OTTAWA—Finding the perfect real Christmas tree will be harder and more expensive this year. Canada, the world’s top exporter of natural Christmas trees, is grappling with a shortage that will likely be exacerbated by historic flooding in British Columbia, where some tree farms are underwater. A phenomenon known as an atmospheric river dumped a month’s worth of rain on the Pacific province in just two days, destroying roads and bridges and leaving some communities cut off from the rest of Canada. Canada exports about 2.3 million Christmas trees per year, with some 97 percent going to the United States. While British Columbia does not export cut Christmas trees, it is a significant domestic supplier. That means shortfalls in that province will have to be made up with supply from elsewhere, leaving fewer Canadian trees for export. “We can’t ship them because all the roads are closed,” said Arthur Loewen, whose …