There has been much talk about the impact of losing the tropical rainforests. Dire predictions have it that the disappearence of these forests would result in higher global temperatures, and reduced rainfall. But would this be the case? Suppose the Amazon rainforst were to revert to pampas-like grasslands-wouldn’t the same amount of plant matter be produced? I mean, in a mature forest, you have massive trees-but only the topes of the trees produce leaves. And growth of the wood mass 9which sequesters the most carbon) is slow. Would grasses (which grow quickly) sequested as much carbon? Would the area of leaf surface, be the same?
the Amazon rain forest survives because it is a forest. the forest provides shade which keeps the soil cooler and provides that material to make soil. when the forest is removed it becomes a poor growing area and only lasts a number of years.
also because it is rain forest there is a very high number of plant and animal species. lots of new discoveries that are useful come from such an environment.
The question is actually difficult to answer unless you pick a specific climate zone and specific biomasses to compare. Realistic dry ton yields for energy crop trees range from 2 to 7 dry tons per acre per year (some higher, many lower). Switchgrass might get you 11-12 dry tons per acre per year, and I’ve heard of a special genetically engineered sorghum that could yield 20 dry tons per acre per year. Another wrinkle, however, is also whether these energy crops need fertilizer, which has its own energy and GHG emissions penalty. Plus, grassy crops need to be harvested each year in order to get the same growth as the prior year, whereas a forest will, for some time anyhow, fix carbon in trunks and leaves without needing annual harvesting.
Forests are probably better: you get more depth and scale for trees. However, a great many areas really can’t support trees. Despite generations of Russians trying, they’ve never managed to block the steppe winds with forests. Likewise, the American Great Plains are really never going to be a good forest zone without a lot of changes.
Wow. This post is absolutely amazing for its utter inability to relate even tangentially to the question. Bravo! IN fact, even if we stretch the point, you stil fail because the reason the soil doesn’t do well is because it’s being overfarmed intensely. It can certainly support plantlife; it’s jsut not especially good for cultivation.
With regard to global climate change, the issue is not carbon production but carbon storage. Forests for the most part store far more carbon in their trunks than grasslands do. When forests are cut down and the wood is burned, that releases the carbon into the atmosphere and increases global levels of carbon dioxide.
While in some grasslands carbon is stored in undecayed plant material in the sod or in the soil, for the most part the plant material is either grazed or decays rapidly. Also, many grasslands are maintained by being regularly burned either by humans or due to lightning. So it doesn’t matter how productive the carbon fixation is, since the carbon is just recycled back to the atmosphere in a matter of months or years.
A nitpick: not simply carbon storage but yearly net carbon storage.
This is something that seems obvious if you look at the huge tree trunks, but what if you annualise it? Those tree trunks are rather spread out, aren’t they? If you take the area covered by a tree and total the amount stored by grass covering that area over the lifespan of the tree, how do they compare? And is it different if the grassland is pasture for animals? And how does a farmed forest compare?
Grass might be able to store more in a year’s growth, but it’s not going to stay stored for longer than a year. The first year, you’ll store carbon, but in all subsequent years, the new carbon you’re storing is offset by the old carbon getting eaten or rotting or otherwise being re-released. This’ll happen eventually with a forest, too, but in the mean time, it’ll spend decades or centuries accumulating.
Very true, which is why I specified net carbon storage.
Yes.
Firstly, as Colibri notes. The forest itself is a massive carbon pool. If you remove the forest all that stored carbon goes straight into the atmosphere. It doesn’t matter whether you replace the forest with grassland or tarmac, that carbon has been liberated.
Secondly the tree canopy and the litter acts to regulate local air temperature and moisture levels, thus enhancing rainfall. Grassland simply can’t achieve that, so your rainfall will decline in most places.
No, no and no… in that order.
The simplest way to understand this may simply be to ask why the landscape is dominated by trees, and not by grass. If grass were just as productive, why do trees dominate? And the answer is that grass isn’t as productive, because if it was then it would dominate. Fire plays a role in maintaining grass dominance, but as Colibri points out, if your system is fire mediated in favour of grass then it ain’t going to be sequestering a lot of carbon anyway. Absent fire, trees have several advantages over grasses that make them more productive
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They have deeper root systems. Even most rainforests suffer short rain free periods when the soil surface dries out. As surface moisture becomes limiting grasses cope by becoming dormant. In contrast the deep rooted trees remain productive year round. This gives trees a massive productive advantage.
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Trees are three dimensional. If you look at a forest canopy the first thing that leaps out at you is that it isn’t uniform. Even when the forest is growing on a flat plain the canopy exists as a series of mounds, highest directly above the trunk, and subsiding towards the periphery. In contrast a grassland is featureless, with a canopy that follows that landscape contours to within a few inches. What that means is that forest has a much higher surface area than grassland. Imagine a floor covered with toilet rolls crammed edge to edge. The surface area of the toilet rolls will be almost identical to the area of the floor. Now push out the centre of each roll so that it forms a pyramid. The floor space remains the same but the surface area of the roll is now at least an order of magnitude larger. And the same is true of a forest compared to a grassland. That extra surface area allows the forest to transpire much, much more than a grassland. It also means that in the tropics where there is excess noon light, the light strikes more canopy and it strikes most of the canopy obliquely, thus being both attenuated to a useable level and exposed to more photosynthetic areas. In high latitudes the low angle sun Is better captured by a tilted canopy. Only in a very small band of the subtropics is the flat canopy of a grassland optimal.
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Trees have larger root systems. Soil simply isn’t homogeneous. Nutrients exist in localised patches where animals have voided waste or old material has decayed and so forth. Grass has a very limited root system, rarely more than a metre from the point where the stem leaves the ground. As a result some grass plants have excess nutrient while plants a stones throw away are limited by a deficit. The huge root systems of trees even out those distribution problems, making a tree amore productive lifeform.
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Trees are big. Seems obvious, but being big is an advantage all by itself. Trees are less affected by diurnal temperature changes. Even in the tropics it gets cold at night during the winter, and it gets damn hot by day. The small mass, and especially the small mass of water, in a grass plant means that it heats up and cools down rapidly, and either the temperature of the plant fluctuates outside optimal ranges or, more often in the case of heat, the plant expends excessive energy trying to keep itself form cooking. In contrast a large tree can retain a relatively stable diurnal temperature just because of its bulk and remains more efficient and productive as a consequence.
There are numerous other reasons why tree are more productive than grasses, all things being equal. Grasses are niche group. A very productive niche group, but a niche group all the same. They largely dominate in areas with nutrient poor soils, erratic rainfall and frequent disturbance. If an area doesn’t have at leats two of those three than trees will usually have a major advantage.
You have to be a bit careful about comparing apples with apples. Most forest trees are at their most productive as submature trees, rather than as coppice shoots harvested annually. For example, the litter production alone in a forest will typically be 5 T/ha and may be as high as 15T/ha, which is considerably more than the 3 T/ha total yield you might expect form a coppice plantation.
From an energy perspective this productivity isn’t very useful, since it’s a pain to try to capture litterfall in any usable form. However it is all still the result of plant productivity.
A nitpick, but this is not necessarily correct. If you use the timber for buildings or whatever, then the carbon is still stored. Only if the timber is burned or left to rot on the surface does the carbon get released.
Forest, no, there just isn’t the water. Savanna, sure. Large areas of the Great Plains have reverted to savanna simply because of the supression of fire, and this is an ongoing process. There si nothing inherent about teh Great Plains that prevents tree growth. Remember that there are vast areas of Australia, for example, that are timbered despite having far lower rainfalls than the Great Plains. The Great Plains, or at least large swathes of it are anthropogenic grassland. Given a cessation of human management and a a suitable seed source and there is no obvious reason why it wouldn’t all revert to savanna with no changes at all.
To be nitpicky, it supported hundreds of thousands, probably millions, of Indian farmers for millenia, so it’s pretty good for cultivation. It just isn’t very good for cultivation practices developed in Asia and Europe.
Only kinda. Mature forests actually have exactly the same yearly net carbon storage as grasslands: absolutely none at all.
The real issue with forests is that they represent a pool. Cut them down, you release all that carbon. Contrary to popular myth forests aren’t “the lungs of the Earth” or any such nonsense. Forests don’t actually produce oxygen, nor do they absorb carbon.
Forests are more productive than grasslands in all but the most unusual circumstances. That remains true whether you annualise it or look at it on a daily or decadal basis. Forests simply photosynthesise more. For example, a good pasture might manage to give you 15 T/ha annually. A really poor forest will produce about 15 T/ha of *wood[i/] annually and another 5 T/ha of litter. A good forest will give you 50 T/ha of wood and 15 T/ha of litter.
Only a tiny, tiny proportion of the trees in a rainforest are suitable for milling, and only a minority of each individual plant is usable. So straight away the vast majority of t he wood will be mineralised. If you use the timber for buildings it will have a residence time of about 50 years on average. Then about 60% of it will be mineralised. The remainder goes to landfill, where about 20% of it is mineralised. So no matter how you look at it the vast majority of the carbon will be released.
Oddly the best option seems to be conversion to either paper or particle board. That allows many more species and much more of each tree to be utilised, and these products are much less likely to be burned as fuel wood or left to decay and almost all of it ends up in landfill.
Except when they lay down peat or coal or…
Very useful, thanks. That really settles it.
True, but those are rather exceptional circumstances.