Trees need to be planted to combat flooding and replace those dying from disease, a conservation group has said.
Concerns are growing about the number of solitary trees in Cumbria contracting diseases such as ash dieback and sudden oak death.
The Woodland Trust said the loss of these "landscape" trees had a large impact.
Trust advisor Pete Leeson said trees retain water and "mitigate against some of the worst flooding".
Trees slow down the flow of water and allow it to "percolate into the ground rather than running over it", he said.
"We haven't been planting young trees in the landscape for the last 50 or 100 years which might replace those in time."
Half the UK's trees are not in woods, Mr Leeson said.
The UK was "poorly wooded", with about 12% tree cover compared with 44% in Germany, he said.
Solitary trees are being celebrated in a £50,000 Heritage Lottery fund project, Cumbria's Top 50 Trees.
Project co-ordinator Iris Glimmerveen said it aimed to "encourage local authorities, landowners and official bodies to look after individual trees".
"This gives real power to the argument that any tree, anywhere, no matter how big or small, can matter to someone and should be protected,'' she said.