I have a layer of thick loose moss covering the substrate of a fee spruce and firs, this is to keep the substrate moist without having to drench the mountain soil rootball too often. Moss keeping the lifelines moist on just 2 junipers. I don't know how long I will leave it, tempting to remove it now that they are actively growing mostly because the trees are more fun to look at without covering. Possible problems with leaving it could be hiding boring beetles which so far I haven't found in my yard but the cause of death of many great old trees where I collect and maybe it could lead to fungal issues. So undecided when to remove it for now, first year doing this.