Carbon storage in our urban forest

We believe that addressing climate change should be our highest environmental priority because it is the cause of many environmental problems.  For example, a recent study found that changes in climate accounted for over half of the significant changes in vegetation all over the world in the past 30 years:  “The climate governs the seasonal activity of vegetation…In humid mid-latitudes temperature is the largest influencing factor in plant growth.  In predominantly dry areas, however, it is the availability of water and in the high altitudes incident solar radiation.” (1) Animals are affected by both changes in vegetation and climate, as exemplified by the shrinking home of the polar bear as Arctic ice melts.

The consensus amongst scientists is that increases in greenhouse gas emissions are the primary cause of climate change and carbon dioxide is the predominant greenhouse gas.  Although the burning of fossil fuels is often considered the biggest source of greenhouse gas emissions, in fact transportation is responsible for only 10% of emissions.  In contrast, deforestation is contributing 20% of greenhouse gas emissions because trees store carbon as they grow and release it into the atmosphere as carbon dioxide when the tree is destroyed.  For that reason—and many others– we are opposed to the destruction of our urban forest.

Because our urban forest is predominantly non-native, native plant advocates are committed to defending the projects that are destroying the urban forest, which puts them in the awkward position of claiming that its destruction will not contribute to climate change.   Here are a few of the arguments used by native plant advocates and the scientific evidence that those arguments are fallacious:

• Since the native landscape in the Bay Area is grassland and scrub, native plant advocates often claim that these landscapes store more carbon than trees.  In fact, trees store far more carbon than the native landscape because carbon storage is largely proportional to biomass.  In other words, the bigger the plant, the more carbon it is capable of storing.  (Carbon storage in plants and soils is explained in detail here.)
• In the Draft Environmental Impact Report for San Francisco’s Natural Areas Program, native plant advocates claimed that destroying the forest and restoring grassland would lower ground temperatures based on a scientific study about the arctic north at latitudes above 50°.  In fact, the point of that study was that snow reflects more light than trees.  The Bay Area is far below 50° latitude and it doesn’t snow here, so that study is irrelevant to the Bay Area.  (That study and its misuse by native plant advocates are reported here.)
• Since most of the urban forest in the Bay Area was planted over 100 years ago, native plant advocates often claim that only young trees store carbon.  Since carbon storage is largely proportional to biomass, mature trees store more carbon than small young trees.  That is illustrated by this graph from the US Forest Service survey of San Francisco’s urban forest.

• The claim that young trees store more carbon is often made in connection with the equally bogus claim that “restoration” projects in the Bay Area will replace non-native trees with native trees.   None of the plans for these projects propose to plant native trees where non-native trees are destroyed because that wasn’t the native landscape.  In any case, native trees don’t tolerate the windy, dry conditions in which non-native trees are growing.  For example, a study of historic vegetation in Oakland, California reported that only 2% of pre-settlement Oakland was forested with trees. (2)

A new study about carbon storage in forests

Now that science has established the reality of climate change, most scientific inquiry has turned to how to stop it and/or mitigate it.  For example, a recent study reports that planting forests where they did not exist in the past, quickly stores far more carbon in the soil than the treeless landscape.  Scientists “…looked at lands previously used for surface mining and other industrial uses, former agricultural lands, and native grasslands where forests have encroached….[they] found that, in general, growing trees on formerly non-forested land increases soil carbon.” (3) 

Here are their specific findings on each type of previously non-forested land:

• “On a post-mining landscape, the amount of soil carbon generally doubled within 20 years and continued to double after that every decade or so.”
• “The changes after cultivation of farm fields was abandoned and trees became established are much subtler, but still significant…at the end of a century’s time, the amount of soil carbon averages 15 percent higher than when the land was under cultivation…”
• “In places where trees and shrubs have encroached into native grassland, soil carbon increased 31 percent after several decades…”

Mainstream environmental organizations such as the Sierra Club claim to be concerned about climate change, yet they are the driving force behind the destruction of the urban forest in the San Francisco Bay Area.  When will they wake up to the fact that advocating for the destruction of the urban forest is irresponsible for an environmental organization in the age of climate change?

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(1)    “A Look at the World Explains 90 Percent of Changes in Vegetation,” Science Daily, April 22, 2013.

(2)    Nowak, David, “Historical vegetation change in Oakland and its implications for urban forest management,” Journal of Arboriculture, 19(5): September 1993

(3)    “Soils in Newly Forested Areas Store Substantial Carbon That Could Help Offset Climate Change,” Science Daily, April  4, 2013.