Fine-resolution mapping of microforms of a boreal bog using aerial images and waveform-recording LiDAR

Boreal bogs are important stores and sinks of atmospheric carbon whose surfaces are characterised by vegetation microforms. Efficient methods for monitoring their vegetation are needed because changes in vegetation composition lead to alteration in their function such as carbon gas exchange with the atmosphere. We investigated how airborne image and waveform-recording LiDAR data can be used for 3D mapping of microforms in an open bog which is a mosaic of pools, hummocks with a few stunted pines, hollows, intermediate surfaces and mud-bottom hollows. The proposed method operates on the bog surface, which is reconstructed using LiDAR. The vegetation was classified at 20 cm resolution. We hypothesised that LiDAR data describe surface topography, moisture and the presence and depth of field-layer vegetation and surface roughness; while multiple images capture the colours and texture of the vegetation, which are influenced by directional reflectance effects. We conclude that geometric LiDAR features are efficient predictors of microforms. LiDAR intensity and echo width were specific to moisture and surface roughness, respectively. Directional reflectance constituted 4-34 % of the variance in images and its form was linked to the presence of the field layer. Microform-specific directional reflectance patterns were deemed to be of marginal value in enhancing the classification, and RGB image features were inferior to LiDAR variables. Sensor fusion is an attractive option for fine-scale mapping of these habitats. We discuss the task and propose options for improving the methodology.Peer reviewe

Plant mediated methane efflux from a boreal peatland complex

Purpose Aerenchymous plants are an important control for methane efflux from peatlands to the atmosphere, providing a bypass from the anoxic peat and avoiding oxidation in the oxic peat. We aimed to quantify the drivers of aerenchymous peatland species methane transport and the importance of this process for ecosystem-scale methane efflux. Methods We measured seasonal and interspecies variation in methane transport rate per gram of plant dry mass at a boreal fen and bog, which were upscaled to ecosystem-scale plant methane transport. Results Methane transport rate was better explained by plant species, leaf greenness and area than by environmental variables. Leaves appeared to transport methane even after senescence. Contrary to our expectations, both methane transport rate and the proportion of plant transport were lower in the fen (with greater sedge cover) than in the bog site. At the fen and bog, average methane transport rate was 0.7 and 1.8 mg g(-1) d(-1), and the proportion of seasonally variable plant transport was 7-41% and 6-90%, respectively. Species-specific differences in methane transport rate were observed at the ecosystem-scale: Scheuchzeria palustris, which accounted for 16% of the aerenchymous leaf area in the fen and displayed the greatest methane transport rate, was responsible for 45% of the ecosystem-scale plant transport. Conclusion Our study showed that plant species influence the magnitude of ecosystem-scale methane emissions through their properties of methane transport. The identification and quantification of these properties could be the pivotal next step in predicting plant methane transport in peatlands.Peer reviewe

Small spatial variability in methane emission measured from a wet patterned boreal bog

We measured methane fluxes of a patterned bog situated in Siikaneva in southern Finland from six different plant community types in three growing seasons (2012-2014) using the static chamber method with chamber exposure of 35 min. A mixed-effects model was applied to quantify the effect of the controlling factors on the methane flux. The plant community types differed from each other in their water level, species composition, total leaf area (LAI(TOT)) and leaf area of aerenchymatous plant species (LAI(AER)). Methane emissions ranged from -309 to 1254 mg m(-2) d(-1). Although methane fluxes increased with increasing peat temperature, LAI(TOT) and LAI(AER), they had no correlation with water table or with plant community type. The only exception was higher fluxes from hummocks and high lawns than from high hummocks and bare peat surfaces in 2013 and from bare peat surfaces than from high hummocks in 2014. Chamber fluxes upscaled to ecosystem level for the peak season were of the same magnitude as the fluxes measured with the eddy covariance (EC) technique. In 2012 and in August 2014 there was a good agreement between the two methods; in 2013 and in July 2014, the chamber fluxes were higher than the EC fluxes. Net fluxes to soil, indicating higher methane oxidation than production, were detected every year and in all community types. Our results underline the importance of both LAI(AER) and LAI(TOT) in controlling methane fluxes and indicate the need for automatized chambers to reliably capture localized events to support the more robust EC method.Peer reviewe

Warming impacts on boreal fen CO2 exchange under wet and dry conditions

Abstract Northern peatlands form a major soil carbon (C) stock. With climate change, peatland C mineralization is expected to increase, which in turn would accelerate climate change. A particularity of peatlands is the importance of soil aeration, which regulates peatland functioning and likely modulates the responses to warming climate. Our aim is to assess the impacts of warming on a southern boreal and a sub-arctic sedge fen carbon dioxide (CO2) exchange under two plausible water table regimes: wet and moderately dry. We focused this study on minerotrophic treeless sedge fens, as they are common peatland types at boreal and (sub)arctic areas, which are expected to face the highest rates of climate warming. In addition, fens are expected to respond to environmental changes faster than the nutrient poor bogs. Our study confirmed that CO2 exchange is more strongly affected by drying than warming. Experimental water level draw-down (WLD) significantly increased gross photosynthesis and ecosystem respiration. Warming alone had insignificant impacts on the CO2 exchange components, but when combined with WLD it further increased ecosystem respiration. In the southern fen, CO2 uptake decreased due to WLD, which was amplified by warming, while at northern fen it remained stable. As a conclusion, our results suggest that a very small difference in the WLD may be decisive, whether the C sink of a fen decreases, or whether the system is able to adapt within its regime and maintain its functions. Moreover, the water table has a role in determining how much the increased temperature impacts the CO2 exchange. This article is protected by copyright. All rights reserved.Peer reviewe

Relationship between aerodynamic roughness length and bulk sedge leaf area index in a mixed-species boreal mire complex

Leaf area index (LAI) is an important parameter in natural ecosystems, representing the seasonal development of vegetation and photosynthetic potential. However, direct measurement techniques require labor-intensive field campaigns that are usually limited in time, while remote sensing approaches often do not yield reliable estimates. Here we propose that the bulk LAI of sedges (LAI(s)) can be estimated alternatively from a micrometeorological parameter, the aerodynamic roughness length for momentum (z(0)). z(0) can be readily calculated from high-response turbulence and other meteorological data, typically measured continuously and routinely available at ecosystem research sites. The regressions of LAI versus z(0) were obtained using the data from two Finnish natural sites representative of boreal fen and bog ecosystems. LAI(s) was found to be well correlated with z(0) and sedge canopy height. Superior method performance was demonstrated in the fen ecosystem where the sedges make a bigger contribution to overall surface roughness than in bogs.Peer reviewe

Heterotrophic and autotrophic soil CO2 respiration in two drained peatland sites after nutrient application

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Impact of long-term water level draw-down on functional plant trait composition of northern peatlands

We measured the following vascular plant functional traits: plant height (cm), leaf size (LS, cm2), specific leaf area (SLA, cm2 g-1), leaf dry matter content (LDMC, mg g-1) and leaf moisture content (g g-1) from the most common species in each research unit. We measured the following Sphagnum traits: capitulum density (number of shoots cm-2), fascicle density (number cm-1), surface density (mg cm-3), capitulum dry mass (mg) and capitulum moisture content (cap_wc, g g-1). In addition, rate of net photosynthesis was measured at four light levels. The data was collected from Lakkasuo mire complex located in Southern Finland (61° 47' N; 24° 18' E). The study includes three sites called rich fen, poor fen, and bog. At each site two experimental units were established in 2000/2001: an undrained control unit and a Water level drawdown (WLD) unit that was surrounded by a 30 cm-deep ditches after a control year. Photosynthesis measurements were carried out during summer 2016, while other traits were sampled during August 2016. We measured vascular plant vegetative height (cm), leaf area (LA, cm2 leaf-1) with a leaf area scanner (LI-3000, LI-COR Inc.), leaf fresh mass and leaf dry mass after the sample was dried at 40 °C for at least 48h (mg leaf-1). Leaf dry matter content (LDMC mg g-1) was calculated from fresh and dry mass, while specific leaf area (SLA, cm2 g-1) was calculated from LA and dry mass. Leaf traits were measured from five replicate plants as an average of a sample of ten fully grown healthy leaves from each plant. Sphagnum moss traits were measured from five replicates of single-species samples. Each sample consisted of two parts: a volume-specific sample collected with a core (diameter 7 cm, area 38.5 cm2, height 3 cm) to maintain the natural density of the stand and an additional sample of ca. 10 individuals, with stems more than 5 cm at length. Before collecting the core in the field, the number of shoots was counted from a 4 × 4 cm square for capitulum density (cap_dens, number of shoots cm-2). The volume-specific sample was cleaned of litter and unwanted species before drying at 40 °C for at least 48h to determine the surface density (surf_dens, mg cm-3). The additional sample of ten moss individuals was divided into capitula and stems (4 cm below capitula). We counted the number of fascicles on the 4 cm stem segments (fasc_dens, number cm-1). The capitula were thoroughly moistened and placed on top of tissue paper for 2 minutes to drain, before weighing them for water-filled fresh mass (cap_fw, mg). The samples were dried at 60 °C for at least 48h to measure the capitulum dry masses (cap_dw, mg). The moisture contents of capitula (cap_mc, g g-1) were then calculated as the ratio of water-filled to dry mass. Height growth (mm growing season-1) was measured in the field with the modified cranked wire method (Clymo 1970) as a difference in height between the beginning (mid-May) and end (mid-October) of the growing season 2017. For both vascular plants and mosses, we measured net photosynthesis rate, with a fully controlled, flow-through gas-exchange fluorescence measurement systems (GFS-3000, Walz, Germany; LI6400, LI-COR, USA). For mosses the living apical parts (~0.5 to 1 cm) were harvested right before the measurement and placed on a custom-made cuvette. For vascular plants, leaves, or in the case of shrubs, segments of branches were enclosed within the cuvette without disturbing the connection to the rooting system. Net photosynthesis rate (A, µmol m-2 g-1 s-1) was measured at 1500, 250, 35, and 0 µmol m-2 s-1 photosynthetic photon flux density (PPFD). The cuvette conditions were kept constant (temperature 20°C, CO2 concentration 400 ppm, flow rate 500, impeller in level 5). Relative humidity (Rh) of incoming air was set to 40% for vascular plants and 60% for mosses; for mosses this setting retained the cuvette Rh at around 80%. The setting enabled mosses to remain moist to ensure photosynthesis but protected the device from excess moisture. The data was collected to find out the impact of long-term WLD on functional traits of vascular plants and mosses, and how this impact is modulated by nutrient status (rich fen, poor fen, bog). We first assess (i) how peatland species differ in their traits and their intraspecific trait variability, to quantify (ii) how WLD impacts community level traits at different peatland sites