THE INTERACTION OF NATIVE AND ADVENTIVE PLANT SPECIES …

The following seven papers were deli1'ered as a symposium on "The Interaction

of Native al1d Introduced Species in New Zealand."

THE INTERACTION OF

PLANT SPECIES

A. J.

NATIVE AND ADVENTIVE

IN NEW ZEALAND

HEALY

Botany Division, Department of Scientific

and Industrial Research, Lincoln

The probable and actual results of theinteraction of native and adventive plantspecies in New Zealand have been and stillare topics of considerable interest and dis-cussion, and over the last one hundred andtwenty years such eminent authorities asJ. D. Hooker, W. T. L. Travers, Chas. Darwin,A. W. Wallace, T. Kirk, T. F. Cheeseman,G. M. Thomson, L. Cockayne, H. H. Allanand C. M. Smith have all contributed paperson different aspects of the subject.

In brief review, it can be stated thatneither Hooker's original thesis that manysmall local genera would ultimately disap-pear owing to the usurping tendencies ofthe Northern Hemisphere adventives, northe later prophecies which forecast large-scale displacement of the native vegetationby "all-conquering" adventives, was ful-filled; the true position has been clearlydescribed by the last four authorities citedabove. The marked diminution in total areaof native vegetation and abundance of somenative species was due, not to inherentsuperiority of the adventive species, but toactual physical destruction of, or modifica-tion of, the primitive vegetation by man andhis associated agents - fire, cultivation,oversowing and grazing animals.

Allan (1931) pointed out that we have inNew Zealand two floras and two vegetations,while C. M. Smith (1957) very pertinentlystated that". . . the local botanist is nowwitnessing the phase of modification of theinitial pioneer communities of adventiveplants, and the initiation of the much moreadvanced phase of communities composedof an amalgam of indigenous and adventiveelements"; both writers made a plea for

intensive study of the second vegetation aswell as the second flora.

With a primary interest in the adventiverather than the native flora, I am impressednot only by the number, abundance andwidespread distribution of adventive plantspecies, but equally so by the tenacity andadaptiveness of a number of the native plantspecies; not only have some persisted de-spite vigorous competition from adventivesand the operation of modifying factors, butthey have demonstrated their ability toadapt themselves to much modified or en-tirely new habitats. The large-scale displace-ment of sown grassland (adventive species)over hill country of higher rainfall districtsin both islands by such native species asAcaena spp., Cassil1ia spp., Leptospermumspp., Paesia scaberula and Pteridium escul-el1tum, and the displacement of *Ulex euro-,tJaeus' communities by native broad-leafforest, result from interactions as significantas any in the world; and just as significantGS the perhaps more spectacular displace-ment of sown grassland communities by*Ulex europaeus and *Erica sp. communi-

ties.

Now I propose to consider several of thesemixed native-adventive species communities,treating the significant species and the gen-eral character of the communities. Thesecommunities are treated in three main cate-gories, based on the probable time of entryof the respective species into the community,and on the site or nature of the significantspeCIes.

'* Adventivc species are marked with an asterisk through-

out.

40 NATIVE AND INTRODUCED SPECIES IN NEW ZEALAND

SIMUI.TANEOUS ENTRY OF NATIVE AND

ADVENTIVE SPECIES

Ferns (Native) versus fern (Advel1tive)

In the cracks of the timbering of creeksand drains about Christchurch exists an arti-ficial habitat frequently occupied by linearfern communities; Pteridium esculel1tumand the European *Dryopteris filix-mas ap-pear simultaneously and over a period ofyears have maintained their relative abun-dance. Surprisingly, Blechnum penna-marinahas entered in some instances, and despitethe disparity in habit characters has held itsstatus for at least six years. By contrast, thegrass' Arrhenatherum elatius has displacedEpilobium spp. in the same habitat in threeyears.

Sedges (N) versus various herbs (A)

The concrete gutters and channels ofurban roadsides present an extensive special-ised habitat. In this habitat a number ofnative species occur as casuals from timeto time, but rarely persist in the permanentplant communities; but in a number oflocalities, Elaeocharis sp., Scirpus ameri-canus and S. cernuus have assumed domin-ance over both annual and perennial adven-tives including' Agrostis spp., 'Alopecuruspratensis, *Crepis capillaris, 'Epilobium sp.,'Juncus bufonius, 'J. articulatus (syn. J.lampocarpus), *Mimulus spp., 'Poa spp.,*Sagina spp. and 'Taraxacum officil1ale. Inthe north, these native species appear unableto displace the related 'Cyperus rotunduswhich grows well in such habitats, anddominance varied in the communities ex-amined.

The artificial habitat of railway ballast,with mechanical disturbance at intervalsand often periodic weedkilling, is one inwhich plant communities with many adven-tives might be expected. This is generallyso, except that in higher rainfall districtsone native species, Carex ternaria (of Cheese-man), has displaced the ruderal type ofadventive element, giving in a short timepure communities devoid of adventives.

Grass (N) versus various herbs (A)

An unusual gutter-crack community inChristchurch shows Poa breviculmis, norm-

ally a grass of shaded, non-urban situations,competing in the open and in an artificialhabitat with *Poa annua, "Sagina ape talaand *5. procumbens. Over a six-year period,P. brevicubnis and *5. procumbel1s haveremained roughly static, "5. apetala has de-creased, while "Poa annua has increased andat the same time developed a stoloniferousperennial habit.

SUBSEQUENT ENTRY OF NATIVE SPECIES INTO

ADVENTIVE PLANT COMMUNITIES

Fern (N) versus various herbs (A)

The berms of ditches, water-races and irri-gation channels along roadsides on the dryCanterbury Plains provide an example of anative fern, Blechnum capense, coming inand establishing in extensive linear colonies,associated with such perennial adventivesas 'Agrostis spp., *Bromus spp., *Dactylisf!.lomerata, *Festuca rubra, *Trifolium hy-bridum and forms of *Mentha piperita.This community might have been placed inthe previous category, but I have seen it inprocess of development along water-wayswhich have had no major bank disturbancefor years.

Climbing shrub and under-shrub (N) versus

spreadil1g shrub (A)

Just as efficiently as it establishes in nativeplant communities, Muehlel1beckia com-plexa enters and competes successfully inmany places through the thousands of milesof planted *Ulex europaeus hedges; it some-times smothers the supporting shrub andreplaces it with a dense mass of inter-twinedbranches. In this community Muehlenbeckiahas come to be regarded as an importantweed, because the tough wirv stems aredifficult to cut, and foul the blades, arms andspindle of the mechanical hedge-trimmer.

In the Cheviot distriCt,. Clematis afoliatahas come into hedgerows of *Crataegusmonogyna and *Lycium ferocissimum, andover a ten-year period has covered many ofthe supporting shrubs. Examination showsseedlings in addition to mature plants ofClp.matis and time alone will show whetherthis two-plant community can maintain it-

HEALY: PLANTS 41

self, or whether the climber will eventuallysmother out the supporting shrub.

Broad-leaved herbs (N) versus grassy-leavedherbs (A)

Domestic lawns and playing greens, oftenwith artificial soil conditions and with re-current close mowing and rolling, are goodcommunities in which to watch the inter-action of native and adventive species.

So successful have been the invadingnative species in this artificial habitat, thatthe acceptance of the so-called "weedgreens" and "weed lawns" by householdersand turf-users alike is a tribute to the abilityof certain native species to adapt themselvesto entirely new conditions and usurp theadventive vegetation. Swards initially of*Agrostis tenuis and *Festuca rubra var.commutata (often with such volunteerplants as 'Bellis perennis, *Cerastiwn spp.,*Hypochaeris radicata, *Plantago lanceolata,*Sagina procumbens, 'Saliva spp. and *Tar-axacum officinale, to mention but a few) areinvaded and sometimes ultimately replacedby one or other of the following natives -Centella unif/ora, Cotula spp., Dichondrarepens, Gnaphalium spp., Hydrocotyleamerica, H. moschata, H. novae-zealandiae,Nertera granadensis (syn. N. depressa), N.setulosa, Oxa/is corniculata, Plantago tri-andra, Pratia angulata, and P. perpusil/a.While representing diverse plant families,these plants have, with one exception, thecommon characteristic of creeping, rootingstems and the ability to increase their dom-inance in an adventive community and apeculiar habitat.

Some of these New Zealand native specieshave shown that they can enter similarcommunities outside New Zealand; andMcClintock (1960) has recently reported thesuccessful growth of some of them in lawnsin the British Isles, with Cotula dioica asprobably the most frequent representative.

Lawns and greens are commended forstudy as being of taxonomic as well asecological interest: are the species ofDichondra, Gnaphalium, Hydrocotyle andOxalis conspecific with those so-named over-seas, are the plants in our lawns truly nativeor truly adventive, or are they in part nativeand part adventive?

SUBSEQUENT ENTRY OF ADVENTIVE SPECIES

INTO NATIVE PLANT COMMUNITIES

Short-lived al1l1ual (A)

annual (N)

Truly ephemeral species in the New Zea-land flora are rare, and the presence of oneas a significant member of a communityseems worthy of mention. On what wouldappear to be denuded hillside basins in partsof Central Otago, there exists in spring toearly summer a mixed community of Myo-surus minimum and the Chilean *Amsinckiaaugustifolia, with occasional relict plants ofan umbellifer (Oreomyrrhus?) and scatteredplants of 'Hordeum 111l1ril1wn.The ephem-eral character of this small plant enables itto retain its co-dominance each year againsta longer-lived, larger species.

versus ephemeral

Al1l1ual and perennial herbs (A) versus an-

nual and perennial herbs and shrubs

The shingly, high-level terraces of themain North Canterbury rivers carry in manylocalities a vegetation characteristicallyannual. but with several perennial herbsand shrubs. One community of commonoccurrence is predominantly adventive:*Acaena ovina, *Aira caryophyIlea, *Bramusmallis, *B. tectorum, *Cerastium glomera-tum, *Erodium cicutarium, *Medicago his-vida. *M. minima, *Rumex acetosella,*Sedum acre, *Stipa variabilis, *Trifolium

arvense, *T. striatum, *Tunica pralifer and*Vulpia spp., in which persist (but do notappear to spread) a meagre representationof natives - Carmichaelia monroi, Dantho-nia spp., Geranium pilosum, Muehlenbeckiaaxil/aris, M. ephedra ides and Tillaea sieberi-ana. This same cOlnmunitv shows modifica-tion under the influence of periodic mowingin golf-course fairways - the native elementpersists with the exception of Til/aea, whilethe adventive element is markedly reducedin number of species.

Tussock grass (A) versus modified low tus-.

sock grassland

Modified low tussock grassland in partsof North Canterbury and Marlborough hasbeen under invasion by a tussock, *Nassella

42 NATIVE AND INTRODUCED SPECIES IN NEW ZEALAND

trichotoma, for about half a century, and allstages of the mixed community from initialappearance to an ultimate replacement com-munity of one species may be found. Thepre-invasion community of relict Poa caespi-tosa and/or Festuca novae-zealandiae tus-socks, sheltering a range of native and ad-ventive herbs was cumulatively weakenedby fire and grazing and a rigorous climaticregime, which promoted a dominance ofshort-lived adventive annuals, an increasein bare ground and corresponding lack ofcompetition, and acceleration of invasion of*Nassella.

The initial stage of establishment of thisadventive was interesting, in that in thescattered tussock phase, the effect was thesame as occurs with increase of the nativetussocks - a greater amount of shelter forthe ground laver and an increase in numberof many of the inter-tussock species, bothnative and adventive. The community wasdynamic, however, not static, owing to thecontinued operation of the modifying fac-tors, and consequently it did not, and couldnot, remain at this stage. This adventivespecies was better adapted than any of thepre-invasion species of the community toincrease its numbers, especially on thesunny north-west hill slopes; and with theprogressive replacement of the other nativeand adventive species, a virtual one species,*Nassella trichotoma community resulted,with occasional relict native or adventiveplants.

This replacement community on the sunnyface then in some instances brought intoexistence a further altered mixed communityon the adjacent shady hill slopes. Because*Nassella was unpalatable, stock were forcedto graze more heavily on the communitieson the sour or shady slopes, which at thatstage were made up of a greater numberof native species and more permanent adven-tive species. The heavier grazing and tramp-ling reduced the vigour of the community,eliminated some species, and created condi-tions which allowed *Nassella successfullyto enter into the community.

Gymnosperm (A) versus modified low tus-sock grassland

Characteristic of the mixed communitiesdiscussed has been the not-too-great dispar-

ity in size of the members and in durationof the life cycle.

The en try of *Larix decidua and *Pinus

murrayana into modified low tussock grass-land has given mixed communities in whichthere are marked differences in size, generalecological characteristics, earliest age ofreproduction and rate of reproduction ofthe significant species. The development ofsuch communities is of necessity slower thanin the types considered earlier, and timealone will reveal their ultimate character.

Herbs and shrubs (A) versus ground andshrub layer, modified native forest

.

The following communities are all foundin remnants of modified native forest andprovide some types which merit study.

Herbs (A) versus ground layer species

Examination of forest remnants in manyNorth Island districts, especially of the typeof small reserves on the flat which havesuffered from grazing and mechanical dam-age from livestock, reveals instances of entryof *Selaginella kraussiana or *Tradescantiafluminel1sis, the latter particularly wherefloods are experienced. Where the ferns andseedlings of native species have been eatenout or trampled out, and a complete coverof one or other of the above-mentionedadventives has developed, it appears impos-sible for the ferns and seedlings of nativespecies to enter into the community. Wheredestruction was not complete, these adven-tives gradually assumed complete domin-ance.

In Riccarton Bush, Christchurch, *Hederahelix and *Iris foetidissima' form exten-sive colonies on the floor of a remnant ofmodified swamp forest and effectively pre-vent the growth of seedlings of nativespecies. In one portion of this bush, a largecolony of ''Iris has come in contact margin-ally with a similar sized colony of Polypo-dium diversifoliwll and over a six-yearperiod neither spread into the other's terri-tory.

Shrubs (A) versus shrub layer species (N)

In some North Island localities, followingstock damage to the ground and shrub layerspecies, *Berberis darwinii has entered inand appears, particularly where the canopy

SAXBY: GR,ISSLAND PLANTS 43

is at all thin, to be capable of slowly gainingdominance over a range of native species.

Another ornamental, *Euonymus ph ello-mana, has in parts of Riccarton Bush as-sumed dominance in the shrub layer and itsseedlings are so abundant on the iloor of theforest that many seedlings of native speciesare smothered. Flooding of the soil duringthe dry summer period may in this instanceassist the native species to playa more vigor-ous part in the community, as it seems thatlack of soil moisture during the summerover a number of years was a factor whichplaced the native members of the forest com-munity as a whole at a disadvantage as com-pared with the adventivespecies.

Climbers (A) versus climbers (N)

A micro-community of the Asiatic ''Lol1i-cera japonica growing with Muehlenbeckiaaustralis and Tetrapathaea tetrandra is par-ticularly interesting, and all three species areintertwined through and over lower trees ofthe canopy: after some years of observationthe final results are still uncertain, althoughrecent indications are that *Lonicera appearsthe most vigorous.

Despite the patchiness of the communitiesdescribed here, the following points mayreasonably be drawn:

SOME INTERACTIONS ON

PLANTS IN NEW ZEALAND GRASSLAND

1. In the interaction of native and adven-tive plant species, representatives ofboth groups have shown their abilityto become relatively permanent mem-bers of mixed communities and to be-come dominants.

2. Certain native species have shown thatthey can adapt themselves successfullyto modified original or even new artifi-cial habitats and communities.

3. Modifying factors may exercise markedeffects on the behaviour of species.

4. Species belonging to both floras anddiffering markedly in botanical rela-tionships, growth habit and originalhabit, have become significant mem-bers of a wide variety of mixed plantcomn1unities.

5. Many mixed communitiesprocess of developmen I.

are still in

REFERENCES

ALLAN,H. H., 1931. The alien flora of New Zealand. N.Z.loam. Agric. 42: 392~394.

MCCLINTOCK,D., 1960. Alien lawn weeds. Journ. Roy.Hortie. Soc. 85: 276-279.

SMITH, C. M., 1957. Changed and changing vcgetation.In Science in Nc\'v' Zealand. Ed. F. R. Callaghan.Govt. Printer, Wellington.

NATIVE AND INTRODUCED

S. H. SAXBY

Department of Agriculture, Wellington

A study of the history of the developmentof grassland in New Zealand is also a studyof a prolonged interaction between nativeand introduced plants in which there hasusually been a strong and deliberately guidedbias towards the supremacy of the intro-duced plants. Because much of the nativevegetation does not meet the requirementsof the farmer, he has tried to replace thiswith plants that do so.

This programme of replacement has beendependent on many aids. Fire, axe, plough,fences, fertilisers and animals have allplayed a parI. Much of this programme wascarried out as described by H. Guthrie-Smithin Tutira: " . . . stamped, jammed, hauled,murdered into grass.". Although this refersto the conversion of bracken fern countryinto grassland, it is typical of the actiontaken under many conditions to destroy the