BEIJING - Dubbed "April Snow," poplar and willow catkins have been blamed for triggering half of Beijing's fires last month, according to the municipal fire department.

The department received 686 fire alarms in April, 346 of which were reportedly caused by highly-flammable catkins.

One severe catkin-triggered fire destroyed 90 vehicles in a parking lot in Chaoyang District. More than 50 fire engines were sent to the scene.

The city government Tuesday mobilized resources and personnel to help clear catkins from residential areas, schools, gas stations, warehouses, bus stops, historical buildings, parks and construction sites.

On Monday and Tuesday alone, over 400 fire engines and 300 water trucks sprayed 8,750 tonnes of water in these key areas to stop catkins floating in the air.

According to the municipal landscaping bureau, poplar and willow trees help combat air pollution. A poplar tree with 20 cm diameter can annually assimilate 172 kg of carbon dioxide, discharge 125 kg of oxygen and stop 16 kg of dust.

Under urban landscaping campaigns in the 1980s, more than 3 million poplar and willow trees were planted along Beijing's urban roads and rivers.

However, most of those planted were female trees which produce massive amounts of catkins.

In addition to causing fire hazards, the catkins cause allergies in urban residents besieged by air pollution.

Du Jianjun, an official at the bureau, said in recent years, the bureau has injected a substance into the trees to change their sex and reduce the amount of catkins produced.

"We can't solve the catkin problem simply by cutting down the trees. The current sex change program takes not only time, but also substantial costs and human resources," he said.

This year, the bureau will treat 400,000 of the 2 million poplar and willow trees in Beijing. Du said.

The bureau plans to solve the catkin problem by 2020.