Threatened by the advance of a desert that already covers two-thirds of Niger, the poor Sahel nation hopes to halt rapid deforestation by promoting natural gas.

Giant billboards, media ads starring local celebrities and door-to-door campaigns extol the virtues of gas and alert people to the ecological dangers of unchecked deforestation.

What is more, the government is offering financial incentives to persuade people to drop age-old energy habits.

The ecological stakes are huge.

The wooded regions of southern Niger have lost a third of their area in the past 25 years, and now cover only one percent of the country, according to the UN Environment Programme.

More than 90 percent of households use wood exclusively for lighting and cooking year round, and for heating during the short but dry Nigerien winter, the country's environmental services say.

Currently, some 200,000 tonnes of wood are burned for energy purposes annually in Niger, "the equivalent of 100,000 hectares (250,000 acres) of destroyed forests", notes forest ranger Ibro Adamou.

That is more than four times the area of Niger's capital Niamey.

As a result of the uncontrolled felling of trees, desert sands have engulfed once arable land in the arid country whose northern region lies beneath Sahara desert sands.

"Before we used to cut down trees five kilometres outside Niamey," says wood vendor Mama Maigari. "Now you have to go 200 kilometres (120 miles), into neighbouring Burkina Faso."

Government incentives for people to switch to gas, begun in 2012, include subsidies that reduce the cost of a gas heater by 35 percent and that of a six-kilo bottle of butane by half.

More price cuts will be introduced soon, sources involved in the drive say.

Businesses in the sector have meanwhile negotiated flexible loan conditions with banks to help people buy gas cookers, says Mahamoud Ali, director of private gas distribution company Gani Gaz.

Without the incentives, most poor households have previously been unable to convert to gas, an abundant resource in Niger, which has produced 44,000 tonnes each year since 2011.

"If Nigeriens would consume all the gas the country produces, it would easily offset environmental damage from (continued) timber cutting," says a UN expert requesting anonymity.

- 'Slowly swallowing fertile land' -

Failure to convert to gas would have both dire and unsustainable consequences, experts agree.

"We live at the desert's doorstep, yet we continue cutting down the little remaining timber," said Moustapha Kadi, who heads a non-governmental organisation that promotes better access to energy resources.

As trees and entire forests disappear, little remains to prevent the desert from "slowly swallowing fertile land... (when) the population is getting bigger and bigger and needs it for farming", the UN expert warns.

At an average of 7.6 children per woman, Niger has the highest birthrate in the world, and its population is forecast to triple to 56 million by 2050.

The population pressure makes arable land even more precious to a country where food shortages are as regular as droughts and other adverse effects of global warming.

Exacerbating food insecurity is the fact that 80 percent of the population live off farms using primitive farming methods.

Faced with "the alarming rate of productive land loss, the battle against desertification has become urgent", said Environment Minister Adamou Cheffou.

The campaign to swap wood for gas has borne fruit, with butane consumption surging by a factor of more than five in three years to 17,000 tonnes in 2015, according to government statistics.

As a result, some 40,000 hectares of woodland are being spared each year, according to forest ranger Amadou.

Nevertheless, Kadi says "a real change in mentality" is needed before habits can change for good -- especially when "some ministers still cook with wood".