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The park service helps narrow down what sort of Juniper it is likely to be:

One of those species is famously long lived (and slow growing):

Reaching ~5-6 meters and living hundreds of years.

I guess the other is similar, though no mention of ages in the wiki article:

Trees grow from the top.

Don't trees grow very slowly in arid areas like that?

The gun could also have been laying down at one point then raised slowly as the tree grew.

Trees grow from the top - from the very tip of the branches. The trunk never moves up, the gun would not raise as the tree grew.

(Grass grows from the bottom, that's why animals can eat it without harming it.)