The Subalpine Scrub of the Hokitika Catchment, Westland

By P Wardle

I. Classification of the Communities

In the following account, the subalpine scrub vegetation is subdivided into categories, each named according to the most abundant dominant species. The following categories are recognised:

• Hoheria glabrata low forest.

• Olearia low forest.

• Olearia colensoi scrub.

• Dracophyllum uniflorum scrub.

• Dacrydium biforme scrub.

• Cliff communities.

Under each heading, there is a description of a typical community, followed by a general account. All the typical communities were described in the top basin of the Toaroha River, while the general accounts have been compiled from observations made throughout the Hokitika catchment.

Hoheria Glabrata Low Forest

Typical Community

Altitude, 3,200ft. Slope, 38°. Aspect, N. E.

Canopy. Pure Hoheria, the trees being 15–20ft tall, with diameters averaging 10in. ^* Stand apparently even-aged.

Ground Cover. Dense Polystichum vestitum. Ranunculus hirtus is the main plant between the clumps of fern. Coprosma depressa and Astelia cockaynei are less common.

Regeneration. Hoheria seedlings up to 3 years old and 6in tall are abundant. Apparent saplings of Hoheria are really coppice shoots from stumps of decadent trees. Saplings of Olearia ilicifolia 1–10ft tall are frequent.

Soil 1in: Matrix medium-dark brown, fine sand, with numerous small schist fragments; loose crumb structure.

2in: Grey zone, probably slightly leached.

>5in: Light brown sand and stones.

The material of the whole profile is loose. Roots are distributed throughout but are densest at 0–1in. Earthworms are present.

General Account

Hoheria glabrata communities are always found on young, deep, moist, welldrained soils, the usual habitats being healed slips, talus slopes and alluvial fans. Due to the rapid erosion, these habitats are extensive, and Hoheria forest comprises a large proportion—perhaps a third—of the vegetation of the scrub belt. The main features are the dominance of Hoheria in the canopy and of Polystichum vestitum in the ground cover. Olearia ilicifolia is usually present, and may be co-dominant. Species less constantly present include Olearia lacunosa, Coprosma ciliata and Phormium colensoi.

Below 2,500–2,800ft the numerous small trees which colonise similar habitats at low altitudes begin to enter; the first are Fuchsia excorticata and Aristotelia serrata. A. community of Hypolepis millefolium, Polystichum vestitum, Danthonia flavescens and Phormium colensoi often develops on young, loose soils both above the altitudinal limits of Hoheria, and in places where the succession to Hoheria forest is delayed.

The pH values of several samples of soil collected in July, 1959, were measured with a Marconi pH meter (Type TF 889/1), using a mixture of approximately 2 volumes of water to 1 of soil. Two samples of soil collected under Hoheria forest at depths of 1–3 inches gave values of 5.1 and 5.3.

(C.f. Hoheria glabrata low forest, Cockayne, 1928, p. 267.)

Olearia Low Forest

Typical Community. Altitude, 2,800ft. Slope, 5°. Aspect, N. E.

Canopy. Co-dominance of Olearia lacunosa, Dracophyllum traversii and Hoheria glabrata. There is also some Olearia ilicifolia. The Dracophyllum traversu is 20–25ft tall and about 10in in diameter. The lower parts of the trunks are inclined downhill; in some trees the trunks are prostrate for a length of 6ft, but no adventitious roots were detected Olearia lacunosa is as tall, and grows to 20in in diameter Hoheria glabrata is a smaller tree.

Shrub Storey. Open, includes Coprosma ciliata and C. pseudocuneata.

Ground Cover This is sparse, due to browsing and trampling. Polystichum vestitum, Coprosma depressa and Uncinia sp are the most important species. There are also Phormium colensoi and Astelia cockaynei.

Regeneration. Seedlings of Olearia ilicifolia and O. lacunosa are common (being comparatively browse-tolerant or unpalatable). There are occasional young plants of Dracophyllum traversii.

Soil. There is a continuous litter of Dracophyllum traversu leaves.

4in: Fine sand, coloured dark brown by humus.

8in: Grey-brown silty sand with numerous, unweathered fragments of schist Roots are mostly between the surface and 6in depth.

General Account

Communities such as the above occupy, like the Hoheria glabrata low forest, deep, moist, freely drained soils But the soils are older with more advanced development of profile, and the communities are more varied and richer. On the old alluvialfan where the typical community is sited, there is considerable variation. Dracophyllum traversu, Olearia lacunosa, O. ilicifolia and Hoheria glabrata may each be clearly dominant, or conversely, absent over small areas. The shrub storey shows similar variations Usually the canopy of Olearia low forest is pierced by trees of Libocedrus bidwillii and occasionally by Podocarpus hallii Olearia colensoi, Senecio elaeagnifolius and Archeria traversii are present where the tree canopy is not continuous.

Olearia Colensoi Scrub.

Typical Community. Altitude, 3,200ft Slope, 45°. Aspect, N. E.

General Shrub Canopy. Olearia colensoi contributes half of the canopy. The plants measure 12ft from ground to apex, but grow nearly horixontally. Archeria traversii, Senecio elaeagnifolius and Olearia lacunosa are also important. There are scattered bushes of Dracophyllum longifolium.

Discontinous Storey of Tallers Shrubs. Dracophyllum traversii forms scattered groups. The stems are up to 10in in diameter and 20ft tall, and their lower parts are inclined.

Ground Cover. Blechnum minus forms a dense sheet, except in the darkest places. Scattered Phormium colensoi and Astelia cockeynei.

Regeneration. No young plants of Dracophyllum traversii, D. longifolium and Archeria traversii were seen. There are occasional seedlings of Senecio elaeagnifolius and Olearia lacunosa. Small seedlings of Olearia colensoi are abundant everywhere, and they succeed in gaps. Occasional small seedlings of Hoheria glabrata do not become established.

Soil. ¼in : Litter.

2in: Dark brown, structureless fine sand.

3in. Medium brown, fine sand, with occasional schist fragments.

>5in: Light brown, fine sand. The proportion of weathering schist particles increases with increasing depth.

General Account

Communities similar to the one described occupy all scrub land sites where the slope is steep, the drainage free, the soil shallow, and the exposure not extreme. They comprise about half of the vegetation of the scrub zone. Their characteristic feature is the dense storey of shrubs, mainly Olearia colensoi, with groups of taller Dracophyllum traversii. Dracophyllum longifolium is usually present, and on drier or warmer sites, such as slight spurs and terrace faces, it may rise to dominance.

On exposed ridges, especially those with northerly aspect, Olearia colensoi scrub passes gradually into communities dominated by Dracophyllum uniflorum. There is a corresponding decrease in height of the scrub, and an increase in the proportions of certain species, notably Dracophyllum longifolium, Olearia lacunosa, Gaultheria rupestris and Coprosma serrulata. On southerly faces low-growing communities of nearly pure Olearia colensoi intervene between tall scrub and grassland.

Occasional slipping of the substratum may explain the occurrence of scattered plants of Hoheria glabrata and Olearia ilicifolia. Gaps often contain herbfield inclusions with Aciphylla (?) colensoi, Celmisia coriacea, Danthonia flavescens (usually the very broad-leaved form), Astelia cockaynet and Phormium colensoi.

In a downhill direction, with the entry of Libocedrus bidwillii, Olearia colensoi scrub passes into high altitude forest. The following pH values were obtained from a soil supporting a community in which Olearia colensoi and Dracophyllum longifolium share dominance:

(C.f. shrub composite scrub of Southern Alps and mountains of north-western district, Cockayne, 1928, p. 278).

Dracophyllum Uniflorum Scrub

Typical Community. Altitude, 3,800ft Slope, 43°. Aspect, N. E. Steep ground on the crest of a ridge.

Shrub Storey. Dense clumps of Dracophyllum uniflorum 2–3 feet tall.

Ground Cover. Mainly Blechnum minus and large moss (genus Dendroligotrichum.) Also Gaultheria sp., Suttonia nummularia, and Schoenus pauciflorus. Occasional clumps of Phormium colensoi, Danthonia flavescens (2 forms), and Celmisia coriacea.

Regeneration. Seedlings of Dracophyllum uniflorum were not seen; but the bushes are propagating by “downhill layering”. One chance seedling of Hoheria glabrata seen.

Soil. ¾in : Black humus.

2in: Dark brown, fine sand.

>5in: Grey-brown, fine sand, containing weathering fragments of schist.

General Features

On exposed ridges, especially those with northerly aspect, Olearia colensoi scrub peters out into Dracophyllum uniflorum communities like the one described. Further, Dracophyllum uniflorum is dominant in the stunted outliers of scrub which occupy rock spurs up to 4,500ft. These high-altitude communities merge into grassland; the shrub component is predominant on steep slopes and shallow soils, the grasses are predominant on gentler slopes and deeper soils (Plate 3, Fig. 2).

(C.f. Dracophyllum scrub: Cockayne, 1928, p. 280.)

Dacrydium Biforme Scrub

Typical Community. Altitude, 2,800ft. Slope, 23°. Aspect, N.E.

Discontinuous Storey of Stunted Trees. Scattered Dacrydium biforme up to 12ft tall, with distinct erect trunks up to 16m in diameter.

Main Shrub Storey. 3–5ft tall, consisting of dense clumps with spaces between. Main species are shrub-form Dacrydium biforme, Dracophyllum longifolium, Olearia colensoi and Archeria traversii. Other species are Nothopanax lineare, N. simplex, Pittosporum divaricatum, Coprosma pseudocuneata and C. colensoi.

Ground Cover and Low Shrubs. The herbs are found mainly between the clumps of bushes, and include Celmisia armstrongii, C. walkeri, Schoenus pauciflorus, Astelia cockeynei, Blechnum minus, Sticherus cunninghamu and Lycopodium scariosum. Danthonia flavescens grows in the widest gaps. Podocarpus nivalis occurs abundantly, and Coprosma serrulata less so, mainly within the shrubby clumps.

Regeneration. No seedlings of Dacrydium biforme were seen, but the bushes propagate freely by downhill layering. All ages of Olearia colensoi and Dracophyllum longifolium are represented.

Soil. ¼in: Humus.

4in: Silty loam, coloured grey by humus. There are a few small schist pebbles. On solid schist, weathering at top.

The upper part of the profile is very wet, and water collected in the pit dug for the examination of the profile. The depth of soil varies; over a fissure in the bedrock it was at least 14m deep, and well drained.

General Account

Dacrydium biforme is conspicuous where the soils are cold and poorly drained. Such conditions occur on gentle slopes where the soil texture is too fine to permit free vertical drainage. On old moraine in the top Toaroha basin, Dacrydium biforme is important on puggy soils, which have developed an alpine gley profile. The following soil description is typical of alpine gley:

Surface flat. Altitude, 2,500ft. The soil forms a matrix between morainic boulders.

¼in: Litter.

4–6in: Greyish brown, fine sand with loose, weak crumb structure. Many roots. Transition to:

3–4in: Bluish-grey, fine sand, puggy and compact, with blocky structure. Weathering pebbles present, and there are also conspicuous reddish-brown blotches which appear to represent the last traces of pebbles destroyed by weathering. Sharp and irregular transition to:

¼in: Reddish-brown, soft iron pan. This gradually lightens in colour to:

2–4in: Light reddish-brown, moderately compact fine sand with weathering pebbles. Gradual transition to:

≥ 4in: Light brown, with a bluish tinge, compact sand. Samples taken from a soil of this type gave the following pH values:

Most of the Dacrydium trees here are growing over morainic boulders which project up to 2 feet above the level of the surrounding ground. They accumulate from several inches to a foot of black raw humus between their roots. A sample of this humus gave a pH value of 4.3.

Where the soil is shallow over solid rock, as in the typical community, the soil water is kept near the surface and conditions again induce Dacrydium biforme scrub. Dacrydium biforme is also common on poorly drained soils in the forest zone, right down to lowland terraces where it is joined by the other Dacrydium spp. characteristic of the “pakihi” habitat.

(C.f. cupressoid-podocarp scrub, Cockayne, 1928, p. 279.)

Cliff Communities.

Under the excessively wet climate of Westland, cliffs cut in moraine and alluvium support dense vegetation. In the scrub zone, the characteristic species are Olearia arborescens, Dracophyllum longifolium, Carmichaelia grandiflora, Coprosma depressa, Phormium colensoi, Danthonia cunninghamii, and Blechnum minus.

II. Autecological Notes on Dominant Species

The following notes summarise the site requirements and features of the regeneration of the most important shrubs. Counts of growth rings form the basis for estimates of growth rate. These counts were made in the field, using a hand lens.

The results were plotted, and estimates of growth rate have been read off. These estimates are merely preliminary approximations; for the slow-growing species, they are gross over-estimates of growth rate, since microscopic examination will certainly reveal more rings than could be detected in the field.

The figures in brackets following the estimates of growth rates are the approximate heights and diameters attainable by mature plants in the scrub zone; but it must be stressed that with all the species discussed, these dimensions vary greatly according to the habitat.

1. Hoheria glabrata is dominant on recent fans, slips and talus slopes between 2,500ft and 3,500ft, but the species ranges from 1,500ft to 4,000ft. Chance seedlings occur nearly down to sea level. Newly germinated seedlings appear in profusion near parent trees, but they do not often survive the attacks of deer. J. T. Holloway (pers. com.) has noted vivipary during wet years, but it is not known whether the seedlings can become established.

The tree is deciduous, and in the summer of 1957–58 the leaves did not expand until December. I have not ascertained the time of leaf-fall, but the total growing season cannot exceed four months. Despite this, H. glabrata is one of the fastest growing subalpine shrubs and trees, as the following estimates show :

Height growth, 1.5in per year (25ft). Growth rings per inch, 36 (24in).

2. Olearia colensoi is the most abundant shrub of the scrub zone, and it occurs in all communities except Hoheria glabrata forest and the higher and more exposed Dracophyllum uniflorum scrub. On steep southerly faces low clumps of O. colensoi

communities which occupy less rigorous habitats, and from dense Danthonia flavescens grassland.

D. uniflorum has the strongest development of the downhill layering habit. Especially on slopes of over 30°, the branches on the downhill side become procumbent, produce adventitious roots, and eventually become separated from the parent plant. With continual layering of the downhill branches, and death of the older uphill shoots, the whole colony migrates downhill.

Estimates of growth rate:

Height growth, 0.4in per year (5ft).

Growth rings per inch, 110 (2in).

These figures suggest that colonies of D. uniflorum can be extremely old.

8. Dacrydium biforme grows on leached soils with poor drainage. The altitudinal range is sea level to 3,000ft, but in the upper Hokitika catchment there are few suitable habitats below 2,000ft. At its upper limits, D. biforme is always a low shrub with prostrate outer branches, while within the forest zone it is nearly always a tree. In an intermediate belt it occurs both as a shrub and as a tree. Plants of intermediate size and form are rare. The shrubby forms show marked downhill layering.

Estimates of growth rate:

Height growth, 0.5in per year (20ft).

Growth rings per inch, 120 (18in).

Some trees which exceed 18in diameter may prove to be well over 1,000 years old.

III Effect of Fire and Herbivorous Mammals on the Subalpine Scrub

A few small areas of scrub near tracks have been burnt. An area in the middle Toaroha, burnt 10 years ago, is a typical example. On a 35° slope Dracophyllum traversii, Olearia lacunosa and O. colensoi were co-dominant before the fire. Phormium colensoi is now the physiognomic species. Polystichum vestitum and Hypolepis millefolium are also important. There is profuse regeneration of shrubs, now 1–3ft tall. Seedlings examined had 6–7 growth rings. The most abundant seedlings are those of the light demanding Olearia ilicifolia which was probably rare or absent in the original scrub. Seedlings of Olearia colensoi are common. There are also hybrids between Olearia lacunosa and O. ilicifolia. Even where this fire extended onto a 70° slope, there is now dense vegetation dominated by Phormium colensoi.

Red deer and chamois both feed within the scrub zone, and their effects were not distinguished from each other. Though Hoheria glabrata is palatable, there does not appear to be much damage to trees in established Hoheria forest; but isolated Hoheria trees are usually badly damaged or killed through removal of bark by chewing and rubbing, and the animals are almost completely preventing regeneration and the establishment of new Hoheria communities on bared areas. The usual community of young alluvial fans and healing slips is now an open one containing Poa cockayniana, Helichrysum bellidioides, Danthonia cunninghamii and browsed, suckering Carmichaelia grandifolia; on steep slopes Hypolepis millefolium is dominant. These communities are being invaded gradually by shrubs, especially the browse-resistant Olearia ilicifolia.

In the true scrub communities the impenetrability and low palatability of most of the species discourage the deer and chamois. Therefore their activity is mostly confined to the formation of sharply defined tracks on the crests of spurs and ridges, leading from the forest to the grassland. Along these tracks, shrubs die and are not replaced. The tracks tend to enlarge into erosion channels, and such grassland species as Danthonia flavescens and Celmisia spp. enter. Away from the tracks there are surviving plants of highly palatable species, notably Nothopanax colensoi and

N. simplex. Along the margins between scrub and grassland, and in places where shrubs are scattered through grassland, less palatable species—especially Dracophyllum uniflorum— are heavily browsed in the winter.

From the point of view of the control of deer and chamois the scrub belt is important. Its position as a barrier between forest and grassland allows the animals to use the uppermost forest almost undisturbed by hunters, who work either from the valley tracks or from the grassland. This uppermost forest is severely damaged throughout the Hokitika catchment.

Hares are present, and in winter they probably make considerable use of the scrub, but their effects were not recognised.

Mature specimens of Hoheria glabrata, Olearia colensoi, O. lacunosa and O. ilicifolia growing within the forest zone are often badly defoliated and, at least sometimes, opossums are responsible. However, no serious damage to the subalpine scrub was noted, although the animals range throughout. There is an exception to this statement; on fault-shattered terrain above the Kokatahi gorge, areas of scrub have died. Opossums are notoriously thick in the forest below, and probably overpopulation has forced numbers of them into the scrub.

Discussion

1. Factors Determining the Grouping of Scrub Species

The ranges of species in the subalpine scrub are apparently determined by their tolerances in respect to several sets of factors, especially soil depth and drainage, altitude and exposure, and light avaiable to the seedlings. A community is formed by species whose tolerances overlap; the richest communities are included in the Olearia colensoi scrub, where there is most overlap (Tables 1 and 2).

There is a close connection between variations in soil and vegetation. The soils which support Hoheria glabrata forest are immature, deep, moist and well-drained. Olearia forest grows on similar, but older, soils; the richer communities, which include the very slow-growing Dracophyllum traversii among the dominant trees, and the more developed soil profiles reflect the longer occupation. Olearia colensoi scrub and Dracophyllum uniflorum scrub usually grow on sites so sleep that the shallow soils cannot develop more than rudimentary profiles, though occasionally one sees deep, brownish-yellow sub-soils which indicate greater maturity of profile. The poorly-drained soils which support Dacrydium biforme have well-developed leached or gley horisons. The fine puggy texture which impedes the drainage of some of these soils may itself be a result of advanced weathering.

It is most probable that differences in nutrient status among these soils influence the vegetation quite so much as do the physical differences. For example, the favourable physical properties of the soils which support Hoheria glabrata forest are possibly reinforced by a better nutrient status. The possibility receives some slight support from the two measurements of p^H, which were 0.3 and 0.4 units higher than the highest readings from soils supporting other communities.

To a certain extent, altitude and exposure have similar effects on the vegetation. Thus, Dracophyllum uniflorum ascends highest and is also dominant on sharp ridge crests. However, Olearia colensoi is the uppermost shrub on moist southerly faces, perhaps because it can compete with dense Danthonia flavescens.

The light demands of seedlings have important bearing on regeneration and succession. Dracophyllum longifolium and D. uniflorum seedlings demand most light for their establishment, and this is in keeping with the preponderance of these species on spur and ridges. Where there is full light and suitable soil conditions, Hoheria glabrata and Olearia ilicifolia seedlings are the most rapidly growing; thus they are able to occupy their preferred sites to the initial exclusion of other shrubs