Restoring natural forests is the best way to remove atmospheric carbon, Lewis et al. 2019
In order to keep global warming under the 1.5C threshold, the IPCC warns that not only must we cut carbon emissions nearly in half by 2030, we must also draw massive amounts of CO2 out of the atmosphere.
The Intergovernmental Panel on Climate Change (IPCC) suggests that around 730 billion tons of CO2 (730 petagrams of CO2, or 199 petagrams of carbon, Pg C) must be taken out of the atmosphere by the end of this century. That is equivalent to all the CO2 emitted by the United States, the United Kingdom, Germany and China since the Industrial Revolution [Lewis 2019: 25].
The IPCC further advises that forests and wooded savannas could store enough carbon to get us a quarter of the way there. “In the near term, this means adding up to 24 million hectares (Mha) of forest every year from now until 2030” [Lewis 2019: 26].
Through the Bonn Challenge, 43 countries have pledged to reforest nearly 300Mha out of a goal of 350Mha by 2030. “However, plantations are the most popular restoration plan: 45% of all commitments involve planting vast monocultures of trees as profitable enterprises” [Lewis 2019: 26], which stores much less carbon than do natural forests. Agroforestry accounts for 21% of pledged land, while natural forest regeneration accounts for 34% of commitments.
While timber plantations technically fit the definition of a forest (greater than 0.5 hectares in area, trees at least five meters high and more than 10% canopy cover, according to UN FAO),
the key components of climate-change mitigation and biodiversity protection are missing. Plantations are important economically, but they should not be classified as forest restoration. That definition urgently needs an overhaul to exclude monoculture plantations [Lewis 2019: 27].
Illustrating vast differences in mitigation potential, Lewis et al. state that “if the entire 350 Mha [of the Bonn Challenge goal] is given over to natural forests, they would store an additional 42 Pg C by 2100. Giving the same area exclusively to plantations would sequester just 1 Pg C or, if used only for agroforestry, 7 Pg C” [Lewis 2019: 27].
The authors make four specific recommendations to ensure more effective climate change mitigation through conservation and restoration efforts:
(1) Countries should significantly increase the proportion of natural forest restoration in their commitments. (Natural forest restoration over an area the size of South Carolina could store 1 Pg of carbon by 2100.)
(2) Natural forest restoration should be prioritized in the tropics, where trees grow fastest and don’t risk countering the albedo effect since there’s never any reflective snow there anyway.
(3) “Target degraded forests and partly wooded areas for natural regeneration; focus plantations and agroforestry systems on treeless regions and, where possible, select agroforestry over plantations.”
(4) Natural forest once restored must be protected.
The global tree restoration potential, Bastin et al. 2019
This study models the total amount of land globally that is suitable for reforestation, finding that there is sufficient space to meet the IPCC’s recommendation of reforestation on 1 billion hectares to limit global warming to 1.5C by 2050. The potential forest land identified in this study excludes urban and agricultural land; rather, it “exists in areas that were previously degraded, dominated by sparse vegetation, grasslands, and degraded bare soils” [Bastin 2019]. Yet for reforestation efforts to meet this potential, time is of the essence. By 2050, climate change will have shrunk the additional amount of land capable of supporting forest ecosystems by about a quarter.
The restoration of trees remains among the most effective strategies for climate change mitigation. We mapped the global potential tree coverage to show that 4.4 billion hectares of canopy cover could exist under the current climate. Excluding existing trees and agricultural and urban areas, we found that there is room for an extra 0.9 billion hectares of canopy cover, which could store 205 gigatonnes of carbon in areas that would naturally support woodlands and forests.This highlights global tree restoration as our most effective climate change solution to date. However, climate change will alter this potential tree coverage.We estimate that if we cannot deviate from the current trajectory, the global potential canopy cover may shrink by ~223 million hectares by 2050, with the vast majority of losses occurring in the tropics. Our results highlight the opportunity of climate change mitigation through global tree restoration but also the urgent need for action [Bastin 2019: 1].
The restoration of trees remains among the most effective strategies for climate change mitigation [Bastin 2019: 1].
More than 50% of the tree restoration potential can be found in only six countries (in million hectares: Russia,+151; United States,+103; Canada, +78.4; Australia, +58; Brazil, +49.7; and China, +40.2), stressing the important responsibility of some of the world’s leading economies.
The significance of retention trees for survival of ectomycorrhizal fungi in clear-cut Scots pine forests, Sterkenburg et al. 2019
Industrialized forestry simplifies forest structure and harms biodiversity. To mitigate this harm, retention forestry has been adopted in places such as Sweden, where this study was conducted. “Retention forestry” avoids clearcutting and instead preserves some 5-30 percent of trees to benefit populations of birds, lichens, fungi and other types of organisms.
The authors focused on the effects of retention on ectomycorrhizal (ECM) fungi (also commonly abbreviated as “EM” fungi), an ecologically important group of species.
ECM fungi represent a large part of the biodiversity in boreal forests. They depend on carbohydrates from their host trees and are vital for forest production, as uptake of nutrients and water by the trees is mediated by the soil ECM symbiosis. ECM fungal mycelium forms a basis for soil food webs. The largely cryptic life of ECM fungi has hampered understanding of their biology and their importance for ecosystem processes, impeding adaptation of forestry to sustain ECM fungal diversity [Sterkenburg 2019: 2].
Aiming to quantify the decline in ECM fungi species abundance and richness in relation to the proportion of trees logged, the authors established an experiment with two levels of trees retained (30% and 60%), which was then compared to unlogged forest (100% retained) and clear-cut forest (0% retained). They found that ECM fungal diversity and relative abundance is preserved in proportion to the amount of retained trees.
“In clear-cuts, ECM fungal relative abundance had decreased by 95%, while ECM fungal species richness had declined by 75%, compared to unlogged plots” [Sterkenburg 2019: 1]. The latter result meant that the less common species of ECM fungi were lost, while the more dominant ones survived. The authors noted that even at the Swedish Forestry Council’s sustainability threshold of 5% tree retention (i.e., 95% logged), some 75% of ECM species are lost. In other words, there’s no significant difference between clearcutting and retaining 5% of the trees in terms of the effects on the number of fungal species lost. To preserve fungal diversity, many more trees must be retained when logging.
This study illustrates the unseen damage to forest ecosystems of intensive logging, as well as the potential challenges of re-growing forests following clear-cutting, given a likely dearth of ECM symbionts to aid sapling development.
Plant-soil feedbacks and mycorrhizal type influence temperate forest population dynamics, Bennett et al. 2017
This study illustrates the important role of soil fungi in tree population dynamics of temperate forests. In general, when a particular plant species dominates an area of land, it attracts species that feed on it. In an experiment conducted in this study, the roots of surviving seedlings had 60% fewer lesions when they were planted beneath a tree species different than their own, compared to when they were planted beneath a member of their own species, “potentially because of increased root damage by antagonists” [Bennett 2017: 2].
However, seedlings inoculated by ectomycorrhizal (EM) fungi, which forms a protective sheath around its host’s roots and also efficiently transferring nitrogen to its host,
had 840% higher survival and 75% lower lesion densities than those of uninoculated seedlings when planted beneath conspecifics [members of the same species], but inoculation had no effect beneath heterospecifics [members of another species]. In contrast, AM seedlings did not benefit from pre-inoculation, nor did pre-inoculation affect lesion densities, regardless of transplant location [Bennett 2017: 2].
Mycorrhizal fungi colonize plant roots, where they transfer nutrients to hosts in exchange for sugar produced through photosynthesis. Ectomycorrhizal fungi form a sheath around tree roots, while arbuscular mycorrhizal (AM) fungi colonize tree roots by penetrating root cell walls. In this study, as noted above, trees with EM fungi symbionts appear to be better protected from pathogens than are AM trees when growing amongst others of their own species. This dynamic affects the population dynamics of the forest by facilitating larger stands of EM trees, while inhibiting the clustering of AM trees.
In summary, “These results suggest that mycorrhizal type, through effects on plant-soil feedbacks, could be an important contributor to population regulation and community structure in temperate forests” [Bennett 2017: 1].
Nitrogen-fixing red alder trees tap rock-derived nutrients, Perakisa & Pett-Ridge 2019
Red alder fix atmospheric nitrogen through a symbiosis with bacteria that colonize their roots. This study showed that when more nitrogen is produced than is needed by the plant, the resulting excess of nitric acid acts to dissolve bedrock minerals in the soil, making them available to plants.
The substantial increase in mineral weathering by N-fixing [nitrogen-fixing] alder helps explain how this species takes up 65% more P [phosphorus] and 200% more Ca [calcium] than non-fixing Douglas-fir. Enhanced access to P is most likely important to N fixers, and is used to increase photosynthetic tissue mass and N-fixing nodule production to support growth. … Ecosystem supplies of both P and Ca can limit nonfixer tree growth where N is abundant, including in our forests. Alder-enhanced uptake of rock-derived Ca and its subsequent redistribution via litterfall may especially benefit bigleaf maple and western red cedar, two nonfixers with consistently high Ca demands that have limited direct access to rock-derived nutrients [Perakisa & Pett-Ridge 2019: 5012].
This study suggests possibilities for increasing forests’ capacity to absorb carbon and mitigate climate change through the ability of red alder (and potentially other nitrogen-fixing trees) to make otherwise limiting nutrients, including nitrogen, phosphorus and calcium, available within the forest ecosystem.
Our finding that an N-fixing tree species can directly access rock-derived nutrients has implications for nutrient supplies that regulate tree growth and C uptake in forests. Inputs of fixed N can increase tree growth in N-limited forests, and could be further stimulated by access to rock-derived nutrients. Where N is already abundant and other nutrients are limiting, supplies of rock-derived nutrients can be even more important to forest growth and C uptake. It is presently unknown whether high rates of N fixation by trees are geographically widespread, and whether N fixers other than red alder can similarly access rock-derived nutrients [Perakisa & Pett-Ridge 2019: 5013].
Climatic controls of decomposition drive the global biogeography of forest-tree symbioses, Steidinger et al. 2019
This article describes three major types of microbial tree symbionts, why they matter, and maps their global distribution.
Microbial symbionts strongly influence the functioning of forest ecosystems. Root-associated microorganisms exploit inorganic, organic and/or atmospheric forms of nutrients that enable plant growth, determine how trees respond to increased concentrations of CO2, regulate the respiratory activity of soil microorganisms and affect plant species diversity by altering the strength of conspecific negative density dependence [Steidinger 2019: 404].
Arbuscular mycorrhizal and ectomycorrhizal fungi and nitrogen-fixing bacteria are the focus of the study.
Plants that are involved in arbuscular mycorrhizal symbiosis comprise nearly 80% of all terrestrial plant species; these plants principally rely on arbuscular mycorrhizal fungi for enhancing mineral phosphorus uptake. In contrast to arbuscular mycorrhizal fungi, ectomycorrhizal fungi evolved from multiple lineages of saprotrophic ancestors and, as a result, some ectomycorrhizal fungi are capable of directly mobilizing organic sources of soil nutrients (particularly nitrogen). Associations with ectomycorrhizal fungi—but not arbuscular mycorrhizal fungi—have previously been shown to enable trees to accelerate photosynthesis in response to increased concentrations of atmospheric CO2 when soil nitrogen is limiting, and to inhibit soil respiration by decomposer microorganisms. Because increased plant photosynthesis and decreased soil respiration both reduce atmospheric CO2 concentrations, the ectomycorrhizal symbiosis is associated with buffering the Earth’s climate against anthropogenic change.
In contrast to mycorrhizal fungi, which extract nutrients from the soil, symbiotic N-fixers (Rhizobia and Actinobacteria) convert atmospheric N2 to plant-usable forms. Symbiotic N-fixers are responsible for a large fraction of biological soil-nitrogen inputs, which can increase nitrogen availability in forests in which N-fixers are locally abundant [Steidinger 2019: 404].
Because increased plant photosynthesis and decreased soil respiration both reduce atmospheric CO2 concentrations, the ectomycorrhizal symbiosis is associated with buffering the Earth’s climate against anthropogenic change [Steidinger 2019: 404].
The study finds that climatic controls on litter breakdown determine fungi type in a given region, where colder climates favor ectomycorrhizal fungi, which are more efficient at extracting nutrients from organic material, and warmer climates favor arbuscular mycorrhizal fungi, which efficiently extract phosphorus from the soil. Warmer climates also favor nitrogen-fixing bacterial symbionts. Based on symbiosis distribution vis-a-vis existing spatial climate gradients, the authors predict changes in forest symbiosis distribution as the climate changes overtime.
To illustrate the sensitivity of global patterns of tree symbiosis to climate change, we use the relationships that we observed for current climates to project potential changes in the symbiotic status of forests in the future. Relative to our global predictions that use the most-recent climate data, model predictions that use the projected climates for 2070 suggest that the abundance of ectomycorrhizal trees will decline by as much as 10%… Our models predict that the largest declines in ectomycorrhizal abundance will occur
along the boreal–temperate ecotone, where small increases in climatic decomposition coefficients cause abrupt transitions to arbuscular mycorrhizal forests [Steidinger 2019: 407].
The authors explain that existing transitions between arbuscular and ectomycorrhizal forests are abrupt due to positive feedbacks maintaining these systems. For instance, the chemical composition of the leaves of trees forming ectomycorrhizal symbioses resists decomposition, meaning that their leaf litter reinforces the presence of ectomycorrhiza in cooler regions. Once a small temperature threshold is breached, however, climate controls on decomposition speed up litter breakdown and favor arbuscular mycorrhizal fungi, along with their tree hosts.
Hydraulic diversity of forests regulates ecosystem resilience during drought, Anderegg et al. 2018
Higher forest biodiversity (specifically plant functional diversity related to water, or hydraulic, transport) engenders greater ecosystem resilience to drought. This is because different species respond differently to water stress – some species slow down their release of water (and heat) through transpiration sooner than others do. Plants’ response to water availability in turn affects the local climate.
Water, carbon and energy exchanges from the land surface strongly influence the atmosphere and climate; these exchanges are dominated by plants in most ecosystems. Plant physiological responses to water stress influence these fluxes, and the resulting land-surface feedback effects influence local weather as well as the regional atmospheric circulation. Furthermore, changes in vegetation physiology and cover can drive shifts in sensible and latent heat fluxes that intensify droughts [Anderegg 2018: 538].
We have documented a fundamental effect of trait variation on ecosystem stability that directly influences the atmosphere and climate system. Temperate and boreal forest ecosystems with higher hydraulic diversity are more buffered to changing drought conditions [Anderegg 2018: 540].
Tree diversity regulates forest pest invasion, Guo et al. 2019
Using data from 130,210 forest plots across the US, this study examines the effects of tree diversity on pest invasions. The authors found that tree diversity increases pest diversity by increasing the variety of host species available (i.e., facilitation), while also decreasing establishment of pests by increasing the number of non-hosts for any given pest species relative to the total number of trees (i.e., dilution). In other words:
The relative proportion of component tree species (hosts vs. nonhosts) plays a key role in determining pest invasions, as indicated by our evidence that host diversity may promote pest diversity while neighboring nonhost species could enhance the associational resistance of host species to nonnative pest invasions [Guo 2019].
More specifically, the study observed a hump-shaped relationship between tree and pest diversity.
Pest diversity increases with tree diversity at low tree diversity (because of facilitation or amplification) and is reduced at higher tree diversity (as a result of dilution). Thus, tree diversity likely regulates forest pest invasion through both facilitation and dilution that operate simultaneously, but their relative strengths vary with overall diversity [Guo 2019].
Other factors that influence pest invasions in forest ecosystems include: “climate, resource availability, spatial scale, and habitat fragmentation related to human disturbances.” Furthermore, “recent analyses indicate that pest species continue to be introduced and spread around the globe. Under climate and land use changes, many tree species could expand, contract, or undergo latitudinal/elevational shifts in their geographical ranges” [Guo 2019].
These findings underscore the importance of biodiversity in maintaining healthy and stable ecosystems, while also highlighting the complexity of ecosystems (given the non-linear relationship between tree and pest diversity) and the challenges that poses for restoration.
Restoration of living environment based on vegetation ecology: theory and practice, Miyawaki 2004
Natural environments have been devastated and destroyed worldwide by recent rapid development, urbanization and industrialization. It is no exaggeration to say that the basis of human life is now threatened (Miyawaki 1982a,b).
We ecologists have been giving warnings against the devastation of nature through study results, and have produced some good effects. Besides criticism, however, we should contribute to the wholesome development of human society by active concern for nature restoration and reconstruction (Miyawaki 1975, 1981) [Miyawaki 2004: 83].
As suggested in these introductory words, Akira Miyawaki is a Japanese ecologist who has dedicated decades of his life to the study and implementation of forest restoration. He emphasizes the importance of restoring barren or degraded land more quickly than the time it takes for natural forest succession to occur, which can be 150-300 years, depending on the regional climate. By contrast, the methods he recommends can yield results within 15-20 years in terms of establishing forests mature enough to protect communities against natural disasters, such as earthquakes and storms. The principles of what has become known as the “Miyawaki Method” are based on mimicking natural forest growth patterns and thus feature: high biodiversity, preference for native species, relatively high planting densities, and healthy soil.
Communities undertaking such restoration efforts must first survey the landscape to determine the “potential vegetation” for the area based on what remains of native tree communities. Next, seeds must be gathered for some 30-50 species of native trees, and then propagated in greenhouses. After a year or two, once the seedlings have strong, well-developed roots, they can be planted. Miyawaki refers to planting events as “festivals” because the community dynamic is important for increasing public understanding of the relevance of ecological restoration and igniting a collective willingness to protect the plantings well into the future.
Miyawaki concludes with these words:
These forests of complex multilayer communities have disaster-mitigation and environmental protection functions in each region. In the Great Hanshin Earthquake, which hit the Kobe district, western Japan in January 1995, there was no damage to trees in Japanese traditional temple forests, the potential natural vegetation, however, huge structures made of non-living materials collapsed, including elevated railways, highways and tall buildings (Miyawaki 1998). On a global scale, natural forests help to avoid global warming by absorbing carbon dioxide. Restoration and regeneration of ecologically diverse forests is inevitable for citizens in every region to survive in the next century, and the next millennium [Miyawaki 2004: 89].
The legacy of 4,500 years of polyculture agroforestry in the eastern Amazon, Maezumi et al. 2018
This study combines archaeology, archaeobotany, palaeoecology and palaeoclimate investigation to shed light on the legacy of pre-Columbian land management practices on today’s Amazon rainforest. Evidence points to a millennial-scale cultivation practice that at once maintained ecosystem integrity while sustaining a large and growing human civilization.
Here, we show that persistent anthropogenic landscapes for the past 4,500 years have had an enduring legacy on the hyperdominance of edible plants in modern forests in the eastern Amazon. We found an abrupt enrichment of edible plant species in fossil lake and terrestrial records associated with pre-Columbian occupation. Our results demonstrate that, through closed-canopy forest enrichment, limited clearing for crop cultivation and low-severity fire management, long-term food security was attained despite climate and social changes. Our results suggest that, in the eastern Amazon, the subsistence basis for the development of complex societies began ~4,500 years ago with the adoption of polyculture agroforestry, combining the cultivation of multiple annual crops with the progressive enrichment of edible forest species and the exploitation of aquatic resources. This subsistence strategy intensified with the later development of Amazonian dark earths, enabling the expansion of maize cultivation to the Belterra Plateau, providing a food production system that sustained growing human populations in the eastern Amazon. Furthermore, these millennial-scale polyculture agroforestry systems have an enduring legacy on the hyperdominance of edible plants in modern forests in the eastern Amazon. Together, our data provide a long-term example of past anthropogenic land use that can inform management and conservation efforts in modern Amazonian ecosystems [Maezumi 2018: 540].
This largely hidden history of the Amazon illuminates a path forward today as humanity grapples with the combined challenges of maintaining food production for a growing global population, while preserving and restoring forests and curbing biodiversity collapse. As suggested here, ecological restoration and agricultural productivity to sustain growing populations are not mutually exclusive enterprises, but in fact can be synergistic.
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 Members of one’s own species.
 Saprotrophic species are those that feed on decaying organic matter.