REVIEW Infertile landscapes on an old oceanic island: the biodiversity hotspot of New Caledonia - Oxford ...
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Biological Journal of the Linnean Society, 2021, 133, 317–341. With 7 figures.
REVIEW
Infertile landscapes on an old oceanic island:
the biodiversity hotspot of New Caledonia
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YOHAN PILLON1*, , TANGUY JAFFRÉ2,3, PHILIPPE BIRNBAUM2,3,4, DAVID BRUY2,3,
DOMINIQUE CLUZEL5, MARC DUCOUSSO1, BRUNO FOGLIANI5, THOMAS IBANEZ6,
HERVÉ JOURDAN7, LOUIS LAGARDE8, AUDREY LÉOPOLD4, JÉRÔME MUNZINGER2, ,
ROBIN POUTEAU2, , JENNIFER READ9 and SANDRINE ISNARD2,3
1
LSTM, IRD, INRAE, CIRAD, Institut Agro, Univ Montpellier, Montpellier, France
2
AMAP, Univ Montpellier, IRD, CIRAD, CNRS, INRAE, Montpellier, France
3
AMAP, IRD, CIRAD, Herbier de la Nouvelle-Calédonie, Nouméa, New Caledonia
4
Institut Agronomique Néo-Calédonien (IAC), équipe SolVeg, Nouméa, New Caledonia
5
ISEA, Université de la Nouvelle-Calédonie, Nouméa, New Caledonia
6
Department of Biology, University of Hawai′i at Hilo, Hilo, HI, USA
7
IMBE, Aix Marseille Université, CNRS, IRD, Avignon Université, Nouméa, New Caledonia
8
TROCA, Université de la Nouvelle-Calédonie, Nouméa, New Caledonia
9
School of Biological Sciences, Monash University, Victoria, Australia
Received 7 May 2020; revised 13 August 2020; accepted for publication 22 August 2020
The OCBIL theory comprises a set of hypotheses to comprehend the biota of old, climatically buffered, infertile
landscapes (OCBILs). Here, we review evidence from the literature to evaluate the extent to which this theory
could apply to the biodiversity hotspot of New Caledonia. We present geological, pedological and climatic evidence
suggesting how the island might qualify as an OCBIL. The predictions of OCBIL theory are then reviewed in
the context of New Caledonia. There is evidence for a high rate of micro-endemism, accumulation of relict
lineages, a high incidence of dioecy, myrmecochory and nutritional specializations in plants. New Caledonian
vegetation also exhibits several types of monodominant formations that reveal the importance of disturbances
on the island. Fires and tropical storms are likely to be important factors that contribute to the dynamic of New
Caledonian ecosystems. Although naturally infertile, there is archaeological evidence that humans developed
specific horticultural practices in the ultramafic landscapes of New Caledonia. Further comparisons between
New Caledonia and other areas of the world, such as South Africa and Southwest Australia, are desirable, to
develop the OCBIL theory into a more robust and generalized, testable framework and to determine the most
efficient strategies to preserve their outstanding biodiversity.
ADDITIONAL KEYWORDS: cyclone – dispersal – metal hyperaccumulation – monodominance – mycorrhiza.
INTRODUCTION 2004). Although the smallest in area (~19 000 km 2),
this hotspot hosts a rich flora of > 3400 native vascular
The archipelago of New Caledonia in the Southwest
plants, characterized by a high level of endemism
Pacific is acknowledged to be one of the biodiversity
(74.7%; Morat et al., 2012; Munzinger et al., 2020),
hotspots of the world (Myers et al., 2000; Lowry et al.,
whereas > 70% of the natural vegetation has been
lost (Sloan et al., 2014). The flora has high endemism
at higher taxonomic levels (genus, family and even
*Corresponding author. E-mail: yohan.pillon@ird.fr order), including a wide range of gymnosperms and
© 2020 The Linnean Society of London, Biological Journal of the Linnean Society, 2021, 133, 317–341 317
318 Y. PILLON ET AL.
‘basal’ angiosperms, and has even been treated as a New Caledonian ecosystems. The Loyalty Islands
floristic subkingdom (Takhtajan, 1969). The supposed (offshore islands east of the main island), owing
archaism of the biota has often been interpreted as a to their Pliocene emergence (< 5 Mya; Maurizot &
Gondwanan heritage (Holloway, 1979; Morat, 1993) Campbell, 2020) and mostly calcareous substrate,
after the island drifted away from Australia and the will not be discussed here.
rest of the Gondwana supercontinent at ~80 Mya
(Aronson & Tilton, 1971; Uruski & Wood, 1991;
McLoughlin, 2001). Nevertheless, geological evidence HOW OLD, CLIMATICALLY BUFFERED
points to a likely submersion after the divergence from AND INFERTILE ARE NEW CALEDONIAN
Australia (Pelletier, 2006), which would imply that the LANDSCAPES?
biota of New Caledonia is entirely the result of long-
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distance dispersal (Grandcolas et al., 2008), and the The formation of New Caledonian landscapes
island could thus qualify as a ‘Darwinian island’ (sensu through the Cenozoic
Gillespie & Roderick, 2002). The exact age of the biota Since its oldest geological record during the
is unclear. There is good evidence for submergence Carboniferous to Early Cretaceous (300–95 Mya),
between 75 and 60 Mya and for continuous emergence New Caledonia was almost continuously separated
since 25 Mya, but there is an interval without any from Gondwana and marked by prominent
geological record between 34 and 25 Mya (Maurizot & endemism shared with New Zealand (Maurizot &
Campbell, 2020). In addition, some landmasses might Campbell, 2020). During the mid-Cretaceous, the
have emerged a few hundred kilometres to the west geodynamic setting changed into a passive margin
between 45 and 50 Mya (Sutherland et al., 2020). All extensional regime. Coniacian–Santonian (90–
endemic plant lineages investigated so far have a 84 Mya) rifting and, finally, Palaeocene oceanization
crown age that is no older than 25 Mya (Pillon, 2012; isolated continent-size areas of thinned continental
Nattier et al., 2017), whereas a few arthropod clades crust termed Zealandia (Luyendyk, 1995; Mortimer
are older and some can be considered to be discordant et al., 2017); meanwhile, the Norfolk Ridge was
with the geological history (Giribet & Baker, 2019). totally drowned.
B e s i d e i t s i n t r i g u i n g b i o g e o g r a p h y, N e w A new north-east-dipping subduction started at ~56
Caledonia has also long been considered as unique Mya (Cluzel et al., 2006) and consumed the oceanic
among islands because of its large exposure of crust generated to the east of Norfolk Ridge during
ultramafic substrates that probably had a profound the Late Cretaceous. When the previously thinned
influence in shaping the flora (Virot, 1956; Jaffré, continental crust of the Norfolk Ridge reached the
1980; Isnard et al., 2016). Ultramafic soils offer trench, it emerged temporarily in the fore-arc bulge
challenging conditions to plant growth and are and, finally, jammed the subduction and resulted in
often covered by a distinctive vegetation (Proctor, obduction. Marine sedimentation continued in the
2003; Kazakou et al., 2008). In New Caledonia, south of the island until the latest Eocene (34 Mya;
this includes a diverse sclerophyllous scrub- Cluzel et al., 1998), before it was overthrust by
heath vegetation locally known as ‘maquis minier’ peridotites. Peridotites are upper mantle rocks
or ‘maquis’ (L’Huillier et al., 2010: pp. 65–80) composed mainly of olivine and pyroxene and termed
that resembles the South African fynbos or the ‘ultramafic’ because of their chemical composition:
Southwest Australian kwongan (Read et al., 2007). low in SiO2 (< 45%) and high in FeO + MgO (> 40%).
Inspired by those ecosystems, Hopper (2009) As a consequence, New Caledonia hosts one of the
developed the OCBIL theory to explain the high largest ultramafic terranes in the world, termed
diversity and ecological uniqueness of these very Peridotite Nappe (Avias, 1967), that covers at
old, climatically buffered, infertile landscapes present about one-third of the surface of the island
(OCBILs), as opposed to the young, often disturbed, and its insular extensions (Belep Islands and the
fertile landscapes (YODFELs) that cover most Isle of Pines). However, the occurrence of peridotite
of Europe and North America. In this article, we remnants throughout New Caledonia (Maurizot
aim to address the extent to which the landscapes & Vendé-Leclerc, 2012) signals the existence of a
of New Caledonia might be considered as old, complete ultramafic cover at the time of obduction.
climatically buffered and infertile. We then discuss The exhumation of previously subducted metamorphic
how its unique flora and ecosystems might satisfy rocks started from 44–38 Mya (Spandler et al., 2005),
the seven main OCBIL predictions formulated by and they arrived near the surface at ~34 Mya (Baldwin
Hopper (2009). We then present the special cases of et al., 2007). At present, they form the rugged Mount
monodominant formations that are common on the Panié ridge (1629 m a.s.l.) in the north of the island. It is
island and three major disturbance factors that likely that, in contrast to the rest of the island, the area
might have played an important role in shaping that now corresponds to unroofed metamorphic rocks
© 2020 The Linnean Society of London, Biological Journal of the Linnean Society, 2021, 133, 317–341
OCBILS OF NEW CALEDONIA 319
was uncovered relatively soon from the peridotites; et al., 1974) resulted in the moderate asymmetrical
however, this hypothesis is not yet constrained by data. uplift (~10 m) and westward tilt of southern New
Owing to the absence of sedimentary record, nothing Caledonia from ~200 kya to the present.
is known of the period (Oligocene) that immediately
followed obduction and was probably marked by
fast uplift and intense erosion. In contrast, tropical The low soil fertility of ultramafic and
weathering of serpentinized peridotites in a relatively non-ultramafic substrates
smooth topography and stable environment was New Caledonia has a very wide diversity of soils over
recorded by the development of lateritic profiles and a a relatively small area: 20 main soil types classified
meandering river system before ~25 Mya (Sevin et al., in eight groups according to the World Reference Base
2012). Therefore, the final emergence of Grande Terre (WRB) for soil resources (Fritsch, 2012). This diversity
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is loosely time constrained after 34 Mya (latest marine is related to the various geological substrates, relief,
sediments underneath peridotites) and before 25 Mya and variability of precipitation, which have significant
(erosion of the earliest regolith). impacts on the vertical development of the soils
After the formation of Early Miocene fringing (Tercinier, 1963; Latham et al., 1978; Fritsch, 2012).
coral reefs along the west coast, slab break-related Here, we describe five main types of soils that account
uplift occurred at ~23 Mya (Sevin et al., 2014); for 70% of the main island area (Fig. 2). The remaining
the corresponding erosion created steep slopes 30% occur almost exclusively on the west coast and
and re-incision of meandering valleys. As a result, consist of a wide diversity of soil types, principally
the large klippes of the west coast were isolated Vertic Cambisol, Vertisol and Haplic Fluvisol. Given
from the main peridotite unit, termed Massif du that the natural vegetation almost completely
Sud (Fig. 1). Meanwhile, large volumes of reworked disappeared in these areas owing to human activities,
laterite filled the former topography and created these soils are not discussed here.
the sedimentary plateaus of the Massif du Sud From the peridotite nappe (ultramafic rocks), and
(Folcher et al., 2015). During the Quaternary under a humid tropical climate, two main soil types
glaciation, variations in sea level repeatedly filled have developed. The most widespread type consists
and re-incised the valleys without greatly changing mainly of iron oxy-hydroxide (Becquer et al., 2001);
the bulk landscape; finally, the fore-arc bulge these soils are Ferralsol (ferritic) (IUSS Working
attributable to the east-dipping subduction of the Group, 2006; Table 1). During weathering, especially
Australian Plate in the Vanuatu Trench (Dubois in areas having well-drained topography, silica and
Figure 1. Simplified geological map of New Caledonia.
© 2020 The Linnean Society of London, Biological Journal of the Linnean Society, 2021, 133, 317–341
320 Y. PILLON ET AL.
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Figure 2. Simplified pedological map of New Caledonia.
cations (Mg 2+, Ca 2+ and K +) are exported, resulting (Jaffré & Latham, 1974). In addition to the very low
in the accumulation of ferrous minerals in soils availability of key nutrients, these soils can also
(Trescases, 1969, 1973; Latham, 1986). In addition, contain exceptionally high levels of nickel, cobalt,
these soils do not contain clay minerals; kaolinite is manganese and chromium (e.g. Lee et al., 1977;
notably absent owing to the low aluminum content Latham et al., 1978). However, their bioavailability
in peridotites (Latham et al., 1978; Becquer et al., would depend on the position of soil along the
2001). Consequently, these soils have a very low toposequence and the modifications undergone by
cation exchange capacity (CEC), mainly determined the soils (erosion, colluvium) in addition to the degree
by the soil organic matter (SOM) content. The pH is of hydromorphy, oxydo-reduction and biological
very acidic and also depends on the SOM content. processes (L’Huillier & Edighoffer, 1996; Becquer
Total and available phosphorus concentrations et al., 2001, 2006; Quantin et al., 2002).
are particularly low, and the low availability is The second soil type is developed mainly on
accentuated by the high retention rate attributable peridotite massifs with high slope and serpentinite
to iron oxides (Becquer et al., 2001; Dubus & Becquer, (Trescases, 1969, 1973). These soils are young, with
2001). Total and exchangeable calcium and potassium a low vertical development owing to rejuvenation by
are deficient; magnesium concentrations are low, but mechanical erosion. Clay minerals as smectite suggest
the soil CEC is mainly occupied by this element. In high CEC, but it is clearly saturated by magnesium.
the humic horizon of the soil surface, calcium and Indeed, exchangeable magnesium can occupy > 80%
magnesium may be in equilibrium, but in deeper of the CEC (T. Jaffré, pers. obs. described by Jaffré &
horizons, magnesium is the only exchangeable cation, L’Huillier, 2010): they are Haplic Cambisol (Magnesic;
particularly in alluvial and colluvial soils and in WRB classification) known as brown hypermagnesian
deep soils with an indured or gravelly upper horizon soils (Table 1). Calcium and potassium are deficient,
© 2020 The Linnean Society of London, Biological Journal of the Linnean Society, 2021, 133, 317–341
Table 1. Principal soil types of New Caledonia
Soil type* Surface Main Characteristics Main constraints Fertility Fertility
(%)† substratum† definition† score†
Chemical Physical
Posic 19.9‡ Ultramafic, No clay mineral, domin- Acidic pH, low CEC, phos- – Very low V
Ferralsol peridotites ance of oxy-hydroxide phorus deficiency, imbalance fertility to
(Ferritic) minerals, acidic pH in the Ca:Mg ratio, low infertility
with pH KCl > pH nutrient content, high metal
water, high anion content
retention rate, CEC
related to soil organic
matter content
Haplic 14.0 Ultramafic, High CEC saturated Phosphorus deficiency, strong Low vertical Very low IV
Cambisol serpentinites by Mg, neutral to imbalance in the Ca:Mg ratio, development fertility
(Magnesic) slightly basic pH, rich low nutrient content, high
in 2:1 clays (ferrous metal content
smectite)
Ferralic 26.9 Various CEC saturation < 20%, Acidic pH, high exchangeable Rejuvenation by Low III
Cambisol volcano- Ca, K and P deficient, Al and available Mn, phos- mechanical erosion fertility
(Dystric) sedimentary acidic pH, kaolinite phorus deficiency, low CEC,
origins dominant clay min- desaturation of CEC
eral, but presence of
unprocessed illite and
vermiculite
Haplic 6.2 Metamorphic, Thin and humic surface Phosphorus deficiency Very low vertical Very low IV
Regosol mica-schist horizon, very soft and development; fertility
and gneiss altered ‘C horizon’ rejuvenation by
(> 1 m depth) mechanical erosion
Haplic 3.5 Siliceous rocks, Acidic pH, CEC desat- Very acidic pH, phosphorus Rejuvenation by Very low V
Acrisols notably uration, marked pro- deficiency mechanical erosion; fertility to
(Rhodic phtanites file differentiation, textural discontinuity infertility
and Albic) bleaching of soil adverse to root
© 2020 The Linnean Society of London, Biological Journal of the Linnean Society, 2021, 133, 317–341
associ- surface horizon, illu- penetration
ated with viated and rubefied B
Podzol horizon, kaolinite clay
dominance
Abbreviation: CEC, cation exchange capacity.
*IUSS Working Group, 2006.
OCBILS OF NEW CALEDONIA
†
As a percentage of the surface of the main island; data adapted from Latham et al. (1978).
‡
This area also includes Petroplinthic Plinthosol described by Fritsch (2012).
321
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322 Y. PILLON ET AL.
whereas concentrations of metals such as nickel, cobalt in the Ca:Mg ratio and to a high content of metals,
and manganese are high, especially when the soils particularly nickel, chromium, manganese and cobalt,
receive matter inputs from peridotites. Although their whose bioavailability may be significant. Given all
natural fertility might be considered slightly better these constraints, the natural fertility of the main
than Ferralsols, their low vertical development and types of soils developed in the New Caledonia main
chemical characteristics, particularly the very high island can be considered very low (Table 1).
magnesium content (Magnesic soils with exchangeable
Ca:Mg ratio < 1) and metals, constrain the mineral
nutrition of plants (e.g. Lee et al., 1977). A tropical oceanically buffered climate
On the other major geological substrates, from Normal annual temperature ranges from 19.2 °C in
metamorphic or volcano-sedimentary origin, young soils July–August to 25.9 °C in February. Annual precipitation
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developed in the ‘Chaîne Centrale’ mountain range, ranges from 300 to 4200 mm, with greater precipitation
representing the largest set of soils in terms of land area on the windward east coast (Météo-France, 2007). There
(Table 1). These soils, classified as Ferralic Cambisol are several indications of prolonged buffered climatic
(Dystric), are characterized by very acidic pH and CEC conditions in New Caledonia, suggesting a Pleistocene
saturation < 20%, i.e. oligotrophic or desaturated soils. climatically stable landscape (Mucina & Wardell-Johnson,
Their natural chemical fertility is considered very low 2011). Pollen records collected from Lake Xere Wapo, in the
because of nutrient deficiency (N, Ca, P and K) and south-east of the main island, revealed that the vegetation
very high acidity, leading to high concentrations of alternated between rain forest and maquis over the last
exchangeable aluminum and manganese. Moreover, 120 kya (Stevenson & Hope, 2005). However, in contrast
unprocessed illite- and vermiculite-clay minerals are to the north-east Australian records of Lynch’s Crater
present, suggesting very young soils, rejuvenated by (Kershaw, 1986), rain forest gymnosperms of the genera
intense gully erosion owing to the slopes. On the north- Dacrydium, Podocarpus and Araucaria and rain forest
east coast, soils have developed on acidic metamorphic angiosperms of the families Cunoniaceae and Myrtaceae
rocks: mica-schist and gneiss. These Haplic Regosols, never disappeared in sediment cores, providing evidence
which are found on the highest topography of New for the persistence of a distant source of rain forest tree
Caledonia, i.e. Mount Panié (1629 m a.s.l.), are located pollen over time (Stevenson & Hope, 2005). Based on the
on very steep slopes. They are young soils owing to present-day distribution of Nothofagus in the area and
systematic rejuvenation, but the bedrock is deeply the prevalence of Nothofagus forest at ~30 000 yr BP in
altered. Under a thin and humic surface horizon, a dated sediments from the nearby Lake Suprin (southern
very soft and altered horizon (‘C’) of > 1 m can develop. New Caledonia), Hope & Pask (1998) suggested that
Naturally covered by montane cloud forest, their natural local temperatures might have been cooler by as little as
fertility is considered very low (Table 1). On the opposite 1–5 °C over this period.
slope, on the north-west coast of the main island, soils Further evidence comes from spatially explicit
with a marked profile differentiation have developed: reconstructions of the Late Quaternary climate, which
Haplic Acrisols (Albic and Rhodic) (Table 1). These soils are increasingly available with the development
are acidic and desaturated (saturation rate ≤ 50%); the of general circulation models. Results from these
shallowest horizons may be bleached (Albic), whereas models indicate that the warm and wet climatic
the B horizons may be enriched in illuviated clay and conditions currently found in the archipelago were less
are reddish (rubefied; Rhodic). Their pedogenesis tends dramatically affected than those of Australia and some
towards podzolization, and these soils are associated nearby islands during the Last Glacial Maximum,
with Podzol s.s. developed on phtanites (black cherts) which occurred ~20 000 yr BP (Poncet et al., 2013;
and siliceous alluvium (Latham et al., 1978). Although Pouteau et al., 2015). If one considers macroclimatic
the soil profile is differentiated, these soils are subject shifts since the Last Glacial Maximum together with
to rejuvenation by mechanical erosion. Their chemical current topoclimatic gradients, the dispersal speed
fertility is very low to nil, and clay migration (i.e. needed to track the mean annual change in temperature
illuviation) can create a discontinuity in the soil texture (past climate change velocity) is much higher in most
that can limit or inhibit root penetration. parts of Australia than in New Caledonia (Fig. 3).
The soils mentioned above often have a low The relative climatic stability in New Caledonia is
vertical development, mainly owing to rejuvenation also reflected in forecasts of future climate change.
caused by a marked topography. Strong alteration Recent bioclimatic envelope modelling of species
and weathering have resulted in low chemical distributions has estimated that up to 15% of the 469
quality, i.e. acidic pH, low CEC, desaturation and low most widespread New Caledonian tree species will be
nutrient concentration. In addition, the pedogenesis likely to have no analogue habitat by 2070 and might
of peridotites and serpentinites has led to an excess then become extinct (Pouteau & Birnbaum, 2016). In
in magnesium associated with a high imbalance comparison, using a similar approach based on the
© 2020 The Linnean Society of London, Biological Journal of the Linnean Society, 2021, 133, 317–341OCBILS OF NEW CALEDONIA 323
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Figure 3. Past climate change velocity since the Last Glacial Maximum in the south-eastern Pacific Ocean (data from
Sandel et al., 2011).
same envelope model, general circulation models and than on continents. This is probably all the more the case
scenarios have found that up to 25% of Banksia species for slopes facing trade winds throughout the year (east
(100 species investigated), thought to be representative coast), on which orographic clouds and showers develop
of diversity patterns of plants in general in Southwest more frequently, thus supporting the subsistence of
Australia, are threatened with extinction by 2080, locally cool and wet microrefugia during global glacial
although the region is much larger (Fitzpatrick et al., periods (Irl et al., 2015).
2008). More generally, analyses of mid- (2046–2065)
and long-term (2081–2100) climate projections have
revealed that average temperature increases in coastal NEW CALEDONIA AND THE PREDICTIONS
and continental regions might exceed those of oceanic OF THE OCBIL THEORY
islands by > 2 °C (Harter et al., 2015).
The question then arises as to why the climate has been Reduced dispersability, increased local
endemism and common rarity
relatively stable in New Caledonia since at least the last
glacial period, whereas a significant portion of Australia Dispersal from parental habitat might be a risky
experienced dramatic climate change. One reason is strategy in OCBILs, where reduced dispersability,
certainly the lower diurnal and annual temperature high numbers of localized rare endemics and strong
amplitudes on islands compared with larger land masses population differentiation are therefore expected
because of the buffering effect of the ocean (Leuschner, (Hopper, 2009). Based on observations on the Southwest
1996). In addition, Irl et al. (2016) and Pouteau et al. Australian flora, Hopper (2009) considered that seed
(2018) showed that, contrary to mainland regions, local dispersal assisted by wind or animals, indicated by the
topography prevails over regional factors in shaping presence of wings or fleshy arils, was uncommon in
the elevational distribution of montane ecosystems OCBILs, although this might be less true in the Cape
on relatively small islands. This finding suggests that flora. Carlquist (1974: p. 215) considered that New
historical global climate change is likely to have caused Caledonian gymnosperms are not easily dispersible
less significant upslope shifts of rain forest habitats on and indicated Zygogynum (Carlquist, 1965: p. 245) and
coastal mountains found on oceanically buffered islands Strasburgeria (Carlquist, 1974: p. 481) as examples
© 2020 The Linnean Society of London, Biological Journal of the Linnean Society, 2021, 133, 317–341324 Y. PILLON ET AL.
of loss of dispersibility. However, he considered this landscapes, although dispersal distances may only be
phenomenon as an insular syndrome. a few metres (Orians & Milewski, 2007).
Several censuses of dispersal syndromes have been Models of costs and benefits of dormancy predict
published for the ultramafic flora of New Caledonia, that non-dormant seeds should be favoured under a
covering rain forest (Carpenter et al., 2003; Ibanez scenario of long-term climatic stability (Dayrell et al.,
et al., 2017) or maquis and its interface with rain forest 2017). Accordingly, the ratio of dormant to non-dormant
(Ititiaty et al., 2020). The dominant means of dispersal seeds found for campo rupestre (an OCBIL in Brazil;
were animals (55–72% of the plant species), followed by Hopper et al., 2016) is the lowest for any vegetation
wind (14–23%) and gravity (7–31%). The proportion of type on Earth (62.5% non-dormant; Dayrell et al.,
taxa dispersed by animals increases along the ecological 2017). For 332 ultramafic species examined in New
succession (Ititiaty et al., 2020), with more animal- Caledonia, 38% were confirmed by experimentation as
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dispersed species being found in the maquis–forest non-dormant and a further 20% could be so (already
edge (64.7%) than in maquis (41.8%). The paucity of identified within the same genus), suggesting that up
vertebrates (particularly mammals) in New Caledonia to 50% of the examined species could be non-dormant
probably has an impact on dispersal syndromes and (Ititiaty et al., 2020).
could explain the relatively high representation Micro-endemism is a well-known feature of the New
of wind-dispersed species compared with similar Caledonian flora (Wulff et al., 2013) and fauna (Caesar
vegetation in the world, as in the lowland rain forest et al., 2017). Based on literature and herbarium
of Malaysia, North Queensland and Costa Rica (Webb specimens, Wulff et al. (2013) estimated that 635 of
& Tracey, 1981; Levey et al., 1994; Seidler & Plotkin, the 2930 plant species assessed are narrow endemic
2006). This is especially the case in maquis, which is species, i.e. known from no more than three locations
open vegetation known to contain a larger proportion separated by > 10 km (Fig. 4). Three hundred and
of wind-dispersed species (Duncan & Chapman, 1999). nine species were recorded from a single location.
The patchy distribution of habitats and the rugged The rate of micro-endemism was higher on ultramafic
topography of New Caledonia would, however, suggest substrates, with 228 species. The few population
that these wind-dispersed seeds (with wings, hairs, etc.) genetics studies published on New Caledonian plants
are generally not dispersed over a very long distance. have revealed well-differentiated populations, e.g.
This is further attested by many rain forest species that in Amborella (Poncet et al., 2013), Araucaria (Kettle
exhibit a strong spatial aggregation (Birnbaum et al., et al., 2007; Ruhsam et al., 2016) and Santalum
2015). Long-distance dispersal is probably an uncommon (Bottin et al., 2005). The micro-endemic Scaevola
phenomenon in New Caledonian rain forests and can be coccinea is an extreme case, with four genetic groups
attributed only to Ducula goliath (Barré et al., 2003), within its range of 12 km × 6 km (Wulff et al., 2017),
a large pigeon (600–720 g) that can swallow large although its pollination system is specialized for small
fruits (> 2 cm in diameter) and fruit bats (Muscarella nectarivorous birds (Wulff, 2012).
& Fleming, 2007), which probably swallow only small
seeds (e.g. Ficus). Most small- to medium-size (< 2 cm in
diameter) fleshy fruits are probably dispersed by small Accentuated persistence: old lineages, old
frugivorous birds, but lizards (mostly diplodactylid individuals
geckos and a few skinks) are not insignificant dispersers If natural selection has favoured limited
in New Caledonia (Sadlier et al., 2014), and both are dispersability of sedentary organisms in OCBILs,
mainly short-distance dispersers. elevated persistence of lineages and long-lived
Some seeds might be dispersed over short distances individuals should be expected (Hopper, 2009). New
by invertebrates, because a high level of myrmecochory Caledonia is rich in relict lineages, such as its iconic
(dispersal by ants) was found in the New Caledonian bird, the kagu (Rhynochetos jubatus). New Caledonia
flora, potentially in 8% of plant species, i.e. 300 species, surpasses all other Pacific islands by the number
among which 113 have already been confirmed to of endemic genera of vascular plants, between 62
possess an elaiosome (lipid-rich appendage, which and 91 depending on taxonomic concepts (Pillon
attracts ants). In particular, 72% of the confirmed et al., 2017), and three endemic plant families:
myrmecochorous species are ultramafic obligates, in Oncothecaceae, Phellinaceae and Amborellaceae
both forest and maquis (Le Yannou-Cateine, 2017), in (endemic order Amborellales), the sister group to
line with a high level of myrmecochory observed on the rest of the angiosperms (Qiu et al., 1999). Most
nutrient-poor soils in Australia or South Africa (Westoby of these endemic taxa are monotypic or species poor
et al., 1991; Mucina & Majer, 2012). Elaiosomes are and can often represent relict lineages rather than
less costly dispersal attributes compared with fleshy offspring of adaptive radiation (Pillon et al., 2017).
fruits and might be promoted in infertile or fire-prone Conifers are also particularly diverse, with almost
© 2020 The Linnean Society of London, Biological Journal of the Linnean Society, 2021, 133, 317–341OCBILS OF NEW CALEDONIA 325
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Figure 4. Major climate refugia and centres of endemism in New Caledonia. Blue areas display 3000 mm isohyets, with
high species richness and endemism of palms interpreted as boundaries of possible Pleistocene refugia (Pintaud et al.,
2001). Purple areas are hotspots of plant micro-endemism (Wulff et al., 2013). Red areas have been projected to be suitable
for at least one species of ‘basal’ angiosperms [ANA (Amborellales, Nymphaeales, Austrobaileyales) grade + Magnoliids]
during the Last Glacial Maximum (Pouteau et al., 2015).
half the diversity of the world’s Araucariaceae, growth, e.g. Agathis ovata (> 700 years old, perhaps
in addition to the ANA grade (Amborellales, up to 1500 years old; Fig. 5C; Enright et al., 2003) and
Nymphaeales, Austrobaileyales), Chloranthales and Araucaria goroensis (650 years, as Araucaria muelleri;
Magnoliids (Pouteau et al., 2015). Some of these Enright et al., 2014). Lifespan in many species is
lineages are particularly ancient, e.g. the stem age further increased by resprouting after fire (Jaffré
of Amborella is between 140 and 256 Mya (Poncet et al., 1998b). In contrast, Cerberiopsis candelabra is
et al., 2019), that of the conifer Austrotaxus between a long-lived pioneer monocarpic tree that can reach
90 and 145 Mya (Leslie et al., 2012) and the fern 20–30 m in height and 40–60 cm in diameter (Veillon,
Stromatopteris ~90 Mya (Pryer et al., 2004). They are 1971), whose lifespan is determinated by flowering.
clearly older than the emergence of New Caledonia
and are evidence of the mismatch between the age of
endemics and the age of islands/ecosystems (Pillon The James effect
& Buerki, 2017). The majority of the endemic genera The James effect has been defined as the ‘natural
(59–85) are found on ultramafic substrates, and selection for genetic, cytogenetic, or phenotypic
25–36 of the endemic genera are restricted to them adaptations that conserve heterozygosity in the face
(or almost so), whereas only three to six avoid them of inbreeding due to small population size’ (Hopper,
(Pillon et al., 2017); conifers also have a strong bias 2009). Adaptations to avoid inbreeding in the flora of
for ultramafic substrates (Jaffré et al., 2010). ‘Basal’ New Caledonia might include pollination by strong-
angiosperms, in contrast, seem to prefer the wettest flying animals. In their study of 123 rain forest
areas, also putative climatic refugia (Fig. 4) (Pouteau tree species, on ultramafic substrates, Carpenter
et al., 2015; Trueba et al., 2017). The flora of New et al. (2003) reported that 6% were pollinated by
Caledonia is largely woody, with a limited diversity birds. Kato & Kawakita (2004) found that 12% of
of herbaceous (short-lived) species, besides the 95 indigenous species from diverse vegetation types
families Orchidaceae, Cyperaceae and Poaceae. Some were bird pollinated, a value close to the Southwest
tree species can be particularly long lived, with slow Australian Floristic Region, which has the highest
© 2020 The Linnean Society of London, Biological Journal of the Linnean Society, 2021, 133, 317–341326 Y. PILLON ET AL.
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Figure 5. A, Ixora margaretiae (Rubiaceae), one of 182 monocaulous plant species endemic to New Caledonia. B, Lethedon
leratii (Thymelaeaceae), a dioecious species, like 21% of the New Caledonian native flora (the highest percentage in the
world). C, an example of ‘maquis minier’, the typical shrubby vegetation on ultramafic substrates, here with some centuries-
old trees (Agathis ovata, Araucariaceae). Pictures: A, C by Y. Pillon; B by J. Munzinger.
proportion of plants pollinated by vertebrates in some flowers, such as those of Geissois (Cunoniaceae;
the world (15%; Hopper & Gioia, 2004). There are Hopkins et al., 2015).
six species of honeyeaters in the archipelago (mostly Another way to increase heterozygosity is through
represented by endemic subspecies), and some sexual systems such as dioecy. New Caledonia has
species are very common in the maquis, where the the highest percentage of dioecious species in the
single barred honeyeater (Glycifohia undulata) world (21%; Fig. 5B; Schlessman et al., 2014). It thus
visits ≥ 26 species, including several Proteaceae substantially surpasses other OCBILs: Southwest
(Barré et al., 2010). Other vertebrates could also Australian Floristic Region (4%; McComb, 1966),
be pollinators. Little is known about diplodactylid Gran Sabana of the Pantepui (4.5%; Ramirez, 1993)
geckos and skinks (locally diverse), but they have and Cape Flora (6.6%; Steiner, 1988) have values
been observed visiting flowers, especially in maquis lower than or comparable to the global average
(e.g. Bauer & Sadlier, 2000; Barré et al., 2010), percentage for angiosperms (6%; Renner & Ricklefs,
and some gecko gut contents showed anthers and 1995). Other floras with a high incidence of dioecy
stamens (Bauer & Sadlier, 1994). Bats can also visit are New Zealand (12–13%; Godley, 1979) and
© 2020 The Linnean Society of London, Biological Journal of the Linnean Society, 2021, 133, 317–341OCBILS OF NEW CALEDONIA 327
Hawaii (14.7%; Sakai et al., 1995), both dominated Nutritional and other biological
by YODFELs. Insularity and wet forest habitat are specialization
probably confounding factors that promoted dioecy in To cope with infertile lands, plants in OCBILs are
New Caledonia. expected to display special nutritional and other
biological traits (Hopper, 2009). The flora of New
Caledonia is indeed remarkable in the diversity of
Prolonged speciation at the margins root symbioses represented. Ectomycorrhizal plants
Hopper (2009) predicted recurrent speciation events are particularly diverse (Table 2) and often locally
in semi-arid areas adjacent to the climatically dominant in maquis, rain forest (on ultramafic
stable OCBIL. There is no semi-arid region in New substrates) and savanna; they are associated with
Caledonia that would be comparable to South Africa a high fungal diversity (Carriconde et al., 2019). At
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and Southwest Australia. Nevertheless, the western least nine species of Cyperaceae (in five genera) can
coast of New Caledonia is much drier (< 1200 mm of produce dauciform roots (Lagrange et al., 2011), which
annual precipitation) and once harboured large areas are structures that are involved in a P-acquisition
of tropical dry forest that have largely vanished today strategy based on carboxylate release in some
(Bouchet et al., 1995). This vegetation has many Australian species (Playsted et al., 2006; Shane et al.,
endemic species, but lower endemism rate (65%). It is 2006). Although this family is predominantly non-
lacking unique endemic genera and is depauperate in mycorrhizal, evidence of root colonization by arbuscular
relict lineages including gymnosperms or magnoliids. mycorrhizas has been found in nine species (Lagrange
This flora seems to be derived from the local rain et al., 2011), with at least some cases of infection in
forest lineages (e.g. Diospyros; Paun et al., 2016). the dauciform roots. Cyperaceae are also associated
The biodiverse main island of New Caledonia also with beneficial rhizospheric bacteria (Bourles et al.,
served as a stepping-stone into the Pacific, where 2020) and often dominate the herbaceous stratum
some of its unique lineages dispersed and gave rise to in maquis. The family Proteaceae, which are mostly
endemic species. For instance, peripheral speciation non-mycorrhizal but produce cluster roots to acquire
is observed on the nearby Loyalty Islands, which are phosphorus (Lambers et al., 2011), are species rich
more recent (< 5 Mya) low coralline islands. Their (~50 species, all endemics, in nine genera) and
biota is largely borrowed from the main island, with present in most ecosystems. Ericaceae (particularly
a small number of endemic species (see Schmid, 1969; subfamily Epacridoideae, with 20 endemic species),
Sadlier & Bauer, 1997; Barré et al., 2006 for the flora, with their specific ericoid mycorrhizas, are abundant
herpetofauna and avifauna, respectively). Several in maquis. Although under-represented on the island
lineages also dispersed and speciated on the volcanic (Pillon et al., 2010) and specifically on ultramafic
islands of Vanuatu and Fiji, e.g. Macadamieae (Mast substrates (Pillon et al., 2019), the nitrogen-fixing
et al., 2008), Geissois (Pillon et al., 2014) and Oxera Fabaceae (associated with symbiotic proteobacteria,
(Barrabé et al., 2015). such as Rhizobium) are represented by > 90 species,
Table 2. List of native ectomycorrhizal species from New Caledonia
Family Species Substrate Main ecology
Casuarinaceae Casuarina collina UM + NUM Riparian, coastal, pioneer
Casuarinaceae Casuarina equisetifolia NUM Coastal
Fabaceae Acacia spirorbis UM + NUM Pioneer
Fabaceae Intsia bijuga NUM Forest
Myrtaceae Arillastrum gummiferum UM Rain forest, maquis
Myrtaceae Melaleuca quinquenervia Mostly NUM + UM Savanna, swamp
Myrtaceae Melaleuca spp. (seven species) UM Maquis, riparian
Myrtaceae Sannantha spp. (four species) UM + NUM Maquis, riparian, pioneer
Myrtaceae Tristaniopsis spp. (13 species) UM (rarely NUM*) Maquis, forest
Nothofagaceae Nothofagus spp. (five species) UM (rarely NUM†) Rain forest
Some doubts exist about the possible occurrence of ectomycorrhiza in some Myrtaceae genera: Eugenia, Metrosideros, Syzygium and Xanthostemon.
Abbreviations: NUM, non-ultramafic; UM, ultramafic.
*On siliceous substrate.
†
A single location (Col des Roussettes).
© 2020 The Linnean Society of London, Biological Journal of the Linnean Society, 2021, 133, 317–341328 Y. PILLON ET AL.
but may be ecologically displaced by actinorhizal regarding nutrients and metals, and these different
Casuarinaceae (nitrogen-fixing species associated strategies can evolve relatively quickly (Pillon et al.,
with symbiotic Frankia; Navarro et al., 1999) that 2014). The selective advantages of hyperaccumulation
can be locally dominant. Casuarinaceae are either are uncertain and probably diverse: chemical defense
ectomycorrhizal (genus Casuarina) or form peculiar (Martens & Boyd, 1994; Boyd & Martens, 1998),
mycorrhizal nodules (Gymnostoma), involving disposal (Baker, 1981), drought resistance (Bhatia
Glomus fungi (Duhoux et al., 2001). Interestingly, the et al., 2005), allelopathy (Boyd & Jaffré, 2001) or a
most widespread, ecologically labile and often locally ‘side effect’ of a carboxylate-releasing P-acquisition
dominant legume species is Acacia spirorbis, one of strategy (Lambers et al., 2015; DeGroote et al., 2018).
only two native Fabaceae that are ectomycorrhizal
(Houlès et al., 2018) with a strong ability to fix nitrogen
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(Vincent et al., 2018). The phylogenetic and species Adaptation to saline soils
diversity of Santalales is high on the island: four A diverse halophytic flora is observed on saline
families, 12 genera, 19 species, with a varying degree palaeoriver systems of the Southwest Australian
of parasitism, including two endemic achlorophyllous flat landscapes, but Hopper (2009) expected that
genera (Daenikera and Hachettea). The rain forests this feature would not apply to other OCBILs. New
of New Caledonia are also home to the only known Caledonia has large areas of mangroves, which,
parasitic gymnosperm, Parasitaxus usta (Feild & however, mostly comprise limited assemblages of
Brodribb, 2005). Terrestrial carnivorous plants are widely distributed species, to which the endemic
represented by Drosera neocaledonica and Nepenthes flora makes little, if any, contribution (Munzinger &
vieillardii, mainly on ultramafic substrates. This Lebigre, 2006).
diversity of nutrient acquisition strategies could be
important in maintaining high local species richness
(α-diversity), as suggested elsewhere (Teste et al., Special vulnerability and enhanced resilience
2017; Tedersoo et al., 2020). OCBIL organisms are expected to have an enhanced
Plants in New Caledonia also differ in their ways ability to survive and persist in small, fragmented
of dealing with the high concentrations of metals populations and to be vulnerable to soil removal (Hopper,
(Ni, Co, Mn and Cr) found in ultramafic soils. The 2009). The current fragmentation of New Caledonian
majority of plants are metal excluders, i.e. they forests at low- and mid-elevation principally results
maintain a low and relatively constant concentration from a combination of anthropogenic impacts, very
of metals in their biomass over a wide range of soil intense at low and medium elevations (Jaffré et al.,
concentrations (Baker, 1981). A few plant species 1998a; Ibanez et al., 2017), and natural topographic
and genotypes have, however, evolved towards and edaphic ruptures owing to a sharp relief and a
hyperaccumulation of specific elements, i.e. they complex geology over relatively short distances (Jaffré,
accumulate high concentrations of metals in their 1993). Fragmentation is also an intrinsic feature of tree
shoots (Baker & Brooks, 1989). Hyperaccumulation is a populations in New Caledonia, where rain forest trees
mechanism involving an enhanced rate of loading and showed high spatial species turnover not correlated
translocation and sequestration of metal in the leaves with the geographical distance (Ibanez et al., 2018). On
(Clemens et al., 2002). Current estimates suggest that mountain tops, temperatures are lower, rainfall and
hyperaccumulation occurs in < 0.2% of angiosperms runoff are higher, insolation is reduced, light is enriched
(Cappa & Pilon-Smits, 2014; Reeves et al., 2018) and with ultraviolet-B and relative humidity decreases
in 1–2% of the ultramafic flora (van der Ent et al., above 1200 m a.s.l. (Nasi et al., 2002). At this elevation,
2015). New Caledonia hosts a particularly rich and the climatic conditions prevailing inside and outside the
phylogenetically diverse set of hyperaccumulator forest do not differ significantly. In particular, because
species, second only to Cuba (Reeves et al., 1999), the moisture of the air remains saturated in both
with 99 hyperaccumulators of Ni, 74 of Mn, 34 of Co cases, the vegetation does not undergo water stress.
and ten of Zn (Jaffré et al., 2013; Losfeld et al., 2015; In contrast, the closed canopy of mid-elevation rain
Gei et al., 2020). Hyperaccumulator species occur in forests sharply dampens the atmosphere, by reducing
both maquis and rain forests and are predominantly the temperature and increasing the relative humidity,
endemic to ultramafic soils. The long time of exposure such that the vapour deficit pressure is 0.35 kPa lower
and the extent of ultramafic substrates are considered than outside (P. Birnbaum, unpublished data). Thus, by
to be important factors explaining the high incidence considering only the micro-climate, we can expect a low
of hyperaccumulation in the flora (Isnard et al., 2016), edge effect at low and high rainfall where forests are
as suggested for Cuba (Reeves et al., 1999). Species more structurally heterogeneous (Harper et al., 2005;
within the same genus can have contrasting behaviour Oliveira et al., 2013) and a maximum effect in the
© 2020 The Linnean Society of London, Biological Journal of the Linnean Society, 2021, 133, 317–341OCBILS OF NEW CALEDONIA 329
mid-climatic domain, which also hosts the most and Araucaria spp. (Araucariaceae; Rigg et al., 2010),
diversified forests (Birnbaum et al., 2015; Chao et al., predominantly on, but not limited to, ultramafic soils.
2016). However, fragmentation leads to numerous Perhaps its most striking manifestation is in species-
other effects unrelated to climate (Laurance et al., rich rain forests of the ultramafic massifs of the main
2018), because a lower immigration rate leads to a slow island. Some monodominant rain forests have relatively
shift in tree communities by filtering species according low species richness, but often the dominant species
to their dispersal ability (Halley et al., 2014; Figueiredo forms an uppermost canopy over a species-rich lower
et al., 2019). In addition, the anthropogenic activities, canopy and understorey that is comparable to that of
concentrated at low elevation, induce additive effects, nearby mixed-canopy rain forests (Read et al., 2000;
such as fire and exotic organisms, at the edge of the Demenois et al., 2017). The monodominant species
driest forest (Malcolm et al., 2017; Blanchard et al., that are best documented, Arillastrum gummiferum
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2019). (Myrtaceae; Demenois et al., 2017), Cerberiopsis
On the ultramafic substrates exploited for nickel, candelabra (Read et al., 2008), Nothofagus spp.
the reconstitution of plant cover after mining is (Nothofagaceae; Read et al., 2000; Read & Jaffré, 2013)
difficult and slow (Jaffré et al., 1977). Sensitivity to and Codia mackeeana (Ibanez & Birnbaum, 2014), are
topsoil removal is suggested by the slower growth somewhat diverse in their functional traits. In common,
of plantlets measured in controlled conditions in they appear to be non-persistent (Connell & Lowman,
soils from mine tailings compared with soil taken 1989) or transient (Newbery et al., 2013) dominants,
from pristine maquis (Houlès, 2017: p. 131). There achieving dominance after large-scale disturbances,
are numerous invasive alien plant species in New such as fires and cyclones; their comparatively poor
Caledonia (Meyer et al., 2006). However, they currently capacity to regenerate in the shaded understorey
cover relatively small areas, and many of them appear makes progress to mixed-canopy rain forests likely
to be the passengers of human-induced disturbances in the absence of further disturbance, at least at low
(e.g. roads, agriculture and forestry) rather than the to mid-elevations (Read et al., 2008, 2015; Read &
drivers, because very few alien species are invasive Jaffré, 2013; Ibanez & Birnbaum, 2014; Demenois
in non-anthropogenic habitats, especially those on et al., 2017). Multiple and divergent traits might
ultramafic substrates (with the notable exception contribute to dominance by these species, as suggested
of Pinus caribaea, an ectomycorrhizal tree naturally for persistent monodominance (Torti et al., 2001; Peh
present on these substrates in its native range). This et al., 2011). Unfortunately, the bulk of the literature
mirrors the high resilience of New Caledonian forests, from elsewhere focuses on persistent monodominance
which include a number of ‘aggressive’ pioneer native (but see Newbery et al., 2013); therefore, comparisons
species (e.g. Acacia spirorbis, Codia spp. and Melaleuca with New Caledonia are limited. At least some of those
quinquenervia), and the high biotic resistance of these persistent monodominant forests are potentially in
forests owing to their high diversity, which offers few OCBILs (Woolley et al., 2008; Fonty et al., 2011). The
niche opportunities for alien species. high incidence of fires and cyclones in New Caledonia
(Hope & Pask, 1998; Jaffré et al., 1998b; McCoy
et al., 1999; Read et al., 2011; Demenois et al., 2017)
has probably contributed to the number of taxa of
THE SPECIAL CASE OF MONODOMINANT non-persistent monodominants and the extent of
FORMATIONS monodominant vegetation. Low soil fertility might
A particular feature of New Caledonian landscapes is interact with the disturbance regime to increase the
the extent and diversity of monodominant vegetation. extent of monodominant vegetation, because slow
Monodominance, where one species contributes ≥ 50% growth potentially reduces the rate of successional
to the canopy (Connell & Lowman, 1989), has been change, increasing the likelihood of interruption by
reported in other tropical regions, often over extensive large-scale disturbance (Read et al., 2006a). Equally,
areas (Connell & Lowman, 1989; Hart, 1990; Torti et al., this disturbance regime probably reduces the likelihood
2001; Peh et al., 2011). However, New Caledonia might of persistent monodominance.
be unusual in the number of taxa from diverse families
that form monodominant canopies. Monodominance
occurs in maquis and forest, including by Gymnostoma MAJOR DISTURBANCE AFFECTING NEW
spp. (Casuarinaceae; Jaffré & Latham, 1974; McCoy CALEDONIAN ECOSYSTEMS
et al., 1999; Navarro et al., 1999), Tristaniopsis spp.
(Myrtaceae; Jaffré & Latham, 1974), Codia spp. Cyclones
(Cunoniaceae; Jaffré, 1980; Ibanez & Birnbaum, 2014), Among the regions with significant areas of OCBILs
Acacia spirorbis (Fabaceae; Jaffré & Latham, 1974) identified by Hopper et al. (2016), New Caledonia
© 2020 The Linnean Society of London, Biological Journal of the Linnean Society, 2021, 133, 317–341330 Y. PILLON ET AL.
and Madagascar (and, to a lesser extent, forests According to Whittaker’s (1975) ‘ecosystems
of eastern Australia and the Western Ghats/Sri u n c e r t a i n ’ d e f i n i t i o n , cl i m a t e ( m e a n a n n u a l
Lanka) are affected by recurrent tropical storms temperature and precipitation) can equally support
and cyclones (on average, three to four events per savannas, maquis or forests on ~45% of New Caledonia
year over the last 40 years in New Caledonia). (excluding Loyalty Islands; see Fig. 6). The vast majority
These events bring strong winds (up to 300 km h−1) of these New Caledonian ‘ecosystems uncertain’ are no
and heavy rainfall. Recurrent cyclonic winds are longer covered by forests and constitute ‘black world’,
likely to shape the structure of New Caledonian where savannas and maquis (depending on substrate)
forests, which exhibit high stem density and low are maintained by recurrent fires (Bond, 2005). Most
canopy height (Ibanez et al., 2019b). Although little maquis and savannas are expected to burn at least
is known about potential adaptation of plants to once every 50 years in New Caledonia (Curt et al.,
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cyclone regimes in New Caledonia (e.g. Read et al., 2015), which is frequent enough to maintain these
2011), trees might invest disproportionally in vegetations.
radial growth compared with growth in height, in The areas of savannas and maquis extend far beyond
order to increase mechanical stability (Blanchard the ‘ecosystems uncertain’ climatic envelope and today
et al., 2016). The recurrent convergent evolution of cover ~75% of New Caledonia. The difference between
small, unbranched trees (i.e. monocaulous;, Fig. 5A), the observed vegetation and the potential vegetation
particularly preponderant in forest understorey, has predicted from climate might be explained, in part, by
been attributed, at least in part, to the peculiarity flaws in bioclimatic envelope definitions and global
of forest structure and recurrence of cyclones (Bruy, climatic data sets. However, feedbacks between soil,
2018). Heavy rains brought by tropical storms and vegetation and fire frequency are also very likely
cyclones also affect ecosystems through the effect to play an important role (Bowman & Perry, 2017).
of erosion and nutrient leaching. Leaching can be Low soil fertility and water availability promote
particularly important on burned areas (e.g. Gay- more open vegetation and higher fire frequency,
des-Combes et al., 2017), and most cyclones occur in which in turn promotes runoff and nutrient leaching.
February–March, i.e. a few months after the end of These processes might be particularly important in
the fire season (September–December). extending ‘black world’ outside its climatic envelope
on ultramafic substrates and/or on slopes and ridges
(Morat et al., 1981; McCoy et al., 1999; Ibanez et al.,
Fire 2013a). Whether or not fire has been an evolutionary
New Caledonia, as for most of the regions with force shaping New Caledonian ecosystems
significant OCBIL areas identified by Hopper et al. remains an open question. To date, the longest
(2016), exhibits recurrent fires. The importance of palaeoecological record suggests that > 50 000 years
fires, relative to soil and climate, in shaping plant ago fire contributed to maintenance of diversity in
functional traits and plant communities in these areas New Caledonian landscapes (Stevenson & Hope,
has emerged as one of the most controversial points 2005). Only 13.5% of the species occurring in savannas
in the OCBIL theory (e.g. Mucina & Wardell-Johnson,
2011; Hopper et al., 2016).
Since Melanesian (~3000 years ago) and European
(1774) arrivals, fire has contributed to the conversion of
forests into open vegetation, i.e. mostly into maquis on
ultramafic and siliceous substrates and into savannas
on non-ultramafic substrates (Morat et al., 1981;
Jaffré et al., 1998b). Also, palaeoecological records
(e.g. charcoal) support the view that natural fires
(i.e. initiated by lightning) shaped New Caledonian
landscapes long before human arrival, but that human Figure 6. Distribution and coverage of areas of New Caledonia
arrival coincides with dramatic changes in fire regime falling into the ‘ecosystems uncertain’ (in black) climate envelope
and vegetation (see review by Cabioch et al., 2008). of Whittaker’s (1975) global biome (see Bond, 2005) based on
Today, fire is recognized as one of the major threats to CHELSEA (Climatologies at high resolution for the earth’s land
New Caledonian flora (Virot, 1956; Jaffré et al., 1998a; surface areas) climatology at ~1 km spatial resolution (Karger
Ibanez et al., 2019a). Each year, ~1–2% of the land of et al., 2017). Climate in ‘ecosystems uncertain’ areas can equally
New Caledonia burns, and 68% of the current 502 Red support maquis, savannas or forests. White areas correspond to
Listed plant species are threatened by fire (Endemia & wet forest climate envelope and green areas to the current wet
RLA Flore NC, 2019). forests (main forest massif in 2010; Jaffré et al., 2012).
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