From carbon sinks to carbon sources

 Tropical forests and their ability to store carbon during El Niño

Tropical forests draw down and store large quantities of CO₂ from the atmosphere. The Amazon rainforest in South America, for example, stores approximately 123 billion tons of carbon, more than is stored in any other terrestrial ecosystem in the world. But these forests are facing a critical challenge. New research shows that tropical forests are taking up less CO2 from the atmosphere, reducing their ability to act as ‘carbon sinks’ and instead becoming sources of carbon. What does this mean for the future of humanity? The Amazon could turn into a source of carbon instead of one of the biggest absorbers of the gas as soon as the next decade, as a result of the damage caused by loggers and farming interests and the impacts of the climate crisis. Research from 2023, found that tropical forests in South America are vulnerable to climate extremes. During an El Niño event, the warm phase of a natural fluctuation in Earth's climate system, South American tropical forests may fail to act as a carbon sink. Finding becomes even more alarming when we consider the increasing frequency and intensity of El Niño events. There have been twice as many "very strong" El Niños in the past 60 years as there were in the 60 years before that. And the US National Oceanic and Atmospheric Administration has recently confirmed that such an El Niño is currently underway.

Changes in tropical forest carbon sink strength during El Niño Southern Oscillation (ENSO) events can indicate future behavior under climate change. Previous studies revealed ˜6 Mg C ha-1 yr-1 lower net ecosystem production (NEP) during ENSO year 1998 compared with non-ENSO year 2000 in a Costa Rican tropical rainforest. Researchers explored environmental drivers of this change and examined the contributions of ecosystem respiration (RE) and gross primary production (GPP) to this carbon sink. For 1998-2000, RE using chamber-based respiration measurements, and estimated GPP in two ways: using (1) the canopy process model MAESTRA, and (2) combined eddy covariance and chamber respiration data. MAESTRA-estimated GPP did not statistically differ from GPP estimated using approach 2, but was ˜ 28% greater than published GPP estimates for the same site and years using eddy covariance data only. A 7% increase in RE (primarily increased soil respiration) and a 10% reduction in GPP contributed equally to the difference in NEP between ENSO year 1998 and non-ENSO year 2000. A warming and drying climate for tropical forests may yield a weakened carbon sink from both decreased GPP and increased RE. Understanding physiological acclimation will be critical for the large carbon stores in these ecosystems.

Tropical forests absorb CO₂ through the process of photosynthesis and convert it into biomass. However, the balance between photosynthesis and respiration is delicate and depends on two factors: temperature and water availability. In hotter and drier conditions, plants close the pores of their leaves to avoid water loss. But closing them effectively cuts off a plant's fuel supply because it is through these pores that they absorb CO₂. This starves plants of the carbon needed for photosynthesis and growth. During El Niño years, which are characterized by high-temperature anomalies, prolonged climate stress leads to reduced forest growth and increased tree mortality. The effects of this are felt for decades as carbon is released back into the atmosphere when the dead trees decompose. Intact tropical forests sequestered almost 50% of the global terrestrial carbon uptake over the 1990s and early 2000s, removing about 15% of CO2 emissions. These carbon sinks are becoming saturated in both Amazonian and African rainforests, with different patterns of change. These rainforests are now taking up a third less carbon than they did in the 1990s, owing to the impacts of higher temperatures (trees have only partially acclimated to recently rising temperatures), droughts and deforestation. This downward trend is likely to continue, as forests come under increasing threat from the climate crisis and exploitation. The typical tropical forest may become a carbon source by the 2060s. 

Tropical rainforests act as net carbon sinks when the amount of carbon gained through the establishment of new trees and tree growth is larger than the amount lost through tree mortality. In these circumstances, the quantity of carbon stored in the biomass increases over time. Findings revealed that during the 2015–2016 El Niño, when temperatures on land were at least a degree higher on average than usual conditions, some of South America's tropical forests effectively stopped absorbing carbon. This raises concerns about the possible impact of the current El Niño on the Amazon and global climate. In the research, team measured over half a million trees across six South American countries over a period of more than 30 years, using tape measures to track their growth. These trees belonged to more than 4,000 different species. This data was used to calculate precise estimates of the amount of carbon stored as a forest's aboveground biomass. The vulnerability of these forests to El Niño conditions was closely linked to their baseline climate. By assuming that rainforests are all hot, wet and biodiverse ecosystems, seasonal drought is a reality for many tropical forests. Conditions in regions at the edge of the Amazon rainforest, for example, tend to be particularly hot and dry. Findings revealed that drier forests at the edge of the Amazon, where trees regularly endure periods of limited water availability, were especially susceptible to extreme El Niño conditions. On average, a 0.5°C increase in temperature caused these forests to lose 0.5% of their aboveground carbon. 

Larger trees suffered the most. While tree mortality rates increased from 1.8% to 3%/year during the El Niño in South American tropical forests as a whole, mortality rates effectively doubled for medium (classified as over 20 cm) and large trees. The fact that larger trees with less dense wood died at much higher rates than small trees and those with high wood density points strongly to hydraulic failure, when intense atmospheric moisture demand snaps the tension in the tree's internal water column rather than slow carbon starvation. These results suggest that adaptation to seasonal drought may not be sufficient to protect tropical forests from extreme events. Climate extremes are possibly already pushing forests at the edges of the Amazon beyond their capacity to adapt, causing catastrophic carbon losses. The researchers of the study monitored tree establishment, growth and mortality in 244 undisturbed forest plots in Africa across 11 countries between 1968 and 2015. This data was then compared with similar measurements from 321 plots in the Amazonian region. The results showed that carbon uptake in the Amazonian region started to decline around 1990, whereas signs of a potential slowdown in Africa appeared in 2010. The uptake of carbon from the atmosphere by tropical forests peaked in the 1990s when about 46 billion tonnes were removed from the atmosphere, equivalent to about 17% of carbon emissions from human activities. By the last decade, that amount had sunk to about 25 billion tonnes, or 6% of global emissions, similar to a decade of fossil fuel emissions from the UK, Germany, France and Canada put together. 

According to the report, by 2030, the carbon sink in Africa will be 14% lower than in 2010-2015, while the Amazonian carbon sink will reach zero by 2035 (meaning that there will be no more net carbon uptake from the atmosphere). The researchers say that the reason for this difference between Amazonian and African tropical forests is because of increasing mean annual temperatures and droughts since 2000 that have reduced tree growth, offsetting the increase in carbon uptake. These reductions are smaller in Africa than in Amazonia. Scientists have warned that 2026 may again be the warmest year on record. Heightening the alarm further is the severity of the current El Niño. Never before has an El Niño begun when oceans are already so warm and air temperatures so high. On top of this is the fact that, over the past three decades, the edges of the Amazon have experienced some of the highest temperatures and most rapid warming the tropics have ever seen. The structural integrity of a forest is compromised when a major climate anomaly occurs before it has recovered from recent, multi-year stress. These compounding factors mean that we risk witnessing tree and carbon losses on scales not yet seen.

Further, the higher carbon gains persisted for longer in Africa than in Amazonia because the warming rate was slower, there were fewer droughts and air temperatures were generally lower (because African forests are located at higher elevations). Generally, trees in Amazonia grow faster and have shorter residence times than those in African forests. According to the report, the carbon sink strength of the world’s two most extensive tropical forests ‘have now saturated’. Reaching emissions reduction targets counts largely on the continuation of a large tropical carbon sink, which are disappearing at a rapid rate and could soon turn into carbon sources by the end of the decade. The protection of these tropical forests as well as faster greenhouse gas emissions will be needed to prevent catastrophic climate changes.  Tropical forests are invaluable assets in the fight against climate change. But South American tropical forests, a once-reliable carbon sink, are vulnerable to intensifying heat and drought. There is a risk that these essential ecological allies will stop acting as a carbon sink as extreme climate conditions become the norm. Preserving tropical forests is thus essential. Their ability to continue acting as carbon sinks hinges on efforts to protect them and a collective commitment to limit global temperature rise. The Amazon's future depends on this, and so does ours. UN climate talks will most likely see many countries coming forward with plans to reach net zero emissions by mid-century. This will be crucial in the fight against anthropogenic global warming.

Muhammad (Peace be upon him) Name

 

ALLAH Names

 

Trees and their absorbing carbon ability

 Trees growth stops but still keep absorbing carbon 

A tree that is photosynthesizing is not necessarily a tree that is growing. It sounds counterintuitive, the two processes seem like they should go hand in hand. But a new study of oak trees has found that growth stops in mid-summer, even as the trees continue to absorb carbon well into autumn. Oak trees may keep capturing carbon long after they stop growing, raising new questions about how much carbon forests can really store. Oak trees has revealed that photosynthesis and wood production are not as closely linked as scientists once believed. The finding could reshape forecasts of how much carbon forests will be able to store in a warmer future. New research have challenged a key assumption about how forests store carbon. Trees do not necessarily keep growing for as long as they keep photosynthesizing. Researchers found that oak trees continue absorbing CO2 well after their annual growth has ended, suggesting forests may store less carbon in wood than many climate models currently predict.

The finding has real consequences for how much carbon forests can be expected to store as the climate warms. The research was led by Mukund Palat Rao, an ecoclimatologist at Lamont-Doherty Earth Observatory, part of the Columbia Climate School. The team combined satellite imagery, treetop CO2 sensors, trunk-mounted growth monitors, tree ring records and temperature data stretching back to 1950. This helped them pull together daily readings of photosynthesis, carbon uptake and actual wood growth across 137 sites in the eastern US and California. The discovery challenges a long standing assumption that higher rates of photosynthesis naturally lead to greater tree growth. If trees continue taking in carbon without turning much of it into new wood, less carbon may remain locked away over the long term. Forests play a major role in slowing climate change because trees remove CO2 from the atmosphere and store much of it in their trunks, branches and roots. Scientists have generally expected that rising atmospheric CO2 levels would boost photosynthesis, leading to faster growth and increased long term carbon storage. The new findings suggest the relationship is more complicated. While trees may continue absorbing additional carbon, much of it does not necessarily become new wood. Instead, that carbon may be used to produce leaves, fuel short lived metabolic processes or serve other functions, reducing the amount of carbon stored in forests compared with previous expectations.

The results could have important implications for climate forecasting. "Right now, most models assume that if you have photosynthesis, you have growth. We find that's not the case," says lead author Mukund Palat Rao, an ecoclimatologist at Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School. "Just because there is more photosynthesis might not necessarily mean more tree growth in the future." The carbon absorbed after growth stops doesn’t disappear. Some of it is stored as starch and used to kick-start growth the following spring, while some goes into new leaves and roots. Moreover, carbon is also burned through cellular respiration to keep the tree alive through winter. Some is released into the soil, feeding microbial communities. What it mostly isn’t doing is being converted into woody biomass, the durable, long-lived form of carbon storage that makes forests valuable as carbon sinks. Carbon stored in wood can remain there for decades, centuries or even millennia depending on conditions. Carbon used to grow leaves or feed soil microbes cycles back into the atmosphere much faster. “Understanding how photosynthesis and growth are linked is very important from the perspective of understanding how forests will store carbon over long time scales,” Rao said. Exactly how much of the post-growth carbon ends up sequestered long-term versus released relatively quickly is still unknown. But the implication is clear: projections that assume a direct line from increased photosynthesis to increased carbon storage are probably too optimistic.

During photosynthesis, plants use sunlight to convert CO2 and water into sugars while releasing oxygen back into the atmosphere. The captured carbon remains inside the plant, but it is not all used to build wood. Because wood stores carbon for such long periods, understanding how much of the carbon captured through photosynthesis ultimately becomes woody biomass is critical for estimating how forests help slow climate change. Scientists had previously suspected that carbon uptake and tree growth were not always synchronized, but there had been too few detailed observations to fully understand why. To investigate, Rao and his colleagues combined several sources of data. They analyzed satellite imagery capable of detecting photosynthesis. They also used instruments that measured CO2 levels in tree canopies every hour and sensors attached to tree trunks that tracked tiny changes in trunk size throughout the day. (Trees tend to expand at night as roots take up water, then shrink slightly in daytime as they transpire water, with the long-term trajectory adding up to growth.) The team also incorporated tree ring records and temperature data. Together, these datasets provided daily measurements of photosynthesis, carbon uptake, and tree growth. The researchers found a clear separation between growth and photosynthesis.

Climate models have generally assumed that more photosynthesis means more tree growth, and more tree growth means more carbon locked away in wood for decades or centuries. Rising CO2 levels, act as a fertilizer, with trees absorbing more, growing faster, storing more carbon and partially offsetting what humans are pumping into the atmosphere. “Right now, most models assume that if you have photosynthesis, you have growth. We find that’s not the case,” Rao said. “Just because there is more photosynthesis might not necessarily mean more tree growth in the future.” At eastern US sites, oak trees typically grew from May through July but continued photosynthesizing into October. About 36% of their annual carbon assimilation occurred after growth had already stopped in late summer. California oaks showed a different seasonal schedule but the same overall pattern. Growth generally occurred between December and April, then slowed during mid summer and ended by August even though photosynthesis continued. Roughly 26% of the trees' yearly carbon uptake happened after growth had ceased. According to Rao, the explanation is straightforward. Tree growth depends on internal water pressure, and that pressure drops quickly during hot, dry conditions. "The moment you have dry and hot conditions, growth activity stops pretty instantly while photosynthesis seems to continue at a slightly decreased rate," says Rao.

Some of the carbon captured after growth ends is saved to help fuel growth when the next growing season begins. The remainder is used to produce new roots and leaves or is oxidized to keep living cells functioning through the winter. Researchers still do not know exactly how much of that carbon eventually becomes long term woody biomass versus how much returns to the atmosphere over shorter time periods. However, the findings suggest that projections of forests growing larger and storing substantially more carbon in a warmer, CO2 rich world may need to be reconsidered. The team also found that the disconnect between photosynthesis and growth became even stronger during years when local weather swung between unusually wet and unusually dry conditions. Because climate change is expected to increase this kind of variability in many regions, the pattern could become more common in the future. Wood growth requires water pressure inside the tree. When conditions turn hot and dry, that pressure drops and growth stops almost immediately. Photosynthesis is more resilient, it keeps running, at a slightly reduced rate, even when the tree can no longer convert the carbon it’s capturing into new wood. “The moment you have dry and hot conditions, growth activity stops pretty instantly while photosynthesis seems to continue at a slightly decreased rate,” Rao said. Rao and his colleagues are now investigating whether similar patterns occur in other tree species, forest ecosystems, and climates. He expects the degree of separation between photosynthesis and growth will vary across different forests, but says many questions remain unanswered. 

"I don't really have answers yet," he says. "There are many questions still left to address." The research also found that the gap between photosynthesis and growth was most pronounced in years when local climates swung between wet and dry extremes. Highly variable conditions, periods of abundance followed by drought, widened the decoupling. Those conditions are expected to become more common as climate change intensifies. The bigger question hanging over all of it is how much forests can actually be relied upon to absorb the carbon humans are producing. The answer, this research suggests, is probably less than the models have been assuming, and the gap between photosynthesis and growth is a significant reason why.

Muhammad (Peace be upon him) Name