We can't determine what's causing the needle tips to turn brown without seeing the tree or sharp photos of the affected parts. Even then, it may be difficult to diagnose the problem because many factors could account for this.
The following websites may provide useful information.
If the problem persists and seems to be progressive, ask a forester or certified arborist to assess the tree's health onsite and recommend a course of action. Or send or take samples of the affected parts to the University of Minnesota Plant Disease Clinic.