[meteorite-list] Meteorite That Doomed Dinosaurs Remade Forests

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 16 Sep 2014 14:52:10 -0700 (PDT)
Message-ID: <201409162152.s8GLqAOc019222_at_zagami.jpl.nasa.gov>


Meteorite That Doomed Dinosaurs Remade Forests
By Daniel Stolte
University of Arizona
September 16, 2014

A post-apocalyptic forest: This post-extinction landscape is lush from
warm weather and ample rain along the Front Range, but there are only a
few types of trees. Extinct relatives of sycamores, walnut trees and
palm trees are the most common. (Image by Donna Braginetz; courtesy of
Denver Museum of Nature & Science)

The impact decimated slow-growing evergreens and made way for
fast-growing, deciduous plants, according to a study applying
biomechanical analyses to fossilized leaves.

The meteorite impact that spelled doom for the dinosaurs 66 million
years ago decimated the evergreens among the flowering plants to a much
greater extent than their deciduous peers, according to a study led by
UA researchers. The results are published
in the journal PLoS Biology.

Applying biomechanical formulas to a treasure trove of thousands of
fossilized leaves of angiosperms - flowering plants excluding conifers
- the team was able to reconstruct the ecology of a diverse plant
community thriving during a 2.2 million-year period spanning the
cataclysmic impact event, believed to have wiped out more than half of
plant species living at the time.

The researchers found evidence that after the event, fast-growing,
deciduous angiosperms had replaced their slow-growing, evergreen peers
to a large extent. Living examples of evergreen angiosperms, such as
holly and ivy, tend to prefer shade, don't grow very fast and sport
dark-colored leaves.

"When you look at forests around the world today, you don't see many
forests dominated by evergreen flowering plants," said the study's lead
author, Benjamin Blonder, who graduated last year from the lab of UA
Professor Brian Enquist with a Ph.D. from the UA's Department of Ecology
and Evolutionary Biology and is now the science coordinator at the UA
SkySchool. "Instead, they are dominated by deciduous species, plants
that lose their leaves at some point during the year."

The study provides much-needed evidence for how the extinction event
unfolded in the plant communities at the time, Blonder said. While it
was known that the plant species that existed before the impact were
different from those that came after, data was sparse on whether the
shift in plant assemblages was just a random phenomenon or a direct
result of the event.

"If you think about a mass extinction caused by catastrophic event such
as a meteorite impacting Earth, you might imagine all species are
equally likely to die," Blonder said. "Survival of the fittest doesn't
apply - the impact is like a reset button. The alternative hypothesis,
however, is that some species had properties that enabled them to survive.

"Our study provides evidence of a dramatic shift from slow-growing
plants to fast-growing species," he said. "This tells us that the
extinction was not random, and the way in which a plant acquires
resources predicts how it can respond to a major disturbance. And
potentially this also tells us why we find that modern forests are
generally deciduous and not evergreen."

Previously, other scientists found evidence of a dramatic drop in
temperature caused by dust from the impact. Under the conditions of such
an "impact winter," many plants would have struggled harvesting enough
sunlight to maintain their metabolism and growth.

"The hypothesis is that the impact winter introduced a very variable
climate," Blonder said. "That would have favored plants that grew
quickly and could take advantage of changing conditions, such as
deciduous plants."

Blonder, Enquist and their colleagues Dana Royer from Wesleyan
University, Kirk Johnson from the Smithsonian National Museum of Natural
History and Ian Miller from the Denver Museum of Nature and Science
studied a total of about 1,000 fossilized plant leaves collected from a
location in southern North Dakota, embedded in rock layers known as the
Hell Creek Formation, in what at the time was a lowland floodplain
crisscrossed by river channels. The collection consists of more than
10,000 identified plant fossils and is housed primarily at the Denver
Museum of Nature and Science.

By analyzing leaves, which convert carbon dioxide from the atmosphere
and water into nutrients for the plant, the study followed a new
approach that enabled the researchers to predict how plant species used
carbon and water, shedding light on the ecological strategies of plant
communities long gone, hidden under sediments for many millions of years.

"We measured the mass of a given leaf in relation to its area, which
tells us whether the leaf was a chunky, expensive one to make for the
plant, or whether it was a more flimsy, cheap one," Blonder explained.
"In other words, how much carbon the plant had invested in the leaf."

In addition to the leaves' mass-per-area ratio, Blonder and his
coworkers measured the density of the leaves' vein networks.

"When you hold a leaf up to the light, you see a pattern of veins
running through it," Blonder said. "That network determines how much
water is moved through the leaf. If the density is high, the plant is
able to transpire more water, and that means it can acquire carbon faster.

"By comparing the two parameters, we get an idea of resources invested
versus resources returned, and that allows us to capture the ecological
strategy of the plants we studied long after they went extinct."

Evergreen plants are more likely to invest in leaves that are costly to
construct but are well-built and last a long time, Blonder explained,
while the leaves of deciduous plants tend to be short-lived but offer
high metabolic rates.

"There is a spectrum between fast- and slow-growing species," he said.
"There is the 'live fast, die young' strategy and there is the 'slow but
steady' strategy. You could compare it to financial strategies investing
in stocks versus bonds."

The analyses revealed that while slow-growing evergreens dominated the
plant assemblages before the extinction event, fast-growing flowering
species had taken their places afterward.

The National Science Foundation awarded Blonder a graduate research
fellowship to pursue this research. Additional funding was provided by
the Geological Society of America.

Blonder said he was inspired to pursue the research project after seeing
a lecture on paleobiology at the UA.

"I had a strong interest in how plants function based on their leaves,
and I was fascinated to learn about applying those biomechanical
principles to reconstruct ecological functions of the past," he said.
"When you hold one of those leaves that is so exquisitely preserved in
your hand knowing it's 66 million years old, it's a humbling feeling."

Benjamin Blonder
UA Sky School
College of Science
bblonder at gmail.com
Received on Tue 16 Sep 2014 05:52:10 PM PDT

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