With all of this intense rain and wind that we’ve been experiencing in Sausalito and throughout the Bay Area, you probably have seen fallen trees in your neighborhood or along the roads.

In fact, the Bay Area has lost thousands of trees over the last few years due to drought and the intensity of these winter storms.

So you may think that it’s best not to plant more trees in Sausalito since they risk falling.

This couldn’t be further from the truth.

Trees and their root systems are critical to stopping our steep slopes here in Sausalito from eroding and turning into mudslides.They are what stop our storm drains from becoming overwhelmed with water and our neighborhoods from flooding.

That’s because trees and their roots slow down and store stormwater in the soil. Their root systems also bind the soil together to prevent erosion and make it possible for the soil to absorb more water.

 

 

Here are more details on how trees reduce soil erosion and stormwater runoff:

When it rains, the tree canopy reduces the intensity of the rainfall. Some of the rain also remains on the leaves and branches. By slowing down the rainfall, the soil has more time to absorb it, rather than the stormwater quickly running off of bare earth (or impervious surfaces such as concrete or asphalt ) into the bay or overwhelming our stormwater drains, which causes flooding.

When the upper canopy cannot hold any more water, the rain drips farther down the tree hitting more leaves and branches. Eventually, the rainwater held on the leaves and branches will evaporate back into the atmosphere.

The stormwater that is not held in the canopy of the tree will drip off the leaves and branches down to the base of the tree. The water hits the leaves and vegetation under the tree, which also slows it down, filters it and helps the soil absorb the water.

The water then enters the soil. Some of it is stored in soil pores – spaces between soil particles. In a process called “transpiration,” the tree pulls the water and nutrients from these macro and micro soil pores into its roots and pulls them up the trunk to nourish the leaves. Any unused water evaporates into the atmosphere.

As transpiration happens almost every day, especially in the growing season, trees continually enable the soil to hold greater amounts of water. At the same time, some of the water infiltrates more deeply into the soil and replenishes our water table.

Trees also need many of the nutrients — which are pollutants for us — found in stormwater runoff such as nitrogen and phosphorus to survive. So they act as a filter – taking these pollutants out of the runoff – and helping prevent the ground water we use from becoming contaminated.

Below are a few graphics showing how trees reduce the volume of stromwater runoff and helps us store valuable water in the ground. The less trees we have and the more impervious our surfaces are, the more water runoff and the less water we are able to add to our ground water table.

These are just a few of the reasons why we need to continue to preserve Sausalito’s tree canopy and continually plant more trees.

This is why we here at Sausalito Beautiful focus a lot of our resources on planting and cultivating trees. If you’d like to get involved in our next tree planting project, please contact us to volunteer.

References:
treecanopybmp.org

USDA 2020 report entitled “Urban Forest Systems and Green Stormwater Infrastructure”

Green Infrastructure Center report entitled “Trees Stormwater Summary Report June 2019”