“Serious natural carbon sequestration, at whatever scale, requires regenerative landscape management practices such as putting in a biodiverse palette of native trees, flowers and grasses and stopping the use of pesticides and synthetic fertilizer.”
Following on from yesterday’s posting on trees…
… a correspondent looks a little further.
Starting with a piece from earlier in the year from BBC Futures:
Planting trees doesn’t always help with climate change
By some estimates, trees can be an enormous carbon sink. A study published in July 2019, led by Thomas Crowther of ETH-Zurich in Switzerland, estimated the world has room for an extra 0.9 billion hectares of forest. Once those trees had matured, they could store 752 billion tonnes of CO2. Planting trees, the team wrote, is “one of the most effective carbon drawdown solutions to date”.
This finding has had immediate, fierce pushback from other climate scientists. In October 2019, the journal Science published four highly critical comments. These argued that the researchers had overestimated the carbon trees could store – by a factor of five. They also highlighted multiple mistakes. For instance, much of the land Crowther described as “available” for tree planting already has plants growing on it, all of them storing carbon, many of which would have to be removed, according to Sonia Seneviratne of ETH-Zurich and her colleagues.
The criticism hit home and, in May 2020, Crowther’s team published an extensive correction, in which they admitted that some of their headline claims were “incorrect” and that the data contained “errors”.
“there are also deeper problems, because trees have more than one way to affect the climate.”
“The first issue is that trees are dark, at least compared to other things that might blanket the land, such as grass or snow. As a result, planting more trees typically makes the land darker. Since dark surfaces absorb more heat, a dark tree-covered surface will trap more of the Sun’s heat – and warm the local climate.
“As a result, there is a delicate balance between trees’ ability to take in CO2, reducing warming, and their tendency to trap additional heat and thus create warming. This means planting trees only helps stop climate change in certain places.
“Specifically, according to a 2007 study that has been repeatedly confirmed, the best place to plant new trees is the tropics, where trees grow fastest and thus trap the most CO2. In contrast, planting trees in snowy regions near the poles is likely to cause a net warming, while planting them in temperate climates – like that of the UK, much of Europe and parts of the US – may have no net effect on climate.”
Tall and old or dense and young: Which kind of forest is better for the climate?
While young forests tend to absorb more carbon overall because trees can be crowded together when they’re small, a tree’s carbon absorption rate accelerates as it ages. This means that forests comprised of tall, old trees – like the temperate rainforests of North America’s Pacific coast – are some of the planet’s biggest carbon storehouses.
Although every backyard vegetable garden absorbs some amount of carbon, a rainforest takes in exponentially more. For this reason, rainforests and other large terrestrial ecosystems made up of dense vegetation are known as “carbon sinks.”
The expansive Amazon tropical rainforest of South America is one of the world’s largest carbon sinks. But on a per-acre basis, the Amazon is not nearly as efficient at absorbing carbon as the coastal temperate rainforest. The Douglas fir forests of Oregon and the hemlock and cedar forests of Alaska store about twice as much carbon per acre as the Amazon. The giant redwoods of Northern California, which store seven times as much, are regarded as the most carbon dense forests in the world.
The problem is most mature trees in the rainforest have been cut down and young ones are not allowed to mature. Outside conservation areas like national parks and wilderness, ancient groves are converted to industrial tree farms by the timber industry. After cutting down every old growth tree it can get its hands on, the industry typically plants a young sapling in its place. The saplings grow for about 40 years on average until the next harvest. Then the cycle repeats again and again.
This business model might be good for timber industry profits, but what does it do to the climate?
Then there is the question of whether shrubs do better than trees:
“While nearly everyone has a pretty good idea of how to cut emissions, fewer are aware of how they themselves could implement natural carbon solutions beyond planting a tree or two. But simply plopping some trees in a lawn or along a parkway is not enough. As I’ve written previously, serious natural carbon sequestration, at whatever scale, requires regenerative landscape management practices such as putting in a biodiverse palette of native trees, flowers and grasses and stopping the use of pesticides and synthetic fertilizer.
“Shrubs can be crucial to this kind of planting, especially in terms of the other ecological benefits they offer. Wherever there is a lawn, a tree and possibly a small garden, or even a tiny strip along the foundations of a building, there should be a native shrub or two, or possibly more. Large properties and farms have nearly unlimited possibilities in the form of hedgerows, shelterbelts or even reconstituted shrub prairies.”
Native Shrubs and Why They’re Essential for Carbon Sequestration
Shrubs are a necessary part of landscaping for carbon sequestration
From a landscaping perspective, shrubs are sort of like the middle children in a very large family: necessarily adaptable, but little thought of or noticed. This is true even scientifically. “Natural Carbon Solutions” explicitly omits shrublands from the calculations, and a 2016 review of scientific literature in “Why Be a Shrub” states that the least studied landscape types are shrublands, while the least studied plants are shrubs. Yet shrubs flourish virtually everywhere and shrublands are increasing across the globe, possibly due in part to climate change. In the American West the new severity of wildfires makes it difficult for forests to regenerate. The replacement is shrubland, or, to use an old term, “barrens.” Is the lack of notice and study because shrubs are so common and ubiquitous, but lack the majesty of trees and the show-offy beauty of flowering annuals and perennials?
The default for parks and private property alike is often faux open woodland or savanna, with widely spaced trees and plenty of grass—and few shrubs.
Because they are missing shrubby layers, what seem to humans like well structured environments are, in fact, missing the complex structure that creates the capacity for higher order ecological relationships—that is, the relationships among three or more species (including plants, animals, fungi and bacteria) that tie an ecosystem together and enable carbon sequestration.
Crucially for wild landscapes of all types, their adaptability means they can re-sprout easily after fires or other disturbances, grow to mature size much faster than trees, and have self-spreading habits such as suckering or rooting where branches touch the ground. It’s often hard to kill a shrub without digging out the roots. Their many leaves and stems make them efficient photosynthesis factories, pulling carbon out of the air, and their roots hook up with the underground biome as contributing partners. They share nutrients and information with other plants, engage in the carbon-sugars-for-nutrients trade with fungi, shelter microbes in return for nitrogen and other nutrients, and thus contribute to a healthy, biodiverse, carbon-sequestering soil system. Some kinds of shrubs even act as nurse species, so that young trees grow better in their company.
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