why are my shrubs turning brown
We cannot see the browning clearly from the photo. Mites, both spruce spider mites or the two-spotted mite, will produce browning and are a likely cause. Tap some affected branches over a white sheet of paper and look for crawling insects the size of a punctuation period. Here is info about them, including control: https://extension.umd.edu/hgic/spider-mites-trees-and-shrubs Slow mites are bad, fast mites are predators and they are beneficial so you don't want to kill them.
Gravel creates a very hot microclimate around plants. That could be a factor.
Also, we cannot see the flare at the bottom of the tree (where it widens before entering the ground). Mulch of any kind should not be touching or piled on the base of the tree, nor should soil be covering the flare of the tree.
You might want to send us photos of the browning that are more close-up; then we may be able to see insect or disease signs.
here are close up photos. we talked to a tree and shrub guy but he was not sure. did tge check for insects 2x and found none.
thank you !
We can't see signs of pests, but the ones noted above would not be visible in a photo.
We think the plants are stressed for multiple reasons. Spruce and juniper have a hard time in our climates and are native to areas with less heat and humidity. The blue star juniper is really a groundcover plant and would get more humidity there, and any browning below (shaded areas brown out) would not be visible. Growing them grafted onto a trunk as a lollipop shape is tough to do well over time. These evergreens are not able to sprout out again from brown, old wood.
If you are married to the idea of the stone (which is hot- even the hosta looks stressed), consider installing an irrigation system.
You might want to search on line for discussion groups of people who are growing topiaries and see what is discussed.
You could also hire a certified arborist to come and make an evaluation of all of the factors, and recommendations of how to proceed.