Why, in Autumn, Do Leaves at the Branch Top Wither Last?

in the subtropical zone many trees become leafless in autumn. Generally, leaves at the lower part of a branch wither earlier than those at its top. What's the reason? The leaves of a free have roughly the same life span. In spring the leaves at the lower part of a branch sprout first and consequently wither earlier. Moreover, the leaves at the branch top get more sunshine, moisture and nutrients. In auW= when the roots stop supplying nutrients, the top leaves can still keep going for a while by consuming the nutrients they have preserved.