Upper right corner of the screen.  There are three tabs, the first one is the Aspect tab.  There are 4 tiny icons.  The one that looks like a branch/weed is the ignore from ecosystems. 

This works great when you have a single tree, and don't want it to mess with the ecosystem or for example, you have a waterplane, and want plants to populate under the water.  Not sure how it will help if the tree is part of the ecosystem. 

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