Student 1: A stand of oaks shows an unusually narrow growth ring for one year. Ring width tracks how much wood a tree adds, which depends on the water available during the growing season. That year's regional rainfall was far below average, so the trees closed their stomata and slowed photosynthesis to conserve water. Less photosynthesis meant less sugar for growth, producing the thin ring. The narrow ring records a drought.

Student 2: The narrow ring formed because a caterpillar outbreak stripped most of the leaves from the oaks early that summer. With little leaf area, the trees captured almost no sunlight and made little sugar, so they added scarcely any wood. Rainfall that year was near normal; the trees simply had no foliage to use it. When the insects vanished the next year, rings returned to their usual width.