By Kieran Guilbert

DAKAR, May 7 (Thomson Reuters Foundation) - A project to plant a wall of trees stretching across Africa aims not only to halt desertification, but also to improve food security, create jobs, and offer young people an alternative to migration and extremism, environmental experts said on Thursday.

The planned Great Green Wall would see a 7,000 km (4,400 mile) strip of vegetation reaching from Senegal in West Africa to Djibouti in East Africa, designed to trap the sands of the Sahara, halt the advance of the desert and restore 50 million hectares of land.

Some 60 million Africans could be forced to leave their homes within five years as their land turns to desert, while two thirds of the continent's arable land could be lost by 2025 due to growing desertification, according to the United Nations.

This could drive young people across Africa into joining militant groups, such as Boko Haram, or attempting to cross the Mediterranean to seek work in Europe, said Camila Nordheim-Larsen of the U.N. Convention to Combat Desertification (UNCCD).

"The Great Green Wall is about more than just planting and counting trees, it is about building resilience in communities and developing sustainable projects to give young people reasons to stay," she told the Thomson Reuters Foundation in Dakar.

"A lack of opportunity is driving them away," she said on the sidelines of a global conference where countries signed up to the initiative are presenting their national action plans.

Presented to the African Union in 2005 by the then Nigerian president, the Great Green Wall initiative received $4 billion of funding from signatories to the U.N. climate deal agreed in Paris last year, and has received pledges from France and the World Bank.

Critics of the project say it is a top-down approach to development, dependant on external funding and management, yet organisations such as the Sahara and Sahel Observatory (OSS) say the initiative has the backing of local communities.

"This project comes from African countries, and there is a will from affected communities," said OSS executive secretary Khatim Kherraz.

Around 15 percent of the wall of trees has been planted, mainly in Senegal, while villages in Burkina Faso, Mali and Niger have made progress in planting vegetation to be used in medicine and for food, according to the United Nations.

"There is a mobilization of local people, who are at the heart of restoration work and choosing what they want to plant," said Nora Berrahmouni of the U.N. Food and Agriculture Organization. (Reporting By Kieran Guilbert, Editing by Katie Nguyen.; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, corruption and climate change. Visit news.trust.org)