Watch out for the roots of your pine trees--adding more soil can smother them, and digging around them can damage them. Trees do tend to suck all the water and nutrients up, leaving little for plants. That said, you can plant in small, shallow pockets between the roots, especailly if they are showing above ground. Lots of groundcovers thrive in dry shade under trees, like Lamium maculatum (Spotted dead nettle), Tiarella (Foam flower), Epimedium (Barrenwort) and Ajuga (Bugleweed). Perennials include Pulmonaria (Lungwort), periwinkle (Vinca minor) and Virginia bluebell (Mertensia virginica); also Corydalis elata, Dwarf goatsbeard (Arucus aethusifolius) and Golden hakone grass (Hakonechloa macra 'Aureola').