In a Swiss forest lab, scientists tracked how beech and oak leaves cool themselves and pinpointed the moment heat and drought push them past their limits.By Bob BerwynIn spring and summer, the canopies of oak and beech forest gather into layers of green. Leaves flicker, shaping the flow of light and air. The effect is almost effortless, a shaded world held in balance. But as heatwaves and droughts intensify, that balance is starting to slip, and the first signs of stress often first appear in leaves before spreading across entire forests.

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