Building skyscrapers slows down the Earth's rotation, lengthening the day by a tiny, tiny amount. Leaves falling have the opposite effect. Both effects are entirely negligible.
Work Step by Step
This situation is similar to the one shown in Figure 8.52, but in reverse. The skyscraper materials that are mined and erected end up farther from the Earth’s rotational axis. Moving mass away from the axis of rotation increases the rotational inertia of the system. No external torques act on the system (Earth + materials), so angular momentum is conserved. The product of rotational inertia and rotational speed stays constant. Since the first went up, the second must go down. The rotational speed slows down, making the day longer. The opposite effect occurs for falling leaves, since the process decreases their distance from the Earth’s rotational axis. Arguably, the falling leaves are just undoing the process that occurred when the tree lifted some of the leaf's raw materials out of the soil.