlepidotum, Schima mertensiana, and Rhaphio­lepis indica v. integerrima) that characterizethe dry forests on Chichijima-retto. Distyliumlepidotum and Shima mertensiana dominate inthe Distylium-Schima dry forest located onhigher areas on Chichijima and Anijima. The

Distylium Dry Forest in the Bonin Islands-SmMIzU 187

FIGURE 10. Fog occurrence in higher areas (Chuosan-higashi-daira) on Chichijima Island.

FIGURE II. Monthly frequency offog occurrence. Cal­culated from data from Ministry of Transport (1969),which was recorded by the U.S. Army from May 1966 toMarch 1967 on Chichijima Island.

-iwOjima

ajima

matojima

hijima

4 6 8 10 12 14 16

Frequency (%)2

I1

I792m

I Kita

I 462m Hah

II

317m ChicI

254m Aniji

rr 235m Otoh

o

3000<

150>

1500

900E 600

450

390

330

270

210

o 270-330mF%iI <270m

5 6 7 8 9 10 11 12 1 2 3----1966 H--1967-

o

";> 30~>.uc 20QI;j

CT~ 10I.L

dominance of Distylium lepidotum increasesand Schima mertensiana disappears in Dis­tylium-Pouteria dry scrub. In contrast, theRhaphiolepis-Livistona dry forest is charac­terized by a high proportion of Rhaphiolepisindica v. integerrima and a lack of Distyliumlepidotum and Schima mertensiana.

Table 2 presents the species compositionof the Distylium-Schima dry forest and the

FIGURE 12. Altitudinal frequency of fog occurrence.Calculated from data from Ministry ofTransport (1969),which was recorded by the U. S. Army from May 1966 toMarch 1967 on Chichijima Island.

Rhaphiolepis-Livistona dry forest. As men­tioned previously, the Distylium-Schima dryforest includes many endemic and endangeredspecies, especially shrub species. In contrast,

Chichijima

~.,::.-

14 :::;:;':':':.::~~ .

. ':'': ,:"

@9'".....

15':'::':);:.

Q:?,:::.':::.;-:: OJ S ,....

16 ~D~

~ Distylium lepidotum

11II Schima mertensiana

~·:::I Rhaphiolepis indicav. integerrima

r==J Other species

Otohtojima

.~\ift1fb.) \D

Anijima

FIGURE 13, Proportion of three dominant species based on the density of canopy trees in dry-type vegetation inChichijima-retto, Numbers above the circles numbered 1-9 show the seedling density (no,/m 2

) of Distylium lepidotum,Key to numbered circles: 1-9, 18-23,29-30: Distylium-Schima dry forest; 10-13,26-28: Distylium-Pouteria dryscrub; 14-17, 24-25: Rhaphiolepis-Livistona dry forest.

Distylium Dry Forest in the Bonin Islands-SHIMIZU

TABLE 2

COMPARISON OF THE SPECIES COMPOSITION OF

Distylium-Schima DRY FOREST AND Rhaphiolepis-LivistonaDRY FOREST

Distylium-Schima DRY FOREST +- TRANSITIONAL -+ Rhaphiolepis-Livistona DRY FOREST

Distylium lepidotum, Schima mertensianaElaeocarpus photiniaefoliusGardenia boninensis, Myrsine maximowicziiGeniostoma glabrum, Psychotria homalospermaPouteria boninensis, Juniperus taxifoliaMachi/us boninensis, Machi/us kobuMetrosideros boninensisBoninia glabra*, Vaccinium boninense*Photinia wrightiana*, Tarenna subsessilis*Viburnumjaponicum v. boninsimense*Osteomeles boninensis*

189

Evodia nishimurae/lex matanoanaCallicarpa glabra*Symplocos pergracilis*Pittosporum chichisimense*Stachyurus macrocarpus*Eurya boninensis*Melastoma tetramerum*

Boninia grisea, Drypetes integerrima/lex mertensii, Ochrosia nakaianaLivistona chinensis v. boninensisOsmanthus insularis, Ardisia sieboldiiPittosporum boninense+Callicarpa subpubescens+Celtis boninensis+Melia azedarach+, Trema orientalis+Fagara boninensis+, Morus australis+Elaeagnus rotundata+Boehmeria boninensis+

Pouteria obovata, Syzygium buxifolium, Neolitsea sericeaHibiscus glaber, Rhaphiolepis indica v. integerrima/lex percoriacea, Ficus boninensis, Ligustrum micranthumCinnamomum insularimontanum, Wikstroemia pseudoretusa

NOTE: Width of dissected blocks shows the distribution range of the speciesenclosed.

• Undergrowth shrub species.+ Pioneer or light-demanding species.

the Rhaphiolepis-Livistona dry forest has nosuch endangered shrub species. Instead, manylight-demanding species, including some pio­neer trees, are mixed in the Rhaphiolepis­Livistona dry forest. In contrast to the Disty­lium-Schima dry forest, which occurs inhigher areas, the Rhaphiolepis-Livistona dryforest is situated on rocky slopes at lower alti­tudes (Figure 5). Judging from species compo­sition and habitat condition, the Rhapiolepis­Livistona dry forest is drier and more unstablethan the Distylium-Schima dry forest.

There are some plots where the speciescomposition is intermediate between Disty­lium-Schima and Rhaphiolepis-Livistona dryforests (Figure 13). These plots occur at pheri-

pheral areas in the distribution range of theDistylium dry forest; almost all of them arelocated at altitudes below 100 m (Figure 9).Seedlings and saplings of Distylium lepidotumare not abundant in these plots. I found somedead trees ofD.lepidotum in the marginal partof this forest. It seems that this intermediateforest is changing from a Distylium dry forestto a Rhaphiolepis dry forest.

Types ofDistribution Patterns ofEndangeredSpecies

Figure 14 shows four distribution patternsof 25 endangered woody species that aremembers of the Distylium dry forest on

TYPE ISymplocos pergracilis

TYPE IIEvodia nishimurae

TYPEmGardenia boninensis

TYPE IVMyrsine maximowiczii

Symplocos pergracilisPittosporum chichisimense*Eurya boninensisStaChYUTUS macrocarpusMelastoma tetramerum*Myrsine okabeanaCallicarpa nishimuraeBoninia crassifolia

Evodia nishimuraeCallicarpa glabraIlex matanoanaPsychotria homalospermaMetrosideros boninensisPittosporum parvifolium

Gardenia boninensisViburnum japonicum

v. boninsimenseBoninia glabraPouteria boninensisGeniostoma glabrumFatsia oligocarpellaSymplocos kawakamii*Santalum boninense

Myrsine maximowicziiPhotinia wrightianaVaccinium boninense

FIGURE 14. Distribution patterns of 25 endangered woody species on Chicbijima Island classified into fourdistribution types. Each type is represented by a typical species. Species with similar distribution patterns are listedbelow the figure. Asterisks indicate species endemic to Cbicbijima.

Distylium Dry Forest in the Bonin Islands-SHIMIZU 191

FIGURE 15. Proportion (%) of three endangered spe­cies based on density of canopy trees in some plots inChichijima-retto, showing the concentrated distributionin Chuosan-higashi-daira on Chichijima.

the Distylium dry forest of Chuosan-higashi­daira.

Almost all the endangered species listed inFigure 14 also occur in Anijima (Figure 16).Only three species (Symplocos kawakamii, Pit­tosporum chichisimense, and Melastoma tetra­merum) out of the 25 are not found onAnijima. The distribution patterns of the 22endangered species on Anijima are classifiedinto two types: widespread type and restrictedtype (Figure 16). Species composition in thedistribution patterns on Anijima does notnecessarily coincide with that on Chichijima(Figure 16). For example, [lex matanoana,Callicarpa glabra, and Evodia nishimurae aredistributed more widely on Anijima than onChichijima, though the density of these spe­cies on Anijima is as low as that in theperipheral areas on Chichijima (Figure 15).

It seems that the distribution of the re­stricted-type species on Anijima is centered onthe ridges and valleys located in the central,highest part of the island (Figure 9). But inAnijima, there is no such place as Chuosan­higashi-daira of Chichijima where all endan­gered species occur together with high density.This is probably because the highest areas ofAnijima are about 50-100 m lower in altitudethan those of Chichijima, thus the habitatconditions offered by Chuosan-higashi-dairado not occur on Anijima.

The species composition of the Distyliumdry forest is poor on Otohtojima. Only a fewof the 25 endangered species appear there.

Damage ofDrought in 1980

A severe drought hit Chichijima-retto in thesummer of 1980 (Shimizu 1982b). The totalprecipitation in May-October 1980 was only18% of normal. This kind of severe droughtseems to happen occasionally, about every 20yr, in the Bonin Islands. Many plants werekilled or damaged by the drought in 1980,especially in the Distylium-Pouteria dry scrub,which grows on shallow soils (Table 3).

Strangely, Distylium lepidotum, the mostdominant species in the Distylium-Pouteriadry scrub, was damaged the most. About 70%of the total Distylium lepidotum individualsstudied in the plots were dead (Table 3). Some

{O'oJ5L°MIE

M: Myrsina maximowiczii

I: Ilex matanoana

E: Evodia nishimurae

LL

L.k---.Lf~

k.-~~:::::::::':~"""'--V

Chichijima-retto. All of these species occurat the highest altitude on Chichijima-retto,called Chuosan-higashi-daira, lying at theeastern side of the main ridge in the center ofChichijima (Figure 10), where fog occurrenceis probably more frequent than at other places.

The distribution of eight of the 25 endan­gered species (type 1) is confined to a smallarea ofChuosan-higashi-daira. The other spe­cies (types 2-4) are distributed more or lesswidely in the Distylium dry forest on Chi­chijima (Figure 14). But even in types 2-4, thedensity ofindividuals ofeach species is highestat Chuosan-higashi-daira and it decreasesgradually with distance from that area (Figure15). Thus, species composition is richest in

192

RESTRICTED TYPE

Viburnum japonicumv. boninsimense

Viburnum japonicumv. boninsimense

Eurya boninensisStachyurus macrocarpusFatsia oligocarpellaMetrosideros boniensisPittosporum parvifoliumSantalum boninenseSymplocos pergracilis

PACIFIC SCIENCE, Volume 46, April 1992

WIDESPREAD TYPE

Ilex matanoana

Il ex ma tanoanaGeniostoma glabrumPsychotria homalospermaPouteria boninensisCallicarpa glabraCallicarpa nishimuraePhotinia wrightiana

Evodia nishimuraeGardenia boninensisMyrsine maximowicziiMyrsine okabeanaBoninia glaraBoninia crassifoliaVaccinium boninense

FIGURE 16. Distribution patterns of 21 endangered woody species on Anijima Island. Each type is represented bya typical species, and species with similar distribution patterns are listed below the figure.

endangered species, such as Symplocos per­gracilis, Callicarpa nishimurae, and Pittospo­rum parvifolium, were also killed or damagedto some degree. Thus, this type of scrubhas degenerated to grass vegetation in someplaces.

Most of the species in the Distylium­Pouteria dry scrub have drought-tolerant fea­tures such as multistem growth forms andsmall, thick, hairy leaves. They survive theusual droughts in midsummer, but they can­not endure occasional severe drought. Thisphenomenon indicates that Distylium lepi­dotum and other members of the Distylium­Pouteria dry scrub are not truly resistantagainst severe drought.

Comparison ofCongeneric Species on OtherIslands

Another cluster of islands, called Iwo-retto(Volcano Islands), exists about 150 km to thesouth of the Bonin Islands. Iwo-retto, which

is composed of three main islands, was pro­duced by volcanic activity in the Quaternary(Fukuyama 1983). Its flora has been formedbasically from colonization by the species ofthe Bonin Islands, together with some ele­ments from mainland Japan and the Mi­cronesian islands (Tuyama 1970, Ohba 1983).Minami-Iwojima (highest altitude 916 m) andKita-Iwojima (highest altitude 792 m) islandshave a true fog zone above 500 m (Kaji andTakiguchi 1983, Kawakubo 1986).

Hahajima (highest altitude 462 m) in theBonin Islands is located about 50 km south ofChichijima. The main ridge, above 400 m,is frequently shrouded by fog (Figure 12).Dendrocacalia-Fatsia mesic scrub (see Figure3) occurs on the ridge together with densethickets of Freycinetia boninensis in places.

Figure 17 shows a comparison of habitatsof congeneric species found in Kita-Iwojima,Hahajima, and Chichijima. Eurya, Fatsia,Viburnum, and Melastoma species grow inthe cloud forest on Kita-Iwojima; Fatsia,

Distylium Dry Forest in the Bonin Islands-SHIMIZU

TABLE 3

DAMAGE FROM DROUGHT IN 1980 IN Distylium-Pouteria DRY SCRUB

RATIO (%) OF INDIVIDUALS

193

NO. OF INDIV.SPECIES NAME STUDIED DEAD DAMAGED HEALTHY

Distylium lepidotum 91 69.2 25.3 5.5Ligustrum micranthum 65 7.7 89.2 3.1Pouteria obovata 48 0 75.0 25.0Osteomeles boninensis 32 0 56.3 43.7Rhaphiolepis indica v. imegerrima 14 0 14.3 85.7Syzygium buxifolium 13 7.7 53.8 38.7Dodonaea viscosa 13 7.7 15.4 76.9Hibiscus glaber 10 0 40.0 60.0Hedyotis grayi 8 50.0 50.0 0!lex percoriacea 7 0 71.4 28.6Pinus lutchuensis 6 100 0 0Boninia glabra 5 0 100 0Vaccinium boninense 4 0 25.0 75.0Wikstroemia pseudoretusa 4 0 75.0 25.0Juniperus taxifolia 4 0 100 0Gardenia boninensis 3 0 33.3 66.7Pandanus boninensis 3 0 0 100Photinia wrightiana 2 0 50.0 50.0Pillosporum parvifolium 2 50.0 50.0 0Schima mertensiana 1 0 100 0Symplocos kawakamii 1 0 100 0Myrsine okabeana 1 0 100 0Callicarpa nishimurae 1 0 100 0

1000

E 800.....,<II

600u::J-- 400

«200

Eurya boninensisFatsia oligocarpellaViburnum japonicum

v. boninsimenseMelastoma tetramerum

Stachyurus macrocarpus

D-S d.t.

Fatsia oligocarpella

Melastoma tetramerumv. pentapetalum

Stachyurus macro carpusv. purnifolius

D-F m.s.

Eurya japonicaFatsia oligocarpellaViburnum japonicum

v. boninsimenseMelastoma candidum

v. alessandrense

Cloud t.

OL..-----'----L---~---.L..-------L.---...a...--

Chichijima Hahajima Kita-Iwojima

FIGURE 17. Comparison of habitats of congeneric species in five genera among Chichijima, Hahajima, andKita-Iwojima islands.

194

Melastoma, and Stachyurus species occur inthe mesic scrub on Hahajima. Species of thesegenera are found also in the Distylium dryforest on Chichijima. Hydrangea macraphyllaf. normalis on Kita-Iwojima and Dendrocaca­lia crepidifolia on Hahajima, both of whichrequire mesic conditions, do not occur onChichijima, probably because the true fogzone that favors these species is now absenton Chichijima.

DISCUSSION AND CONCLUSIONS

The results given above offer some cluesregarding the origin of the Distylium dryforest and the occurrence of so many endan­gered species in this type of forest in Chi­chijima-retto: Distylium dry forest is distrib­uted widely over the central parts of Chi­chijima and Anijima, with scattered patchesin the peripheral areas. Small patches ofDistylium dry forest are poor in species com­position because of the absence of manyendangered species. All of the 25 endangeredspecies selected in this study are preservedin the highest area on Chichijima, calledChuosan-higashi-daira. The three islands ofChichijima-retto were united into a large is­land (about three times as large as the presentarea) during the last Ice Age, when sea levelwas at least 100 m lower than at present. Thepresent distribution of the Distylium dry for­est seems to be related to the frequency of fogoccurrence. Distylium lepidotum and manyother members of the Distylium dry forest arenot resistant to severe drought. The Distyliumdry forest located below 100 m altitude seemsto be changing to a Rhaphiolepis dry forest, thedrier vegetation type. Mesic-type forests takethe place of Distylium dry forest in deep soils,irrespective of altitude.

In general, the geological history of oceanicislands begins as submarine volcanic activity.A large, high mountainous island with simpletopography is formed first. After volcanicactivity ceases, the island becomes lower andsmaller because of subsidence of the basallandmass and continuous erosive action, whichmakes for an intricate topography. The islandfinally becomes flat and low and covered by

PACIFIC SCIENCE, Volume 46, April 1992

the sea. This historical trend ofoceanic islandsshould be applicable to the Bonin Islands, butthere is no direct geological evidence.

Clouds are usually generated by an as­cending air current around oceanic islands.Thus, high islands always have a cloud zone(fog zone) in their mountainous areas abovesome altitude (usually about 500 m and above).Here one usually finds cloud forests (Kajiand Takiguchi 1983). It can be said that thehigher and larger an island, the more frequentand abundant is its cloud cover.

Large trees in mesic-type forests (Elaeo­carpus-Ardisia mesic forest) in the BoninIslands were selectively cut. The forests ondeep soils were almost completely cleared forsugarcane fields by the 1900s (Katahira 1981).But the dry-type forests on shallow soils wereless damaged, probably because the dry habi­tat conditions were not suitable for agricul­ture. (This assumption is supported by thefact that Anijima, where Distylium dry forestspreads over almost all the island, has neverbeen inhabited by humans [Yasui 1988].) Asa result, most of the Distylium dry forest areashave been preserved in rather natural condi­tion in Chichijima-retto. Thus the presence ofendangered species in the Distylium dry forestseems to be a natural pattern that still existsat present in the Bonin Islands.

Based on the results and assumptions men­tioned above, I propose a hypothesis to ex­plain the origin of the Distylium dry forest andthe occurrence of endangered species in Chi­chijima-retto (Figure 18):

STAGE 1: A large, high island was formed.The environment was simple and the vegeta­tion was not so diversified. A mesic scrubexisted in the fog zone on the upper part ofthe island.

STAGE 2: The island began to shrink aftervolcanic activity ceased. Vegetation diversi­fied into several types according to the ha­bitats formed by erosion. Distylium-Schimadry forest occurred at the lower part of the fogzone or just below the fog zone. This forestgrew on shallow soils in habitats characterizedby both frequent fog occurrence and periodicaridity. Mesic-type forest was established ondeep soils.

STAGE 3 (Present): The island decreased in

Distylium Dry Forest in the Bonin Islands-SHIMIZU 195

LITERATURE CITED

CARLQUIST, S. 1965. Island life-A naturalhistory of the islands of the world. TheNatural History Press, New York.

ELTON, C. S. 1958. The ecology of invasion byanimals and plants. John Wiley & Sons,New York.

FUKUYAMA, H. 1983. GeologyofMinami-IwoVolcano (in Japanese with English summa­ry). Pages 31-50 in Environment Agency,ed. The nature of Minami-Iwojima Island.Japan Wildlife Research Center, Tokyo.

KAJI, M., and S. TAKIGUCHI. 1983. Vegetationstructure of Minami-Iwojima Island, theBonin Islands (in Japanese with Englishsummary). Pages 191-221 in EnvironmentAgency, ed. The nature ofMinami-IwojimaIsland. Japan Wildlife Research Center,Tokyo.

KATAHIRA, H. 1981. Characteristics of landuse on Chichijima Island before World WarII (in Japanese). Pages 155-162 in Re­port on the assessment of the nature ofOgasawara (2). Tokyo Metropolitan Gov­ernment, Tokyo.

dry forest. This trend of aridification has beenaccelerated by the roughly 100-m rise of sealevel after the last Ice Age. The trend musthave been interrupted during several Ice Agesin the Quaternary.

STAGE 4: As subsidence of the islands con­tinues, the Distylium dry forest will almostdisappear on the flat, low islands. Most of theendangered species will become extinct whenstage 4 is reached. Flat land with deep soilswill be covered with a dwarfed mesic-typeforest with simple species composition. Onlya few members of the Distylium dry forest willsurvive in the Rhaphiolepis dry forest that willhave displaced the Distylium dry forest onshallow soils.

Pollen analysis might support this hypothe­sis, but there is no bog with sediments suitablefor pollen analysis. Germination experimentswith some species of the Distylium dry forestand investigations of similar dry forestson islands other than Chichijima-retto areplanned for future research.

altitude below the usual fog zone and the truemesic scrub disappeared. Some members ofthe mesic scrub survived in the Distylium­Schima dry forest and became endangeredspecies. The island was dissected into threeislands by the rise in sea level and the openingof two straits. The Distylium dry forest be­came fragmented into several patches. Somespecies, sensitive to reduced fog frequency,disappeared from small pheripheral patchesof this forest. These sensitive species survivenow only in the highest areas on Chichijimaand in the central part of Anijima, where theyare endangered species. The Distylium dryforest is changing to a Rhaphiolepis dry forestat low altitude. The Distylium-Pouteria dryscrub has diversified from the Distylium­Schima dry forest in dry habitats, which firstappeared earlier in this stage. Some speciesendemic to Chichijima-retto (Pittosporumparvifolium, Callicarpa nishimurae, Myrsineokabeana, Symplocos kawakamii, Boniniacrassifolia) speciated in the dry scrub from thecongeneric species in the Distylium-Schima

D-Fm.s.

STAGE2E_~m, '. -D-Sd.f.

~ -200m-R-Ld.f.

FIGURE 18. The hypothetical process of the origin ofDisty/ium dry forest and the occurrence of endangeredspecies in Chichijima-retto.

STAGE 1

_~=~e-p:TAGE 4_.<:::::...._-----'-

196

KAWAKUBO, N. 1986. Vegetation of the upperpart of Kita-Iwojima including Sanman­tsubo area (in Japanese). Ogasawara­kenkyu-nenpo 10: 63-70.

KURODA, T. 1979. Rocks of the Bonin Islandswith special reference to boninite (in Japa­nese). Ogasawara-kenkyu-nenpo 3: 20-28.

MINISTRY OF TRANSPORT. 1969. General sur­vey for the reconstruction project onOgasawara-shoto (airport construction) (inJapanese). Ministry of Transport, Tokyo.

NATURE CONSERVATION SOCIETY OF JAPANand WORLD WILDLIFE FUND JAPAN .. 1989.Present situation of endangered plant spe­cies in Japan (Japanese Red Data Book).Tokyo.

OHBA, H. 1983. The higher plant flora ofMinami-Iwojima Island (Ins. San August­ino), the Volcano Group ofIslands, Japan(in Japanese with English summary). Pages61-143 in Environment Agency, ed. Thenature of Minami-Iwojima Island. JapanWildlife Research Center, Tokyo.

OHTA, Y, and N. YONEKURA. 1987. Coastline(in Japanese). Pages 70-72 in Japan Associ­ation for Quaternary Research, ed. Explan­atory text for Quaternary maps of Japan.University of Tokyo Press, Tokyo.

ONO, M., S. KOBAYASHI, and N. KAWAKUBO.1986. Present situation ofendangered plantspecies in the Bonin (Ogasawara) Islands.Ogasawara Res. 12: 1-32.

SHIMIZU, Y. 1981. Estimation of the totalnumber of mature individuals of 67 woody

PACIFIC SCIENCE, Volume 46, April 1992

species on Chichijima, the Bonin Islands (inJapanese). Pages 55-60 in Report on theassessment of the nature of Ogasawara (2).Tokyo Metropolitan Government, Tokyo.

---. 1982a. Occurrence of trees, saplingsand seedlings of75 woody species in naturalforests on the Bonin (Ogasawara) Islands(in Japanese). Ogasawara-kenkyu-nenpo6:42-52.

---. 1982b. Influence of the drought in1980 on the vegetation of Chichijima (inJapanese). Pages 31-37 in Report on theassessment of the nature of Ogasawara (3).Tokyo Metropolitan Government, Tokyo.

---. 1989. Ecological characteristics offorest vegetation on oceanic islands,Ogasawara (in Japanese). Pages 159-203in A. Miyawaki, ed. Vegetation of Japan­Okinawa & Ogasawara. Shibundo, Tokyo.

---. 1990. Endangered plant species onthe Bonin Islands (in Japanese). Heredity44(10): 5-6.

TAMURA, T., and T. IMAIZUMI. 1981. Terrestri­al geography (in Japanese). Pages 70-84 inReport on the assessment of the nature ofOgasawara (2). Tokyo Metropolitan Gov­ernment, Tokyo.

TUYAMA, T. 1970. Plants of Ogasawara (inJapanese). Pages 109-141 in T. Tuyamaand S. Asami, eds. Nature in the BoninIslands. Hirokawa-shoten, Tokyo.

YASUI, T. 1988. Flora ofAnijima Island in theBonin Islands. Ogasawara-kenkyu-nenpo12:1-14.