There is an altitude control for the eco in the environment tab.  Just move the slide so the minimum doesn go all the way to -1 on the left. The exact value depends on your terrain, a few trials will lead you to the right choice.
The 'decay' option helps you avoid populating areas where there is already an object. Eg you have a house in your scene, you wouldnt want the trees to grow through the house so you turn on this optins and the tree population will decrease as they get close to the house.

"I paint that which comes from the imagination or from dreams, or from an unconscious drive. I photograph the things that I do not wish to paint, the things which already have an existence."
Man Ray, modernist painter
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