Estimating methane emissions using vegetation mapping in the taiga–tundra boundary of a north-eastern Siberian lowland

Figures & data

Fig. 1. (a) Regional and (b) Local position of the study site. The Indigirka lowland is located in the north area of the watershed (Google Maps). WorldView-2 satellite image covered an area of 10 × 10 km (true colour). The main branch of the Indigirka River lies in the upper part of the image. The locations of the Kodac site (K), B site (B), local surveyed plot for training (circle) and test pixels (rectangular) are plotted on the map. Each survey point contains 3–4 quadrats for vegetation surveys. Vegetation was mapped within a target area (red line) without hills and cloudy areas.

Fig. 2. (a) True colour image and (b) vegetation map showing nine vegetation classes on the site scale (400 × 400 m) at site K (Kodac). The lake class does not appear at this scale, and therefore the map was covered by eight classes only, as shown in (b).

Table 3. Methane emissions at the local scale were calculated using the observed CH₄ flux, and the area of each vegetation class obtained from the vegetation map. In situ CH₄ flux (230 observation in total) was observed from 3 July to 9 August 2009–2016. The sedge-dominated wetland contributes 70% of total emissions.

Class	Area (km^2)	Coverage%	Observed mean CH4 flux (mg m^−2 d^−1)^a	SE	N	Days	Estimated local CH4 emission (10^6 g CH4 month^−1)	CH4 contribution%
Willow	26	27	0.3	0.3	17	5	0.3	0.2
Cotton-sedge	22	23	114	11	90	36	78	70
River	14	15	2.6^b	0.3	18	2	1.1	1.0
Emergent	9.2	9.5	96	29	14	4	27	25
Shrub	8.0	8.3	0.8	0.7	8	3	0.2	0.2
Bare-land	6.8	7.0	−3.4	2.2	7	2	−0.7	−0.6
Lake	4.6	4.8	25	3.4	3	2	3.6	2.8
Tree	4.0	4.1	−0.1	0.2	49	23	0.0	0.0
Sphagnum	1.8	1.9	38	8.9	24	17	2.2	1.9
Total	96		37^c	9.8	230	94	111

^aMethane flux observed by the calculation of the average of chamber-measured values. Data shown are averages ± standard error (SE), with the repeated number of total flux measurements (N) and the observation days (Days).

^bRiver methane flux was estimated from river dissolved CH₄ concentrations and transfer velocity (Supplementary Material: Text S1, Fig. S7).

^cAverage local CH₄ flux per square metre per day (mg CH₄ m⁻² d⁻¹).

Fig. 3. Vegetation, relative elevation (a) and soil moisture (b) along a 50-m transect. Soil moisture was measured as the averaged volumetric water content at the surface (0–8 cm) by time domain reflectance on 12 July 2012, 15 July 2014 and 12 July 2015. Sphagnum spp. and cotton-sedge areas are hatched to illustrate potential CH₄ sources.

Table 1. Classes assigned by visual observation and final classes based on satellite image mapping. Plant species composition classes corresponding to microtopographies were categorised into six classes. Because of the similarity of species compositions, structures and spectral features of satellite images among the shrub-dry, shrub-wet and dry-tussock classes, and between the willow and alder classes, these classes were combined into the shrub class and the willow class, respectively. Bare-land, river and lake classes were added, whereupon the landscape was finally grouped into nine classes.

Microtopographic classes		Vegetation classes based on plant species composition				Vegetation classes for satellite-data based mapping
		Plant community		Moss layer	Shrub and herb layer
		Classes	Clusters^a
Mound (High)	Tree	Tree	I, II, III	Green-moss^b	Larix cajanderi, Betula nana, Ledum palustre, Vaccinium vitis-idaea, Vaccinium uliginosum	Tree
	Shrub	Shrub-dry^c	VI, VII	Green-moss^b	Betula nana, Ledum palstre, Rubus chamaemorus	Shrub
		Shrub-moist^c		Green-moss/Sphagnum-dry^d	Salix pulchra, Vaccinium uliginosum
		Dry-tussock^c		Sphagnum-dry^e	Carex aquatilis, Salix pulchra
Wet area (Low)	Sphagnum	Sphagnum	XII	Sphagnum-wet^f	Carex aquatilis, Eriophorum vaginatum, Salix pulchra,	Sphagnum
	Cotton-sedge	Cotton-sedge	XI	Sphagnum-wet^f, Moss-wet^g	Eriophorum angustifolium, Comarum palstre	Cotton-sedge
		Willow	IX	Green-moss^b, Moss-wet^g	Salix boganidensis	Willow
		Alder	VIII	Green-moss^b	Alnus fruticosa
		Emergent	X	Moss-wet^g	Equisetum spp. Arctophila fulva, Carex chordorrhiza	Emergent
		Bare-land		No	No	Bare-land
		Lake/River		No	No	Lake/River

^a Clusters of plant species composition in local scale (Table S2).

^b Green-moss (Tomentypnum nitens, Hylocomium splendens, Aulacomnium turgidum).

^c Classes of shrub-dry, shrub-moist and dry-tussock are distinguished at the site scale (Table S1).

^d Green-moss/Sphagnum-dry mixed (Green-moss [as above] and Sphagnum spp. [S. warnstorfii and S. girgensohnii]).

^e Sphagnum-dry (Sphagnum spp. [S. warnstorfii, S. girgensohnii, and S. balticum]).

^f Sphagnum-wet (S. squarrosum and S. angustifolium).

^g Moss-wet (Drepanocladus spp., Warnstorfia spp.).

Table 2. Area and coverage of classification at the site scale (400 × 400 m).

Class	Area (km^2)	Coverage (%)
Cotton-sedge	0.045	28
Willow	0.035	22
Tree	0.023	14
Shrub	0.021	13
Sphagnum	0.015	9.1
River	0.013	7.9
Bare-land	0.006	3.5
Emergent	0.004	2.2
total	0.160

Fig. 4. Vegetation map showing the nine classes at the local scale (10 × 10 km) obtained by the MLC-DTC (maximum-likelihood classification and decision tree classification combined) method.

Fig. 5. In situ CH₄ flux measured by the chamber method in 2009–2016 during the summer growing season (3 July–9 August 2009–16, n (total) = 230). Only river CH₄ flux was derived from dissolved CH₄ concentration using the diffusive fraction with the transfer velocity method. Details are shown in Text S1 and Fig. S7.