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measurements are available to calibrate the be particularly tricky at regional and global wood biomass. Continuing research will models. scales and across biodiverse ecosystems. be needed to develop more­efficient cano­ A comparison (Fig. 1) of that earlier result The mapping of individual tree canopies by py­classification algorithms. In the meantime, with Brandt and colleagues’ findings in the species will probably remain at the top of the Brandt and colleagues have clearly demon­ western Sahel, for example, shows that the Earth­observation research community’s wish strated the potential for future global mapping previous study tended to underestimate the list for some time6. of tree canopies at submetre scales. number of trees in the drier regions (areas In the years ahead, remote sensing will with annual rainfall of less than 600 milli­ undoubtedly provide unprecedented detail Niall P. Hanan is with the Jornada Basin metres). Moreover, the previous estimates about vegetation structure as data from a Long-Term Ecological Research Program, provided no information on the location and range of sources — including light detection Department of Plant and Environmental size of individual trees within each square kilo­ and ranging (lidar), radar and high­resolution Sciences, and Julius Y. Anchang is in the metre, whereas Brandt and colleagues provide visible and near­infrared sensors — become Department of Plant and Environmental detailed information on the location and size more readily available7. Satellite­derived Sciences, New Mexico State University, of every individual canopy. The improvement Las Cruces, New Mexico 88003, USA. provided in the latest study can also be seen “Never before have trees e-mails: [email protected]; in the much higher level of detail it gives for [email protected] the wetter regions (those with annual rainfall been mapped at this level greater than 600 mm), and shows local spatial of detail across such variability in trees that is presumably associ­ a large area.” ated with contrasting soil types, water availa­ bility, land use and land­use history. There are, of course, caveats and limita­ high­resolution data on tree canopy size and 1. Brandt, M. et al. Nature 587, 78–82 (2020). tions to Brandt and colleagues’ work and the density could contribute to the inventory 2. Crowther, T. W. et al. Nature 525, 201–205 (2015). potential for scaling up their approach to a and management of forests and woodland, 3. Axelsson, C. R. & Hanan, N. P. Biogeosciences 14, 3239–3252 (2017). global analysis. Successful canopy detection deforestation monitoring, and assessment 4. Reichstein, M. et al. Nature 566, 195–204 (2019). declined drastically below a canopy diameter of the carbon sequestered in biomass, tim­ 5. Engler, R. et al. Forest Ecol. Mgmt 310, 64–73 (2013). of 2 m, owing to constraints imposed by the ber, fuel wood and tree crops. The ability to 6. Draper, F. C. et al. Trends Ecol. Evol. https://doi. org/10.1016/j.tree.2020.08.003 (2020). spatial resolution of the imagery, and consist­ map the size and location of individual tree 7. National Academies of Sciences, Engineering, and 3 ent with earlier work . Although we can expect canopies using such satellite data will com­ Medicine. Thriving on Our Changing Planet: A Decadal further improvements in the spatial resolu­ plement information available from other Strategy for Earth Observation from Space (Natl Academies Press, 2018). tion of satellite images, it becomes pertinent instruments that provide data for tree height, to ask what minimum canopy size is needed vertical canopy profiles and above­ground This article was published online on 14 October 2020. to characterize woody­plant communities in various regions. For global tree­canopy map­ Astronomy ping, if we assume that the computational and storage challenges associated with large data volumes can be overcome, the biggest roadblock would lie in developing efficient A fast radio burst approaches for automated classification and delineation of canopies. Brandt and col­ in our own Galaxy leagues’ deep­learning method required an input of approximately 90,000 manually digi­ Amanda Weltman & Anthony Walters tized training points. This approach becomes untenable for work on a global scale, and The origins of millisecond­long bursts of radio emissions, more­automated (unsuper vised) methods for known as fast radio bursts, from beyond our Galaxy have been extracting information from satellite imagery 4 enigmatic. The detection of one such burst from a Galactic would be necessary . A related problem is the ability to distin­ source helps to constrain the theories. See p.54, p.59 & p.63 guish between what might look like one large canopy and adjacent, overlapping canopies of different individual trees. To improve can­ Sometimes, being an astrophysicist is an should help us to work out the mechanisms opy separation, Brandt et al. used a weighting exercise in international detective work. that drive these spectacular events. scheme in training their convolutional neural Piecing together the evidence is complicated, The name ‘fast radio bursts’ is a good network, but still resorted to a ‘canopy clump’ because observations are often made after a description of what they are: bright bursts of class to describe aggregated canopy areas of key event, the experiments are not generally radio waves with durations roughly at the milli­ more than 200 m2, suggesting that the sepa­ repeatable and, when it comes to telescopes, it second scale. First discovered7 in 2007, their ration approach was not always effective. For is all about location, location, location. Three short­lived nature makes it particularly chal­ application in wetter regions, where overlap­ papers1–3 in this issue of Nature report exactly lenging to detect them and to determine their ping canopies in woodlands and forests are this situation in the detection of a phenom­ position on the sky. The smorgasbord of theo­ common, canopy delineation and separation enon called a fast radio burst (FRB) coming ries8 that has been proposed to explain FRBs methods will need refinement and automation from a source in our Galaxy. Intriguingly, the has, until recently, outpaced our discovery of to be feasible at global scales. FRB was accompanied by a burst of X­rays4–6. actual FRB events. The majority of these the­ Yet more challenging is the identification of The discovery was made and understood by ories invoke some kinds of stellar remnant as tree species. Although feasible, on the basis piecing together observations from multiple FRB sources. In particular, highly magnetized of canopy colour, shape and texture5, it will space­ and ground­based telescopes, and young neutron stars known as magnetars have Nature | Vol 587 | 5 November 2020 | 43 ©2020 Spri nger Nature Li mited. All rights reserved. ©2020 Spri nger Nature Li mited. All rights reserved. News & views Galactic magnetar that has been measured so Debris from far — which potentially solves a key puzzle in previous flare Magnetic Shock-wave this field. Before the discovery of FRB 200428, field line front the absence of X­ray and γ­ray bursts from Gyrating repeating FRBs lent weight to certain magne­ electron tar theories of the origins of FRBs. But because no bright radio bursts had been observed Magnetar Flare coming from Galactic magnetars, it seemed Collision X-rays unlikely that magnetars could be FRB sources Radio at all. The discovery of FRB 200428 proves that Newly ejected waves electrons and magnetars can indeed drive FRBs. Moreover, particles FRB 200428 is the first Galactic radio burst that is as bright as the FRBs observed in other, nearby galaxies, which also provides much­ needed evidence that magnetars could be the Figure 1 | A potential mechanism for the formation of fast radio bursts. A bright, millisecond­long burst sources of extragalactic FRBs. of radio waves, known as a fast radio burst (FRB), has been detected1–3 coming from a highly magnetized Intriguingly, there are several mechanisms stellar remnant (a magnetar) in our Galaxy. The radio waves were accompanied by X­ray emissions4–6. One by which magnetars can drive FRBs, each of 9,10 possible mechanism for the formation of such an FRB is that the magnetar produces a submillisecond­ which has a distinct observational signature. long flare of electrons and other charged particles, which collides with particles that had been emitted The new results thus open up a host of excit­ from previous flares (note that the collision occurs a great distance away from the magnetar; this distance ing problems to explore. For example, what is not shown to scale). The collision generates an outward­moving shock front, which in turn produces huge theoretical mechanism could give rise to magnetic fields. Electrons gyrate around the magnetic field lines, and thereby emit a burst of radio waves. such bright, yet rare, radio bursts with X­ray The shock wave also heats the electrons, which causes them to emit X­rays. counterparts? One promising possibility is that a flare from a magnetar collides with the emerged as leading candidates, because their the discovery of FRB 200428; these findings surrounding medium and thereby generates a strong magnetic fields could act as ‘engines’ are described by Bochenek et al.2 on page 59. shock wave9,10 (Fig. 1). Observations of nearby that drive FRBs. However, Bochenek and colleagues found the rapidly star­forming galaxies will be crucial for A key approach to testing these progenitor FRB to be about 1,000 times brighter than had finding events similar to FRB 200428, to help theories is to associate FRBs with other astro­ been announced by the CHIME/FRB Collab­ pin down the actual mechanism. nomical phenomena. It is therefore crucial to oration. This discrepancy was resolved after Other magnetar­driven FRB mechanisms narrow down the potential positions of FRBs the CHIME/FRB Collaboration recalibrated would produce accompanying neutrino to small regions of the sky, so that the associ­ its data, whereupon it found the brightness to bursts11.

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