Distributional Dynamics in the Hawaiian Vegetation!

DIETER MUELLER-DOMBOIS 2

ABSTRACT: Vegetation ecology is usually divided into two broad research areas, floristic/environmental gradient analysis and studies of vegetation dy namics. The early influential American ecologist Clements combined the two areas into a dynamic system for classifying vegetation. His succession and climax theory, however, was later severely criticized. A new approach to the study of distributional dynamics, called "landscape ecology," focuses on the dynamics of spatial vegetation patterns. There is a spatial hierarchy rule, which implies greater stability of species and community patterns when one considers larger area units versus smaller ones. It is argued that this rule is frequently transgressed in biotically impoverished areas, like the Hawaiian Islands, where certain dominant plant species have become established over unusually broad areas and habitat spectra. A further point made is that with "species packing" successional patterns change from auto-succession, where the dominant species retains dominance by in situ generation turnover (termed chronosequential monoculture), via "normal" succession (i.e., displacement ofdominants by other dominants over time [termed chronosequentialpolyculture]), to small-area patch or gap dynamics (termed chronosequential gap rotation). Examples of the three spatially different succession paradigms are given for Hawaii, and the point is made that chronosequential monocultures cannot be expected to last, but change to chronosequential gap rotation with the invasion of alien dominants. Before the invasion of alien dominants, certain native dominants seem to have segregated into races or varieties by evolutionary adaptation to successional habitats. Finally, the concept ofclimax is discussed as having two meanings: (I) permanency of community type, which can only be observed for the aggregate assemblage of smaller communities in a larger space, such as occupied by a biome; (2) the mode of organic production in ecosystem development. The mode seems to occur between 1000 and 3000 yr in the Hawaiian rainforest biome on volcanic soils. Thereafter, productivity declines with acidification and soil nutrient impoverishment over a million years and more. This amounts to a retrogression in the course of primary succession.

RESEARCH IN VEGETATION ecology is usually environmental gradients, implies some stabil divided into two broad areas, the study of ity in terms of plant distribution patterns. distribution of species and communities and Succession is traditionally defined as any the study of plant succession and vegetation time-related change ofvegetation in the same dynamics. Distribution is a spatial/geograph area except those recognizable as phenolog ic concept. It relates to the classification and ical or evolutionary changes. In a more re mapping of species and plant communities stricted sense, therefore, plant succession re and, particularly when done in relation to lates to any chronosequential change in spe cies distribution in a given area, site, or habitat. Clements (1916, 1928) tried to com bine the two concepts by recognizing any 1 Manuscript accepted 2 May 1991. 2 Department of Botany, University of Hawaii at spatially defined community also in a succes Manoa, Honolulu, Hawaii 96822. sional and dynamic context. He did this by 221 222 PACIFIC SCIENCE, Volume 46, April 1992

analyzing the spatial side-by-side differences that emerged from observing spatial/dynamic of communities in a macro-climatic region patterns in the Hawaiian Islands. They are and arranging them into hypothetical chro summarized under three subheadings dealing nosequences. Clements's dynamic classifi with (1) dynamics in the hierarchy of vegeta cation ran into trouble because he assumed tion units, (2) changing patterns of plant .that all physiographically constrained com succession, and (3) the concept ofclimax in an munities ofa region or biome converge into a island context. uniform climatic climax community, given enough time. This proved to be unrealistic (see Dynamics in the Spatial Hierarchy Mueller-Dombois and Ellenberg 1974). Another dynamic concept with a spatial BASIC CONSIDERAnONS. Perhaps one of the implication is the "pattern and process" con better known formalized vegetation hierar cept, perhaps most effectively introduced by chies is the one developed by Braun-Blanquet Watt (1947). This concept postulates what is (1928), which was patterned after the taxo sometimes referred to as "cyclic" succession nomic system of classifying organisms (Krebs 1985) by contrasting it to the so-called (Mueller-Dombois and Ellenberg 1974). In "directional" succession of Clements. This this system, the basic unit is the "association," "cyclic" succession infers a rotation ofsmaller which was treated as analogous to the taxon community patterns within larger community "species" in the organismic classification sys units; the larger units can be formations or tem. Egler (1947), when studying Hawaiian biomes. The primary mechanism in cyclic vegetation, expressed his dissatisfaction with successions may be the demographic behavior the association concept and opted instead for of the component species, which grow, ma Gleason's (1926) "individualistic" concept of ture, and die in an otherwise relatively perma the plant community. One reason for Egler's nent or stable community. In tropical forest dissatisfaction was the dynamic nature of the biomes this concept is now often interpreted Hawaiian vegetation. Changes or shifts in as "gap dynamics." vegetation patterns can easily be expected at For the purpose of integrating the concept the spatial level of releve analysis. A plot or of gap dynamics with that of succession, reIeve of 20 m by 20 m, once established in a Pickett and White (1986) proposed the term Hawaiian forest, scrub, or grassland, may "patch dynamics." This liberates the cyclic have a different species composition in just a succession concept from a preconceived spa few years. By extension, the plant association, tial definition. This may be useful for under a vegetation unit analytically based on a standing the relationship between the primari number of floristically similar releves and ly "temperate fire-succession" dynamics and used for mapping floristically defined vegeta the primarily "tropical gap-rotation" dy tion patterns at large map scales (such as namics. However, the two often occur at 1 : 10,000 to 1 : 50,000), may require frequent quite different spatial scales and are asso spatial readjustments in such a dynamic envi ciated with different disturbance regimes. ronment. Spatial scales and configurations ofdynam Starting instead from a small-scale map ic phenomena are an important area of re projection, Hawaiian vegetation is relatively search, now often referred to as "landscape stable. For example, the altitudinal bound ecology" (Forman 1986, Urban et al. 1987). aries of major vegetation zones have not Such focus should be particularly important changed over several decades. Troll's (1959) to the vegetation ecology of islands, because comparison of the mountain biomes of spatial/dynamic shifts of vegetation patterns Hawaii with those on Mt. Kinabalu and are often dramatic in island ecosystems. These other tropical mountains still holds. The dis spatial/dynamic shifts need attention for an tribution of dominant species and plant-life appropriate interpretation in conservation bi forms can still be used as indicators ofclimatic ology, land-use policy, and vegetation man gradients unique to oceanic island mountains. agement. Only global warming or cooling may change In this paper, I emphasize a few concepts that. Distributional Dynamics in Hawaiian Vegetation-MUELLER-DoMBOIS 223

A STABLE SCHEME FOR DYNAMICS RESEARCH Moreover, the azonal system includes IN HAWAII. A small-scale overview of the smaller-area landscape units, which can be Hawaiian Islands provides for a stable frame projected only on large-scale maps. It incor work that may serve for an analysis ofdynam porates all the strand and coastline com ic patterns. The very isolated Hawaiian archi munities found on the leeward Hawaiian Is pelago forms a 300-km stretch of eight high lands, the communities on recent lava flows, islands, which continues northwest for anoth and the Hawaiian bog vegetation. er 2000 km in a sequence of low leeward islands. The latter are either atolls or rock TRANSGRESSIONS OF THE HIERARCHY RULE. bluffs. They are also the older islands, ranging As presented here, a hierarchy of vegetation in age from 10 to 25 million years, while the units may start with ecological or vegetation high islands are the younger ones. The high zones in the form ofzonal and azonal ecosys islands range in age from the currently build tems. Within these large zonal units, one can ing volcanic mountains on the most southern recognize smaller-area ecosystems such as and largest island, Hawaii, to Kauai, with those 30 principal terrestrial ecosystems de surfaces formed 3 million years ago. All high scribed by Fosberg (1972) for the Hawaiian islands intercept the northeast trade winds Islands. These are mostly physiognomic units and thereby receive orographic rainfall. This such as forest, scrub, and grassland, and is their most reliable moisture source and also because of the limited island flora, many of provides for a pronounced windward/ them are species-dominance types (sensu leeward effect. The two youngest islands, Whittaker 1962). They include, among others, Hawaii and Maui, rise above the prevailing such designations as M etrosideros woodland cloud belt and trade-wind inversion (above with Gleichenia (=Dicranopteris), Acacia koa 2000 m) into a cool-tropical alpine environ forest, Aleurites forest, Psidium cattleianum ment. forest, Leucaena scrub, and Heteropogon This three-fold segmentation into leeward, grassland, to name a few. But the question windward, and high-altitude climates pro is, how permanent or dynamic are these vides for an outline of zonal (i.e., primarily units? Can they be considered as climax climatically controlled) ecosystems. Ten such communities? zonal ecosystems or landscapes were mapped Egler (1947) objected not only to using the by Ripperton and Hosaka (1942) at an inter association concept, but also to using phy mediate map scale range of 1 : 100,000 to 1 : 1 siognomic criteria for classifying vegetation million. Because of this relatively small and on Oahu. In his words, the application of generalized scale, their map is still mostly physiognomic criteria "does extremely seri valid. Supplied with some modified vegetation ous injustice to the recognition of natural designations, this map served well for starting areas" in Hawaii. Instead, he offered his a program of selecting natural areas for bio concept of "land-type" (defined as part of a logical conservation in Hawaii (Mueller zone with uniform vegetational potentialities) Dombois and Gagne 1975; see Table 1). as an intermediate-level class unit in the vege Notice that each of the 10 zones could be tation hierarchy above his individualistic characterized by a few dominant species in community and below the level of vegetation 1975. Within these 10 zones occur a larger zone. number of smaller-area vegetation types. With this, Egler tried to bring permanency Fosberg (1972) described about three times as or stability into his classification. His objec many as "principal Hawaiian ecosystems." tion to the use of physiognomic criteria was The most recent classification, by Gagne and based on the frequently made implication of Cuddihy (1990), described 106 community using vegetation physiognomy as an indicator types, most ofwhich can be grouped into this ofclimate. Egler (1942, 1947) emphasized that zonal and the following azonal ecosystem in Hawaii several physiognomic types ofvege scheme. tation such as grassland, scrub, and forest In the azonal scheme, physiographic con coexist often side by side in the same climatic straints override those of the zonal climates. zone; and moreover, that the causes of such 224 PACIFIC SCIENCE, Volume 46, April 1992

TABLE I

ECOLOGICAL ZONATION SCHEME FOR THE HAWAIIAN ISLANDS*

I. Zonal ecosystems (controlled dominantly through macroclimate) 1. Xerotropical (leeward lowland to submontane) A. Savannah and dry grassland (Prosopis savannah and Heteropogon-Rhynche/ytrum grassland) B. Dryland sclerophyll forest (or scrub) (Metrosideros-Diospyros open forests; displacement vegetation: Leucaena scrub and forest) C. Mixed mesophytic forest (woodland or scrub). CI, low phase; C2, high phase. (Acacia koa open forests; displacement vegetation: Psidium guajava, Eugenia cumini forests and woodlands) 2. Pluviotropical (windward lowland to upper montane) DI. Lowland rainforest (Metrosideros forests) (largely displaced, few remnants only) D2. Montane rainforest (Metrosideros-Cibotium and dominantly Cibotium forests) D3. Upper montane rain- or cloud forest (Cheirodendron or Acacia koa-Metrosideros mixed forests) 3. Cool tropical (upper montane to alpine; only on Maui and Hawaii) EI. Mountain parkland and savannah (Acacia koa-Sophora chrysophy//a tree communities, Deschampsia tussock grassland) E2. Subalpine forest and scrub (Sophora-Myoporum tree communities, Styphe/ia- Vaccinium-Dodonaea scrub communities) E3. Sparse alpine scrub (Styphe/ia, Vaccinium) and moss desert (Rhacomitrium /anuginosum) II. Azonal ecosystems (controlled largely through edaphic factors) (not mapped) 4. Coastline ecosystems (Richmond and Mueller-Dombois 1972) including leeward Hawaiian Islands (Gagne and Cuddihy 1990) • Windward (beach, dune, and rock substrates; Scaevo/a scrub, Pandanus and Hibiscus forests, marshes, and mangroves • Leeward (beach, dune, and rock substrates; mangroves, coastal Prosopis forests, Scaevo/a scrub, atolls, and rock islands) 5. Bogs and swamps • Low- and mid-elevation bogs and swamps • Montane bogs (dwarf Metrosideros) 6. Geologically recent ecosystems • Vegetation on new volcanic surfaces (e.g., Stereocau/on lichen-Nephro/epis fern-Metrosideros seedling and sapling stages, young Metrosideros forest with Machaerina sedge and Lycopodium clubmoss-G/eichenia [=Dicranopteris] and Sad/eria ferns) • Lava tubes and other recent geoiogical features (Howarth 1981) 7. Aquatic ecosystems • Freshwater lakes • Streams • Coastal brackish and marine ponds

*The modified vegetation designations of Mueller-Dombois and Gagne (1975) are here summarized for the 10 zonal ecosystems with the letter symbols as mapped on an intermediate scale map by Ripperton and Hosaka (1942). Information synthesized from earlier works: Hillebrand 1888, Rock 1913, Egler 1939, Ripperton and Hosaka 1942, Krajina 1963, Knapp 1965, Doty and Mueller-Dombois 1966, Mueller-Dombois and Krajina 1968, Fosberg 1972, Mueller-Dombois 1981.

fragmentation and physiognomic/structural cipal terrestrial ecosystems, one-third are differentiation are many. This led him to clearly dominated by alien species. conclude a "highly dynamic condition" for Is this trend foreshadowing the doom ofall the Hawaiian vegetation. native vegetation? Many botanists thought so Some of the most obvious dynamic trans at Egler's time and continue to think so today. gressions of the spatial hierarchy rule are in But Egler himself was more optimistic based the Hawaiian zonation scheme itself. Four of on his ecological insights. the 10 zones (A, B, C, and DI) are largely characterized by displacement vegetation Changing Patterns ofPlant Succession dominated by alien species in the xerotropical and pluviotropical lowland to submontane Disturbance regimes have changed in the environments. Among Fosberg's (1972) prin- Hawaiian Islands and they are continuing to Distributional Dynamics in Hawaiian Vegetation-MUELLER-DoMBOIS 225 change. The lowland to submontane environ species that determine the vegetational struc ments have been actively converted to other ture of island ecosystems. Two of the best uses, and the remnant natural areas were first Hawaiian examples are the indigenous tree subjected to frequent fires by the Hawaiians species M etrosideros polymorpha and Acacia and later to heavy grazing and browsing from koa. On the youngest Hawaiian mountain, a variety of feral ungulates introduced by Mauna Loa, Metrosideros polymorpha is dis Europeans (Cuddihy and Stone 1990). As of tributed from sea level to 2500 m elevation, late, significant success in controlling the where it forms the tree line ecosystem. It spread and impact offeral ungulates has been thereby traverses a mean annual temperature accomplished even in remote areas (Stone and range from 23° to 8°C and rainfall regimes Scott 1985).. from subhumid to wet and then dry. On However, by no means are human-related Mauna Kea and Haleakala, Metrosideros disturbances, such as the introduction of un does not extend its range quite as high, but it gulates and their uncontrolled spread into still is the dominant forest tree species on their Hawaii's natural vegetation, logging, burn wet, windward slopes. On the older islands ing, land-use conversion, and land-abandon (Lanai, Molokai, Oahu, and Kauai), Met ing, the only drastic disturbances. Drastic rosideros polymorpha is also still the dominant natural disturbances, such as volcanic activi tree of the montane rainforest. The species ty, drought, extremely wet periods, infrequent accomplished this enormous ecological suc hurricanes, naturally caused fires, and land cess by its ability to undergo auto-succession slides, are equally important. These natural (i.e., the turnover of many generations of its disturbances have shaped the evolution ofthe own kind). Bog pollen analysis has shown that original Hawaiian vegetation. Because of the it has accomplished autocsuccession for at archipelago's historical isolation, this original least the past 10,000 yr (Selling 1948). Stand vegetation can be characterized as biogeo structure and ecosystem dynamics research graphically impoverished but secondarily en led to the conclusion (Mueller-Dombois 1986, riched by evolution. 1987) that a process ofsynchronized mortality In addition to the taxonomic enrichment ofcohort stands and subsequent rejuvenation by evolution resulting in endemism, "species was responsible for Metrosideros's succes packing" has occurred and continues to occur sional persistence. by the introduction of alien species to the Acacia koa likewise has maintained its Hawaiian Islands. The biogeographic theory dominance on all high islands by auto-succes of MacArthur and Wilson (1967) predicts a sion. Exceptions are where stands have been one-to-one species extinction through species logged and habitats actively converted to invasion to oceanic islands after reaching a alien tree plantations or ranchland. On the certain equilibrium. This theory greatly over latter, Acacia koa stands are still present simplifies the spatial distribution dynamics of today, but many are senescing and dying, vegetation. because their reproductive cycle has been Egler (1947) in his island vegetation studies interrupted by cattle feeding preferentially on preferred the term vegetation change over the Acacia koa seedlings and young suckers. term plant succession, because he felt that Several alien invaders have also become knowledge was insufficient to predict whether dominant species forming naturalized mono a change in island vegetation was progressive cultures. One of the earlier tree invaders or retrogressive. Although this still holds with introduced by the Hawaiians is Aleurites regard to alien species invasion, it is neverthe moluccana. This species became naturalized less possible now to predict some general by invading most of the steep gulches on the trends in the spatial dynamics of succession older high islands from near sea level to patterns. 500 m elevation. From here it did not spread into other habitats most likely because the CHRONOSEQUENTIAL MONOCULTURES. Suc characteristics that make it a superior compet cessful plant invaders often become dominant itor in the ravine bottoms are ineffective in 226 PACIFIC SCIENCE, Volume 46, April 1992 other habitats with different disturbance re Clements 1916) and then in Europe (Tansley gimes. The infrequent, but periodically recur 1929, Liidi 1930). Both these continental areas ring torrential stream activity must allow this are depauperate of tree species, Europe even species to maintain a closed canopy in gulches more so than North America, relative to the by episodic reproduction and fast growth of continental tropics. Yet, progressive forest saplings into the canopy. This trait, however, successions involve typically a chronose did not allow this species to invade forest quence of dominant tree species (e.g., first stands on ridges and spurs, which are often commonly stands ofpine, which then become occupied by Acacia koa, or those on the slopes invaded by hardwoods, such as poplar and next to the gulches, now often occupied by birch, which in turn become invaded by Psidium guajava and other introduced tree spruces and firs). Egler (1954) contributed to species. this picture of"relay floristics" his concept of Another example of a naturalized alien "initial floristic composition" implying that monoculture is the Leucaena leucocephala later successional dominants may already be scrub, which ranked so high in spatial domi present in a latent stage at the beginning of a nance that it became an indicator of a zonal secondary succession. Each of these stages ecosystem (zone B, Table 1). During the mid may form tree associations, in which one or 1980s this species was subject to a severe and the other species is dominant (Curtis and widespread dieback (lkagawa 1987), but it McIntosh 1951, Daubenmire 1968). Thus, the remained in the area. typical temperate forest succession can be According to Wester (in press), Leucaena described as a chronosequential polyculture. leucocephala was introduced before 1837, but It is analogous to the long-standing European it did not become spatialy dominant until the practice of crop rotation, an empirically es 1920s. Egler (1947) pointed out that the bota tablished agro-biological principle that pro nist Forbes did not mention the species as a motes agricultural sustainability. prevailing element of the lowland plant asso Because of the initial biotic simplicity in ciations in a survey done just after 1919, but geographically isolated ecosystems, such as that it was actively spreading into dry grass islands and mountains, auto-successions are land communities in the 1940s. Thus, al rather persistent in such isolated environ though giving the impression of a self-per ments, leading to chronosequential mono petuating monoculture, Leucaena scrub prob cultures. With the evolution of endemic spe ably only dominated as first-generation cies or, much more rapidly, with invasion of stands. The dieback was probably the result alien species, new successionally functional ofa chain reaction ofcauses involving predis species are added. This may lead to the devel position from stand senescence (Mueller opment of chronosequential polycultures. Dombois 1983), drought during the last giant Whether or not this will be so with Leucaena El Nino of 1982-1983 as a dieback trigger leucocephala, now that it has lost its domi (Mueller-Dombois 1986), and the introduc nance, is still open to question. More likely, tion of a psyllid (Heteropsylla cubana) in the dieback stands will become a patch mosaic April-June 1984 (Nakahara and Lai 1984) as oflocally more restricted successional species. an aggravator and hastening cause ofdieback. In the moister segments of Leucaena habitat, Although seedlings were observed under the another alien tree, Schinus terebinthifolius, mature Leucaena canopy (Egler 1947), the seems to assert a new dominance, 'whil~ in succession now seems to lead to different drier habitat segments the tall bunchgrass species becoming dominant after Leucaena Panicum maximum has invaded. In some lo dieback (lkagawa 1987). calities the native woody plants Sida fallax and Dodonaea viscosa have reasserted a cer CHRONOSEQUENTIAL POLYCULTURES. The tain successional dominance in Leucaena die concept of plant succession arose from the back stands on Waialae Iki Ridge (Honolulu). study of temperate zone vegetation, first in Chronosequential polycultures along a gra North America (Cowles 1899, Cooper 1913, dient of soil aging have developed in Hawaii Distributional Dynamics in Hawaiian Vegetation-MUELLER-DoMBOIS 227 in the subalpine environment. Indirect evi sine lessertiana, and Metrosideros polymor dence comes from comparing the subalpine pha, and occasionally two deciduous spe ecosystems on the high Hawaiian mountains. cies, Reynoldsia sandwicensis and Erythrina Metrosideros polymorpha prevails in the sub sandwicensis. These forests have been de alpine environment of Mauna Loa, but So scribed in more detail by Hatheway (1952) phora chrysophylla is slowly increasing in den and Wirawan (1974). Wirawan found them to sity. Sophora chrysophylla is the distinct domi have remained stable 20 yr after Hatheway's nant ofthe tree line ecosystem on the geologi first detailed analysis, but alien tree species cally older Mauna Kea, where Metrosideros such as Schinus terebinthifolius had also re has long since disappeared. Similar observa mained in the absence offurther disturbance. tions can be made in seasonal submontane Fosberg's mixed lowland forest belongs to environments, but not in the rainforest the same dynamic category. He wrote biome, where Metrosideros has persisted but (Fosberg 1972: 32) that "At present this forest appears to have evolved into successional presents an irregular aspect more a mosaic of races (Stemmermann 1983, 1986). small patches of other forests than an inte Chronosequential polycultures have not yet grated entity. However, Pandanus, Eugenia been described for secondary successions in malaccensis, Eugenia cumini, and Hibiscus Hawaii, although the current invasion of the tiliaceus and others" (including Eugenia alien Myrica faya in Hawaii Volcanoes Na jambosa, Psidium guajava, P. cattleianum, tional Park may represent such a case. The Persea americana, Samanea saman, Termi N-fixer Myrica is explosively invading an nalia catappa, Calophyllum inophyllum) "re open Metrosideros forest in the seasonal sub produce themselves effectively and with their montane ecosystem (Vitousek and Walker associates do form a recognizable forest 1989), where it appears to become the new belt." This mixed forest belt has largely dis dominant. Myrica has already formed a placed the former Psidium guajava-dominat monodominant stand of about 1 ha, approx ed forest belt (or zone). Notice that this imating 1% of the area of the seasonal sub displacement of one dominant by several montane ecosystem. In the adjacent sub associates resulted from "species packing," humid segment of closed Metrosideros mostly with alien tree species. Cibotium rainforest, Myrica has become a significant canopy associate. In this subhumid Climax in an Island Context rainforest environment and the seasonal sub montane ecosystem, only a new lava flow may The concept of climax has been much de permit Metrosideros polymorpha to reassume bated in the literature. Egler (1947), from his pioneer dominance, while lesser disturbances experience with island vegetation, rejected the may provide Myrica with a further advantage concept as totally misleading. However, in the to dominate in secondary succession. most recent treatment ofHawaiian vegetation by Gagne and Cuddihy (1990), the climax CHRONOSEQUENTIAL GAP ROTATION. This concept has been resurrecte,d. In the conceptu concept relates to the successional turnover of al statement of their classifying philosophy, tree species in multispecies tropical forests, these authors stated (Gagne and Cuddihy which typically occurs in single tree-fall gaps 1990: 51), "Distinguishing between climax (Brockaw 1985). communities and successional communities This form of small-area successional rota can be difficult. Indeed, one might argue that tion appears to have evolved in native Hawai all of our vegetation is successional, in that ian forests containing several canopy species, our islands are either growing volcanically or such as Fosberg's (1972) mixed mesophytic eroding back to sea level, with profound forest. Canopy trees include at least six ever ecological and vegetational consequences in green species, such as Diospyrosferrea, Acacia either case." This shows that a powerful koa, Eugenia malaccensis (another Hawaiian concept cannot simply be dismissed, and that introduction), Osmanthus sandwicensis, Myr- instead, clarification is needed. 228 PACIFIC SCIENCE, Volume 46, April 1992

CLIMAX AS A PATCH MOSAIC. Whittaker ages in the same rainforest climate. The other (1953) emphasized the need to separate the high islands, in addition, form a continuing concept of climax from its physiographic age sequence, which starts with Hawaii at the connotation and to treat it as a pattern of southeast end of the chain, and continues to population structures. By that he meant that Kauai at the northwest end, where rainforest to be considered climax a vegetation must soils exceed 3 million yr ofage. display a species assemblage that perpetuates Plotting forest biomass and biophilic nutri itself in the same area. The geographic site of ent availability over this substrate age se the area must therefore be large enough to quence (which forms a hypothetical primary allow for the complete growth cycle of all successional sequence) reveals that a physio its component species. If tree species repro graphicjedaphic climax is reached at 1000 to duce and grow under their own canopy or in 3000 yr. This time range marks the mode (i.e., small gaps as is the case in most multispecies the high point or climax) of organic produc tropical forests, community permanency may tion in the Hawaiian rainforest biome. There be maintained over a few tens of hectares is enough evidence to show that a physio under protection and with allowance of a graphicjedaphic climax in the form of a pro generous buffer zone. However, if communi duction mode is followed by a retrogression ties form a patch mosaic consisting of differ of forest site capacity. This retrogression is ent successional stages, for example those due to the constantly high rainfall regime, developing after disturbances, such as fire or which causes cation leaching and, in associa hurricanes, the climax concept must include tion with increasing acidity, aluminum toxici these successional stages, and the frequency ty and, under poor drainage, also iron and and scale of such disturbances must be taken manganese toxicity (Mueller-Dombois 1990). into consideration. In this case the distur Such processes of accelerated soil aging are bance regime would set the spatial limit for probably typical for most humid tropical what may be considered the climax communi environments, although high-stature forest ty. Thus, a climax community may be consid species that could cope withsuch soil nutrient ered a patch mosaic of different dynamic limitations evolved in the continental but not stages that only become constant or stable in the isolated island tropics. at a larger level of the spatial hierarchy, that This process ofcontinuous change or devel of a vegetation zone or biome. opment supports those who have seriously Egler (1947) in his rejection of the climax critiqued the climatic climax concept. It fur concept went even farther. On the island of ther clarifies that it is necessary, when speak Oahu, he considered as relatively permanent ing of climax in vegetation studies, to distin only three broad floristic areas, which he guish between ecosystem development and called strand, xerotropical, and pluviotrop persistence of vegetation patterns or ical. He certainly had a point, because alien communities. species may even change the dominance rela tionships at the most generalized level in Conclusions the vegetation hierarchy. Transgressions of the spatial hierarchy ap THE PHYSIOGRAPHIC CLIMAX. Clements pear to be typical for isolated islands. This is (1916, 1928) had postulated a climatic climax documented by dominant species extending into which all physiographically constrained through more than one ecological zone. Good communities of a climatic zone would con Hawaiian examples are Metrosideros poly verge, given enough time. Ecosystem develop morpha and Acaciakoa. Such spatial trans ment in Hawaii emphasizes that this much gressions also imply larger patterns ofvegeta criticized, but persuasive concept should be tion dynamics than occur in floristically richer interpreted with caution. regions. The maintenance of such larger pat The island of Hawaii displays a series of terns of monodominance is accomplished lava flows and pyroclastic surfaces ofdifferent through auto-succession. Distributional Dynamics in Hawaiian Vegetation-MUELLER-DoMBols 229

Evolution, however, works against auto documented temperate forest successions succession and the maintenance of chronose were either successions following fire or suc quential monocultures. This is indicated in cessions on abandoned fields. Unfortunately Hawaii by different races of Metrosideros in the humid tropics, forest destruction by fire polymorpha participating in the generation often encourages invasion of pyrophytic turnover of these forests in successionally grasses. Where aggressive tropical pioneer older rainforest ecosystems. trees, fast-growing species of Cecropia, Introduction of alien species occasionally Melochia, Trema, and Albizia, are not readily results in the biological invasion ofnew domi available as secondary pioneers, such areas nant species. These seem to behave in ways are converted by frequently recurring fires similar to the indigenous dominants. Good that arrest forest succession or the redevelop examples are Leucaena leucocephala and ment of a chronosequential polyculture. Myricafaya. However, none of these species, including Prosopis chilensis, Psidium guajava, and Psidium cattleianum, have managed to ACKNOWLEDGMENTS extend their ranges over such broad habitat spectra as Metrosideros polymorpha and Aca This paper was prepared for and presented cia koa. Instead, their initial establishment in at the V International Congress ofEcology in the form of naturalized monocultures seems Yokohama with research and travel support to become fragmented, during their genera from NSF Grants BSR-87-18004 and 89 tional turnover, by successional species, re 18382. I thank my wife, Annette Mueller sulting in mosaics of patches. These succes Dombois, for word processing and valuable sional species are often aliens that have been discussions. I also thank Kanehiro Kitayama present in the general area for a long time. and Donald Drake for reviewing the manu They also include indigenous species as long script. as prior physical disturbances (such as habitat conversion and subsequent abandonment) have not eliminated them totally from such LITERATURE CITED areas. In such cases superior dispersal mecha nisms of introduced tree species are often the BRAUN-BLANQUET, J. 1928. Pflanzensoziol decisive factor favoring aliens over natives. ogie. Springer-Verlag, 1st ed., Berlin, Thus, the trends in the distributional dy 1928; 2d ed., Vienna, 1951,631 p.; 3d ed., namics ofthe Hawaiian vegetation seem to go Vienna, New York, 1964, 865 p. from large-area chronosequential monocul BROCKAW, N. V. L. 1985. Treefalls, re tures to mosaics ofpatches, which may persist growth, and community structure. Pages in the form of chronosequential gap rotation 53-69 in S. T. A. Pickett and P. S. involving many canopy species. White, eds. The ecology of natural dis Spatial dynamic patterns seem to exclude turbance and patch dynamics. Academic what was considered to be a typical succession Press, New York. in forest environments ofthe north temperate CLEMENTS, F. E. 1916. Plant succession: An zones (i.e., the chronosequential appearance analysis of the development of vegetation. of dominant tree species forming larger-area Carnegie Institute, Washington. 512 p. successional stages). Such chronosequential ---. 1928. Plant succession and indica polycultures provided the basis for Clements's tors. H. W. Wilson Co., New York. 453 p. succession theory. Although his climatic cli COOPER, W. S. 1913. The climax forest ofIsle max theory was certainly an exaggeration, Royale, Lake Superior, and its develop large-area uniform successional behavior has ment. Bot. Gaz. 55: 1-235. been well documented. COWLES, H. C. 1899. The ecological relations Successional patterns are a function ofboth ofthe vegetation on the sand dunes ofLake the ecological properties of the available spe Michigan. Bot. Gaz. 27: 95-117, 167-202, cies and the disturbance regime. Many of the 281-308,361-391. 230 PACIFIC SCIENCE, Volume 46, April 1992

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