Der er ikke nok træer i verden til at opveje samfundets CO2-udledning - og det vil der aldrig være

En morgen i 2009 Jeg sad på en knirkende bus, der snoede sig op ad en bjergside i det centrale Costa Rica, lys i hovedet af dieseldampe, mens jeg greb mine mange kufferter. De indeholdt tusindvis af reagensglas og prøvehætteglas, en tandbørste, en vandtæt notesbog og to tøjskift.

Jeg var på vej til La Selva Biologiske Station, hvor jeg skulle bruge flere måneder på at studere det våde, lavlandsregnskovs reaktion på stadig mere almindelige tørkeperioder. På hver side af den smalle motorvej, træer blødte ind i tågen som akvareller til papir, giver indtryk af en uendelig urskov badet i skyer.

Mens jeg så ud af vinduet på det imponerende landskab, Jeg spekulerede på, hvordan jeg nogensinde kunne håbe på at forstå et så komplekst landskab. Jeg vidste, at tusindvis af forskere over hele verden kæmpede med de samme spørgsmål, forsøger at forstå skæbnen for tropiske skove i en verden i hastig forandring.

Vores samfund kræver så meget af disse skrøbelige økosystemer, som kontrollerer tilgængeligheden af ​​ferskvand for millioner af mennesker og er hjemsted for to tredjedele af planetens terrestriske biodiversitet. Og i stigende grad, vi har stillet et nyt krav til disse skove – for at redde os fra menneskeskabte klimaændringer.

Planter optager CO ₂ fra atmosfæren, forvandler det til blade, træ og rødder. Dette hverdagsmirakel har ansporet håbet om, at planter - især hurtigtvoksende tropiske træer - kan fungere som en naturlig bremse på klimaændringer, opfanger meget af CO ₂ udsendes ved afbrænding af fossile brændstoffer. Verden over, regeringer, virksomheder og velgørende naturbeskyttelsesorganisationer har lovet at bevare eller plante et massivt antal træer.

Men faktum er, at der ikke er nok træer til at opveje samfundets CO2-emissioner - og det vil der aldrig være. Jeg foretog for nylig en gennemgang af den tilgængelige videnskabelige litteratur for at vurdere, hvor meget kulstof skove kunne optage. Hvis vi absolut maksimerede mængden af ​​vegetation, al jord på Jorden kunne rumme, vi ville binde nok kulstof til at kompensere for omkring ti års drivhusgasemissioner med de nuværende rater. Efter det, der kunne ikke være nogen yderligere stigning i kulstoffangsten.

Alligevel er vores arts skæbne uløseligt forbundet med skovenes overlevelse og den biodiversitet, de indeholder. Ved at skynde sig at plante millioner af træer til kulstoffangst, kunne vi utilsigtet beskadige selve skovens egenskaber, der gør dem så vigtige for vores velvære? For at besvare dette spørgsmål, vi skal ikke kun overveje, hvordan planter optager CO ₂ , men også hvordan de giver det robuste grønne grundlag for økosystemer på land.

Hvordan planter bekæmper klimaændringer

Planter omdanner CO ₂ gas til simple sukkerarter i en proces kendt som fotosyntese. Disse sukkerarter bruges derefter til at bygge planternes levende kroppe. Hvis det opfangede kulstof ender i træ, den kan låses væk fra atmosfæren i mange årtier. Når planter dør, deres væv undergår forfald og inkorporeres i jorden.

Mens denne proces naturligt frigiver CO ₂ gennem respiration (eller vejrtrækning) af mikrober, der nedbryder døde organismer, en brøkdel af plantekulstof kan forblive under jorden i årtier eller endda århundreder. Sammen, landplanter og jord rummer omkring 2, 500 gigatons kulstof - omkring tre gange mere, end der opbevares i atmosfæren.

Fordi planter (især træer) er så fremragende naturlige lagerhuse for kulstof, Det giver mening, at en forøgelse af mængden af ​​planter over hele verden kan trække ned atmosfærisk CO ₂ koncentrationer.

Planter har brug for fire grundlæggende ingredienser for at vokse:lys, CO ₂ , vand og næringsstoffer (som nitrogen og fosfor, de samme elementer, der findes i plantegødning). Tusindvis af videnskabsmænd over hele verden studerer, hvordan plantevækst varierer i forhold til disse fire ingredienser, for at forudsige, hvordan vegetationen vil reagere på klimaændringer.

Dette er en overraskende udfordrende opgave, i betragtning af at mennesker samtidig ændrer så mange aspekter af det naturlige miljø ved at opvarme kloden, ændring af nedbørsmønstre, hugge store områder af skov i små fragmenter og introducere fremmede arter, hvor de ikke hører hjemme. Der er også over 350, 000 arter af blomstrende planter på land, og hver enkelt reagerer på miljømæssige udfordringer på unikke måder.

På grund af de komplicerede måder, hvorpå mennesker ændrer planeten, der er en masse videnskabelig debat om den præcise mængde kulstof, som planter kan optage fra atmosfæren. Men forskere er enige om, at landøkosystemer har en begrænset kapacitet til at optage kulstof.

Hvis vi sikrer, at træerne har nok vand at drikke, skove bliver høje og frodige, skabe skyggefulde baldakiner, der udsulter mindre træer af lys. Hvis vi øger koncentrationen af ​​CO ₂ i luften, planter vil ivrigt absorbere det - indtil de ikke længere kan udvinde nok gødning fra jorden til at opfylde deres behov. Ligesom en bager laver en kage, planter kræver CO ₂ , nitrogen og fosfor i særlige forhold, efter en bestemt opskrift på livet.

I erkendelse af disse grundlæggende begrænsninger, forskere vurderer, at jordens landøkosystemer kan rumme tilstrækkeligt med ekstra vegetation til at absorbere mellem 40 og 100 gigatons kulstof fra atmosfæren. Når først denne yderligere vækst er opnået (en proces, der vil tage et antal årtier), der er ingen kapacitet til yderligere kulstoflagring på land.

Men vores samfund hælder i øjeblikket CO ₂ ud i atmosfæren med en hastighed på ti gigaton kulstof om året. Naturlige processer vil kæmpe for at holde trit med syndfloden af ​​drivhusgasser, der genereres af den globale økonomi. For eksempel, Jeg beregnede, at en enkelt passager på en returflyvning fra Melbourne til New York City vil udlede omtrent dobbelt så meget kulstof (1600 kg C), som der er indeholdt i et egetræ på en halv meter i diameter (750 kg C).

Fare og løfte

På trods af alle disse velkendte fysiske begrænsninger for plantevækst, der er et voksende antal storstilede indsatser for at øge vegetationsdækningen for at afbøde klimakrisen - en såkaldt "naturbaseret" klimaløsning. Langt størstedelen af ​​disse bestræbelser fokuserer på at beskytte eller udvide skovene, da træer indeholder mange gange mere biomasse end buske eller græsser og derfor repræsenterer et større kulstoffangstpotentiale.

Alligevel kan grundlæggende misforståelser om kulstoffangst i landøkosystemer have ødelæggende konsekvenser, resulterer i tab af biodiversitet og en stigning i CO ₂ koncentrationer. Dette virker som et paradoks - hvordan kan plantning af træer påvirke miljøet negativt?

Svaret ligger i den subtile kompleksitet af kulstoffangst i naturlige økosystemer. For at undgå miljøskader, vi skal undlade at etablere skove, hvor de naturligt ikke hører hjemme, undgå "perverse incitamenter" til at fælde eksisterende skov for at plante nye træer, og overvej, hvordan frøplanter plantet i dag kan klare sig i løbet af de næste årtier.

Inden der foretages nogen udvidelse af skovens habitat, vi skal sikre, at træer plantes det rigtige sted, fordi ikke alle økosystemer på land kan eller bør understøtte træer. Plantning af træer i økosystemer, der normalt er domineret af andre typer af vegetation, resulterer ofte ikke i langsigtet kulstofbinding.

Et særligt illustrativt eksempel kommer fra skotske tørveområder - store områder af land, hvor den lavtliggende vegetation (for det meste mosser og græsser) vokser i konstant fugtig, fugtig jord. Fordi nedbrydningen er meget langsom i den sure og vandlidende jord, døde planter akkumuleres over meget lange perioder, skabe tørv. Det er ikke kun vegetationen, der er bevaret:Tørvemoser mumificerer også såkaldte "mosekroppe" - de næsten intakte rester af mænd og kvinder, der døde for årtusinder siden. Faktisk, Britiske tørveområder indeholder 20 gange mere kulstof, end der findes i landets skove.

Men i slutningen af ​​det 20. århundrede, nogle skotske moser blev drænet til træplantning. Tørring af jorden gjorde det muligt for træplanter at etablere sig, men fik også tørvens henfald til at fremskynde. Økolog Nina Friggens og hendes kolleger ved University of Exeter vurderede, at nedbrydningen af ​​tørv frigjorde mere kulstof, end de voksende træer kunne optage. Klart, tørvemarker kan bedst beskytte klimaet, når de overlades til sig selv.

Det samme gælder græsarealer og savanner, hvor brande er en naturlig del af landskabet og ofte brænder træer, der er plantet, hvor de ikke hører hjemme. Dette princip gælder også for arktiske tundraer, hvor den oprindelige vegetation er dækket af sne hele vinteren, reflekterer lys og varme tilbage til rummet. Planter højt, mørkbladede træer i disse områder kan øge optagelsen af ​​varmeenergi, og føre til lokal opvarmning.

Men selv plantning af træer i skovhabitater kan føre til negative miljømæssige resultater. Fra perspektivet af både kulstofbinding og biodiversitet, alle skove er ikke lige - naturligt etablerede skove indeholder flere arter af planter og dyr end plantageskove. De rummer ofte mere kulstof, også. Men politikker rettet mod at fremme træplantning kan utilsigtet tilskynde til skovrydning af veletablerede naturlige levesteder.

A recent high-profile example concerns the Mexican government's Sembrando Vida program, which provides direct payments to landowners for planting trees. Problemet? Many rural landowners cut down well established older forest to plant seedlings. This decision, while quite sensible from an economic point of view, has resulted in the loss of tens of thousands of hectares of mature forest.

This example demonstrates the risks of a narrow focus on trees as carbon absorption machines. Many well meaning organizations seek to plant the trees which grow the fastest, as this theoretically means a higher rate of CO ₂ "drawdown" from the atmosphere.

Yet from a climate perspective, what matters is not how quickly a tree can grow, but how much carbon it contains at maturity, and how long that carbon resides in the ecosystem. As a forest ages, it reaches what ecologists call a "steady state"—this is when the amount of carbon absorbed by the trees each year is perfectly balanced by the CO ₂ released through the breathing of the plants themselves and the trillions of decomposer microbes underground.

This phenomenon has led to an erroneous perception that old forests are not useful for climate mitigation because they are no longer growing rapidly and sequestering additional CO ₂ . The misguided "solution" to the issue is to prioritize tree planting ahead of the conservation of already established forests. This is analogous to draining a bathtub so that the tap can be turned on full blast:the flow of water from the tap is greater than it was before—but the total capacity of the bath hasn't changed. Mature forests are like bathtubs full of carbon. They are making an important contribution to the large, men begrænset, quantity of carbon that can be locked away on land, and there is little to be gained by disturbing them.

What about situations where fast growing forests are cut down every few decades and replanted, with the extracted wood used for other climate-fighting purposes? While harvested wood can be a very good carbon store if it ends up in long lived products (like houses or other buildings), surprisingly little timber is used in this way.

Tilsvarende burning wood as a source of biofuel may have a positive climate impact if this reduces total consumption of fossil fuels. But forests managed as biofuel plantations provide little in the way of protection for biodiversity and some research questions the benefits of biofuels for the climate in the first place.

Fertilize a whole forest

Scientific estimates of carbon capture in land ecosystems depend on how those systems respond to the mounting challenges they will face in the coming decades. All forests on Earth—even the most pristine—are vulnerable to warming, changes in rainfall, increasingly severe wildfires and pollutants that drift through the Earth's atmospheric currents.

Some of these pollutants, imidlertid, contain lots of nitrogen (plant fertilizer) which could potentially give the global forest a growth boost. By producing massive quantities of agricultural chemicals and burning fossil fuels, humans have massively increased the amount of "reactive" nitrogen available for plant use. Some of this nitrogen is dissolved in rainwater and reaches the forest floor, where it can stimulate tree growth in some areas.

As a young researcher fresh out of graduate school, I wondered whether a type of under-studied ecosystem, known as seasonally dry tropical forest, might be particularly responsive to this effect. There was only one way to find out:I would need to fertilize a whole forest.

Working with my postdoctoral adviser, the ecologist Jennifer Powers, and expert botanist Daniel Pérez Avilez, I outlined an area of the forest about as big as two football fields and divided it into 16 plots, which were randomly assigned to different fertilizer treatments. For the next three years (2015-2017) the plots became among the most intensively studied forest fragments on Earth. We measured the growth of each individual tree trunk with specialised, hand-built instruments called dendrometers.

We used baskets to catch the dead leaves that fell from the trees and installed mesh bags in the ground to track the growth of roots, which were painstakingly washed free of soil and weighed. The most challenging aspect of the experiment was the application of the fertilizers themselves, which took place three times a year. Wearing raincoats and goggles to protect our skin against the caustic chemicals, we hauled back-mounted sprayers into the dense forest, ensuring the chemicals were evenly applied to the forest floor while we sweated under our rubber coats.

Desværre, our gear didn't provide any protection against angry wasps, whose nests were often concealed in overhanging branches. Men, our efforts were worth it. Efter tre år, we could calculate all the leaves, wood and roots produced in each plot and assess carbon captured over the study period. We found that most trees in the forest didn't benefit from the fertilizers—instead, growth was strongly tied to the amount of rainfall in a given year.

This suggests that nitrogen pollution won't boost tree growth in these forests as long as droughts continue to intensify. To make the same prediction for other forest types (wetter or drier, younger or older, warmer or cooler) such studies will need to be repeated, adding to the library of knowledge developed through similar experiments over the decades. Yet researchers are in a race against time. Experiments like this are slow, painstaking, sometimes backbreaking work and humans are changing the face of the planet faster than the scientific community can respond.

Humans need healthy forests

Supporting natural ecosystems is an important tool in the arsenal of strategies we will need to combat climate change. But land ecosystems will never be able to absorb the quantity of carbon released by fossil fuel burning. Rather than be lulled into false complacency by tree planting schemes, we need to cut off emissions at their source and search for additional strategies to remove the carbon that has already accumulated in the atmosphere.

Does this mean that current campaigns to protect and expand forest are a poor idea? Emphatically not. The protection and expansion of natural habitat, particularly forests, is absolutely vital to ensure the health of our planet. Forests in temperate and tropical zones contain eight out of every ten species on land, yet they are under increasing threat. Nearly half of our planet's habitable land is devoted to agriculture, and forest clearing for cropland or pasture is continuing apace.

I mellemtiden the atmospheric mayhem caused by climate change is intensifying wildfires, worsening droughts and systematically heating the planet, posing an escalating threat to forests and the wildlife they support. What does that mean for our species? Again and again, researchers have demonstrated strong links between biodiversity and so-called "ecosystem services"—the multitude of benefits the natural world provides to humanity.

Carbon capture is just one ecosystem service in an incalculably long list. Biodiverse ecosystems provide a dizzying array of pharmaceutically active compounds that inspire the creation of new drugs. They provide food security in ways both direct (think of the millions of people whose main source of protein is wild fish) and indirect (for example, a large fraction of crops are pollinated by wild animals).

Natural ecosystems and the millions of species that inhabit them still inspire technological developments that revolutionize human society. For eksempel, take the polymerase chain reaction ("PCR") that allows crime labs to catch criminals and your local pharmacy to provide a COVID test. PCR is only possible because of a special protein synthesized by a humble bacteria that lives in hot springs.

As an ecologist, I worry that a simplistic perspective on the role of forests in climate mitigation will inadvertently lead to their decline. Many tree planting efforts focus on the number of saplings planted or their initial rate of growth—both of which are poor indicators of the forest's ultimate carbon storage capacity and even poorer metric of biodiversity. Vigtigere, viewing natural ecosystems as "climate solutions" gives the misleading impression that forests can function like an infinitely absorbent mop to clean up the ever increasing flood of human caused CO ₂ emissioner.

Heldigvis, many big organizations dedicated to forest expansion are incorporating ecosystem health and biodiversity into their metrics of success. A little over a year ago, I visited an enormous reforestation experiment on the Yucatán Peninsula in Mexico, operated by Plant-for-the-Planet—one of the world's largest tree planting organizations. After realizing the challenges inherent in large scale ecosystem restoration, Plant-for-the-Planet has initiated a series of experiments to understand how different interventions early in a forest's development might improve tree survival.

But that is not all. Led by Director of Science Leland Werden, researchers at the site will study how these same practices can jump-start the recovery of native biodiversity by providing the ideal environment for seeds to germinate and grow as the forest develops. These experiments will also help land managers decide when and where planting trees benefits the ecosystem and where forest regeneration can occur naturally.

Viewing forests as reservoirs for biodiversity, rather than simply storehouses of carbon, complicates decision making and may require shifts in policy. I am all too aware of these challenges. I have spent my entire adult life studying and thinking about the carbon cycle and I too sometimes can't see the forest for the trees. One morning several years ago, I was sitting on the rainforest floor in Costa Rica measuring CO ₂ emissions from the soil—a relatively time intensive and solitary process.

As I waited for the measurement to finish, I spotted a strawberry poison dart frog—a tiny, jewel-bright animal the size of my thumb—hopping up the trunk of a nearby tree. fascineret, I watched her progress towards a small pool of water held in the leaves of a spiky plant, in which a few tadpoles idly swam. Once the frog reached this miniature aquarium, the tiny tadpoles (her children, as it turned out) vibrated excitedly, while their mother deposited unfertilised eggs for them to eat. As I later learned, frogs of this species (Oophaga pumilio) take very diligent care of their offspring and the mother's long journey would be repeated every day until the tadpoles developed into frogs.

It occurred to me, as I packed up my equipment to return to the lab, that thousands of such small dramas were playing out around me in parallel. Forests are so much more than just carbon stores. They are the unknowably complex green webs that bind together the fates of millions of known species, with millions more still waiting to be discovered. To survive and thrive in a future of dramatic global change, we will have to respect that tangled web and our place in it.

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