Evaluation of eddy covariance latent heat fluxes with independent lysimeter and sapflow estimates in a Mediterranean savannah ecosystem

Pérez-Priego, Óscar; El-Madany, Tarek S.; Migliavacca, Mirco; Kowalski, Andrew S.; Martín, M. Pilar; Carrara, Arnaud; Kolle, Olaf; Martín, M. Pilar; Pacheco‐Labrador, Javier; Moreno, Gerardo; Reichstein, Markus

doi:10.1016/j.agrformet.2017.01.009

Cited by 72 publications

(77 citation statements)

References 69 publications

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“…The yellowing grasses in senescence lose their vigor during the dry and hot summer. Being that the VIs derived from the tree are relatively constant during the year, grasses contribute a large proportion of VIs ( Figure A6) and GPP in rainy seasons, but trees contribute more during dry periods [49].…”

Section: Characterizing Variatoin and Drivers Of Structural And Physimentioning

confidence: 99%

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Using Near-Infrared-Enabled Digital Repeat Photography to Track Structural and Physiological Phenology in Mediterranean Tree–Grass Ecosystems

Luo

El-Madany

Filippa³

et al. 2018

Remote Sensing

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Tree-grass ecosystems are widely distributed. However, their phenology has not yet been fully characterized. The technique of repeated digital photographs for plant phenology monitoring (hereafter referred as PhenoCam) provide opportunities for long-term monitoring of plant phenology, and extracting phenological transition dates (PTDs, e.g., start of the growing season). Here, we aim to evaluate the utility of near-infrared-enabled PhenoCam for monitoring the phenology of structure (i.e., greenness) and physiology (i.e., gross primary productivity-GPP) at four tree-grass Mediterranean sites. We computed four vegetation indexes (VIs) from PhenoCams: (1) green chromatic coordinates (GCC), (2) normalized difference vegetation index (CamNDVI), (3) near-infrared reflectance of vegetation index (CamNIRv), and (4) ratio vegetation index (CamRVI). GPP is derived from eddy covariance flux tower measurement. Then, we extracted PTDs and their uncertainty from different VIs and GPP. The consistency between structural (VIs) and physiological (GPP) phenology was then evaluated. CamNIRv is best at representing the PTDs of GPP during the Green-up period, while CamNDVI is best during the Dry-down period. Moreover, CamNIRv outperforms the other VIs in tracking growing season length of GPP. In summary, the results show it is promising to track structural

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Section: Characterizing Variatoin and Drivers Of Structural And Physimentioning

confidence: 99%

“…650 mm and 400 mm, respectively. The rain falls typically from November to May with a very dry summer [49].…”

Section: Sites Description Instrument Set-up and Data Sourcesmentioning

confidence: 99%

Using Near-Infrared-Enabled Digital Repeat Photography to Track Structural and Physiological Phenology in Mediterranean Tree–Grass Ecosystems

Luo

El-Madany

Filippa³

et al. 2018

Remote Sensing

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“…Ciais et al, 2005;Zhao and Running, 2010;Wolf et al, 2013;Zscheischler et al, 2014a, d, c) where water stress directly forces plants to close their stomata to limit transpiration, reducing photosynthesis. Similarly, Jung et al (2017) found that water availability is a much bigger control on the inter annual variability of GPP (IAV, which is controlled to a large degree by extreme events) compared to a smaller temperature control on a global In contrast to the small response of GPP to heat, however, R eco generally increased during most high temperature extreme events (Fig. 3).…”

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confidence: 99%

“…Finally, we aim to bridge the gap between local site level studies and global assessments,which most often are based on models (e.g. Cramer et al, 1999;Friedlingstein et al, 2006) or upscaling studies (Beer et al, 2010;Jung et al, 2017) by providing some helpful benchmarks for the models and their underlying assumptions (Canadell et al, 2000;Williams et al, 2009). …”

Section: Objectives Of This Studymentioning

confidence: 99%

Impacts of droughts and extreme temperature events on gross primary production and ecosystem respiration: a systematic assessment across ecosystems and climate zones

Buttlar¹,

Zscheischler²,

Rammig³

et al. 2017

Preprint

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Abstract.Extreme climatic events, such as droughts and heat stress induce anomalies in ecosystem-atmosphere CO 2 fluxes, such as gross primary production (GPP) and ecosystem respiration (R eco ), and, hence, can change the net ecosystem carbon balance.However, despite our increasing understanding of the underlying mechanisms, the magnitudes of the impacts of different types of extremes on GPP and R eco within and between ecosystems remain poorly predicted. Here we aim to identify the major 5 factors controlling the amplitude of extreme event impacts on GPP, R eco , and the resulting net ecosystem production (NEP).We focus on the impacts of heat and drought and their combination. We identified hydrometeorological extreme events in consistently downscaled water availability and temperature measurements over a 30 year time period. We then used FLUXNET eddy-covariance flux measurements to estimate the CO 2 flux anomalies during these extreme events across dominant vegetation types and climate zones. Overall, our results indicate that short-term heat extremes increased respiration more strongly than 10 they down-regulated GPP, resulting in a moderate reduction of the ecosystem's carbon sink potential. In the absence of heat stress, droughts tended to have smaller and similarly dampening effects on both GPP and R eco , and, hence, often resulted in neutral NEP responses. The combination of drought and heat typically led to a strong decrease in GPP, whereas heat and drought impacts on respiration partially offset each other. Taken together, compound heat and drought events led to the strongest C sink reduction compared to any single-factor extreme. A key insight of this paper, however, is that duration matters most: 15 for heat stress during droughts, the magnitude of impacts systematically increased with duration, whereas under heat stress without drought, the response of R eco over time turned from an initial increase to a down-regulation after about two weeks.This confirms earlier theories that not only the magnitude but also the duration of an extreme event determines its impact. Our study corroborates the results of several local site-level case studies, but as a novelty generalizes these findings at the global scale. Specifically, we find that the different response functions of the two antipodal land-atmosphere fluxes GPP and R eco can 20 also result in increasing NEP during certain extreme conditions. Apparently counterintuitive findings of this kind bear great potential for scrutinizing the mechanisms implemented in state-of-the-art terrestrial biosphere models and provide a benchmark for future model development and testing.

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“…Their findings were confirmed by a comprehensive cross-validation analysis using FLUXNET 2 towers. In the latest study of Jung et al (2017), they investigate the dependencies of changes in temperature and water availability on the interannual variability of carbon fluxes both locally and globally using their upscaled data products and process-based global land models.…”

Section: Deals With Empirical Upscaling Of Monthly Average Values Of mentioning

confidence: 99%

Upscaled diurnal cycles of land-atmosphere fluxes: a new global half-hourly data product

Bodesheim¹,

Jung²,

Gans³

et al. 2018

Preprint

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Abstract. Interactions between the biosphere and the atmosphere can be well characterized by fluxes between the two. In particular, carbon and energy fluxes play a major role for understanding biogeochemical processes on ecosystem level or global scale. However, the fluxes can only be measured at individual sites, e.g., by eddy covariance towers, and an upscaling of We analyze the results for a set of FLUXNET tower sites showing the suitability of our approaches for this upscaling task.Finally, we have selected one approach to calculate the global half-hourly data products based on predictor variables from remote sensing and meteorology at daily resolution as well as half-hourly potential radiation. In addition, we provide a derived product that only contains monthly average diurnal cycles, which is a lightweight version in terms of data storage that still enables to study the important characteristics of diurnal courses globally. We recommend to primarily use these monthly 15 average diurnal cycles, because they are less affected by the impacts of day-to-day variation, observation noise, and shortterm fluctuations on subdaily scales compared to the plain half-hourly flux products. The global half-hourly data products are available at https://doi.org/10

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Evaluation of eddy covariance latent heat fluxes with independent lysimeter and sapflow estimates in a Mediterranean savannah ecosystem

Cited by 72 publications

References 69 publications

Using Near-Infrared-Enabled Digital Repeat Photography to Track Structural and Physiological Phenology in Mediterranean Tree–Grass Ecosystems

Using Near-Infrared-Enabled Digital Repeat Photography to Track Structural and Physiological Phenology in Mediterranean Tree–Grass Ecosystems

Impacts of droughts and extreme temperature events on gross primary production and ecosystem respiration: a systematic assessment across ecosystems and climate zones

Upscaled diurnal cycles of land-atmosphere fluxes: a new global half-hourly data product

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