The Woodland Trust said the rate of creating new woods in the UK was continuing to fall despite calls for increasing cover from government.
Levels have more than halved from more than 12,000 ha of new planting six years ago. The last planting season alone has seen a fall to 5,000 ha.
Chief executive Sue Holden said: "These figures are truly worrying, but should be seen as a clarion call to us all to reverse the downward trend as a matter of urgency.
"Trees are not a luxury but essential for our future quality of life. They are a key component to environmental benefits, such as carbon storage, air-quality control, shade and shelter, surface flood-water management and soil conservation."
Holden welcomed government pledges for a tree-planting campaign but feared with new planting rates so low the net result "may be a loss of woodland cover in the UK".
She said: "We want to help government achieve its ambitious plans, but need to see real commitment to support woodland creation."
Trees outside woods meanwhile were dying or being felled for safety reasons, she said. Forestry Commission research showed between 1980 and 1997 England saw a 64% drop in trees.