Forest density is increasing across much of the world after decades of decline, according to a new study by scientists from the United States and Europe. The change, which is being dubbed the Great Reversal by the authors, has important, has positive implications for carbon capture and climate change.

The research, carried out by teams from the University of Helsinki and New York's Rockefeller University, shows that forests are thickening in 45 of 68 countries, which together account for 72 per cent of global forests. Traditionally, environmentalists have focused their concern solely on the dwindling extent of forested areas, but the authors believe new evidence of more dense forest – made up of more and larger trees – could be crucial in reducing atmospheric carbon, which is linked to climate change.

Forests are often referred to as the planet's lungs, acting as huge carbon sinks that absorb carbon dioxide from the atmosphere as they grow and trap large amounts within their biomass and surrounding soil. In countries from Finland to Malaysia, the thickening has taken place so quickly that it has reversed the carbon losses caused by forested areas continuing to shrink during the period studied (1990-2010). In other places, including the Brazilian rainforest and parts of Africa, increasing density has partially offset the toll of deforestation caused by logging and other human activities.

With the Great Reversal, the study's authors believe a tipping point has been reached, with countries now able to pursue policies to boost their forests' thickness and carbon capacities dramatically. Jesse Ausubel, a director at the Rockefeller University and a co-author, said: "The enlarging forests in almost 50 nations studied may signal the start of a welcome and necessary restoration."

Aapo Rautiainen, lead author of the report, and based at Helsinki University, said: "The reversal occurred in Europe much earlier, then a little bit later in North America, and it has now spread to certain parts of Asia. So that is a positive sign."

He hopes policy-makers will take note: "The carbon-storage question is important as there is growing political interest in using forests as a part of climate mitigation policy.... There is a wide range of different ways you can manage forests – density is a decisive factor in carbon storage in addition to area."

Professor Pekka Kauppi of Helsinki University, a co-author of the study, said: "People worry about forest area, and that's quite correct. But if you want to know the carbon budget, it cannot be monitored observing only the changes in area. It is more important to observe this change in forest density."

Commenting on the study, Mette Loyche Wilkie, co-ordinator of the UN's Global Forest Resources Assessment 2010 report, confirmed that in some countries "the growing stock per hectare is increasing – and so is the carbon sequestered". She noted that a recent UN report observed this trend occurring "globally". She added that the change was uneven, and most notable in Europe, where forests had grown in density by over 6 per cent in the past decade, and North America.

Environmentalists expressed concerns, however, that much of the increasing density is driven by huge new monoculture plantations. For example, China's ambitious reforestation programme has added three million new hectares (nearly eight million acres) to the country's forests every year over the past decade, but green campaigners believe this is predominantly composed of one species – eucalyptus.

Planted forests are certainly playing a major role. Every year, more than 10 million hectares of forest are planted worldwide, either on newly felled woodland or reclaimed land. Species that grow faster and taller are often preferred, even where this entails importing new species, with the effects on density not seen until these reach "middle age".

Bustar Maitar, who works on Greenpeace's rainforest campaign in Indonesia, expressed concerns over the loss of biodiversity, saying: "There is a carbon capture, but it's mostly the timber plantations. Timber plantations are ecologically quite different from the forest. The solution is to stop cutting down natural forests."

Though the study, entitled A National and International Analysis of Changing Forest Density, does not itself consider biodiversity, the authors concede there is a balance to be struck. "Almost always there are trade-offs. Harmonising with other goals for forests is always difficult," says Professor Kauppi. "They have to serve many purposes – whether it's beauty, like the English countryside where the important priority is the landscape, or biodiversity, or protection, there are many things. It always has to be balanced, but the carbon budget is important."

The report's lead author, Mr Rautiainen, added: "In some regions, of course, the emphasis on monoculture plantations is very important, but there are also possibilities of managing semi-natural or natural forests. You can't directly infer worsening or improving biodiversity from forest density."

While for much of the world thickening forests are a new phenomenon, in Europe this has been occurring since the Second World War. According to a German study in the Forest Policy and Economics journal in 2006, forest density has almost doubled in Western Europe over that time, primarily because of modern, intensive forest management, and the spectacular growth of major plantations.

In the rest of the world, where the thickening trend is only now emerging, the increase is slower, currently at around 1 per cent each decade in South America and parts of Asia and Africa. However, in a country the size of Brazil, which has more than 500 million hectares of already dense forest, even a small shift means millions of additional tons of carbon are trapped in the remaining rainforest.

The authors believe the change is also being wrought by other, less divisive factors, including more sustainable government forestry practices. Concerns over desertification and soil and water protection, together with policies favouring wildlife conservation and forests as recreational spaces, are prompting better woodland management, which allows existing forest to grow thicker in many countries.

There has also been a major expansion of forest-conservation schemes, with 94 million hectares of global forest placed under legal protection since 1990. "If you have a big area of conserved forest you will probably end up with increased density because of conservation alone, because when the forest is not utilised for wood then the trees can grow and become bigger," said Mr Rautiainen. "That is also a part of the increasing density picture, along with the introduction of plantations and the management of other forests."

And in poorer countries economic development has brought changes such as the diminishing use of wood as a household fuel – which exerts a heavy burden on forest resources, and results in shorter rotations of timber crops. Academics have long predicted, based on precedents in the rich world, that a host of such changes – which include the arrival of modern agricultural methods and rising living standards – would reduce encroachment on forests. This study offers early indications that these predictions are coming true, at least in certain regions.

Bizarrely, even some polluting human activities may also be boosting growth. Ms Wilkie said that UN studies suggest "there may also be some increase in the growth rates (and carbon uptake) due to changes in the atmospheric composition or the climate in some countries". According to one study in Nature Geoscience, increased emissions of carbon dioxide and airborne nitrogen may have helped recent tree growth in Europe through increasing fertility, though the effect is uncertain.

Clearly there remain major concerns for environmentalists. Although the report says that the area covered by trees has expanded in Europe over the past decade by 2 per cent, and marginally in North America, deforestation continued globally at a pace of 13 million hectares every year in the past decade.

In Indonesia, rampant exploitation means the rainforest is getting smaller and thinner every year, while environmentalists have little faith in the government's new moratorium on logging, which began on 20 May. There are concerns, too, over a loosening of regulations on logging in the Brazilian Amazon last month.

Things are not always as they seem in the world of global forestry. Last week, at an international summit that his country is hosting on rainforest sustainability, President Nguesso of Congo-Brazzaville announced an initiative to plant one million hectares of trees by 2020. But The Independent on Sunday has received a memo from Global Witness suggesting that the country "has already marked out approximately 80 per cent of its forests for industrial-scale logging".

The debate over the benefits of the trend to thicker forests comes as UN World Environment Day, which is marked today, launches a series of events to celebrate the value of the world's forests. Announcements gave no indication of whether the word "forests" included plantations.

"With so much bad news available on World Environment Day, we are pleased to report that, of 68 nations studied, forest area is expanding in 45, and density is also increasing in 45," said Professor Kauppi.

The study is published by the online, peer-reviewed journal PLoS One.

Retreating rainforests

The news from scientists that forests across the world are thickening is certainly welcome. There has been little for environmentalists to celebrate with regard to the rainforest since the birth of the green movement. There is no doubt the pressure applied by activists and a concerned public has contributed to the reversal in forest density decline. However, today's news does not mean the problem of retreating rainforests – due to, for example, unsustainable logging – just disappears. It is essential we do not ease off.

Governments, particularly in poorer countries, remain under intense pressure to tap into the lucrative rewards offered by resource-rich ancient forests. Some countries will be more able to withstand this than others. Where major concessions are granted, there will continue to be disastrous implications for biodiversity and degradation, as well as indigenous populations.

Plantations may bring benefits as well as dangers, but are no substitute for the sustainable management and conservation of natural forest.

Gathering information on the consequences of forest exploitation is expensive – for swathes of the Congo basin there exists no information. But what this Helsinki-Rockefeller study does demonstrate is that, with greater awareness of forest density, forests can be managed to ensure they remain fertile and absorb more carbon dioxide. This can make an enormous contribution to the battle against climate change.

World Environment Day