Jesse Smith

The Future of Forests

![Figure][1] CREDIT: DLILLC/CORBIS Forests have had a pervasive influence on the evolution of terrestrial life and continue to provide important feedbacks to the physical environment, notably climate. Today, studies of the worlds forests are taking place against a backdrop of unprecedented change, largely resulting either directly or indirectly from human activity. In this special issue, we focus particularly on the future of forests in light of these changes. Current research on the relationships of forests and climate are considered in a Review by Bonan (p. [1444][2]), which provides an overview of how climate and forests are connected through physical, chemical, and biological processes that affect the carbon cycle, the hydrologic cycle, atmospheric composition, and the flow of solar energy and heat through the Earth system. In this video presentation, Jerry Franklin, Review author Gordon Bonan, and Perspective author Valerie Kapos discuss the importance of understanding the influence of forests on climate and some of the challenges of global forest governance. For scientists interested in forest dynamics (the turnover of individual trees and species over time), long-term forest plots are yielding field data on processes that take place over time scales longer than a research career. Until recently, though, the development of predictive models of forest dynamics lagged behind observation. In a Perspective, Purves and Pacala (p. [1452][3]) explain how advances in the mathematics of forest modeling and the ecological understanding of forest communities are generating exciting new possibilities for mapping future trajectories of forests over times from decades to centuries. At longer time scales, pollen and macrofossil records, along with genetic data, have revealed past movements of species as climates changed, which in turn provide pointers to the direction of future change, as discussed by Petit et al. in a Perspective (p. [1450][4]). Three further Perspectives deal with aspects of sustainable forest management. Miles and Kapos (p. [1454][5]) consider the question of incentives for “avoided deforestation” in the context of the recent Bali conference on climate change; Canadell and Raupach (p. [1456][6]) discuss how carbon sequestration can protect against the effects of climate change; and Chazdon (p. [1458][7]) considers how forests and their ecosystem services can be restored on degraded lands. In another Perspective, Agrawal et al. (p. [1460][8]) spotlight some recent trends in forest governance and ownership, which in effect define the limits and opportunities for sustainability. The three News reports take a look at how humans have reshaped wooded landscapes across the globe. Stokstad (p. [1436][9]) takes stock of a large-scale assessment of Amazonian biodiversity in regenerating forests and tree farms. Koenig (p. [1439][10]) examines the precariousness of the extensive rainforests in the Democratic Republic of the Congo. Morell (p. [1442][11]) reports on the success of preservation efforts in Chinas Hengduan Mountain Region, one of the richest temperate forest ecosystems. Forests and trees have been intimately bound up with the emergence and cultural development of our own species. Their future, and that of human society, depends ever more on how humans treat them in the coming decades. [1]: pending:yes [2]: /lookup/doi/10.1126/science.1155121 [3]: /lookup/doi/10.1126/science.1155359 [4]: /lookup/doi/10.1126/science.1155457 [5]: /lookup/doi/10.1126/science.1155358 [6]: /lookup/doi/10.1126/science.1155458 [7]: /lookup/doi/10.1126/science.1155365 [8]: /lookup/doi/10.1126/science.1155369 [9]: /lookup/doi/10.1126/science.320.5882.1436 [10]: /lookup/doi/10.1126/science.320.5882.1439 [11]: /lookup/doi/10.1126/science.320.5882.1442

Measuring Earth's carbon cycle

![Figure][1] The OCO-2 satellite can measure photosynthesis, as well as the amount of CO2 in the atmosphere, and so will shed new light on the carbon cycle. PHOTO: BEAWIHARTA/REUTERS/NEWSCOM One of the crowning achievements of modern environmental science is the Keeling curve, the detailed time series of the concentration of atmospheric carbon dioxide (CO2) begun in 1958 that has enabled deep insights into the mechanisms of global climate change. These measurements were difficult to make for most of their 60-year history, involving the physical collection of air samples in flasks at a small number of sites scattered strategically around the globe and the subsequent analysis of their CO2 inventories in a handful of laboratories throughout the world. The Orbiting Carbon Observatory-2 (OCO-2) mission was designed to circumvent those limitations by providing a platform with which atmospheric CO2 can be measured spectrally from space over large geographic areas, thereby offering an unprecedented capability to study, in great detail, the processes that affect the concentration of the gas over a variety of spatial and temporal scales. The satellite can also measure solar-induced fluorescence, a proxy for photosynthesis, which provides valuable information about the biological processes that affect atmospheric CO2. In this issue, a collection of Research Articles presents the initial results from OCO-2, covering the detection of CO2 emissions from specific point sources; measurements of CO2 variations associated with El Nino, on land and at sea; and solar-induced fluorescence measurements of photosynthesis for determining gross primary production by plants. With its impressive collection of observational capabilities, OCO-2 will enable measurements of atmospheric CO2 to be made with sufficient precision, resolution, and coverage to faithfully characterize its sources and sinks globally over the seasonal cycle, a long-standing goal in atmospheric and climate science. [1]: pending:yes

Aqueous azobenzene switching

Photoswitches Photoswitching molecules can enable targeted delivery of drugs in the body, but the body is best penetrated by near-infrared radiation (wavelengths between 700 and 900 nanometers). Most photoswitching processes in molecules require shorter wavelengths, and high light intensities are needed to trigger two-photon processes. Dong et al. extensively substituted azobenzene molecules so that they were soluble in water and could photoswitch from the trans isomer after single-photon irradiation at 720 nanometers. The half-life of the resulting cis isomer was 1 second. The benzene rings were substituted with methoxy and pyrrolidine groups, as well as a fused dioxane ring, so that the molecule remained protonated at physiological pH while exhibiting good stability in water. J. Am. Chem. Soc. 10.1021/jacs.7b06471 (2017).

Taking tabletop tomography to extremes

Optical Imaging The practice of slicing through a sample at various depths with extreme ultraviolet (XUV) light to build up a three-dimensional picture of it is usually confined to synchrotron facilities. Intense infrared laser pulses impinging onto a gas cell of noble atoms generates high harmonics and provides access to the short wavelengths of XUV light with a tabletop source. Fuchs et al. show that filtering and focusing the high harmonics can narrow the range of wavelengths to produce a coherent XUV source. They demonstrate extreme coherent tomography by building up a three-dimensional image of a structured semiconductor sample with a depth resolution of 24 nanometers, providing an example of a tabletop laser source for highly spatially and temporally resolved coherent imaging applications. Optica 4 , 903 (2017).

Subtleties of growing iron sulfides

Nanomaterials![Figure][1] Pyrrhotite (Fe7S8) prepared from the precursor phenyl disulfide PHOTO: J. M. RHODES ET AL., CHEM. MATER. 10.1021/ACS.CHEMMATER.7B03550 (2017)

Drone technology for whale health

Microbiota It is hard to obtain biological samples from whales. However, whales do shed lots of material as oily slicks behind them and in their massive exhalations, or blows, at the surface. Exhalations contain tissue debris and respiratory microorganisms. Apprill et al. used a small drone furnished with a Petri dish and a 96-well plate to capture exhaled material from 28 humpback whales off Vancouver Island, Canada, and Cape Cod, USA. 16S ribosomal RNA sequencing of bacteria and archaea revealed that animals from the two populations have diverse, distinctive, and yet surprisingly consistent core microbiomes in common with each other and with small, toothed cetaceans: bottlenose dolphins. Fortunately, in this study, no known cetacean respiratory pathogens were detected. These data offer a glimpse into what a healthy microbiota state might look like for a baleen whale. mSystems 10.1128/mSystems.00119-17 (2017).

Sorting out submarine canyons

Marine Geology Submarine canyons, often found directly off the coastline, are common but understudied features of Earth. Despite their prevalence, what determines their location and morphology remains a mystery. Smith et al. used a massive data set from the entire western coast of the United States to determine what variables best correlated with underwater canyon formation. They found that the distribution of rock type and sediment flux were the most important factors for explaining the location of submarine canyons. Geophys. Res. Lett. 10.1002/2017GL075139 (2017).

A shocking discovery

Impact Events Shocked quartz—whose crystalline structure is deformed along planes inside the crystal, a result of sudden high pressure and heating—long has been considered to be an unequivocal signature of the impact of an extraterrestrial object such as a meteorite. This favorite tool of geologists searching for proof of an impact may not be so foolproof after all, though. Chen et al. simulated the pressure and temperature caused by an idealized lightning strike on rocks and found that lightning can produce the same conditions and effects caused by impact events. Therefore, the presence of shock features in quartz should not be taken as unequivocal evidence for an extraterrestrial impact. Geophys. Res. Lett. 10.1002/2017GL073843 (2017).

Notch signaling schools thymic DCs

Immunology Conventional and plasmacytoid dendritic cells in the thymus are important for the establishment of central tolerance. They induce autoreactive T cell death and generate regulatory T cells. Although these cells are believed to have an extrathymic origin, Martin-Gayo et al. uncovered evidence that human early thymic progenitors (ETPs) are another source. From postnatal thymocytes, ETPs differentiate into CD123+ common dendritic cell progenitors that can reconstitute dendritic cells in immunodeficient murine thymi. JAG1-mediated Notch signaling in medullary niches is critical for this process. The idea is that thymic progenitors, which escape strong Notch signaling via ligands such as DLL1 and DLL4 and then encounter JAG1, become myeloid-primed by up-regulating GATA2 and turning off GATA3. J. Exp. Med. 10.1084/jem.20161564 (2017).

Elliptical galaxies spinning on end

Galaxy Structure Most elliptical galaxies have a flattened shape and rotate around their shortest axis, like a wheel. However, some galaxies, known as prolate rotators, are elongated and spin around their longest axis, like a spindle. Tsatsi et al. searched a large survey of galaxy kinematics to discover eight previously unknown examples. They estimate that around 10% of elliptical galaxies are prolate rotators, but the fraction is higher at higher masses. They use simulations to show that prolate rotators can form if two massive spiral galaxies merge in a polar orientation, potentially explaining the evolutionary history of these systems. Astron. Astrophys. 606 , A62 (2017).