Witness trees tell how ecosystems of Silicon Valley have changed

Featured Image: Today’s cities are built on land once occupied by very different ecosystems (Photo: Pixabay)

Article: Beller, E.E., M. Kelly and L.G. Larsen. 2020. From savanna to suburb: Effects of 160 years of landscape change on carbon storage in Silicon Valley, California. Landscape and Urban Planning 195: 103712. https://doi.org/10.1016/j.landurbplan.2019.103712

Picture the street where you live. Maybe you see single-family homes, or apartment blocks, a road running alongside, sidewalks, a nearby park. Now imagine time running in reverse: buildings and stores change their faces one by one, people and cars pass in vintage styles. As the clock winds backward, the street becomes a dirt road trod on by horses pulling people in carriages; a little further and you may be standing in a field of corn, or a pasture dotted with cows. Go far enough back, and the place where your house stood may have been a forest of beech trees, or a sandy desert of blooming cacti. A new study by Erin E. Beller and colleagues for the first time scientifically tackles an important aspect of the story of our urban landscape: how do ecosystems change from the time prior to first permanent settlement to today, after growing into a densely developed modern city?

Carbon accounting

Ecosystems on land – areas like forests, grasslands, or coastal marshes – play a critical role in the planet’s carbon cycle as plants exchange gases with the atmosphere and store carbon in their tissues and in the soil, helping regulate the earth’s climate in the long-term. Today, ongoing land cover change is still an important part of human activity spurring climate change, though lately swamped by larger annual contributions of CO2 from burning fossil fuels.

The oak savanna ecosystem of coastal California (Photo: Linda Tanner)

Land clearance for intensive human use is often accompanied by a large drop in total carbon stored in vegetation, such as from the loss of trees. What is less clear is how conversion of land to specifically urban areas might change the amount of carbon stored in biomass in the landscape. Because turning an area from a native ecosystem to a city involves a host of shifts like the introduction of imported water in parallel with changes in tree cover, urban areas may host more or less plant biomass than the original conditions that prevailed before settlement. Along with this uncertainty, it isn’t clear how much room cities might have to manage and maximize the benefits provided by their local ecosystems – like storing away climate-changing carbon emissions, or providing habitat for native species.

Paper maps of a silicon valley

The co-authors of the study examined ecosystem change since about 1850 in the highly urbanized Silicon Valley region of California, encompassing the modern tech-hub city of San Jose and its surrounding towns. This area, with small-scale Spanish settlement starting in the late 1700s, was formerly covered in oak savanna and oak woodland, the iconic Californian landscape of large wide-spaced oak trees surrounded by seasonally dry grasslands. These ecosystems, now much more rare in the state due to clearance for agriculture and cities, are uniquely adapted to the region’s unusual Mediterranean climate, with cool wet winters followed by warm dry summers. These ecosystems in their natural state straddle the boundary between what might be formally called forests and grasslands. Because biomass on this type of land can be limited by drought and recurring natural fires, uncertainty reigns as to whether conversion of these ecosystems to urban land would tend to push the total amount of tree biomass higher or lower.

To understand the kind of landscape that existed prior to Euro-American settlement in the area, the authors delved into a variety of historical archive data, including old land surveys which often used conveniently placed “witness trees” as land boundary markers. With this data in hand on the size and species of these witness trees – along with a variety of other historical data sources such as paintings, maps, and photographs – the authors were able to estimate the extent, location, and density of trees in the pre-settlement ecosystems of oak savanna and –woodland, as well as native meadows and riverside forests, in Silicon Valley prior to settlement.

The study authors used historical survey data, maps, and photos of the Silicon Valley region, including the city of San Jose, to map the locations and types of ecosystems that once made up the landscape (Photo: GoodFreePhotos)

The authors then compared their maps, and the biomass they contained, to the current distribution around Silicon Valley. Their models show that overall the region may have gained 14% more biomass from the historical condition, or may have lost a substantial amount (40-60%). The outcome depends upon how much biomass might have been present in the original woodlands, which at present is still uncertain. However, the authors showed that biomass distribution clearly had changed; many areas of the city (that used to host denser woodlands) lost biomass, but many areas that had formerly had low tree cover gained biomass. These shifts largely depended on the soil and plant communities that their reconstructions showed had existed prior to settlement. In contrast, the urbanized area itself, with scattered street and backyard trees, tended to be more homogenous in biomass distribution than the pre-urban landscape had been.

Looking ahead

Today, the Silicon Valley region is mostly urban land uses like residential, commercial, and transportation areas, with only 9% remaining as natural open space. This research project suggests that areas like Sunnyvale and San Jose, with lower tree cover and biomass than the historic baseline, might be good candidate areas to begin for efforts to “re-oak” and restore more native vegetation to the region. This study also shows that many parts of urban Silicon Valley, like Palo Alto, may have gained biomass over the 160 years, running against some popular intuition about the relative poverty of urban landscapes compared to “natural” areas. Emerging research, like the study by Beller and colleagues, contributes to a larger picture that urban areas are not ecological “dead” zones, but part of a landscape historical continuum – areas worth protecting, and worth investing in for the benefits of the human and non-human life that call them home.

 

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Andrew Trlica

Andrew Trlica

Recent PhD out of Boston U's Dept. of Earth & Environment. Interested in landscapes defined by the human presence, and how humanity’s choices connect our landscapes to the causes and consequences of climate change. My own research has dealt with the urban carbon cycle and urban forest, the urban heat island, agricultural practices and soil quality, and disturbed land reclamation. Twitter/Insta: @places_we_made

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