The notion that forests remove carbon dioxide from the air and prevent global warming has some complications, says a new study. There’s a kind of forest that does remove carbon dioxide, but does not help prevent global warming because it heats up so much itself.

Forests can directly absorb and retain heat, and, in at least one type of forest, these effects may be strong enough to cancel out a good part of the benefit in lowered carbon dioxide, says a discovery by chemistry researchers at the Weizmann Institute (WI), Israel.

For the past 10 years, WI has been operating a research station in the semi-arid Yatir Forest, a pine forest at the edge of the Negev Desert.

This station is part of a worldwide project comprising over 400 stations, called FLUXNET, which probes the link among forests, the atmosphere and climate around the globe.

The contribution of the Yatir station, says Dan Yakir, professor at the Environmental Sciences and Energy Research Department, is unique as it is one of very few in the semi-arid zone, which covers over 17 per cent of the Earth’s land surface.

Forests counteract the greenhouse effect by removing heat-trapping carbon dioxide from the atmosphere and storing it in living trees.

Over the years of measurement, Yakir’s group has found that the semi-arid forest, even though not as luxuriant as temperate forests farther north, is a surprisingly good carbon sink - better than most European pine forests and about on par with the global average.

This was unexpected news for a forest sitting at the edge of a desert, and it indicated that there is real hope for the more temperate forests if things heat up under future global change scenarios.

But forests do more than just store carbon dioxide, and Yakir, together with Eyal Rotenberg, decided to look at the larger picture - the ‘total energy budget’ of a semi-arid forest.

The first hint they had that other processes might be counteracting the cooling effect of carbon dioxide uptake came when they compared the forest’s albedo - how much sunlight is reflected from its surface back into space - with that of the nearby open shrub land.

They found that the dark forest canopy had a much lower albedo, absorbing quite a bit more of the sun’s energy than the pale, reflective surface of surrounding areas, said the institute release.

In a cloudless environment with high levels of solar radiation, albedo becomes an important factor in surface heating.

These findings were published in the Friday edition of Science.