The Seasonal Growth of Eucalyptus regnans F. Muell

Abstract

At Wallaby Creek, Vic. (altitude 670 m), Eucalyptus regnans seedlings 1-6 m in height usually commence growth in early September, reach maximum growth in midsummer and cease growing in late May. The active growing season when shoot growth exceeds 12 mm per month is 7-9½. months. Slow growth occurs in midwinter in Melbourne (altitude 47 m) and is not affected by day length.

The growth rates of seedlings of associated understorey species are, in descending order of magnitude: Acacia dealbata, E. vegnans, Prostantheva lasianthos, Pomadevvis aspeva, Acacia melanoxylon. Their growth periods are similar, except for that of A. melanoxylon which is much shorter.

Leaves of E. regnans mature in 2½-3½ months. In larger seedlings some of the leaves produced are initiated during the growing season. The size of the mature leaf depends partly on the season of its emergence.

In winter and spring, die-back of apical shoots occurs in most seedlings owing to weevil attack and fungal infection. Insects may severely damage young leaves, but rapid recovery in full light takes place by the development of axillary and accessory buds.

Maximum diameter growth rates tend to occur in November just prior to the main peak of shoot extension. Further increases in diameter may occur in April, and slight shrinkage may take place in late summer and midwinter.

The period of active growth can be correlated with the period over which the air temperatures at 1.3 m above ground exceed a daily mean of 5.0-7.5°C, a daily maximum of 12.8° and a weekly maximum exceeding 15.5°. Growth,resumption may be delayed until weekly minimum air tempera- tures rise above - 2°. The mean temperatures at 1.3 m are roughly the mean of air temperatures from the top and base of the shoot. The mean temperatures at the mean commencement and cessation of growth differ by only 0.6-1.6°C, whereas the day length differs by 1½ hr.

Growth rates correlate strongly with mean temperatures (particularly mean maximum tempera- tures) in spring and early summer. The wide deviations which occur in late summer and early autumn may be attributable to depletion of soil water reserves.