Can planting trees really reduce CO2 in atmosphere?

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Discussion Overview

The discussion centers on the potential of planting trees to reduce atmospheric CO2 levels, exploring the roles of mycorrhizal fungi, carbon sequestration, and the carbon cycle. Participants examine the complexities of carbon storage in trees and soil, as well as the implications for climate change mitigation.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants note that most land plants rely on mycorrhizal fungi for nutrient uptake, raising questions about the effectiveness of trees in sequestering CO2 given that fungi also respire CO2.
  • There is a discussion about the "residence time" of carbon in trees and fungi, with some arguing that the carbon absorbed by trees is eventually released back into the atmosphere upon decomposition.
  • One participant suggests that at steady-state, forests like the Amazon may have a negligible effect on atmospheric CO2 levels, as equilibrium implies no net change.
  • Others point out that while trees sequester carbon during their growth, they also release it upon death, complicating the overall impact on atmospheric CO2.
  • Some participants highlight the importance of soil as a long-term carbon sink, suggesting that the carbon stored in soil may be more significant than that in living biomass.
  • There is mention of other organisms, such as algae and plankton, that also utilize CO2, indicating that a balanced ecosystem is crucial for effective carbon sequestration.
  • One participant emphasizes that the carbon locked in tree tissues, particularly in older trees, cannot be easily reversed by fungal respiration, suggesting a more complex interaction between photosynthesis and respiration.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of tree planting as a strategy for CO2 reduction, with no consensus reached on the overall impact of trees and fungi in the carbon cycle.

Contextual Notes

The discussion reveals limitations in understanding the dynamics of carbon sequestration, including the roles of various organisms, the complexity of the carbon cycle, and the effects of equilibrium states on atmospheric CO2 levels.

Steve Aplin
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From mycology books and articles, I read that most land plants (> 80 percent) on Earth live in symbiosis with mycorrhizal fungi. In this symbiosis, plants provide glucose to fungi and fungi provide water and nutrients to the plants. Without fungi, plants cannot harvest nutrients in sufficient quantity directly from the substrate.

Seeing as trees "eat" CO2 and fungi breathe it out, is the notion that humans could sequester our CO2 garbage by planting trees not somewhat suspect?
 
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Steve Aplin said:
is the notion that humans could sequester our CO2 garbage by planting trees not somewhat suspect?
In what way? You've omitted "residence time" of sequestered carbon in the trees and fungi from your consideration.
 
@Bystander: in the sense of warding off climate change due to anthropogenic CO2. Whatever amounts that trees do absorb, we are exceeding that amount with fossil fuel emissions.

Every tree will eventually turn into CO2 anyway, just due to saprotrophic fungi. But trees cannot grow without mycorrhizal fungi, which are respirators. I guess we'd have to know the rate at which the fungi that are in symbiosis with trees breathe out CO2, versus the rate at which trees eat CO2 and turn it into cellulose/hemicellulose.
 
Surely a tree sequesters carbon while growing and releases it when it dies. It seems to me that at steady-state, even the Amazon jungle has basically zero effect on atmospheric CO2.
 
insightful said:
Surely a tree sequesters carbon while growing and releases it when it dies. It seems to me that at steady-state, even the Amazon jungle has basically zero effect on atmospheric CO2.
Yes, but ANYTHING at equilibrium has no effect on the atmosphere.
 
Steve Aplin said:
From mycology books and articles, I read that most land plants (> 80 percent) on Earth live in symbiosis with mycorrhizal fungi. In this symbiosis, plants provide glucose to fungi and fungi provide water and nutrients to the plants. Without fungi, plants cannot harvest nutrients in sufficient quantity directly from the substrate.

Seeing as trees "eat" CO2 and fungi breathe it out, is the notion that humans could sequester our CO2 garbage by planting trees not somewhat suspect?
I would think that your model needs a slight improvement, but more on that later. As has been said, if in equilibrium, a forest will remove a certain amount of carbon from the atmosphere while living, and upon death return some of that carbon through decomposition. In, fact just looking at it only that way, you yourself ( and it can be said of any living thing ) are a short term carbon resevoir while living. You, for example, have certainly more carbon within you body now than when you were a small baby. But the carbon cycle is much more complicated than that.

But, getting back to trees. forests, and how plants and their influence become a major carbon sink.
the bigger picture is what is important here for climate models regarding the carnon cycle.

It really is not the biomass of the living, but the storage of the dead.
The forest soil is actually where the carbon sink is in the long term.
If the forest ceases to exist, the carbon within will have a good chance of being returned to the atmosphere as carbon dioxide.

For a preliminary, please read:
https://www.physicsforums.com/wiki/Soil_carbon https://www.physicsforums.com/wiki/Carbon_sink is an important function of terrestrial https://www.physicsforums.com/wiki/Ecosystem . https://www.physicsforums.com/wiki/Soil contains more https://www.physicsforums.com/wiki/Carbon than https://www.physicsforums.com/wiki/Plant and the https://www.physicsforums.com/wiki/Atmosphere_of_Earth combined.[1] Understanding what maintains the soil carbon pool is important to understand the current distribution of carbon on Earth, and how it will respond to environmental change. While much research has been done on how plants, free-living microbial https://www.physicsforums.com/wiki/Decomposer , and soil minerals affect this pool of carbon, it is recently coming to light that https://www.physicsforums.com/wiki/Mycorrhizal_fungi —symbiotic fungi that associate with roots of almost all living plants—may play an important role in maintaining this pool as well. Measurements of plant carbon allocation to mycorrhizal fungi have been estimated to be 5-20% of total plant carbon uptake,[2][3] and in some ecosystems the https://www.physicsforums.com/wiki/Biomass_(ecology) of mycorrhizal fungi can be comparable to the biomass of fine roots.[4] Recent research has shown that mycorrhizal fungi hold 50 to 70 percent of the total carbon stored in leaf litter and soil on forested islands in Sweden.[5] Turnover of mycorrhizal biomass into the soil carbon pool is thought to be rapid[6] and has been shown in some ecosystems to be the dominant pathway by which living carbon enters the soil carbon pool.[7]
https://en.wikipedia.org/wiki/Mycorrhizal_fungi_and_soil_carbon_storage

Although the figure is frequently being revised upwards with new discoveries, over 2,700 gigatonnes (Gt) of carbon is stored in soils worldwide, which is well above the combined total of https://www.physicsforums.com/wiki/Atmosphere (780 Gt) or https://www.physicsforums.com/wiki/Biomass (575 Gt), most of which is https://www.physicsforums.com/wiki/Wood . Carbon is taken out of the atmosphere by plant https://www.physicsforums.com/wiki/Photosynthesis ; about 60 Gt annually is incorporated into various types of https://www.physicsforums.com/wiki/Soil_organic_matter (SOM), including surface https://www.physicsforums.com/wiki/Plant_litter ; about 60 Gt annually is https://www.physicsforums.com/wiki/Carbon_respiration or oxidized from soil.[2]
Soil carbon is the last major pool of the carbon cycle. The carbon that is fixed by plants is transferred to the soil via dead plant matter, including dead https://www.physicsforums.com/wiki/Root , https://www.physicsforums.com/wiki/Leaf , and fruiting bodies. This dead organic matter creates a substrate which decomposes and respires back to the atmosphere as https://www.physicsforums.com/wiki/Carbon_dioxide or https://www.physicsforums.com/wiki/Methane , depending on the availability of https://www.physicsforums.com/wiki/Oxygen in the soil. Soil carbon is also https://www.physicsforums.com/wiki/Oxidation by https://www.physicsforums.com/wiki/Combustion and returned to the atmosphere as carbon dioxide.
https://en.wikipedia.org/wiki/Soil_carbon

As you can see from above, the soil carbon vastly surpasses the biomass carbon and atmospheric carbon.
 
Last edited by a moderator:
Plants are a kind of living carbon sink. Wood and xylem is primarily sugar polymers: lignin and cellulose, which are only reversible via a few organisms, and this does not happen in living tree tissues. PS: Sugars have a lot of carbon in them.

So if a tree is 100 years old it is usually large with lots of wood. If it accreted rings of growth for 100 years - the innermost wood ring is made from carbon dioxide that was converted into sugar 100 years ago. The carbon has been locked in there for that long. Bristlecone pine trees can live for literally thousands of years. See: https://en.wikipedia.org/wiki/Bristlecone_pine

That means in a very simplistic way that the idea of fungal respiration (giving off carbon dioxide) reversing all of photosynthesis right away in living trees or plants cannot possibly be correct. Plus fungi often undergo anaerobic respiration - turning sugars into complex organic molecules. Those exudates become bound into soils by leaching the molecules away from growing roots. So the carbon is "parked" out of immediate reach.

Also in bogs and swamps, trees that deadfall often end up preserved in sphagnum moss exudates for loooong periods. Check out bog oak pictures:
http://mtss-woodblog.blogspot.com/2013/02/celtic-irish-bog-oak.html
 
http://www.ancientkauri.co.nz/zealand_ancient_swamp_kauri_wood
 

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