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
occur in moist, forb-rich* grassland above the continuous scrub. The altitudinal range is about 2,000–3,800ft.
O. colensoi is relatively short lived (probably it does not exceed 60 or 100 years), but it regenerates profusely in gaps. The following growth estimates were made:
Height growth, 2.5in per year (12ft).
Growth rings per inch, 40 (4in).
3. Olearia ilicifolia grows on young soils from near sea level to 4,000ft. Scattered plants colonise fans and slips at the same time as Hoheria glabrata. and since the seedlings tolerate shading by Hoheria, it gradually increases in importance in the Hoheria forest. Due to the ready dispersal of the seeds, seedlings also colonise scattered gaps in tall scrub. They occasionally become established on sheltered tussock slopes well above the limits of continous scrub, where, in 1957, they were severely damaged by frost.
Estimates of growth rate: Height growth, 6in per year (25ft). Growth rings per inch, 14 (20in).
4. Olearia lacunosa occurs occasionally in the Hoheria low forest, and is one of the dominants of the Olearia low forest and related communities, containing Libocedus bidwillii. In Olearia colensoi scrub, it is common on northerly aspects and in a stunted form becomes abundant in communities transitional to Dracophyllum uniflorum scrub. It accompanies Dacrydium biforme only on less poorly drained sites. The seedlings occur mainly in gaps.
Estimate of growth rate:
Height growth, 1.5in per year (25ft).
Growth rings per inch, 36 (24in).
5. Dracophyllum traversii grows from 1,200ft to 3,800ft, on soils which are well drained. The seedlings, which appear to be shade-tolerant, are not common. The sparing reproduction, however, is compensated by longevity. In some places large isolated plants of D. traversii stand in grassland; it is possible that these remain from continuous scrub, the shorter-lived shrubs having died. On steep slopes groups of D. traversii occur, in which the lower prostrate parts of the trunks are connected. It remains to discover whether these groups all rise vegetatively from a single parent plant, or whether grafting between separate plants has occurred.
Estimate of growth rate:
Height growth, 0.5in per year (25ft). Growth rings per inch, 50 (16in).
Thus, a tree 12in in diameter is likely to be at least 300 years old.
6. Dracophyllum longifolium grows on all except the youngest soils. It is abundant from 2,500–3,500ft, but does extend from sea level to 4,000ft. It is slowgrowing, and the lower parts of the stems tend to be prostrate. The unbranched stem of one sapling, growing among scrub on a 40° slope, was 17ft long, and the first 6ft lay on the ground and bore adventitious roots. Such downhill layering often results in vegetative propagation. Seedlings apparently reach maturity only under full light or slight shade. The species is abundant where exposure, excessive drainage or water-logging reduce the density of the shrub canopy and so improve illummation. Thus, it is conspicuous in drier variations of Olearia colensoi scrub, in the transition between Olearia colensoi and Dracophyllum uniflorum scrub and in Dacrydium biforme scrub. It is also abundant in the mosaics of scrub and tussock which occupy gentle slopes. A stunted form occurs in Sphagnum bogs.
7. Dracophyllum uniflorum is dominant on exposed ridges and at high altitudes. The seedlings are light demanding, so that the species is excluded from the scrub
[Footnote] * Forb-a herbaceous plant which does have a grassy habit.
Fig. 1.—The basin at the head of the Toaroha River. Note the change of slope corresponding to the upper limit of the scrub. The flat at the left is occupied by tussock grassland, and the top of the terrace supports bog. Fig. 2.—The upper limits of scrub on a northern aspect at the Toaroha Saddle (3,840ft) The shrub component is almost solely Dracophyllum uniflorum. Fig. 3.—Mosaic of grass and scrub on morainic area in the top Toaroha basin. The tall shrubs are Dracophyllum longifolium (left) and Dacrydium biforme (centre)
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.