Amazon trees may absorb far less carbon than previously thought: study

The capacity of the Amazon rainforest to absorb carbon dioxide from the atmosphere is predicted to increase with climate change, but a computer modelling study published in Nature Geoscience suggests that these increases may be far more modest than expected.

Depending on how key nutrient cycles are represented in climate models, the Amazon carbon sink may be half the size predicted based on current trends, the study reports — a finding that has serious implications for the escalating climate crisis.

One of the few apparent silver-linings of increasing atmospheric carbon dioxide (CO₂) levels has been a predicted simultaneous boost to plant growth — carbon dioxide is one of the key ingredients that fuels photosynthesis, so adding more if it to the air allows plants to photosynthesize more, and produce increased energy for growth — a process known as “CO₂ fertilization.”

Higher atmospheric carbon dioxide levels are also forecasted to help plants use water more efficiently by allowing them to keep the pores in their leaves, known as stomata, closed for longer — a potential help against climate change intensified drought.

These effects have caused some scientists to anticipate dramatic increases in crop yield, as well as an expansion in the capacity of trees to absorb carbon dioxide from the atmosphere and to mitigate human emissions. But these predictions rely on the assumption that forest growth rates, while enhanced by increased carbon, are not limited by other factors, for example, lack of soil nutrient availability.

The phosphorus limit

In fact, scientists have discovered a battle going on in Amazon soils. Phosphorus — an essential plant nutrient leached from rock — is in short supply there. But at the same time, this element is in high demand from plants and microbes, while soil minerals naturally bind with it and lock it away.

Amazon soils are rich in clays, which contain iron and aluminium oxides that chemically bind and store phosphorus in the soil , putting it out of reach of living organisms that desperately need the element.

“While higher CO₂ stimulates forest growth by making it easier for [trees] to take in CO₂ … that growth will be constrained, over both short and long time periods, by the availability of the nutrients needed for tree growth, mainly nitrogen and phosphorus,” explained Christopher Neill, an ecosystem ecologist at Woods Hole Research Center in Massachusetts, USA. This phosphorus-imposed growth constraint could have serious repercussions for current estimates of Amazon forest carbon sequestration.

Field testing Amazon forest carbon storage models

The United Nations Intergovernmental Panel on Climate Change (IPCC) estimates that natural processes, such as photosynthesis, currently absorb just under a third of all human GHG emissions. The Amazon rainforest is a major contributor to this ecosystem service and — although there have been hints that its capacity for absorbing carbon has been reduced by climate change and deforestation — a growing Amazon carbon sink is still a key component of models used to forecast warming and evaluate mitigation strategies.

To date, global photosynthesis rates have risen in line with increases in atmospheric CO₂ concentrations — meaning more growth and more carbon storage — but whether this global pattern will continue to hold true for the tropics and particularly the Amazon, one of the oldest ecosystems on Earth, is unclear.

“A lot of our climate and ecosystem models that are applied to [predict] what [will be] happening in the future have these assumptions built into them … but since [these carbon storage expectations have] never been tested, it’s really urgent that we have a large-scale experiment in the tropics and the Amazon” to ground truth our sequestration hypotheses, said Katrin Fleischer a post-doctoral researcher at the Technical University of Munich in Germany, who led the current modelling study.

Fleischer is part of an international research team preparing to do this field work. They’re setting up a large-scale experiment to measure the actual response of Amazon forest patches to elevated atmospheric CO₂ levels. The so-called AmazonFACE project will use free-air carbon dioxide enrichment (FACE) to artificially increase CO₂ levels around groups of Amazon trees over a period of 15 years, and monitor above- and below-ground processes to get a full picture of how individual trees and whole ecological communities might respond to rising carbon dioxide levels.

Prior to starting the experiment, the researchers collected baseline data on tree growth, plant nutrition, leaf development and root growth, as well as soil nutrient cycling of carbon, nitrogen and phosphorus at the AmazonFACE site near Manaus, in the Brazilian Amazon. They plugged this data into 14 different climate models that included different elements involved in the soil nutrient cycle, including nitrogen and phosphorus, and looked for varying levels of adaptability in plant responses to increased carbon dioxide for photosynthesis.

All the models supported the hypothesis that atmospheric concentration increases of up to 200 parts per million above current levels would have a positive effect on plant growth, however the increases were, on average, smallest in the six models that took account of phosphorus limitations in the calculations. These models predicted that the Amazon would absorb between 46 and 52 percent less carbon compared to models that did not consider phosphorus limitation.

“The [computer] study is slightly unusual [and innovative] in that it involved running a range of different ecosystem models in advance of a [field] experiment, to better inform which processes will be most important to measure [during] the [15-year AmazonFACE] experiment,” explained Lucas Cernusak, a plant physiologist at Australia’s James Cook University, who wasn’t part of the modeling study.

The nutrient-limited Amazon

The Amazon rainforest is more than 30 million years old and was never subjected to ice age glaciers, so most of its soils are deep and highly weathered. As a result, phosphorus is severely depleted in most areas. However, phosphorus is required by plants for protein synthesis, cell division, and energy metabolism, making it essential for growth. In addition, many plants produce phosphorus-rich seeds, fruit or pollen, making reproduction another phosphorus-demanding activity.

Even though Amazon soils are known to be nutrient poor, you wouldn’t know it to look at the thriving forest. That’s because plants there have evolved myriad compensatory strategies to win the phosphorus battle: growing larger more extensive root networks, releasing sugars that alter soil pH and free phosphorus from grasping clay, and producing enzymes that activate nutrient-cycling microbes or liberate the nutrient directly from the soil or leaf litter.

“It’s a very productive forest, but it’s been adapted to really [efficiently] recycle those few nutrients that it has,” said Fleischer.

However, these survival strategies have a cost: “Carbon is the currency that plants can use,” to succeed in multiple ways, Fleischer explains. In the Amazon’s case, plants must invest carbon to get sufficient phosphorus in return, but this trade off means that the plants “have less carbon to invest in [producing] wood,” she said.

So, higher levels of atmospheric CO₂ — a product of human-induced climate change — should theoretically offer plants sufficient carbon to extract phosphorus from the soil while also investing in more woody plant matter. But that’s only assuming there is enough phosphorus available in the soil to extract.

In the computer study, models that allowed some flexibility in how plants balance this trade-off, demonstrated a moderate boost to plant growth under elevated atmospheric CO₂ levels: The models generated a range of predictions for how much extra carbon would be stored in plant material under high-CO₂ conditions, from an increase of just 5 grams per square meter per year all the way up to 140 grams.

However, when compared to field records from the 2000s at the Manaus field site, the authors found that plant carbon sequestration increased by just 23 grams per square meter per year, suggesting that the more conservative models may be closest to the truth — at least for this particular patch of forest.

Climbing carbon, fixed phosphorus: into the unknown

“The key insight from this paper stems from the fact that the model results vary very widely,” said Neill, because each [computer] model approximates the cycling of phosphorous in different ways. This generates hypotheses that can be tested [on the ground] by the AmazonFACE experiment. “It’s a great use of models… to guide [field] experiments that lead to new insights.”

“The crucial next step will be to conduct the FACE experiment, which will be the first stand level CO₂ manipulation experiment in a hyper-diverse tropical forest, to see if the models are actually on track with reality,” agrees Cernusak. However, FACE experiments are expensive to set up, and AmazonFACE is currently seeking the remaining funding needed to begin the work.

“We know so little about this place, how it works, how it evolved over millions of years, which species are where… how do they [interact] as a community? It’s so complex,” said Fleischer. Meanwhile, the historic Amazon forest is being rapidly altered by escalating climate change, along with other human disturbances. “We’re losing it in front of us without really having understood it,” she concluded.

Citation:

Fleischer, K., Rammig, A., De Kauwe, M. G., Walker, A. P., Domingues, T. F., Fuchslueger, L., … & Haverd, V. (2019). Amazon forest response to CO₂ fertilization dependent on plant phosphorus acquisition. Nature Geoscience, 12(9), 736-741.

Banner image caption: Scientists don’t know whether growth enhancements seen in temperate climates will also occur in the Amazon rainforest, where plant growth may be severely limited by low phosphorus availability in soils. Image by Ben Sutherland found on Flickr CC by 2.0d.

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