As if trying to scratch the sky’s underbelly, California redwoods (Sequoia sempervirens) stretch their arboreal fingertips toward the heavens. But how do their highest leaves stay hydrated so far from their roots? The air is drier at the summit, while sunlight is more intense, so evaporation should be more potent. Plus, gravity fights liquid as it scales the tree’s water highway, known as xylem. The key may rest in the redwoods’ leaves, which are not all built the same, according to a study published online in Functional Ecology. The tallest redwoods grow in northern California, where the researchers noticed water pooling on the surface of treetop leaves during the region’s signature foggy mornings. Redwood leaves can absorb moisture from the fog, according to prior work, but questions remained over where the water goes. Is it immediately used for photosynthesis, or can the tree squirrel it away? To find out, the researchers collected leaves at different heights along redwood trees. They then soaked each leaf with water before placing it in a pressure chamber to see how much water could be slowly squeezed out. Pinnacle leaves held onto their water better than those closer to the base. The crucial difference was the xylem (red-stained core in leaf cross-sections). This water conduit inhabits more space in lower leaves (left panel, 28 m high), but becomes thinner toward the tree’s apex. In contrast, water-storing “transfusion tissue” (stained blue) bulks up in the upper reaches (right panel, 104 m high), allowing these leaves to hold up to five times more water than they could use in a single day. This ability to sap and store air moisture may contribute to why redwoods surveyed in foggier northern California grow up to 30 m taller than those in the drier south.
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