Forests store a lot of carbon both aboveground in vegetation, and belowground in the soil. To implement effective forest management policy, policy makers must understand the forest’s carbon budget, including whether it is a net source or sink. There is currently a good understanding of carbon efflux rates from soils and leaves, but not as much research is focused on flux from tree stems (including internal respiration), which could comprise a large portion of forest carbon budgets (Janssens et al. 2001). Tree stem respiration differs from other forms of respiration, which are typically controlled by biology (i.e. microbes consuming soil organic matter). Tree stem respiration is primarily controlled by physiological processes, and therefore can provide important information about factors that control the autotrophic portion of respiration.
While one study reported no relationship between stem flux and temperature (Saveyn 2008), to date most studies have shown a correlation between tree stem carbon dioxide (CO2) and environmental factors. Some have seen an exponential relationship between tree stem flux rate and temperature (Zeng et al., 2000; Bowman et al., 2005; Maier et al., 1998). Others have reported a diurnal hysteresis relationship between tree stem respiration and temperature (Lavigne, 1987; Lavigne et al., 1996; Damesin et al., 2002; Ryan et al., 1995; Bowman et al., 2005).
Here we sought to determine whether the eosFD was suitable for measuring CO2 efflux through tree bark and into the atmosphere. In September 2017 in Beaverbank, NS, we installed an eosFD horizontally at breast height on a birch tree. The eosFD was inserted into the collar prior to attachment to the tree to ensure that the silicone we used could support the eosFD. After applying silicone to the collar, we held the collar and eosFD to the tree while the silicone hardened to ensure a good seal. Once the silicone had started to harden, one person continued to hold the collar/eosFD to the tree while another attached two lengths of metal wire to the top of the eosFD and wrapped each length around the tree. This was to guarantee that the eosFD stayed in place and was constantly sealed to the tree stem. We then placed foam under the wire so that we did not damage the tree.
eosFD attached to the tree
Meteorological data was collected from the nearest Environment Canada weather station so that we could compare the flux data with environmental factors.
Diurnal Pattern clear over 10 days
The eosFD successfully captured tree stem respiration signals over the measurement period.
Figure 1. Tree stem flux and atmospheric temperature over a week in September 2017.
As shown in Figure 1, there was a clear diurnal pattern where the tree stem flux was highest around midnight, and at the lowest around noon. These peaks and troughs in tree stem flux were anti-correlated with atmospheric temperature, which was highest mid–day and lowest at night and can be seen more clearly in Figure 2 focusing only on a few days of the whole deployment. This is consistent with Yan et al. (2008), where they also saw a peak in stem flux at night-time, when temperature was lower.
Figure 2. Diurnal tree stem flux over a 2-day sampling of the whole measurement period.
The eosFD can provide a means for researchers to continuously monitor tree stem respiration, and better understand the rates and mechanisms of this potentially important part of ecosystem exchange. Less is known about internal stem respiration and its correlation to environmental factors than stem flux (Teskey et al. 2017), so the eosFD should be used in conjunction with other measurement techniques, like measuring O2 uptake in situ (Angert and Sherer, 2011; Trumbore et al. 2013). These other measurements will give a better idea about the amount of CO2 being transported away through the vascular tissue, xylem, and is not captured in the traditional CO2 gas exchange chamber flux measurement.
Here, we mounted the eosFD horizontally using a regular PVC collar, but we recommend for longer deployment periods that a collar is constructed to vertically mount the eosFD to prevent moisture from pooling on its membrane.
Thank you to Jocelyn Egan from Environment and Climate Change Canada for writing this article on her own study using the eosFD.
Please contact us if you’re interested in using eosFD technology at your site to study stem or soil fluxes!
The eosFD is a revolutionary soil CO2 flux sensor that uses Eosense’s patented Forced Diffusion technology to measure soil gas flux directly.