I hope you're really lucky and these are white pines you're talking about! If so, they are very garden friendly trees and my absolute favorite tree! I have an extensive garden under white pines and it is a very thriving garden. I do suggest that you lay soaker hoses under the pine to make watering easy and efficient and then plant your plants with their roots near the hose.
You might find the following articles about pines and my garden of interest. My garden under the pines has been extended considerably since I wrote these articles but they should give you some ideas....
On White Pines:
http://www.icangarden.com/document.cfm?task=viewdetail&itemid=3950
On gardening under them:
http://www.icangarden.com/document.cfm?task=viewdetail&itemid=3484
http://www.icangarden.com/document.cfm?task=viewdetail&itemid=3524
http://www.icangarden.com/document.cfm?task=viewdetail&itemid=3532