Likely safe. But it is better to leave them on the grass. Leaving clippings on the grass can reduce your fertilizer use in half.

EXTREMELY beneficial IMHO!

All things in moderation, of course, just don't pile up too much and not too close to the trunk.

if the mat is too thick ... it literally becomes a mat ... and water repellent .. and stinks ....

as noted.. in moderation .. what the heck ... you can always remove it ...

ken

Be careful with clippings from pesticide-treated-lawns. I can't remember the toxicity, but it is harmful to gardens whether it be trees, or perennials and annuals.

Otherwise it's fine but as Ken says there are 'matting' issues and everything needs good air circulation from the root systems (to the canopy above).

Dax

If you let the clippings dry out a bit first, and use them like any other mulch - no deeper than 4" total mulch depth and no mulch in the 2-4" next to the trunk - they are fine. As said above, leaving them on the lawn is frequently the best plan, but if you can't or don't want to, then they are a good mulch, applied thinly or dry. If you pile them thickly when wet, they pack down, shed water, go anaerobic, and get slimy and stinky! It's NOT nice moving them then, although you won't really die of it, just wish it were over NOW!

Wood chip mulch is far better. What they do is promote fungal growth which is essential to trees' health. Grass clipping tend to promote bacterial growth. Grass clipping is helpful though when you put fresh grass clipping in a bucket of dechlorined water for a day then pour around wood chip mulch to help release nitrogen to feed the trees. Protozoa grow rapidly in the bucket feeding on grass clippings and they chew through fungi in the wood chips releasing nutrients to the roots.

Of course, it is pointless if somebody uses all kinds of chemicals on the lawn around the trees.

I use them quite successfully, but I let them dry first, then gather them up with a pull behind the yard sweeper. (The clippings are essentially hay at that point.)