Automated measurements of greenhouse gases fluxes from tree stems and soils: magnitudes, patterns and drivers

Barba J., Poyatos R., Vargas R. (2019) Automated measurements of greenhouse gases fluxes from tree stems and soils: magnitudes, patterns and drivers. Scientific Reports. 9: 0-0.
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Doi: 10.1038/s41598-019-39663-8

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Tree stems exchange CO 2 , CH 4 and N 2 O with the atmosphere but the magnitudes, patterns and drivers of these greenhouse gas (GHG) fluxes remain poorly understood. Our understanding mainly comes from static-manual measurements, which provide limited information on the temporal variability and magnitude of these fluxes. We measured hourly CO 2 , CH 4 and N 2 O fluxes at two stem heights and adjacent soils within an upland temperate forest. We analyzed diurnal and seasonal variability of fluxes and biophysical drivers (i.e., temperature, soil moisture, sap flux). Tree stems were a net source of CO 2 (3.80 ± 0.18 µmol m −2 s −1 ; mean ± 95% CI) and CH 4 (0.37 ± 0.18 nmol m −2 s −1 ), but a sink for N 2 O (−0.016 ± 0.008 nmol m −2 s −1 ). Time series analysis showed diurnal temporal correlations between these gases with temperature or sap flux for certain days. CO 2 and CH 4 showed a clear seasonal pattern explained by temperature, soil water content and sap flux. Relationships between stem, soil fluxes and their drivers suggest that CH 4 for stem emissions could be partially produced belowground. High-frequency measurements demonstrate that: a) tree stems exchange GHGs with the atmosphere at multiple time scales; and b) are needed to better estimate fluxes magnitudes and understand underlying mechanisms of GHG stem emissions. © 2019, The Author(s).

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A synthesis of bias and uncertainty in sap flow methods

Flo V., Martinez-Vilalta J., Steppe K., Schuldt B., Poyatos R. (2019) A synthesis of bias and uncertainty in sap flow methods. Agricultural and Forest Meteorology. 271: 362-374.
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Doi: 10.1016/j.agrformet.2019.03.012

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Sap flow measurements with thermometric methods are widely used to measure transpiration in plants. Different method families exist depending on how they apply heat and track sapwood temperature (heat pulse, heat dissipation, heat field deformation or heat balance). These methods have been calibrated for many species, but a global assessment of their uncertainty and reliability has not yet been conducted. Here we perform a meta-analysis of 290 individual calibration experiments assembled from the literature to assess calibration performance and how this varies across methods, experimental conditions and wood properties (density and porosity types). We used different metrics to characterize mean accuracy (closeness of the measurements to the true, reference value), proportional bias (resulting from an effect of measured flow on the magnitude of the error), linearity in the relationship between measurements and reference values, and precision (reproducibility and repeatability). We found a large intra- and inter-method variability in calibration performance, with a low proportion of this variability explained by species. Calibration performance was best when using stem segments. We did not find evidence of strong effects of wood density or porosity type in calibration performance. Dissipation methods showed lower accuracy and higher proportional bias than the other methods but they showed relatively high linearity and precision. Pulse methods also showed significant proportional bias, driven by their overestimation of low flows. These results suggest that Dissipation methods may be more appropriate to assess relative sap flow (e.g., treatment effects within a study) and Pulse methods may be more suitable to quantify absolute flows. Nevertheless, all sap flow methods showed high precision, allowing potential correction of the measurements when a study-specific calibration is performed. Our understanding of how sap flow methods perform across species would be greatly improved if experimental conditions and wood properties, including changes in wood moisture, were better reported. © 2019 Elsevier B.V.

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Correction to: Below-ground hydraulic constraints during drought-induced decline in Scots pine (Annals of Forest Science, (2018), 75, 4, (100), 10.1007/s13595-018-0778-7)

Poyatos R., Aguadé D., Martínez-Vilalta J. (2019) Correction to: Below-ground hydraulic constraints during drought-induced decline in Scots pine (Annals of Forest Science, (2018), 75, 4, (100), 10.1007/s13595-018-0778-7). Annals of Forest Science. 76: 0-0.
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Doi: 10.1007/s13595-019-0825-z

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The article was published without the submitted data availability statement linking readers to a public repository. Due to publication modifications, the information appears missing in the original article. The following corrects previous version of the statement: Data availability The datasets generated and/or analysed during the current study are available in Zenodo Repository (Poyatos et al. 2018). The datasets were not peer reviewed. The original article has been corrected. © 2019, INRA and Springer-Verlag France SAS, part of Springer Nature.

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