Actually, no.
it's about room for the plants to breathe.
when too many plants are all jammed together... there's often problems... with lack of air-flow, which may create likely conditions for fungus diseases and... I don't know, the plants just aren't happy and none of them will look their best.
I'd avoid suggesting that the tree is "sucking" out the nutrients... as I stated earlier, the plants evolved to fill that niche. as long as you are top-dressing with compost and mulch each year (and leaving the oak leaves where God intended)... there shouldn't be nutrient issues.
