yellow needle tips

It's Spring here in South America and some needles of my cascade red pine are turning yellow on the tips. New growth (this year's) is blue/green with no yellowing; but old needles have visibly damaged tips. I have searched online, and at other bonsai forums, but apparently could be from "normal"...

Greetings all (and specifically Vance). I am a novice and welcome your comments/advice. The following is an update on the Mugo I purchased approx. 2 years ago from a nursery in Ottawa, CA. Based on advice from Vance and others here (thanks), I cut off ~1/4 of the almost completely sand base and...