What chemical process occurs when dead leaves make a mark on concrete?
The shape of the leaf is consistent with a type of maple tree leaf, and the brown coloring is characteristic of the water soluble tannins that would be extracted by water. For comparison, here are two photographs, one of tanic acid in water and one of a dried mixture of tannins. The photos were taken from this Wikipedia article.  Although the tannic acid in the photograph below was extracted from different parts of a redwood tree, it is a ubiquitous tannin found in most hardwood tree leaves and the color is characteristic of many such tannins.