Photosynthesis is the process by which plants harvest sunlight to produce sugars from carbon dioxide and water. It is the primary source of energy for all life on Earth; hence it is important to ...understand how this process responds to climate change and human impact. However, model-based estimates of gross primary production (GPP, output from photosynthesis) are highly uncertain, in particular over heavily managed agricultural areas. Recent advances in spectroscopy enable the space-based monitoring of sun-induced chlorophyll fluorescence (SIF) from terrestrial plants. Here we demonstrate that spaceborne SIF retrievals provide a direct measure of the GPP of cropland and grassland ecosystems. Such a strong link with crop photosynthesis is not evident for traditional remotely sensed vegetation indices, nor for more complex carbon cycle models. We use SIF observations to provide a global perspective on agricultural productivity. Our SIF-based crop GPP estimates are 50-75% higher than results from state-of-the-art carbon cycle models over, for example, the US Corn Belt and the Indo-Gangetic Plain, implying that current models severely underestimate the role of management. Our results indicate that SIF data can help us improve our global models for more accurate projections of agricultural productivity and climate impact on crop yields. Extension of our approach to other ecosystems, along with increased observational capabilities for SIF in the near future, holds the prospect of reducing uncertainties in the modeling of the current and future carbon cycle.
An analysis of the sensitivity of synthetic aperture radar (SAR) backscatter (σ o ) to crop and soil conditions was conducted using 57 RADARSAT-2 C-band quad-polarized SAR images acquired from April ...to September 2009 for large fields of wheat, barley, oat, corn, onion, and alfalfa in Barrax, Spain. Preliminary results showed that the cross-polarized σ HV o was particularly useful for monitoring both crop and soil conditions and was the least sensitive to differences in beam incidence angle. The greatest separability of barley, corn, and onion occurred in spring after the barley had been harvested or in the narrow time window associated with grain crop heading when corn and onion were still immature. The time series of σ o offered reliable information about crop growth stage, such as jointing and heading in grain crops and leaf growth and reproduction in corn and onion. There was a positive correlation between σ o and the Normalized Difference Vegetation Index for onion and corn but not for all crops, and the impact of view direction and incidence angle on the time series was minimal compared to the signal response to crop and soil conditions. Related to planning for future C-band SAR missions, we found that quad-polarization with image acquisition frequency from 3-6 days was best suited for distinguishing crop types and for monitoring crop phenology, single- or dual-polarization with an acquisition frequency of 3-6 days was sufficient for mapping crop green biomass, and single- or dual-polarization with daily image acquisition was necessary to capture rapid changes in soil moisture condition.
Climate change is predicted to increase both drought frequency and duration, and when coupled with substantial warming, will establish a new hydroclimatological model for many regions. Large-scale, ...warm droughts have recently occurred in North America, Africa, Europe, Amazonia and Australia, resulting in major effects on terrestrial ecosystems, carbon balance and food security. Here we compare the functional response of above-ground net primary production to contrasting hydroclimatic periods in the late twentieth century (1975-1998), and drier, warmer conditions in the early twenty-first century (2000-2009) in the Northern and Southern Hemispheres. We find a common ecosystem water-use efficiency (WUE(e): above-ground net primary production/evapotranspiration) across biomes ranging from grassland to forest that indicates an intrinsic system sensitivity to water availability across rainfall regimes, regardless of hydroclimatic conditions. We found higher WUE(e) in drier years that increased significantly with drought to a maximum WUE(e) across all biomes; and a minimum native state in wetter years that was common across hydroclimatic periods. This indicates biome-scale resilience to the interannual variability associated with the early twenty-first century drought--that is, the capacity to tolerate low, annual precipitation and to respond to subsequent periods of favourable water balance. These findings provide a conceptual model of ecosystem properties at the decadal scale applicable to the widespread altered hydroclimatic conditions that are predicted for later this century. Understanding the hydroclimatic threshold that will break down ecosystem resilience and alter maximum WUE(e) may allow us to predict land-surface consequences as large regions become more arid, starting with water-limited, low-productivity grasslands.
Frequency and severity of droughts are projected to increase in many regions, and their effects on temporal dynamics of the terrestrial carbon cycle remain uncertain. Ecosystem net primary ...productivity (NPP) is a key component of the carbon cycle, and rainfall use efficiency (RUE=NPP/precipitation) is an important measure of ecosystem stability and resilience. Here we investigated the temporal patterns of NPP and RUE and their key driving climate factors, during the early 21st century drought for four biomes in China: Needleleaf forest, Broadleaf forest, Woody savannas, and Grassland. Estimates of regional-scale NPP were based on the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) MOD17 NPP product. Our results confirmed recent findings that the impact of current-year precipitation on NPP was confounded by an array of biotic and abiotic factors. Whereas, the RUE responded strongly to variations in current- and previous-year drought for all the four biomes and the four biomes combined. We found that a dry year preceded by a wet year resulted in the highest RUE, and conversely, a wet year preceded by a dry year resulted in the lowest RUE. This was attributed to the legacy effect of precipitation changes in both wet and dry years, and to the resilience of the biomes in the dry years. Based on these results, we developed and validated a model of RUE based on the Palmer Drought Severity Index (PDSI) of both current and previous years which works well for these four biomes and all biomes combined. This model is particularly useful for understanding the impact of prolonged drought at the landscape scale because it is based on accessible satellite data and available meteorological data and the results have been tested across four major biomes.
•The annual NPP was derived from MODIS global data set (MOD17A3).•The impact of current-year precipitation on NPP was weak across biome types.•The RUE responded strongly to variations in current- and previous-year drought.•We developed and validated a model of RUE based on two-year PDSI.
Global warming may intensify the hydrological cycle and lead to increased drought severity and duration, which could alter plant community structure and subsequent ecosystem water and carbon dioxide ...cycling. We report on the net ecosystem exchange of carbon dioxide (NEE) of a semidesert grassland through a severe drought which drove succession from native bunchgrasses to forbs and to eventual dominance by an exotic bunchgrass. We monitored NEE and energy fluxes using eddy covariance coupled with meteorological and soil moisture variables for 6 years at a grassland site in southeastern Arizona, USA. Seasonal NEE typically showed a springtime carbon uptake after winter‐spring periods of average rainfall followed by much stronger sink activity during the summer rainy season. The two severe drought years (2004 and 2005) resulted in a net release of carbon dioxide (25 g C m−2) and widespread mortality of native perennial bunchgrasses. Above average summer rains in 2006 alleviated drought conditions, resulting in a large flush of broad‐leaved forbs and negative total NEE (−55 g C m−2 year−1). Starting in 2007 and continuing through 2009, the ecosystem became increasingly dominated by the exotic grass, Eragrostis lehmanniana, and was a net carbon sink (−47 to −98 g C m−2 year−1) but with distinct annual patterns in NEE. Rainfall mediated by soils was the key driver to water and carbon fluxes. Seasonal respiration and photosynthesis were strongly dependent on precipitation, but photosynthesis was more sensitive to rainfall variation. Respiration normalized by evapotranspiration showed no interannual variation, while normalized gross ecosystem production (i.e., water use efficiency) was low during drought years and then increased as the rains returned and the E. lehmanniana invasion progressed. Thus, when dry summer conditions returned in 2009, the potential for ecosystem carbon accumulation was increased and the ecosystem remained a net sink unlike similar dry years when native grasses dominated ecosystem structure.
The Soil Moisture Active Passive (SMAP) mission is one of the first Earth observation satellites being developed by NASA in response to the National Research Council's Decadal Survey. SMAP will make ...global measurements of the soil moisture present at the Earth's land surface and will distinguish frozen from thawed land surfaces. Direct observations of soil moisture and freeze/thaw state from space will allow significantly improved estimates of water, energy, and carbon transfers between the land and the atmosphere. The accuracy of numerical models of the atmosphere used in weather prediction and climate projections are critically dependent on the correct characterization of these transfers. Soil moisture measurements are also directly applicable to flood assessment and drought monitoring. SMAP observations can help monitor these natural hazards, resulting in potentially great economic and social benefits. SMAP observations of soil moisture and freeze/thaw timing will also reduce a major uncertainty in quantifying the global carbon balance by helping to resolve an apparent missing carbon sink on land over the boreal latitudes. The SMAP mission concept will utilize L-band radar and radiometer instruments sharing a rotating 6-m mesh reflector antenna to provide high-resolution and high-accuracy global maps of soil moisture and freeze/thaw state every two to three days. In addition, the SMAP project will use these observations with advanced modeling and data assimilation to provide deeper root-zone soil moisture and net ecosystem exchange of carbon. SMAP is scheduled for launch in the 2014-2015 time frame.
The circadian clock plays a significant role in many aspects of female reproductive biology, including estrous cycling, ovulation, embryonic implantation, onset of puberty, and parturition. In an ...effort to link cell-specific circadian clocks to their specific roles in female reproduction, we used the promoter that controls expression of Steroidogenic Factor-1 (SF1) to drive Cre -recombinase–mediated deletion of the brain muscle arnt-like 1 ( Bmal1 ) gene, known to encode an essential component of the circadian clock (SF1- Bmal1 ⁻/⁻). The resultant SF1- Bmal1 ⁻/⁻ females display embryonic implantation failure, which is rescued by progesterone supplementation, or bilateral or unilateral transplantation of wild-type ovaries into SF1- Bmal1 ⁻/⁻ dams. The observation that the central clock, and many other peripheral clocks, are fully functional in this model allows the assignment of the implantation phenotype to the clock in ovarian steroidogenic cells and distinguishes it from more general circadian related systemic pathology (e.g., early onset arthropathy, premature aging, ovulation, late onset of puberty, and abnormal estrous cycle). Our ovarian transcriptome analysis reveals that deletion of ovarian Bmal1 disrupts expression of transcripts associated with the circadian machinery and also genes critical for regulation of progesterone production, such as steroidogenic acute regulatory factor ( Star ). Overall, these data provide a powerful model to probe the interlocking and synergistic network of the circadian clock and reproductive systems.
Significance This work demonstrates that specific peripheral clocks play unique and discrete roles in specific aspects of reproductive biology. Our use of a cell-specific conditional knockout model, in coordination with ovary transplant technology, permits examination of a peripheral clock without the impacts of off-target deletions that might indirectly impact reproductive function. In this case, we show that the molecular circadian clock, found in ovarian steroidogenic cells, is crucial for normal female reproduction, specifically embryonic implantation. The observation that implantation can be rescued by a single ovary with normal molecular clock machinery i.e., brain muscle arnt-like 1 (BMAL1) may provide direction for clinical intervention strategies when aberrant circadian oscillations are influencing fertility.
Grasslands across the United States play a key role in regional livelihood and national food security. Yet, it is still unclear how this important resource will respond to the prolonged warm droughts ...and more intense rainfall events predicted with climate change. The early 21st-century drought in the southwestern United States resulted in hydroclimatic conditions that are similar to those expected with future climate change. We investigated the impact of the early 21st-century drought on aboveground net primary production (ANPP) of six desert and plains grasslands dominated by C₄ (warm season) grasses in terms of significant deviations between observed and expected ANPP. In desert grasslands, drought-induced grass mortality led to shifts in the functional response to annual total precipitation (PT), and in some cases, new species assemblages occurred that included invasive species. In contrast, the ANPP in plains grasslands exhibited a strong linear function of the current-year PT and the previous-year ANPP, despite prolonged warm drought. We used these results to disentangle the impacts of interannual total precipitation, intra-annual precipitation patterns, and grassland abundance on ANPP, and thus generalize the functional response of C₄ grasslands to predicted climate change. This will allow managers to plan for predictable shifts in resources associated with climate change related to fire risk, loss of forage, and ecosystem services.
Significance Acetaminophen toxicity is significantly influenced by the hepatocyte circadian clock through its control of xenobiotic metabolizing systems. We have found that, although the central ...circadian clock can influence detoxification through glutathione biosynthesis, the autonomous hepatocyte circadian clock also controls major aspects of acetaminophen (APAP) bioactivation. One mechanism by which APAP bioactivation is controlled is through the clock’s regulation of cytochrome P450-dependent activity through NADPH-cytochrome P450 oxidoreductase.
The diurnal variation in acetaminophen (APAP) hepatotoxicity (chronotoxicity) reportedly is driven by oscillations in metabolism that are influenced by the circadian phases of feeding and fasting. To determine the relative contributions of the central clock and the hepatocyte circadian clock in modulating the chronotoxicity of APAP, we used a conditional null allele of brain and muscle Arnt-like 1 ( Bmal1, aka Mop3 or Arntl ) allowing deletion of the clock from hepatocytes while keeping the central and other peripheral clocks (e.g., the clocks controlling food intake) intact. We show that deletion of the hepatocyte clock dramatically reduces APAP bioactivation and toxicity in vivo and in vitro because of a reduction in NADPH-cytochrome P450 oxidoreductase gene expression, protein, and activity.
Synthetic Aperture Radar has shown its large potential for retrieving soil moisture maps at regional scales. However, since the backscattered signal is determined by several surface characteristics, ...the retrieval of soil moisture is an ill-posed problem when using single configuration imagery. Unless accurate surface roughness parameter values are available, retrieving soil moisture from radar backscatter usually provides inaccurate estimates. The characterization of soil roughness is not fully understood, and a large range of roughness parameter values can be obtained for the same surface when different measurement methodologies are used. In this paper, a literature review is made that summarizes the problems encountered when parameterizing soil roughness as well as the reported impact of the errors made on the retrieved soil moisture. A number of suggestions were made for resolving issues in roughness parameterization and studying the impact of these roughness problems on the soil moisture retrieval accuracy and scale.
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