Eddy covariance measurements of the forest floor CO2 exchange in two contrasting forest stands in boreal Sweden

<p>Boreal forests cover a large portion of land surface area in the northern hemisphere and greatly affect the global carbon (C) cycle and climate. Since these forests exchange carbon dioxide (CO<sub>2</sub>) with the atmosphere in different vertical layers, many different CO<sub>2</sub> sources and sinks exist within the complex forest stand. The forest floor (soil and understory vegetation) may act as an important component of the C budget in a forest stand, however its contribution may vary from negligible to determining the inter-annual variability of ecosystem C balance. To date, there are a limited number of studies that have directly quantified the CO<sub>2</sub> fluxes over a forest floor using the eddy covariance (EC) method primarily due to challenges and potential violation of underlying assumptions when applying this method in the trunk space where turbulence characteristics are complicated, intermittent, and not in accordance with universal theories.</p><p>In this study, we installed two identical EC flux systems at two contrasting boreal forests (sparse pine stand vs. a dense mixed pine-spruce stand) in Sweden to measure the forest floor CO<sub>2</sub> exchange. We developed site-specific ideal cospectral models for the below-canopy fluxes in the trunk space under the well-mixed condition as defined by the standard deviation of the vertical wind speed. Spectral correction to the half-hourly fluxes was performed based on the newly fitted cospectral models at each site. Chamber measurements of CO<sub>2</sub> fluxes during the growing season were conducted to compare with the estimates from the below-canopy EC data.</p><p>Our below-canopy cospectral models show that more high-frequency signals (small eddies) occurred in the forest trunk space compared to the ideal above-canopy cospectral model. The high-frequency contribution was greater in the dense pine-spruce forest compared to the open pine stand. The spectral corrected CO<sub>2</sub> fluxes measured by the EC method agreed well with the concurrent chamber measurements. The EC results revealed that the forest floor of the two contrasting stands acted as net CO<sub>2</sub> sources during the 3-year period (2017-2019). This study highlights that by applying a data correction based on site-specific below-canopy cospectral models, the EC method can be used in the trunk space to accurately measure the net CO<sub>2</sub> exchange between the forest floor and the atmosphere and thus to improve our understanding of the role of the forest floor in the ecosystem-scale C budget in the boreal forest region.</p>

Neural adaptation accounts for the dynamic resizing of peripersonal space: evidence from a psychophysical-computational approach

Interactions between the body and the environment occur within the peripersonal space (PPS), the space immediately surrounding the body. The PPS is encoded by multisensory (audio-tactile, visual-tactile) neurons that possess receptive fields (RFs) anchored on the body and restricted in depth. The extension in depth of PPS neurons’ RFs has been documented to change dynamically as a function of the velocity of incoming stimuli, but the underlying neural mechanisms are still unknown. Here, by integrating a psychophysical approach with neural network modeling, we propose a mechanistic explanation behind this inherent dynamic property of PPS. We psychophysically mapped the size of participant’s peri-face and peri-trunk space as a function of the velocity of task-irrelevant approaching auditory stimuli. Findings indicated that the peri-trunk space was larger than the peri-face space, and, importantly, as for the neurophysiological delineation of RFs, both of these representations enlarged as the velocity of incoming sound increased. We propose a neural network model to mechanistically interpret these findings: the network includes reciprocal connections between unisensory areas and higher order multisensory neurons, and it implements neural adaptation to persistent stimulation as a mechanism sensitive to stimulus velocity. The network was capable of replicating the behavioral observations of PPS size remapping and relates behavioral proxies of PPS size to neurophysiological measures of multisensory neurons’ RF size. We propose that a biologically plausible neural adaptation mechanism embedded within the network encoding for PPS can be responsible for the dynamic alterations in PPS size as a function of the velocity of incoming stimuli. NEW & NOTEWORTHY Interactions between body and environment occur within the peripersonal space (PPS). PPS neurons are highly dynamic, adapting online as a function of body-object interactions. The mechanistic underpinning PPS dynamic properties are unexplained. We demonstrate with a psychophysical approach that PPS enlarges as incoming stimulus velocity increases, efficiently preventing contacts with faster approaching objects. We present a neurocomputational model of multisensory PPS implementing neural adaptation to persistent stimulation to propose a neurophysiological mechanism underlying this effect.

Coupling processes and exchange of energy and reactive and non-reactive trace gases at a forest site – results of the EGER experiment

To investigate the energy, matter and reactive and non-reactive trace gas exchange between the atmosphere and a spruce forest in the German mountain region, two intensive measuring periods were conducted at the FLUXNET site DE-Bay (Waldstein-Weidenbrunnen) in September/October 2007 and June/July 2008. They were part of the project "ExchanGE processes in mountainous Regions" (EGER). Beyond a brief description of the experiment, the main focus of the paper concerns the coupling between the trunk space, the canopy and the above-canopy atmosphere. Therefore, relevant coherent structures were analyzed for different in- and above canopy layers, coupling between layers was classified according to already published procedures, and gradients and fluxes of meteorological quantities as well as concentrations of non-reactive and reactive trace compounds have been sorted along the coupling classes. Only in the case of a fully coupled system, it could be shown, that fluxes measured above the canopy are related to gradients between the canopy and the above-canopy atmosphere. Temporal changes of concentration differences between top of canopy and the forest floor, particularly those of reactive trace gases (NO, NO2, O3, and HONO) could only be interpreted on the basis of the coupling stage. Consequently, only concurrent and vertically resolved measurements of micrometeorological (turbulence) quantities and fluxes (gradients) of trace compounds will lead to a better understanding of the forest-atmosphere interaction.

ExchanGE processes in mountainous Regions (EGER) – overview of design, methods, and first results

To investigate the energy, matter and reactive and non-reactive trace gas exchange between the atmosphere and a spruce forest in the German mountain region, two intensive measuring periods were conducted at the FLUXNET site Waldstein-Weidenbrunnen in September/October 2007 and June/July 2008. They were part of the project "ExchanGE processes in mountainous Regions" (EGER). Beyond a brief description of the experiment and links to the already published results of both experiments, the main focus of the paper is the problem of the coupling of the trunk space, the canopy and the atmosphere. Therefore, the relevant coherent structures were analyzed in different canopy levels and an already published coupling classification was applied to gradients and fluxes. It could be shown that fluxes above the canopy are only related to the gradient between the canopy and the atmosphere in the case of a fully coupled system. Changes in the concentration of especially reactive trace gases (NO-NO2-O3 and HONO) could only be interpreted together with the coupling stage. Finally it was pointed out that the combination of air chemical measurements with micrometeorological turbulence measurements is urgently needed to understand the biosphere-atmosphere interaction.

Aerosol fluxes and dynamics within and above a tropical rainforest in South-East Asia

Atmospheric aerosol measurements were conducted near Danum Valley, in the Malaysian state of Sabah, North-East Borneo, as part of the OP3 and ACES projects, in April and June/July 2008. Here, aerosol fluxes and diurnal variability in and above the rainforest canopy were examined in order to gain an understanding of their behaviour in the surface layer of the South-East Asian rainforest. Aerosol fluxes were calculated by eddy covariance from measurements above the rainforest canopy on the Global Atmosphere Watch (GAW) tower. Upward fluxes were seen on most mornings between 09:00 and 11:00 local time and this could be attributed to venting of the nocturnal boundary layer as it broke up in the morning. Measurements were also conducted within the canopy and trunk space at a nearby site. Profiles in aerosol number concentrations were investigated using GRIMM Optical Particle Counters (OPCs) at various levels within the rainforest canopy and trunk space, as well as a single OPC on a vertically moving platform. These showed an overnight increase in larger particles (1–20 μm) at all levels, but much more prominently near the top of the canopy, which could be attributed to fog formation. At ground level, number concentrations in this size range correlated with enhancements in biological aerosol concentrations, measured using a Wide Issue Bioaerosol Spectrometer (WIBS) located near the forest floor, suggesting that coarse particle number concentrations were dominated by biological aerosols. A comparison of particle number concentrations (in the size range 0.5–1.0 μm) between above canopy and the trunk space showed correlations, despite turbulence data suggesting persistent decoupling between the two measurement sites. These correlations often relied on a shift of the particle time-series against each other, implying a time delay in observations between the sites, which varied according to time of day. This lag time was shortest during the middle of the day by a significant margin. This was not observed for aerosols larger than 1.0 μm. Further evidence of daytime coupling between above canopy and the trunk space in terms of aerosol measurements is implied by comparison of measurements from an Aerosol Mass Spectrometer (AMS) at the GAW tower and simultaneous bag sampling at the in-canopy site, subsequently analysed with the AMS. Transport of particles through the canopy seems to occur through large-scale, sporadic turbulent events, suggesting that the coupling between the canopy space and the air above is due to these ventilation events.

Nitrous oxide emissions from a beech forest floor measured by eddy covariance and soil enclosure techniques

Spring time nitrous oxide (N2O) emissions from an old beech (Fagus sylvatica L.) forest were measured with eddy covariance (EC) and chamber techniques. The aim was to obtain information on the spatial and temporal variability in N2O emissions and link the emissions to soil environmental parameters. Mean N2O fluxes over the five week measurement period were 5.6±1.1, 10±1 and 16±11 μg N m−2 h−1 from EC, automatic chamber and manual chambers, respectively. High temporal variability characterized the EC fluxes in the trunk-space. To reduce this variability, resulting mostly from random uncertainty due to measuring fluxes close to the detection limit, we averaged the fluxes over one day periods. The variability in the chamber measurements was much smaller and dominated by high small scale spatial variability. The highest emissions measured by the EC method occurred during the first week of May when the trees were leafing and the soil moisture content was at its highest. If chamber techniques are used to estimate ecosystem level N2O emissions from forest soils, placement of the chambers should be considered carefully to cover the spatial variability in the soil N2O emissions. The EC technique, applied in this study, is a promising alternative tool to measure ecosystem level N2O fluxes in forest ecosystems. To our knowledge, this is the first study to demonstrate that the EC technique can be used to measure N2O fluxes in the trunk-space of a forest.

A simple model to estimate exchange rates of nitrogen dioxide between the atmosphere and forests

A simple model (2layer) was constructed that describes the exchange of the reactive gases NO, NO2 and O3 between forest and the atmosphere. The model uses standard equations to describe exchange processes and uptake of gases. It also takes into account reactions taking place in the trunk space between NO and O3 and photolysis of NO2. All equations are solved analytically leading to a scheme efficient enough to allow implementation in a large scale dispersion model such as the EMEP model. The model is tested on two comprehensive datasets obtained in a coniferous forest and a deciduous forest. The model calculations of NO2 and O3 fluxes to the forest were compared with observations of these fluxes. Although the comparison is often not perfect some of the striking features of the observed fluxes i.e. upward fluxes of NO2 were simulated quite well. The impact of chemical reactions between O3, NO and NO2 in the trunk space appear to have a significant effect on the deposition rate of O2. This is especially true during the night and more so for forests emitting large amounts of NO.