July, 1928 
The Queensland Naturalist. 
83 
NOTES ON THE GROWTH OF THE GREY MAN- 
GROVE (Avicemria) IN THE UPPER 
BRISBANE RIVER. 
By C. J. J. WATSON. 
The following regarding the growth of the Grey 
Mangrove (Avicennia officinalis) along the river at 
Chelmer may be of interest. 
Since about April this year I have noticed that about 
fifty per cent, of the above mangroves in certain parts 
have been slowly dying and are now quite dead. I 
was at a loss to understand this at first, especially as 
dead trees were in many cases standing alongside trees 
in robust growth. On closer investigation I discovered 
that the small flood in January this year had deposited 
a considerable quantity of silt on the roots of the dead 
trees. This, being kept continually wet by the tides, 
seems to have had the effect of completely excluding the 
air from the buried pneumatophores or breathing roots, 
and completely smothering the trees. Trees which had 
all or portion of their roots exposed to the air were quite 
unaffected. 
The submersion of the trees for over a week in dirty 
fresh water does not seem to have had an ill effect, and 
the silt did not seem to have any noticeable effect until 
a month or more had passed aftei^ the flood. 
Since I came, to live at Chelmer I have been puzzled 
to know' why only comparatively small Avicennias are 
to be found in the river reaches here. The trees seem 
to thrive on the banks and grow very rapidly, some hav- 
ing grown over ten feet in height in the past two years. 
In spite of this, trees are seldom met with over six inches 
in diameter, while on the sea shore I have seen old 
hollow trees up to two feet in diameter, possibly a hun- 
dred years old. I should sqy that twenty years would 
produce 9 to 12 inches diameter at the rate of growth 
here. 
The explanation seems to be in the recurring floods. 
Large floods carrying and depositing very large quanti- 
ties of silt on backwater corners such as this, would prob- 
ably completely* exterminate these trees for the time 
being, until the tides again left the fruits on the mud 
banks. I believe that the largest of these trees now 
growing probably started life after the 1893 flood, pos- 
sibly after the 1898. 
