Researchers have warned that villagers in Papua New Guinea could be facing future food shortages because of a dramatic increase in population.

PNG's population has more than doubled in the last 30 years, and it is affecting how the nation's subsistence farmers are using land.

Geographer Bryant Allen has been studying village food production in the foothills of the Torricelli Mountains — near PNG's northern coast — for almost 45 years.

His research has shown people are using more land and farming it more intensively.

Tumam boys clear land for farming. ( ABC News: Eric Tlozek )

"People have extended their food production by shortening the time they leave the land in fallow, the time they leave the land uncultivated, and by more than doubling the time they cultivate it," Mr Allen said.

Mr Allen first visited the village of Tumam, in East Sepik Province, as a PhD student in 1971, where he mapped the villagers' many food gardens using only a measuring tape and a compass.

"As a result of that, I surveyed all the gardens in 1971 and since then I've been back a number of times looking at the shifting cultivation system, how people produce food," he said.

The village's population has gone from around 400 in the 1980s to more than 700 today.

Geographer Bryant Allen has been studying village food production in the foothills of the Torricelli Mountains for almost 45 years. ( ABC News: Eric Tlozek )

Dr Allen said the population boom had forced people — who rely entirely on subsistence farming for their food — to find new ways of farming to feed more people.

"They have done that by planting sweet potato, which they never used to cultivate, and the sweet potato cultivation goes on for up to five years after they've planted yams and bananas, which are their staple crop," he said.

"It's not a stable situation because the population is still increasing and if the population doubles in the next 25 years, then we'll have 1,400 people here and it will be a serious problem providing food for them."

The increased demand for food has put pressure on traditional systems of land allocation in Tumam village, where land is passed down from fathers to sons and each family has access to a limited part of the forest.

Geographer Bryant Allen and friend Bansis Morris plant a coconut palm to mark Bryant's 45-year association with Tumam village. ( ABC News: Eric Tlozek )

Villager Bansis Morris said if the population kept increasing, families would not have enough access to land to grow enough food for themselves.

"Then we might have a fight between us. It's time now to think properly [about this]," he said.

"Maybe we need some help from the Department of Health to teach us about family planning and all that to control our child birth so we might have land."

Papua New Guinean agriculture researcher John Sowei, who is doing a PhD on the cultivation of sago — a staple crop — in nearby Nuku district, has noticed similar trends in population growth and its effect on agriculture.

"We need to look at how best we can assist village people to effectively manage the resource at the community level to be able to provide enough food to feed the increasing number of people," he said.

More than 5 million people in PNG rely solely on agriculture for food and income.

Mr Allen's research has shown people are using more land and farming it more intensively. ( ABC News: Eric Tlozek )

Their ability to feed themselves has allowed villages to continue growing despite a lack of development investment.

But Mr Allen fears this may not hold true for much longer.

"The risks are that people won't be able to provide enough food to support themselves," he said.

"So they won't be able to be taken for granted anymore, or if they're ignored they'll just suffer from lack of food."