As Matt Kirwan walks through Maryland’s Blackwater National Refuge, his rubber boots begin to squish. With each step the land beneath him turns from dry ground to increasingly soggy mud. The trees around him go from tall and full of leaves or needles to short, bare and pale white. Partway out, ankle deep in water, Kirwan stops. “At this point we’ve transitioned from being in the forest, to actually being in a full-fledged marsh,” explains the Virginia Institute of Marine Science ecologist. “This ground is now too salty and too wet to support living trees.” Kirwan is standing in the...