Good day, dear Take in Mind readers! Yes, perhaps we slightly exaggerated the scope of the discovery in the title of this post. It would be more appropriate to quote the modest original title of the scientific publication “An unexpectedly large count of trees in the West African Sahara and Sahel”. But the fact still remains: An international team of scientists led by the Danish scientist Rasmus Fensholt revealed over 1.8 billion individual trees (13.4 trees per hectare), with a median crown size of 12 m2 over a land area that spans 1.3 million km2 in the West African Sahara, Sahel, and sub-humid zone.
The canopy cover increases from 0.1% (0.7 trees per hectare) in hyper-arid areas, through 1.6% (9.9 trees per hectare) in arid and 5.6% (30.1 trees per hectare) in semi-arid zones, to 13.3% (47 trees per hectare) in sub-humid areas. The mapping of woody plants at the resolution level of single trees was achieved by the use of satellite data at very high spatial resolution (0.5 m) from DigitalGlobe satellites, combined with modern machine-learning techniques.
This research is revolutionary in several areas. First, the scientists have shown that deep learning, combined with a high spatial resolution satellite imagery, can detect individual trees over large areas. In this case, on an area of about 1.3 million square km (an approximately two Ukraine’s or slightly less than four Germany’s territories), almost every tree with a crown diameter of two meters and more was “identified” (in total, as we mentioned above, more than 1.8 billion).
Of course, there are various technical difficulties to convert this method to a universal one. For example, the study had to use a very large number of training samples to achieve high quality across landscapes. Training samples also had to cover a range of different satellite images: acquisition dates, dust, clouds, burned areas, solar zenith and viewing angle, off-nadir, sensor systems, and image boundaries all affect the visibility of tree crowns. All of these variations need to be considered when training the model, which requires a vast amount of training data and makes training time-consuming. Specific for this study, Fensholt and his colleagues had to manually mark nearly 90,000 trees in the images. But we hope that the advanced technologies of the satellite imagery with the increased resolution will solve many of these problems.
Secondly, this work changes our understanding of the ecology of the Sahara and its environs. It is already clear that the Sahara is not only dunes and Alhagi. The trees are the most important indicator of the natural state and allow us to study the qualitative, quantitative, and temporal changes in the ecosystem. In addition, such types of studies provide the most important information for the development of agriculture in desert areas. As the authors claimed, “…In a longer-term perspective, knowledge based on individual trees will improve long-term monitoring, environmental assessments and information-driven land-use policies. As such, a database on dryland trees will be an important baseline for policy-makers and stakeholders, as well as initiatives that aim at protecting and restoring trees in arid and semi-arid lands in relation to mitigating degradation, poverty and climate change … “. Or in other words, we can better appreciate the consequences of own “creative” activities.
Lastly, this work is a small, but a very big step towards a global and very detailed treemap. At least technically, such a project seems possible now. We will not be surprised that one day scientists will find out that the prototype of Pandora’s world (by James Cameron’s “Avatar”), where all the trees are connected into one unique organism, is not found hundreds of light-years away from Earth, but it is here, in our native planet. Do you agree with us?
The link of the original article: Brandt, M., Tucker, C.J., Kariryaa, A. et al. An unexpectedly large count of trees in the West African Sahara and Sahel. Nature 587, 78–82 (2020). https://doi.org/10.1038/s41586-020-2824-5
Featured image by Norbert Graube from Pixabay.
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