Good morning Bernadine, 

Thank you for your question regarding black ants where you would like to plant an Austrian Pine. 

Ants are not generally considered harmful to trees and mostly coexist peacefully or have a mutually beneficial relationship. However, it is true that ants may sometimes eat the roots of plants that are near their nest. Please see the below links about ants and trees, as well as the information about ants and landscape plants.

Trees Get By With Ant Aids - https://askabiologist.asu.edu/plosable/trees-aided-ants

Ants and Trees, A Lifelong Relationship - https://harvardforest.fas.harvard.edu/sites/default/files/ellison/NEants/Ellison_2014_AmForests.pdf

Ants and Landscape Plants - https://planttalk.colostate.edu/topics/insects-diseases/1420-ants-landscape-plants/

Once you have reviewed the above information, hopefully you will be able to decide whether you are comfortable planting the tree despite the ants or you want to eradicate the colony before planting the tree. If you choose to eradicate the ants, the best way to do so would be to use an insecticidal bait that the ants take back into the colony. https://extension.colostate.edu/topic-areas/insects/ants-in-the-home-5-518/

Please don't hesitate to contact us with any further questions. 

Happy Gardening!

Michelle D

Colorado Master Gardener Volunteer Apprentice