Guessing...

Shadowing of any sprouting weed seeds by covering of pine needles slow to break down. Straw and most mulches do the same, I believe...particularly if they prevent the freeflow of air and water.

Possible test...

Clear a small area of pine needles, and scratch in some weed seeds, possibly dandelions. Recover with pine needles up to 2 inches all around, leaving a 4 inch diameter circle free of covering. Water as needed, especially in the beginning.

I would think, depending on how much sunlight strikes the spot, this would produce fine healthy plants...virtually free of cost.

The same principles apply to building a healthy compost pile...layers of different types of plant material...sticks and light branches, for air; green material; brown material; a shovelful of earth; some water...then repeat. Do not pack down as air is important, too.

My best,

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