VVARDlE: SECRETARY ISLAND

SOILS AND VEGETATION

27

OF SECRETARY ISLAND

P. WARDLE

Botany Division, Department of Scientificand Industrial Research, Lincoln

Western Fiordland provides a unique com-bination of geology, topography, climateand vegetation. The rocks throughout aregranites, schists and gneisses which arecxtremely hard and resistant to weathering,softer sedimentary rocks being found onlyin the extreme south-west. The raw glacialtopography developed during the Pleisto-cene has scarcely been modified, and ischaracterised by irregular summits, precipi-tous fiord walls, and spurs worn by ice. Theclimate is cxcessively wet, and though cool,is equable. Exposure to prevailing westerlywinds is an important factor on headlandsand on the 'tops'. Up to an altitude of about3000 ft. the vegetation consists of forest andscrub, nearly all the communities being mix-tures of beeches, podocarps and broad-leaved hardwoods, while the open tops areoccupied by tussock grasslands and herb-fields.

As elsewhere in New Zealand, the char-acter of the vegetation varies according toaltitude, but there are few areas outsideFiordland where the effects of rock, soiltype, and topography are so apparent. I hadthe opportunity to investigate these effectswhen Professor Baylis invited me to join hisexcursions to Secretary Island. We had timeto examine only the eastern end of the islandfrom Blanket Bay to the summit ridge,which rises to neady 4000 ft., the landscapeconsisting in part of rolling, ice-worn terrainand in part of steep slopes which are brokenhere and there by precipitous ledges. Thewhole area is more or less sheltered fromthe prevailing winds, except on westernaspects near the summit ridge. Our investi-gations were practically confined to appar-ently stable communities of long standing,and consequently I shall be making no refer-ence to certain important processes of soildevelopment and plant succession which arecharacteristic of Fiordland; for example,colonisation of landslide scars.

The chart (Fig. I) summarises my results.Edaphic classes are arranged along the hori-zontal axis, and the altitudinal grad<\tion inclimate is represented by the vertic~l axis.Needless to say, this representation 'greatlyoversimplifies matters. For instance, althoughthe cdaphic classes shown on the lef! of thechart are more fertile than those 'on theright, the various factors of draina¥e, soildepth and so on are too complex! to beadcquately portrayed as a linear se\iuence.Th~ chart also does not illustrate thellimpor-tance of aspect and exposure at high alti-.tudes. I have attempted to put relateli plantcommunities adjacent to each other ion thetable wherever possible, and have shqwn theexistence of transitions with brokeri lines.In addition, there are transitions b,tweenIA and IVA, and between IliA and IVA. ClassV communities, generally, intergrade withClass VII communities. I expect al$o thatClass I communities intergrade with ClassVII, though I saw no instances on SecretaryIsland.

A brief description of the plant communi-ties follows:

Class I. Community growing on a p','at soilwith relatively high pH and the water tablenear the surface.

IA (Nothofagus cliffortiodes-piJdocar-pus dacrydioides forest) is base<l on asingle stand in which mountain! beech(Nothofagus cliffortioides) is th~ maincanopy species, but kamahi (Weininanniaracemosa) and rata (Metrosiderosumbel-lata) are also important. The ,uniquefeature of the community, however: is thepresence of kahikatea (Podocarpu~1 dacry-dioides ).

Class II. Communities occupying \veaklyweathered sands which are deep, moist andwell-drained. Such soils are developed onrecently consolidated alluvium and tilus.

28 FIORDLAND ECOLOGY

IIA (Hoheria forest) is dominated bymountain ribbonwood (Hoheria glabrata),though silvcr beech (Nothofagus men-ziesii) is present as a scattered overstorey.Beneath the trees, there is a dense growthof Polystichum vestitum.

lIB (Olearia-Hoheria scrub) is astunted ridge-crest variant of IIA, and isdistinguished by the co.dominance ofOlearia colensoi.

Class III. Communities occupying moder-ately weathered and moderately leachedsandy loams which are deep, moist and well-drained.

In IlIA (Weinmannia/Cyathea forest)kamahi (W einmannia racemosa) is domi-nant, but the most characteristic featureis the abundance of the tree fern Cyathea .

smithii. Melicytus ramiflorus and Hedy-carya arborea were seen only in this co'm-munity.

IIIB (Nothofagus menziesii/Polysti-chum forest) is a small stand at 1700 ft.dominated by silver beech, and with densePolystichum vestitum on the floor.

Class IV. Communities occurring on soilswhich are of considerable depth, but lessfreely drained than soils of Class III. Thesomewhat impeded drainage is due either tothe fine soil texture or to moderate slope.

IVA (Not hofagus-W einmannia-Podocarpforest) is the densest and tallest forestcommunity on the island. Dominance isshared by silver beech, mountain beech,kamahi, rimu (Dacrydium cupressinum),and miro (Podocarpus ferrugineus), andthere is a vigorous development of Blech-num discolor, except on wetter groundwherc it is replaced by B. capense. Senecioreinoldii, O/caria oporina and Dracophyl-lum IOl1gifolium occur along the littoralfringe of the forest.

There is a gap in my observations onClass IV soils until 1000 ft. By this alti-tude, many of the low altitude species havebecome rare or absent, and co-dominanceis shared by silver beech and kamahi (lVB,Nothofagus menziesii- Weinmannia forest).IVC (Nothofagus menziesii/Olearia

forest) is the most extensive type of sub-alpine forest. Silver beech is the only large

tree, and usually forms an op~n canopyover an understorey of subalpine scrub inwhich Olearia colensoi is the maIn species.Above 2800-3000 ft. the forest passes

into mixed snow-tussock and Iiherbfieldcommunities (lVD).

Class V. Communities occupying soils whichare underlain by solid rock at alidepth of6-24 in., and which therefore are jTIuch lessfreely drained than soils of Class ~V.

VA (Dacrydium intermedium iforest) isa stunted forest community in iwhich D.intermedium is usually domil)ant, butmountain beech, Dacrydium

~

biforme,kamahi and manuka (Leptospel,1nUm sco-parium) may rise to co-dominailce. Thereis a thick mantle of bryophytes dominatedby Dicranoloma billiardieri. Dacrydiumbiforme and manuka not onlY1ioccur astrees, but in this and related communitiesthey also form extensive patch~s of semi-prostrate stems a few inches tall, whichare adventitiously rooted in thellmoss.

VB (Nothofagus cliffortioides-Dacry-dium biforme scrub) is the

Ii subalpinescrub equivalent of VA, being distin-guished especially by the absence ofDacrydium intermedium.

j!

VC (Nothofagus cliffortioidis-N. men-ziesii-Dacrydium biforme scrub) is almostidentical with VB, but soils are deeper,and silver beech is present, ofteh growingas a prostrate, layering shrub. Ii

Class VI. Communities occuPYin~ the veryshallow soils which are developed" over rockridges. At lower altitudes there m~y be up to12 in. of peaty soil overlying theil rock, butabove 2000 ft. soil is mostly c~nfined tocrevices.

IiVIA (Dacrydium intermediJm scrub)is floristically closely similar to :v

I

A, but thedominant species are stunted ,to form amore or less open scrub. VIBlicommuni-ties, like VB communities, are mainly dis-tinguished by the absence of Dacrydiumintermedium.

Ii

Class VII. Communities which

1

11CCUpy wethollows and drainage channels. i

VIlA (Dacrydium biforme folest) is bogforest dominated by Dacrydiw'n biforme.This community grows on w~ter-logge<\

AS c. SNow.ND /USSOr.1< /

CRASS LAND //

/,/

//

//

vcN.CLIFfORTIOIDES-

N I.iENZIESII-

D.BIFORME SCRUB

m e ::~::::::::::~::;:::::::::::i:t::::::::::::::::::::::::::;:::::::;::t:::;:;

I......................................................-.......-.....-......-MIl( E D SHRUB C,RASS .:.:.:.:.:.:.:.;.:.:.:.:.:.:.:.;.;.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.

COM ,",UNITIES Of'~:5:::::::::~:;:i::::::::ii::::::::::::::;:;~:::::::::::::::::::;:::;:;:;:::

O N ::::.'.':-"-":c..:-"RAINACiE CHANNELS, :.;.:.:.:.:.:.:.;.:.:.:.:.:.:.:.;.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.;.:_:m..............._.....................................................-.......................................................................

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3000,

-------

2500 WcN.MENZ IESII/OLEARIAfOREST

:YB ~N,CLIf"FORTIOIOES_ IDBlfORME SCRUB. I

I,

. =.:SCHOEN1J!" SWAMp'

W.SCRUBN.CLlffORTIOIDES& D.BlfORMEUSUALLY DOMINANT.

YITADACRYDIUM

BlfORME fOREST.

I :nr.IN.MENZIEStr/

r::Y~TlC~UM !.~RE!T

II

----------

mAWEINMANN IA/CYATHEA fOREST

II

I II Ir .!Y.B r XAI N.MENZIESII- IDACRYDIUMIWEINt,II"NNIA fOREST. I INTERMEDIUM

I IfOREST.

II

I I

.I'.lADACRYDIUM

INTERMEOOM SCRUB.

1000

500.......

SEA

LEVEL

/.- 'n RRACEJ..

]II :nr =WET, PEATY LOAM,RELATIVELY HICH

PH.

DEEP, WEAkLYWEATHERED, WEL:..-

DRAINED SOIL

DEEP, MODERATELY

WEATHEIlED, WELL-

DRAINED SOIL.

DEEP SOIL,DRAINACE SOME-

WHAT IMPEDE.D.

SHALLOW SOIL,

DRAINAc.E !t,IIPEDED

BY ROCk.

SHALLOWON ROC k

SOIL

RIDCoESWET HOLLOWSAND DRA'NAc.E

CHANNELS.

SWAMP

FIGURE Tabular altitude. Stipplingof distribution of plant communities in relation to soil andtial sites which were not encountered on Secretary Island.

representationI.

fEETA.S.L.

3000.

'2!tOO.

'2000,

150Q

1000

500.

~IOTHOFACU5 SPI>

DACRYO!UM BlfOfUAC

HOHERIA CiLA8RATA

SENECIO BENNETT"

METROSIDEROS

UMBELLATA

WEINMANN'A RACEMOS",

PODOCARPUS HALLU

CYATHEA SMITHIL

DAe. CUPRESSINUt.!

ASCARINA LUCIDA

RIPOc.oNUM $CANOEN$.

HEDYCARYA AR80REA

OLEARIA OPORtNA

UPPER ALTITUDINAL

SPECIES LlM'TS

indicates poten-

30 FIORDLAND

peats or peaty loams, which are coveredby thick moss (consisting mainly ofDicranoloma, not Sphagnum).

VIIB includcs mixed scrub/grass com-lTIunities of drainage channels. These com-munities are wet variants of grassland andsubalpine scrub, rather than bog.

Class VII!. VIIIA and B include communi-ties growing in swamps, with deep peats andwater lying on the surface. These werc notrecorded in any detail.

The relationship between soil and vegeta-tiOD on Secretary Island is summcd up inthe foliowing paragraph: .

The communities included in Classes I-IVare taller, denscr, and floristically richerthan thosc in Classes V-VIII, and thc lcavesof most species belonging to the former

- conl~unities ar'e green or dark green, and ofmedium size, in contrast to the small yel-lowish or brownish leavcs of most speciesbelonging to the latter communities. Soilsof Classes I-IV arc thus evidently morefertile than those of Classes V-VIII. How-ever, chemical analyses of seven soil sam-ples, carried out at Rukuhia Soil ResearchStation, indicated relatively high nutrientcontents only undcr the Nothofagus cliffor-tioides-Podocarpus. dacrydioides and theWeinmannia/Cyathea communities.. Underthe remaining communi tics fertility appearsto be primarily rclated to the depth of soilavailable for exploitation. Soils of ClassesV-VIII are either actually shallow bccauseof solid rock lying near the surface, or effec-tively shallow because of high water tables,and this may be sufficient explanation oftheir infcrtili ty.

IkoLOG1'

Values for total exchangeablclbases for theseven soils are as follows;

WeinmanniajCyathea forest .I~

Nothofagus cliffortioides-Podocarpusdacrydioides forest ...m l~...

Dacrvdium inlermedium scrub 11....NotHofagus menziesiijOlearia for~stH h

. .Io erza torest ~:...

Dacrydiwn intermediwn forest L...

Noth,ofagus clijfortioides-DacrydiUjl1lblforme scrub t..

... 26.3

18.04.12.11.10.9

0.6

In the first two examples, comparativelyhigh fertility would be expeded from thepresence of'species such as MeJicytus rmnijflorus and Podocarpus dacrydioides, but inthe remaining five examples

~' ere is obviR

o!-,sly no consistent relationshi:

between th~vIgour of the stand and base tatus. Thereis, however, good agreement be1tween vigourof stand and depth of soil i'tvailable forexploitation.

Ii

Wright and Miller (1952) discuss a widerange of soils and types of vegetation ofsouth-west Fiordland. Our Secretary Islandsoils appear to correspond widl fivc of theil'series, but on the whole show less acccumu'lation of peat. Whereas Wrig!1t and Millc]'record accumulation to dcpths of up to 24inchcs, Sccretary Island soils!, usually hadonly 0-6 inches of peat, and there wasseldom morc than 12 inches. :This may bebecause our area is somewhat

I

warmer andless exposed than the extreme south-westcorner of Fiordland, where IWright andMiller based their work. . I . .'A fuller account of the plant'commullltles

is being prepared for later publication.

REFERENCEn

."WRIGHT, A. C. S. and MILLER, R. 8., 1952. SOlIs ofSouth-:vest Fiordland. Soil Bur

l

'fau Bull. (n.s.)7. WellIngton, N.Z. , q