We here at Our City Forest love talking to local residents, neighbors and random passers-by when we’re serving the communities of the Silicon Valley. We get asked a lot of questions about who we are and what we do. In general these interactions are friendly but sometimes, when the hoses are running and the water buckets are filling we get asked:
Why are you planting that tree? Don’t you know we’re in a drought?
Why are you watering? There’s a drought going on!
The idea that we should not invest in watering and planting our trees and shrubs during a drought is a misconception. Trees and shrubs can be responsibly planted and maintained during drought, even one as historic and severe as the current one. Newly planted, young trees only require between 10-20 gallons of water every week to maintain whereas lawns require approximately 62 gallons for every 10-square-foot patch weekly. Mature trees can be sustained on less water than a lawn if watered using deep-root techniques 2-4 times per month. Trees are also long-term investments and provide greater benefits to things like property value, energy and water use reduction as well as air pollution reduction.
Trees, especially drought-tolerant, native species, also provide critical services that directly attack some of the consequences of drought. Native trees send out deep, water-seeking roots into the soil to tap into local groundwater and draw it to the surface. This water is redistributed laterally and vertically through the root system in a process called hydraulic redistribution. Trees do this to prevent root death and retain soil moisture. In the process of redistributing water along root systems, trees filter water and make some available to plants with shallower root systems. As the plants use water in photosynthesis it is released from the leaves in a process called transpiration, which cools the surrounding air. In semi-arid, Mediterranean climates, like ours, this cooling can be 80% dependent on water redistribution by trees. Tree cover also prevents evaporation from rivers, reservoirs and soil, saving water for drinking and agriculture.
Tree cover shades and cools the surrounding area. In cities this is particularly important. If you’ve ever stepped onto asphalt on a summer day you have some idea of why. Urban landscapes are made of materials that absorb sunlight and heat more than soils or planted landscapes, making cities hotter than the surrounding landscape. Scientists call this “the heat island effect”. Tree cover can cut the temperature by about 10 degrees F in heat islands, meaning trees can be the difference between a comfortable 80 and a hot 90.
Benefits like these don’t happen overnight. Trees are a long-term investment, requiring thought and tender care throughout their lives. If we want to reap the long term benefits of trees we need to water and care for the trees we already have and plant trees for the future. We also need to avoid planting things, like lawns, that waste water and provide no drought benefits. Drought in California is cyclical. There’s strong evidence that cyclical drought has been a part of California for the past 7000 years, if not the entirety of California history, with some droughts lasting decades. Planting and maintaining trees to help us cope with drought is essential for the long-term health of our cities and towns.