NEW YORK — A United Nations report released Thursday shows that there is still a multibillion dollar funding gap for preserving tropical forests, which are key to fighting climate change. A new fund that aims to raise $100 million for firefighting in South America, home to the Amazon rainforest, was announced this week at New York Climate Week and the U.N. General Assembly. But the funding excludes vast swathes of forests in Africa and Southeast Asia. As the fund launched, wildfires spread across the Amazon during the peak of its dry season. On Wednesday, smoke blanketed the city of Manaus in Brazil, causing air quality to hit unhealthy levels, according to IQAir, an air quality monitoring service. “We’re at the possibility that we might lose the Amazon altogether, and once we lose the Amazon, we don’t get it back — the next couple of years are really going to matter,” said Kumar Garg, president of Renaissance Philanthropy, one of the groups steering the new fund.