Welcome to ATP!
You don't say where you live - that is always good to know when trying to answer questions like this. Mature Ponderosa Pines can have lateral roots 150 feet away from the trunk. They don't compete well with other shrubs or plants as they require too much of the available nutrients.
I hate to say this but I would start pulling all the plastic. Not only is the plastic blocking water from getting to the roots, it will also cause the soil to sour and compact. Ponderas can't take compacted soil either - they need a loose, coarse soil to be healthy.
Plastic is never a good idea for any plant.
Daisy
