A Brief Account of the Re-establishment of Vegetation on Tarawera Mountain since the Eruption of 1886.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 59, 1928, Page 60

A Brief Account of the Re-establishment of Vegetation on Tarawera Mountain since the Eruption of 1886. By E. Phillips Turner, F.R.G.S. [Read before the Wellington Philosophical Society, 23rd November, 1927; received. by Editor, 19th December, 1927; issued separately, May 12th, 1928.] Plates 16—19. On June 10th, 1886, occurred the Tarawera eruption which destroyed the incomparable and world-famous Pink and White Terraces at Lake Rotomahana, and which scattered volcanic mud or ash over about six thousand square miles of country. I was, at the time, an assistant on the survey of the railway line which now gives access to Rotorua, and during the two days following the eruption I tramped from our camp at Ngatira, via Rotorua and the Blue and Green Lakes, light through to Tarawera Lake. Though in the vicinity of Rotorua Township the volcanic deposit was little more than a film, it increased rapidly in thickness, and between the Blue Lake and Lake Tarawera (Fig. 1) the mud was so deep that I sank almost to my hips whilst trudging through it. Previous to the eruption, at several places between Rotorua and Lake Tarawera there had been patches of beautiful forest, but now there was no vestige of live vegetation to be seen, as the whole country was buried under a huge thick sheet of mud out of which a few mud-plastered, blasted tree-trunks alone gave proof of the previous existence of the forest. In 1889–1900, from 14 to 15 years after the eruption, I was engaged in surveying lands which had been purchased by the Crown from the Maori owners in the vicinity of Rotorua, the Blue Lake, Lake Tarawera, Lake Rotomaliana, etc., and I then found that many of the forest trees and large shrubs in this locality had merely been stripped of their small branches and leaves at the time of the eruption, and that they now bore profuse and healthy foliage. Indeed many places where the vegetation had previously been Pteridium esculentum or low scrub now bore a completely new growth of P. esculentum, Coriaria arborea or Aristotelia serrata which were growing on the ejected mud or on sears that had been caused by the ejected mud having slid from the sides of steep hills. In the year 1900 on the ash and mud-covered lands abutting on to Lake Tarawera (on the West) and in the vicinity of the new Lake Rotomahana and the celebrated Waimangu Geyser there was a considerable pioneer growth of Arundo conspicua, Coriaria arborea, and Pteridium esculentum. These lands prior to the eruption bore a profuse growth composed mainly of Leptospermum scrub and Pteridium esculentum. I made the ascent of Mount Tarawera in 1900 and, as far as my memory serves, there was then no new vegetation on the western slopes of the mountain, but I regret that I did not make any notes on the matter at the time.

The first account of the new vegetation on the land covered with the matter ejected at the Tarawera eruption of 1886 was given by L. Cockayne in the 1910 edition of New Zealand, Plants and Their Story (3 p. 54), but only Coriaria and Arundo are therein mentioned. In 1913 and again in 1915 and 1916, B. C. Aston (1, 1a) visited Tarawera Mountain, and found its western slopes were carrying a growth of small trees, shrubs, and herbaceous plants, which were abundant near the shores of the Lake, but gradually lessened as the mountain was ascended. In a very interesting account of his Re-establishment of Vegetation on Tarawera Mountain since the Eruption of 1886. visits Aston showed that of the ninety-one species of plants he recorded as growing on the north-western flank of Mount Tarawera 58% were plants which had become established there through the agency of wind; 26% had been spread by birds, and fourteen species were difficult to account for. In January of the present year (1927) I took the opportunity during a short holiday to make a hurried ascent of Mount Tarawera,

which I had not visited since I ascended it in 1900. I was of course immensely impressed with the great changes which had occurred since my first visit, for now near the shore of Lake Tarawera there was a young forest with trees from 7.5 to 12.2 metres high, and above the forest, shrubs and herbaceous plants occupied the ground more or less plentifully right to the summit of the mountain, which at its highest peak is 1149.4 metres above the sea, or 834.7 metres above the mean level of the Lake. The point at which I started my ascent on the present occasion is about midway along the Eastern shore of a large bay at the southern end of Tarawera Lake (vide map), and I followed up a flat-floored steep-walled and narrow valley, which has been eroded since the big eruption of 1886 and extends nearly to the rift-like crater which split the mountain. The soil of the valley—as in other parts of the mountain—is a coarse sand mixed with large quantities of scoria and rock from the size of a pea to blocks many pounds in weight. The origin of the soil has been both rhyolite and andesite, stones and particles of the latter preponderating. In heavy rains a large volume of water descends from the higher slopes, but the soil is so loose and gravelly that probably as much water flows under the surface as above it. In the lower part of the valley (Figs. 2, 3) the chief components of the scrub growth which intermittently occupies the floor are Coriaria arborea, with Leptospermum scoparium and L. ericoides in lesser quantity. Raoulia australis forms numerous mats on the floor, and Pimelia prostrata is also present with a similar habit of growth. On the sides of this part of the valley the scrub formation is more varied and contains in addition Coriaria, Aristotelia Serrata, Nothopanax arboreum, Hebe, salicifolia var., Fuchsia excorticata, and the Two Leptospermums. Scrambling over these small trees Muehlenbeckia australis is often seen, whilst between them there may be Pteridium esculentum. At an altitude of about 518.3 metres (204.2 metres above Lake Tarawera) the mat-plants Raoulia australis, R. tenuicaulis, R. glabra, and Pimelia prostrata occur as scattered colonies on the fine loose scoria or sand. In these colonies there are often other plants, e.g., Hypochoeris radicata, Gaultheria oppositifolia, G. rupestris, G. antipoda, the two Leptospermums, and Danthonia semiannularis var. On gravelly soils or soils composed of coarse sands, as has been elsewhere related by Cockayne (2, 3, 4 and 5) and Aston (1, 1a) these mat-plants, by their action in arresting the movement of the sand particles and also their habit of decaying at the center of the mat, form suitable spots for the establishment of hardy pioneer plants with an erect habit of growth such as those just named. Near this locality (Fig. 4) there are a few small clumps of Coriaria scrub. One of these was composed of C. arborea about 3.5 metres high with two small trees of Pittosporum tenuifolium in the middle 6.9 metres high. On these trees the epiphytic fern Cyclophorus serpens was growing. Around and under the shade of this clump there was quite a dense carpet-growth of Danthonia, Hypochoeris radicata, the grass Holcus lanatus, Acaena Sanguisorbae, Aira caryophyllae, Pteridium esculentum, Centaurium umbellatum, Epilobium pubens, E. sp., Raoulia glabra, R.

australis, Lagenophora petiolata, Pimelia prostrata, moss, Muehlen-beckia axillaris, Microtis uniflora, Polypodium diversifolium, young pittosporums, Pomaderris phylicaefolia, and Anagallis arvensis. Occurring sporadically at this altitude there were also Gaultheria oppositifolia, G. antipoda, Epilobium melanocaulon, two Leptospermums, Pteridium esculentum, Hebe salidfolia var., Dracophyllum subulatum, Thelymitra longifolia, Hypochoeris radicata, Trifolium dubium, Celmisia longifolia, Gahnia gahniaeformis, Leucopogon Fraseri, L. fasciculatus, Gaultheria rupestris, and Erigeron canadensis. These plants extend down to the lake, and, in lessening quantity, up the mountain for over 100 metres. At an altitude of about 853 metres above sea-level the same scattered low shrubs and the same Raoulias, grasses, and Pimelia are found in lesser number. It is interesting to state that in 1900 (14 years after the eruption) the loose scoria at this altitude was still quite warm at about 12 centimetres under the surface, but on the present occasion it was quite cold. Looking into the crater-rift at this point (Figs. 5, 6) one could see, perched at various places on its perpendicular walls, shrubs of Coriaria and Gaultheria oppositifolia—the former probably the result of carriage by birds, the latter by wind. The walls of the crater at this place were too steep to allow one without ropes to descend to test the temperature of the rock and soil, but I was told that at some places in the crater steam is still emitted. At an altitude of about 915 metres above sea-level there is a slope of fine scoria lying at an angle of about 33°. Though the scoria here slides downhill when walked upon, there are established on it odd patches of Raoulia australis, with an occasional Gaultheria, and Hypochoeris. On this loose and unstable slope, where the moisture reaches the surface, there are occasional little oasis-like compact colonies composed mainly of Raoulia glabra, with some R. australis, and a little Hypochoeris, Trifolium dubium, T. repens, a minute Raoulia (apparently a hybrid between R. glabra and R. australis), Pimelia prostrata, and a minute Epilobium which I did not identify. In connection with this special ability of the Gaultheria to colonize unoccupied new lands, I may state that when I was making a topographical survey of the Tongariro National Park, in 1908, I found impressions of Gaultheria rupestris leaves to be common in three distinct strata of soil which must have been ejected at various times from one of the volcanoes in that locality when they were in a very active condition long ages ago. On the summit of the mountain (the southern part, about 1,120 metres, was all I had time to visit) I found an intermittent and sparse vegetation composed of stunted Gaultheria oppositifolia, G. rupestris, an apparent hybrid between G. oppositifolia and G. rupestris, Celmisia longifolia var. gracilenta, Leptospermum scoparium, Dracophyllum subulatum, Danthonia semiannularis var., and Hypochoeris, all of which have probably been established through the agency of wind. The shrubs appear to have become established here since

Aston's visit. I may mention that snow not infrequently covers the mountain in winter, and it may occasionally lie on the summit for as long as a week or perhaps even more. The rainfall in this locality is between 128 and 154 centimetres (50 and 60 inches), and it is very well distributed, no month being without rain, so the climate favours vegetation. The loose and coarse nature of the soil and its great porosity have already been referred to above. With respect to the fauna of this part of the mountain it may be said that blackbirds and thrushes and the indigenous tui, all frugivorous birds, were seen or heard on the lower slopes. Two wild horses were seen, and also recent excreta of rabbits and wild pigs. The native pipit was occasionally seen even high up the mountain. Of the animals, rabbits might occasionally carry grass seeds or the pappus-furnished seeds of the composites or the barbed seeds of Acaena; but pigs and horses would take a far smaller part in the dispersion of such seeds. All the plants observed by me on and near the summit of the mountain were such as have either very light, small, or pappus-furnished seeds, and this fact confirms the observations of others that wind is definitely the chief agent in the extension of vegetation on a virgin soil of the nature and under the conditions described as existing in this locality. The drupe-bearing Pimelia prostrata was found as high as about 850 metres, but I am of opinion that the seeds of even this plant may have here been dispersed by wind after their surrounding pulpy matter has been desiccated. The pulpy-fruited Coriaria and Coprosmas of the lower slopes are undoubtedly spread mainly by birds. Twenty-five years after the eruption of Krakatau near Java, Ernst (4, 6) found that the three factors instrumental in the re-establishment of vegetation on the island were sea currents, wind, and birds. The determination of the agencies of distribution was in his case, however, by no means easy, but he gives sea currents as being probably responsible for 39 to 72%; wind, 16 to 30%; and birds 10 to 19%. Krakatau is distant 11½ miles from the nearest land with vegetation. Though after the eruption of 1886 the western slopes of Tarawera Mountain wore an isolated area, there were no sea currents to bring plants from other regions; hut wind and birds exercised practically the same relative influence as they did in connection with the establishment of the new flora of Krakatau. Willis (7) before referring to the classical case of Krakatau (Ernst 6) gives the case of Ritigala in Ceylon. It is an isolated mountain 765 metres high and separated from a wet zone area by a plain 40 miles wide with a dry climate. The summit of the mountain has a wet climate and is populated by 103 wet-zone plants. Of the 103 plants, 49 had light fruits or spores suited to carriage by wind, 24 were suited to carriage by birds, and 30 were doubtful. That wind should be such a preponderant factor in the dispersal of plants which have seeds with a special mechanism (e.g., pappus hairs), or which are very light is not surprising when one remembers that loess soils have been formed by accumulations of dust which has

Fig. 1.—Rift-like crater at south end of Mt. Tarawera from southern extremity of Lake Tarawera, showing flanks of mountain covered with volcanic ejecta—a few months after the eruption of 1886. Fig. 2.—About 48 metres above Lake Tarawera, in large, flat-floored wash-out, looking west towards lake. On floor is Coriaria, on sides are Coriaria, Aristotelia, Nothopanax arboreum, Fuchsia, Hebe salicifolia var., and two Leptospermums. On high spur on right is a dense and vigorous growth of above, with Metrosideros, Knightia, Melicytus, Beilschmiedia, tree-ferns, &c. Lake Tarawera is 314.5 metres above sea-level.

Fig. 3.—From same spot as Fig. 1, but looking towards south end of Mt. Tarawera, The tall conical tree is Eucalyptus globulus. The dark mass near summit of mountain is rocky cliff. Fig. 4.—From the same spot as Fig. 3, but looking north-west across Lake Tarawera. A clump of Pittosporum tenuifolium (about 7 metres high) and Coriaria (about 5 metres High). Under the shrubs is a carpet of Danthonia, Hypochoeris, Acaena sanguisorbac, &c. Frequent patches of Raoulia, shrubs becoming sparser.

Fig. 5.—Near top of the big rift on southern end of Mt. Tarawera, looking easterly across crater-rift. On cliffs are odd shrubs of Coriaiia, and Gaultheiia oppositifolia, the latter occasionally on the scoria slopes. Altitude roughly 853 metres.

Fig 6.—Shewing the rift-like crater extending down the south-western flank. Taken in February, 1927, from about one mile from the easternmost end of Lake Rotomahana. Vegetation mainly Coriaria aiborea and Pteridium esculentum. Fig. 7—Taiawera mountain as in February, 1927. New vegetation at Wairoa Village at the western end of Lake Tarawera. On the left are chiefly Aristotelia, Weinmaunia iacemosa, Coriaiia arboiea, Copiosma robusta, Pteridium, &c. On the right the tree is an exotic, its species not noted at the time.

been transported by wind from mountains distant sometimes several hundreds of miles. Dust discharged at the eruption of Tarawera Mountain in 1886 was carried by wind and deposited as far as sixty miles from the mountain. On one occasion when on a steamer travelling to Sydney and about 500 miles from land, I saw thistle pappus floating in the air, though the pappus was not close enough for me to make sure that a seed was attached to it. Even such heavy matter as sand on a desert surface is transported hundreds of miles. The establishment of the vegetation on this mountain during the forty years which have expired since the eruption in 1886 has probably on the whole been somewhat retarded by wild pigs, horses, and rabbits, as these animals will, when food is scarce, nibble off most unpalatable plants which, even if not destroyed, may be prevented from bearing fruit: moreover, the coarse sandy nature of the soil on the slopes of the mountain is such that it is much disturbed when walked over by animals, and consequently the establishment of plants on it is, in this way, further slightly retarded. In other localities I have observed that wild pigs and horses spread brier (both animals will eat the berries), but in the particular locality herein dealt with I saw no brier to be spread. Other animals and man have had no detectable effect on the re-establishment of the vegetation on the locality now dealt with. Though wind has undoubtedly been the chief factor in the repopulation with plants of the virgin area of Tarawera Mountain, this repopulation has taken place from localities close to the virgin area, as has been shown by Cockayne (5) in his third edition of New Zealand Plants and Their Story. Owing to shortness of time I could not explore the country laterally from the route by which I ascended the mountain; but there is no doubt that the re-establishment of plants on the south-western slopes of the mountain is taking place far less quickly than it is on the north-western slopes, and also, along the route taken by me, there are far fewer genera and species than those noted by Aston on the north-west. North of the point where I landed, along the lake, and sometimes extending to about 200 metres in altitude up the mountain, a young forest is forming. The components of this forest have been fully given in Aston's paper, but I may add that at present it is just emerging from a condition of even growth, and the tall-growing trees Metrosdieros tomentosa, Beilschmiedia tawa, Knightia excelsa, and Weinmannia racemosa are now shooting above the smaller trees Aristotelia serrata, Pittosporum tenuifolium, Nothopanax arboreum, Fuchsia excorticata, Geniostoma ligustrifolium, Dodonea viscosa, tree ferns, and shrubs, and will form the top story of an ultimate high forest. Of the leading trees the Knightia is prominent by reason of its evenly conical growth-form. Of the plants noted there were nine exotic species, and of these three had become established through the agency of wind, three through the agency of animals, and three were of doubtful origin. It is interesting to state that in the shrubby growth shown in Fig. 3, I found a solitary blue-gum (Eucalyptus globulus) which is an

exotic. This tree was about 12.2 metres high, and as there is no blue-gum growing at a closer point than Wairoa, about five and a-half miles away and across the lake, it seems reasonable to conclude that the seed must have been carried by wind to the spot where the young tree is now growing. The age of this tree would probably not exceed about 13 years, as the blue-gum is of exceptionally quick growth. Shortness of time prevented my making a more comprehensive examination of the new surface of Tarawera, but the foregoing observations are probably worth recording for use by future students of what is going on in that region. Literature Referred to. 1. Aston, B. C., 1916. “The Vegetation of the Tarawera Mountains, New Zealand.” Trans. N.Z. Inst., vol. 48, 1915. 1a. Aston, B. C., 1916. “The Vegetation of the Tarawera Mountain, New Zealand: Part 1, the North West Face.” Journ. of Ecology, vol. 4. No. 1. 2. Cockayne, L., 1908. “Report on Botanical Survey of Tongariro National Park, Lands Department Report, Wellington. 3. Cockayne, L., 1910. New Zealand Plants and Their Story, p. 54, Wellington. 4. Cockayne, L., 1919. New Zealand Plants and Their Story, 2nd Ed.: pp. 194-198. 5. Cockayne, L., 1927. New Zealand Plants and Their Story, 3rd Ed.: p. 104. 6. Ernst, A., 1908. The New Flora of the Volcanic Island of Krakatau (trans, by A. C. Seward), Cambridge. 7. Willis, J. C., 1922. Age and Area, Cambridge.