Being rodents, squirrels have huge olfactory bulbs, which they use to sniff out bulbs and other delicacies growing in your garden. It's very quick and easy to toss a little pepper on the flower bed. The next time Mr. Squirrel comes sniffing to see what you've planted for supper, he'll get a nose full of pepper in his exquisitely sensitive little rodent nose and leave you alone. The downside is that you have to reapply it after each rain. As for the trees, you might consider getting in touch with your inner Bull Conner and pulling out the garden hose.
I have a Crossman pump air rifle. It came from Walmart and is not expensive.
It shoots BB’s or 17 caliber pellets. It came with a scope but I took it off. It has a dayglo green front sight and is easy to get on target.
Some poopoo it because it is cheap and perhaps not as finely machined as more expensive brethren.
If pumped 10 times, it easily kills noxious squirrels. If pumped 5 times it burns a cat’s ass enough to embed a strong memory.
It is very accurate within the 50 foot zone where the intruders are and when pains are taken accurate for a much greater distance.
Practice make perfect and that is true with this rifle.