"Trees - Protectors Against a Changing Climate." - Journal Publication.

This article is a much abridged version of a full paper [with 145 references] published (4-3-24) in the journal Ecological Civilization.

Beech trees (Fagus sylvatica) showing “Crown Shyness” (Clayfield Copse, Reading, UK).

“Trees are poems that the earth writes upon the sky. We fell them down and turn them into paper that we may record our own emptiness.” — Kahlil Gibran (1883–1931).


1. Tree Huggers.

Although the term “tree hugger” is used, generally pejoratively, to describe someone acutely concerned with protecting the natural environment, its true origin marks the brutal slaughter in 1730, of 294 men and 69 women, members of the Bishnoi Hindu community, who were trying to defend the sacred khejri trees (Prosopis cineraria) in their village from being cut down by soldiers, to provide timber and charcoal to build a new royal palace. By literally hugging the trees, they paid the ultimate price, but this action resulted in a royal decree that no trees could be felled in any Bishnoi village.

It is through this lens that environmentalism and social justice might best be viewed, with trees serving not only as powerful symbols of the current ecological crisis, but as real and essential protectors against a changing climate, and all other symptoms of the current condition of human ecological overshoot.


2. Trees and Forests.

There are estimated to be about 3 trillion trees on Earth, or about half the number that existed before the dawn of human civilization. Trees are vital to at least four major biogeochemical cycles, namely, the carbon, water (hydrological), nitrogen and oxygen cycles, and hence their significant presence or loss impacts dramatically upon the global climate. Since the end of the last ice age, the overall extent of forested land has been reduced from 6 billion to 4 billion hectares, principally from the expansion of agriculture. The majority of trees are present in forests, which currently cover about a third of the Earth’s habitable land surface, and provide habitat for 80% of land based biodiversity.

Forests affect the climate, not only in terms of capturing carbon, as is mostly focussed upon by policymakers, but in maintaining biodiversity, generating clouds and increasing albedo (thus causing cooling), influencing rainfall and weather patterns, and other factors. The loss of trees, therefore, weakens our chances of meeting climate and biodiversity targets.


3. Forest Loss - deforestation and forest degradation.

Forest loss represents the sum of deforestation and forest degradation: deforestation is where trees are removed completely for changes in land-use, such as for agriculture, mining, or urbanisation, and are not expected to re-grow; while forest degradation is a more general “thinning” of trees, e.g. from logging, shifting agriculture, or wildfires, and which can be expected to recover eventually. However, it cannot be overstated that, in parts of the globe, particularly the tropics, the deterioration of forests is severe, as a result of a confluence of different human impacts, which vary from region to region in their relative proportion and scale. Thus, globally, around one quarter of annual forest loss is from deforestation: in Latin America, the major driver of deforestation is clearing forest to graze cattle, mainly by corporations, while in Southeast Asia it is “freeing-up” land to grow crops, for example, tree plantations to produce palm oil.

The Brazilian Amazon is found to have been a net emitter of CO2 for the past two decades. Although the Amazon as a whole, which extends across 9 different nations, has absorbed a net 1.7 billion tonnes of CO2 equivalent in the past 20 years, the Brazilian portion alone has emitted a net 3.6 billion tonnes during the same period. On the basis of satellite monitoring data, it is concluded that the best chance for preserving the Amazon, and its ability to buffer against climate change, lies in placing formally protected areas and lands in the charge of indigenous peoples.

It has been estimated that forests keep Earth around half a degree (Celsius) cooler than without them, and help to stabilise the climate. This is due to a combination of effects, including carbon dioxide uptake, and evapotranspiration, but also the release of volatile organic compounds (VOCs) from trees, which create aerosols and aid cloud formation, with a consequent increase in albedo (thus causing global cooling). Thus, we can neither reach “net-zero” emissions targets, nor keep global heating to within the limit of 1.5 ^oC above pre-industrial levels, if deforestation is allowed to continue. In fact, according to the Special Report on Global Warming of 1.5 °C of the Intergovernmental Panel on Climate Change, an increase in the global forest area of 10 million square kilometres (equal to the land area of the United States) by 2050 is necessary to avoid exceeding 1.5 ^oC.

4. Should we simply Plant Trees, on a Colossal Scale, to Offset Deforestation and Avert Climate Change?

Since forest eco-systems are far more complex than mere assemblies of trees, simply establishing new tree plantations might prove an incomplete, and potentially counterproductive, strategy in regard to stabilising climate and promoting biodiversity. The global area of near natural peatland (over 3 million km2) sequesters an annual 0.37 Gt of CO2, while peat soils contain over 600 Gt of carbon, which is as much as 44% of all soil carbon on Earth, and more than the amount of carbon stored in all other vegetation types, including the world's forests. The need to preserve grasslands has been emphasised, and it was noted that savannahs are not “inferior forests” but store large amounts of carbon in their soils. Indeed, savannahs hold more carbon belowground than do forests. Thus, planting trees on peatlands and grasslands can lead to an overall release of carbon into the atmosphere.

Clearing established forests for tree plantations is especially detrimental, not only in respect of carbon storage but also biodiversity loss. Moreover, many tree planting projects have failed, in part from a tendency to target the numbers of trees planted, with insufficient regard for environmental impacts such as water demand, and a lack of follow up monitoring and legal protection, to ensure that the trees survive. Thus, if planting is to be done, judicious choices must be made at each stage of the process. This is aptly summarised in the context of the European Commission and European Environment Agency’s “Map My Tree” platform, aimed to assist with planting 3 billion new trees, which acknowledges the need for: “Planting the right trees in the right place and for the right purpose.”

Thus, a paper on the potential for global tree restoration sparked controversy, in regard to the prospect of planting trees over the 0.9 billion hectares that might be available globally, without encroaching on existing forests, or urban and agricultural areas. While it was inferred that thus planting close to one trillion trees over such a large area would absorb two thirds of anthropogenic carbon dioxide emissions, this met with strong dissent on various grounds, to which the original authors responded, and then issued an erratum, in which they revised down the amount of carbon that might be removed by global forest and woodland areas.

Furthermore, they emphasised that, overall, tree restoration is not more important than all other methods of environmental conservation, but that “climate change is an extremely complex problem with no simple fix and that it will require a full combination of approaches.”


5. Proforestation.

Proforestation is the primary strategy of protecting existing forests from human disturbance, so that they can continue to grow naturally to reach their full ecological potential, thus maximising the absorption and storage of carbon, while increasing biodiversity and structural complexity, including soil, mycorrhizal fungi, insects, plants, lichens etc. The critical factor of timescale also applies, since active reforestation, or even natural regeneration strategies, assisted or otherwise, will take time to grow new biomass, whereas proforestation builds on what is already there, particularly the larger trees. In one forest, these number only 3% of the total trees present, but account for 42% of its carbon storage; hence, preserving them exerts an immediate, positive impact. Indeed, due to the tardiness of our actions to repair the Earth and its climate, severe restrictions to the cutting of mature trees must actually be enabled globally.

There is some controversy over whether an intact standing forest continues to absorb carbon once it has become fully established, or reaches a kind of “saturation” state, but there are strong indications that the accumulation process remains ongoing on a scale from decades to centuries. Intact forests also may accumulate half or more of their carbon content as soil organic carbon or in standing and fallen trees which eventually decay and contribute more soil carbon. Some older forests are found to continue accumulating soil organic carbon, while soil organic matter is bound more tightly in older, rather than younger, forests. During the period 2001-2019, global forests were found to have sequestered about twice as much CO2 as they emitted, and to provide a “carbon sink” that absorbs a net annual 7.6 billion tonnes of CO2. This is 1.5 times more carbon than is emitted by the United States each year. However, keeping forests intact offers far more than just carbon sequestration and accumulation, but also contributes to maintaining biodiversity, ecosystem services, and building forest and community resilience.


6. Natural Regeneration.

In contrast to proforestation, in which existing forests are preserved intact to reach their full ecological potential, natural regeneration is the natural re-growing of forest on land from which trees have been removed (e.g. through logging or agriculture), or the expansion of current forest area. Thus, woodlands are restocked by trees that develop from seeds that fall and germinate in situ. The prospects, and also challenges for using natural regeneration as a means for large scale reforestation in the tropics – where deforestation is particularly severe - have been surveyed in detail. This is particularly important, since to achieve necessarily challenging forest and landscape restoration goals will require cost-effective natural regeneration at the global scale.

In regard to natural regeneration, Rewilding Britain propose the following strategy:

“Natural regeneration should be our default approach to woodland expansion. Most natural regeneration takes place within a couple of hundred metres of existing trees, even for highly mobile seeds like birch. We propose a Three Step Natural Regeneration Hierarchy as a practical model for decision making. This should be part of a broader rewilding approach where species-rich mosaics of woodland, scrub and grassland habitats are allowed to regenerate over large landscapes. The hierarchy starts with natural regeneration as the default approach, with tree planting as a support option where the natural regeneration of diverse habitats will not happen without it.

Step 1. Let nature lead: Allow natural regeneration as a default approach unless a natural mix of trees and shrubs are unable to establish or would take too long to arrive.

Step 2. Give nature a hand: Kick-start the process by assisting natural regeneration (next section) if needed.

Step 3. Plant trees: Plant locally sourced tree saplings only where still considered necessary.”


7. Assisted Natural Regeneration.

Assisted natural regeneration (ANR), sometimes termed “managed regrowth”, can provide a cost-effective method for forest restoration, regenerating biodiversity and ecosystem services, in areas at intermediate degrees of degradation, while simultaneously generating income for rural livelihoods. The approach utilises either residual seeds and plants, already available at the specific site to be restored, or as dispersed from neighbouring vegetation. Low-cost methods are employed, to assist in the natural re-growth of vegetation, for example: providing fences to keep cattle, deer and other animals from grazing on new growth, selectively removing vegetation that can threaten the survival of resprouting saplings; avoiding practices, such as mechanical disturbance, logging and burning; thinning of competing vegetation, as required, to promote the growth of tree saplings; and, if and where necessary, planting seedlings. ANR has most often been used to enhance tropical forests, but is now being used, more widely, to restore forests across a range of ecosystems.

The following summary has been given, to gauge the appropriateness of ANR for a given situation: “Assisted natural regeneration doesn’t work for every landscape; it’s critical to assess the local context. For example, ANR works best in areas that are not highly degraded but are surrounded by forest remnants and where seeds are living in the soil. Where intensive farming and overgrazing have heavily degraded or compacted the soil, tree planting/seeding usually makes more sense.”


8. Trees and One-Planet living – a Summary Set of Guidelines.

• Protect existing mature (primary) forest ecosystems – stop deforestation - and allow them to grow to their full potential (this has an immediate effect).

• Natural regeneration, assisted if necessary. Expand areas around mature forests, allow secondary forests to grow (cheaper, easier, and richer in biodiversity than planting).

• If planting is to be done, plant mixtures of “native” tree seedlings (“whips”) or seeds in areas where forest previously existed; avoid projects that convert grasslands or peatlands to forest, or clearing established forest to grow tree plantations, e.g. for palm oil.

• Avoid introducing undesirable “alien” species, of trees or parasites.

• Mangroves, and ocean “forests” of kelp and seaweed to be established and protected.

• Still need to reduce carbon dioxide (and other GHG) emissions at source, and not rely on natural climate solutions alone to continue with Business as Usual. The overall problem is “overshoot”.


9. Conclusions.

There are estimated to be about 3 trillion trees on Earth, or about half the number that existed before the dawn of human civilization. Trees are vital to at least four major biogeochemical cycles, namely, the carbon, water, nitrogen and oxygen cycles. In addition to absorbing carbon, and releasing oxygen through photosynthesis, trees are critical for maintaining biodiversity, providing habitat for 80% of land based wildlife, feeding the soil, generating clouds and increasing albedo (thus causing global cooling), influencing rainfall and weather patterns.

The loss of trees, weakens our chances of reaching climate and biodiversity targets, and so proforestation and other practices to stringently preserve the functionality of and holistically restore forest ecosystems, must be adopted as a matter of urgency, paying due attention to soil, and species diversity including mycorrhizae; not being limited to insouciant “tree planting” solutions. Indeed, due to the tardiness of our actions to repair the Earth and its climate, severe restrictions to the cutting of mature trees must actually be enabled globally. It has been concluded that the best chance for preserving the Amazon, and its ability to buffer against climate change, lies in placing formally protected areas and lands in the charge of indigenous peoples.

Nonetheless tree growing [with a focus on (assisted) natural regeneration, rather than just tree planting] alone is not enough to offset climate change. It must be integrated with all other forms of land, wetland, grassland and agricultural land protection and restoration. Taken together, such Natural Climate Solutions (NCS) could provide more than one-third of the cost-effective climate mitigation needed by 2030 to keep within the 2 ^oC global heating limit.

It is also critical to reduce emissions, not only of CO2 but all other greenhouse gases, at source (and not rely on NCS alone to continue with Business as Usual) by implementing both low-carbon, renewable energy, and energy demand reduction strategies, such as insulating buildings, relocalisation, and local food growing.

Finally, even climate change is not the problem, but a symptom of ecological overshoot, which is underpinned by a human behavioural crisis that needs to be addressed.