Storm Eowyn toppled more than 10,000 trees at National Trust properties, the conservation charity has said.
Winds of up to 100mph on Friday, coupled with heavy rain which softened the ground, caused the loss of historically important trees across Northern Ireland and northern England.
The National Trust has appealed for donations to pay for the clean-up and recovery operation which will include extensive replanting.
Northern Ireland, where a red warning was issued for the first time in its history, was worst affected by the storm.
Around 10,000 trees were lost at the 900-acre site at Mount Stewart, County Down, as winds blew through the formal gardens, estate and woodlands.
At Rowallane Garden, in Saintfield, County Down, “irreplaceable specimen trees” – some over 250 years old – were lost, leaving areas of the estate “almost unrecognisable”, the trust said.
In Northumberland, the losses at the estates at Cragside and Wallington were still being assessed as the bad weather continued into this week.
At Cragside, work is under way to remove a 150-year-old 45-metre high tree which threatened to block a stream when it fell.
Among the losses at Wallington were 200-year-old beeches which were planted as part of the original garden designs.
Sheila Das, the National Trust’s head of gardens and parks, said: “Our ranger and garden teams are assessing the aftermath but it’s already clear we’ve lost some very significant and precious trees, which is hugely sad.
“Mature trees, some with veteran qualities and important histories, give such character to our gardens and they connect us to the people who planted them, as well as playing a vital role in the ecosystems of our landscapes.
“We know climate change is making extreme storms like Eowyn more likely, posing a real threat to gardens and landscapes everywhere.
“Storm Eowyn reminds us that the world is changing and that we need to adapt the places we care for, to ensure that people and nature can thrive.”
Donations can be made via the National Trust website to the Climate Action Appeal: www.nationaltrust.org.uk/climate-appeal.