From savanna to suburb: Effects of 160 years of landscape change on carbon storage in Silicon Valley, California

Landscape changes such as urbanization can dramatically affect the provision of ecosystem services such as carbon storage. However, while cities have been shown to store substantial amounts of carbon in soils and vegetation, we have little information from long-term studies about how contemporary carbon storage in urban areas compares to carbon storage in the natural ecosystems that characterized these landscapes prior to urbanization. We used historical archival sources and land-cover data to quantify and map historical tree carbon storage in the now-urban Santa Clara Valley, California, USA prior to substantial Euro-American modification (ca. 1850) and to analyze change in the amount and distribution of carbon storage over the past ca. 160 years. We estimate that total tree carbon storage in the study area was similar to 784,000 to 2.2 million Mg (13.6-38.1 Mg C/ha) when the region was characterized by oak savanna and woodland habitats, compared to similar to 895,000 Mg C (15.5 Mg C/ha) today. This represents a non-significant gain of 14% to a significant loss of 60% depending on scenario. We also demonstrate changes in the spatial distribution of carbon on the landscape, as losses in carbon storage in areas of former oak woodland were partially offset by gains in carbon storage in historical habitat types that historically had few or no trees. This challenges the hypothesis that aboveground carbon storage increases with urbanization in Mediterranean-climate ecosystems due to irrigation and tree planting. Our study demonstrates the utility of using pre-1900s historical sources to reconstruct changes in ecosystem services such as carbon storage over century time scales.