Structure, function and value of street trees in California, USA

This study compiled recent inventory data from 929,823 street trees in 50 cities to determine trends in tree number and density, identify priority investments and create baseline data against which the efficacy of future practices can be evaluated. The number of street trees increased from 5.9 million in 1988 to 9.1 million in 2014, about one for every four residents. Street tree density declined from 65.6 to 46.6 trees per km, nearly a 30% drop. City streets are at 36.3% of full stocking. State-wide, only London planetree (Platanus × hispanica) comprises over 10% of the total, suggesting good state-wide species diversity. However, at the city scale, 39 communities were overly reliant on a single species. The state's street trees remove 567,748 t CO2 (92,253 t se) annually, equivalent to taking 120,000 cars off the road. Their asset value is $2.49 billion ($75.1 million se). The annual value (USD) of all ecosystem services is $1.0 billion ($58.3 million se), or $110.63 per tree ($29.17 per capita). Given an average annual per tree management cost of $19.00, $5.82 in benefit is returned for every $1 spent. Management implications could include establishing an aggressive program to plant the 16 million vacant sites and replace removed trees, while restricting planting of overabundant species. Given the tree population's youth there is likely need to invest in pruning young trees for structure and form, which can reduce subsequent costs for treating defects in mature trees.