Environmental sciences Trees are good carbon sinks
In the future, forests can generate CO2 save shorter than expected
Forests are one of the most important carbon sinks. Around a quarter of man-made carbon emissions are bound in them every year. So they play an outstanding role, the CO2-Concentration affecting the atmosphere. Scientists working with Kailiang Yu have now analyzed in a study from the journal "PNAS" (see Primary source) Long-term data from 1955 to 2018 on the quality of almost 700 forest areas in different climatic zones, forests of cold, temperate and tropical regions in Europe and in North and South America. They determined the time it takes for carbon to be absorbed by forests before it is released back into the atmosphere, the so-called carbon flow rate. There was an increased carbon loss and a shortened carbon turnover time due to increased tree mortality. The authors state that ongoing climate changes are likely to affect plant growth and forest death rates. Therefore, earth system models that couple the carbon cycle with climate fluctuations should, according to the authors, take into account tree mortality more closely.
- Prof. em. Dr. Christian Körner, Emeritus at the Institute for Botany, Department of Environmental Sciences, University of Basel, Switzerland
- Dr. Cornelius Senf, Research Associate at the Institute for Silviculture, University of Natural Resources and Life Sciences, Vienna, Austria
- Prof. em. Dr. Martin Heimann, former director of the Department of Biogeochemical Systems, Max Planck Institute for Biogeochemistry, Jena and Research Director, Division of Atmospheric Sciences, Department of Physics, University of Helsinki, Finland
- Prof. Dr. Harald Bugmann, Professor at the Institute for Terrestrial Ecosystems, Department of Environmental Systems Science, ETH Zurich, Switzerland
Prof. em. Dr. Christian Körner
Emeritus at the Institute for Botany, Department of Environmental Sciences, University of Basel, Switzerland
“Most of the carbon bound in the earth's vegetation is in tree trunks. It is often expected that trees will be affected by the CO2-Enrichment of the atmosphere grow faster and therefore the atmosphere of CO2 relieve. Apart from the fact that to this day not a single tree growing in a near-natural forest with experimentally increased CO2-Offer actually grew faster, the basic assumption is fundamentally wrong that one can say something about the carbon stock in the forest from a growth rate . The authors of this study have precisely identified the sore point of the previous, popular argumentation and modeling. For the formation of reserves in forests, it is not the rate of carbon turnover that is decisive, but rather its mean time in the system. Therefore, one should never infer a change in the carbon stocks in forests from the growth rate of trees. It's like in the economy, where nobody would think of equating turnover with capital. "
“The authors developed a solid data set from forest inventories, from which they can peel out the connection between growth and stock on a large spatial scale. It turns out that without taking into account the mortality of trees at the same time, the actual stock cannot be calculated and predicted in models. The mean life expectancy of trees cannot be determined experimentally because of the long periods of time, which is why even models for this only contain vague estimates, if at all. The authors of this article show that accelerated growth (for whatever reason - they guess increased CO2which I rather doubt as an explanation), as expected, results in a shortening of the service life. The trees simply go through their life cycle faster. Only recently this was also shown in a large data set for boreal conifers . Since almost half of the biosphere biomass carbon is bound in tropical forests, any prognosis for this system also depends on how long trees live. The fact that the development of lianas has been proven to be faster today will statistically shorten this lifespan and thus rather lower the carbon stock . "
“The insight that faster growth tends to shorten the length of time carbon remains in the forest system is taking off incredibly slowly and with difficulty. This PNAS contribution is therefore most welcome in terms of a realistic assessment of the potential ability of forests to store more carbon. Perhaps one day many well-meaning biologists and modelers of the carbon cycle will understand that turnover (which includes growth) is not the same as capital (stock), even if it is about carbon and not about money. "
Dr. Cornelius mustard
Scientific project assistant at the Institute for Silviculture, University of Natural Resources and Life Sciences, Vienna, Austria
“As far as I can judge the methods, I don't see any methodological fallacies. The study examines an unprecedented network of data points of this size, which represents a significant step towards the global understanding of tree mortality. "
"Current global vegetation models do not take into account that increasing climate change leads to higher tree mortality, which means that the CO absorbed by the trees2 is saved shorter than expected. Many vegetation models therefore predict that global forests are an important global CO2-Sink because it is expected that trees will grow better and faster in the future. The potential of the sink can, however, be weakened or even reversed by higher mortality. This has not yet been taken into account in the models. "
“The underlying processes apply globally, so that a transfer to regions that were not measured is entirely permissible. The fact that more and more trees are dying from droughts and heat waves is a global phenomenon and, as such, is very well established. "
When asked what the specific effects on climate change and CO2-Budget could be:
“The potential of global forests as a CO2-Sink could be overestimated, which in turn means that less CO2 removed from the atmosphere by trees is believed to be. Politicians should therefore not rely on reforestation as the only or most important climate protection measure, but should actively reduce CO emissions2 to reduce."
Prof. em. Dr. Martin Heimann
Former Director of the Department of Biogeochemical Systems, Max Planck Institute for Biogeochemistry, Jena and Research Director, Division of Atmospheric Sciences, Department of Physics, University of Helsinki, Finland
“In the terrestrial biosphere, the residence time of the carbon is from the CO2- Absorption during photosynthesis up to respiration by microorganisms is an important parameter, as it increases the storage capacity of atmospheric excess CO2 co-determined. "
“In the study, the authors convincingly determined from long-term forest inventories the residence time in the living biomass, i.e. in trees, at a number of locations around the world and examined its dependencies on the local climate. One of the main results is that the carbon retention time is shortened as the vegetation grows, for example due to CO2-Fertilization due to the increasing atmospheric CO2-Concentration, and thus the carbon storage capacity of the forests is impaired. "
“A comparison with today's“ state-of-the-art ”earth system models shows that although they can partially depict the effect, they cannot correctly reflect the observed dependency of the dwell time on different climates. This is an important finding that must be taken into account when developing the next generation of models. "
“A general reduction in the residence time of the carbon would have a negative impact on future CO2- Impact storage capacity of the land biosphere. However, the study only deals with the living biomass in trees. The dwell time in other ecosystems and in the dead biomass (leaves, dead wood, soil) cannot be recorded with the selected method of the study. "
“In addition to the carbon retention time, there are a number of other processes, some of which are poorly understood, that reduce global CO2-Control the storage capacity of the land biosphere today and in the future under changing climates. In this context, the direct influence of humans through land use and land management must also be taken into account. The study sites for the study are all near-natural forests. However, effects from different forest management and changes in biodiversity are also very important for CO2-Image of the land biosphere. "
Prof. Dr. Harald Bugmann
Professor at the Institute for Terrestrial Ecosystems, Department of Environmental Systems Science, ETH Zurich, Switzerland
“In principle, the study is methodologically sound, although it should be noted that the data set used contains a very large bias. In addition, about 67 percent of the data come from the period after 1990, i.e. they only cover the last 25 years or so, which makes the analysis of temporal trends very difficult. Because of the supplement, it is not possible to determine exactly how the statistical analyzes were carried out and whether they are really reliable. "
“It is known that carbon storage in terrestrial systems is not constant: deforestation releases carbon, and the 'residual carbon sink' according to the IPCC implies that many terrestrial ecosystems absorb carbon, which is also clearly evident in many inventories. The 'previous models' take this into account. In my opinion, this statement in the paper is incorrect. What is correct, however, is that so far only a few studies have attempted to analyze and quantify changes in the “conversion rate” of carbon over a large area. The study by Yu et al. is very valuable from this perspective. "
“The areas examined are almost exclusively on the American continent: 100 percent of the tropics, 86.3 percent of temperate zones and 97 percent of cold areas. Africa and Asia are completely absent from the analysis; only 28 of the 695 areas examined (4 percent) come from Europe. It is a valuable case study. Global statements cannot be derived from the data used. "
“The assumption that the CO2Content leads to an increased turnover rate (higher mortality), was analyzed and quantified for the first time almost ten years ago . Yu et al. find that CO2 contributes most as an 'explanatory' variable, but at the same time they warn against confusing correlation with causality. According to our earlier analyzes, it should be the case that trees that show a greatly increased growth in youth (for example due to the CO2Fertilization), earlier mortality will be experienced in later life. However, this is different from that of Yu et al. found increased mortality in 'old-growth' stands, whose trees are typically already relatively old and not yet have significant CO in their youth2- Have experienced fertilization. "
“Overall: It is likely that climate change and CO2-Fertilization will lead to an increased rate of carbon turnover in terrestrial ecosystems, which will reduce the amount of carbon stored in terrestrial ecosystems. This can become an important feedback mechanism in the climate system, but it has so far only received insufficient attention in Earth system models. The study by Yu et al. is a piece of the puzzle on the way to a better understanding and better prognosis of these effects. "
Information on possible conflicts of interest
Prof. Dr. Harald Bugmann:"I don't see any conflicts of interest."
Prof. em. Dr. Martin Heimann: "No."
All other: No information received.
Yu K et al. (2019): Pervasive decreases in living vegetation carbon turnover time across forest climate zones. PNAS. DOI: 10.1073 / pnas.1821387116.
References cited by the experts
 Grains C (2017): A matter of tree longevity. Science Vol. 355, Issue 6321, pp. 130-131. DOI: 10.1126 / science.aal2449.
 Büntgen U et al. (2019): Limited capacity of tree growth to mitigate the global greenhouse effect under predicted warming. Nature Communications 10, 2171. DOI: 10.1038 / s41467-019-10174-4.
 Körner C (2009): Responses of Humid Tropical Trees to Rising CO2. Annual Review of Ecology, Evolution, and Systematics, Vol. 40: 61-79. DOI: 10.1146 / annurev.ecolsys.110308.120217.
 Bugmann H et al. (2011): Will the CO2 fertilization effect in forests be offset by reduced tree longevity? Oecologia 165: 533-544. DOI 10.1007 / s00442-010-1837-4.
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