Do tree or grass systems sequester more carbon?

This seems to be a controversial point. Which sequesters more carbon, forests or grasslands? In unmanaged systems? In managed systems? Which sequesters carbon more quickly? Which eventually holds the most? Does it differ in the tropics as opposed to temperate climates?

In my area of of the world (Colorado high plains/ high desert) would more carbon be sequestered by planting drought tolerant trees into grasslands? Or by leaving the grasslands as they are? Or by managing them in some other way?

In terms of carbon per acre, grass will definitely sequester more carbon (denser root systems and vegetative mass). But I think your question is more complicated by climate. In the boreal forests, grass grows weakly or not at all, while tree systems thrive. And so in the boreal forests, trees sequester more carbon than grass does. I'd say the answer to your question is that the plant that grows the best will sequester the most carbon.

In your climate, I would suspect the best management plan would be meadow preservation — removing dewatering species like Lodgepole that invade meadow edges and reduce the grasslands. Water is going to be your limiting factor for vegetative growth, and grasslands will only really thrive in wet areas in the West. Native Americans used to routinely burn meadows to fill in this function, which is another option (Rx'd burns). But then you get back into the carbon question with burns…

In terms of carbon per acre, grass will definitely sequester more carbon (denser root systems and vegetative mass).

Kyle, would you say that grassland sequesters more carbon per year, or overall? (In other words, could it be possible that grassland starts off fast, but slows down sooner then forest does?)

You're gonna hate me, but again I'm gonna say it depends… a natural, lightly grazed grassland won't have many opportunities to regrow itself multiple times over a season. But a well-managed, intensively grazed pasture can regrow that carbon over and over again, cycled through animals and their manure, and quickly build carbon-sequestering topsoil. Similarly, an intensively managed forest can regrow its trees over and over again, sequestering the carbon in building lumber, hugel beds, or rotting on the forest floor. I don't have enough knowledge to help explain all of the variables, but it's incredibly contextual (climate, soil, etc).

Here is an example study looking into the conversion of pasture to forests.. You can see there's a lot of variables involved just from the abstract:

Variation in carbon concentrations over sites in volcanic soils could be well predicted for both pastures (R2 = 0.96) and forests (R2 = 0.93) on the basis of soil mineralogy, while for sedimentary soils, clearly less variation could be explained (R2 = 0.14 for pastures and 0.39 for forests). The dominant factor explaining changes in carbon stocks following pasture to forest conversion was pasture age. Forests, paired with pastures less than 10 years old, had on average 9.3 Mg ha−1 less soil carbon than the pastures, while forests paired with pastures between 20 and 30 years old had on average 18.8 Mg ha−1 more soil carbon and forest paired with pastures older than 30 years had on average 15.8 Mg ha−1 more carbon than the pastures.

I will try to find the link, but there is recent evidence that, probably largely because of symbiotic relationships with fungal populations, old growth forests sequester much more carbon than earlier thought, and that it might not be such a good idea to chop all the old growth just to put in something younger.

There are species suited to pasture that grow quickly and accumulate biomass, such as hemp, alfalfa, and buckwheat, that are harvested at or above ground level, and whose root systems stay in the ground. Also, I would imagine ruminants would leave the roots in the ground, allowing for even faster cycling of nutrients and sequestration of carbon.

-CK

How do you define sequestering carbon?

On a yearly/daily basis a forest and a grassland absorb at the same amount of carbon from the air, the forest probably absorb a tiny bit more because it has more levels of leave, and the forest floor is now dark, but overall it is pretty close.

Next is how much carbon is locked up in the above ground living plant? A grassland might have 10 years or 10ft of carbon trapped in the living plant. But after a season or maybe 10, it then returns to the atmosphere. A tree in a forest can trap that carbon in living tissue for a couple hundred years, maybe 2,000yrs, that to me is sequestering carbon, vs yearly rotting.

Next is the below ground carbon trapped in the living matrix, in a forest tree, some of those taproot and support roots are centuries old, obviously like grass, they have hair root and leaves that die yearly.

Carbon trapped in the living non-plant matrix. I like to think of fungi as really extensions of plant root so we have to count their mass too, additional their cells are more similar to plants with alot of carbon, vs animal/microbes which are more nitrogen based vs carbon based. So in that sense forest fungi store more carbon than grassland cows and earthworms, I would even go on to say that trees and fungi tissues are much better at storing carbon when compared to humans/cow tissues.

Carbon trapped as dead plant matter, I would say that forest trees tissues breakdown a bit slower than grassland plant matter. Thus forest effectively trap carbon for longer, because the carbon is in a form (lignin) that is much harder to breakdown than say grassland starch. When a human/animal eat carbs/carbon, we use alot of it for movement and less to build mass, but a fungi, stores more of it as cell wall mass and very little for movement similar to plants.

Native Grassland tend to be in places with limited water, so while the leaves might be more efficient maybe, they are stunted and turned off because water limits the growth rate of the grassland, when compared to a well water forest.

Now I will say that as an animal a grassland is very good for me, it is shorter and the carbon is in less complex so it support more animals/bacteria and less fungi. I can hunt herbivores, carnivores, birds, because we all eat the much simpler grassland starch vs the hard to digest lignin. The forest however caters to less bacteria/animals and more so to fungi, so I prefer grasslands/mix land.

Now when it comes to which one makes more biochar/charcoal which is what I call real carbon sequestering due to the fact that said form of carbon is locked up for 100,000's of years. I am not too sure, I have a feeling that forest produce more and send it downstream to be deposited on the meandering river flood plains (grassland) and to the bottom of the sea.

Peat field also store carbon for quite awhile in boggy situations, i also see that as more of a high water forest area vs a more limited water grassland.  

Something to consider when burning grasses, very little ash is produced. It mostly leaves behind charcoal. To test this, try burning a handfull of it. It does not burn completely. You can observe this for yourself. Burning grasslands therefore takes carbon out of the carbon cycle. Charcoal takes an extremely long time to break down and form CO2. But CO2 is formed when microbes and fungi break down plant matter. The more often you burn your grass, the more carbon you will sequester.

I don't think either of them "sequesters" CO2, other than the amount that is temporarily embodied in the plants.

What I mean is, if you have 100 acres of mature grassland and 100 acres of mature forest, they both release CO2 into the atmosphere at roughly the same rate as they absorb it.  Granted, things like forest fires, etc. might temporarily move a tiny amount of the carbon into the soil, but it's not significant and it's generally not permanent (on a geological scale).

A forest, especially something like a rain-forest, has more Carbon tied up in the plants at any given time than a grasslands.

The above, of course, assumes that neither is degraded/damaged.  Mankind is currently destroying/damaging/degrading rainforests to the point that it appears they are emitting FAR more CO2 than they are absorbing.

https://www.theguardian.com/environment/2017/sep/28/alarm-as-study-reveals-worlds-tropical-forests-are-huge-carbon-emission-source

Of course, conflicting studies claim that the rainforests are currently absorbing significant amounts of CO2.
https://phys.org/news/2017-02-carbon-uptake-amazon-forests-region.html

This is a good question.

I think that they both capture about the same amount of solar energy and Carbon from the air. So in a sense they are the same.
They do differ in that grass gives up their captured carbon to microbes, and animals (more productive human food system, but we aren't asking about that).
But trees are stingy and keep on bio-accumulating it for centuries as the trees get taller, they also mostly feed mushroom vs animals/microbes.
And to me mushroom have better feed conversion ratio than animal/microbes and pound for pound mushroom have more carbon than animal, so better carbon storage

Personally I think that more charcoal production (forest fire, grassland forest, man-made biochar, etc) is the key to making the carbon non bio degradable, swamps could also create peat/coal too.

So we really have 3 parts
1. Long term storage as 'charcoal/biochar/peat/etc" (grassland wins)
2. short term storage as 'living matter/roots/mushroom/dropped leaves/trunks" (trees win)
3. carbon absorption from the air by the amount/type of plants. (equal)

If we focus only on how to get the most/best carbon capturing plants system we will have to look at limiting factors.
Sunlight, Temperature, Water, Mineral Availability, Pest/Animal Pressure, Fertility.

Sunlight/Temp
The closer one is to the equator the better the quality of sunlight/temp is.
Temperature is also moderated by proximity to bodies of surface water and transpiration of plants and elevation.

Water
Earthworks to slow down the flow of water so that it can be available for longer period of time is good.
Windbreaks also help, so does leaf litter, humus, bichar and other plant systems
Plants mainly demand water so that they can extract the bio available minerals in it, so the more bio available minerals there is the less water requirements.
Deep rooted plants,

Mineral Availability
Biochar, forest fire charcoal, humus, dead roots, etc will increase the amount of available mineral trapped in the 'activated charcoal'
Depth of plant roots too
Nearby Nitrogen fixing and phosphorus mining legumes/etc
Trading with more efficient mycelium 'roots'  
More water (earthworks\etc)