Assessment of Carbon Storage by Sacramento's Urban Forest

t the forefront of combating climate change, California has adopted laws like AB32 (Global Warming Solutions Act) that reflect ambitious solutions to slow down the trend of increasing emissions. A statewide urban forest planting initiative in California is estimated to account for a significant percentage of the state's total targeted 170 million tons (Mt) reduction of CO2 (i.e., 6 Mt of CO2 annually for 50 million new trees). However, there is no baseline for statewide urban forest reductions, which makes it impossible to determine how much, if any of these projected reductions are "additional" to a business-as-usual (BAU) baseline. Quantifying CO 2 storage in the urban forest for 1990 to the present and predicting future storage to 2020 are required to establish a baseline. Reductions that go above and beyond this baseline would be deemed "additional" and worth reporting. The Urban Forest Project Protocol (Vers. 1.1) guides accounting and reporting practices for new tree planting projects, but it does not consider sector-wide efforts to conserve carbon stored in existing trees.;This paper presents an approach that combines remotely sensed data and field inventories to develop 1990-2020 baselines for CO2 storage and sequestration by Sacramento's regional urban forest. The first section of this document focuses on determining and explaining the variability among estimates of CO2 storage from four sets of allometric equations for the same ground sample of 640 trees. Limited open-grown urban tree species biomass equations have necessitated use of forest-derived equations with diverse conclusions on the accuracy of these equations to estimate urban biomass and carbon storage. A second goal is to compare the variability found in CO 2 stored and sequestered per hectare among estimation approaches for Sacramento's urban forest with the variation found among six other cities. Substantial variability was observed among the four approaches. Storage estimates differed by a maximum of 29% and ranged from 38-49 Mg/ha. The two sequestration estimates differed by 55%, ranging from 1.8-2.8 Mg/ha. To put these numbers in perspective, they amounted to about one-tenth and one-quarter of the maximum differences in CO2 storage and sequestration rates among six cities, respectively. i-Tree Eco produced the lowest storage estimates, perhaps because it relied exclusively on forest-based equations and applied a 0.80 correction factor to open-grown trees. The storage estimates produced by i-Tree Streets and CUFR Tree Carbon Calculator (CTCC) were the highest, while Urban General Equations produced relatively low estimates of CO2 storage. Eco produced lower estimates of CO2 sequestration rates than the CTCC across a range of species. Eco's reductions for tree condition and projected mortality may partially explain the difference. The results support the conclusion that applying UGEs to remotely sensed data that accurately classify broadleaf, conifer and palm tree types in the Sacramento region is likely to produce conservative results compared to results from urban-based species-specific equations.;In the second section of this document tree canopy cover (TCC, m 2) TCC was mapped and measured for years 1990, 2000, and 2007 using Thematic Mapper and Quickbird imagery. Results from field studies conducted in 1994 (Sacramento Urban Forest Ecosystem Study) and 2007 (UFORE) were used to calibrate the model. The resulting baseline for 1990 to 2020 shows a gradual increase in CO2 storage and sequestration. This trend reflects maturing tree canopy in older neighborhoods, as well as emerging canopy with new development. A potential 15-year planting scenario of 5 million trees results in 381,389 additional trees, 0.51% additional tree canopy cover (TCC) increase, 927.33 additional treed ha and 14,049 additional t of carbon dioxide sequestered or...