The British like short and bushy trees

Experts at Cambridge University have found the traditional "wedding cake" shape - a wide bottom tapering elegantly to a pointed top - is slowly giving way to one that is squat and bushy.

They say the change is due to extra nitrogen in the air caused by pollution.

The nitrogen acts like fertiliser, making the trees grow more branches.

But, far from being seen as environmental damage, the trend is bringing comfort and joy to England's Christmas tree growers - mainly based in East Anglia.

Christmas tree facts About 65 million Christmas trees are currently growing across Britain British growers sell about seven million trees a year, some of which are exported An additional one million trees are imported into Britain from abroad.

Dr David Hanke, from the University of Cambridge, led a three-year study in which spruce trees were sprayed with a range of chemicals to see how they were affected by pollution.

"We found that nitrogen caused a big rise in levels of a hormone called cytokinin that causes Christmas trees to grow more branches," said Dr Hanke.

"The level goes up five-fold, which is quite a large increase."

Farms were releasing nitrogen compounds into the atmosphere from fertilisers and animal sewage, said Dr Hanke.

Pigs and battery chickens were two of the biggest culprits. Christmas trees easily absorb ammonia, which contains nitrogen, given off by their waste.

"It's coming down in the rain and being blown over the trees," said Dr Hanke.

But, Dr Hanke warned, not everyone was going to cheer the growth of the shorter and fatter trees.

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"The Germans are going to be hopping mad," he said. They prefer sparser trees, often regarded as too spindly in Britain.

Roger Hay, secretary of the British Christmas Tree Growers' Association, said the effects of pollution on Christmas trees might even be helpful to the industry.

"We have to make Christmas trees bushier by pruning and shearing, so this will save the growers some work," he said.