Remote sensing of canopy nitrogen at regional scale in Mediterranean forests using the spaceborne MERIS terrestrial chlorophyll index

Loozen Y., Rebel K.T., Karssenberg D., Wassen M.J., Sardans J., Peñuelas J., De Jong S.M. (2018) Remote sensing of canopy nitrogen at regional scale in Mediterranean forests using the spaceborne MERIS terrestrial chlorophyll index. Biogeosciences. 15: 2723-2742.
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Doi: 10.5194/bg-15-2723-2018

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Canopy nitrogen (N) concentration and content are linked to several vegetation processes. Therefore, canopy N concentration is a state variable in global vegetation models with coupled carbon (C) and N cycles. While there are ample C data available to constrain the models, widespread N data are lacking. Remotely sensed vegetation indices have been used to detect canopy N concentration and canopy N content at the local scale in grasslands and forests. Vegetation indices could be a valuable tool to detect canopy N concentration and canopy N content at larger scale. In this paper, we conducted a regional case-study analysis to investigate the relationship between the Medium Resolution Imaging Spectrometer (MERIS) Terrestrial Chlorophyll Index (MTCI) time series from European Space Agency (ESA) Envisat satellite at 1ĝ€km spatial resolution and both canopy N concentration (%N) and canopy N content (Nĝ€gĝ€mĝ'2, of ground area) from a Mediterranean forest inventory in the region of Catalonia, in the northeast of Spain. The relationships between the datasets were studied after resampling both datasets to lower spatial resolutions (20, 15, 10 and 5ĝ€km) and at the original spatial resolution of 1ĝ€km. The results at higher spatial resolution (1ĝ€km) yielded significant log-linear relationships between MTCI and both canopy N concentration and content: r2ĝ€ Combining double low line ĝ€0.32 and r2ĝ€ Combining double low line ĝ€0.17, respectively. We also investigated these relationships per plant functional type. While the relationship between MTCI and canopy N concentration was strongest for deciduous broadleaf and mixed plots (r2ĝ€ Combining double low line ĝ€0.24 and r2ĝ€ Combining double low line ĝ€0.44, respectively), the relationship between MTCI and canopy N content was strongest for evergreen needleleaf trees (r2ĝ€ Combining double low line ĝ€0.19). At the species level, canopy N concentration was strongly related to MTCI for European beech plots (r2ĝ€ Combining double low line ĝ€0.69). These results present a new perspective on the application of MTCI time series for canopy N detection. © Author(s) 2018.

Llegeix més

Rhizosphere microorganisms can influence the timing of plant flowering

Lu T., Ke M., Lavoie M., Jin Y., Fan X., Zhang Z., Fu Z., Sun L., Gillings M., Peñuelas J., Qian H., Zhu Y.-G. (2018) Rhizosphere microorganisms can influence the timing of plant flowering. Microbiome. 6: 0-0.
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Doi: 10.1186/s40168-018-0615-0

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Background: Plant phenology has crucial biological, physical, and chemical effects on the biosphere. Phenological drivers have largely been studied, but the role of plant microbiota, particularly rhizosphere microbiota, has not been considered. Results: We discovered that rhizosphere microbial communities could modulate the timing of flowering of Arabidopsis thaliana. Rhizosphere microorganisms that increased and prolonged N bioavailability by nitrification delayed flowering by converting tryptophan to the phytohormone indole acetic acid (IAA), thus downregulating genes that trigger flowering, and stimulating further plant growth. The addition of IAA to hydroponic cultures confirmed this metabolic network. Conclusions: We document a novel metabolic network in which soil microbiota influenced plant flowering time, thus shedding light on the key role of soil microbiota on plant functioning. This opens up multiple opportunities for application, from helping to mitigate some of the effects of climate change and environmental stress on plants (e.g. abnormal temperature variation, drought, salinity) to manipulating plant characteristics using microbial inocula to increase crop potential. © 2018 The Author(s).

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Higher capability of C3 than C4 plants to use nitrogen inferred from nitrogen stable isotopes along an aridity gradient

Luo W., Wang X., Sardans J., Wang Z., Dijkstra F.A., Lü X.-T., Peñuelas J., Han X. (2018) Higher capability of C3 than C4 plants to use nitrogen inferred from nitrogen stable isotopes along an aridity gradient. Plant and Soil. : 1-11.
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Doi: 10.1007/s11104-018-3661-2

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Background and aims: The nitrogen isotope composition (δ15N) of plants in arid and semiarid grasslands is affected by environmental factors, especially water availability. Nevertheless, it is unclear whether the response of δ15N to water availability differs between C3 and C4 photosynthetic pathways. Methods: We investigated plant δ15N of coexisting C3 and C4 species as a function of aridity along a 3200 km aridity gradient across the arid and semi-arid grasslands of northern China. Results: Aridity was positively correlated with plant δ15N values in both C3 and C4 plants and also in the entire plant community, whereas soil bulk δ15N values increased first and then decreased along the aridity gradient. The N uptake by C4 plants appeared to be more affected by competition pressure of neighboring plants and soil microbes than for C3 plants along the transect. Conclusions: The decoupled relationship between plant and soil δ15N values indicated that variations in vegetation and soil δ15N values were driven by differential biogeochemical processes, while different soil N sources were used for plant growth along the climatic gradient. The advantage of C3 plants in the use of N may counteract the competitive advantage that C4 plants have over C3 plants due to their higher water use efficiency under drier conditions. These findings can help understand why C4 plants do not completely replace C3 plants in drier environments despite their higher water use efficiency. © 2018 Springer International Publishing AG, part of Springer Nature

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Effects of extreme drought on plant nutrient uptake and resorption in rhizomatous vs bunchgrass-dominated grasslands

Luo W., Xu C., Ma W., Yue X., Liang X., Zuo X., Knapp A.K., Smith M.D., Sardans J., Dijkstra F.A., Peñuelas J., Bai Y., Wang Z., Yu Q., Han X. (2018) Effects of extreme drought on plant nutrient uptake and resorption in rhizomatous vs bunchgrass-dominated grasslands. Oecologia. : 0-0.
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Doi: 10.1007/s00442-018-4232-1

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Both the dominance and the mass ratio hypotheses predict that plant internal nutrient cycling in ecosystems is determined by the dominant species within plant communities. We tested this hypothesis under conditions of extreme drought by assessing plant nutrient (N, P and K) uptake and resorption in response to experimentally imposed precipitation reductions in two semiarid grasslands of northern China. These two communities shared similar environmental conditions, but had different dominant species—one was dominated by a rhizomatous grass (Leymus chinensis) and the other by a bunchgrass (Stipa grandis). Results showed that responses of N to drought differed between the two communities with drought decreasing green leaf N concentration and resorption in the community dominated by the rhizomatous grass, but not in the bunchgrass-dominated community. In contrast, negative effects of drought on green leaf P and K concentrations and their resorption efficiencies were consistent across the two communities. Additionally, in each community, the effects of extreme drought on soil N, P and K supply did not change synchronously with that on green leaf N, P and K concentrations, and senesced leaf N, P and K concentrations showed no response to extreme drought. Consistent with the dominance/mass ratio hypothesis, our findings suggest that differences in dominant species and their growth form (i.e., rhizomatous vs bunch grass) play an important nutrient-specific role in mediating plant internal nutrient cycling across communities within a single region. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature.

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Geothermally warmed soils reveal persistent increases in the respiratory costs of soil microbes contributing to substantial C losses

Marañón-Jiménez S., Soong J.L., Leblans N.I.W., Sigurdsson B.D., Peñuelas J., Richter A., Asensio D., Fransen E., Janssens I.A. (2018) Geothermally warmed soils reveal persistent increases in the respiratory costs of soil microbes contributing to substantial C losses. Biogeochemistry. : 1-16.
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Doi: 10.1007/s10533-018-0443-0

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Increasing temperatures can accelerate soil organic matter decomposition and release large amounts of CO2 to the atmosphere, potentially inducing positive warming feedbacks. Alterations to the temperature sensitivity and physiological functioning of soil microorganisms may play a key role in these carbon (C) losses. Geothermally active areas in Iceland provide stable and continuous soil temperature gradients to test this hypothesis, encompassing the full range of warming scenarios projected by the Intergovernmental Panel on Climate Change for the northern region. We took soils from these geothermal sites 7 years after the onset of warming and incubated them at varying temperatures and substrate availability conditions to detect persistent alterations of microbial physiology to long-term warming. Seven years of continuous warming ranging from 1.8 to 15.9 °C triggered a 8.6–58.0% decrease on the C concentrations in the topsoil (0–10 cm) of these sub-arctic silt-loam Andosols. The sensitivity of microbial respiration to temperature (Q10) was not altered. However, soil microbes showed a persistent increase in their microbial metabolic quotients (microbial respiration per unit of microbial biomass) and a subsequent diminished C retention in biomass. After an initial depletion of labile soil C upon soil warming, increasing energy costs of metabolic maintenance and resource acquisition led to a weaker capacity of C stabilization in the microbial biomass of warmer soils. This mechanism contributes to our understanding of the acclimated response of soil respiration to in situ soil warming at the ecosystem level, despite a lack of acclimation at the physiological level. Persistent increases in the respiratory costs of soil microbes in response to warming constitute a fundamental process that should be incorporated into climate change-C cycling models. © 2018 Springer International Publishing AG, part of Springer Nature

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Revisiting the role of high-energy Pacific events in the environmental and cultural history of Easter Island (Rapa Nui)

Margalef O., Álvarez-Gómez J.A., Pla-Rabes S., Cañellas-Boltà N., Rull V., Sáez A., Geyer A., Peñuelas J., Sardans J., Giralt S. (2018) Revisiting the role of high-energy Pacific events in the environmental and cultural history of Easter Island (Rapa Nui). Geographical Journal. : 0-0.
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Doi: 10.1111/geoj.12253

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Pacific islands are spread over thousands of kilometres of the Pacific Basin and are characterised by similar ecological features but very diverse geologic origins, from steep volcanoes to flat coral atolls. Several climatic phases have been shared across the region within the last 1,000 years. Numerous and abrupt societal and cultural changes during the same period have been described for islands separated by thousands of kilometres. Conspicuous societal changes have been exclusively attributed to the main climatic patterns (changes in precipitation and temperature). The possible role of tsunamis and the occurrence of large volcanic eruptions as regional societal modulators, however, have traditionally received little attention from archaeologists, mainly due to the difficulty of recognising them in the sedimentary and geomorphological records. We explore the potential influence of the most important high-energy events in the Pacific on Polynesian societal changes, with a special focus on Easter Island. For example, the extreme Samalas eruption in AD 1257 may have been an indirect driver of the sudden population decline, land degradation and decreased food resources on many Pacific islands between AD 1250 and 1300, and the Kuwae eruption in AD 1450 may have triggered the synchronous end of long voyaging expeditions across the Pacific. Important palaeo-tsunamis have had unquestionable impacts on coastal and seafaring societies. A direct effect of the main eruptions of the last millennia (AD 1257 and 1453) on Easter Island has not yet been identified by any record, but we have calculated the likelihood of destructive tsunamis with an estimated period of recurrence for large events of less than a century. This insight is new and needs to be taken into account to complement what we already know about Easter Island's cultural history and archaeological sites, especially those in vulnerable coastal locations. © 2018 Royal Geographical Society (with the Institute of British Geographers).

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A methodology to derive global maps of leaf traits using remote sensing and climate data

Moreno-Martínez Á., Camps-Valls G., Kattge J., Robinson N., Reichstein M., van Bodegom P., Kramer K., Cornelissen J.H.C., Reich P., Bahn M., Niinemets Ü., Peñuelas J., Craine J.M., Cerabolini B.E.L., Minden V., Laughlin D.C., Sack L., Allred B., Baraloto C., Byun C., Soudzilovskaia N.A., Running S.W. (2018) A methodology to derive global maps of leaf traits using remote sensing and climate data. Remote Sensing of Environment. 218: 69-88.
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Doi: 10.1016/j.rse.2018.09.006

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This paper introduces a modular processing chain to derive global high-resolution maps of leaf traits. In particular, we present global maps at 500 m resolution of specific leaf area, leaf dry matter content, leaf nitrogen and phosphorus content per dry mass, and leaf nitrogen/phosphorus ratio. The processing chain exploits machine learning techniques along with optical remote sensing data (MODIS/Landsat) and climate data for gap filling and up-scaling of in-situ measured leaf traits. The chain first uses random forests regression with surrogates to fill gaps in the database (> 45% of missing entries) and maximizes the global representativeness of the trait dataset. Plant species are then aggregated to Plant Functional Types (PFTs). Next, the spatial abundance of PFTs at MODIS resolution (500 m) is calculated using Landsat data (30 m). Based on these PFT abundances, representative trait values are calculated for MODIS pixels with nearby trait data. Finally, different regression algorithms are applied to globally predict trait estimates from these MODIS pixels using remote sensing and climate data. The methods were compared in terms of precision, robustness and efficiency. The best model (random forests regression) shows good precision (normalized RMSE≤ 20%) and goodness of fit (averaged Pearson's correlation R = 0.78) in any considered trait. Along with the estimated global maps of leaf traits, we provide associated uncertainty estimates derived from the regression models. The process chain is modular, and can easily accommodate new traits, data streams (traits databases and remote sensing data), and methods. The machine learning techniques applied allow attribution of information gain to data input and thus provide the opportunity to understand trait-environment relationships at the plant and ecosystem scales. The new data products – the gap-filled trait matrix, a global map of PFT abundance per MODIS gridcells and the high-resolution global leaf trait maps – are complementary to existing large-scale observations of the land surface and we therefore anticipate substantial contributions to advances in quantifying, understanding and prediction of the Earth system. © 2018 Elsevier Inc.

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Seasonal and diurnal variations of plant isoprenoid emissions from two dominant species in Mediterranean shrubland and forest submitted to experimental drought

Mu Z., Llusià J., Liu D., Ogaya R., Asensio D., Zhang C., Peñuelas J. (2018) Seasonal and diurnal variations of plant isoprenoid emissions from two dominant species in Mediterranean shrubland and forest submitted to experimental drought. Atmospheric Environment. 191: 105-115.
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Doi: 10.1016/j.atmosenv.2018.08.010

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We tested the effect of increasing drought conditions in the Mediterranean Basin on isoprenoid emissions for the coming decades by analyzing their effect experimentally on the dominant Mediterranean species Erica multiflora in a Garraf shrubland and Quercus ilex in a Prades forest in Catalonia (Spain). Drought was simulated in Garraf using automatically sliding curtains to decrease the amount of soil moisture by 5% and in Prades by partial rainfall exclusion and runoff exclusion for a 25% decrease. We measured photosynthetic rates (A), stomatal conductance (gs) and rates of isoprenoid emission in the morning and at midday for four seasons and determined the relationship of emission rates with environmental conditions. Terpenes were emitted by both species, but only E. multiflora emitted isoprene. α-Pinene and limonene were the most abundant terpenes. Isoprenoid emissions increased with air temperature and generally decreased as the amount of soil moisture increased. The results of this study suggest that higher isoprenoid emissions can be expected in the warmer and drier conditions predicted for the coming decades in the Mediterranean region. © 2018

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Ecosystem thresholds, tipping points, and critical transitions

Munson S.M., Reed S.C., Peñuelas J., McDowell N.G., Sala O.E. (2018) Ecosystem thresholds, tipping points, and critical transitions. New Phytologist. 218: 1315-1317.
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Doi: 10.1111/nph.15145

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[No abstract available]

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Thirsty tree roots exude more carbon

Preece C., Farré-Armengol G., Llusià J., Peñuelas J. (2018) Thirsty tree roots exude more carbon. Tree Physiology. 38: 690-695.
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Doi: 10.1093/treephys/tpx163

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Root exudation is an important input of carbon into soils and affects plant and soil communities, but little is known about the effect of climatic factors such as drought on exudation, and its ability to recover. We studied the impact of increasing drought on root exudation and its subsequent recovery in the Mediterranean tree species Quercus ilex L. in a greenhouse study by measuring the amount of total organic carbon in exudates. The amount of exudation per unit root area increased with drought duration and was 21% higher under the most extreme drought scenario compared with the non-droughted control. The amount of root exudation did not differ between the treatments following 6 weeks of re-watering, indicating a strong capacity for recovery in this species. We concluded that drought could affect the amount of root exudation, which could in turn have a large impact on microbial activity in the rhizosphere, and alter these microbial communities, at least in the short term. This tree species may be able to return to normal levels of root exudation after a drought event, but long-term exudate-mediated impacts on Mediterranean forest soils may be an unforeseen effect of drought. © The Author(s) 2018. Published by Oxford University Press. All rights reserved.

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