Does the droppings from a cedar tree make plants under it not grow?

There are yellowish things that come off of the tree. I have set out bulbs under the tree and nothing is growing. I planted the bulbs on Feb. 14th of this year.Does the droppings from a cedar tree make plants under it not grow?
Two main reasons prevent other plants to grow down a cedar (or pine or eucaliptus) tree: a) lack of light and so forth heat and b) yes, the resines of these kind of trees are caustic, they 'burn' or create a bad environment for other plants to compete with itself