I have a bunch of these little guys all over my young potted plants/trees right now. I wasn't real concerned until they started attacking one of my various Jujube trees that is starting to leaf.
Are these ants distructive?
Do you have aphids on your tree? Most ants don't attack plants, but do tend to aphids for their honeydew.
Do an image search on Monomorium minimum to see if they look like the native little black ant.
Here is a link that might be useful: Monomorium minimum
I have some Aphids, but not on my Jujube(s)...I think? I will have to take a closer look.