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CHANGES
IN RAINFALL PATTERNS SPUR PLANT GROWTH, CARBON ABSORPTION
ACROSS U.S.
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A
NASA-funded study finds that changing rainfall patterns over
much of the United States in the last century have allowed
plants to grow more vigorously and absorb more carbon dioxide
from the atmosphere.
In
the presence of water and sunlight, plants take in carbon
dioxide (CO2) during photosynthesis to create fuel, glucose
and other sugars, for building plant structures. Better understanding
of biological and physical processes that contribute to carbon
uptake by plants will help scientists predict climate change
and future levels of CO2, a heat-trapping gas in the atmosphere.
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"The
changes in the hydrologic cycle is one of the mechanisms that
is often overlooked in the recent debate over carbon sequestration
in the United States," said Ramakrishna Nemani, a researcher
at the University of Montana's School of Forestry, and lead
author of the study that appears in an issue of Geophysical
Research Letters later this month.
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Scientists
have noticed that the U.S. terrestrial carbon sink, an effect
where carbon is drained from the air and stored in the land,
has been increasing since the latter part of the 20th century.
Previous research has claimed this rise may be due to an observed
greening of the U.S. as a result of forest re-growth, as well
as greater concentrations of atmospheric CO2 and warming temperatures.
For
the first time, however, this study suggests that changing
rainfall patterns may play a bigger role in plant growth and
carbon absorption. Computer model results showed that on average
from 1950 to 1993 higher humidity combined with an eight percent
increase in precipitation has led to a 14 percent increase
in plant growth in the U.S. The data over that time period
also show increases in cloud cover, minimum temperatures,
soil moisture and stream flows, which are all signs of a changing
hydrologic cycle.
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Whether
or not shifting rainfall patterns result in a positive uptake
of carbon by land ecosystems depends on complex interactions
that include plant physiology, and both the magnitude and
timing of changes that impact the water cycle.
Between
1950 and 1993, in general, the minimum temperatures in the
spring have become warmer, and autumns have become wetter,
which have combined to lengthen the growing season for plants.
A longer growing season means plants pull carbon from the
air for a greater period of time. In addition, the magnitude
of precipitation on average has gone up in the conterminous
U.S., except over the Pacific Northwest.
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"Most
people only think of the idea that more water means more growth,
but really plants benefit from more water in a number of ways,"
said Steven Running, a co-author of the paper, who is also
a researcher at the University of Montana's School of Forestry.
When
the air is wetter, plants can open special cells in their
leaves without losing much water to the air, increasing CO2
uptake while reducing the amount of water needed to grow.
Additionally, wetter soils promote decomposition of dead plant
materials, releasing nutrients needed for plant growth. Also,
higher humidity in the spring helps maintain higher night
temperatures, which makes for more frost-free days and lengthens
the growing season.
The
authors found that without enhanced rainfall and humidity,
CO2 increases and temperature changes have a lesser effect
on plant growth.
Greenhouse
gases warm the air, and warmer air can hold more water, which
impacts the hydrologic cycle. Changes in the hydrologic cycle
may mean more rainfall in some regions and less in other places,
affecting plant growth and carbon absorption, which in turn
affects future concentrations of greenhouse gases, Nemani
said.
The
study was funded by the Vegetation Ecosystem Modeling and
Analysis Project and the Moderate-resolution Imaging Spectroradiometer
science team, under NASA's Earth Science Enterprise.
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