JUST when it seemed as if the environmental dangers facing the planet could not get any worse, scientists have come up with a new threat - the Scots pine.

An unlikely candidate on any green campaigner's hit list, it now appears that Scots pinus sylvestris could be akin to a 20- metre ''chimney'' pumping out harmful emissions which wreak as much environmental damage as similar outpourings from factories and motor vehicles. Finnish researchers found that in the right conditions Scots pine trees can release smog-making nitrogen oxides directly into the air.

These trigger chemical reactions in the atmosphere and, as well as smog, help create acid rain.

An estimated 136,000 hec-tares of Scotland is covered with Scots pinewoods, up by 30,000 hectares in the past 15 years, with most in the Grampians and northern and western Highlands.

The largest and longest-lived tree in the Caledonian Forest, the Scots pine is the most widely distributed conifer in the world, with a natural range that stretches from beyond the Arctic Circle in Scandinavia to southern Spain and from Scotland to the Okhotsk Sea in eastern Siberia.

They can grow to 36 metres, but in most of the pinewood remnants in Scotland today the largest trees are about 20 metres tall.

Scots pine usually lives up to an age of 250 to 300 years in Scotland, although a tree in one of the western pinewood remnants was recently discovered to be more than 520 years old. The development, and its impact on future planting decisions, was greeted with caution by the Forestry Commission. A spokesman said: ''We would need to look at the balance of disadvantages against advantages, not least the role of forests as carbon sinks.

''Trees and forests provide a wide range of benefits in addition to those relating to air quality, and Scots pine is a species native to the UK that forms valuable forest

ecosystems.''

In a country renowned for its dismal weather, the Finnish research offers one minor encouragement. The scientists, led by Pertti Hari, from the Helsinki University, discovered Scots pine shoots emit higher levels of the gases when exposed to sunlight.

Ultraviolet radiation ap-peared to be the key factor triggering the release of nitrogen oxides. The researchers studied young trees enclosed in special chambers made of ultraviolet-transparent quartz glass. Nitrogen oxide concentrations went up when shoots were placed in the chambers and decreased when they were removed.

The scientists wrote in the journal Nature: ''Our findings suggest that global nitrogen oxides emissions from boreal coniferous forests may be comparable to those produced by worldwide industrial and traffic sources.''

They said the source of the nitrogen oxides was unclear. It could arise from plant metabolism, or the effect of sunlight hitting pine needle surfaces.

The oldest Scots pine, the 550-year-old, was discovered by scientists working on a research project in Glen Falloch, Glen Avon, Glen Einig, and Glen Loyne.

Its age was confirmed in tests at the Roslin laboratories near Edinburgh. The medieval tree was found by a Forestry Commission researcher, who said he uncovered seven others at least 500 years old.