3.21 Intellectual The Intellectual
Forests are just as fragile and sensitive as humans, which in turn weakens the potential | of forest carbon sequestration Image source: pixabay.com
Introduction
21 March 2022 is the tenth International Day of Forests. Compared with afforestation, the International Day of Forests pays more attention to the deep and complex interaction between forests and people's livelihoods, emphasizing the long-term coexistence of humans and forests.
In the context of climate change, numerous studies have found that forests are as fragile and sensitive as humans, which in turn weakens the potential of forests to sequester carbon; if not properly protected, forests may even be reduced from a valuable "carbon sink" to another source of greenhouse gas emissions.
Written by | Wu Yunong
Editor-in-charge | Feng Hao
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In the Amazon, forests rain for themselves and rivers fly.
Scientists have found that the formation of clouds and rain in the Amazon is related to the organic material released by forests [1]. Trees act like "sweating", releasing organic molecules such as plant fragments, pollen, fungal spores, etc., and reacting with compounds in the air to produce particles between 20-200 nanometers in diameter, which are both seeds of clouds and key factors in rainfall. At the same time, the roots of the trees draw water from the soil, evaporating it into the atmosphere through the stomata of the leaves, and replenishing the water vapor for the clouds [2].
Clouds from the Amazon, combined with the evaporating water vapor on the surface of the equatorial sea, crash into the Andes and ride the trade winds south to form a "flying river". Every day, the Amazon rainforest injects about 20 billion litres of water into this "heavenly river", which is almost equivalent to the flow of the Amazon River [3]. This "flying river" nourishes glaciers, grasslands, and farmland in the heart of the South American continent, and makes this leading region of South America's economic development more comfortable and livable [4].
This is the "magic" of natural forests -- not just thousands of trees, but a key component of the Planet's ecosystem. Trees, along with the soil and life beneath their feet, carry about 80% of the world's terrestrial biodiversity and are an important helper for humanity in the fight against climate change.
On March 18, Peking University's Conservation Ecology team published a long article in the journal Science [5] saying that natural forests can better support biodiversity conservation and achieve ecosystem services such as surface carbon storage, soil conservation, and water conservation than simple plantations [6].
In the context of climate change, forests are as fragile and sensitive as humans, and if not properly protected, they could accelerate the climate crisis. In the Case of the Amazon Rainforest, for example, years of fires and tree deaths have turned this "magical" forest into a "carbon source" for net carbon dioxide emissions.
Protecting and restoring natural forests and mitigating climate change are saving both forests and ourselves.
1 "Jagged" in the forest world: natural forests vs plantations
At present, "forest restoration" as an important "nature-based solution" is highly expected by the international community to increase the area of forests to absorb and store carbon dioxide in response to global climate change.
Against this backdrop, a large number of afforestation campaigns are in full swing. In January 2020, at the World Economic Forum in Davos, China, the United States and other countries passed the "One Trillion Trees" initiative, and in 2011, the German government and the International Union for Conservation of Nature (IUCN) launched the "Bonn Challenge" to restore 350 million hectares of forest by 2030.
So what kind of forest restoration pathways can help us achieve our climate goals more efficiently?
In general, the path of forest restoration can be divided into two categories: planting a relatively simple plantation forest and restoring natural forests. In 2019, a study published in Nature analyzed the differences in carbon sequestration in selecting different forest restoration pathways. The study found that the carbon sequestration capacity of natural forests is six times higher than that of sustainablely managed agroforestry complex land and 40 times higher than that of planted forests [7].
Figure 1 Comparison of carbon sequestration potential of natural forests, agroforestry complex land and plantation farms| source[7]
This is also in line with the IPCC's Special Report on Climate Change and Land, released in the same year. The report notes that protecting existing forests is a faster, better, and more "affordable" way to stabilize the global climate than planting new trees [8].
In addition, a large number of studies have pointed out that afforestation is not the most effective means of reducing atmospheric carbon dioxide content, and may even be counterproductive, for example, afforestation in high latitudes may reduce the reflection of the surface to solar radiation, resulting in warming [9]; it takes tens or hundreds of years for regenerated forests to reach the carbon sequestration capacity of primary forests, and plantation tree species are single and often felled again before maturity.[10]
The latest study published in Science by the Conservation Ecology Team of Peking University further analyzes the "advantages and disadvantages" of natural forests and plantation forests, providing a basis for choosing a scientific path for forest restoration.
The research team believes that a large number of current forest restoration projects aimed at ecosystem services have the assumption that "all forests are available" and build simple plantations by planting one or a few tree species, but the assumption that "plantation forests are equally effective as natural forests" has not yet been rigorously tested. To fill this gap, the study aggregated nearly 26,000 data from 264 field studies in 53 countries and regions around the world, comparing the effectiveness of planted and natural forests in four key ecosystem services and biodiversity: surface carbon storage, soil conservation, water conservation, and timber production.
Figure 2 Assessment of plantations (compared to natural forests) in biodiversity and ecosystem services A- Distribution of data sources and volume of data; B- Relative effectiveness of plantation forests compared to mature natural forests (above), relative effectiveness of plantation forests compared to natural forests of older or abandoned forests (below); C - Relative effectiveness of plantations compared to restored natural forests, including timber production (below) ;D-Restored natural forests compared to the average annual yield of timber from major single-tree plantations worldwide| Source [ 11]
The study found that natural forests are more valuable than plantation forests in the three ecological goals of biodiversity conservation and surface carbon storage, soil conservation, and water conservation; however, plantation forests are significantly more effective in wood production than natural forests. Therefore, the goal of timber production can be achieved through the efficient use of plantations, reducing the deforestation of natural forests with higher ecological benefits, thereby indirectly providing ecological benefits.
The team also found that there are many plantations around the world that are older or abandoned and no longer used for timber production, and restoring them to natural forests can bring ecological dividends more economically.
The comparative photos given by Hua Fangyuan in Figure 3 can intuitively see the huge difference between natural forests and plantation landscapes. A-Natural forest in Daxiangling Provincial Nature Reserve, Sichuan Province; B-Source of Liushan Plantation | in Hongya County, Sichuan Province[11]
2 Climate change is driving down the "ceiling" of forest carbon sequestration
In a sense, "forest restoration" is the "remedy" of human beings.
Although it is not too late, the cycle of "ten years of trees, one hundred years of tree people", in reality, is far more than ten years.
The study found that even in tropical and subtropical regions with superior hydrothermal conditions, it takes about 66 years for the regenerated forest to recover to 90% of the average surface biomass of primary forests [12]. Competition for sunlight, moisture, and nutrients among individual trees as newly planted saplings grow into forests further limits the forest's ability to sequester carbon [13].
So, what is the current carbon sequestration capacity of natural forests?
Forests' long-term carbon sequestration capacity depends on its ability to conserve carbon in the form of biomass, and in scientific research, surface biomass is often used to measure the level of carbon stored in forests. Natural disturbances, such as hurricanes, fires and droughts, can reduce a forest's long-term carbon sequestration capacity. Global climate change exacerbates the erosion of forests by this natural disturbance.
The IPCC Special Report on Climate Change and Land states that the risk of wildfires will continue to rise if global temperature rises by 1.5°C, and the risk of vegetation destruction and wildfire loss will reach dangerous levels if the temperature rises to 3°C [14].
A study published in Nature Climate Change in 2017 [15] established an analytical framework to comprehensively assess the disturbing effects of six impact factors on forests, namely non-biological (fire, drought, wind, snow and ice) and biological (insects and pathogens).
Figure 4 Distribution of direct, indirect and interactive effects of climate change on forest disturbances. The number of observations in the literature that support the impact factor, reflected as the width and percentage of the arrows in the figure, indicates the relative prominence of the impact factor. The center shows the total results for all impact factors. Direct effects refer to the direct impact of climate change on the interference process, indirect effects describe the impact of climate change on the interference process through the impact on vegetation and other ecosystem processes, and the interaction effect refers to the influence of this influence factor by the change of other influencing factors, which in turn affect the interference process| Source[15]
The study found that as the climate warms, the frequency of occurrence of five other disturbances worldwide, in addition to snow and ice, is likely to increase. Warmer and drier climatic conditions will fuel fires, droughts and pests, while warmer and wetter climatic conditions increase the threat of winds and pathogens to forests. Widespread interactions between various factors can amplify disturbances and increase forest sensitivity to climate, for example, drought and wind disturbances clearly contribute to the emergence of insect and fire disturbances.
The worrying "future" is moving from research forecasts to reality.
In early March, the IPCC report Climate Change 2022: Impacts, Adaptation and Vulnerability showed that droughts, fires, pest outbreaks and human activities triggered by climate change are all exacerbating tree deaths [16].
The report mentions that between 1945 and 2007, in three regions of Africa and North America, up to 20 per cent of trees were dead from climate change. Between 1984 and 2017, wildfires in the western United States increased by 900 percent, half of which are attributable to human-induced climate change; wildfires in the Arctic, Australia, Africa, and parts of Asia continue to increase. The global fire season is being extended.
Un Environment also noted in February that climate change is causing wet tropical forests that used to be almost non-wildfires to burn. Between 2002 and 2016, an average of 423 million hectares of land were burned each year, roughly the size of the entire European Union [17]. Wildfires directly cause forest death, causing carbon fixed in trees to be released.
At the same time, the "fragmentation" of forests has also caused forest degradation, further reducing the carbon sequestration capacity of forests.
A 2015 study published in Nature Communications found that the storage biomass of forests within 1.5 km of the forest edge was on average 15% lower than that of the forest interior, and if the range was compressed to 500 meters from the forest edge, the forest stored biomass within the range was 25% lower than the interior. Based on this, the team believes that the IPCC may have overestimated the carbon stocks of tropical forests by nearly 10% [18].
Even without natural disturbances, climate change will still weaken the potential of forests to sequester carbon. In the case of forests in North America, for example, a study of changes in surface biomass over time over different climatic conditions shows that, even without taking into account the effects of natural disturbances on forests, under the high emission scenario [19], the average level of surface biomass in North American forest stocks will be only 78% of the current level by 2080, while the destruction of natural disturbances may be worse [20].
It can be said that climate change is "finding ways" to lower the "ceiling" of carbon sequestration in forests.
3Can tropical virgin forests be a lifesaver?
So, there are no natural forests that can help us?
As a key node in the global carbon cycle and water cycle, tropical forests should be the precious "carbon sinks" of the earth.
Currently, the global terrestrial region absorbs about 30% of human-generated carbon dioxide emissions [21], while about 40%-50% of the carbon fixed by terrestrial vegetation is stored in tropical forests [22].
As early as 2011, a landmark study published in the journal Science concluded that tropical forests absorbed 15% of total human carbon dioxide emissions between 1990 and 2007 [23].
However, as natural disturbances increase, trees die and degrade, tropical forests are also quietly becoming "carbon sources".
In March 2020, a study published in the journal Nature stated that the ability of tropical forests to absorb carbon dioxide from the atmosphere peaked in the 1990s and has been declining ever since.
A study published in February also found that over the past five years (2015-2019), the average annual carbon loss in tropical forests is 2.1 times higher than in the early 21st century (2001-2005) [25].
Figure 5 Four grouped bar charts (from left to right) show the average annual forest carbon loss | source for the four periods 2001-2005, 2006-2010, 2011-2014, and 2015-2019, respectively[25]
Among them, forest fragmentation is still an important cause of carbon loss. A study using a high-resolution (30 m) satellite map of forest cover found that 19 percent of tropical forests are within 100 meters of forest edges, with about 50 million forest debris throughout the tropics. This marginal effect is responsible for a total of about 10.3 billion tonnes of carbon emissions, equivalent to 340 million tonnes per year, or about 31% of the current annual carbon emissions from tropical deforestation [26].
The overall situation of tropical forests is not optimistic, and the conclusions of scientists' research on different forest areas are also "mixed":
The Amazon rainforest, which has attracted the most attention, has been receiving bad news in recent years. A growing body of research suggests that climate change and deforestation, which have led to years of fires and tree deaths in the Amazon rainforest, have reduced the "lungs of the earth" from carbon dioxide strippers to emitters [27-29].
What is even more unfortunate is that the natural degradation of forests has replaced the deforestation caused by human activities and become the most important factor in carbon emissions in the Amazon rainforest. The study found that between 2010 and 2019, the Amazon region of Brazil released a cumulative 445 million tons of carbon and absorbed 378 million tons of carbon, which is a net carbon dioxide emission. As forest degradation exceeds deforestation, the forest surface biomass associated with forest degradation (73 per cent) decreases three times that of deforestation (27 per cent) [30].
But unlike the vanished hopes of the Amazon rainforest, Africa's montane forests are opening up new possibilities.
A 2020 study published in the journal Nature [31] found that Africa's mountain rainforests stored higher concentrations of carbon in the Biamasun rainforest.
More than 100 scientists participated in the study, measuring 72,000 trees, recording trunk diameters, tree species and tree heights, obtaining data on 244 structurally intact African tropical forests spanning 11 African countries and comparing them with 321 forest plots in the Amazon.
They found that for 30 years as of 2015, the carbon stock of aboveground biomass in Tropical Montane Forests in Africa has been stable, about 0.66 tons of carbon per hectare per year, in contrast to the long-term decline in carbon storage in Amazon forests, where forest degradation and death are the main causes of this difference. The study also notes that although carbon sinks in Africa's mountain forests have also begun to decline after 2010, they have been delayed compared to forests in the Amazon region.
Simon Lewis, co-author of the paper from the University of Leeds and University College London in the United Kingdom, said in an interview that the results of this study are surprising considering that mountain forests face challenges, including high altitudes, strong winds, steep slopes, low temperatures, prolonged cloud soaking and soil staining, which are often thought to limit tree growth [32].
"Trees in Africa live longer, so they have more time to accumulate carbon." Lewis also mentioned that studies [33] have shown that Lowland Forests in Africa are also an important forest carbon sink, which may mean that Africa's forests as a whole remain strongly "resilient" in the face of climate change in recent decades.
4Forest protection, the line is coming
The call for the protection of Africa's primary forests and the Congo Basin fundraising initiative were among the key actions of the COP26 climate conference in Glasgow, Uk, last November.
The initiative aims to raise US$ 500 million over the next five years to protect important carbon sinks such as forests and peatlands in the Congo Basin, which account for about 10% of the world's tropical forests [34]. In return, the Democratic Republic of the Congo has undertaken to publish the results of audits of all logging contracts and to strengthen the review of illegal contracts for delaminated forests. However, in February this year, some media questioned that the relevant commitments had not been implemented [35].
Also during COP26, 141 countries, including China, covering 90% of the world's forest area, signed the Glasgow Leaders' Declaration on Forests and Land Use, pledging to work together to halt and reverse forest loss and land degradation by 2030.
It can be said that the protection of forests is already a globally recognized priority in the fight against climate change. The effectiveness of policy intervention has long been apparent. For example, in protecting existing forests, Brazil has successfully reduced deforestation in the Amazon between 2004 and 2012 through legislation and the establishment of a monitoring system. The loosening of policies in recent years has once again led to an increase in the associated logging rate [36]. In terms of forest restoration, China's policies such as returning farmland to forests in recent decades have also had a clear effect [37], and measures taken in southern China to refine afforestation in marginal areas of agricultural activities have also produced short-term carbon storage [38].
The year 2021-2030 is a decade of China's gradual transformation and completion of the "carbon peak" climate commitment, a key decade for the global response to climate change and the mitigation of greenhouse gas emissions, and a "decade of ecosystem restoration" for the United Nations.
Without emission reduction measures and climate change, forests and human beings will face more unknown risks and harms, and protecting existing forests and choosing more scientific and efficient ways to restore forests may help us unlock a more optimistic future. Let the forest rain for itself and let the river continue to fly.
Acknowledgements: Thanks to Yanlei Feng, a PhD student at the University of California, Berkeley, for discussing this article.
Author Bio
Wu Yunong is ChinaDialogue's climate change strategy communications officer.
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