As summer vacation evaporates, a dry spell, such as the one we had early this month, triggers tulip trees to shut down some of their leaves, which become bright yellow flags signaling the final month of the tree's growing season.

Yellow leaves are sometimes visible on tulip trees during early summer droughts, but those fresh, young leaves are much less apt to surrender than are the aging leaves of August.

Old leaves lose some of their ability to stabilize water in their cells, rendering them more vulnerable during rainless stretches. Even a brief drop in soil moisture can trigger leaf loss in late summer.

Tulip trees have several other weaknesses that are often typical of fast-growing trees. Seedlings can perish in the winter when rabbits chew off the succulent bark and buds. Saplings are sensitive to late frosts, can be killed by even small fires and often fall prey to deer that devour their leaves, twigs and branches. Frail limbs can succumb to hail, heavy snow and vines, inviting fungi to invade the exposed creamy white wood. Scale insects suck the sap.

But rapid growth has it advantages: Tulip trees can outpace their predators to become the tallest hardwoods on the East Coast. Two of them planted at Mount Vernon by George Washington in 1785 now stand more than 130 feet high.

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