As the moisture decreases, what happens to the amount of vegetation in an area is that it also decreases.
So, the more humidity there is, the more the plants are going to thrive and prosper, and the more of them there will be. However, if the weather is quite dry, there will be less vegetation. Take rainforests and deserts as your example: there is a lot of vegetation in rainforests, but not much in deserts.
