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Drought Sensitivity of the Amazon Rainforest
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Amazon forests are a key but poorly understood component of the global carbon cycle. If, as
anticipated, they dry this century, they might accelerate climate change through carbon losses and
changed surface energy balances. We used records from multiple long-term monitoring plots across
Amazonia to assess forest responses to the intense 2005 drought, a possible analog of future events.
Affected forest lost biomass, reversing a large long-term carbon sink, with the greatest impacts
observed where the dry season was unusually intense. Relative to pre-2005 conditions, forest subjected
to a 100-millimeter increase in water deficit lost 5.3 megagrams of aboveground biomass of carbon per
hectare. The drought had a total biomass carbon impact of 1.2 to 1.6 petagrams (1.2 × 1015 to
1.6 × 1015 grams). Amazon forests therefore appear vulnerable to increasing moisture stress, with the
potential for large carbon losses to exert feedback on climate change.
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The Genetic Architecture of Maize Flowering Time
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Flowering time is a complex trait that controls adaptation of plants to their local environment in
the outcrossing species Zea mays (maize). We dissected variation for flowering time with a set of
5000 recombinant inbred lines (maize Nested Association Mapping population, NAM). Nearly a
million plants were assayed in eight environments but showed no evidence for any single largeeffect
quantitative trait loci (QTLs). Instead, we identified evidence for numerous small-effect QTLs
shared among families; however, allelic effects differ across founder lines. We identified no
individual QTLs at which allelic effects are determined by geographic origin or large effects for
epistasis or environmental interactions. Thus, a simple additive model accurately predicts flowering
time for maize, in contrast to the genetic architecture observed in the selfing plant species rice
and Arabidopsis.
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A-maize-ing Diversity
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Analysis of a new maize resource reveals that a large number of genetic loci with small effects may underlie the wide variation seen in traits such as flowering time.
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Phenology Feedbacks on Climate Change
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A longer growing season as a result of climate
change will in turn affect climate through
biogeochemical and biophysical effects.
SCIENCE VOL 324
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Risks of Climate Engineering
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Observations indicate that attempts to limit climate
warming by reducing incoming shortwave radiation risk
major precipitation changes.
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Seasons and Life Cycles
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A conceptual framework. This table is a guide to determining how individual species are responding to an extended growing
season by observing the duration of peak season. The life history of a species—from the onset of greening through the end of
senescence—is illustrated by the length of the solid lines. Each case represents a shift in the timing (columns) and duration
(rows) of one or more species in a hypothetical three-species community that includes an early-, mid-, and late-season species.
The growing season begins when the first species greens and ends when the last species senesces. The peak season (gray shaded
area) occurs when all species have started and none have completed their life history. Reproductive life history events likely
begin before the peak season and are completed before its end. The final row and column list changes that can be observed
through frequent observations of surface greenness.
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The Last Glacial Maximum
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We used 5704 14C, 10Be, and 3
He ages that span the interval from 10,000 to 50,000 years ago
(10 to 50 ka) to constrain the timing of the Last Glacial Maximum (LGM) in terms of global
ice-sheet and mountain-glacier extent. Growth of the ice sheets to their maximum positions
occurred between 33.0 and 26.5 ka in response to climate forcing from decreases in northern
summer insolation, tropical Pacific sea surface temperatures, and atmospheric CO2. Nearly all
ice sheets were at their LGM positions from 26.5 ka to 19 to 20 ka, corresponding to minima in
these forcings. The onset of Northern Hemisphere deglaciation 19 to 20 ka was induced by an
increase in northern summer insolation, providing the source for an abrupt rise in sea level. The
onset of deglaciation of the West Antarctic Ice Sheet occurred between 14 and 15 ka, consistent
with evidence that this was the primary source for an abrupt rise in sea level ~14.5 ka.
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Pleistocene Megafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America
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Although the North American megafaunal extinctions and the formation of novel plant communities are
well-known features of the last deglaciation, the causal relationships between these phenomena are
unclear. Using the dung fungus Sporormiella and other paleoecological proxies from Appleman Lake,
Indiana, and several New York sites, we established that the megafaunal decline closely preceded
enhanced fire regimes and the development of plant communities that have no modern analogs. The loss
of keystone megaherbivores may thus have altered ecosystem structure and function by the release of
palatable hardwoods from herbivory pressure and by fuel accumulation. Megafaunal populations
collapsed from 14,800 to 13,700 years ago, well before the final extinctions and during the BøllingAllerød
warm period. Human impacts remain plausible, but the decline predates Younger Dryas cooling
and the extraterrestrial impact event proposed to have occurred 12,900 years ago.
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Megafaunal Decline and Fall
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Declines in North American megafauna
populations began before the Clovis period
and were the cause, not the result, of
vegetation changes and increased fires.
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Warming Up Food Webs
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How do predator-prey interactions influence Warming Up Food Webs ecosystem responses to climate change?
VOL 323 SCIENCE
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