Management of significant trees and landscape in our heritage environment has been presenting some fascinating challenges to staff at the Port Arthur Historic Site.
Visitors to Port Arthur will notice that some heavy pruning has been undertaken on the silver poplars on the road up to the Asylum. The canopy reduction work has been in response to one of the poplars failing at ground level in early January.
This poplar was part of a group of five (that may originally have been a group of eight) planted around 60 – 70 years ago. An aerial photograph from 1942 shows a different group of mature trees growing in this location (most likely crack willows), which probably dated to the late 19th century after the penal settlement had closed.
Prior to this, it is unlikely that trees would have been grown along this road, as landscaping traditions of the time would have required the Separate Prison to sit prominently on the high ground, as a foreboding presence to remind the convicts of the punishment for bad behaviour. Surveillance would have been another important consideration.
The remaining poplars were inspected by an expert from the Royal Tasmanian Botanical Gardens, both visually and using a technique called sonic tomography, also known as PICUS testing (where radio waves are passed between sensors around the trunk of the tree to determine the density of wood and any structural weaknesses).
Two of the four poplars were found to be in a poor condition and removal was suggested as a necessary measure to prevent these trees from failing. It was felt that as this would compromise the integrity of the whole group, removal should extend to all the poplars and a replacement avenue planted (using a different species to poplar).
However, a second assessment was made by two consultants from ENSPEC that drew attention to the fact that the sudden removal of another four (or even two) trees in this area would result in dramatic changes to wind patterns, and serious consequences for the surrounding vegetation (including parts of the Memorial Avenue, and two neighbouring oak trees dating from the late 19th century).
Further examination of the PICUS tests revealed that the trees could be retained in the short term if some weight reduction was carried out in their canopies, so that a protective screen would still exist, although modified, to provide some wind buffering to the nearby trees.
The poplars will be removed in stages over the coming years, allowing the surrounding trees the chance to adjust to changing wind profiles. A replacement strategy is still being developed for this part of the site, but it is obvious that any avenue along this road needs to be managed as a group of important structural plantings that should be unified, but also integrated with its surroundings.