How old are the Rimu?
One of the most common questions the Society is asked about Craigieburn is how old are the rimu trees on the reserve? Providing an accurate age on the trees can be quite difficult but recently the Society has been able to make a comparison with a large rimu that was struck by lightning in an area of catchment land near to Craigieburn. The rimu had begun to burn and was felled by the Rural Fire Authority to ensure there was no further risk of fire. From that tree a ring was removed and prepared to allow the Society to determine its age. By counting the rings of the damaged rimu and measuring its circumference a comparison has been able to be made to the rimu at Craigieburn.
As a tree grows it will produce new layers of wood around the trunk and below the level of bark. Those layers (rings) provide a cross-section that show the annual growth of the tree. Generally one layer of wood grows each year, and a layer will consist of two colors of wood. The lighter layer is the growth during spring and summer, while the darker wood is from the autumn and winter. The sample slab was prepared by sanding so that the growth rings were visible for counting and the circumference measured. The count was undertaken a number of times with the average number of rings being 493 at a diameter 67 centimetres.
There are 52 rimu trees within the Craigieburn Reserve which have all had their circumference measured and their diameter calculated (D=C/π). By comparing the sample count against the diameter of the living trees at Craigieburn the Society is able to give a reasonably accurate age of the trees on the reserve. The largest rimu at Craigieburn has a diameter of 100 centimetres which in comparison with the sample ring would make it a staggering 737 years old! Five other rimu at Craigieburn range in age from 640-670 years old and 39 of the rimu are between 340-580 years old.
One of the interesting aspects of Craigieburn is that the rimu are divided into two distinct areas. The upper western site adjacent to the track and Wakari Road are much smaller, and it’s thought that a fire disturbed the bush canopy creating a gap for the trees to establish. By using the sample slab the Society can date that disturbance to around 1650-1700 AD.