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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - MEMETRE (From processes to modelling of methane emissions from trees)

Teaser

Summary

Contribution of trees to the CH4 budget of forest ecosystems has long been overlooked due to the perception that trees do not play a role in the CH4 dynamics. Recent evidence shows that whole forest ecosystems may act as sources of CH4 despite the soils being sinks, and more specifically that CH4 emissions from tree stems may substantially contribute to the net CH4 exchange. Despite the growing evidence of the capacity of trees to emit CH4 in different climatic zones, the process understanding is incomplete. MEMETRE project fills the gap and provides state-of-the-art laboratory and field measurements to identify the mechanisms, their drivers and spatio-temporal variability. Furthermore, the process understanding will be used to construct a soil-tree-atmosphere process model of CH4 cycling in forest ecosystems, which allows to evaluate the role of trees to the CH4 budget at forest and global scales.

The project has a strong experimental base necessary to obtain the process level information for the modelling. We use novel enclosure methods in specifically designed laboratory setups and in the field to investigate gas exchange of tree seedlings and mature trees. We utilize existing research infrastructure sites in Europe (e.g. ICOS, Integrated Carbon Observation System) to guarantee state-of-the-art facilities and supporting tree-physiological and environmental data for our CH4 flux measurements. Simultaneously, we work towards a process-based model that includes 1) CH4 production and consumption processes in the soil, 2) gas transport processes in tree, and 3) in-situ CH4 production processes in trees. Constant interplay between the experimental work and the process model development guarantee success of the project.

Work performed

Since the start of the project, we have built a novel system to measure the shoot and soil gas exchange of tree saplings within a controlled-environment chamber (Work package 1). This unique system allows us to study the response of CH4 emissions and transport to environmental variables. With the setup we conducted two measurement campaigns: one focused on the diurnal cycle of CH4 emissions from Scots pine saplings (WP1, Task 1.2); the second aimed to assess CH4-emitting processes in Betula species (B. nana, B. pubescens, B. pendula) (WP1, Task 1.2, 1.3). For Scots pine, the initial results confirmed that the leaves of the pine trees emitted CH4 in light, and the leaf-level CH4 emissions depended on photosynthetically active radiation (PAR), and were independent of UV radiation. For Betula species, B. nana showed strong conductivity of CH4 from the soil to the atmosphere via leaves. The other two Betula species showed markedly less conductivity. The results suggest that Betula species differ in the capacity to transport soil-derived CH4. In addition, we developed an automated multi-chamber shoot enclosure system for field measurements (to be utilized in Work package 2). A prototype of this system was built at the Viikki greenhouse facility and will be employed for a 13C pulse labelling experiment in Spring 2020.
The CH4 diffusivity experiments (Task 1.3) were completed in Spring 2019. These experiments demonstrated that CH4 transport in tree stems does not occur in the xylem water, indicating that the export of soil CH4 through tree stems is limited to the gas phase diffusion in aerenchymes. These experiments provided us diffusivity values for CH4 in pine, spruce and birch stems, which will be utilized as parameter values in gas transport modelling (WP3, Task 3.2; years 2020-2021).
Seventeen decomposing fungal species have been tested for CH4 production and at least five of them produced CH4 (Task 1.4.). We have also pre-tested the link between CH4 oxidation and N2 fixation in both forest soil and in decayed sawdust. Based on those results, moisture level controls N2 fixation in both of the tested materials. We aim to repeat the setup in different CH4 concentrations ja then continue by inoculating the systems with both CH4 producing and non-producing fungi. Aim is to include both pathogenic and non-pathogenic decomposing fungi.

Field measurements were conducted in summers 2018 and 2019, in Skogaryd, southern Sweden, and PallasjÃ¤rvi, northern Finland, respectively (WP2). Both consisted of three week-long measurement campaigns where tree stem and shoot CH4 emissions were measured. In Skogaryd, soil redox potential was measured simultaneously, and in KenttÃ¤rova, also soil samples were collected for the analysis of CH4 cycling microbes (Task 2.2, 2.3).

In initial field measurements, we discovered that different CH4 analysers showed drastically different apparent CH4 fluxes due to interferences by volatile organic compounds (VOC). We therefore conducted an extensive instrument comparison experiment that evaluated the potential error from VOC interferences in Fourier-transformed infrared spectroscopy and laser absorption spectroscopy based instruments produced (Kohl et al. 2019).

In work package 3 progress has been made towards regional landscape estimation of the CH4 exchange of boreal forests (WP3, Task 3.4). Forest floor CH4 flux data and soil moisture were combined with airborne laser scanning and imaging to construct a topography-based modelled CH4 flux map of the forest (Vainio et al., submitted). In 2019-2020 this map will further be complemented with CH4 fluxes from trees to eventually provide a landscape level map of the whole-forest CH4 exchange.

Final results

The gas flux measurement systems are the first implementations of this kind. They allow high frequency (<10 min) continuous measurements of leaf level trace gas and volatile organic compound exchange. Building of the growth chamber setup allows for the first time to assess the diurnal variability of leaf-level CH4 exchange of small trees under fully controlled light, temperature, humidity and CO2 conditions. Automatically controlled and programmed conditions, online measurements of CH4 and CO2 fluxes, including stable C-isotopes (13C-CH4, 13C-CO2) allowing to follow a tracer in the soil-plant system. This setup has been presented in European Geosciences Union General Assembly 2019 (Pihlatie et al., 2019, Geophysical Abstracts), and is shortly presented in Kohl et al. (2019), and several manuscripts in preparation.

Our field measurements of stem and shoot CH4 emissions at Skogaryd (southern Sweden) and KenttÃ¤rova (northern Finland) add critically needed field data from boreal forests. To our understanding, such comprehensive CH4 flux measurements from tree canopies, tree stems and forest floor are only conducted within our group, and available data exist currently only from the two campaigns mentioned above and from our earlier measurements at HyytiÃ¤lÃ¤ SMEAR II station (southern Finland).