In a drear-nighted December,
Too happy, happy tree,
Thy branches ne'er remember
Their green felicity.
This year, the poet is being proved wrong. For those who study Britain's seasons and the increasingly topsy-turvy world of the natural calendar know many trees are remaining stubbornly in leaf despite the onset of "drear-nighted December".
The Woodland Trust, which oversees the national phenology project and the popular Springwatch and Autumnwatch initiatives, believes parts of Britain could be poised for their first "green Christmas", with some particularly verdant species having foregone their traditional autumnal show of reds, browns, russets and yellows.
The reason can be traced to the balmy days of October when temperatures were 2.5C above normal. That was followed by an exceptionally bright November with sunshine levels 50 per cent above the 30-year average.
The phenomenon is most clearly being observed in southern climes, but in milder northern areas, swaths of woodland are retaining foliage.
Jill Attenborough, of the Woodland Trust, said the greening of winter was most prevalent in urban areas which are typically a degree or two warmer than the frost-prone countryside. "Our impression is that 2005 is proving to be particularly late for leaf fall in many cities," she said. "We considered 2001 to be a late autumn, but 2005 points to being even later."
While trees from more delicate species, such as ash, walnut and horse chestnut have shed their leaves, others - particularly oaks - were still clinging to theirs. "We would love to know how many green and leafy native trees are spotted over Christmas," Ms Attenborough added.
Barry Champion, head gardener at the National Trust's Trelissick Gardens near Truro in Cornwall, believes autumn has been hanging on later and later for the past 30 years but this year was exceptional.
"Whether this is a 30 or 40-year cyclical thing or global warming, I don't know. My gut feeling is that climate change is here and happening but it is impossible to prove because we have not kept proper records."
Dr Philip Gates, of the University of Durham, said trees shed their leaves to preserve energy during the dark days of winter when there is not enough light to photosynthesise. "By the end of the growing season most deciduous trees' leaves are getting eaten by insects and damaged by fungal infections," he said. "Losing these leaves is the best way a tree has of preserving its resources, withdrawing the nutrients back into itself."
Trees cut the leaves off their branches with a specialised hormone, leaving the appendages hanging by their "natural plumbing", rendering them vulnerable to frost and wind
But the change in the natural rhythms could also affect the wider ecology of the forest floor, reliant on leaf fall in December to enrich the soil. Scientists have yet to understand what impact the delay in this "natural composting" process might have.
It could hit woodland flowers such as the bluebell which exploit narrow windows' of opportunity in the natural calendar - between the warming of the soil and the closure of the woodland canopy - to put on their colourful display.
There is mounting evidence that spring is coming earlier each year. For each one degree rise in Celsius, spring advances by six days and frogspawn has already been spotted in Cornwall.
But the Woodland Trust believes the advent of spring in 2006 could be delayed because the warm autumn might lead to later spring leafing. The effect could be exaggerated by the cold winter forecast in the new year. But a mild winter could counterbalance that and lead to another early spring.
Sightings of green trees this Christmas can be logged at www.bbc.co.uk/springwatch