On Newport 's south side, historic and opulent mansions sit on an avenue with mature trees that help cool the neighborhoods, clean the air and foster wildlife. On the city's poorer North End, where some streets are dotted with subsidized housing, big trees are a lot fewer and far between. “ People in this neighborhood aren’t receiving the benefits of trees,” said Natasha Harrison, executive director of the Newport Tree Conservancy. “They need more.” The conservancy has planted hundreds of native trees to create a healthier forest in a 30-acre city-owned park, Miantonomi Memorial Park. Fifteen volunteers met staff from the conservancy at the park in the northern part of the city on Wednesday for Earth Day. Many native trees in the park’s forest have been crowded out by invasive plants, killed by disease or eaten by deer, rabbits and squirrels.…