Mitigating Global Warming by Storing Carbon in Biodiverse Forests
Bernhard Schmid
Remote Sensing Laboratories, Department of Geography, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
“Our work demonstrates that planting multispecies forests instead of monocultures could strongly reduce global warming”
Our planet, Earth, differs from other planets by the presence of a thin layer of life on its surface, called the biosphere. This biosphere maintains biogeochemical cycles, soil formation, and climatic conditions. It constantly removes carbon dioxide from the atmosphere so that the concentration of this “greenhouse gas” is now less than 0.05% instead of more than 3% around 2.2 billion years ago (Rye et al. 1995). Responsible for the capture of carbon are so-called autotroph organisms — from single-celled blue-green algae up to huge forest trees — that use the carbon to produce organic compounds. In the process, these organisms also produce oxygen. Thus, at the time mentioned above, oxygen concentration in the atmosphere had already risen to similarly high levels as we have today.
Over the past century, human activities, especially the burning of fossil fuels, have led to an increase of the carbon dioxide concentration in the atmosphere and a corresponding increase in global temperatures. At the same time, we are slowly destroying the biosphere by reducing the number of species of other living beings through extinctions. However, in the 1980s, when I started my scientific career, it was uncertain whether and how a reduction in biodiversity might affect the capacity of ecosystems, such as grasslands and forests, to capture and store carbon, thereby affecting global temperatures. It was even believed that monocultures of highly productive plant species in agriculture and forestry could convert more carbon dioxide to biomass than did species mixtures, which included less productive species, because the latter were thought to compete for resources needed by the more productive species.
Following the Rio Earth Summit in 1992, where international conventions about climate and biodiversity were drafted, researchers began conducting experiments to explore potential consequences of biodiversity loss for ecosystem functioning. To their surprise, they found out that it was not the monocultures that could capture and store the largest amounts of carbon, but rather the mixtures of several species. Consequently, the widely held view that monocultures are good for production and that conservation of biodiversity can only be traded off for reduced production has been reversed. Further experiments that aimed to challenge the new results, consistently found that biodiversity had positive rather than negative effects on productivity. At the same time, biodiversity had positive effects on most other ecosystem functions and services in these experiments, including carbon storage, maintenance of healthy soils, buffering of extreme climatic events, and maintenance of biodiversity itself.
I had the chance to participate in some of the largest biodiversity–ecosystem functioning experiments in the world, the EU BIODEPTH project (Hector et al., 1999), the Jena Experiment (Weisser et al., 2017), and the BEF-China experiment (Huang et al., 2018), which showed that irrespective of whether they were grassland or forest ecosystems, reductions in species diversity resulted in equivalent reductions in ecosystem biomass production, and the effect was stronger if a system had already low species richness at the start. For example, in the BEF-China experiment, monocultures captured less than 50% of the amount of carbon captured by mixtures of more than 15 tree species, whereby both monocultures and mixtures were assembled from the same species pool and planted at the same time with the same total starting biomass, same total density, and on same-sized plots. Critics adhering to the old view of a trade-off between production and biodiversity conservation pointed out that in nature and managed forests only the best monocultures or low-diversity mixtures will occur and thus the experimental results may not apply. However, in natural forests next to the BEF-China experiment, stands composed of around 15 tree species also stored about twice as much carbon above and below ground than did stands of only five tree species (Liu et al., 2018). The main explanation for these biodiversity effects lies in the division of labor among different species, allowing a diverse mixture of species to better assimilate environmental resources and build up biomass. Some of this biomass likely escapes full decomposition back to CO2 and thus carbon remains locked up in the ecosystem for prolonged periods. So, what about managed forests? Would it indeed be better to plant species mixtures instead of the best monocultures, if we want to remove as much carbon dioxide as possible from the atmosphere and thus combat global warming? The answer is yes; and that is what we presented in the article that awarded us the national Frontiers Planet Prize for Switzerland (Feng et al., 2022).
Global forests can store huge amounts of carbon, thus reducing carbon dioxide concentrations in the atmosphere and counteracting global warming. However, most afforestation programs use monoculture plantations (e.g. > 85% of the area afforested in China over the past 60 years, which is more than 10% of the country’s surface; Hua et al, 2016, Yu et al. 2022). In our work, we used a “quasi-experimental” protocol matching monocultures and mixtures by species, stand density and stand age to assemble a worldwide dataset of forest stands. Because these were managed, we could assume that they were composed of species selected by foresters for good performance regarding biomass production and thus carbon storage. Already mixtures of two species had more than 20% higher biomass than monocultures. These results show that planting diverse forests in afforestation programs can have clear benefits in terms of climate mitigation, while at the same time maintaining biodiversity of trees and associated organisms.
Changing afforestation policies form monoculture to mixture plantations can be implemented immediately, and indeed has already started in the region of our BEF-China experiment. The choice of tree species can be optimized for high mixture performance, including biomass production and carbon storage. Our work shows that this can be done with species differing in leaf morphology and leaf life span, for example mixing evergreen with deciduous trees. Furthermore, in both our previous work in European grassland (Zuppinger-Dingley et al., 2014) and in the recent work in Chinese forests, we found abundant genetic variation within species that can be used as a resource for selection and breeding. That is, some genotypes within species are particularly suitable for mixed planting, whereas others may grow better in monoculture. Thus, in the next years we will use those genotypes for next-generation afforestation experiments. If the equivalent of 800 tons of carbon dioxide can be stored in a diverse forest per hectare (Liu et al., 2018) and given current costs per ton of carbon dioxide produced, unprecedented win-win situations in economic, climate-mitigation, and biodiversity-conservation terms may be achieved by implementation of the results of the presented work. Once planted, the mixed forests will immediately start to capture carbon, at a rate expected to be twice as fast as monoculture forests.
Photo 1. One of the authors (Bernhard Schmid) working with students at a field site in Gutianshan, Zhejiang, China. Here a tree is dissected into parts to measure the diameter, volume, and biomass of branches. These measurements are then used to fit prediction equations allowing the estimation of volume and biomass from new diameter measurements of trees that cannot be harvested.
Photo 2. One of the authors (Bernhard Schmid, upper left) working with students at a field site in Gutianshan, Zhejiang, China. Here a tree is dissected into parts to measure the diameter, volume, and biomass of branches. These measurements are then used to fit prediction equations allowing the estimation of volume and biomass from new diameter measurements of trees that cannot be harvested.
References
Feng, Y., Schmid, B., …, Wang, S., Fang, J. (2022). Multispecies forest plantations outyield monocultures across a broad range of conditions. Science 376: 865-868.
Hector, A., Schmid, B., …, Lawton, J.H. (1999). Plant diversity and productivity experiments in European grasslands. Science 286: 1123-1127.
Hua, F., Wang, X., Zheng, X., Fisher, B., Wang, L., Zhu, J., Tang, Y., Yu, D.W., Wilcove, D.S. (2016). Opportunities for biodiversity gains under the world’s largest reforestation programme. Nature Communications 7: 12717 (11 pages).
Huang, Y., Chen, Y., …, Ma, K., Niklaus, P.A., Schmid, B. (2018). Impacts of species richness on productivity in a large-scale subtropical forest experiment. Science 362: 80-83.
Liu, X., Trogisch, S., …, Wirth, C., Schmid, B., Ma, K. (2018). Tree species richness increases ecosystem carbon storage in subtropical forests. Proceedings of the Royal Society London B 285: 20181240. 10.1098/rspb.2018.1240
Rey, R., Kuo, P.H., Holland, H.D. (1995). Atmospheric carbon dioxide concentrations before 2.2 billion years ago. Nature 378: 603-605.
Weisser, W.W., …, Schulze, E.-D., Schmid, B., Eisenhauer, N. (2017). Biodiversity effects on ecosystem functioning in a 15-year grassland experiment: patterns, mechanisms, and open questions. Basic and Applied Ecology 23: 1-73.
Yu, Z., Ciais, P., Piao, S., Houghton, R.A., Lu, C., Tian, H., Agathokleous, E., Kattel, G.R., Sitch, S., Goll, D., Yue, X., Walker, A., Friedlingstein, P, Jain, A.K., Liu, S., Zhou, G. (2022). Forest expansion dominates China’s land carbon sink since 1980. Nature Communications 13: 5374 (12 pages).
Zuppinger-Dingley, D., Schmid, B., …, Flynn, D. (2014). Selection for niche differentiation in plant communities increases biodiversity effects. Nature 515: 108-111.
