How many forests do we have left on Earth? It depends on how you look at it

Defining the planet’s tree cover is still unclear, finds Dr Antje Ahrends

What do you think of when you think of a forest? Tall trees and shade? It may surprise many to hear that areas covered by small and sparse trees – and even areas entirely void of trees – can be labelled ‘forest’ according to widely accepted international criteria. This is problematic for conservation and climate change mitigation.

The United Nations Food and Agriculture Organization defines forest as an area larger than 0.5 hectares with trees that are higher than 5 m and a canopy cover of more than 10 per cent.

This includes areas with immature trees that have the potential to reach the height and cover thresholds and even areas that are tempo- rarily unstocked as a result of clearcutting or natural disasters.

Taking this to an extreme – if all the global forests were cut down in an instant – as long as there is the intention to regenerate these areas within the next five years no ‘forest’ would have been lost.

With such an all-encompassing definition there is a risk of confusion and of a false sense of security. An example of this is the multi-billion pound investment in tree planting in China.

The scale of China’s afforestation efforts is unparalleled – both globally and in history. According to national and international reports, every year China is gaining more forest than the rest of the world combined. However, research at the Royal Botanic Garden Edinburgh (RBGE) has shown that the success of this undertaking depends on how one defines ‘forest’. If the definition follows the above criteria, China has gained approximately 40,000 km2 of forest per year over the last decade – more than half the size of Scotland. However, if ‘forest’ is defined as an area with tall trees and shade then the annual gains were only around the size of the Outer Hebrides.

Another example of this is the recent discovery of more than four million square kilometres of ‘forest’ in the world’s deserts.

The study – published in the magazine Science earlier this year – reports that there are vast expanses of previously undetected tree cover in the world’s drylands. The size of these newly discovered forests is equivalent to the tropical moist forest in Amazonia. Indeed, in a blog one of the authors described their discovery as “the equivalent of another Amazon rainforest that we never knew existed”. The generic use of the word ‘forest’ in these statements may give rise to misperceptions.

Sparse tree cover in drylands fulfils many important ecological and social functions. But, it is certainly very far from the Amazon rainforest in terms of its value for biodiversity and timber and its contribution to carbon storage, water and climate regulation.

Today, scientists are in a better position than ever to monitor the

fate of global tree cover. The field of Earth Observation is advancing rapidly and generating data with an unprecedented spatial resolution.

Optical technologies now provide imagery with sub-metre resolution, and radar technologies can even go down to a sub-centimetre level.

At the same time computing power and speed have increased exponentially, allowing near real-time analysis of extremely large volumes of earth observation data.

In 2013, scientists from the University of Maryland used remotely sensed imagery to monitor recent changes in global tree cover. According to their study, there are now around 40 million square kilometres with tree cover – corresponding to approximately 30 per cent of the terrestrial surface. However, less than half of this area consists of dense tree cover. The distinction is important, and technology allows us to make it.

We are also in a position to use climatic models to infer where there may have been forests historically. RBGE estimates that more than half of the world’s dense forests have already been lost in the past 10,000 years.

Definitions really do matter. All the studies on tree cover change need a transparent definition to make them interpretable.

The use of the term ‘forest’ for vegetation ranging from scattered low density planted trees to tall dense natural Amazonian rainforest is clearly lumping together very different things. Without addressing this point it will remain unclear on a global scale what tree cover is there, what is being gained, and what is being lost. Dr Antje Ahrends is head of genetics and conservation, Royal Botanic Garden Edinburgh