Hi,
Not a good idea to add the tree tag to a National Park... 
Not all the National Park is covered by trees leading some mappers to make holes where there are not tree .. thus making holes in the National Park
At d=some place the trees may extend beyond the National Park .. mappers then extend the National Park... 

Best? Make a new multipolygon relation for the trees... Can still have some extend the National Park where the outer ways are common to both tree are and Park area. 

I have added a crude tree relation for this area. Please make changes to suit. https://www.openstreetmap.org/relation/17613045 