Forest Deterioration in The Tararuas due to Deer and Opossum.
The inner hills east of the main divide of the Tararua Mountains, with the exception of the high tops (4,000ft. to 5,000ft.) are nearly all forested. Mountain scrub, mainly leatherwood (Olearia colensoi), occurs in extensive areas in the northern reaches. In the valleys the main species are clinker beech (Nothofagus truncata.) and red beech (N. fusca), either or both, together with rimu (Dacrydium cupressinum) and rata (Metrosideros robusta). Kamahi (Weinmannia racemosa) is more or less abundant throughout and there are numerous species of under-storey and lower undergrowth. Above 2,000 ft. elevation the forest tends to be predominantly silver beech (Nothofagus menziesii), which is the only tall tree reaching the timber line (4,000ft.).
In an undisturbed state, as I knew it in the summer of 1930–31, the forest had a dense undergrowth of mosses, ferns, various herbs and shrubs. The undergrowth was particularly dense at elevations above 2,000 ft. The undergrowth is now rapidly disappearing and in some places forest trees are also
going. At the time of my early visits damage done to the forest vegetation was already appreciable, although, perhaps, not impressive to a casual observer. The animal population which was doing damage was then already great. It was common to see groups of deer on the high country in summer throughout the length of the Tararuas. In the Orongorongo Ranges there were numerous goats. Opossums were plentiful throughout. These animals and depredations were briefly noted in my paper dealing with the vegetation of the Tararua Mountains (Trans. Roy. Soc. N.Z., vol. 68, pp. 250–324—see especially pp. 278–280).
In the course of my investigations in the Tararuas in those days I made fairly detailed records of the vegetation as then found in some of the localities. In my present investigations I have revisited some of these localities as sample areas on which any changes that have occurred in the intervening period could be readily observed. Thus I have visited, in company with Mr. M. E. Roberts, the Ruamahanga-Ruapai Creek area, and, with. Mr. A. P. Druce, the Mount Holdsworth and Bull Mound–Hell's Gate areas in the headwaters of the Tauwharenikau River.
The top of Hell's Gate is 3,900ft. above sea-level. It is 300ft. above the saddle on Mount Alpha side and 600ft. above the saddle on Bull Mound side. Down to 3,300 ft. level it has an area of about 600 acres. This area was under mature silver-beech forest with a proportion of over-mature trees and comparatively few young trees. There was heavy undergrowth, hardly passable off the track, and consisting of a large number of species of which, the following were in abundance: Danthonia ounninghamii, Phormium colensoi, Astelia cockaynei, A. nervosa, Olearia colensoi, Senecio elaeagnifolius, Coprosma foetidissima, C. pseudocuneata, Nothopanax colensoi. The following were very common: Olearia lacunosa, O. arboresens, Coprosma parviflora, C. colensoi, Pittosporum rigidum, Suttonia divarioata, Griselinia littoralis, Nothopanax sinclairii, Fuchsia excorticata, Polystichum vestitum.
At the time, the barking of small trees attributable to deer and involving particularly the species of Nothopanax and Griselinia was noted. On my first visit to Hell's Gate in the summer of 1930–1931 the vegetation, to all appearances, was undisturbed. Thinning of undergrowth and trampling of ground on the top was observed on my 1933–1934 summer visit. This was progressively more extensive in the following 1934–1935 and 1936–1937 summers. Changes then were still not appreciable enough to catch a casual eye. A very different scene confronted me on my last visit in January of this year. Tree tops were reduced or thinned, many trees standing dead. Viewed from Bull Mound across the gully, old logs of fallen trees lying on what appeared to be bare ground were clearly visible between growing trees. On close inspection we found on the slopes facing Bull Mound (to the east) all undergrowth less than 8ft. tall to be virtually completely dead. Of the taller plants, occasional trees of Griselinia and Olearia lacunosa were still alive. Of smaller plants a number of species could be listed. None of them, however, exceed half an inch in stature or else were grazed down to that height. The common species to be found there, forming small patches, are Nertera dichondraefolia, Poa breviglumis, Ranunculus hirtus, Isolepis aucklandicus, a few liverworts and mosses. In general, however, the ground is now bare of living plants and the surface is bare for a hundred yards in every direction, only the standing trees of silver beech obstructing the view beyond. The top of Hell's Gate is in much the same state. On the face sloping towards Mount Alpha and, so far as I could judge from the view obtained and from actually descending to the saddle, right up to Alpha timber-line the same conditions. obtain, except that occasional shrubs, particularly species of Olearia, are still alive.
What happened in this area may be fairly satisfactorily reconstructed from past observations and from such as those made by me on my last visit in adjoining areas. I choose Mount Holdsworth as an example Here the tussock vegetation immediately above the timber-line is in a very good state of preservation, but the forest undergrowth immediately below the timber-line (4,000ft.) down to about 3,000ft. is appreciably damaged by deer, opossums and, to some extent, by pigs. It is quite easy to distinguish the teeth marks left by deer from those of opossums. Besides, the habits of feeding of the two animals are distinctive, one reaching from the ground to a height of seven or eight feet, the other over the tops of individual shrubs and small trees. There are other tell-tale distinctions. The only obvious activity by pigs seems to be confined to rooting in the
ground generally and particularly under such plants as Astelia and Phormium. There is, on the whole, very little evidence of pigs.
Practically every species of undergrowth is here eaten extensively by deer, which feed essentially on young growth within reach. This applies to Astelia, Phormium, Danthonia, Coprosma foetidissima, Seneoio elaeagnifolius, Olearia arborescens, but apparently not O. lacunosa or O. colensoi. At the time of our visit here we did not actually see any deer, but relatively fresh droppings and fresh footprints were seen literally everywhere.
No herbaceous plants were observed as having been eaten by opossums, but every species of shrub and small tree was found to be extensively eaten by it. Numerous plants of small tree species are here dead or in a dying state, being completely or partially defoliated by this animal. Olearia colensoi (leatherwood) and, apparently, O. lacunosa do not escape. Particularly striking examples of this were seen on Mount Matthews, where the animal was feeding on the young shoots of leatherwood. It escaped my observation, but Dr. K. A. Wodzicki, of this department, informs me that he had seen in the Mount Ross area (Haurangi Mountains) ground littered with pieces of young shoots of silver beech. This phenomenon, in view of all the known circumstances, could be attributed only to the opossum. Here is, therefore, a suggestion to explain the sudden appearance of numerous dead and dying trees of this species on Hell's Gate, the animal paying it special attention in the absence of other growth.
On my earlier visits this particular slope of Mount Holdsworth had much the same dense vegetation as Hell's Gate. I judge that the state of the vegetation on Holdsworth now is in a similar state to what it was on Hell's Gate three to five years ago. In other words, I expect the vegetation on Holdsworth to suffer the same fate as that on Hell's Gate within five years unless the animal population is much reduced. I am not in a position to estimate the animal population. However, I record that within a day's walk through the forest in the neighbourhood of Hell's Gate we counted twelve deer actually seen.
The vegetation on Haukura ridge leading to Mount Ruapai is in much the same state as on Mount Holdsworth. Here the timber line is less well-defined, dropping down well below the scrub-line (4,000ft.). The scrub, as seen in February, 1946, is in a severely browsed state and the ground is largely devoid of all smaller growth. Immediately above the scrub-line on Haukura II there are two areas of some five acres each almost completely stripped of soil, which was here one and a-half to two feet deep, as may be judged from surviving clumps of tussock perched high above the present surface. It can be readily demonstrated that these eroded areas are a development of the last ten years or so as the result of trampling by deer. There is much ground in the neighbourhood which is now being heavily trampled, with many tussocks of snow grass (Danthonia flavescens) pushed over and dying.
A somewhat similar soil loss occurs on Bull Mound, which is cast and just over the saddle from Hell's Gate. Here at an elevation of about 3,600 ft. is a wide ridge across the top of which straddles a boggy area about four acres in extent. This area is surrounded by silver beech forest. Over the area a number of species of alpine bog plants occur. Especially abundant are Oreobolus pectinatus, Gaymardia cilliata, and Carpha alpina. The first two species are too short for deer to graze while the latter is closely grazed and is mostly not much more than one inch tall. At the time of my first visit Carpha was more abundant and over six inches tall. On top of the area there was a tarn about 30 feet square and some two feet deep of clear water. Close by, along a narrower part of the ridge of the main boggy area, there were two other tarns of about the same size. The condition of the tarns had deteriorated somewhat on my later visits. In February, 1937, the three tarns were deer mudwallows. The tarn on the main area and the one nearest to it have now completely disappeared, their place is occupied by bare rock. The third has not yet completely disappeared. There is some mud left in it. The stones in place of the tarns are bleached ashy-white and are similar to those throughout the area under the peaty soil. The soil, consisting of a black peaty horizon 12 to 18 inches deep over an ashy-white horizon 6 to 12 inches deep, is now stripped off in belts five to ten yards wide on the three dipping sides of the area. Around the top tarn a wide strip of ground is now devoid of plants and the soil from here is washed by rain over the whole area. I was not aware in the past of any direct evidence that this boggy ground was once wooded. Now, however, numerous ancient tree stumps
and their roots project over the consolidated soil. The vegetation surrounding the other vanished tarn, which was mainly snow grass (Danthonia flavescens) and turpentine wood (Dracophyllum urvilleanum), has now disappeared, leaving behind dead remains sitting over some six inches of hard, compacted peat.
On Mount Matthews (3,079ft.), the vegetation is similarly heavily affected by animals. I cannot state to what extent it is worse than on my visit there in January, 1933. It is, however, definitely in an unhealthy state. Here the animals are principally goats and opossums. There is also some evidence of pigs. We did not see any undoubted indication of the presence of deer. Although we did not actually see any goats during the day we were there, relatively fresh droppings were literally all over the mountain along our route, especially above the 2,000 ft. level. This animal appears to eat everything within reach, which is, in general, any shrub or other plant up to four feet high. The windward faces, which are not necessarily “sunny” faces, are most thoroughly eaten. The only indication there is now that any undergrowth ever grew there is the presence of fairly numerous seedlings an inch or so high or at most young plants cropped to that height. The consolidation of soil is everywwhere in evidence. There is a wholesale destruction of bryophytes (mosses and liverworts) as well as all other herbaceous cover. The increase in the size of extensive slips on the mountain is probably closely connected with the increased run-off associated with this phenomenon. Opossums do not appear to be nearly so abundant as on Mount Holdsworth, but their depredations are nevertheless quite a common sight.
Deer and opossums cannot be blamed for all the damage done to vegetation in the Tararuas. I am well aware, for example, that the larva of a moth, tentatively identified as that of the genus Carposina, is responsible for periodic widespread killing of leatherwood (Olearia colensoi) and for attacking seriously Olcai ia lacunosa. It was active at Hell's Gate and Holdsworth on my last visit there. Another example is a widespread disease of Coprosma foetidissima on Holdsworth caused by an ascomycetous fungus. Still other examples are periodic stripping of tree crowns by snow. A rare gale, such as that of 2nd February, 1936, is known to smash forest over miles of hillside in a few hours. All these factors can be accounted for and dismissed from the present consideration of the animal damage.
I am aware that the Internal Affairs Department has deer shooters periodically in the Tararuas engaged on the reduction of the deer population. However, it appears to me that the present rate of deer destruction is quite inadequate, and that this activity, in one form or another, must be extended urgently to opossums. The denudation of the steep hills receiving some 100 inches of rain per annum must bring about disastrous consequences upon the rivers and plains of the Wairarapa if not checked in time.