I learned about "log mulching" in dry Denver from Bill Mollison's first book, and there I used it to create a damp habitat with a small pond beneath my green ash tree. Here about 4 years ago a friend of a friend dropped off a lot of logs of oak and Brazillian Pepper, and I arranged them to create two large beds out front that previously just contained a few drought resistant roses in what had been chronically dry soil. They do wonders to help the soil behind them retain both rain water plus the 6 or so times per year I water both beds. Plus they make neat mushrooms and the soil beneath them is teeming with earthworms. Not only have the roses and papayas behind them have benefited, each winter they support a lush crop of nasturtiums, and in summer a crop of Cosmos sulfurea. When the logs finally do collapse I'll add them to the soil in both beds and try to get more logs sections to continue the effort. One bed also have quite a few Chia (Salvia hispanica) that for a few months I mistakenly thought was the VERY unlikely self sown seedling of the interspecies Salvia hybrid 'Indigo Spires, which is sterile.