Urban forests in California

Urban areas in the United States (US) are facing numerous environmental pressures such as poor air and water quality and biodiversity loss (Duh et al., 2008). The impacts of climate change multiply these pressures (Petek, 2022), which can impact human health. The WHO estimates that air pollution from fossil fuels was responsible for 193,000 deaths in the US in 2015 (Lelieveld et al., 2019). Healthy urban forests, such as those in California, have been identified as an effective way of mitigating these impacts.

In 2010, urban areas constituted only 5% of the state of California's land base (21,280 m²), but were home to 35.2 million people, 95% of the population. These areas were estimated to contain 173.2 million trees, with their canopy covering 15% of the total urban area, which is the lowest among all US states.

This study uses a combination of field inventory (to describe forest structure) and theoretical assumptions to quantify the effect of urban trees on: GHG emissions (carbon storage and CO₂ removal), air quality, flood management and energy savings from reduced heat exposure. The monetary value and susceptibility to invasive pests of the urban tree stand was also assessed.

The urban trees stored 103 MMT (million metric tonnes) CO₂ in total per year, removing 8.5 MMT CO₂ from the atmosphere annually. This yearly removal is from 7.2 MMT CO₂ sequestered and 1.3 MMT CO₂ emissions avoided from building energy savings.

The trees provided a net air pollution reduction of 3,537 t/year (tonnes per year). The air pollutant with the greatest reduction was ozone, with 11,293 t/year, followed by PM10, with a total of 7,030 t/year reduced. 6,481 t/year and 2,331 t/year of NO₂ and SO₂, respectively, were avoided and removed. Biogenic volatile organic compounds (BVOCs) and volatile organic compounds (VOCs) are ozone precursors, and the amount of BVOCs emitted by trees surpassed the amount of VOCs avoided from energy savings, resulting in a net increase of 23,599 t/year.

Trees can reduce energy required for heating by reducing wind speeds and air infiltration. Annual energy savings from natural gas heating were 2.18 million Gigajoule (GJ) due to urban trees in California, and savings were particularly high in coastal areas due to sea breezes. Shade provided by tree canopies and evapotranspiration cooling (when trees transpire, they release water, which evaporates, cooling the air) reduced air conditioning requirements. Annual electricity and savings from air conditioning reduction were 3,851 Gigawatt hours (GWh), and savings were greatest inland due to hot and dry summers. These heating and cooling savings are estimated to reduce energy spending by US$568.6 million.

Urban trees intercepted 196 million m³ of rainfall per year, resulting in annual stormwater management reduction costs of US$324.6 million. The trees further added US$7.2 billion to the value of homes, although as discussed elsewhere this could exacerbate existing inequalities. In total, annual eco-services were estimated at US$8.29 billion. It was estimated that 32.8% of the trees in Southern California were susceptible to a particular invasive pest (the ISHB-FD complex), the impacts of which would require roughly 10 years to correct. Assuming half the susceptible trees were lost, it could cost US$15.9 billion for their removal and replacement, as well as US$0.6 billion in lost eco-services annually, whilst if the 80% of trees were lost it could cost US$25.4 billion for removal and replacement, and US$1 billion in lost eco-services.

The study noted that, while the benefits from 173.2 million urban trees are substantial, there is potential to plant more trees in California, California has the lowest urban tree cover of 90.8m²/capita among US states.

Climate change is increasing the risk of fires and other hazards, while population growth in California is causing a rise in invasive pests and water and energy demand issues, presenting multiple challenges. Therefore a transition to more stable and resilient urban trees is required. McPherson et al. (2017) note that pests such as ISHB-FD complex will attack multiple types of tree, so having a maximum of 10% of species from one genus (5% if it has a large spread of species across the Western Hemisphere) is recommended. Ongoing evaluation of species that are climate-ready (i.e. tolerant to stressors such as heat, drought, extreme winds and pests) and well-adapted to city conditions will also be necessary. It is important to note that different regions have different climatic conditions, trees, pests, air pollution levels, and therefore different needs, so, while the conclusions of resilient trees which maximise air pollution reduction are relevant to other regions, the optimal tree choices will differ.

The value added to homes may not be beneficial for all people. If trees are unevenly distributed through the community they may exacerbate inequalities: more trees in certain areas can reflect green gentrification and increased property prices may make land inaccessible to those of lower socio-economic status meaning they don't reap the social and health benefits of the green spaces and risk displacement (Jelks et al., 2021). It is therefore important that all members of the community are considered in the scale-up of any intervention, so that everyone can benefit equally, and those of lower socio-economic status are not further marginalised. Rigolon et al. (2020) found that early and meaningful engagement with marginalised residents of communities is critical in giving those individuals a voice in the design and a sense of ownership of the green spaces. A community outreach intervention piloted by the California Department of Parks and Recreation in a new Los Angeles park employed the promotores model, which uses community members as outreach ambassadors, and was found to aid community involvement in park programming and enhanced park use by previously underrepresented groups (Loukaitou-Sideris and Mukhija, 2020).