Get some wood chips, spread them around the tree (2 inch to 6 inches deep) and keep away from the trunk 6 inches. Next year, do the same thing. Mulching is a technique for covering the root area of Trees with a protective material.
This helps retain water, reducing water loss by up to 30%. Mulch also supresses weeds, which compete with young trees for nutrients and water. Organic Mulches also slowly add organic material to the soil and encourage beneficial soil organisms.
Coarse wood chips make excellent mulch, when applied properly. Most cities offer free mulch, from their tree trimming and maintenance operations. Call, and get a load delivered. This Mulch tends to derive from many varieties of trees, rather than the single source you usually find in purchased bags. Variety is better. Although some Mulches are considered Allelopathic (like Black Walnut Eucalyptus etc. "can kill plants") all of the coarse wood chip mulches, applied properly, will be beneficial to your trees. You want coarse material (chips, not sawdust!) so that air can penetrate, and it can dry out and not support mold or fungi.