TIMMINS: VEGETATION SUCCESSION ON MT. TARAWERA

MT TARAWERA: 1. VEGETATION TYPES ANDSUCCESSIONAL TRENDS

SUSAN M. TIMMINS

Biological Sciences Department, University of Waikato, Hamilton, New Zealand.Present address: Department of Lands and Survey, Private Bag, Wellington,

New Zealand.

SUMMARY: Vegetation succession on Mt Tarawera, Rotorua, New Zealand, a recentlyerupted volcano, was studied. Field data from 70 plots were collected and analysed by aclustering algorithm. The plots formed an altitudinal and successional series which includedbare scoria, herbfield, mixed hardwood, scrub, kamahi forest and tawa forest. Because of thedifferential effects of the 1886 eruption, different successional trends are being followed on thenorth-west and south-east faces of the mountain.

KEYWORDS: volcanic areas; plant establishment; plant succession; Mt. Tarawera.

INTRODUCTION

Mt Tarawera comprises a group of rhyolitic cumu-lodomes 24 km east-south-east of Rotorua in theNoith Island of New Zealand (Figs. 1, 2).

It was formed by the extrusion of successive rhyo-litic domes and flows during the late Pleistoceneperiod and has been known to erupt four times (Cole,1976). During the Kaharoa eruption at about 1324AD ± 56 years (Lawlor, 1980), the three centraldomes of the Mt Tarawera complex; Wahanga,Ruawahia and Tarawera were formed (Cole, 1965)(Fig. 3).

The most recent eruption occurred in 1886 and,although initially rhyolitic, the ejecta became pre-dominantly basaltic ash and lapilli. Half a cubic kilo-metre of material was ejected from the mountainonto the surrounding landscape extending 16 km toboth the north and south and about 24 km to thenorth-east. Comparatively little was deposited to thewest (Thomas, 1888). The ejected material formed alayer overtopping the existing domes and reached itsgreatest thickness, 61 m, near the craters (Burke,1964). Although the eruption resulted in little overallchange to the form of the mountain, an impressive7 km fissure opened up along the south-west-north-east axis of the complex. The events of the eruptionare described in detail by Smith (1886), Hutton(1887), Thomas (1888) and others (see Cole, 1966).

The 1886 eruption of Mt Tarawera is the most sig-nificant to have taken place in New Zealand sinceEuropean settlement. Records of the vegetation be-fore, shortly afterwards and at intervals followingthe eruption offer the unique chance to study plantestablishment and successions on a new volcanicsurface.

VEGETATION HISTORY

Pre-eruption vegetationPrior to the 1886 eruption, stands of forest, similar

to extant virgin stands in the Rotorua Lakes district,grew on the lower slopes of the mountain reachingan altitude of about 610 m on the talus slopes atthe bottom of the dome cliffs. These probablycomprised rimu (Dacrydium cupressinum), rata

FIGURE 1. Obligue aerial view across Mt Taraweracomplex from above the southern arm of Lake Tara-wera, looking north-east. From the left backgroundto the right hand foreground the domes are:Wahanga, Ruawahia obscuring the lower Plateau,Tarawera. (photo: D. L. Homer, N.Z. GeologicalSurvey).

New Zealand Journal of Ecology 6: 99-105 © New Zealand Ecological Society

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100 NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 6, 1983

FIGURE 2. Location of Mt Tarawera study area.(Source; Forest Service Map, series 65).

(Metrosideros robusta), totara (Podocarpus totara)with Hall's totara (Podocarpus hallii) also commonat higher altitudes, overshadowing a mixture of tawa(Beilschmiedia tawa), mangeao (Litsea calicaris),rewarewa (Knightia excelsa), hinau (Elaeocarpusdentatus) and kamahi (Weinmannia racemosa) (Nich-olls, 1959). On the higher slopes of the mountain,Kirk (1872) found shrub community of manuka(Leptospermum scoparium), kanuka (.Leptospermumericoides) and kohuhu (Pittosporum tenuifolium)in the deep ravines, and young growth of Ry t i do -sperma sp. and Deyeuxia sp. on the open land thathad been burnt over. On the summit (1092 m) hefound dwarf shrubby vegetation in sheltered placesbut scanty, depauperate vegetation in the open andonly 70 species were collected at about 914 m. Indeed,Smith (1886) described the plateau-like summit thus:

"The surface was formed by a confused mass ofrhyolitic rocks, broken and jagged. Generally quite

bare of vegetation, but here and there a littlebracken or moss, or occasionally a weather beatenshrub, relieved the sombre grey of the rocks."The immediate effects of the eruption on the Tara-

wera vegetation were described by Pond and Smith(1886), Smith (1886), Thomas (1888), Hill (1910) andTurner (1928). Most of the Tarawera vegetation wasutterly destroyed by the eruption, although there wasa gradient of decreasing damage away from thecraters. On the lower slopes some trees were not kill-ed but merely stripped of their leaves and smallbranches. They were thus able to resprout and pro-duce seed again the following year (Aston, 1916).

Post-eruption vegetationPlant recolonisation on- Mt Tarawera has been

documented by various authors, the most com-prehensive accounts being those of Aston (1915,1916), Turner (1928) and Burke (1964). Twenty-sevenyears after the eruption, Aston visited the north-westslopes and found revegetation was proceeding. pole-sized pohutukawa (Metrosideros excelsa), rewarewaand kamahi, and young trees and shrubs coveredthe cliffs along the shores of Lake Tarawera and, ina few places, extended further up the lower flanks.High up the mountain, a sparse shrub growth pre-dominantly of tutu (Coriaria arborea), manuka andkoromiko (Hebe stricta var. stricta) had establishedalong with stunted individuals of kamahi, Gaultheriaspp., monoao (Dracophyllum subulatum) and others.

FIGURE 3. Mt Tarawera study area showing domesand lava flows. (Source: NZMS1 N77; Cole, 1965).

TIMMINS: VEGETATION SUCCESSION ON MT. TARAWERA 101

Vegetation cover, height and species diversity de-creased with increasing altitude. On Ruawahia sum-mit the only plant recorded was Raoulia sp.

Forty-one years after the eruption, Turner (1928)recorded that the mixed hardwood forest along theshores of Lake Tarawera had in places extended to180 m up the mountain side. At around 800 m a.s.l.on the south-west slopes simple vegetation communiincluded emergent kohuhu, were relatively common.Also, mat communities were formed by species suchas Raoulia spp. and Pimelea prostrata. These matplants bound the fine loose scoria together, provid-ing stability and eventually litter to enable otherpioneer plants to establish.

Burke (1964) studied the south-eastern flanks ofMt Tarawera which included Tarawera Ridge dome,Southern lava flow, Plateau dome and its adjacentcrater. He recorded forest grading from tawa-domin-ant at lower altitudes to kamahi-dominant at highaltitudes. Bushline, defined by Burke (1964) as theupper limit of complete cover of forest vegetation,was formed by shrubby many-branched kamahi,broadleaf (Griselinia littoralis) and other species. Itvaried in altitude around 853 m a.s.l. Above this,patches of shrubs, especially monoao, manuka andGaultheria spp. grew while on the dome top therewas a sparse covering of grasses, herbs, mosses andlichens overtopped in places by stunted monoao andGaultheria oppositifolia. There was little vegetationin the crater. Burke (1964) listed 63 vascular speciesin the crater and a total of 110 vascular species abovebushline.

Present vegetationAlthough the summit at 1111 m is well within the

altitudinal range for forest for the district, there isas yet only sparse shrubland and herbfield on muchof the mountain top. Gradations in successionalstage and stature of the vegetation are readily seenwhen climbing the mountain and in Burke's (1964)vegetation descriptions. The establishment and re-covery of vegetation on the volcano is documented intwo parts. This paper provides a description of cur-cent vegetation types and successional trends on MtTarawera from the 300 m level to the summit, whilethe second paper (Clarkson and Clarkson, 1983)gives a detailed analysis of successional trends andrates of change on one part of the mountain-thehigh domes. Botanical nomenclature for this paperis given in Appendix 2 of Clarkson and Clarkson(1983).

METHODS

Sixty-five plots covering all the vegetation typesrecognised on Mt Tarawera from reconnaissance

trips, aerial photographs and the forest type map(Nicholls, 1965: N77) were sampled using Druce's(1959) sampling method. Sample sites had to beaccessible, homogeneous, 2 m away from tracks orridge tops, have a constant "lope and aspect, and beof at least 100 by 100 m, the latter constraint im-posed by the need to furnish field data for use inLandsat data analysis (Timmins, 1981, 1982). Canopyspecies composition, cover and height were assessedby eye. The physical conditions of the site were alsorecorded: altitude, topography and exposure, slopeand aspect, soil type and drainage.Vegetation stands were named following Atkinson's

(1962) scheme. The first, floristic, name indicated themajor canopy plants and the second, structural namewas based on the dominant plant growth forms and/or non-plant surfaces such as rocks or bare scoria.Burke's (1964) five forest plots were also re-

sampled and the combined total of 70 plots wereanalysed by average linkage plot cluster analysis.using the Biomedical Computer Package (BMDP-77)program, BMD: P2M (Dixon and Brown, 1977).Plots were clustered on the basis of their similarity inspecies composition, measured by euclidean distance(Orloci, 1978). This resemblance measure was chosenfrom the four measures offered by BMDP-77 as themost appropriate for the Tarawera data which werecontinuous with no extreme outliers (Williams, 1971).A dendrogram was constructed from the similaritymatrix of the cluster analysis.

DESCRIPTION AND ANALYSIS OF VEGETATION

Vegetation groupsThe floristic dendrogram prepared by plot cluster

analysis is depicted in Figure 4. At the arbitrarily-selected euclidean distance of 40, eight major groupswere defined.

Cluster 1: Scoria land. Ninety percent of the coverwas scoria. Grass/herb-lichen ring growths contri-buted the other 10% cover. The dominant plant ofthese rings was invariably a cushion or mat plantsuch as Muehlenbeckia axillaris or Raoulia albo-serirea.

Cluster 2 : Scoria-shrubland mixed hardwoods.These plots lacked a single dominant species; similarspecies were present in many plots, although the pro-portions varied. A herbfield of mosses, particularlyRacomitrium lanuginosum, lichens, small herb andherb-like species, sub-shrubs and a high percentageof scoria (20-40% of cover) characterised plots 43through 26. Stunted scattered shrubs of monoao, tutu.snow berry (Gaultheria antipoda), G. oppositifoliaand Olearia furfuracea were also present. Plots 36 to6 were characterised by a mixture of taller hardwood

102 NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 6, 1983

FIGURE 4. Floristic site cluster dendrogram. Hatching denotes lines which join a cluster at a highersimilarity level than the previous linkage. The euclidean distance is a measure of dissimilarity-thelower the euclidean value, the greater the level of similarity.

TIMMINS: VEGETATION SUCCESSION ON MT. TARAWERA 103

shrub species: tutu, manuka, O. furfuracea. Copros-ma tenuifolia. kamahi and broadleaf (the latter twospecies present as shrubs), together comprising20-70% of canopy cover.

Cluster 3: Kanuku scrub. The dominant kanukacontributed 25-50% cover at a height of 5-8 m.Various shrub species, predominantly of the Erica-ceae and Epacridaceae families were associates. Thisvegetation type was the stably community on thenorth-western flanks of Mt Tarawera at 400-500 ma.s.l.

Cluster 4: Manuka scrub. Manuka, the dominantspecies, provided 45-65 % of the canopy cover atabout 4-6 m in height. Other species includedOlearia furfuracea, kohuhu, five-finger (Pseudopanaxarboreus) and kamahi.

Cluster 5: Manuka-kamahi treeland. Kamahiwas co-dominant with manuka, both contributing40 % of the canopy cover but the kamahi was 1-2 mtaller than the manuka. The lack of manuka seed-lings in this stand suggested that in the future broad-leaved species will dominate.

Cluster 6: Kamahi forest. These were simple-structured stands of nearly pure pole kamahi. Insome plots, species other than kamahi also contri-buted to the canopy. These species were local andincluded (in order of importance): broadleaf,kanuka, kohuhu, tawa, five-finger, Coprosma tenui-folia and rewarewa. The strong chaining of thecluster 6 plots (Fig. 4) was a function of the per-centage kamahi cover; a highly significant inverserelationship was found between percentage kamahiand the dissimilarity value (r (17) = -0.9, p < 0.001).The gradient in percentage kamahi cover was relatedto altitude and thus to distance from the 1886eruption crater.

Cluster 7:. Kamahi-tawa forest. Tawa and kamahiwere co-dominants and there was plentiful regenera-tion of both tawa and mangeao. In this forest emer-gent dead trunks of Hall's totara, killed by theeruption (Burke, 1964) were seen on Tarawera ridge.

Cluster 8: Tawa forest. The canopy was dominat-ed by tawa but a few mature mangeao trees werealso present. There were many tawa saplings andthe rather open forest floor was dominated by tawaand mangeao seedlings. Saplings of broadleaf andmahoe (Melicytus ramiflorus), and seedlings of whitemaire (Nestegis lanceolata), hinau and the occasionalkahikatea (Podocarpus dacrydioides) also occurred.

Miscellany. These plots represented isolated occur-rences of particular combinations of species. Forexample, the vegetation of plot 58, a thicket ofrangiora (Brachyglottis repanda) through whichkamahi was beginning to emerge, had colonised aslip in a kamahi stand. A stand of mahoe and

putaputaweta (Carpodetus serratus) within kamahiforest (plot 4) reflected disturbance associated witha frequently changing water course.

INTERPRETATION

The above groups form a successional, as well asan altitudinal sequence, both the result of recentvolcanic activity. First, bare scoria (cluster 1) is in-vaded by mat or cushion plants, particularly Muehlen-beckia axillaris and Raoulia spp. which stabilise theash and lapilli and improve water relations. Thisprovides a seed bed for low growing species to es-tablish and form grass/herb-lichen ring growths. Thenumber and diversity of species in the rings increas-es with time (cluster 2).

Later, stunted hardwood shrubs develop aroundthe edge of the rings (cluster 2). The shrubs pro-gressively invade the centre of the rings as the her-baceous plants die (cluster 2). These shrub ringsare usually discontinuous but with time they toocoalesce, particularly with the advent of tutu, toform a relatively complete cover of hardwood scrubwhich is eventually replaced by kamahi forest (cluster6). In some of the kamahi forest plots there werevarious relics of a scrub community. A few tututrees were present, many dead tutu branches andspindly Corokia buddleioides and Gaultheria spp.bemg shaded out by kamahi. These features indi-cated that previously a mixed hardwood scrub(cluster 2) had occupied the site.

In some areas, kamahi may have developed undera near pure manuka canopy (cluster 4-5). An areasouth-east of Tarawera ridge which is now manuka-kamahi treeland was described as manuka scrub byBurke (1964). According to Burke (1964) the manukascrub developed after fire, unrelated to the earliervolcanic activity.

With decrease in altitude the proportion of otherspecies in the kamahi forest increases until tawaforest, which occurs on the lower slopes of the Tara-wera ridge and southern domes below an altitude of670 m and 610 m respectively, is reached. This latterforest type largely survived the eruption except forsmall pockets where there are now pole stands oftawa.

As the upper limit for tawa in the district is 823 m(Nicholls, 1963), with time it could be expected thattawa forest will extend beyond its present limits.However, kamahi forest will probably dominate athigher altitudes. Kamahi is commonly an early suc-cessional forest species at lower altitudes, but at high-er altitudes, low fertility-low altitude sites and south-ern districts, kamahi forest can be the stable vege-taton (Wardle, 1966). Other important componentsare likely to be broadleaf and Hall's totara. The

104 NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 6, 1983

latter has not yet established on the dome tops(Clarkson and Clarkson, 1983). However, the nearbyMakatiti dome top supported a Hall's totara-kamahi.broadleaf forest before the Hall's totara were felled(Nicholls, 1959, 1980).

An exception to the expected pattern of successionon Mt Tarawera, as described above, was seen onthe dome tops. Stunted individuals of tutu, monoao,Gaultheria spp., broadleaf, Coprosma spp. andkamahi were establishing in boulder crevices (seeClarkson and Clarkson, 1983). This occurrenceeclipses some earlier stages, and thus accelerates thepace of succession considerably over the progress viaring growth.

COMPARISON OF THE NORTH-WESTAND SOUTH-EAST FACES

The vegetation and rate of succession. differed oneither side of the central fissure. Ericalean speciesand manuka were important shrubs in the scoria-shrubland communities of the north-west occurringin definite clumps; tutu was more important on thesouth-east. However, on the north-western slopes,stands of tall (2-3 m) tutu interspersed with essen-tially bare scoria have developed in the last 8-10 yrs.

Kamahi was more important on the south-east thanon the north-west. On the latter, kanuka was com-mon as an early successional tree and formed maturestands at the base of the north-western faces of thedomes, the pre-eruption vegetation type at this site(Nicholls, 1980). Only very occasionally were seed-lings of kamahi found in the scoria-shrubland of thenorth-western flanks.

Rewarewa was more important in the forests andtreelands of the north-west than the south-east andwas found in association with kamahi, kanuka andkohuhu.

The origin of these distinctions lies in the patternof the 1886 eruption. The vegetation on the north-western flanks of the mountain, which suffered heavy

. deposits of ash, was completely destroyed by theeruption. Pyroclastic flows, funnelled by topographic’gaps', would have rushed from the crater down themountain side to Lake Tarawera, sticky Rotomahana<mud' would have caused even greater devastationby adhering to the vegetation.

By contrast, large tracts of forest less than 4 kmto the east of the fissure, were virtually unaffected bythe ejecta (Nicholls, 1963; Fig. 3). The forests in thesmall dips of the ridge and southern domes wouldalso nave been protected, particularly on the lee side,from the base surges, the ground hugging shockwaves that would have coursed down the mountainat a speed of greater than 150 km/hr (R. M:. Briggs,pers. comm.). Furthermore, regeneration since the

eruption has been much slower on the north-westthan on the south-east because the two areas vary indistance from propagule source, The north-westernflanks are isolated from direct link with seed sourceby Lakes Tarawera and Rotomahana, except for thenarrow land isthmus between the two (Fig. 2). Onthe south-east the tracts of forests at the base of themountain plus numerous individual trees closer tothe chasm which survived the eruption (Burke, 1964)have provided ready propagule sources for the rapidrecovery and spread of forest there. Sproutingstumps of tawa, mangeao, mahoe and kamahi, rem-nants of the 1886 eruption, have also been reportedon the south-east (Burke, 1964).

Although reasons for the differential revegetationof the north-west and south-east faces can be ad-vanced, further study is required to explain all theobserved variation in succession on Mt Tarawera. Itis clear that as a result of the 1886 eruption, thevegetation of Mt Tarawera forms both an altitudinaland a successional series. However, the subsequentregeneration is proceeding by more than one pathwaycontrolled, at least in part, by history of the site, seedsource availability, microclimate and substrate.

ACKNOWLEDGEMENTSThis paper was derived from part of an M.Sc. thesis

submitted to the University of Waikato, Hamilton (Tim-mins, 1981), partly funded by a Mobil Oil New ZealandLtd Environmental Grant Award 1979. It was writtenwhilst in the employ of Physics and Engineering Labora-tary, DSIR and I gratefully' acknowledge that opportun-ity. Thanks are due to Dr Alan Edmonds for supervision,Mr Kerry Hollingsworth for help in the field, Mr WarrenBurke for use of his forest plot data and Te Arawa TrustBoard for allowing access to their land. Ms BeverleyClarkson. Mr Tony. Druce.. and Drs Bruce Clarkson,Peter Wardle and Ian Atkinson provided valuable criti-cisms of the manuscript.

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