How trees survive hot summers ☀️

Two children look at a young street tree’s roots before it is planted.

San Francisco has a Mediterranean climate, which means we have long, dry summers and a short, intense, rainy winter. You may have heard us talk about planting drought-adapted trees, and wondered, “What does that mean exactly?” Well, here’s how!

💧First, let's talk about how trees use water. All trees need water for key functions like photosynthesis, nutrient transport, and structural support (drooping leaves are often one of the first signs of water stress in a tree). 

💦 Trees "drink" water through a process called transpiration: They release water from their leaves, allowing it to evaporate. This creates a negative pressure that pulls water from the soil into the tree's water column and all the way through its roots, trunk, and branches. Pretty cool, huh? 

🌿 One of the primary ways trees survive dry summers is by controlling transpiration. The leaf cells that control transpiration are called stomata. The stomata can open to increase, or close to decrease transpiration. During the summer, trees close their stomata more, which reduces water loss in the tree. 

🍃 Another adaptation for drought tolerance is small, thick, waxy leaves, like the coast live oak. The smaller size reduces the leaf's surface area and the thick, waxy coating acts as a barrier against the sun's dry heat. The end result is less transpiration and more drought resilience. 

🪴 Additional drought tolerance strategies include water storage in the tree's trunk or roots that it can draw upon slowly during dry periods, as well as deep rooting to access groundwater. 

🍂 Perhaps the most extreme leaf adaptation is drought deciduousness. Drought deciduousness is when trees drop their leaves during dry periods, stopping transpiration entirely. For example, the California buckeye, one of the native trees we plant, is drought deciduous. 

🌳 Isn't it amazing how trees have learned to adapt to adverse conditions?