MAINE — The health of the iconic evergreen trees that define the Pine Tree State is at risk from climate change. Researchers across the state have begun planting tree species from mid-Atlantic states to replace these trees as temperatures rise, extreme weather intensifies and pests spread. They believe this practice, called assisted migration, will keep Maine’s forests functioning amid climate change. The migration would happen over time without human planting, but moving trees now will prevent ecosystems from collapsing if weather conditions get more extreme, researchers said. Pine, spruce, cedar, ash and beech are some of the softwood tree species expected to decline here in coming decades. Hardwoods such as poplar, maple and oak are poised to take their places. How Maine people adopt these new species on their land — if they do — will make a big difference for the country’s most forested state, researchers said.