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Relation to Sunshine.
The temperature belts as just described must be distinguished from vegetational formations such as scrub and forest. The latter may or may not coincide with the former and since they are often characterised by a single dominant species, they may depend on any of the numerous controlling environmental factors. The dominant species of the scrub formation on the Tararuas is usually Olearia Colensoi, while of the forest of the higher altitudes it is almost exclusively Nothofagus Menziesii. Since O. Colensoi penetrates far down into the forest, it is obvious that this timber line depends entirely on the presence or absence of N. Menziesii.
On the Tararuas, particularly on their western side, the timber line is very irregular, varying from 1200 m. to 500 m. The lowest point to which the timber line drops occurs in the neighbourhood of Baber's Saddle, about 30 km. south of the Manawatu Gorge. Here the scrub formation, which includes some Weinmannia racemosa, Podocarpus Hallii, and Phyllocladus alpinus reduced to scrubby state, is to be found alongside the forest of warm temperate belt. It may be noted that the variation in altitude of the timber line is in no way connected with altitudinal range of the temperature belts, which remain constant throughout, with the exception perhaps that they appear to be a little higher in the northern portion of the ranges.
The incidence of heavy fog on the crests of the mountains was discussed in the introductory part. It remains to point out that the lower level of the fog is observed to coincide very closely with the timber line. This indicates that the intensity of illumination is reduced by the fog for more than half the time to much below the requirements of tree species, particularly Nothofagus Menziesii, which could enable them to compete successfully against scrub. Measurements with a photoelectric cell show that the direct mid-day sunshine is reduced by about 200 times immediately underneath cloud ceiling, while in the fog it may be reduced by 1,000 times
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and more. Examination of the uppermost branches of N. Menziesii taken from various altitudes of lowered timber line in sheltered places shows annual radial increment of about .2 mm. for the first three or four years. In similar specimens taken from the timber line at maximum altitude (1200 m.), where sunshine is much more abundant, the increment is about 1 mm. At the same time the trees at this altitude show every indication of having reached the coldest level at which they can compete satisfactorily. They are of low stature, hardly 3 m. high or much lower, with flat crowns and long spreading boughs. Examples of these are commonly to be seen in the central and eastern ranges, where they are bordered by the meadows of the higher belt. The timber line is quite different where it is more or less appreciably lowered. The line is very broken, odd patches of trees jutting out frequently well above the general level and occasionally quite separating from the main body of the forest. The trees themselves are, as a rule, perfectly formed, standing with their crowns two to three times above the scrub species, which are here some 3 or 4 m. tall.
The regeneration of forest at timber line at its maximum altitude is a continuous process. There is plenty of light reaching the floor. When the old trees die, seedlings quickly take their place. The timber line there is thus fairly fixed. On the other hand, the lowered timber line is always slowly advancing upwards until it is cut off from the rear and the process begins anew. Under the scrub, within a few metres of the forest, tree seedlings appear in great numbers, some of which survive the competition for light. Seedlings are also numerous for a few metres within the forest, but farther back they seldom survive the dense shade. The scrub species, particularly Olearia Colensoi, are much more shade-tolerant and can exist where tree seedlings perish. The result of this is that when the old trees begin to die their place is quickly occupied by scrub and the new timber line is formed below. Occasionally a patch of scrub may be adversely affected in some way, as, for example, by severe attack of a certain moth caterpillar on O. Colensoi; N. Menziesii is then able to establish quickly in a cleared space. The lowered timber line is thus continually fluctuating under the severe competition for light.