Forests in Flux
Special Online Collection

Forests provide vital goods and services including food, fuel, and fiber, and are also major reservoirs of global carbon. In a special section of the 13 Jun 2008 issue, Science examined the future of the world’s forests against a backdrop of climate change and intensifying human activity. News reports looked at how humans have reshaped wooded landscapes across the globe; a Review explained how forests influence climate through physical, chemical, and biological processes; and a series of Perspectives discussed studies of past forest change, predictive models of forest dynamics, and aspects of sustainable forest management. A related Editorial discussed forest research priorities for the 21st century and Science Careers highlighted career opportunities in forest ecology. On the Web, a video presentation explored some of the challenges of global forest management and understanding the influence of forests on climate, and an all-forests podcast featured interviews and stories on seed dispersal and tree resilience, being a forest ecologist, growth of an ancient date seed, and preserving forest biodiversity.


Plants Seek Higher Ground

Recent climate change has induced biological and ecological responses from animals and plants throughout the world. One of the telltale fingerprints of global warming is a change in the distribution of species at their latitudinal or altitudinal extremes. In a Report in the 27 Jun 2008 Science, Lenoir et al. presented a 100-year survey of the distribution of 171 forest plant species along the entire elevation gradient (0 to 2600 meters above sea level) of six mountain ranges in west Europe. Their data show that climate warming has resulted in an average upslope movement of plant species’ optimal elevation of about 30 meters per decade. The team also found that although species generally displayed different rates of movement, those species restricted to mountain habitats and short-lived grassy species showed more pronounced shifts in their optimum elevation. Overall, the results demonstrate that climate change affects the spatial core of the distributional range of plant species, in addition to their distributional margins. Lead author Dr. Jonathan Lenoir discussed the findings in a related podcast interview.


Abrupt Andes Uplift

The elevation histories of mountains are important for understanding tectonic processes as well as ancient climate change. The uplift of mountain belts like the central Andes -- Earth’s second largest -- is generally assumed to reflect the gradual shortening and thickening of Earth’s crust. Across the Andes, an oceanic tectonic plate drives against the western edge of South America as the plate dives beneath it. Because the crust is less dense than underlying rock, that thickening buoys it, causing the mountains to rise. In the 6 Jun 2007 Science Garzione et al. reviewed recent evidence of the elevation history of the Andes (see the related News story by R.A. Kerr) that modifies this picture. New geochemical paleoelevation data combined with existing isotopic data indicate that before about 10 million years ago (Ma), the elevation of the Andes was moderate and relatively stable. Between ~10 and 6 Ma, the Andean plateau shot up abruptly by 1.5 to 2.5 kilometers, perhaps triggered by the detachment of dense lithosphere at the base of the crust.


Unexpected Lipid Regulator

The synthesis of lipids in the liver is regulated by transcriptional pathways under the influence of carbohydrate intake. In a Report in the 13 Jun 2008 Science, Lee et al. reported an unexpected addition to the list of transcription factors involved in mammalian lipid metabolism: a protein called XBP1. This protein is a key regulator of the unfolded protein response in the endoplasmic reticulum -- a process that arrests protein synthesis and removes malformed proteins from the organelle. The team found that XBP1 expression in normal mice was induced by the feeding of carbohydrates and corresponded with the expression of several genes involved in fatty acid synthesis. Moreover, they showed that deletion of the Xbp1 gene in the adult mouse liver markedly reduced liver lipid production, thus lowering blood cholesterol and triglyceride concentrations. Elucidating the exact mechanism by which XBP1 helps regulate lipid synthesis in response to carbohydrate uptake will require further investigation. Nevertheless, an accompanying Perspective by J.D. Horton noted that "[t]he finding provides new opportunities to develop treatments for conditions such as hyperlipidemia and fatty liver disease in humans."


A Greener Greenland

Melting of the Greenland ice sheet is expected to cause much of the rise in sea level that will occur as global climate continues to warm. One approach to understanding the future stability of the ice sheet is to look at how it was affected by climate change in the past. In a Report in the 20 Jun 2008 Science, de Vernal and Hillaire-Marcel used pollen records preserved in marine sediment off southwest Greenland to assess the changes in the vegetation, climate, and ice volume over the past million years. The data indicate that large changes in ice extent and vegetation accompanied changes in temperature and suggest that the Greenland ice sheet was much smaller during previous warm periods than it is today. The presence of abundant spruce pollen indicates that a coniferous forest developed during an interglacial period some 400,000 years ago and provides a time frame for the development and decline of forest ecosystems over a nearly ice-free Greenland. An accompanying Perspective by E.J. Steig and A.P. Wolfe discussed the findings.


Ozone Recovery and Climate

In the past several decades, the westerly winds of the southern hemispheric jet stream have accelerated on the poleward side of the jet. This acceleration has been attributed to the combined effects of increased greenhouse gases and the depletion of stratospheric ozone and has been predicted to continue. However, stratospheric ozone is expected to recover in the first half of the 21st century due in large part to the 1987 Montreal Protocol, which banned the manufacture of ozone-destroying chemicals called chlorofluorocarbons. How might this affect the westerly winds in the Southern Hemisphere? In the 13 Jun 2008 Science, Son et al. report that a recent set of climate models that take stratospheric chemistry into account project the summer tropospheric westerly winds will weaken, not strengthen as previously predicted, owing to the gradual disappearance of the ozone hole over Antarctica. This would have important implications for climate in the Southern Hemisphere, as the westerly jet stream directly affects sea surface temperatures, the extent of sea ice, the variability of storm tracks, location of arid regions, the strength of wind-driven ocean circulation, and the exchange of heat and carbon dioxide between the atmosphere and ocean. A ScienceNOW story by P. Berardelli highlighted the Report.


Resolving Bird Relationships

Resolving the evolutionary relationships among birds has been difficult in part because of their apparently rapid divergence early in their evolutionary history, and also because of a relatively limited number of DNA samples. In a Report in the 27 Jun 2008 Science Hackett et al. constructed a new phylogeny of birds after analyzing DNA sequences from 19 independent loci in 169 species, representing all major extant groups. While many established relationships are upheld by their analysis, there are several surprising results that challenge current classifications and alter our understanding of trait evolution; for example, some diurnal birds evolved from nocturnal ancestors, and a family of birds capable of flight (tinamous) falls in the same group as flightless ratites, which include kiwis and ostriches. In a podcast interview, corresponding author Dr. Rebecca Kimball discussed the additional finding that songbirds and parrots share a closer kinship than previously thought. A related News Focus by E. Pennisi highlighted the study and other large-scale efforts to build family trees based on molecular data.


Molecular Flagellum Clutch

Many bacteria in the environment live either as motile planktonic individuals or within sessile multicellular clusters called biofilms. Planktonic bacteria move by way of one or more rotating flagella -- whip-like molecular machines composed of some 30 proteins. A motor, located at the base of each flagellum, uses the proton motive force to power rotation of an extracellular helical filament and propel the bacterium through the environment. In a study described in the 20 Jun 2008 Science, Blair et al. identified a novel component of the motor that allows the bacterial cell to arrest flagellar motion and is co-regulated with biofilm formation. The component -- a protein called EpsE --works in a manner similar to that of a clutch, by disengaging motor force-generating elements in cells from the flagellar rotor as the bacteria are imbedded in a sticky biofilm matrix. The protein was discovered in the soil bacterium Bacillus subtilis, but similar clutch proteins may be involved in mediating the transition between motile and nonmotile lifestyles in other bacterial species. An accompanying Perspective by R.M. Berry and J.P. Armitage highlighted the Report. And in a related podcast interview, senior author Dr. Daniel Kearns discussed how the findings may prove useful in clinical settings for understanding and disrupting the development of some bacterial diseases.


Tryptophan-Accelerated Flow

Many proteins involved in life processes such as photosynthesis, nitrogen fixation, and aerobic respiration, promote rapid electron transfer over surprisingly long distances. One mechanism that has been proposed to account for such efficient transfer rates is the transient oxidation and/or reduction of amino acids along the path from donor to acceptor. In a Report in the 27 Jun 2008 Science, Shih et al. provided experimental validation of this concept using a semisynthetic system in which a redox-active metal complex is chemically tethered at a strategic location on the protein’s surface, where it is poised to undergo photochemically initiated electron transfer with a redox partner. In this case, the researchers engineered mutant versions of the bacterial protein azurin bearing a tryptophan, tyrosine, or phenylalanine residue between a copper electron donor and a rhenium electron acceptor. Although the intervening tyrosine and phenylalanine residues had little effect, transient absorption spectroscopy in the visible and infrared revealed that the tryptophan accelerated electron transfer by more than a factor of 100. An accompanying Perspective by J.M. Bollinger Jr. considered the mechanism(s) by which proteins might regulate electron relay.


An Eccentric Pulsar

Millisecond pulsars are rapidly spinning neutron stars that emit lighthouse-like beams of radio waves with periods shorter than 10 ms. Most of these pulsars have a companion star that is a white dwarf, and all known millisecond binary pulsars have perfectly circular orbits. Now, in a Research Article in the 6 Jun 2008 Science (published online 15 May), Champion et al. report the discovery of a millisecond pulsar in the galactic disk that has a highly eccentric 95-day orbit about a solar-like star, thus challenging current models of pulsar formation. As noted in an accompanying Perspective by E.P.J. van den Heuvel, possible explanations for the high eccentricity include recycling of the pulsar in a globular cluster followed by ejection into the galactic disk, initial formation from three rather than two stars, and/or disruptive effects from gravitational forces. An additional curiosity is the mass of the newly discovered pulsar, which is about 30% larger than that of other known binary neutron stars. Long-term and higher-precisions timing of the pulsar should provide further insights into the nature of this unique binary system.


Anticipatory Microbes

In nature, environmental changes such as variations in temperature, pH, and oxygen level do not typically occur in isolation, but in a temporally coupled and nonrandom manner. An organism that is capable of learning correlated cues can then exploit them in order to anticipate vital changes in the environment -- for example by preparing for nutrient fluctuations or mounting protective responses. In a Research Article in the 6 Jun 2008 Science (published online 8 May), Tagkopoulos et al. reported that even simple microbes can learn to exploit predictable sequences of environmental signals and prepare for future environmental changes ( see the related ScienceNOW story by R. Kwok). The researchers first used computer simulations to show that virtual evolving organisms garnered more energy if they could "learn" that certain signals preceded the arrival of food and launch a preemptive metabolic response. The researchers then looked for evidence of this ability in Escherichia coli bacteria and found that upon exposure to a large temperature change (as might occur after transition from the outside environment into the mammalian body), the bugs shift from aerobic to anaerobic respiration in anticipation of the low oxygen levels in the gastrointestinal tract. When the bacteria were instead exposed to high oxygen levels after an increase in temperature, they "learned" to turn off their low-oxygen responses in fewer than 100 generations. An accompanying Perspective by N.S. Baliga highlighted the study.



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In Science Signaling

Multipurpose Adaptor Protein

Mutations in the von Hippel-Lindau tumor suppressor gene VHL occur in various inherited and sporadically occurring tumors. The protein encoded by this gene -- pVHL -- bears no known enzymatic activities but interacts with numerous protein partners. In a Perspective in the 17 Jun 2008 issue, I.J. Frew and W. Krek discussed how the identification of distinct pVHL-containing multiprotein complexes has led to a refined portrait of pVHL tumor suppressor function. In general, pVHL acts as an adaptor that links various enzymatic activities to various target proteins to control diverse transcription-dependent pathways, as well as transcription-independent cellular processes -- namely, deposition of the extracellular matrix and regulation of the microtubule cytoskeleton. The authors note that "[a]n exciting challenge will be to determine which molecular functions of pVHL are important for tumor suppression in which tissue types and which spectra of gene mutations affect which pVHL functions." This knowledge could lead to the development of new therapeutic strategies treat VHL-associated tumors.

Also in Science Signaling this month:
--N.A. Lambert discussed the notion that G proteins in cells may function as both activated heterotrimers and dissociated subunits (24 Jun 2008)
--E.M. Adler and A.M VanHook, in a podcast, discussed how microvesicles carrying an oncogenic form of the epidermal growth factor receptor provide a mechanism for lateral spread of the malignant phenotype (17 Jun 2008)
--N. Balduc et al. explained how a single domain confers different subcellular localizations of the homeodomain protein KN1 (10 Jun 2008)