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New Zealand Journal of Botany

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A review of Leptospermum scoparium (Myrtaceae)in New Zealand

J. M. C. Stephens , P. C. Molan & B. D. Clarkson

To cite this article: J. M. C. Stephens , P. C. Molan & B. D. Clarkson (2005) A review ofLeptospermum scoparium (Myrtaceae) in New Zealand, New Zealand Journal of Botany, 43:2,431-449, DOI: 10.1080/0028825X.2005.9512966

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New Zealand Journal of Botany, 2005, Vol. 43: 431–4490028-825X/05/4302-0431 © The Royal Society of New Zealand 2005

431

A review of Leptospermum scoparium (Myrtaceae) in New Zealand

J. M. C. STEPHENS

P. C. MOLANB. D. CLARKSON

Department of Biological SciencesUniversity of WaikatoPrivate Bag 3105Hamilton, New Zealandjms6@waikato.ac.nz

Abstract Information about Leptospermum sco-parium (Myrtaceae), the most widespread and im-portant New Zealand indigenous shrub species, isreviewed. L. scoparium is a variable species, requir-ing more study of the genetically based differencesbetween New Zealand populations and the affinityof these populations to Australian populations andother closely allied Australian species. Improvedunderstanding of the species' variation will assistboth its conservation roles and economic uses, andthe need to sustain genetically distinct varieties isemphasised. Ecologically, the species has a dominantrole in infertile and poorly drained environments,and a wider occurrence as a seral shrub species insuccessions to forest where it may be regarded as awoody weed of pasture or a useful species for ero-sion control, carbon sesquestration, and vegetationrestoration. The main economic products derivedfrom the species are ornamental shrubs, essentialoils, and honey. The species' development as anornamental plant and further definition of the phar-macologically active components are recommendedas priority areas for research.

B04037; Online publication date 5 May 2005Received 16 September 2004; accepted 29 March 2005

Keywords Myrtaceae; Leptospermum scoparium;manuka; tea tree; New Zealand; taxonomy; chemo-taxonomy; ecology; history; ornamental; essentialoils; honey; pharmacology

INTRODUCTION

Information about Leptospermum scoparium isspread throughout much literature. L. scoparium isthe most widespread and important indigenous shrubspecies in New Zealand, and has probably undergonethe most varied development as an economic plant inthe indigenous flora. Bibliographies compiled by Or-win ( 1974) and Williams (1981) provide backgroundmaterial to this review. Whilst all material that refersto L. scoparium in passing has not been included,each section draws upon the principal publications.Chemical control aspects are not included.

The taxonomy, morphology, distribution, habi-tats and plant communities, and other biologicalassociations in which it occurs are considered here,along with its historic and current uses as a sourceof essential oils and honey, and for ornamental shrubdevelopment. The majority of early research consid-ered the ecological position of L. scoparium, eitheras a dominant species where environmental stress isextreme, or as a seral species in disturbed habitatswhere the species was recognised as a significantweed of recently cleared forest for pasture develop-ment. Recognition of the genetic and phenotypicvariation exhibited by the species led to studiesisolating various components of this diversity, rang-ing from morphological to chemotaxonomic treat-ments. Commercial development as an ornamentalshrub and the identification of medicinal essentialoil and honey components have motivated most ofthe recent research, and a collation of this materialis warranted.

We consider it timely to review the biology of L.scoparium as a basis for further economic develop-ment of the species and conservation of geneticvariation.

432 New Zealand Journal of Botany, 2005, Vol. 43

BIOLOGY AND ECOLOGY

Taxonomy, morphology, anatomy, cytologyLeptospermum scoparium J.R. et G.Forst. (manuka,kahikatoa, tea tree, red tea tree) is a member of theMyrtaceae. This family contains at least 133 generaand more than 3800 species, and has evolutionarycentres in Australia, Southeast Asia, and Centraland temperate South America. Myrtaceae are char-acterised by a half-inferior to inferior ovary, usu-ally numerous stamens, entire leaves containing oilglands, internal phloem, and vestured pits on thexylem vessels (Wilson et al. 2001).

Until recently Myrtaceae was divided into twosubfamilies, the capsular Leptospermoideae and thefleshy-fruited Myrtoideae. An extensive review ofthe Myrtaceae inflorescence structure confirmed thisdivision; the Leptospermoideae contained seven al-liances including the Leptospermum alliance, whichwas further subdivided into the Leptospermum andCalothamnus suballiances (Briggs & Johnson 1979).However, cladistic analysis of morphological andanatomical characters concluded that the subfamiliesshould be discarded, as the fleshy-fruited Acmenaalliance did not group within the Myrtoideae sub-family (Johnson & Briggs 1984). A cladistic re-evaluation of non-molecular characters confirmeda high level of homoplasy within Myrtaceae andlimited support for any clade (Wilson et al. 1994).

Molecular analysis placed further doubt on thetraditional taxonomic groupings. Sequences of thechloroplast matK gene analysed in association withnonmolecular data revealed that the Leptospermumalliance was polyphyletic and, thus, an invalid taxo-nomic concept (Wilson et al. 2001). The sequencingof two chloroplast regions for 31 species within theLeptospermum suballiance revealed a monophyleticgrouping of eight genera and the suballiance wasconsidered a valid taxonomic unit (O'Brien et al.2000). However, the same study concluded thatLeptospermum is polyphyletic and should be dividedinto at least four genera: the persistent-fruit group,the East Australian non-persistent-fruit and WestAustralian non-persistent-fruit groups, and Lept-ospermum spinescens separated as a fourth genus.L. scoparium was not included in this analysis butits fruit morphology allies it to the persistent-fruitgroup.

Analysis of leaf anatomy of 40 species of Lept-ospermum showed that L. scoparium has the typi-cal xeromorphic structure of the genus (Johnson1980). The wood anatomy (Johnson 1984; Patel1994) and the pollen morphology (McIntyre 1963)

of L. scoparium also support the species genus clas-sification.

A comprehensive taxonomic revision of the genusLeptospermum listed 79 species (Thompson 1989),which has been increased to 83 with later additions(Dawson 1997a). L. scoparium is one of 13 speciesincluded in the Leptospermum myrtifolium subgroup,the defining characteristics of which are deciduoussepals and persistent strongly wooded fruit-valves(Thompson 1989). The Australian species withinthis sub-group are extremely difficult to define; L.continentale and L. rotundifolium were recentlyelevated by Thompson (1989) from L. scopariumvarieties to species rank. The species L. juniperi-num and L. squarrosum have both been recorded asvarieties ofL. scoparium (Thompson 1989), and theendemic TasmanianL. scoparium var. eximium couldbe considered to warrant species status, displayinglignotuber development which is not found in NewZealand's L. scoparium (Bond et al. 2004).

Initially three species of Leptospermum were re-corded as endemic to New Zealand; the widespreadLeptospermum scoparium and L. ericoides, and L.sinclairii restricted to Great Barrier Island (Allan1961). Revision of Leptospermum led to the transferof L. ericoides to Kunzea as K. ericoides (A.Rich.)J.Thompson (Thompson 1983). L. sinclairii wasincluded in synonomy to this species, and a newname combination of K. sinclairii (Kirk) W.Harriswas later published without supporting material(Connor & Edgar 1987). Accordingly, L. scopariumis now considered to be the only indigenous memberof Leptospermum in New Zealand. The species isnot endemic to New Zealand as indicated by Allan(1961), as it also occurs naturally in mainland Aus-tralia from the southern coast of New South Walesto western Victoria and is widespread in Tasmania(Thompson 1989).

The time of arrival of L. scoparium in New Zea-land is uncertain. Leptospermum pollen has beendated to the Paleocene (Fleming 1975), though therepresentatives in the upper Cretaceous and olderTertiary beds should be interpreted to represent typepollen and not individual species (Couper 1953,1960). Thompson (1989) suggested that Leptosper-mum may have originated in the dry Miocene con-ditions in Australia and that L. scoparium dispersalto New Zealand occurred relatively recently, as thespecies is not a primitive Leptospermum and cannothave been present earlier in New Zealand. War-dle (1991) recorded L. scoparium as the only NewZealand species to release seed overwhelmingly inconcert after fire, a serotinous feature common in the

Stephens et al.—Review of Leptospermum scoparium in NZ 433

Australian flora. Further evidence for the recent evo-lution of the genus Leptospermum is provided by in-complete sterility barriers and the number of putativehybrids in Australia (Thompson 1989). Neverthelessonly three Australian species are tetraploid, indicat-ing that polyploidy has not been a major influence atthe evolutionary centre of the genus (Dawson 1990).A number of defined Leptospermum hybrids exist(Harris 2000), and one wild flowering intergenichybrid has been reported, Kunzea sinclairii × L.scoparium (Harris et al. 1992). Intergenic hybridshave also been produced from controlled crosses,but neither the Kunzea sinclairii × L. scoparium northe Kunzea aff. ericoides × L. scoparium hybridshad flowered after five years (de Lange & Murray2004).

The following description is drawn from thosegiven by Allan ( 1961 ) and Thompson ( 1989). L. sco-parium is a variable shrub or small tree usually about2 m tall but occasionally reaching 4 m or more, anddwarfed in exposed situations. Bark is close andfirm, with young stems bearing a silky pubescencebut soon becoming glabrous. Leaves are highly vari-able both in size and shape, 7-20 × 2-6 mm, rangingfrom broadly elliptical to lanceolate, coriaceous withincurved margins, rigid, acuminate pungent apex,petiole, young leaves glabrous. Flowers are white orrarely pink or red, axillary or occasionally terminalonbranchlets, usually solitary and sessile, 8-12 mmdiam., flowering Oct-Feb. Hypanthium is usuallyglabrous with a distinct pedicel, expanded upper andbroadly turbinate. Sepals are deciduous, oblong tobroadly deltoid. Petals are 5 × 4-7 mm suborbicularand slightly clawed. Stamens occur in bunches of5-7(-9), 2.5-3.5 mm long. Style is inset with a largestigma, often reduced or absent. Ovary is 5-locular,each ovary containing about 100 ovules. Fruits arewoody persistent 5-valved capsules 6-9 mm diam.,distinctly exserted beyond receptacle rim. Matureseeds are 2-3.5 mm long, irregularly narrowly lin-ear-cuneiform or sigmoid, curved, striate.

Leptospermum scoparium is an andromonoeciousspecies; however, the variation in percentage ofperfect flowers is mostly environmentally produced(Primack & Lloyd 1980). Overall control of flower-ing is determined by temperature and day-length.L. scoparium flowering is initially activated by along-day flowering cue, although bud developmentis restrained by cool temperatures throughout winterleading to spring flowering when the temperaturerestraint is lifted (Zieslin & Gottesman 1986).

ThroughoutNewZealandL. scoparium is normallydiploid with 22 chromosomes (Dawson 1987,1990),

but two triploid and one tetraploid cultivars havebeen described (Dawson 1990) and wild aneuploidshave been recorded (P. de Lange pers. comm.).

Intraspecific variationCockayne (1919, p. 73) wrote "Leptospermum sco-parium .…. presents a diversity of forms which areseemingly impossible to classify. Some, it is true, aredistinct races, but most are probably unfixed hybridsbetween races not yet defined by the plant-classi-fier". This statement fairly represents the variabilitydisplayed by this species. In attempts to classify thespecies several wild varieties have been describedin New Zealand. Cheeseman (1925) agreed withCockayne (1919) and listed one species, disputingthe earlier classification of four varieties by Hooker(1867). Allan (1961) described two varieties, men-tioned a further four, and suggested that the formsmay either result from habitat-modification or begenetically determined. The uncertainty regardingthe cause of L. scoparium variability is reinforcedin the genus revision (Thompson 1989). Webb etal. (1988) discussed two varieties of L. scoparium,var. incanum and var. linifolium, and also listed thenaturalised Australian species L. laevigatum as oc-curring in New Zealand.

Leptospermum scoparium var. scoparium waslisted by Allan (1961) to represent the species de-scription and is considered widespread. L. scopariumvar. incanum (Cheeseman 1925; Allan 1961; Webbet al. 1988) has lanceolate-linear leaves c. 8 mmlong, rose-tinted petals, and is common especiallyin the far north of the North Auckland BotanicalDistrict. L. scoparium var. prostratum (Allan 1961)has a prostrate growth form and characteristicallyappears on mountains. L. scoparium var. myrtifolium(Allan 1961) has smaller more ovate recurved leavesand is widespread. L. scoparium var. parvum (Allan1961) is recorded from the Wellington District, andis a small shrub with very small flowers and hairyleathery foliage. L. scoparium var. linifolium (Al-lan 1961) has linear-lanceolate leaves and is alsorecorded as widespread. Webb et al. (1988) placedL. scoparium var. incanum and L. scoparium var.linifolium together.

Variable morphological characteristics inL. sco-parium in New Zealand have been examined. Yinet al. (1984) studied variation of L. scoparium us-ing leaf material from 182 herbarium specimenscovering most of the natural range of the species,a common garden experiment, and a field analysisof natural populations. The herbarium specimensrevealed significant correlations of leaf morphology

434 New Zealand Journal of Botany, 2005, Vol. 43

with latitude, distance from coast, and annual andwinter temperatures. Their common garden experi-ment established that leaf dimensions and plantmorphology had a significant genotypic basis (Yinet al. 1984). Measurement of seven morphologicalcharacteristics of the populations grown in com-mon conditions by Yin et al. (1984) also revealedconsiderable within-population genetic variability(Wilson et al. 1991). Genotypic variation has alsobeen shown for growth form (Harris 1994), leafvariation (Harris 2002), tolerance of soil acidity(Berninger 1992), soil fertility response (Lyon et al.1971), root anatomy (Cook et al. 1980), and freezingresistance (Greer et al. 1991; Decourtye & Harris1992). Genotypic variance within a population ingrowth habit, leaf size, leaf density, and stem andfoliage colour were revealed when a populationwas grown under common conditions by Porter etal. ( 1998). The flowering times within a population,among adjacent populations and geographicallywidely separated populations, and between seasonsare highly variable (Primack 1980). This variabilityalso has a genetic component; both age at first flow-ering and period of flowering differed in a commongarden experiment (Yin et al. 1984).

The ability of the species to respond phenotypi-cally to different environments was shown whenBurrell (1965) transplanted seedlings of L. sco-parium from Central Otago to Dunedin, where theyimmediately produced larger leaves but remainedtypical of the ecotype. Another example of the spe-cies' phenotypic plasticity was provided by Gaynor(1979), who showed that branching height in thefield was correlated with soil depth.

Burrell (1965) noted thatL. scoparium in CentralOtago retained intact capsules until opening wasinduced by either drought or fire, and later studieshave shown that the rate of capsule splitting differsbetween populations. Genetic control of capsulesplitting was confirmed in a common garden experi-ment, and it was hypothesised that the differencebetween New Zealand populations had arisen fromrapid selection by regular fire disturbance since thearrival of people (Harris 2002). However, a SouthIsland field study showed that population differencesof capsule splitting related to a much longer historyof fire exposure in the regions displaying serotiny(Bond et al. 2004).

Distribution, habitats, and plant associationsAlthough the time of arrival of L. scoparium in NewZealand is uncertain, current opinion suggests arelatively recent dispersal from Australia (Thompson

1989). The distribution within New Zealand wouldhave been restricted until the land clearance associ-ated with human settlement vastly increased the areaof low-nutrient environments to which the specieswas adapted in Australia (Thompson 1989).

Leptospermum scoparium has two main eco-logical roles in New Zealand vegetation: permanentdominance of extreme environments or as a seralspecies (Burrows 1973; Wardle 1991). Permanentdominance occurs on sites that are unfavourable forthe development of climax forest as they are too wet,dry, cold, exposed, infertile, or unstable. The seralrole is on disturbed sites, where L. scoparium is anearly woody species in the succession to forest. Thisrole has been greatly extended by human disturbance(Molloy 1975; Wardle 1991).

Five vegetation communities, the Northland gum-lands, Waikato wetlands, East Coast regenerativeseral scrub, North Island Volcanic Plateau heath-lands, and Westland pakihi swamps, which containL. scoparium as a major component and occupy largeareas, are considered in detail. The common woodydicotyledonous members of these L. scoparium-dominated communities are listed in Table 1.

Soils too wet and infertile for the establishmentof climax forest are widespread throughout NewZealand, ranging from gumland in Northland tomire in Southland, upon all of which L. scopariumdominates (Burrows et al. 1979). The Northlandgumlands are typically leached infertile clays withperched water tables and sand podzols sustainingL. scoparium heathland (Esler & Rumball 1975;Beever 1988; Enright 1989; Wardle 1991). Whilstmuch of this land has been cleared and drained forfarmland, significant remnants remain. The 16000year old Ahipara plateau (Wardle 1991) and theNgarura swamp in the Waipoua forest (Burns &Leathwick 1996) are examples of self-maintainingL. scoparium heathland in this region. Studies of thefar north (Enright 1989), Waipoua Forest heathlands(Burns & Leathwick 1996), and heath near Kaikohe(Esler & Rumball 1975) listed 10 woody dicotyle-donous species in two or more reports. Two of these10 species are invasive introductions, Hakea sericeaand Ulex europaeus; the remaining eight form theendemic community in this environment (Table 1).

Waikato oligotrophic lowland mires exhibit arange of infertility yet all support permanent L.scoparium populations (Burrows et al. 1979; War-dle 1991). A comparison of three of these Waikatoenvironments, the extreme Kopuatai bog (Irving etal. 1984), the intermediary Moanatuatua bog (Bur-rows et al. 1979; Clarkson 1997), and the relatively

Stephens et al.—Review of Leptospermum scoparium in NZ 435

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more fertile Whangamarino fen (Clarkson 1997),reveals five other woody dicotyledons present withL. scoparium. Of these, the endemic species Epacrispauciflora was noted in every report and the intro-duced Salix cinerea in two. Erica lusitanica occurredonly at Kopuatai, but was included because of itsinvasive abilities (Table 1).

Leptospermum scoparium is prevalent on infer-tile leached Westland pakihi soils, and a numberof widespread communities have been studied inWestland in the northern area (Rigg 1962; Burrowset al. 1979; Norton 1989), the central area (Bur-rows et al. 1979), and the southern reaches (Mark &Smith 1975). The communities differ according tolatitude yet a common theme is found. One woodydicotyledonous species was always recorded withL.scoparium in the northern area, five species in thesouthern study bordering with forest, and in centralWestland the reported species were the same as thosefound in both the other areas. Thus, a typical vegeta-tion community of Westland pakihi contains only 4of the 19 endemic species found in association withL. scoparium, and the exceptionally invasive Ulexeuropaeus was included despite being recorded onbetter-drained ridges in one report (Table 1). How-ever, L. scoparium dominance may be replaced bylarger forest species in the pakihi areas provided fireis infrequent and the environment is not exception-ally infertile (Williams et al. 1990).

In south Westland (Wardle 1974) and Fiordland(Wardle et al. 1973) lowland swamps are primehabitats, and montane raised mires in Fiordland(Burrows & Dobson 1972; Mark et al. 1979) andOtago (Johnson et al. 1977) also carry L. scoparium.Following fire in a Southland bog, L. scopariumdominated the environment rapidly (Johnson 2001).In association with swamp-like environments L.scoparium dominates lake shorelines around thesouthern lakes where it survives temporary submer-gence (Johnson 1972; Mark et al. 1977; Robertsonet al. 1991). In these conditions the species differ-entiates specialised aeration tissue, aerenchyma, insubmerged roots allowing long-term dominance inwaterlogged environments (Cook et al. 1980).

Areas too high and cold for the establishmentof climax forest occur in both main islands (War-dle 1991). L. scoparium occurs above the tree line(Wardle 1963; Gibbs 1966) and on upland peat andgley soils of both main islands (Burrows et al. 1979),and is frequent on inhospitable sites at low and highaltitude in south Westland (Burrows 1964; Wardle1977) and Southland (Burrows 1964; Burrows et al.1979), and in montane scrubland on Stewart Island

436 New Zealand Journal of Botany, 2005, Vol. 43

(Wells & Mark 1966). However, at high altitude inOtago growth is limited to warmer microclimates(Wilson et al. 1989). Comparison of the five studiesof montane flora in the central North Island, the Ran-gipo Depression (Rogers & Leathwick 1994) and thevolcanic slopes of Mt Tongariro (Atkinson 1981) athigh altitude, and at lower altitude the widespreadignimbrite pumice plain near Tokoroa (McQueen1961), the Waipapa Ecological Area (Leathwick1987), and Pureora mountain mires (Clarkson 1984),demonstrates greater diversity as forest species re-turn to more hospitable environments. Four endemicwoody dicotyledonous species are noted as commonthroughout the region, and the invasive heathers Cal-luna vulgaris and Erica lusitanica were recorded atthe higher and lower sites, respectively (Table 1).

In coastal environments throughout New ZealandL. scoparium andK. ericoides are found on areas tooexposed for forest (Morton & Miller 1968; Molloy1975). L. scoparium occurs on sites as diverse as theedge of mangrove swamps in the Auckland region(Wardle 1991), the Cape Reinga district in the farnorth (Wheeler 1963), Farewell Spit in Nelson (Bur-rows 1973), and the coastal cliff zones around south-ern Wairarapa and Wellington (Burrows 1973).

Areas too infertile for the establishment of forestoverlap with the above categories, as the environ-ments are the same. Oligotrophic mires and swamps,extreme coastal and altitudinal sites, and heavilyleached soils have all been discussed. L. scopariumis also present in other situations: the geothermicheated environments of the central North Island(Wells & Whitton 1966; Given 1980), edaphicallydry pumice in the central North Island (Elder 1962),and as a consistent understorey on poor gleyed soilsin forested areas (Burrows 1973). L. scoparium isalso tolerant of South Island ultramafic soils (Lyonet al. 1971; Lee et al. 1975, 1983; Lee 1992).

Natural unstable environments also lend them-selves to colonisation by L. scoparium. Landslidesin Fiordland forests are rapidly covered by L. sco-parium in a seral role (Mark et al. 1964). L. sco-parium establishes on braided river beds (Burrows1973) and unconsolidated coastal deposits where itis a woody pioneer (Wardle 1991).

Soils too dry for climax forest vegetation presenta more complicated picture. Typically these areasoccur in the eastern rain shadow of the New Zea-land mountain ranges (Wardle 1991). L. scopariumoccurs as a dominant species in relatively higherrainfall areas of these regions, but becomes uncom-mon in dryer situations where Kunzea ericoidesdominates (Wardle 1971, 1991). In Otago where

yearly rainfall is less than 650 mm, K. ericoidesis more common, interspersed with L. scopariumon boggy land (Burrell 1965). Self perpetuationof K. ericoides/L. scoparium scrub occurs wherebroadleaf forest establishment is either prevented(Wardle 2001) or retarded by site conditions (Dob-son 1979).

In contrast to the permanent L. scoparium popu-lations, seral communities also exist and form asignificant proportion of the species' modern range.L. scoparium is found in moist forested regionsforming similarly aged stands in a nursery role forclimax vegetation following fire or other disturbance(Burrows 1973; Payton et al. 1984) where it maypersist for more than a century (Mark et al. 1989).The species also establishes easily in open under-grazed pasture (Grant 1967), and its presence inthis situation indicates unsustainable clearance offorest or scrub to establish pasture (Bascand 1973).L. scoparium may be the initial woody pioneer onmoderately fertile well-drained soil due to prodi-gious seed set and rapid germination and growth(Mohan et al. 1984a,b). The species has an over-riding germination response to full light spectracoupled with an inhibition by far-red wavelengthstypical of pioneer species on disturbed sites (Herronet al. 2000; McKay et al. 2002). L. scoparium seeddoes not exhibit dormancy, and the unshed seed incapsules is probably the main reservoir of seed as thesoil seed bank is non-persistent (Mohan et al. 1984a).Accordingly, L. scoparium scrub regeneration andre-establishment, which has been a feature of NewZealand hill farming, can be avoided with suitableland management such as the fertilisation and reten-tion of a heavy sward (Levy 1970). Communities ofL. scoparium are not permanent in regions wherethe rainfall is adequate to allow the establishment ofclimax broadleaved forest (Esler & Astridge 1974;Wardle 1991), and replacement by K. ericoides andsubsequent establishment of forest has been recordedin Canterbury and Otago (Burrows 1961 ; Molloy &Ives 1972; Dobson 1979; Allen et al. 1992), KapitiIsland (Esler 1967), and the Hauraki Gulf islands(Atkinson 1954; Bellingham 1955; Esler 1978). TheEast Coast region of the North Island provides anexample of these communities where large areas ofcoastal and lowland hillsides are covered with denseseral scrub, established since forest clearance forpasture development. Three types of L. scopariumscrub were described in the Motu Ecological Dis-trict by Clarkson et al. (1986): L. scoparium, L.scoparium/Coprosma spp./Hebe spp., and L. sco-parium/Kunzea ericoides scrub. Generally the same

Stephens et al.—Review of Leptospermum scoparium in NZ 437

situation exists around the East Cape (Regnier etal. 1988; Clarkson & Clarkson 1991; Whaley etal. 2001). Ten endemic woody dicotyledons wererecorded in two or more studies (Table 1). Most ofthese species are associated with regenerating for-est expected in a seral L. scoparium environment.Whilst Ulex europaeus was only recorded in onestudy, distribution is widespread and this species isaccordingly included.

Where forest on steep slopes has been clearedfor pasture establishment the land is often prone toerosion. The value of L. scoparium as protectivescrub is now recognised, as it provides rapid (Smaleet al. 1997) and excellent protection from shallowlandslides (Watson & O'Loughlin 1985), and thepresence of mature stands assists erosion control(Bergin et al. 1995). L. scoparium foliage can inter-cept a significant amount of rainfall (Burke 1981),as much as 40-50% in a storm event (Aldridge &Jackson 1968). Together with soil binding by rootsthis rainfall interruption is effective in erosion con-trol. Carbon accumulation by L. scoparium is rapidand similar to that of plantation forestry (Scott et al.2000).

The variety of environments in which L. sco-parium occurs indicates the species' wide ecologicalamplitude, and a large population of any one en-demic woody dicotyledonous species is not found inassociation with L. scoparium throughout its entirerange. Epacris pauciflora and Dracophyllum spp. arecommon on the infertile lowlands, the seral shrubcommunity contains a selection of early successionalforest species, and the montane environments carrya mixture of Dracophyllum spp. and hardier forestspecies.

The same situation is seen with the introducedshrubs. Ulex europaeus, Cytisus scoparius, and Te-line monspessulana are widespread throughout NewZealand, particularly in scrubland on low fertilityhill country, yet these species do not inhabit poorlydrained environments in appreciable numbers (Royet al. 1998). Hakea sericea and H. salicifolia areboth common in wet lowland environments as farsouth as northern Westland (Roy et al. 1998), butneither is found on the East Coast. The introducedEricaceae species remain locally distributed apartfrom Erica lusitanica, which is now widespread onlow-fertility wet soils throughout New Zealand (Royet al. 1998).

A number of direct plant associations also occurwithL. scoparium. The rare non-green orchid Gas-trodia minor shares mycorrhizae with L. scopariumalong with other plant species and is distributed

throughout New Zealand (Wardle 1991). L. sco-parium acts as a host for the widely distributedlarge-leafed mistletoe Ileostylus micranthus (Molloy1975), butI. micranthus exhibits low host specificityand is most frequently found in association with Co-prosma spp. (Patel 1991). The widespread parasiticdwarf leafless mistletoe Korthalsella salicornioidesattaches preferentially to Leptospermum and Kunzea(Stevenson 1934), but is also found with other en-demic species and has been recorded in associationwith introduced Erica spp. (Bannister 1989).

INSECT ASSOCIATIONS

Insect associations may be divided into two sec-tions: the species involved in the pollination of L.scoparium and the insect pests.

PollinationMuch of New Zealand's insect-pollinated flora hasinconspicuously coloured flowers, which has beenhistorically attributed to the lack of specific insectassociations (Godley 1979; Lloyd 1985; Wardle1991). The small white flowers of L. scoparium areclassified as open-access with a dish/bowl shape and,typical of this type, are visited by a range of insectpollinators (Newstrom & Robertson 2005). Heine(1937) recorded representatives from the ordersColeoptera and Diptera. A detailed study of montaneL. scoparium visitors revealed a range of insects ar-riving in a structured pattern (Primack 1978). Openflowers were visited by large tachinid and calliphoridflies at dawn, followed by a great variety of smallDiptera with increasing temperature. In fine weatherindigenous Hymenoptera visited flowers from midmorning. The bees and flies ended visits in the lateafternoon, and in the early evening in settled weathermoths (Pyralidae, Geometridae, Noctuidae) andcraneflies (Tipulidae) were recorded. Nocturnalmoth visits have been noted (Newstrom & Robertson2005). The introduced honey-bee (Apis mellifera)also collects both pollen and nectar (Butz Huryn1995). These observations confirm the non-specificpollinators associated with L. scoparium.

Insect pestsThe principal insect pests associated with L. sco-parium in New Zealand are the scale insects, orderHomoptera. Of the 17 species listed by Hoy (1961),most are distributed throughout New Zealand atlow levels of infestation. The three most commonlyfound species are the endemic Coelostomidia wai-

438 New Zealand Journal of Botany, 2005, Vol. 43

roensis and introduced Eriococcus orariensis andE. leptospermi. The condition commonly describedas manuka blight is associated with infestation bythe introduced insect species and the developmentof a covering of sooty mould on the resultant hon-eydew.

Coelostomidia wairoensis is distributed through-out the North Island and the northern and easternSouth Island and is associated with Capnodiumelegans, one of the fungi that produce the visuallydiagnostic sooty mould on the stems of infestedplants. C. wairoensis has not been reported to killL. scoparium (Hoy 1961). Eriococcus orariensis,also associated with Capnodium spp., is commonthroughout New Zealand but absent in wetter regionsand the sub-alpine belt (Wardle 1991). E. orariensiswas introduced involuntarily from Australia in themid 20th century, where it does not cause widespreaddeath of the principal host species Leptospermumjuniperinum in the southern and eastern areas ofthe mainland or L. scoparium in Tasmania. Onceintroduced into New Zealand it was deliberatelyspread and brought about a rapid eradication oflarge areas of L. scoparium. The removal of plantnutrients by the scale insect weakens the plants sothat they are unable to survive environmental stress,and reduced photosynthetic efficiency may resultfrom the covering of foliage by the sooty mould(Hoy 1961). The virulence of E. orariensis has beensignificantly reduced by the subsequent spread of theentomogenous fungus Myriangium thwaitesii, andthe revival of L. scoparium has been as spectacularas the initial decline (Hoy 1961). However, there isno record of resistant forms of L. scoparium in theliterature. This disease complex does not affect theextensive cultivar plantings in the British Isles, as theprimary pest Eriococcus orariensis has not been in-troduced (Dawson 1997b). Eriococcus leptospermi,often found together with E. orariensis throughoutNew Zealand, inhabits the bark surface towards thestem tips. Heavy infestation does not lead to plantdeath and E. leptospermi appears to be immune toM. thwaitessi (Hoy 1961).

Manuka beetles (Pyronota spp.) are also wide-spread, often found on light sandy montane soilsassociated with grassland on forest margins. Thesespecies appear to be non-specific feeders, oftenpreferring grass root material to L. scoparium(Thomson et al. 1979), and may have a role in pol-lination (Heine 1937). The leaf-feeding manukamoth (Declanafloccose) is also common, and otherwidespread insect pests are webworm (Heliostibesatychioides), the wood-boring larvae of the longhorn

beetle (Ochrocudus huttoni) and the gall-formingmite (Aceria manukae) (Molloy 1975), and the intro-duced wood-borer (Amasa truncates) (Brockerhoff& Bain 2000).

FUNGAL ASSOCIATIONS

Many fungi have been noted in association withL. scoparium; but published records are far fromcomprehensive. A search of the New Zealand Fun-gal Herbarium database (www.landcareresearch.co.nz/research/biodiversity/fungiprog/) revealed 699L. scoparium-hosted specimens. Ascomycota arerepresented by 15 orders comprising 71 genera andspecies; Basidiomycota 9 orders and 195 genera andspecies; Deuteromycotinaby 21 Hyphomycetes and8 Coelomycetes. Details of their ranges throughoutNew Zealand are not complete.

Leptospermum scoparium ectomycorrhizae andendomycorrhizae (vesicular-arbuscular) are fre-quent yet the number of partners is unknown, andendomycorrhizae infection appears to be more com-mon (Moyersoen & Fittler 1999). Although Hawks-worth et al. (1995) identified the order Glomales(Ascomycota) as the most common endomycorrhizalsymbiont, the herbarium collection does not includeany specimen from this order, probably due to thedifficulties associated with classification and labora-tory growth. However, Baylis (1971) successfullyinfected L. scoparium with an endomycorrhiza inlaboratory conditions, and in a study of five SouthIsland sites dominated by Nothofagus, Pinus ra-diata, or podocarp/broad-leaved forest, 5 of the 12endomycorrhizal symbionts described were foundin association with L. scoparium, whereas only10—36% of the infections were ectomycorrhiza atfour of these sites (Cooper 1976). L. scoparium isone of the principal ectomycorrhizal hosts in NewZealand's endemic flora, with Kunzea ericoides andNothofagus spp. (Hall et al. 1998). L. scopariumectomycorrhizal infection appears to be determinedby the presence of the appropriate inoculum andalternative host plant species, particularly Nothofa-gus spp. (Moyersoen & Fitter 1999). The ectomy-corrhizal species recorded in New Zealand haverecently been reviewed, listing 22 Basidiomycotaand 6 Ascomycota families in association with L.scoparium (Orlovich & Cairney 2004). The invasivebasidiomycete Amanita muscaria, often found withL. scoparium (Ridley 1991), is considered able todisplace the native species (Orlovich & Cairney2004).

Stephens et al.—Review of Leptospermum scoparium in NZ 439

The principal role of L. scoparium mycorrhizalpartners is the improvement of phosphorus uptake(Baylis 1971; Johnson 1976; Hall 1977) allowingrapid growth and exploitation of available light(Wardle 1991). The level of mycorrhizal infectioncorrelates with available phosphorus and growthconditions (Baylis 1975; Cooper 1975; Hall 1975).The development of L. scoparium ectomycorrhizaemay also facilitate the growth and succession ofNothofagus spp. seedlings (Baylis 1980).

A narrow but characteristic range of L. scopariumendophytic fungi was reported from a study in theAuckland province (Johnston 1998). Phyllostictaspp., in association with Diploceras leptospermi andCoelomycete, dominated natural populations, witha range of other species present in insignificant andvariable proportions. The opportunist endophytesBotryosphaeria and Alternaria sp. were prominent inplanted L. scoparium stands, indicative of a host un-der environmental pressure (Johnston 1998). Withinnatural sites neither the species diversity nor the vari-ability of infection rate could be explained by anyobvious correlation with plant age or any environ-mental factor (Johnston 1998). Phyllosticta specieswere specific to L. scoparium and not recorded onKunzea ericoides, yet D. leptospermi was presenton both species (Johnston 1998) in contradiction toan earlier report (Bagnall & Sheridan 1972). Thefungal species' pathogenicity is unknown though sixof the families represented grow as epiphytes and areassociated with leaf wounds (Johnston 1998). Cap-nodium spp. are also associated with L. scopariumfoliage and bark, but as a result of Eriococcus spp.invasion (Hoy 1961).

TRADITIONAL AND HISTORIC USES OFLEPTOSPERMUM SCOPARIUM

Traditional usesSix entries for L. scoparium are listed in the diction-ary of Maori plant names, manuka and kahikatoabeing the most common (Beever 1991). Manukais most frequently used throughout New Zealand,and kahikatoa is common in Northland. The wordkahikatoa also translates as a weapon made of L.scoparium (Williams 1975), and the plant name maybe derived from this association. An alternative sug-gestion is that manuka was used as a generic namefor the two common seral Myrtaceae species in NewZealand, the names kahikatoa and kanuka represent-ing L. scoparium and Kunzea ericoides, respectively

(T. Roapers. comm.). The common names of tea treeand red tea tree are explained by use of the leavesfor a tea and the red colour of the wood. The Maoriand common names for K. ericoides are manuka,kanuka, tea tree, and white tea tree, again indicatingbeverage use and wood colour, and often leading tounderstandable confusion and misidentification. Ti-tree is an incorrect name for both species, and refersto species of Cordyline (Brooker et al. 1987).

Maori usedL. scoparium for food, medicine, andtimber. Pia manuka, the sugary gum found occasion-ally on young branches, was considered a delicacyand given to infants, or was used to alleviate coughsin adults (Crowe 1981). Brooker et al. (1987) listeda number of traditional medicinal uses. A decoctionof leaves was taken, applied as a salve, directlychewed, or the vapours inhaled. The bark was usedin a similar way to alleviate bronchial complaints.The tough wood was harvested for implement mak-ing, and a review of museum artefacts illustratedseven tools made from the plant's timber (Cooper& Cambie 1991).

The first recorded European use was during JamesCook's voyages, whenL. scoparium leaves were ini-tially used as a tea substitute, and later employed asan antiscorbutic in brewing beer (Cooper & Cambie1991 ). Whalers continued to rely upon L. scopariumas a tea substitute (Brooker et al. 1987), giving rise totea tree as a common name, and early settlers becameso attached to the concoction that the importationof Chinese tea was considered unnecessary by oneauthor (Crowe 1981). L. scoparium has continuedto be valued for firewood and charcoal, and is oftenused for smoking fish.

Ornamental useLeptospermum is a genus of ornamental worthiness,and has been cultivated since its introduction to Eu-rope. The greatest numbers of cultivars have beenbred fromL. scoparium. Approximately 150 namedcultivars have been derived from L. scoparium,whilst the balance of the genus is represented byabout 20-30 cultivars (Dawson 1997a).

Material collected during James Cook's first voy-age of discovery included both L. scoparium and K.ericoides, both of which were incorrectly assignedto the genus Philadelphus in the unpublished Primi-tae Florae Novae Zelandiae, prepared by Solander(Harris 2001). Three species were listed as growingat Kew Gardens by 1789, and prior to this four spe-cies of greenhouse Philadelphus were offered to thepublic in the late 1770s (Cooper & Cambie 1991).The specimens were subsequently reclassified cor-

440 New Zealand Journal of Botany, 2005, Vol. 43

rectly as either L. scoparium orK. ericoides (Harris2001). L. scoparium was first described in 1776 frommaterial collected from Dusky Sound, Fiordland, byJ. R. & G. Forster during Cook's second voyage. By1896 L. scoparium was acclimatised in Cornwall anddescribed as a favourite conservatory plant (Cooper& Cambie 1991).

The discovery and use of rare wild variants hasenhanced the range of L. scoparium cultivars avail-able. Outstanding single white- or pink-floweredspecimens have been identified in the wild, and anumber of double white- or pink-flowered plants dis-covered and propagated throughout the 20th century.A red-flowered plant was found in the wild twice,leading to the release of another set of cultivars.Wild prostrate forms have also been used (Dawson1997a).

These unusual wild plants have been developedby deliberate hybridisation, and Lammerts (1945)pioneered controlled breeding in California. Sub-sequent horticulturists have increased the range ofL. scoparium cultivars, notably E. F. Jenkins andSons (Victoria, Australia), J. Hobbs (Auckland, NewZealand), Duncan and Davies (New Plymouth, NewZealand), and G. Hutchins (Essex, England) (Daw-son 1997b). This development allowed Dawson(1997b) to list 23 outstanding named L. scopariumcultivars. More recently, Harris (2000) recorded thedevelopment of four named inter-specific cultivars,all having L. scoparium as one parent and one ofthe Australian species L. rupestre, L. spectabile, orL. polygalifolium as the other, and a L. rotundifo-lium × L. scoparium hybrid has been bred (Bicknell1995).

Studies have also been completed to improve thehorticultural qualities of L. scoparium. Pot plantshave been developed (Bicknell 1985), cut flower life(Bicknell 1995) and flowering cue (Zieslin & Gottes-man 1986) investigated, frost hardiness considered(Greer et al. 1991; Decourtye & Harris 1992), andtolerance to soil acidity studied (Berninger 1992).Nonetheless, one of the greatest drawbacks of cul-tivated L. scoparium in New Zealand is manukablight, and a cultivar resistant to the scale insect pesthas not yet been developed.

Leptospermum scoparium has been extensivelyplanted in the milder areas of the British Isles as asemi-hardy garden plant and is described as ubiqui-tous in Ireland gardens (Cooper & Cambie 1991).Despite the extent of garden planting in the Brit-ish Isles naturalisation is only reported at TrescoAbby, Isles of Scilly, where groves of self-sownseedlings are found (Bean 1973). It is probable

that naturalisation has occurred in the milder areasof England and Ireland but has not been reported.The species has also naturalised in Hawai'i, whereescapes from garden plantings have colonised dis-turbed wet forest areas and become a significantweed (Wagner et al. 1990).

Essential oils

Essential oils distilled from the leaves of L. sco-parium have received considerable commercial at-tention during the last decade. The New ZealandPhytochemical Register - Part III (Cambie 1976)lists earlier research that identified these oils.

An analysis of 16 commercial samples of L. sco-parium essential oil revealed 100 components, ofwhich 51 were identified and made up about 95% ofthe content. The oils fell into three major sections,triketones approximately 20%, sesquiterpene hydro-carbons 60-70%, and monoterpene hydrocarbonsabout 5% (Christoph et al. 1999), in contrast to about75% monoterpene hydrocarbon α-pinene present inKunzea ericoides (Perry et al. 1997a).

A review of the essential oils of New Zealandsuggested that L. scoparium oils would differ be-tween and within natural populations (Douglas et al.1994), and this was confirmed by the variation of thecomponent essential oils of natural populations ofL.scoparium grown in a common garden experiment(Perry et al. 1997b). Two plants from each popula-tion were sampled: the East Cape population con-tained a high triketone level, high levels of α-pineneand β-pinene monoterpene hydrocarbons were foundin Northland populations, and the balance of popula-tions contained a complex mix of sesquiterpene andoxygenated sesquiterpene hydrocarbons. AustralianL. scoparium samples grown in the same commonenvironment had a higher monoterpene level thanthe New Zealand populations. The L. scopariumchemotypes reported matched the morphologicaltypes to some degree (Perry et al. 1997b).

Porter & Wilkins ( 1998) showed a similar patternto those reported by Perry et al. (1997b), describingfour groups of oil profiles found in wild popula-tions: triketone-rich in the East Cape; monoterpene-,linalool-, and eudesmol-rich in Nelson; monoter-pene- and pinene-rich in Canterbury; and triketone-,linalool-, and eudesmol-deficient in the rest of NewZealand. The average composition of L. scopariumessential oil was defined as 3% monoterpenes,

60% sesquiterpenes, and 30% oxygenated ses-quiterpenes and triketones.

A detailed field study of New Zealand L. sco-parium populations confirmed the presence of L.

Stephens et al.—Review of Leptospermum scoparium in NZ 441

scoparium chemotypes: monoterpene-enriched areasin Northland and the West Coast, triketone-enrichedin East Cape and Marlborough, and sesquiterpene-rich oils throughout the rest of the country. Elevenchemotypes were recognised by the division ofthe major oil types referred to above and subdivi-sion of the sesquiterpenes and oxysesquiterpenes(Douglas et al. 2004). The triketone-enriched oilshave been found to carry the greatest antibacterialactivity (Christoph et al. 2000), and are marketed asManex™.

A chemotaxonomic analysis of Leptospermumhas been completed. In dealing with species alliedto L. scoparium, Brophy et al. (1999) showed thatAustralian L. scoparium populations in Victoria andTasmania had different essential oil profiles from theNew Zealand populations; in particular, triketoneswere not found. The persistent woody-fruited groupof Leptospermum established by Thompson (1989)was not amended, and in general the L. scopariumessential oils did not differ in comparison with thisgroup; however, the authors concluded that L. sco-parium is a variable taxon that may require division(Brophy et al. 1999).

Nevertheless, within-population variation of es-sential oil content was shown in a study of L. sco-parium grown in a common garden experiment fromseed collected from five wild plants within a 5 m2

area (Porter et al. 1998). The oil profiles of bothyoung and mature plants differed within and betweenseasons, and the principal component responsiblefor most variation differed between plants wheneversampled. The morphology of the plants also differedmarkedly. The need for extensive sampling over aperiod of more than one growing season to producereliable essential oil profile data for chemotaxo-nomic or variety selection was acknowledged.

Manuka honeyCockayne (1916) recognised L. scoparium as a ma-jor source of superfluous honey produced by theintroduced honeybee, reflecting the abundance ofthe plant and the surplus nectar production. ButzHuryn (1995) reviewed the literature in detail. L.scoparium honey has a distinctive flavour, colour,and consistency and until recently was used solelyfor culinary purposes.

A number of studies analysing the antibacte-rial activities of New Zealand honeys have beencompleted. Whilst many honey types containedsignificant levels of antibacterial activity due to en-zyme-produced hydrogen peroxide, onlyL. scopari-um (manuka) honey often contained a relatively high

level of non-peroxide activity (Molan et al. 1988;Allen et al. 1991). The non-peroxide antibacterialactivity was considered linked to the floral source(Molan & Russell 1988). However, L. scopariumhoney samples have demonstrated a considerablerange of potency of non-peroxide antibacterial activ-ity. A typical agar diffusion assay study reporting amean antibacterial activity of 18.6 units, equivalentto the activity of 18.6% w/v phenol, for 19 L. sco-parium honey samples, with a standard deviationof 8 units (Allen et al. 1991). The variability wasinitially attributed to sample misidentification orprocessing differences (Allen et al. 1991), and laterto a regional difference in phytochemical composi-tion or concentration (Molan 1995). The non-perox-ide antibacterial activity of L. scoparium honey wasnamed the Unique Manuka Factor (UMF®), leadingto the development of a range of medical productsfrom L. scoparium honey containing high levels ofUMF®.

Attempts to identify the active component re-sponsible for the non-peroxide antibacterial activitypresent inL. scoparium honey have continued unsuc-cessfully. Two approaches have been investigated:seeking correlation between the antibacterial activityand the trace organic substances to determine thehoney's floral source identified by GC-MS, and theidentification of the fraction responsible for the non-peroxide activity. Identification of the components ofL. scoparium honey confirmed that the constituentsare different from those found in the antibacterialL. scoparium essential oil (Tan et al. 1988); thetriketones found responsible for the antibacterialactivity of the essential oil (Christoph et al. 2000) arenot found in the honey. The identified phytochemi-cal components of L. scoparium honey are similarregionally throughout New Zealand, accordingly notaccounting for the non-peroxide variability (Tan etal. 1989; Wilkins et al. 1993; Weston et al. 2000).However, the phytochemical components of NewZealand L. scoparium honey differ from those ofthe Australian L. polygalifolium honey that alsoexhibits a non-peroxide antibacterial effect (Yao etal. 2003).

Extraction of potentially active organic fractions(Russell et al. 1990) was followed by the isolationof the phenolic components (Weston et al. 1999),oligosaccharides (Weston & Brocklebank 1999),and antibacterial bee peptides (Weston et al. 2000).These components were found to account for littleof the non-peroxide activity, and Weston (2000)suggested that the non-peroxide antibacterial effectwas an additional peroxide effect and that the assay

442 New Zealand Journal of Botany, 2005, Vol. 43

developed to remove peroxide from honey (Molan& Russell 1988) was consistently failing.

However, the residual hydrogen peroxide in L.scoparium honey has been shown not to accountfor the non-peroxide antibacterial activity by chemi-cal manipulation (Snow & Manley-Harris 2004).Furthermore, a difference in the mode of action ofdissimilar honeys has been indicated in studies thatdetermine the minimum inhibitory concentrationsfor control of wound-infecting bacterial species(Willix et al. 1992). Methicillin-resistant Staphylo-coccus aureus responded to the same concentrationsof L. scoparium honey and a honey with activity dueto hydrogen peroxide, yet vancomycin-resistant En-terococcos faecium required approximately doublethe concentration of the latter honey to be inhibitedcompared with the L. scoparium honey (Cooper etal. 2002). Additionally, the peroxide antibacterialactivity of honey is not effective against all bacterialspecies. Heliobacter pylori was inhibited by a 5%solution of L. scoparium honey but was not inhib-ited by a 40% solution of a honey with activity dueto hydrogen peroxide, whereas both honeys wereequally effective against Staphylococcus aureus(Al Somal et al. 1994). These observations indicatethat an as yet unknown agent other than hydrogenperoxide significantly contributes to the antibacterialactivity of L. scoparium honey.

CONCLUSIONS

Leptospermum scoparium is tolerant of infertile en-vironments, thriving in a wide range of marginal anddisturbed environments. The seral habitat in NewZealand has been greatly extended by human vegeta-tion disturbance, and due to its invasive nature, thespecies has been regarded as an agricultural woodyweed. Recent studies have altered the perception ofL. scoparium. The species' role in erosion control,carbon sequestration, and vegetation restorationby succession, along with the commercial value ofthe essential oils, honey, and ornamental varieties,make further examination of the species necessaryand timely.

The taxonomic status of the species needs to bethoroughly clarified in New Zealand and Australia.In all probability L. scoparium is an undefined spe-cies aggregate in New Zealand (P. de Lange pers.comm.) The relationship of L. scoparium populationswithin New Zealand, populations within Australia,between New Zealand and Australian populations,and the closely related Australian species, should

be investigated. The Australian tea tree Melaleucaalternifolia (Myrtaceae), which is used for essentialoil production, has received just such a comprehen-sive treatment (Rossetto et al. 1999; Lee et al. 2002).Conventional techniques such as uniform environ-ment studies and essential oil profiles, with modernmolecular genetic studies, should be included in sucha study.

The increased seral range of L. scoparium hasmost likely allowed gene flow between previouslyisolated populations in New Zealand. The geneticproperties of unique populations, for example, thedwarf population identified on the Kaikoura coast(Harris 1994), maybe lost due to interbreeding. Theneed to sustain genetic integrity of New Zealandspecies has been recognised (Simpson 1992; At-kinson 1994), andL. scoparium is certainly anotherexample of a species exhibiting a combination ofphenotypic and genotypic variation throughout itsnatural range.

A revised systematic treatment of L. scopar-ium would resolve many questions surroundingthe species. The regional differences of essentialoils profiles and honey non-peroxide antibacterialactivity may relate to genetic differences betweenpopulations, but this awaits experimental confirma-tion. Horticultural cultivar development may beenhanced, particularly the search for a plant resist-ant to manuka blight. Agronomic development ofthe species as a crop plant providing an abundantand reliable source of the pharmacologically ac-tive essential oils and honey could be pursued. Theconservation and, where necessary, the repopulationof genetically unique varieties could be activelypromoted to ensure the survival of the entire geneticspectrum of this interesting and valuable species.

ACKNOWLEDGMENTS

We would like to gratefully acknowledge the constructiveadvice, revisions and suggestions of W. Harris, an initialreferee of the manuscript. Any errors of interpretationremain our own.

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