There are several possibilities how to archive it. In attached effect I've measured the distance from the object to each item and then with help of the easyOut function interpolated the falloff between original and "overridden" scale values. Let me mention, that's not taking in account the shape of used object (not sure if that's something desired).
There's also the usual way by using two Forest objects. You can use your object as an Object Exclude Area, adjust the Area - Scale Falloff, then make a copy of such Forest object, disable the Object Exclude Area and use the first Forest object as an Forest Exclude Area (to fill the space under the object).
Also, in case of using the V-Ray renderer, a possible workaround would be by using the VRayDistanceTex map to drive the Transform - Scale Map.
Hope you'll find it useful.