Pine needles make fine mulch and are extensively used in the South and are sold in bales and bags.. Some feel that they acidify the soil but in an issue of Organic Gardener this is disputed. A soil sample will determine if the area needs lime. The pine needles are light and airy and let rain and air through so can be piled up to 3 inches but like other mulches should not be piled very high up the base of the tree. If you intend to do extensive annual plantings around the trees the pine needles are not as easy to work with as hardwood mulches. They do decompose so may have to be replaced regularly.