Climate change is the environmental issue of our time. The fact that the climate is warming is indisputable and the consequences of the changes are becoming more evident. Much of California has warmed over 3⁰ F since 1980.
Consequences of Climate Change
The impact of climate change on biotic and abiotic realms has been far-reaching:
- Sea Level Rise: Temperatures in Polar Regions have increased the most because the ice is melting and sunlight that was reflected by the ice is now absorbed by the darker surface. Melting ice has raised sea levels between 1993 and 2017 on average 3.1 mm (1/8th inch) per year at an accelerating rate. The Intergovernmental Panel on Climate Change (IPCC) predicts that sea levels will rise .8 meter (2.6 feet) by the end of the century. Coastal cities are flooding during high tides and storm surges. Islands are disappearing.
- Warming Ocean: Marine life is dying in warming waters and coral reefs are dying because the water becomes more acidic as it absorbs more carbon dioxide (CO₂).
- Extreme Weather Events: The increase in the frequency and severity of droughts, hurricanes, tornados, heat waves, etc. is attributed to climate change. These events kill plants and animals. Extreme temperatures will eventually make some places in the world uninhabitable for most life.
- Wildfires: Increased frequency and intensity of wildfires all over the world are caused by global warming and associated drought.
Given the life-threatening conditions created by the warming planet, it seems a small quibble to argue about whether or not the landscape must be transformed into some semblance of what it was in the 14th century, prior to global explorations and colonization by Europeans. We are doing next to nothing to address the causes of climate change, yet we are spending approximately $25 billion per year on such “restorations” of historical landscapes. When these projects kill trees, they make climate change worse. California is considered a leader in addressing climate change in the US. Yet, when calculating carbon loss to meet stated targets for reduction, California does not include carbon loss in the trees that are destroyed.
Causes of Climate Change
There is nearly universal agreement in the scientific community that climate change is caused by greenhouse gasses emitted by the activities of humans.
Note that “forestry” (more accurately described as “deforestation”) contributes more greenhouse gas emissions than transportation. In both cases, carbon dioxide (CO₂) is the specific greenhouse gas that is emitted by these sectors of the economy. In the case of transportation cars, airplanes, ships, etc. are using fossil fuels that emit CO₂ when burned. In the case of deforestation, the CO₂ that is stored by trees during their lifetime is released into the atmosphere as a greenhouse gas when the tree is destroyed and its wood decays. And the loss of the trees means there will be less carbon storage in the future. Even if new trees were planted, less carbon would be stored because carbon storage is largely a function of biomass; that is, bigger trees store more carbon:
Carbon Storage and Sequestration in San Francisco’s Urban Forest
Forests cover 31% of the land area on Earth and annually 75,700 square kilometers (18.7 million acres) of the forest is lost as a result of wildfire, clearing for agriculture and grazing, and logging for timber. For the past 25 years, we have also been destroying trees just because they aren’t native. In California we destroy eucalyptus, Monterey pine and cypress outside their small native range, and a few other non-native species. In the Southwest we destroy tamarisk trees that were planted to control erosion. On the East Coast we destroy ailanthus (tree of heaven). In Florida we destroy malaleuca trees. Native plant advocates call these trees “invasive,” but a more accurate description is that they are successful trees, well adapted to current climate conditions. There are probably many other non-native trees on the long hit list of native plant advocates.
Other benefits of trees
Trees are valuable members of our communities for many reasons in addition to storing carbon.
- Trees provide the windbreak that makes our parks and open spaces comfortable in windy coastal locations.
- Trees are a visual and sound screen around our urban parks and residential properties.
- Trees remove particulates from the air, reducing the air pollution that makes urban environments unhealthy.
- The San Francisco Bay Area is very foggy during summer months. Tall trees condense the fog, which falls to the ground as rain, adding 10 inches of annual precipitation in East Bay eucalyptus forests and 16 inches of annual precipitation in San Francisco’s eucalyptus forests.
- Forests transpire water from their leaves that falls back to earth as rainfall. Where forests are destroyed, rainfall decreases significantly.
- Trees stabilize the soil with their roots, preventing erosion on steep hillsides that become unstable when trees are destroyed.
- The roots of trees absorb rainfall that would otherwise run off the land without being absorbed into the soil. The run off washes the top soil away, clogging rivers and streams and reducing the fertility of the soil.
We don’t need to speculate about the consequences of destroying trees because there are many specific examples of the negative impact of destroying large numbers of trees. Here are two examples, one modern and one historical.
The island nation of Comoros, off East Africa, once had an extensive cloud forest, a forest in which trees are often surrounded by low-level cloud cover. Cloud forests, such as the eucalyptus trees shrouded in fog on Mount Sutro in San Francisco, condense large amounts of moisture out of the clouds that then falls onto the ground. Fog drip in San Francisco’s eucalyptus forests adds sixteen inches of rainfall each year in those forests.
The delicate ecosystem on Comoros was disrupted when the cloud forests were cleared to make way for farmland. Between 1995 and 2014 about 80% of the remaining forest was cut down. The loss of trees disrupted the rainfall cycle on the islands. The moisture that the cloud forest was condensing from the fog was lost to the ground when the trees were destroyed. That ground moisture was then no longer transpired back into the air by the trees that had been destroyed, resulting in less rainfall. The disruption caused waterways to dry out, and left once-fertile soil exposed to erosion, with the loss of nutrients in the soil that remains. Comoros has lost 40 permanent rivers in the last 50 years. There is no longer enough water for agriculture or the daily household needs of the population.
Restoring forests is a challenge, and cloud forest can be particularly difficult. “It’s impossible to replace it,” said a cloud forest specialist at the University of York in England. “You need to save them before they’re gone.” Comoros could be a lesson for those who are cutting down the cloud forest on Mount Sutro and elsewhere in the Bay Area. Disrupting the rainfall cycle could make our drought even more extreme.
Icelanders appreciate their trees because they have few of them. Iceland was heavily forested, mostly with birch trees, when the Vikings arrived in the 9th century. Within 100 years, settlers cut down 97% of original forests to build housing and make way for grazing pastures. Now only 0.5% of the Iceland’s surface is forested, despite extensive reforestation efforts since the 1950s. Lack of trees means there isn’t vegetation to protect the soil from erosion and to store water, leading to extensive desertification.
Reforestation efforts in Iceland did not attempt to restore native birch forests because they store little carbon and they are not useful for timber. Seeds of pine and poplar from Alaska were introduced, but growth has been slow because the soil is nitrogen poor and the climate is very cold. The growth rate is estimated to be only one-tenth of the growth rate of tropical forests in the Amazon.
Both of these examples illustrate that when forests are destroyed, they are not easily replaced. Much like the historical landscape, we can’t go back. Nature is dynamic. It moves forward, not back.
Consequences of deforestation in San Francisco Bay Area
San Francisco has one of the smallest tree canopies—only 14%–of any major city in the Country:
The small urban forest in San Francisco is storing carbon that would otherwise be released into the atmosphere as greenhouse gas, contributing to climate change. “Carbon sequestration is the process by which atmospheric carbon dioxide is taken up by trees, grasses, and other plants through photosynthesis and stored as carbon in biomass (trunks, branches, foliage, and roots) and soils. The sink of carbon sequestration in forests and wood products helps to offset sources of carbon dioxide to the atmosphere, such as deforestation, forest fires, and fossil fuel emissions.” (US Forest Service)
Carbon capture by above ground vegetation is proportional to biomass. Because Blue Gum eucalyptus is the largest and most common tree in San Francisco, most carbon storage in San Francisco’s urban forest is in eucalyptus trees, according to an inventory done by the US Forest Service, as illustrated by this graph of the inventory.
Carbon storage by tree species in San Francisco
All other trees in San Francisco inventoried by US Forest Service are also non-native because there are few native trees in San Francisco. There are few native trees in San Francisco because they are not well adapted to challenging conditions. The wind is strong and constant. The soil is sand, rock, or clay. It doesn’t rain for 7 months of the year. The trees that were planted in the San Francisco Bay Area in the 19th century by European settlers were non-native because they were the species that could survive these harsh conditions.
The non-native trees that are being destroyed by public land managers in the San Francisco Bay Area will not be replaced because the goal of the land managers is to restore grassland that existed prior to the arrival of Europeans at the end of the 18th Century. All the benefits of trees and forests, including carbon storage will not be replaced.
Forests store more carbon than grassland
Native plant advocates defend the destruction of our urban forest by making the inaccurate claim that grassland stores more carbon than trees. While it is true that more carbon is stored in the soil than in above-ground vegetation, it does not follow that the soil in grassland contains more carbon than the soil in forests. The US Department of Agriculture report, “Considering Forest and Grassland Carbon in Land Management” (2017) graphically illustrates that forests in the US store far more carbon per hectare than any other land type and grasslands store the least amount of carbon per hectare of undeveloped land in the Western United States:
The differences in carbon storage per hectare in Western and Eastern United States are caused by differences in climate, soil, and specific vegetation types. The USDA report also makes these statements about the value of forests for carbon storage:
- The conversion of forest to non-forest should be avoided to preserve carbon storage, “Because mature forest stands are more likely to be carbon rich from the high volume of tree biomass and recovery takes a long time through afforestation…Further, soil carbon generally declines after deforestation from accelerated decomposition of organic matter such as litter and tree roots.”
- “Across forest systems, the ‘no harvest’ option commonly produces the highest forest carbon stocks. Managed stands have lower levels of forest biomass than unmanaged stands…” In other words, from the standpoint of maximizing carbon storage, leave the forest alone!
- “Fuel-reduction treatments lower the density of the forest stand, and, therefore, reduce forest carbon.” Again, the message is leave the forest alone!
- “…carbon emissions from prescribed fire, the machinery used to conduct treatments, or the production of wood for bioenergy may reduce or negate the carbon benefit associated with fuel treatments…”
I am mystified by the obsession with native plants. Still, I respect everyone’s horticulture preferences. If you prefer native plants, by all means, plant them. We make just one request: quit destroying everything else because the loss of our urban forest is contributing to climate change and depriving our communities of the many benefits of trees and forests.