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Serrano-Ortiz P.,CSIC - Estacion Experimental De Zonas Aridas | Serrano-Ortiz P.,Instituto Interuniversitario del Sistema Tierra en Andalucia | Oyonarte C.,University of Almeria | Perez-Priego O.,Instituto Interuniversitario del Sistema Tierra en Andalucia | And 8 more authors.
Oecologia | Year: 2014

Climate change may alter ecosystem functioning, as assessed via the net carbon (C) exchange (NEE) with the atmosphere, composed of the biological processes photosynthesis (GPP) and respiration (R eco). In addition, in semi-arid Mediterranean ecosystems, a significant fraction of respired CO2 is stored in the vadose zone and emitted afterwards by subsoil ventilation (VE), contributing also to NEE. Such conditions complicate the prediction of NEE for future change scenarios. To evaluate the possible effects of climate change on annual NEE and its underlying processes (GPP, R eco and VE) we present, over a climate/altitude range, the annual and interannual variability of NEE, GPP, R eco and VE in three Mediterranean sites. We found that annual NEE varied from a net source of around 130 gC m-2 in hot and arid lowlands to a net sink of similar magnitude for alpine meadows (above 2,000 m a.s.l) that are less water stressed. Annual net C fixation increased because of increased GPP during intermittent and several growth periods occurring even during winter, as well as due to decreased VE. In terms of interannual variability, the studied subalpine site behaved as a neutral C sink (from emission of 49 to fixation of 30 gC m-2 year-1), with precipitation as the main factor controlling annual GPP and R eco. Finally, the importance of VE as 0-23 % of annual NEE is highlighted, indicating that this process could shift some Mediterranean ecosystems from annual C sinks to sources. © 2014 Springer-Verlag Berlin Heidelberg.

Perez-Priego O.,University of Granada | Perez-Priego O.,Max Planck Institute for Biogeochemistry | Lopez-Ballesteros A.,University of Granada | Lopez-Ballesteros A.,CSIC - Estacion Experimental De Zonas Aridas | And 8 more authors.
Plant and Soil | Year: 2015

Aims: Gas exchange measurements on individual plants depend largely on chamber systems, and uncertainties and corrections in current flux calculation procedures require further assessment. Methods: We present a practical study with novel methods for analyses of flux uncertainties in an original chamber design excluding soil fluxes and allowing simultaneous measurements of whole-plant photosynthesis and transpiration. Results: Results indicate that random errors caused by IRGA noise and the lack of criteria to optimize the time window (TW) of chamber enclosure lead to significant flux uncertainties (12 %). Although enclosure should be rapid to minimize plant disturbances, longer TWs (3 min) increase confidence in flux estimates. Indeterminate stabilization periods in existing calculation protocols cause significant systematic errors. Stabilization times were identified via the change-point detection method, and flux uncertainties were reduced. Photosynthesis was overestimated by up to 28 % when not correcting the evolving CO2 molar fraction for water vapour dilution. Leakage can compromise flux estimates, but was negligible (ca. 2 %) here due to the large chamber-headspace and relatively small values of both collar contact length and closure time. Conclusions: A bootstrapping, resampling-based flux calculation method is presented and recommended to better assess random errors and improve flux precision. We present practical recommendations for the use of whole-plant chambers. © 2015, Springer International Publishing Switzerland.

Cazorla A.,Instituto Interuniversitario del Sistema Tierra en Andalucia | Cazorla A.,University of Granada | Husillos C.,Institute Astrofisica Of Andalucia | Anton M.,University of Extremadura | And 2 more authors.
Solar Energy | Year: 2015

Sky imagers have been used for cloud detection and classification in the last years, and one of the applications of these instruments is the use of cloud information in forecast algorithms for solar power technologies. These algorithms depend on an accurate classification of the complete sky dome cloud cover, but most system fail in the proximity of the sun due to saturation in the images. This work proposes a new method for cloud detection with sky imagers using images taken with different exposure times and applying an adaptive threshold to each one. The use of multiple exposure times avoids the saturation of the image in the vicinity of the sun position, while the adaptive threshold applied to the images helps in the accurate detection of cloud coverage, especially in the circumsolar area. The method is tested with a commercial sky imager, paying special attention to the detection of clouds close to the sun position. A case study is analyzed, showing an accurate detection of clouds in the vicinity of the sun. The method is also validated using statistical values for data recorded during almost one month which cover a great variety of cloudiness cases. For this purpose, the detection of clouds in the sun position is compared against the reduction of the direct normal irradiance (DNI) with respect to a modeled DNI. © 2015 Elsevier Ltd.

Perez-Priego O.,CSIC - Institute for Sustainable Agriculture | Perez-Priego O.,Instituto Interuniversitario del Sistema Tierra en Andalucia | Perez-Priego O.,University of Granada | Testi L.,CSIC - Institute for Sustainable Agriculture | And 5 more authors.
Agroforestry Systems | Year: 2014

The accurate assessment of respiration by woody vegetation, still a challenge in plant productivity models, is generally a problem of correctly scaling-up the process from organs to the whole plant. We used a large (41.6 m3), canopy chamber to enclose mature olive trees and to measure aboveground respiration (R ag) under natural environmental conditions in an irrigated olive orchard in Córdoba (Spain). The 3-year study assessed nocturnal and seasonal R ag patterns in terms of temperature (T), plant dry matter composition, and phenology. The relative contributions of maintenance and growth respiration to R ag were determined empirically via an independent experiment. Although short-term variations in R ag rates were explained mainly by T variations, over seasonal time-scales this relationship was modulated by the vegetative composition of the olive trees and the contribution of growth respiration to R ag when the plants, in different seasons, allocated most of the new assimilates to actively growing shoots, flowers or fruits. Leaf mass and fruit load were the main determinants of R ag, which was weakly affected by differences in woody biomass since woody tissue respiration accounted for just 15 % of R ag. Respiration in olive trees during fruit setting periods is composed of approximately 30 % growth and 70 % maintenance. This study provides an independent evaluation of how, and to what degree, seasonally varying plant organ composition determines total respiration. Improved modelling of ecosystem respiration can be achieved by accounting for plant biological patterns characterising energy-requiring growth and maintenance processes, since biochemical kinetics alone cannot explain the observed seasonal variability. © 2014 Springer Science+Business Media Dordrecht.

Serrano-Ortiz P.,University of Granada | Serrano-Ortiz P.,Instituto Interuniversitario del Sistema Tierra en Andalucia | Were A.,CSIC - Estacion Experimental De Zonas Aridas | Reverter B.R.,Federal University of Paraiba | And 5 more authors.
Journal of Arid Environments | Year: 2015

In the present climate change context it is important to understand the carbon balance seasonality of Mediterranean areas, that will suffer important changes in precipitation according to the last climate change predictions. This work analyzed the seasonality of carbon exchanges of three Mediterranean ecosystems according to a variety of water and temperature regimes due to differences in altitude (alpine, subalpine and lowland). Results show that the timing and duration of the growing season depended on temperature at the alpine site, while the dependence on water availability increased as altitude decreased. Thus, maximum values of net carbon uptake occurred in late spring for the alpine and subalpine sites (up to 60 and 30 gC m-2 month-1 respectively) whereas the lowland site absorbed carbon throughout winter (up to 30 gC m-2 month-1). Similarly increases in aridity conditions resulted in monthly increases in carbon emissions during dry periods. Thus from May to October, the lowland emitted up to 60°gC°m-2°month-1, the subalpine emitted half that with a delay of two months, whereas the alpine site continued with slight uptake sequestration. Finally, the EVI could be used to provide reasonably accurate estimates of photosynthesis (R2 around 0.6) but this relation varies depending on the site. © 2014 Elsevier Ltd.

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