The colour of the soil itself influences the temperature it reaches after a few hours exposure to the summer sun. Anyone who has gone barefooted when walking from the beach to the carpark understands this relationship between colour and energy absorption. The soil itself is a good insulator and just a few centimetres down below the surface the temperatures are much cooler. Leaf litter is an even better insulator and the soil below leaf litter is much cooler than nearby exposed soil. Rocks, being solid, conduct heat more rapidly than soil which contains air spaces.
Wood is a relatively good insulator because of the air spaces it contains.
Therefore a log of wood will keep the soil underneath cooler and moister than the exposed soil nearby. You can use your hand as a crude temperature guage to feel the relative temperatures of nearby surfaces. Feel the temperature of the air and then press your hand on exposed soil or rocks to compare the temperature. If you have some digging impliment dig down a few centimetres and compare the sub-surface temperature with that of the surface. Compare the temperature of exposed soil with the surface of leaf litter or any other mulch.