TANE 24, 1978 BOTANICAL FEATURES OF THE MOKOHINAU ISLANDS by A.E. Esler Botany Division, DSIR, Private Bag, Auckland SUMMARY The vegetation of the islands is very depleted. Burning and grazing have left pohutukawa {Metrosideros excelsa) and ngaio (Myoporum laetum) as the only large woody plants on Burgess Island and the neighbouring islets. Burning has promoted 2 monocots — flax (Phormium tenax) on the western islets where there is no grazing, and Scirpus nodosus (and some grassland) on Burgess Island where livestock have not allowed flax to establish. A relic piece of bush on Fanal Island is supplying seeds for the spread of forest there. The Mokohinau Islands have about 112 species of native plants and about 80 naturalised species. INTRODUCTION Perhaps the earliest written comment on the plant life of the Mokohinau Islands was by F. Sandager, a lighthouse keeper. In a paper on birds (Sandager 1889) he mentioned as prominent plants Metrosideros, Pittosporum, Myoporum, Coprosma, Hebe, Carmichaelia, Olearia, Phormium, Disphyma, the ferns Pteridium aquilinum (bracken) and Adiantum aethiopicum, and grasses and sedges. Mary E. Gillham visited the islands in August, 1957, described the plant communities, drew a generalised vegetation map, and listed the plant species (Gillham 1960). My paper supplements the earlier accounts and gives islands of occurrence for each plant species listed. The opportunity was taken to visit the islands with C.R. Veitch (Wildlife Service), A.R. Thorpe (Hauraki Gulf Maritime Park) and G. Kuschel (DSIR) from 27 February till 2 March, 1978. Two and a half days were spent in the field visiting seven islands in the group. This included about three hours on Fanal Island for which there was no previous botanical account. HISTORY There is no evidence of permanent Maori occupation of Burgess and the western islets but it is known that visits were made annually to harvest mutton birds. The lighthouse began operating in 1883 and there has been continuous occupation from that time. Goats were released at an early date and sheep and cattle have been grazed on Burgess Island. Dairy cattle still graze regions a, c, and 187 Hg. I. Map of Mokihinau Islands. 188 d (Fig. 1). During World War II Burgess Island played a role in the defence of the country. Photos in a paper on the geology of the Mokohinau Islands (Fleming 1950) show some of the buildings used by men stationed there. All the islands have been subject to burning. Gillham recorded that Burgess Island was fired about every 3 years in an attempt to suppress the tussocky sedges. She also mentioned fires on the 2 larger of the western islets in 1932 started by fishermen. From the vegetation it seems that they have been burnt many times. There is evidence of Maori occupation on Fanal Island 5km to the south-west, and the state of the vegetation suggests that there has been burning up till a few decades ago. It is likely that there are kiore (Rattus exulans) on all islands we visited. PHYSICAL FEATURES The islands are composed of pale coloured rhyolite eroded into steep cliffs on nearly all sides. Where the topography is not influenced directly by marine erosion the land has fairly gentle slopes. The lighthouse at 107m above sea level on a plug of andesite stands well above the surrounding country. This intrusion is cliffed on the seaward margin and slopes inland fairly regularly to a minor valley near the centre of Burgess Island. There is a boulder beach at the landing, a sandy beach at Maori Bay and some small rocky beaches in sheltered places on the same island and on Trig Island. VEGETATION The vegetation on all of the islands has been grossly modified. On Burgess Island grassland and communities of Scirpus nodosus predominate. Pohutukawa has persisted in a few places. The western islets are flax-dominant with pohutukawa increasing in many parts. Lizard Island is mainly shrubby. On Fanal Island enough forest escaped burning to provide the beginnings of a mixed forest. The remainder of the island has flax and shrubland. Burgess Island can be divided into 5 regions on the basis of its vegetation (see Fig. 1). a. is predominantly Scirpus nodosus, bracken and Muehlenbeckia complexa with some patches of buffalo grass (Stenotaphrum secundatum). This region contains most of the pohutukawa on the island. The 1960 air photos show forest on the headland south of Pohutukawa Gully and two trees (or clumps of trees) on the slope facing the channel nearby. This slope now has about a 50% cover of pohutukawa trees about 4m tall. Pohutukawa is establishing also in two patches between this channel and the landing. Most of the outcrops between the landing and the Blowhole have young pohutukawa trees. It seems that bracken will replace the Scirpus nodosus and this may eventually give way to pohutukawa. There is already vigorous growth of young pohutukawa in the bracken on the 189 ridge. This region has not been grazed for many years. b. is dominated by Scirpus nodosus. Some Leptocarpus similis grows along the creek on the western margin. This area is grazed. c. is almost covered in buffalo grass (Stenotaphrum secundatum) filling all the valley and crossing a line drawn between the lighthouse and the landing. South of this line there is rank mixed pasture with cocksfoot (Dactylis glomerata), paspalum (Paspalum dilatatum), prairie grass (Bromus unioloides), ratstail (Sporobolus africanus), Muehlenbeckia complexa and Scirpus nodosus. The upper slope below the lighthouse in the direction of the keepers' houses has a cover of bracken. On a scarp above the boulder beach there is a very minor patch of bush containing Coprosma macrocarpa, ngaio (Myoporum laetum), kawakawa (Macropiper excelsum), Parsonsia heterophylla, Sicyos angulata and a few ferns. This is the only community of this nature on Burgess or the adjoining islets. Buffalo grass is the dominant feature of this part of the island. It grows to a metre tall and was aptly described by Gillham as "mattresslike". Cattle graze only the more turfy parts of the sward which arc fairly limited in area. As buffalo grass spreads it eliminates nearly all other herbaceous plants in its path on wet soils and dry, in exposed places and in shelter. At the landing only a narrow strip of Muehlenbeckia complexa separates the buffalo grass from the wave-washed boulder beach. d. has a grazed mixed sward with some patches of buffalo grass. Scirpus nodosus and Cyperus ustulatus become more important on the irregular topography. The main pasture species are paspalum, cocksfoot, prairie grass, rye grass (Lolium perenne), Yorkshire fog (Holcus lanatus) and sweet vernal (Anthoxanthum odoratum). On the drought-prone rocky outcrops there are greater quantities of Notodanthonia spp., the annual grasses (Vulpia, Briza, Aira), the annual legumes (Lotus subbiflorus and /.. angustissimus), some other annuals such as allseed (Polycarpon tetraphyllum) and Portulaca oleracea together with a few natives Rhagodia triandra and ice plant (Disphyma australe). On the steep slopes inside the Cauldron ice plant and taupata (Coprosma repens) cling to the cliffs in fairly large quantities. e. has Scirpus nodosus with abundant Poa anceps (fine-leaved form), Adiantum aethiopicum, some cocksfoot and Doodia media. Cassinia retorta is prominent in a few places and there are limited patches with Notodanthonia racemosa and annual grasses. The slopes above the cliffs are rocky with Cyperus ustulatus, Scirpus nodosus, some Cassinia retorta, ice plant and a few bushes of ngaio. Goats were eliminated from this part of the island some years ago and cattle do not venture across the narrow rocky ridge which connects this region to the remainder of the island. The western islets are capped with flax and some pohutukawa. On Trig Island there are only a few pohutukawa trees. Islet 1 has more flax than pohutukawa but the small cap in Islet 2 is composed mainly of pohutukawa. The northern extension of Island 3 has more than a 50% cover of pohutukawa and is well on 190 the way to a complete cover. There is much less pohutukawa on the remainder of the island. On all of these islets stunted Scirpus nodosus with some Cassinia retorta forms small patches where the soil is too shallow for flax to flourish. On these patches and on the rocky outcrops, pohutukawa can establish freely. Where the flax is dense and up to 3 m tall pohutukawa has no chance of invading unless the flax loses vigour. There are insufficient sources of seed for Coprosma macrocarpa and ngaio to be effective in suppressing flax. It seems that most of the seeds of Coprosma macrocarpa are eaten by kiore as soon as they ripen. Where the slope changes at the top of the cliffs a few other species can be found. There is some ngaio. On Islet 3 there are two stands of ngaio, the only places on the Mokohinau Islands where the species could be mapped as a community. The cliffs provide some footholds for ice plant, taupata and Chionochloa bromoides. [The Chionochloa is referred to by Gillham as Carex sp. in error.] Lizard Island has a windswept community of taupata, ngaio and Hymenanthera novae-zelandiae to 1.5 m tall surrounding a patch of Cyperus ustulatus. In some places the islet has a turfy fringe of Salicornia australis and ice plant grading into shrubland. Fanal Island may have been grassy at one time with some bush persisting in the large gully. Shrubland and young forest have developed in some parts, and communities of flax in others. There is flax in the central valley and on the sides of the large valley near the coast. The northern valley which we did not see appears from the 1960 air photos to have flax too.