Biogeochemistry of disturbances in the Canadian boreal forest

Background

CO2, CH4, N2O, and water vapor are the major greenhouse gases (GHGs) that are usually found in the boreal soil. Generally, well-aerated boreal soils are source of CO2 and N2O and sinks CH4 (Morishita et al. 2015). Release and uptake of these GHGs are controlled by biological processes (by methanotrophs and methanogens), which are highly influenced by soil temperature, moisture, and substrates (Paul 2007). Therefore, disturbances such as fire and logging that change soil properties can change GHGs dynamics.

Fire is a major disturbance in the boreal forest which is becoming more frequent as a result of anthropogenic climate change (Kasischke et al. 1995). Intensity of fire determines how much organic matter is burned and often accelerates soil thawing. It, thus, increases the depth of active soil layer in the summer following the fire. Chronosequence studies (for example, O’Neill et al. 2006) suggest that decreased root and microbial activities after fire decreases CO2 flux. Although scarce, a few studies have reported that CH4 fluxes increased and N2O fluxes decreased two years after fire (Matson et al. 2009). The increased CH4 emission is probably due to soil thawing which might increase organic matter decomposition and increase CH4 emission from the soil (UNEP 2012). In contrast to the previous findings, some other studies, for example, Hayes et al. (2014), suggest that thawing might also increase CO2 emission from soil. Even though the underlying processes of fire as a driver of these mechanisms are poorly understood, it received relatively little attention (McNamara et al. 2015).

Like fire, logging is also considered as a major disturbance in the boreal forest. Logging opens the canopy and either increases or decreases soil temperature depending on the season. In addition to direct removal of forest carbon, by increasing soil temperature during summer time, there is evidence that logging increases the emission of CH4 and CO2 from boreal soil (Kulmala et al. 2014). Logging also changes soil moisture, water table, and bulk density. It has been observed that well-drained soil facilitates CH4 oxidation and, by contrast, increased soil moisture and bulk density can produce CH4 (Sundqvist et al. 2014).

Almost no studies have actually assessed the flux of water vapor from post- logged (or -fire) sites. However, it can generally be assumed that as logging increases the height of water table and decreases transpiration fluxes from vegetation, therefore, emission of water vapor from soil should increase.

Diurnal variation of GHG fluxes in fire and logging sites of boreal forest should be pronounced. Recently burned/logged sites should differ in GHG fluxes than older stands. To my knowledge no prior research has specifically examined diurnal variation of soil GHG flux in boreal forest. However, a recent study conducted by Tagesson et al. (2012) in wet tundra ecosystem found no diurnal variation of CH4 flux from soil. This contrast with results of an eddy covariance study in a northern hardwood forest in Ontario, which found a strong diurnal pattern of whole-forest CH4 flux (Wang et al. 2013).

Despite the importance of these interactions, there is still limited knowledge on the effect of fire and logging on the GHG fluxes (Morishita et al. 2015). Additionally, our understanding so far is mainly based on periodic measurements using static closed chambers which are prone to underestimation of fluxes (Pihlatie et al. 2013). In this study I, therefore, aim to improve our understanding on how fire and logging affects the soil CO2, CH4, H2O vapor fluxes, with a special attention to diurnal differences, as a function of stand age using dynamic soil chamber and laser-based high precision real time gas analyzer.

Method

Soil CO2, CH4, and H2O vapor concentrations are measured with an Ultraportable GHG Analyzer (UGGA) from Los Gatos Research (LGR) connected to a LI-COR 8100A soil cuvette interfaced with a homebuilt system (we call it LiGER). LiGER controls the chamber opening and closing, records real time soil temperature and moisture, and air temperature within chamber.

To address the spatial heterogeneity within each plot, I’ve set up 5 to 6 gas collars (20cm diameter) per plot. At each gas collar at least 3 measurements (5-10 minutes/measurement) are taken. Additionally, pH, Oxidation Reduction Potential (ORP), and texture of soil from the closest location of each gas collar are determined.

Using the slope of the concentrations measured at each gas collar, GHG fluxes will be calculated using the methods detailed in Ueyama et al. (2015).