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Mythbusting Forests

July 23rd, 2020 No comments

Despite the multiple evidence that afforestation is not a solution for mitigating the increased atmospheric CO2 [1], there are still lobbies and multimillionaire clubs willing to plant millions of trees at the global scale, and spreading myths about the benefits of trees and large afforestation programs. Recently (17 July 2020), William Bond gave a talk at Oxford University to bust these myths. Here is his talk, and below is a summary of the top 5 myths.

Myth 1. Forest are ancient, non-forests are caused by deforestation. There is evidence of ancient species-rich grasslands and shrublands in many parts of the world (from Cerrado in Brazil, to grasslands in Africa, shrublands in Mediterranea ecosystems, etc.). In fact animal grazers evolved long ago (long before humans could deforest) in grasslands. There are also evidences of many tropical forests that were thought to be ancient and are not (e.g., youtube). This myth has deep roots in the western culture [2].

Myth 2. Oxygen comes from trees: cutting down forest will deprive us of air to breath. Oxygen is more ancient than forests! The atmospheric concentration of oxygen during much of the evolutionary history of plants, before the rise of dense tropical forests, has been higher than current level (21%). Fire requires oxygen to burn, and there has been fire since early colonization of land plants [3]. Statements like the Amazon provides 20% of our oxygen are wrong; the Amazon consume about as much O2 as it produces; O2 is ancient, it doesn’t depend on trees (see details: link1 & link2). There are lots of reasons to be preserve the Amazon, but running out of oxygen isn’t one of them.

Myth 3. Forests ‘make rain’: plant trees to get more water. W. Bond note that many city dwellers and some climatologists suggest that planting trees would increase water supply, but farmers, which have daily experience with land management, says that planting trees dries up rivers. A catchment experiment in South Africa unambiguously show that catchments with tree plantations get drier compared with those under natural shrublands (Wyk 1987). Maybe some catchments, given their size, climate and topography, may generate their own rainfall (as often suggested for the Amazon), but this doesn’t seems a general rule. Planting trees will not ‘make rain’.

Myth 4. The biggest store of terrestrial carbon is in tropical forests. Tropical forests store about 225 Pg C, while boreal soils store ca. 1300 Pg C. So, from the C perspective, it is more important to conserve boreal soils (peatlands, etc.) than tropical forests! Obviously tropical forest need to be conserved for their biodiversiy. But you better forget about planting trees, and start thinking in conserving boreal peatlands as their destruction would release high amount of CO2 to the atmosphere.

Myth 5. Forests equate with biodiversity. There are examples where planting trees implies a loss of biodiversity (Abreu et al. 2017, Phifer et al. 2017). When comparing savannas and forest for the same rainfall, there are no differences in biodiversity (Murphy et al. 2016). In addition, many of the global biodiversity hotspots are open non-forest ecosystems or mosaics of forest and open ecosystems. So the myth cannot be hold. In fact, landscape mosaics of forest and non-forest are highly diverse landscapes [4].

References

[1] Afforestation is not the solution to mitigate CO2, jgpausas.blogs.uv.es/2019/10/17/

[2] Pausas J.G. & Bond W.J. 2019. Humboldt and the reinvention of nature. J. Ecol. 107: 1031-1037. [doi | jecol blog | jgp blog | pdf]  

[3] Pausas J.G. & Keeley J.E. 2009. A burning story: The role of fire in the history of life. BioScience 59: 593-601 [doi | OUP | pdf | post]

[4] Pausas J.G. & Bond W.J. 2020. Alternative biome states in terrestrial ecosystems. Trends Plant Sci. 25: 250-263. [doi | sciencedirect | cell | pdf]