If they’re putting them on trees using tape, tacks, or nails, this is detrimental to the trees. Please restrict your art to non-living surfaces where it can do no damage. Which reminds me, I need to get down to the Chaco Canyon Cafe block and remove the year-old++ yarn bomb that has been suffocating a tree on that block. The yarn is beautiful, but the tree is more so; it needs to breathe.
I don’t think 2 square inches of tape & 8 1/2 x 11 inches of paper is that damaging…. but I’m not a arborist. In Japan they bind whole trunks with straw. I dunno but just stay off my lawn!!!!