You might like a look at this Chanell,
http://wbrc.org.uk/atp/Estimating Age of Oaks - Woodland Trust.pdf
Ancient trees are very special over here, most of them have preservation orders on them.
You can grow stuff under pines, but its a bit of a battle. The surface roots usually form a tight mass and its very dry. Best to put down some Geotex and build a raised bed, you need to water a lot more than you would normally till plants get established.
The tree will know there is an extra source of water & will try to put roots into the new soil, hence the Geotex.
Woodland plants like Hellebores, Wood Anenomies, Cyclamen, Lungwort, Primroses, Periwinkle etc. should do ok, but you'll need to clear fallen needles off the new bed.
Keep the bed clear of the trunk though
Here's an ancient Oak from my College.