COLOMBO — When a study published in January flagged the risks to Himalayan forests from excessive atmospheric nitrogen pollution, it relied on an often-overlooked member of the ecosystem to assess the level of the threat: lichens. A composite organism of algae and fungi, lichens, like plants, need nitrogen in order to thrive. But too much of it — either in the air, in the form of ammonia, or when it settles on soil and vegetation, in a process known as deposition — can be damaging. And that was the message these bioindicators were sending back, with headline findings that up to 85% of Himalaya…