Plant Ecology 163: 155–176, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

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Forest vegetation across the tropical Pacific: A biogeographically complexregion with many analogous environments

Dieter Mueller-DomboisDepartment of Botany, University of Hawaii, 3190 Maile Way, Honolulu Hawaii 96822, U.S.A.www.botany.hawaii.edu/pabitra/

Key words: Biogeographic gradients, island types, profile hierarchy, biome-level profiles, landscape-level profiles,stand-level profiles, PABITRA net

Abstract

The tropical realm of the Pacific contains many islands with closely similar (analogous) environmental settings.Due to the ‘filter effect’ of the ocean, these are occupied by historically different species assemblages. This resultsin a unique biogeographic complexity not found in any of the continental tropical regions. The paper presentsa hierarchical approach to the study of vegetation in analogous environments with data illustrated by a series ofdiagrams. It begins with differentiating island types and climatic zones. Then it focuses on Pacific-wide biomes,thereafter on island landscape profiles, and finally, on stand-level profiles as a method to compare forest structureand composition among islands in analogous environments at the scale of relevés. The conclusions emphasizethe urgent need for intensifying conservation-oriented research and capacity building in all island countries ofthe Pacific (not only Hawaii) to protect their indigenous biodiversities for sustainable uses and the health of theirecosystems. Attention is also drawn to the scientific advantage of using the Pacific-Asia Biodiversity Transect(PABITRA) Network, since it connects the island areas across biogeographic boundaries in form of an experimentaldesign. This design refers to analogous island environments filled with different sets of biodiversities, which followa trend of impoverishment with indigenous founder species from west to east.

Introduction

The Pacific islands typically are small landmasses thatare variously scattered across a vast ocean which com-prises about one third of the earth’s planetary surface.Most Pacific islands are in the tropical realm, theocean belt defined by the tropics of cancer in theNorthern Hemisphere and Capricorn in the SouthernHemisphere (Figure 1).

For comparing biogeographically differentiatedvegetation in analogous environments, the Pacific is-lands offer many possibilities. The main reason is thatthe ocean has served as a ‘filter’ of plant dispersal.Thus, each island region can be considered a separatebiogeographic province.

North of the equator, in the western Pacific, are theislands of Micronesia including the Carolines, Mari-anas, Bonin and Marshall Islands. In the tropical north

central Pacific are the Hawaiian Islands. From hereeastward is a vast region, devoid of islands, underlainby the East Pacific Rise. This is a submarine geo-logical boundary, where oceanic plates are constantlyrenewed and move apart in opposite (east-west) direc-tions. This also forms the major biogeographic bound-ary in the Pacific (Bartlott et al. 1996). It separates theoceanic islands with neotropical vegetation (the Revil-lagigedos, Clipperton, Cocos, the Galàpagos, Desven-turadas, and Juan Fernandez Islands) from those cov-ered primarily with vegetation of paleotropical ori-gin. In addition to this major biogeographic bound-ary, there are several other biogeographic boundarieswithin the paleotropical province of the Pacific islandregion. This paleotropical province refers to the is-land areas west of the eastern demarcation line of thePolynesian triangle. West of the Polynesian triangleand below the equator, are the islands of Melanesia

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(Fiji, Vanuatu, New Caledonia, the Solomon and Bis-marck Islands). The equator is not considered a majorbiogeographic boundary in the Pacific.

For comparing biogeographically differentiatedvegetation in analogous environments, it is useful tofocus on a hierarchical approach, from the general tothe specific. This is best illustrated with data in a seriesof easy to understand diagrams. The focus thus will beinitially on biogeographic gradients and island types,followed by examples of biome-level island profiles,then landscape-level vegetation profiles, and finally,stand-level profiles of rain forests in Fiji and Hawaii atthe relevé level. The paper concludes with outlining anew cooperative research project, called the PABITRAnet (the Pacific-Asia Biodiversity Transect Network).

Biogeographic gradients and island types

With increasing knowledge of the Pacific biota, anumber of biogeographic classification schemes havebeen devised and mapped at an overview scale. Someof the better known ones were reviewed by Stoddart(1992), who concluded his review with a proposalof his own, based on island type and precipitationpatterns (aspects that will be discussed below).

Figure 2 incorporates most of the traditional bio-geographic classification schemes in form of sectionmaps, the numbering of which follows an approximatetrend of impoverishment of the indigenous founderfloras from west to east. Thus, section map 1, West-ern Melanesia, is floristically the richest island area,followed by section map 2, Eastern Melanesia, whichincludes Fiji. From here, further floristic attenuationtrends go south (section map 3, the subtropical islandsin the New Zealand region), north (section map 4,Micronesia), and east (section map 5, Western Poly-nesia). The paleotropical outlier regions are, EasternPolynesia (map 7), and Northern Polynesia (map 8,the Hawaiian Islands). Finally, the neotropical out-lier island areas are in section map 9 (which includesthe Galàpagos) and section map 10 (which includesthe subtropical Juan Fernandez Islands). This biogeo-graphically oriented map system was used as outlinefor the chapters in the Pacific vegetation book byMueller-Dombois & Fosberg (1998).

The floristic attenuation trends are not only de-termined by distance of islands from biotic sourceareas and island size as implied in the island bio-geography theory of MacArthur and Wilson (1963,

1967). As Stoddart (1992) pointed out, island type andprecipitation level play a major role.

Two broad groups of Pacific islands are generallyrecognized, the continental and the oceanic islands.Continental islands are those that once were part ofcontinents and that separated as fragments throughtectonic movements. They are often rather large is-lands, such as New Zealand and New Caledonia, butthey can also be very small, such as ‘Eua Island insouthern Tonga, which separated about 40 millionyears ago from Norfolk Ridge near New Caledonia.‘Eua then moved as a ‘terrane’ (continental fragment)brushing Fiji 5 million years ago, and thereafter settledin its present position next to Tongatapu in southernTonga (Yan & Kroenke 1993). Oceanic islands arethose that arose from the bottom of the ocean and thatwere never connected to a continent. They are usuallyclassified into three types, the volcanic high islands,the coral-derived low islands, and the raised limestoneislands. The volcanic high islands can be further sepa-rated into plate-boundary and intraplate islands (Nunn1994). The intraplate islands are those that arose fromso-called ‘hot-spots’ within the oceanic plate. Theyare usually derived from basaltic volcanism and origi-nate as basaltic ‘shield volcanoes’. Examples are theHawaiian Islands, Samoa, and the Society Islands.Plate boundary islands arise where an oceanic plateis subducted under another lithospheric plate, usuallyat the Andesite Line. Typically, they are recognizedas steep-sided andesitic ‘composite volcanoes’ (Fig-ure 3). Examples are the northern Mariana Islands andthe western Tongan Islands.

Low coral-derived islands are either atoll or reefislands that arose from coral growth on the submergedflanks of an eroded volcanic island. Raised limestoneislands can be reef islands that formed at a higherstand of sea level or they can be tectonically raisedformer low islands. They often consist of hard marinederived limestone called ‘makatea’ and may also con-sist in part of volcanic soil segments. An example isthe island of Guam.

Elevation and type of substrate are the major de-terminants of island types, and these two physicalparameters are also determinants of floristic richness.Raised limestone islands are much richer floristicallythan atoll islands, and volcanic high islands usuallycontain several biomes and thus are floristically richer,as a rule, than raised limestone islands. Moreover,atoll islands contain floras which are adapted to oceandispersal or dispersal by ocean-roaming birds. Theirbiogeographic relationships are similar to the strand

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Figure 3. Cross-section of subduction zone indicating the formation processes for the two kinds of volcanic high islands, the intraplate andplate-boundary islands (adapted with permission from Macdonald et al. 1986).

floras of the volcanic high islands. They are veryunlike those on the volcanic substrates inland.

A third major biogeographic determinant is thelevel of precipitation. Figure 4 displays the meanannual precipitation pattern across the Pacific basin.There are three wet zones, with more than 5 m an-nual rainfall. One is in the SW Pacific centering onthe Santa Cruz Islands in Eastern Melanesia, anothernorthwestward in Micronesia, centering on the EasternCarolines, and a third one in the central Pacific, southof Hawaii, affecting the northern Line Islands of Kiri-bati. Low islands in wet zones are floristically richerthan those in the dryer zones. This has been clearlyestablished for the atoll and reef islands by Fosberg(1956, and further supported by Stoddart 1992 andby Manner 1995). Some less disturbed atolls in thePacific wet zones support rain forests on soils derivedfrom marine organisms (Mueller-Dombois & Fosberg1998). Increased floristic richness may also be true forthe raised limestone and volcanic high islands in thetwo western Pacific wet zones. Although high islandsin dryer zones affected by tradewinds may be just asrich floristically or richer on account of their addedleeward environments.

Examples of biome-level island profiles

Biomes are generally understood as large ecosys-tems that are in the same climate zone and whoselandscapes are covered with the same prevailing (orpotential) physiognomic vegetation type. In Walter’s(1985) terminology, these are ‘zonobiomes’. If a ma-jor extreme type of substrate is also involved, onemay recognize ‘pedobiomes’. This term applies wellto atoll and reef islands as a group. Walter also distin-guished ‘orobiomes’, i.e., altitudinal mountain biomesas separate from zonobiomes, i.e., latitudinal lowlandbiomes. Biomes can also be described as vegetationzones (Walter & Breckle 1999).

The only important difference to continental bio-mes is that island biomes are discontinuous. There area number of Pacific-wide biomes (Mueller-Dombois& Fosberg 1998), such as upland rain forests, fern-savannas, fresh-water wetlands, mangroves, andstrand forest-scrub formations, to name a few. Wher-ever they occur on individual islands, they can beconsidered as fragments of the larger Pacific-widebiomes. Some examples follow.

Figure 5 outlines the biomes of the Galápagos Is-lands, described as vegetation zones by Itow (1985,1992). Here, four major biomes are recognized in

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Figure 5. Map of profile of vegetation zones on Santa Cruz and Baltra, Galapagos Islands (recomposed from Itow 1985, 1992, with permissionfrom Pacific Science).

topographic sequence as A = the arid lowland zone,B = the transition zone, C = the moist upland zone,and D = the highland zone. These zones apply to all is-lands of the Galàpagos, except the smaller islands. Thesmaller ones may lack the highland and/or the moistzones. On the windward side of Santa Cruz Island,the moist zone is further subdivided into the Scalesiaforest zone, the brown Frullania epiphytic zone, andthe Miconia scrub zone. These zones are not on allislands of the Galàpagos.

The four major biomes are characterized by cli-mate diagrams, two describe the climate of the aridlowland zone (Baltra and CDRS), one the transitionzone (Caseta at 200 m a.s.l.). Note that the moistzone at Media Luna has a mean monthly precipitationin excess of 100 mm. This uniformly high monthlyprecipitation renders the moist upland zone to be anal-ogous to mesic rain forest environments elsewhere inthe tropics.

At the same level of scale one can compare themajor Galàpagos biomes with the major Hawaiian

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biomes. Figure 6 shows the Hawaiian island chainin profile with island ages in millions of years (Ma)and maximum elevations in meters (m). Note that thetradewinds blow from the northeast, which here re-sults in strong windward and leeward differences oneach mountainous island, a climate feature similar tothat of the Galàpagos Islands. The elevations, how-ever, reach much higher in the Hawaiian Islands. Thetreeless highland (fern-sedge) zone in the Galàpagosbegins already at about 600 m from where it extendsupwards to the maximum height of 864 m on SantaCruz Island. In contrast, the high altitude zones in theHawaiian Islands begin at about 2000 m and are hererestricted to the two southernmost islands, Maui andHawaii.

Figure 7 shows these zones mapped as 6 = cool,dry subalpine zone, and 7 = cool, dry alpine zone.These high-altitude zones are totally unlike the Galà-pagos highland zone, which lacks trees because noneof the indigenous species evolved to invade this windyalternately wet and dry habitat. The other five Hawai-ian zones may be roughly analogous: the arid lowlandzone of the Galàpagos compares to the hot, dry low-land zone of the Hawaiian Islands at least in part.The transition zone of the Galàpagos may compare tozones 2 and 3 in the Hawaiian Islands. The moist zoneon Santa Cruz has some similarity with the rain forestzones (4 and 5) in the Hawaiian Islands but only withtheir dryer ecotones.

Figure 8 is a climate diagram map of the HawaiianIslands. A truly climatic desert environment occursat NW Hawaii (Kawaihae), which is comparable tothat of the northern lowland on Santa Cruz (at Baltra).Seasonally dry climates are indicated for several lee-ward stations (e.g., South Point and Honolulu) whichare somewhat comparable to the transition zone inthe Galàpagos Islands. The moist lowland zone onwindward O‘ahu may be somewhat comparable to theScalesia forest zone on Santa Cruz. Thus, one canfind approximately analogous climatic environmentsin these two archipelagoes. But they are not exactlyalike.

A comparison of the Hawaiian climate with thatof Eastern Melanesia Figure 9 clarifies that EasternMelanesia is more uniformly moist with rain forestclimates prevailing over the whole region. Althoughthere are seasonally dry leeward climates in New Cale-donia (Noumea) and Fiji (Labasa), they are not asarid as those in leeward Hawaii. However, wind-ward Hawaiian climates and rain forest climates inMelanesia are quite comparable.

Landscape-level vegetation profiles

Vegetation profiles at the landscape level are usefulfor interpreting larger scale, i.e. more detailed maps.All maps, whether they are topographic base maps,climate, vegetation, geology, or soil maps, are merelytwo dimensional, i.e., flat-area, projections. These arenever easy to interpret. A profile adds the third dimen-sion and therefore lends itself easily to interpretingvegetation as ecosystems. Moreover, landscape pro-files can display complex data in a most user friendlyway. Unfortunately, vegetation ecologists have notmade enough use of landscape-level profiles as toolsof data display.

Only few island vegetation studies have thus farpresented landscape-level profiles. Here, I will presentjust two examples from the subtropical islands in theNew Zealand region.

Figure 10 displays the southern mountain area ofLord Howe Island, which includes the summit plateauof Mt. Gower (875 m). This topographic vegetationprofile displays nearly all the major vegetation typesfound on this small (about 2 × 10 km) volcanic highisland. The former roaming ranges of feral goats andferal pigs are indicated as an added feature. The goatsand most of the pigs have been removed since LordHowe was declared a World Heritage Area in 1982.

Figure 11 represents a cross-section through theNW part of Norfolk Island, characterizing the vege-tation around Mt. Bates (318 m) and Mt. Pitt (316 m).This is the only natural area left on this small (35 km2)island. An added feature is the display of vegeta-tion ‘200 years ago’ before the introduction of alieninvasive species gained territory as displayed in thevegetation patterns of ‘today’.

Can we find analogous environments in these topo-graphically very different subtropical islands? Theanswer is yes. The lowland climates of both islandsare of the same type: subtropical oceanic withoutany pronounced dry season. Thus, the two islandsshow the same ‘zonobiome’. They differ in that Nor-folk Island lacks an ‘orobiome’. Lord Howe, there-fore, has a richer habitat spectrum and also a greaterbeta-diversity.

The subtropical lowland rain forest on Lord Howecontains several community types: (1) Drypetes-Cryptocaria forest (the most wide-spread type),(2) Cleistocalyx-Chionanthus forest, (3) Lowlandmixed-species forest without distinctly dominanttrees, (4) Two types of Howea palm forests, and

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Figure 7. Hawaiian vegetation zones (adapted with slight modifications from Knapp 1965 with permission from Fischer Verlag).

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Figure 9. Climate diagram map of Eastern Melanesia (from Mueller-Dombois & Fosberg 1998, p. 87).

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Figure 10. Topographic vegetation profile across southern mountain area of Lord Howe Island (adapted with permission from Pickard 1976,1983).

(5) Pandanus forsteri forest (Mueller-Dombois & Fos-berg 1998: 170 ff.).

The subtropical lowland rain forest on Norfolk Is-land has been characterized by Gilmour and Helman(1989) as comprising three intergrading communitytypes: (1) An Araucaria dominated forest in drier,more wind-exposed positions, (2) A Rhopalostylispalm and Cyathea tree fern dominated forest inmoister positions, and (3) A mixed-species hardwoodforest with Araucaria overstory in intermediate po-sitions. Principal indigenous hardwood tree speciesinclude Celtis paniculata, Pouteria costata, Neste-gis apetula, Elaeodendron curtipendulum, Disoxylumbijugum, and Lagunaria patersonia.

Among interesting differences of the two sub-tropical island forests is that Lord Howe lacks anygymnosperm, while Norfolk Island is dominated byAraucaria. Another is that Lagunaria patersonia is in-digenous to both islands, but on Norfolk island it is thesecond most dominant and wide-spread tree species,while on Lord Howe it is among the rarest and thereconfined to the last fresh-water swamp.

Stand-level profiles of island rain forests

Enlarging the scale of landscape-level profiles leads tothe more detailed scale of stand-level profiles. These,

when done accurately with measurements, take someextra time in the field. But they are an excellentsupplement to plot and relevé sampling.

Beard (1946) pioneered the method, which is stillthe best for illustrating the structural composition oftropical rain forests at the relevé level (see Mueller-Dombois & Ellenberg 1974). At this level of scale, onecan make accurate comparisons of forest stand sam-ples in similar (analogous) and different environments.

I will illustrate this by few published examplesof island rain forests. For the International Biologi-cal Program (IBP) in the 1970s, we established an80 ha ‘forest dynamics plot’ in the Kilauea rain for-est next to Hawaii Volcanoes National Park. From alow-flying aircraft we obtained a large-scale (1:1,500)colored air photograph, which we mapped by recog-nizing four recurring structural patterns: (1) An Acaciakoa dominated canopy, (2) Canopy gaps filled withtree ferns and an occasional young Acacia koa tree,(3) Other openings dominated by Metrosideros poly-morpha trees of mostly low and intermediate structure,and (4) Recent windfall gaps created by tall senescentAcacia koa trees (Figure 12).

A checklist of all plant species was prepared, alltrees were measured along four transects in twenty6 × 100 m plots, canopy arthropods, wood-boringbeetles, forest birds, introduced rodents and predators,

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Figure 12. Vegetation map of 80 ha forest dynamics plot in Kilauea rain forest, Island Hawaii (from Mueller-Dombois et al. 1981, p. 255).

were quantitatively assessed, and feral pig effects wereevaluated from activity signs (Mueller-Dombois et al.1981). This 80 ha forest segment is now used as a per-manent plot for periodic monitoring and re-assessmentof biotic change.

Four stand-level profiles were drawn based on50 × 6 m rectangular map projections. Two are re-produced here. Figure 13 is the stand profile of plot5 (see Figure 12 for location), which is dominated bya 20–25 m high canopy of Acacia koa. Up to 5 mtall Cibotium tree ferns dominate the undergrowth.

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Figure 13. Stand-level profile, of 6 by 50 m strip in Kilauea rain forest, plot 5, Figure 12. Acacia koa dominated canopy segment (drawn byR. G. Cooray, in Mueller-Dombois et al. 1981, p. 252).

Other native trees include Metrosideros polymorpha(in the 10–18 m subcanopy). Among smaller treesare Pelea volcanica, Coprosma rhynchocarpa, Cheiro-dendron trigynum, Myrsine lessertiana, Ilex anom-ala, and Coprosma rhynchocarpa. Figure 14 shows alarge canopy gap filled dominantly with juvenile Met-rosoderos polymorpha. This is plot 20 on the map(Figure 12). These diagrams convey the structure oftwo of the four recognized recurring patterns in theKilauea forest. This forest is one of the more diversenative Hawaiian rain forests.

The Fijian rain forests are certainly more com-plex and more densely stocked with taller trees. Thisis clear from the next two stand-level profiles. Fig-ure 15 represents a mixed-species foothill forest in VitiLevu’s lowland wet zone of the Navua River Catch-ment (Figure 16). There are 17 tree species over 5 mtall. The emergent canopy (15–25 m) contains at least10 species on a 60 × 7 m rectangular plot. Figure 17shows the profile of a mono-dominant Fijian upland

rain forest. This is the saucau (Palaquium hornei) for-est representing a sample taken in the upper RewaRiver Catchment. The Palaquium species clearly dom-inates the 15–25 m canopy, while the undergrowth ismade up of a mixed-tree species assemblage, not un-like that of the mixed-species foothill forest. This mayindicate that the mono-dominance of Palaquium repre-sents an earlier successional stage. However, dynamicrelationships can only be established through furtherresearch.

Conclusions

It would be of considerable scientific value to docu-ment the functional differences associated with dif-ferent sets of biodiversities that occupy analogousenvironments. But unlike the Hawaiian rain forest, theFijian rain forest has not yet received its fair share ofmore intensive ecological research. This applies also

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Figure 14. Stand-level profile of 6 by 50 m strip in Kilauea rain forest, plot 20, Figure 12. Juvenile Metrosideros polymorpha dominated canopygap (drawn by R. G. Cooray, in Mueller-Dombois et al. 1981, p. 254).

to most other high island archipelagoes in the Pacific.For Fiji, this has been well documented by Julian Ash(1992), who concludes his review on the vegetationecology of Fiji with a dim forecast.

The emphasis on forest research in Fiji, so far,has been on timber volume surveys. Little researchemphasis has been spent on understanding forest func-tions in terms of sustainable production, or on theupland forest’s role in regulating Fiji’s freshwater flow,or its value as a natural heritage of the Fijian people.

Such conservation-oriented research has now be-come very urgent throughout the Pacific islands be-cause sustainable and improved uses of the islands’biological and freshwater resources are the key to anadequate survival of their human populations. The re-lationship of islanders to their renewable resourceshas evolved historically into a sophisticated resourcemanagement system. In Hawaii, this is known as theahupua‘a system.

An ahupua‘a is a vertical land segment, a livinglandscape, that includes an upland forest (wao na-hele), a more open transition zone (kula), a lowlandagricultural zone (wao kanaka), and a coastal or shore-line zone (kahakai) as a self-sufficient human supportsystem. The freshwater flow is an important key tothe proper functioning of this complex ecosystemconsisting of four ecological production zones.

The Pacific islands offer an experimental layoutfor documenting the functional differences associatedwith different sets of biodiversities in analogous en-vironments. This layout is captured in the PABITRAnet (the Pacific-Asia Biodiversity Transect Network)displayed in Figure 18.

PABITRA is a cooperative international researchprogram that combines both, the horizontal andvertical approaches to ecosystem studies (Mueller-Dombois et al. 1999). The horizontal approach relatesto the comparative analyses of the indigenous uplandforests as ecological reserves and watershed covers.

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Figure 16. Map of Viti Levu, Fiji’s largest island, with climate diagrams representative of wet zone, dry zone, and upland zone (dry-wet zoneboundary after Cochrane 1969).

The vertical approach relates to the study of inter-actions of the upland forests with the lowland andcoastal ecosystems as connected downslope over shortdistances by the freshwater flow, and by gradients oftemperature and precipitation.

PABITRA originated in 1997 within the Taskforceon Biodiversity of the Pacific Science Association (Ki-

tayama & Mueller-Dombois 1997; Mueller-Dombois1998). It is considered the island branch of DIWPA(Diversitas in the Western Pacific and Asia), a researchnetwork coordinated by the Center for EcologicalResearch at Kyoto University. DIWPA in turn is con-sidered the western hemisphere branch of the globalprogram DIVERSITAS, which is currently promoted

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by the international science organizations located inParis, including IUBS, ICSU, UNESCO, and SCOPE.More detailed information on PABITRA is found onthe Internet under www.botany.hawaii.edu/pabitra/.

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