Species will have to find new homes

If nothing is done, the report says, then by 2100 the country's oak trees could have succumbed to imported disease.

Admitting that this is part of a worst-case forecast, it says recurrent droughts could by then have wiped out the alders, leaving room for invasive knotweed to colonise the space they used to occupy.

Other tree species at risk include the ash and the Caledonian pine. The expansion of the Sahara desert in Africa would prove an insuperable obstacle for migrating birds, and so warblers would no longer be heard in the UK.

Butterflies would be reduced in species and numbers, and the bluebell woods of an English spring would be only a memory.

The report acknowledges that none of this is inevitable. But it believes the changes will happen without action now to put woodlands "onto a more sustainable footing".

Connecting corridors

It says that means restoring what has been lost, protecting what has survived, and extending woodlands to overcome the isolation caused by agriculture and urban development.

Often this will not mean planting new woodlands, but simply providing corridors to link separate existing patches: the corridors would serve as routes for wild creatures to travel to and fro.

The Woodland Trust's chief executive, Mike Townsend, said: "Climate change is happening now, and at a faster rate than at any time since the last ice age, and it is already having an impact on our woods and the plants and animals that live there."

Woods may change radically

The Trust says it has evidence to show the changes in seasonal patterns that are already happening. Oak trees, for example, are coming into leaf ten days earlier than they did in the 1980s, and ash, horsechestnut and lime trees are also affected.

Frogs and other amphibians are spawning earlier in some parts of the country, and migrant birds are arriving earlier, or in some cases spending the winter here instead of flying south in the autumn, which is itself lasting longer than it used to.

The Trust is appealing to people to help to keep track of the effects of climate change. It says observations of natural changes will build on data collected since 1736, and will help it to monitor what is happening now and improve predictions of what is to come.

Together with the Centre for Ecology and Hydrology, it has launched a website for the UK Phenology Network which allows users to examine local trends, and to make online recordings of key events.