In a city, a row of trees does more than make a sidewalk look inviting. Leaves catch some of the tiny particles carried by traffic, while branches provide shade that keeps pavement from heating as quickly. On a summer afternoon, shaded pavement can be much cooler than nearby asphalt in full sun. That difference matters because hot pavement warms the air around it and can make a neighborhood uncomfortable.

Tree roots also change what happens after a storm. Roots create spaces in the soil where rainwater can soak in instead of rushing across a sidewalk into a drain. The water that stays in the ground can later help the tree grow. During a dry spell, however, young trees may need extra water because their roots have not reached very far.

For these reasons, city planners often choose tree species carefully. A good street tree must tolerate heat, compacted soil, and pollution, but it must also fit beneath wires and beside buildings. Planting a tree is a small action with benefits that can last for decades, as long as people care for it while it becomes established.

Nell had watched the lighthouse lamp blink out every evening for three weeks. The old generator coughed, the wires had cracked, and the harbor committee had decided to wait until spring to make repairs. “Boats have GPS now,” Mr. Vale said at the meeting. Nell looked through the window at the fishing boats returning in the fog and thought about how small a screen might seem from far out on dark water.

On Saturday, she carried a toolbox up the lighthouse stairs. She could not fix the generator, but she found a hand-crank lantern in a cabinet marked EMERGENCY. At sunset, Nell climbed to the gallery and turned the crank until her arm ached. The lantern made a narrow circle of light, not bright enough to guide a boat by itself, but bright enough to show that someone was watching.

For six nights, Nell kept the lantern turning. On the seventh night, the harbor committee arrived with new wire, a battery, and three volunteers. Mr. Vale tightened the last connection while Nell held the lantern. When the repaired lamp swept across the fog, a horn sounded from the water. Nell smiled, but she did not put away the hand-crank lantern. She knew a town’s most important tools were sometimes the ones people remembered to use before they were needed.