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Prolonged suppression of ecosystem carbon dioxide uptake after an anomalously warm year
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Terrestrial ecosystems control carbon dioxide fluxes to and from
the atmosphere1,2 through photosynthesis and respiration, a balance
between net primary productivity and heterotrophic respiration,
that determines whether an ecosystem issequestering carbon
or releasing it to the atmosphere. Global1,3–5 and site-specific6 data
sets have demonstrated that climate and climate variability influence
biogeochemical processes that determine net ecosystem carbon
dioxide exchange (NEE) at multiple timescales. Experimental
data necessary to quantify impacts of a single climate variable,
such as temperature anomalies, on NEE and carbon sequestration
of ecosystems at interannual timescales have been lacking. This
derives from an inability of field studies to avoid the confounding
effects of natural intra-annual and interannual variability in temperature
and precipitation. Here we present results from a fouryear
study using replicate 12,000-kg intact tallgrass prairie monoliths
located in four 184-m3 enclosed lysimeters7
. We exposed 6 of
12 monoliths to an anomalously warm year in the second year of
the study8 and continuously quantified rates of ecosystem processes,
including NEE. We find that warming decreases NEE in
both the extreme year and the following year by inducing drought
that suppresses net primary productivity in the extreme year and
by stimulating heterotrophic respiration of soil biota in the subsequent
year. Our data indicate thattwo years are required for NEE
in the previously warmed experimental ecosystems to recover to
levels measured in the control ecosystems. Thistime lag caused net
ecosystem carbon sequestration in previously warmed ecosystems
to be decreased threefold over the study period, compared with
control ecosystems. Our findings suggest that more frequent
anomalously warm years9
, a possible consequence of increasing
anthropogenic carbon dioxide levels10, may lead to a sustained
decrease in carbon dioxide uptake by terrestrial ecosystems.
Vol 455| 18 September 2008
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Aeolian process effects on vegetation communities in an arid grassland ecosystem
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Many arid grassland communities are changing from grass dominance to shrub
dominance, but the mechanisms involved in this conversion process are not completely
understood. Aeolian processes likely contribute to this conversion from
grassland to shrubland. The purpose of this research is to provide information
regarding how vegetation changes occur in an arid grassland as a result of aeolian
sediment transport. The experimental design included three treatment blocks, each
with a 25 × 50 m area where all grasses, semi-shrubs, and perennial forbs were
hand removed, a 25 × 50 m control area with no manipulation of vegetation cover,
and two 10 × 25 m plots immediately downwind of the grass-removal and control
areas in the prevailing wind direction, 19◦ north of east, for measuring vegetation
cover. Aeolian sediment flux, soil nutrients, and soil seed bank were monitored on
each treatment area and downwind plot. Grass and shrub cover were measured on
each grass-removal, control, and downwind plot along continuous line transects as
well as on 5 × 10 m subplots within each downwind area over four years following
grass removal. On grass-removal areas, sediment flux increased significantly, soil
nutrients and seed bank were depleted, and Prosopis glandulosa shrub cover increased
compared to controls. Additionally, differential changes for grass and shrub
cover were observed for plots downwind of vegetation-removal and control areas.
Grass cover on plots downwind of vegetation-removal areas decreased over time
(2004–2007) despite above average rainfall throughout the period of observation,
while grass cover increased downwind of control areas; P. glandulosa cover increased
on plots downwind of vegetation-removal areas, while decreasing on plots downwind
of control areas. The relationships between vegetation changes and aeolian
sediment flux were significant and were best described by a logarithmic function,
with decreases in grass cover and increases in shrub cover occurring with small
increases in aeolian sediment flux
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Coupling of Vegetation Growing Season Anomalies and Fire Activity with Hemispheric and Regional-Scale Climate Patterns in Central and East Siberia
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An 18-yr time series of the fraction of absorbed photosynthetically active radiation (fAPAR) taken in by
the green parts of vegetation data from the NOAA Advanced Very High Resolution Radiometer
(AVHRR) instrument series was analyzed for interannual variations in the start, peak, end, and length of
the season of vegetation photosynthetic activity in central and east Siberia. Variations in these indicators of
seasonality can give important information on interactions between the biosphere and atmosphere. A
second-order local moving window regression model called the “camelback method” was developed to
determine the dates of phenological events at subcontinental scale. The algorithm was validated by comparing
the estimated dates to phenological field observations. Using spatial correlations with temperature
and precipitation data and climatic oscillation indices, two geographically distinct mechanisms in the system
of climatic controls of the biosphere in Siberia are postulated: central Siberia is controlled by an “Arctic
Oscillation–temperature mechanism,” while east Siberia is controlled by an “El Niño–precipitation mechanism.”
While the analysis of data from 1982 to 1991 indicates a slight increase in the length of the growing
season for some land-cover types due to an earlier beginning of the growing season, the overall trend from
1982 to 1999 is toward a slightly shorter season for some land-cover types caused by an earlier end of season.
The Arctic Oscillation tended toward a more positive phase in the 1980s leading to enhanced high pressure
system prevalence but toward a less positive phase in the 1990s. The results suggest that the two mechanisms
also control the fire regimes in central and east Siberia. Several extreme fire years in central Siberia were
associated with a highly positive Arctic Oscillation phase, while several years with high fire damage in east
Siberia occurred in El Niño years. An analysis of remote sensing data of forest fire partially supports this
hypothesis
VOLUME 20
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Illuminating the Modern Dance of Climate and CO2
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Records of Earth’s past climate imply higher atmospheric carbon dioxide concentrations in the future
19 SEPTEMBER 2008 VOL 321 SCIENCE
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Soil Temperature following Logging-Debris Manipulation and Aspen Regrowth in Minnesota: Implications for Sampling Depth and Alteration of Soil Processes
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Soil temperature is a fundamental controller of processes influencing the
transformation and flux of soil C and nutrients following forest harvest. Soil
temperature response to harvesting is influenced by the amount of logging
debris (biomass) removal that occurs, but the duration, magnitude, and depth
of influence is unclear. Logging debris manipulations (none, moderate, and
heavy amounts) were applied following clearcut harvesting at four aspendominated
(Populus tremuloides Michx.) sites in northeastern Minnesota, and
temperature was measured at 10-, 30-, and 50-cm depths for two growing
seasons. Across sites, soil temperature was significantly greater at all sample
depths relative to uncut forest in some periods of each year, but the increase
was reduced with increasing logging-debris retention. When logging debris
was removed compared to when it was retained in the first growing season,
mean growing season soil temperatures were 0.9, 1.0, and 0.8°C greater at
10-, 30-, and 50-cm depths, respectively. These patterns were also observed
early in the second growing season, but there was no discernible difference
among treatments later in the growing season due to the modifying effect of
rapid aspen regrowth. Where vegetation establishment and growth occurs
quickly, effects of logging debris removal on soil temperature and the processes
influenced by it will likely be short-lived. The significant increase in
soil temperature that occurred in deep soil for at least 2 yr after harvest
supports an argument for deeper soil sampling than commonly occurs in
experimental studies.
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Climate-induced changes in the small mammal communities of the Northern Great Lakes Region
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We use museum and other collection records to document large and extraordinarily rapid
changes in the ranges and relative abundance of nine species of mammals in the northern
Great Lakes region (white-footed mice, woodland deer mice, southern red-backed voles,
woodland jumping mice, eastern chipmunks, least chipmunks, southern flying squirrels,
northern flying squirrels, common opossums). These species reach either the southern or
the northern limit of their distributions in this region. Changes consistently reflect
increases in species of primarily southern distribution (white-footed mice, eastern
chipmunks, southern flying squirrels, common opossums) and declines by northern
species (woodland deer mice, southern red-backed voles, woodland jumping mice, least
chipmunks, northern flying squirrels). White-footed mice and southern flying squirrels
have extended their ranges over 225 km since 1980, and at particularly well-studied sites
in Michigan’s Upper Peninsula, small mammal assemblages have shifted from numerical
domination by northern species to domination by southern species. Repeated resampling
at some sites suggests that southern species are replacing northern ones rather than
simply being added to the fauna. Observed changes are consistent with predictions from
climatic warming but not with predictions based on recovery from logging or changes in
human populations. Because of the abundance of these focal species (the eight rodent
species make up 96.5% of capture records of all forest-dwelling rodents in the region and
70% of capture records of all forest-dwelling small mammals) and the dominating
ecological roles they play, these changes substantially affect the composition and
structure of forest communities. They also provide an unusually clear example of change
that is likely to be the result of climatic warming in communities that are experienced by
large numbers of people.
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Linking climate change to lemming cycles
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The population cycles of rodents at northern latitudes have puzzled
people for centuries1,2
, and their impact is manifest throughout the
alpine ecosystem2,3
. Climate change is known to be able to drive
animal population dynamics between stable and cyclic phases
4,5
,
and has been suggested to cause the recent changesin cyclic dynamics
of rodents and their predators
3,6–9
. But although predator–rodent
interactions are commonly argued to be the cause of the
Fennoscandian rodent cycles
1,10–13
, the role of the environment in
the modulation of such dynamics is often poorly understood in
natural systems
8,9,14
. Hence, quantitative links between climatedriven
processes and rodent dynamics have so far been lacking.
Here we show that winter weather and snow conditions, together
with density dependence in the net population growth rate, account
for the observed population dynamics of the rodent community
dominated by lemmings (Lemmus lemmus) in an alpine Norwegian
core habitat between 1970 and 1997, and predictthe observed absence
of rodent peak years after 1994. These local rodent dynamics are
coherentwith alpine bird dynamics both locally and over all ofsouthern
Norway, consistent with the influence of large-scale fluctuations
in winter conditions. The relationship between commonly available
meteorological data and snow conditions indicates that changes in
temperature and humidity, and thus conditions in the subnivean
space, seem to markedly affect the dynamics of alpine rodents and
their linked groups. The pattern of less regular rodent peaks, and
corresponding changes in the overall dynamics of the alpine ecosystem,
thusseemslikely to prevail over a growing area under projected
climate change.
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Impact of disturbed desert soils on duration of mountain snow cover
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Snow cover duration in a seasonally snow covered
mountain range (San Juan Mountains, USA) was found to
be shortened by 18 to 35 days during ablation through
surface shortwave radiative forcing by deposition of
disturbed desert dust. Frequency of dust deposition and
radiative forcing doubled when the Colorado Plateau, the
dust source region, experienced intense drought (8 events
and 39–59 Watts per square meter in 2006) versus a year
with near normal precipitation (4 events and 17–34 Watts
per square meter in 2005). It is likely that the current
duration of snow cover and surface radiation budget
represent a dramatic change from those before the
widespread soil disturbance of the western US in the late
1800s that resulted in enhanced dust emission. Moreover,
the projected increases in drought intensity and frequency
and associated increases in dust emission from the desert
southwest US may further reduce snow cover duration
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The Historical Dynamics of Socio-ecological Traps
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Environmental degradation is a typical unintended
outcome of collective human behavior. Hardin’s
metaphor of the ‘‘tragedy of the commons’’ has become a
conceived wisdom that captures the social dynamics leading
to environmental degradation. Recently, ‘‘traps’’ has gained
currency as an alternative concept to explain the rigidity of
social and ecological processes that produce environmental
degradation and livelihood impoverishment. The trap metaphor
is, however, a great deal more complex compared to
Hardin’s insight. This paper takes stock of studies using the
trap metaphor. It argues that the concept includes time and
history in the analysis, but only as background conditions and
not as a factor of causality. From a historical–sociological
perspective this is remarkable since social–ecological traps
are clearly path-dependent processes, which are causally
produced through a conjunction of events. To prove this point
the paper conceptualizes social–ecological traps as a process
instead of a condition, and systematically compares history
and timing in one classic and three recent studies of social–
ecological traps. Based on this comparison it concludes that
conjunction of social and environmental events contributes
profoundly to the production of trap processes. The paper
further discusses the implications of this conclusion for policy
intervention and outlines how future research might generalize
insights from historical–sociological studies of traps.
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The influence of conversion of forest types on carbon sequestration and other ecosystem services in the South Central United States
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This paper develops a forestland management model for the three states in the South Central United States (Arkansas,
Louisiana, and Mississippi). Forest type and land-use shares are estimated to be a function of economic and physical variables.
The results suggest that while historically pine plantations in this region have been established largely on old agricultural land,
in the future pine plantations are likely to occur on converted hardwood-forest lands. This shift in the supply of land for
plantations could have large effects on above-ground carbon storage and other ecosystem services. Subsidies of approximately
$12–27 per ha per year would maintain the area of hardwood forests and reduce carbon emissions from the above-ground and
product pool carbon stocks over the next 30 years. Across the several scenarios considered, results suggest that maintaining
hardwoods could be an efficient carbon sequestration alternative.
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