What is wrong with these trees?

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I may have blocked it a little bit due to the thick circling, but why do these trees have this lightness at the bottom, and the rest of the tree is practically darker? With lightmapping (and this new lightmapping plugin I’m using in the screenshot) it sticks out and makes my game look very unrealistic. How do I get rid of the lightness?

Just as xortrox said; select your terrain object and go to the settings, there should be a section just for trees.