Traffic on the Forest Road will be diverted over the coming three days as tree surgeons take down the trees.
Guernsey Water says the pines are coming to the end of their life and are a threat to the concrete reservoirs that sit alongside them.
These hold 22 million gallons of drinking water and serve 10,000 homes in the southern parishes.
Tree surgeons will take three trees down over the coming days, with diversions in place for drivers using the main thoroughfare between the airport and St Martins.
Carl Falla is the company's capital delivery manager:
“Our priority is ensuring the continued provision of drinking water to islanders, however, we always consider biodiversity within our works. It is regrettable that the existing pines will need to come down, however these trees are reaching the end of their life and do pose a very real threat to the security of the tanks”
In their place, the utility says it will create a pond with associated planting. It will both attract wildlife and act as a mini reservoir during periods of heavy rainfall.
The tree felling is part of maintenance being carried out on the reservoirs, which are approaching their centenary.
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