What a nuisance for you - I'm afraid that you'll need to take it out of the pot, replace the growing medium and replant the tree back in the pot.

The ants will be building a nest in the roots - and this could well kill the tree.

You could try ant traps in the top of the pot first, but if they've started to make the nest, it's better to repot.

Hi Guest,

Generally speaking ants are useful in a garden, and form an important part of the whole ecosystem, but they can become problematic in pots.

Ants don't attack or destroy the roots of a plant, but what they can do, particularly in a pot, is to create large air pockets so that when you water the pot from above the water runs straight through the ant channels and the plant roots remain completely unwatered.

If possible you should try to dunk the entire pot in water for an hour or so - put it into a pond if you have one - this will disperse the ants pretty effectively. Then repot the plant, loosening up and replacing some of the compost so as to remove the ant channels. When returning the pot to it's position make sure it is on "feet" - either purpose made or just small blocks, anything to break the direct contact with the soil/ground beneath and prevent a new ant colony setting up home.

Given that you have a specimen tree in your pot it may be too large to submerge or to unwieldy to easily move around, in which case Ecocharlie sell something called "No Ants":

http://www.ecocharlie.co.uk/grow-your-own/pest-and-disease-control/nemasys-no-ants.html

which is an environmentally friendly nematode that you simply water into the pot/soil etc., and which will effectively drive the ants out of the pot and the immediate area surrounding it.