The 2018 European heatwave led to stem dehydration but not to consistent growth reductions in forests

Heatwaves exert disproportionately strong and sometimes irreversible impacts on forest ecosystems. These impacts remain poorly understood at the tree and species level and across large spatial scales. Here, we investigate the effects of the record-breaking 2018 European heatwave on tree growth and tree water status using a collection of high-temporal resolution dendrometer data from 21 species across 53 sites. Relative to the two preceding years, annual stem growth was not consistently reduced by the 2018 heatwave but stems experienced twice the temporary shrinkage due to depletion of water reserves. Conifer species were less capable of rehydrating overnight than broadleaves across gradients of soil and atmospheric drought, suggesting less resilience toward transient stress. In particular, Norway spruce and Scots pine experienced extensive stem dehydration. Our high-resolution dendrometer network was suitable to disentangle the effects of a severe heatwave on tree growth and desiccation at large-spatial scales in situ, and provided insights on which species may be more vulnerable to climate extremes

The 2018 European heatwave led to stem dehydration but not to consistent growth reductions in forests

Publisher Copyright: © 2022, The Author(s).Heatwaves exert disproportionately strong and sometimes irreversible impacts on forest ecosystems. These impacts remain poorly understood at the tree and species level and across large spatial scales. Here, we investigate the effects of the record-breaking 2018 European heatwave on tree growth and tree water status using a collection of high-temporal resolution dendrometer data from 21 species across 53 sites. Relative to the two preceding years, annual stem growth was not consistently reduced by the 2018 heatwave but stems experienced twice the temporary shrinkage due to depletion of water reserves. Conifer species were less capable of rehydrating overnight than broadleaves across gradients of soil and atmospheric drought, suggesting less resilience toward transient stress. In particular, Norway spruce and Scots pine experienced extensive stem dehydration. Our high-resolution dendrometer network was suitable to disentangle the effects of a severe heatwave on tree growth and desiccation at large-spatial scales in situ, and provided insights on which species may be more vulnerable to climate extremes.Peer reviewe

Impact of hydrophobic coating on mass transport losses in PEFCs

\u3cp\u3eExperimental results about the impact of hydrophobic coating inside commercial gas diffusion layers (GDL) of polymer electrolyte fuel cells (PEFC) were obtained with four different combinations of PTFE-loadings on both anode and cathode. The measurements were performed on differential PEFCs which were characterized with high resolution neutron radiography (NR) combined with two electrochemical methods: a pulsed gas analysis (PGA) and limiting current density measurements. Using the combination of the neutron radiographs with these two electrochemical measurements, a new insight on the impact of water on mass transport losses was obtained. Under high humidification conditions, GDLs without hydrophobic coating do not accumulate more water than coated GDLs, but the water distributes differently and accumulates in the region under the channel of the flow field. Cells with high PTFE coating show higher mass transport losses despite the relatively small quantity of water, which can be explained by a different morphology of water accumulation.\u3c/p\u3

Impact of Hydrophobic Coating on Mass Transport Losses in PEFCs

Experimental results about the impact of hydrophobic coating inside commercial gas diffusion layers (GDL) of polymer electrolyte fuel cells (PEFC) were obtained with four different combinations of PTFE-loadings on both anode and cathode. The measurements were performed on differential PEFCs which were characterized with high resolution neutron radiography (NR) combined with two electrochemical methods: a pulsed gas analysis (PGA) and limiting current density measurements. Using the combination of the neutron radiographs with these two electrochemical measurements, a new insight on the impact of water on mass transport losses was obtained. Under high humidification conditions, GDLs without hydrophobic coating do not accumulate more water than coated GDLs, but the water distributes differently and accumulates in the region under the channel of the flow field. Cells with high PTFE coating show higher mass transport losses despite the relatively small quantity of water, which can be explained by a different morphology of water accumulation.ISSN:0013-4651ISSN:1945-711

Statistical Analysis of Isothermal Cold Starts of PEFCs: Impact of Gas Diffusion Layer Properties

In this paper, we present experimental results about the influence of a hydrophobic coating of gas diffusion layers (GDL) and the existence of a micro porous layer (MPL) on the cold start capability of Polymer Electrolyte Fuel Cells (PEFC). The experiments were performed on five different cell configurations with an active area of 1 cm2 including GDLs of 0, 5, 20 wt% PTFE with and without MPL. Two different experiments were realized: First, a statistical analysis of more than 700 isothermal cold starts was performed to analyze the stochastic freezing behavior of supercooled water inside the fuel cells. Second, a certain number of cold starts were investigated with high-resolution neutron radiography in order to understand the implications of the water distribution on the freezing mechanism. Especially at low coating loads, it was found that cell-to-cell variations are more dominant than variations of the materials. In contrast, a significantly reduced variability between the individual cells as well as a general reduction of the probability of cell failure was observed at high coating loads. The observed effects can be mainly explained by changed morphology of the investigated materials. Regarding the existence of an MPL, only minor effects were observed which could be assigned to a different local distribution of water in the porous layers.ISSN:0013-4651ISSN:1945-711

Advanced water management in PEFCs: diffusion layers with patterned wettability::III. operando characterization with neutron imaging

\u3cp\u3eFollowing our two previous publications on material synthesis and on ex situ characterization, we present an experimental in situ study to evaluate the effects of using gas diffusion layers with patterned wettability at the cathode side of polymer electrolyte fuel cells. The operando performance was assessed using traditional electrochemical diagnostics (such as polarization curves) combined with the pulsed gas analysis (PGA) method, which allows measuring the mass transport losses. Neutron radiography was performed simultaneously in order to image the water distribution during operation. Using this methodology, the effects of changing the pattern, including a microporous layer (MPL), and varying the operation conditions (temperature and relative humidity of the cathode gas) have been systematically evaluated. It has been confirmed that water redistributes according to the engineered pattern and that the power density is significantly increased thanks to reduced mass transport losses under various conditions.\u3c/p\u3

Advanced water management in PEFCs: Diffusion layers with patterned wettability: III. Operando characterization with neutron imaging

Following our two previous publications on material synthesis and on ex situ characterization, we present an experimental in situ study to evaluate the effects of using gas diffusion layers with patterned wettability at the cathode side of polymer electrolyte fuel cells. The operando performance was assessed using traditional electrochemical diagnostics (such as polarization curves) combined with the pulsed gas analysis (PGA) method, which allows measuring the mass transport losses. Neutron radiography was performed simultaneously in order to image the water distribution during operation. Using this methodology, the effects of changing the pattern, including a microporous layer (MPL), and varying the operation conditions (temperature and relative humidity of the cathode gas) have been systematically evaluated. It has been confirmed that water redistributes according to the engineered pattern and that the power density is significantly increased thanks to reduced mass transport losses under various conditions.ISSN:0013-4651ISSN:1945-711

Engineered water highways in fuel cells:radiation grafting of gas diffusion layers

\u3cp\u3eA novel method to produce gas diffusion layers with patterned wettability for fuel cells is presented. The local irradiation and subsequent grafting permits full design flexibility and wettability tuning, while modifying throughout the whole material thickness. These water highways have improved operando performance due to an optimized water management inside the cells.\u3c/p\u3