Fred Pearce is the author of The New Wild, which challenged the conventional wisdom that native species are inherently superior to non-native species and the closely related assumption that all non-native species are competitors of native species. The New Wild is the most effective of the many critiques of invasion biology, which made his latest book required reading (for me).
A Trillion Trees: Restoring Our Forests by Trusting in Nature (1) examines the popular notion that planting one trillion trees around the world can deliver us from the death grip of climate change. Once again, Fred Pearce challenges the conventional wisdom. The claim that planting a trillion trees can compensate for our continuing burning of fossil fuels is an oversimplification, but with much truth at its core.
Forest Accounting: Debits and Credits
Pearce begins by reminding us of the ecological value of forests and the role they play in reducing greenhouse gas emissions causing climate change. In addition to absorbing carbon dioxide from the atmosphere during photosynthesis, forests also release water into the atmosphere in the form of water vapor. Trees pump moisture from the ground into their leaves where excess moisture is transpired from pores (stomata) in leaves, moistening the atmosphere and returning moisture to the Earth as rain. In a time and place where extreme drought is a major issue, this is a strong argument for retaining our forests.
On a global scale, forests are responsible for carrying moisture from coastal forests irrigated by moist sea breezes into drier regions on streams of moisture transpired by forests, Pearce calls flying rivers. Where coastal forests are destroyed, this moisture delivery system is interrupted, resulting in drought in interior regions. Observational data confirms this cycle: “Air coming from forested areas delivered more than twice as much rain as deforested areas. Forests make rain; taking them away creates if not deserts, then certainly aridity.”
The aerial river of moisture transpired by forests is carried by the wind and forests contribute to the wind. Transpiration emits buoyant water vapor that condenses to water as it rises and cools. Liquid water takes up much less space than water vapor, causing a pressure drop where water vapor becomes liquid water, resulting in wind. Some scientists believe that this “biotic pump” creates stronger wind than the winds that are created by cool ocean breezes meeting hot continental air. These theories are controversial, but Pearce finds them credible.
All trees emit volatile organic compounds, commonly abbreviated VOCs. VOCs neutralize a chemical known to neutralize methane, resulting in increased methane emissions from forests. Methane is the most potent greenhouse gas, although it does not persist in the atmosphere as long as carbon dioxide.
Although forests create their own cooling environment with shade and moisture, they also absorb heat from the sun. Albedo is the technical term used to measure light reflection and absorption. Because dark colors absorb heat and light colors reflect heat, this balance of cooling and heating factors varies. In Northern and Southern latitudes where winter snows reflect sunlight, dark forest canopies absorb more sunlight than treeless snow-covered ground. Likewise, desert sand reflects more light than dark forest canopy. Measuring the net effect of the many intervening factors such as albedo on climate change is controversial, even speculative at this time.
Evaluating Planting Projects
Pearce visited tree planting projects around the world and concluded that many are counterproductive in the short term and others are not sustainable in the long term.
- Some projects are planting plantations of fast growing trees such as eucalyptus and pine with the intention of logging them within about 10 years to produce timber, pulp, or biofuels. The short term objectives of these projects do not address the long term problem of climate change.
- Some projects are planting single tree species that aren’t necessarily well adapted to local conditions. The resulting monoculture is more vulnerable to disease, insects, and changed climate conditions.
- Many huge projects exist only on paper. Elaborate plans don’t necessarily produce new forests.
Israel’s strong commitment to planting trees on its desert land illustrates the pros and cons of tree-planting projects. Trees are important in Jewish culture. Jews around the world celebrate an annual holiday of trees, Tu BiShvat. A national nonprofit group created in 1901 bought land to support the Zionist cause and has planted 250 million trees on a quarter-million acres in the desert in what is now Israel.
One such project has planted 4 million Aleppo pines on the slopes of Mount Hebron near Tel Aviv since 1964. Aleppo pines grow naturally in wetter Mediterranean regions. They cope with drought by growing only briefly during spring rains in Israel and are dormant during most of the dry, hot year. There are limits to this adaptation. A year-long drought in 2010 killed 10% of the forest. Because of the slow growth of the forest, the carbon storing capacity of the forest has yet to match the heat the forest absorbs that would otherwise be reflected by light-yellow desert sands. Scientists who study this forest do not expect the forest to attain net cooling advantage for another 80 years. There is some doubt that the forest will live that long, given rapidly rising temperatures and associated drought.
Deforestation and Rewilding
Pearce also visited places where forests are being destroyed in Indonesia, South America, and Africa. In Indonesia, the economic value of the trees themselves is the primary motivation for destroying forests. In Brazil, the primary goal of deforestation is to convert forests to pastures for livestock and agricultural fields for commodity crops that feed animals.
Developed nations have exported much of their agricultural and animal production to undeveloped nations. As agricultural land in developed nations is abandoned, forests have regenerated. In New England, for example forest cover was only 30% by the mid-19th century after 200 years of timber exploitation and clearance of agricultural land. Industrialization brought farmers into cities and marginal agricultural land was abandoned. Today most of New England is forested again. As that transition from an agricultural to an industrial economy was made in developed nations, forests in undeveloped nations were destroyed to produce agricultural products exported to developed nations.
In Brazil, the rewilding of agricultural land is already occurring. Forests were cleared and seeded with grass for cattle pastures, but the poor soil is quickly exhausted and grass won’t grow after a couple of years. These abandoned pastures recover their forests even on exhausted soil, but they won’t be mature rainforests again for many years. When forests cleared for agricultural crops lose most of their rainfall, agricultural crops fail.
Pearce visited agricultural communities in Africa that have figured out that it isn’t necessary to destroy forests in order to grow crops. Farmers had to ignore the dictates of their government to clear their land before planting crops to learn that planting crops within groves of trees is just as productive. The trees provide shade and moisture that shelter crops as well as create a more comfortable home for the community.
Forests that are embedded in indigenous communities are more safeguarded than forests in so-called protected areas, where indigenous people have been evicted. If forests are sustainably used by the community, the community has a direct economic interest in its preservation. When indigenous people are evicted from forests, a handful of salaried rangers can’t provide the same level of surveillance, making forests more vulnerable to poaching and corrupt encroachment. People have tended their forests for eons and community forestry is an extension of that relationship. They understand the forest as no outsider could.
Pearce believes that protecting the forests we have and allowing forests to regenerate naturally where agricultural land can be abandoned is preferable to planting trees because:
- Planting trees where trees have never grown in the past is not likely to create a sustainable forest. If soil and climate conditions have not supported trees in the past, it is probably an unsuitable location for trees.
- Huge projects that plant millions of trees are often creating monocultures of a single species of fast-growing trees. Such monocultures are vulnerable to pathogens, insect infestations, and changes in climate. Forests that regenerate naturally are more diverse, although they aren’t necessarily the same species as in the past because of epidemics of pests and pathogens. “However clever the foresters were, the planted trees were less well suited to the space they were occupying than those chosen by nature” and “Natural regeneration helps species to shift and adapt to climate change.”
- Newly planted trees require more support than a forest that is regenerating from roots and seedbanks. They must be irrigated while they are establishing the fungal networks that give them access to moisture in the soil. They don’t benefit from moisture and carbon resources shared by their mature neighbors. They aren’t members of an existing, sharing community of trees.
Much of what is done in the name of “conservation” is destructive. Pearce makes a strong case for natural recovery rather than active intervention in natural processes: “Most of the fifteen-percent increase in forest cover across the eastern United States in the past four decades has come from natural regeneration rather than planting.” However, urban areas in America have lost 175,000 acres of trees cover each year for the past decade, according to the US Forest Service. We have experienced such loss of our urban forests here in the San Francisco Bay Area.
Bringing it home
Pearce’s message is consistent with my personal experience based on observations of tree-planting projects in the San Francisco Bay Area. Our urban forests are being destroyed for many reasons: to make way for development, to reduce fire hazards, and to eradicate non-native trees. Little planting is done when trees are destroyed. The tree species that are planted are often not suitable for the location. When new trees aren’t irrigated regularly, they don’t survive.
More effort should be devoted to preserving forests because replacing them is largely a fiction.
- Fred Pearce, A Trillion Trees: Restoring Our Forests by Trusting in Nature, Greystone Books, 2021. All quotes in this article are from this book.