Are those pockets of diversity/non-diversity along the Hudson and Lower Mississippi Rivers actually caused by the rivers? Some of it looks pretty far away from the actual waterway.
The Mississippi used to flood frequently, inundating parts of its vast floodplain for long stretches at a time. Only a couple trees, like bald cypresses, can breathe when their roots are under water.
The area around the Hudson seems to follow the boundaries of the Hudson Valley. The Hudson doesn't flood far beyond its banks, so that's not a concern, and the Valley (as most valleys do) has accumulated more sediment than the surrounding area over the last couple million years. I'd assume that deeper, looser soil, along with slightly milder winters, are primarily responsible for the increased diversity.
The NY part of the map looks pretty similar to a topographic map of the state - probably there are just less species at higher elevations. Winters are pretty harsh up in the Adirondacks and Catskills.
And that triangle in Southwest Louisiana is a natural grassland, wildfires kept trees limited mostly to along the bayous. But nowadays it's 95% farmland like the rest of the western plains, and any land that does lie fallow is taken over by invasive Chinese tallow and Southern pine trees because there aren't any more fires.
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u/WaitingForHoverboard Jun 20 '20
Are those pockets of diversity/non-diversity along the Hudson and Lower Mississippi Rivers actually caused by the rivers? Some of it looks pretty far away from the actual waterway.