Answer: Mulch actually serves two purposes when placed around a tree. First, it suppresses weeds which is important - weeds can compete for moisture and nutrients, and removing weeds growing next to trees can sometimes damage the bark, especially if you use a weed whacker. Mulch also helps slow water evaporation. As you have found, mulch that's placed too close to a tree trunk, or too high on the trunk can cause rot. So, keep the mulch but rake it back away from the bark on the trunks and don't let it be piled high (like the cone of a volcano). You can replace the mulch every year but it isn't necessary. Mulch will decompose and will release nutrients into the soil. I think I would just monitor the mulch, raking it back when necessary and removing some if it becomes thicker than about 3 inches.
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