The benefits of mulch to trees in the urban environment are numerous, as pointed out in a paper written by Linda Chalker-Scott of Washington State that summarizes the scientific studies of mulch over the last 50 years. The benefits range from disease suppression, to nutrient supplementation and water conservation. Mulching newly planted trees has been shown to have a significant positive impact on their survival. Over 40 years ago, a researcher discovered that “the increase in survival of mulched plants more than compensated for the extra cost. Given that mulch is readily available, it is typically just a labor issue, and what plant doesn’t love your labor.”
Of course, not all mulch is equal. Coarse woody mulch seems to have the most benefits and living mulch (e.g. turf) the least. Negative consequences of coarse woody mulch are close to non-existent. There is the small possibility it could transport an invasive plant and/or pathogen, but mulch is generally inhospitable to such invaders.
It turns out mulch also protect soils from extreme temperatures in that soils can be kept cooler in hot conditions and warmer in cold conditions. Other important benefits described in the paper are that mulch provides needed soil nutrients, protects soil from compaction, and control against pests and abiotic syndromes like lawn mower damage. Now you understand the magic of mulch, so go out and MULCH!