Monitoring million trees LA: Tree performance during the early years and future benefits

Abstract

Million Trees LA (MTLA) is one of several large-scale mayoral tree planting initiatives in the United States, striving to create more livable cities through urban forestry. This study combined field sampling of tree survival and growth with numerical modeling of future benefits to assess performance of MTLA plantings. From 2006 to 2010 MTLA planted a diverse mix of 91,786 trees. Survivorship rates of 79.8%, 90.7%, and 77.1% for street, park and yard trees were relatively high compared to other studies. Growth rates averaged 0.99 and 1.1 cm DBH per year for street and yard trees. They were similar to rates for the same species in Claremont, California, U.S., and trees in other subtropical urban forests. Projected over 40 years, the amounts of CO2 stored per tree planted per year (20.1 kg), avoided emissions (27.7 kg), rainfall interception (1.5 m3), and air conditioning savings (47.4 kWh) exceeded estimates from a previous assessment. One reason is that MTLA has planted more larger-stature trees than anticipated. Avoided CO2 emissions from energy savings were relatively large because trees were judiciously located for building shade. Park tree plantings were projected to store the most CO2 (42.0 kg per tree per year) because of their large-stature and high survival rate. Although MTLA has not reached its goal of planting 1 million trees, early results suggest that it is achieving success in terms of tree survival, growth, and performance. Continued success will depend on proper tree care practices, strategically selecting and locating new trees, monitoring threats, and adapting to challenges that arise.