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Keywords:

  • Alnus glutinosa ;
  • lenticels;
  • methane (CH4);
  • stem CH4 emissions;
  • tree-mediated methane emissions;
  • wetland

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information
  • Recent studies have confirmed significant tree-mediated methane emissions in wetlands; however, conditions and processes controlling such emissions are unclear. Here we identify factors that control the emission of methane from Alnus glutinosa.
  • Methane fluxes from the soil surface, tree stem surfaces, leaf surfaces and whole mesocosms, pore water methane concentrations and physiological factors (assimilation rate, stomatal conductance and transpiration) were measured from 4-yr old A. glutinosa trees grown under two artificially controlled water-table positions.
  • Up to 64% of methane emitted from the high water-table mesocosms was transported to the atmosphere through Aglutinosa. Stem emissions from 2 to 22 cm above the soil surface accounted for up to 42% of total tree-mediated methane emissions. Methane emissions were not detected from leaves and no relationship existed between leaf surface area and rates of tree-mediated methane emissions. Tree stem methane flux strength was controlled by the amount of methane dissolved in pore water and the density of stem lenticels.
  • Our data show that stem surfaces dominate methane egress from Aglutinosa, suggesting that leaf area index is not a suitable approach for scaling tree-mediated methane emissions from all types of forested wetland.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Wetlands occupy c. 5% of the Earth's land area (Prigent et al., 2007) and are the single largest natural source of methane (CH4) emissions to the atmosphere, representing 20–40% of the global CH4 budget (Cicerone & Oremland, 1988; Denman et al., 2007). Methane produced by methanogenic Archaea in anoxic wetland sediment and soil (Conrad, 1989) is known to be released to the atmosphere via three pathways: pore water diffusion, ebullition (bubbles) and transport through aerenchyma of herbaceous plants. However, there is growing evidence that woody plants represent a fourth pathway for emission of soil-produced CH4 (Gauci et al., 2010). Rice et al. (2010) estimate that CH4 transport through trees releases up to 80 Tg CH4 yr−1 globally to the atmosphere. Pangala et al. (2013) recently demonstrated that tree-mediated emission is the dominant emission pathway in forested tropical peat swamps.

Methane emission from the trunks of trees was first proposed by Schütz et al. (1991) and later confirmed by mesocosm experiments (Rusch & Rennenberg, 1998; Vann & Megonigal, 2003; Garnet et al., 2005; Rice et al., 2010) and field studies in forested wetlands (Terazawa et al., 2007; Gauci et al., 2010; Pangala et al., 2013). These investigations have mostly confirmed that plant-mediated CH4 emission is not limited to herbaceous plants but also is important in trees adapted to wet soil, because the latter facilitate O2 supply to their roots through the formation of aerenchymatous tissue, adventitious roots and hypertrophied lenticels (Megonigal & Day, 1992; Kozlowski, 1997). However, little is known at present about the factors and processes that control tree-mediated CH4 emissions from wetlands. Evidence to date suggests that CH4 transport in trees is driven mainly by diffusion and activated when soil CH4 concentration exceeds atmospheric concentration (Rusch & Rennenberg, 1998; Terazawa et al., 2007). There is presently a lack of direct evidence for tree-mediated CH4 transport via pressurised gas transport or transpiration, mechanisms which are known to drive CH4 transport in a range of herbaceous plant species (e.g. Conrad, 1989; Grünfeld & Brix, 1999) and CO2 transport in trees (e.g. Teskey & McGuire, 2005; McGuire et al., 2007).

Only a few physiological and environmental factors (e.g. pore water CH4 concentration, atmospheric CO2 concentration, stomatal conductance, leaf temperature and wood specific density) have been identified that influence tree-mediated CH4 emissions (Vann & Megonigal, 2003; Garnet et al., 2005; Pangala et al., 2013) in contrast to herbaceous plant-mediated CH4 emissions, which are known to be affected by a range of interacting biotic and abiotic factors (Whiting & Chanton, 1992, 1996; van Bodegom et al., 2001; Megonigal et al., 2004; Dittert et al., 2006). In general, the factors that drive tree-mediated CH4 emissions remain poorly understood as do the relative contributions of stem and leaf surfaces to total CH4 emissions from trees. Garnet et al. (2005) and Rice et al. (2010) expressed tree-mediated CH4 emission rates as a function of leaf surface area, and in the latter case used leaf area index (LAI) to estimate tree-mediated CH4 emissions at a global scale (Rice et al., 2010). Other studies have expressed tree-mediated CH4 emissions as a function of stem surface area (Rusch & Rennenberg, 1998; Terazawa et al., 2007; Gauci et al., 2010; Pangala et al., 2013) although no study to date has explicitly quantified CH4 emissions from stem lenticels. The capacity for lenticels to mediate CH4 egress from trees has been only assumed thus far because of their well-established role in stem aeration (McBain et al., 2004).

This study investigated mechanisms of CH4 emission from Alnus glutinosa (common alder), a key wetland tree species inhabiting waterlogged soil throughout Europe. Stem CH4 emissions from A. glutinosa have been confirmed previously by Rusch & Rennenberg (1998) and Gauci et al. (2010). The aims of this study were to: evaluate the capacity of A. glutinosa to mediate CH4 emissions; determine the relative proportions of CH4 transport through leaves and stems of A. glutinosa; and establish the main factors that control CH4 egress from A. glutinosa.

Materials and Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Mesocosms with controlled water-table levels

The study was conducted using 48 mesocosms, each containing a single Alnus glutinosa (L.) Gaertner sapling. The mesocosms consisted of a cylindrical container constructed of polyvinyl chloride (PVC) (diameter 36 cm × height 55 cm). A 5-cm drainage layer was formed at the bottom of each pot using 10 mm gravel. This material was overlaid with a 45-cm-thick mixture of 95% commercial sphagnum peat and 5% top soil (MANRO South, Cambridgeshire, UK). The layers were separated using a woven polyester fabric, which impeded root growth into the drainage layer and prevented the overlying peat soil from blocking the drainage layer. Each mesocosm was inoculated with 200 g of peat soil from a spring-fed peatland situated in the Flitwick Moor Nature Reserve, Bedfordshire. Three-year old Aglutinosa saplings purchased from Hedge Nursery, Hereford, UK, were planted in the peat mixture in 2010. The mesocosms were divided into two treatments based upon water-table position: one at the soil surface (HW) and the other 25 cm below the soil surface (LW). The 24 replicates of each treatment were arranged randomly outdoors in a nonshaded area of the Open University campus in Milton Keynes, UK (Fig. 1a).

image

Figure 1. (a) Mesocosms containing Alnus glutinosa saplings, the static chamber used to measure (b) whole mesocosm CH4 emissions, and (c) stem CH4 emissions.

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Water-table levels were maintained at the desired soil depth in the mesocosms using the method reported by Araya et al. (2010), which involved controlling water levels using a reservoir tank and two float chambers fitted with ball-valves. Ball-valves regulated water flow from the reservoir tank to the 0.1 m3 float chambers and subsequently into the mesocosms thereby automatically regulating water-table levels to compensate for evaporative losses. The float chambers were connected by branching hose pipes (diameter 1.25 cm) to the bottom of the individual mesocosms. Water-table levels in the control chambers and the mesocosms were set using Total Stations® surveying equipment (T705; Leica Geosystems®, St Gallen, Switzerland).

The water supplied to the reservoir tank (1.5 m3) was kept anoxic through contact with dried sugar beet shreds held within a porous sack and renewed monthly at a rate of 5 kg m−3 of water (Araya et al., 2010). The dried sugar beet deoxygenated the water but also introduced acetate, a known methanogenic substrate, to the mesocosms. Analysis of water samples from the reservoir showed a 92% reduction in dissolved O2 (from 0.25 mmol at the inlet to 0.02 mmol at the outlet) and an increase in acetate concentration from below detection limit at the inlet to 0.18 mmol at the outlet. The latter concentration of dissolved acetate is comparable to quantities that commonly occur naturally in peatland soil (Shannon & White, 1996) and should have enhanced production of CH4 in the mesocosms, facilitating assessment of gas transport mechanisms and pathways in A. glutinosa.

Methane measurements

Methane emission from the soil surface, stems, leaves and the whole mesocosm were measured using headspace static chambers at the peak of the growing season (12–13 and 24–25 July and 6–7 and 20–21 August 2011). At the time measurements were conducted, average soil and air temperatures were 16.7 ± 0.06°C and 26.5 ± 0.56°C, respectively, average relative humidity was 63 ± 3.16%, and photosynthetically active radiation (PAR) was 1.85 ± 0.09 mol m−2 h−1 (maximum PAR = 2.84 mol m−2 h−1; measured at 20 cm above the top of the canopy). Static chambers used to measure CH4 flux from the soil surface were based on a design described by Boardman et al. (2011) and were constructed from PVC pipe, consisting of a soil collar (8.2 cm diameter × 7 cm height; enclosed soil surface area 52.8 cm2) permanently inserted 4 cm into the soil in each mesocosm and a removable headspace chamber (8.2 cm diameter × 25 cm height) equipped with a pressure regulator and sampling ports in the transparent lid. The removable headspace chamber was attached to the soil collar during each deployment.

The static chamber used to measure CH4 emissions from the whole mesocosm (i.e. tree plus soil CH4 emissions; Fig. 1b) was constructed of reinforced transparent sheets of gas-impermeable fluorinated ethylene propylene (FEP) film (Adtech Ltd, Gloucestershire, UK) fitted on a cylinder constructed of wire mesh (36 cm diameter × 150 cm height; enclosed soil surface area 1000–1007 cm2). During sampling the chamber was covered with a clear Perspex® lid (Perspex Distribution Tamworth, Tamworth, UK) fitted with gas-sampling ports and a 12 V battery-powered fan. The chamber enclosed the entire tree and soil surface. Stem surface CH4 emissions were measured at two stem heights (2–12 and 12–22 cm above the soil surface) using a modified version of the method employed by Rusch & Rennenberg (1998). The stem chambers were constructed of clear Perspex® cylinders (11 × 17 cm; Fig. 1c). Tree height and the presence of branches prevented stem sampling at three heights above the soil surface in most cases and consequently, measurement of stem CH4 emissions from the 22–32 cm height interval was possible only for four trees. Relationships between stem height and rates of stem CH4 emission established from these four trees were used to scale stem CH4 fluxes from the other trees where measurements were possible at only two height intervals.

Leaf static chambers were constructed from reinforced transparent sheets of gas-impermeable FEP film fixed onto a frame of four adjustable solid aluminium rods attached to flat Perspex® sheets (6 × 6 cm) fitted with gas sampling ports on one end. The Perspex® sheet attached to the branch was cut into two halves and contained a central opening (diameter = 2 cm) to enclose the branch. A gas-tight seal was achieved by attaching gas impermeable closed-cell foam strips (3 cm wide) onto the branch to which the Perspex® leaf chamber was fitted. The FEP film was permanently fixed to one end of the Perspex® sheet and the other end was attached to the Perspex® sheet using an elastic chord and putty (Terostat IX; Teroson, Henkel, Germany). The elastic cord and putty enabled the chamber to exclude a large portion of the main branch, enclosing only 8–10 leaves during each deployment. Leaf petioles and the branch to which the petioles were attached could not be excluded using this technique. Pressure was controlled in the chambers via a small needle hole (0.8 mm) perforated through a resealable membrane (Suba Seal; Sigma-Aldrich, St Louis, MO, USA).

Measurements were performed between 09:00 and 16:00 h on each sampling day, with emissions being measured from 12 trees per treatment on each day (24 trees in total). Changes in CH4 concentration in the static chambers were measured by cavity-ring down laser spectroscopy (Los Gatos Research RMA-200 Fast Methane Analyser; Los Gatos Research, Mountain View, CA, USA). Each flux chamber was fitted with inlet and outlet valves that were connected to the cavity-ring down analyser using gas impermeable tubing. Gas from the chambers was circulated through the analyser to perform real-time continuous measurements of CH4 within the chambers. Chambers were closed for 5 min during each measurement. No tailing, flattening or saturation of rate of increase of CH4 within the chamber was observed. Methane flux rate was determined by least squares linear regression analysis of changes in CH4 concentration with time in the chambers. The minimum flux that could be detected by this method based upon instrument sensitivity and chamber volume was 0.1–0.3 μg CH4 m−2 h−1.

Diel patterns in CH4 emission from the soil surface, whole mesocosms and stem surfaces of A. glutinosa were investigated on 26 and 27 July 2011. Sampling was conducted during a 48-h period in 4-h sampling intervals (06:00–10:00, 10:00–14:00, 14:00–18:00, 18:00–22:00, 22:00–02:00 and 02:00–06:00 h) using the static headspace chambers described above. Six of the HW mesocosms were used, which contained A. glutinosa saplings that had a similar height, stem diameter, and CH4 emission rate from stems and soil. During diel measurements, the day and night air temperatures were 23.4 ± 0.98 and 15.7 ± 0.5°C, respectively, but the soil temperature stayed relatively constant between day and night (16.4 ± 0.04–16 ± 0.06°C).

One millilitre of unfiltered pore water was collected monthly from the soil in each mesocosm for analysis of dissolved CH4 concentration. Samples were collected using pore water samplers constructed from 0.64 cm polytetrafluoroethylene (PTFE) tubing (Cole-Parmer, London, UK) filled with glass wool (Plastipak; Becton Dickinson, Franklin Lakes, NJ, USA). The pore water samplers were permanently fixed at three depths (10, 20 and 30 cm below the soil surface) in each mesocosm at the beginning of the experiment. The concentration of CH4 extracted from pore water was analysed by cavity-ring down laser spectroscopy using the method of Baird et al. (2010). Dissolved concentrations of CH4 were calculated using Henry's law.

Tree physiology measurements

Net CO2 assimilation, transpiration and stomatal conductance were measured from fully expanded leaves using a CIRAS-II portable photosynthesis system (PP System, Amesbury, MA, USA) and a Parkinson leaf chamber which enclosed 2.5 cm2 of leaf surface area. During each sampling period, leaf gas exchange and stem CH4 emissions were measured simultaneously. Stem lenticel density was estimated using 2 × 2 cm grids on individual stems at two stem heights. The term ‘stem lenticel density’ represents only lenticels and not hypertrophied lenticels because the latter structures were not observed on trees from any of the HW mesocosms. Tree height, stem diameter, stem surface area, leaf surface area, and number of branches and leaves were measured fortnightly. Stem surface area was estimated based upon stem circumference measured at intervals of 10 cm along the height of the tree and by considering the tree stem as a truncated cone. Branch surface area not enclosed within the leaf static chamber was also factored into stem surface area estimations. Leaf surface area of each branch was estimated using the product of the measured maximum width and length of 10–15 leaves per branch and a correction factor determined by estimating leaf surface area using graphing paper. Leaf surface area of each tree was then estimated using the leaf surface area determined per branch, and the number of branches and leaves per tree. Whole tree photosynthesis, transpiration and stomatal conductance were estimated by multiplying the corresponding net fluxes by leaf surface area. (Please note: root growth, root density and root structure were not measured in this study).

Flux calculations

Soil emissions were estimated by multiplying measured soil CH4 fluxes by the soil surface area of each mesocosm after deducting tree basal area. Tree-mediated CH4 emissions were estimated by subtracting soil emissions from the measured whole mesocosm CH4 emissions. Tree emissions calculated by this approach were subsequently compared to stem CH4 emissions measured using stem chambers (i.e. after establishing the relationship between stem emissions and stem height above the soil surface).

For the trees with three stem chamber measurements, a power function relationship was observed between stem CH4 emission rate and stem sampling height for three of the four trees studied, which when used to estimate whole tree emissions, provided flux values that were very similar to tree-mediated CH4 emissions estimated by subtracting soil emissions from whole mesocosm CH4 emissions (Supporting Information Table S1). One tree exhibited a linear relationship between stem CH4 flux and height of measurement; however, total tree CH4 flux calculated using this relationship differed significantly from tree-mediated CH4 emissions determined from whole mesocosm flux (Table S1). Therefore, CH4 fluxes measured along the length of the tree stem were estimated using a regression model, which assumed that stem CH4 emissions varied with height according to the power function relationship.

Statistical analyses

Methane fluxes are reported as the overall means of fortnightly measurements conducted in July and August, 2011 (± SE). Statistical analyses were conducted using SPSS software v19 (SPSS, Chicago, IL, USA) and a significance level of < 0.05. All datasets met the assumptions of normal distribution and homogeneity of variance and were tested using Shapiro–Wilk's Test and Levene's Test, respectively. Variations between HW and LW mesocosm CH4 emissions and stem (at both stem heights) and soil emissions over time and diel variations in CH4 fluxes over 48-h periods were tested using a general linear model (ANOVA repeated measures). Tukey's HSD Test ( 0.05) was used for comparison of means. Relationships between whole mesocosm CH4 emissions, stem CH4 fluxes, whole tree assimilation, stomatal conductance, transpiration, stem diameter, leaf surface area, pore water CH4 concentration, stem lenticel density, PAR, air and soil temperature were evaluated using regression models. Regression models were also used to evaluate relationships between stem CH4 fluxes, whole mesocosm CH4 emissions and independent variables measured during the diel variation experiment. The relative contributions of all the independent variables measured (whole tree assimilation, stomatal conductance, transpiration, stem diameter, leaf surface area, pore water CH4 concentration, stem lenticel density, PAR, air and soil temperature) to stem CH4 emissions and whole mesocosm CH4 emissions were determined using stepwise multiple regression analysis. All independent variables were first tested for multicollinearity and homoscedasticity. As pore water CH4 concentrations at 20 and 30 cm soil depth were highly correlated (= 0.97), the latter data for 30 cm depth below the soil surface were excluded from the stepwise multiple regression analysis.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

The trees grown under HW conditions (water-table at the soil surface) developed visible morphological features, including leaf chlorosis, leaf abscission, formation of adventitious roots, stem thickening and increased number of stem lenticels within 3 wk of transplanting. The density of lenticels in July 2011 in the HW treatment trees was 1.67 ± 0.1 cm−2 (between 2–22 cm stem height) compared to 0.85 ± 0.3 cm−2 in trees grown under LW conditions (water-table 25 cm below the soil surface).

Mesocosm CH4 emissions

The average soil CH4 flux rate from the HW mesocosms was 0.78 ± 0.02 mg m−2 h−1, which was significantly larger (< 0.001) than fluxes from LW mesocosms (−5.31 ± 0.48 × 10−3 mg m−2 h−1) where only CH4 uptake occurred at the soil surface. Tree stems also did not emit CH4 in the LW mesocosms and CH4 emission from leaves was not detected in either the LW or HW mesocosms (i.e. the change in CH4 concentration in leaf flux chambers was below the instrument detection limit of c. 2 ppbv).

In HW mesocosms, rates of stem CH4 flux (expressed per stem unit area) were significantly larger than soil CH4 fluxes (< 0.01; Fig. 2). Stem CH4 fluxes (2–22 cm stem height) averaged 1.94 ± 0.06 mg m−2 h−1 compared to average soil CH4 emission rates of 0.78 ± 0.02 mg m−2 h−1. Stem CH4 fluxes measured at each individual stem heights (2–12 and 12–22 cm above the soil surface) were larger than soil CH4 fluxes in all the HW mesocosms (Fig. 2). Mean CH4 fluxes at 2–12 cm stem height were significantly larger than fluxes at 12–22 cm stem height during both July and August. Rates of CH4 flux from soil exhibited minimal variation between the different HW mesocosms (0.694–0.948 mg m−2 h−1), but there were significantly large variations in stem CH4 flux rate at both stem sampling heights (1.39–2.72 mg m−2 h−1 at 2–12 cm height and 1.27–2.38 mg m−2 h−1 at 12–22 cm height). Both soil and stem CH4 fluxes measured in the HW mesocosms were greater than CH4 emission rates reported for in situ forested wetland ecosystems where both sources were measured (Terazawa et al., 2007; Gauci et al., 2010), most likely resulting from elevated concentrations of acetate in the mesocosm supply water, which would have stimulated soil methanogenesis.

image

Figure 2. Average methane (CH4) fluxes measured in the high water-table mesocosms (HW) (= 24) during the observation period July and August 2011. Bars represent CH4 fluxes measured from stem surfaces of Alnus glutinosa at 2–12 and 12–22 cm height above the soil surface (expressed per stem unit area) and the soil surface (expressed per soil unit area). Error bars represent the mean + SE.

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The mean contributions of CH4 flux from Aglutinosa and the soil surface to whole mesocosm emissions were 0.121 ± 0.0046 and 0.077 ± 0.0023 mg h−1, respectively (Table 1). Approximately 61 ± 3% of CH4 emissions from the mesocosms resulted from transport through Aglutinosa. The remaining 39 ± 3% of CH4 flux was released from the soil surface with transport occurring most likely via diffusion through pore water (Table 1). Ebullition was not detected from any of the mesocosms during flux measurements. Tree stems between 2 and 22 cm height above the soil surface released c. 37 ± 5% of total tree-mediated CH4 flux (Table 1).

Table 1. Summary of mesocosm methane (CH4) fluxes (mg h−1 ± SE) for different emission pathways in the high water-table treatment mesocosms containing Alnus glutinosa saplings
 High water-table mesocosmsPercentage contribution
mg h−1%
  1. a

    Percentage contributions to whole mesocosm CH4 emissions.

  2. b

    Estimated by subtracting total soil CH4 emissions from measured whole mesocosm CH4 emissions.

  3. c

    CH4 emissions measured along the length of A. glutinosa were estimated using a regression model, which assumed that stem CH4 emissions varied with height according to the power function relationship as described in the Materials and Methods section.

  4. d

    Percentage contributions to total tree-mediated CH4 emissions estimated using b(subtracting total soil CH4 emissions from measured whole mesocosm CH4 emissions).

Total soil CH4 emissions0.077 ± 0.003339 ± 3a
Estimated total tree-mediated CH4 emissionsb0.121 ± 0.003661 ± 3a
Whole mesocosm CH4 emissions0.197 ± 0.0069100a
Estimated total tree-mediated CH4 emissionsc0.139 ± 0.003871 ± 3.8a
Stem CH4 emissions at 2–12 cm stem height0.0260 ± 0.002922 ± 2.7d
Stem CH4 emissions at 12–22 cm stem height0.0185 ± 0.002815 ± 2.3d

Controls on tree-mediated CH4 emissions

During the diel flux experiment (i.e. 48-h measurement campaign), no relationship was observed between light intensities and whole mesocosm CH4 emissions or directly measured stem CH4 fluxes (Fig. 3). Methane emissions from stems at two heights (Fig. 3) and whole mesocosms showed no marked diel variation (> 0.05). Day and night CH4 emissions from the whole mesocosms averaged 0.19 ± 0.011 and 0.17 ± 0.01 mg h−1, respectively (a difference of c. 10.5%; Table 2; although not statistically significant (> 0.05)). Air temperature rose rapidly in the morning of both days during the diel experiment, reaching a maximum of 27.5°C by 13:00 h, but soil temperature remained relatively constant (16.4 ± 0.04–16 ± 0.06°C; day and night temperature). Weak relationships were observed between several of the measured variables (air and soil temperature, whole tree stomatal conductance and transpiration) and stem and whole mesocosm CH4 emissions (Table 3).

Table 2. Rates of methane (CH4) flux (mg h−1 ± SE) from Alnus glutinosa (= 6) measured during the day and at night
 DayNightPercentage difference
(mg h−1 per mesocosm)(%)
  1. Day and night time data represent the mean of measurements performed during the periods 10:00–18:00 h and 22:00–06:00 h, respectively.

  2. a

    Estimated by subtracting total soil CH4 emissions from whole mesocosm CH4 emissions.

Tree-mediated CH4 emissionsa0.112 ± 0.00630.098 ± 0.005613
Stem height (2–12 cm)0.0274 ± 0.00120.0245 ± 0.001111
Stem height (12–22 cm)0.0230 ± 0.0090.0211 ± 0.00109
Whole mesocosm CH4 emissions0.190 ± 0.0110.170 ± 0.0110.5
Table 3. Relationships between stem methane (CH4) emissions (mg m−2 h−1) from Alnus glutinosa, whole mesocosm CH4 emissions (mg h−1 per mesocosm) and measured variables during a 48-h day : night cycle
Measured variablesRangeRelationship between 2–12 cm stem CH4 emissions and variable (R2)Relationship between 12–22 cm stem CH4 emissions and variable (R2)Relationship between whole mesocosm CH4 emissions and variable (R2)
  1. a

    < 0.05% uncertainty.

Whole tree assimilation (mmol h−1)24.3 ± 3.61= 0.0033x + 2.41 (0.09)= 0.0037x + 2.02 (0.08)= 0.0003x + 0.175 (0.07)
Whole tree stomatal conductance (mol h−1)295 ± 28.9= 0.0004x + 2.37 (0.16)a= 0.0004x + 1.99 (0.10)= 0.00002x + 0.175 (0.04)
Whole tree transpiration (mol h−1)2.54 ± 0.41= 0.0307x + 2.41 (0.11)a= 0.046x + 1.99 (0.16)a= 0.0014x + 0.177 (0.02)
PAR (mol m−2 h−1)1.21 ± 0.21= 0.0440x + 2.44 (0.06)= 0.045x + 2.05 (0.04)= 0.0043x + 0.176 (0.05)
Soil temperature (oC)16.2 ± 0.04= 0.292x − 2.25 (0.14)a= 0.181x − 0.831 (0.03)= 0.0277x − 0.267 (0.11)
Air temperature (oC)19.3 ± 0.67= 0.0216x + 2.08 (0.14)a= 0.015x + 1.82 (0.05)= 0.002x + 0.143 (0.10)a
image

Figure 3. Average methane (CH4) fluxes measured over a 48-h cycle. Bars represent CH4 fluxes measured from stem surfaces of Alnus glutinosa at 2–12 and 12–22 cm height above the soil surface (expressed per stem unit area) and soil surface (expressed per soil unit area). Error bars represent the mean + SE.

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During the fortnightly measurements conducted between 09:00 and 16:00 h whole tree stomatal conductance and assimilation ranged from 276–717 mol h−1 and 28.6–70 mmol h−1, respectively, with maximum rates observed between 12:00 and 14:00 h. However, stem CH4 emissions did not peak during this period consistent with the results of the diel flux experiment (Fig. 3) which exhibited no relationship with time of day. We also observed no significant relationships between stem and whole mesocosm CH4 emission rates and leaf physiological factors (i.e. whole tree stomatal conductance, assimilation and transpiration; Table 4). Similarly, neither leaf surface area nor PAR or soil and air temperature displayed any relationship with variations in stem or whole mesocosm CH4 emission rates (Table 4).

Table 4. Relationships between stem methane (CH4) emissions (mg m−2 h−1) from Alnus glutinosa, whole mesocosm CH4 emissions (mg h−1 per mesocosm) and measured variables between 09:00 and 16:00 h during the observation period July and August 2011
Measured variablesRangeRelationship between 2–12 cm stem CH4 emissions and variable (R2)Relationship between 12–22 cm stem CH4 emissions and variable (R2)Relationship between whole mesocosm CH4 emissions and variables (R2)
  1. *, < 0.05% uncertainty; **, < 0.01% uncertainty; ***, < 0.001% uncertainty.

  2. a

    Stem lenticel density measured at 2–12 cm height above the soil surface.

  3. b

    Stem lenticel density measured at 12–22 cm height above the soil surface.

  4. c

    Stem lenticel density measured between 2 and 22 cm height above the soil surface.

Pore water CH4 concentrations (mg l−1)10 cm below the soil surface
11.1 ± 0.20= 0.238x − 0.516 (0.39)***= 0.210x − 0.608 (0.41)***= 0.015x + 0.033 (0.31)**
20 cm below the soil surface
12.6 ± 0.26= 0.208x − 0.483 (0.52)***= 0.141x − 0.0328 (0.32)**= 0.014x + 0.024 (0.48)***
30 cm below the soil surface
12.5 ± 0.25= 0.229x − 0.723 (0.57)***= 0.152x − 0.165 (0.34)**= 0.015x + 0.017 (0.48)***
Stem lenticel density (lenticels cm−2)1.90 ± 0.12a   
1.45 ± 0.10b= 0.563xa + 1.0631 (0.77)***= 0.540xb + 0.954 (0.71)***= 0.042xc + 0.127 (0.69)***
1.67 ± 0.10c   
Stem diameter at the base (cm)4.17 ± 0.03= 0.996x − 2.02 (0.22)*= 0.749x − 1.39 (0.16)*= 0.051x − 0.016 (0.11)
Whole tree assimilation (mmol h−1)51.4 ± 2.14= −0.0027x + 2.27 (0.006)= −0.0039x + 1.93 (0.02)= −0.0002x + 0.205 (0.004)
Whole tree stomatal conductance (mol h−1)510 ± 22= −0.0003x + 2.27 (0.006)= −0.0007x + 2.09 (0.06)= −0.00003x + 0.210 (0.01)
Whole tree transpiration (mol h−1)4.06 ± 0.26= 0.048x + 1.93 (0.02)= 0.043x + 1.56 (0.03)= 0.0027x + 0.186 (0.02)
Leaf surface area (m2)1.08 ± 0.04= −0.525x + 2.69 (0.09)= −0.464x + 2.23 (0.09)= −0.029x + 0.2629 (0.06)
PAR (mol m−2 h−1)1.85 ± 0.09= −0.210x + 2.52 (0.07)= −0.119x + 1.96 (0.03)= −0.0121x + 0.22 (0.05)
Air temperature (°C)26.5 ± 0.56= −0.037x + 3.11 (0.08)= −0.017x + 2.17 (0.02)= −0.003x + 0.282 (0.11)
Soil temperature (°C)16.7 ± 0.06= −0.038x + 2.77 (0.0008)= −0.097x + 3.35 (0.007)= 0.0131x − 0.021 (0.02)

Pore water CH4 concentrations varied with depth in the HW mesocosms with the highest concentrations measured at 20 and 30 cm below the peat surface, averaging 12.6 ± 0.26 and 12.5 ± 0.25 mg l−1, respectively (Table 4). A positive linear relationship was observed between stem CH4 emissions measured at 2–12 cm height and pore water CH4 concentrations at 20 cm soil depth (R2 = 0.52; Table 4) and 30 cm (R2 = 0.57; Table 4) in all HW mesocosms. Similar relationships also were observed between pore water CH4 concentration at both soil depths and stem emissions measured at 12–22 cm height and whole mesocosm emissions (Fig. 4a; Table 4).

image

Figure 4. The relationship between whole mesocosm methane (CH4) emissions and (a) pore water CH4 concentrations measured at 20 cm soil depth, and (b) stem lenticel density at 2–22 cm height of Alnus glutinosa above the soil surface during the observation period July and August 2011. The regression equations are: (a) = 0.014 × (pore water CH4 concentration) + 0.024; and (b) = 0.042 × (stem lenticel density) + 0.127.

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Although stem and whole mesocosm CH4 emissions increased at higher pore water CH4 concentrations, our data suggest that controls other than soil CH4 concentration are important in determining variations in stem CH4 emission rate when water-table levels are situated close to the soil surface. Stem diameter variations between the trees were minimal, averaging 4.17 ± 0.03 cm and only a weak relationship existed between stem diameter and stem CH4 emissions at both measurement heights (Table 4). However, significant positive linear relationships were observed between rates of stem CH4 flux and stem lenticel density in the HW mesocosms for both the 2–12 cm (R2 = 0.77; < 0.001; Table 4; Fig. 5a) and 12–22 cm (R2 = 0.71; < 0.001; Table 4; Fig. 5b) stem height intervals. A similar relationship also was observed between whole mesocosm CH4 emission rates and stem lenticel density measured between 2 and 22 cm height (Table 4; Fig. 4b). Stepwise multiple linear regressions on data pooled from the HW mesocosms show that pore water CH4 concentrations at 20 cm soil depth (= 0.004; β = 0.402) and stem lenticel density at 2–12 cm height (< 0.001; β = 0.587) contributed significantly to differences in stem CH4 emissions, collectively accounting for 84% (< 0.001) of the variation (Table S2). Stepwise multiple regression analysis also suggested that c. 79% (< 0.001) of variation in whole mesocosm CH4 emission rate was explained by differences in the concentration of CH4 dissolved in pore water at 20 cm soil depth (= 0.002; β = 0.402) and lenticel density between 2–22 cm stem height (< 0.0001; β = 0.607; Table S2). Equations for estimating stem CH4 emissions at 2–12 cm stem height and whole mesocosm CH4 emissions as a function of pore water CH4 concentration at 20 cm soil depth (X) and lenticel density (between 2–12 and 2–22 cm stem height for stem CH4 emissions and whole mesocosm CH4 emissions, respectively) (Y) obtained using stepwise multiple regressions are:

image

Figure 5. The relationship between stem methane (CH4) emissions and stem lenticel density at (a) 2–12 cm height, and (b) 12–22 cm height of Alnus glutinosa above the soil surface measured in July and August 2011. The regression equations are: (a) = 0.563 × (stem lenticel density) + 1.0631; and (b) = 0.540 × (stem lenticel density) + 0.954.

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Methane emission from Alnus glutinosa

Our results demonstrate that tree-mediated CH4 emissions are a major pathway for CH4 egress in A. glutinosa from the HW mesocosms and that stem surfaces are responsible for most of the tree-mediated CH4 emissions (Fig. 2; Table 1). Approximately 61% of the HW mesocosm CH4 emissions resulted from CH4 venting to the atmosphere through Aglutinosa (Table 1) consistent with the findings of Pangala et al. (2013) that 62–87% of net ecosystem CH4 flux originated from tree stem surfaces in a tropical peat swamp forest. However, the relative contribution of tree-mediated CH4 emissions to ecosystem emissions may vary in natural wetlands depending on factors such as tree species, stand density, height, stem diameter, wood specific density (Pangala et al., 2013), water-table depths and the area of soil surface emitting CH4, and thus should be assessed in situ.

We did not detect CH4 emissions from leaf surfaces of A. glutinosa but large emissions were measured from stem surfaces, consistent with previous studies by Rusch & Rennenberg (1998) and Gauci et al. (2010). These findings collectively suggest that stem surfaces are the principal point of CH4 egress from A. glutinosa. Notably, Garnet et al. (2005) and Rice et al. (2010) reported tree-mediated CH4 emission rates as a function of leaf surface area, suggesting that leaves may be a factor in CH4 transport through Taxodium distichum, Fraxinus latifolia, Populus trichocarpa and Salix fluviatilis. These contrasting observations may be the result of differences between tree species in anatomical, morphological and physiological characteristics (A. glutinosa vs T. distichum, F. latifolia, P. trichocarpa and S. fluviatilis), gas transport mechanisms (i.e. molecular diffusion vs pressurised CH4 transport) and development stage (i.e. leaves may be the principal surface of CH4 egress in younger seedlings due to smaller stem surface area). Nevertheless, the absence of CH4 emissions from the leaf surfaces of A. glutinosa and lack of a relationship between leaf surface area and stem or whole mesocosm CH4 emissions (Table 4) suggest that LAI is not a suitable scaling metric for estimating tree-mediated CH4 emissions from all types of forested wetland.

Controls on tree-mediated CH4 emissions

The uptake of CH4 by soil and absence of stem CH4 emissions in the LW mesocosms and significant CH4 fluxes from the HW mesocosms indicate that water-table level was a dominant control on CH4 production and release. This finding is consistent with the long-standing view that water-table position strongly regulates soil CH4 production and consumption in wetlands (Grünfeld & Brix, 1999 and references within). Notably, variations in stem CH4 emissions in the mesocosms were largely independent of soil and air temperature, which provides an opportunity to evaluate other variables that may influence rates of stem CH4 flux.

During fortnightly measurements in the HW mesocosms some variables controlled stem and whole mesocosm CH4 emissions more strongly than others. Leaf surface area, whole tree transpiration, assimilation and stomatal conductance did not display a significant relationship with stem and whole mesocosm CH4 emissions. However, stem diameter at the tree base explained up to 22% of emission variations (Table 4). Pore water CH4 concentration and stem lenticel density exhibited strong relationships with stem CH4 emissions (Table 4) and collectively explained up to 84% of variation in emission rates (Table S2). Stem lenticel density, in particular, strongly influenced stem and whole mesocosm CH4 emission rates (Figs 4b, 5; Tables 4, S2). These findings suggest that variations in tree-mediated CH4 emissions are controlled primarily by differences in pore water CH4 concentration and the number of stem lenticels per unit area on wetland-adapted trees.

Transport of soil-produced gases (i.e. N2O and CH4) from the root zone through plant aerenchyma followed by release to the atmosphere through stem surfaces generally is attributed to lenticels because of their well understood role in aerating plant stems (Rusch & Rennenberg, 1998; McBain et al., 2004). This study conclusively demonstrates for the first time that the number of stem lenticels exerts an important control over rates of stem CH4 flux (Figs 4b, 5; Tables 4, S2), confirming the importance of these adaptive structures as exit points for CH4 egress from flood-tolerant trees (Rusch & Rennenberg, 1998; Purvaja et al., 2004; Terazawa et al., 2007). This finding has particular significance because formation of lenticels on stems, roots and root nodules – including hypertrophied lenticels – has been reported in many species of flood-tolerant trees (Kozlowski, 1997), including on aerial roots, knees and pneumatophores of mangroves and T. distichum (Pulliam, 1992; Purvaja et al., 2004).

While our study demonstrates a strong positive relationship between stem lenticel density and tree-mediated CH4 emissions in A. glutinosa, further work is required to determine whether such a relationship is common in other tree species. Lenticel presence, number, type, degree of opening, development stage and area vary between tree species (Langenfeld-Heyser, 1997; Kalachanis & Psaras, 2007). Moreover, the development stage of a tree species (Lendzian, 2006; Kalachanis & Psaras, 2007), which commonly is affected by external factors and environmental conditions, also impacts the formation of lenticels (Kuo-Huang & Hung, 1995). Any change in stem lenticel density may influence development of stem and root aerenchyma tissues, and thus potentially alter rates of CH4 transport.

Mechanisms of CH4 transport through A. glutinosa

Leaf physiological factors did not display a strong relationship with stem and whole mesocosm CH4 emissions between 09:00 and 16:00 h (fortnightly measurement; Table 4); however, weak positive relationships were observed between stem CH4 emission rates and whole tree stomatal conductance and transpiration during the diel flux experiment (Table 3). These relationships, although weak, indicate that leaf gas exchange may influence tree-mediated CH4 emissions, a suggestion also proposed by Garnet et al. (2005) for CH4 fluxes from T. distichum. These factors may also have contributed to transport and emission of CH4 through stem surfaces via the transpiration stream as a result of lateral and radial diffusion of CH4 within stems. The difference between stem and whole mesocosm CH4 emissions during day and night periods (< 13%; Table 2) offers evidence in favour of a small contribution to CH4 transport via transpiration stream in A. glutinosa; nonetheless, the observed diel variation could also be due to additional mechanisms such as changes in wind speed (enhanced venturi-induced convection or mechanical disturbance), air and soil temperature (affecting solubility of CH4 and diffusion capacity) and pressurised CH4 transport (Schütz et al., 1991). There was no evidence of substantial pressurised CH4 transport in the A. glutinosa saplings. If pressurised CH4 transport was an important process, tree-mediated CH4 fluxes should have decreased at night similar to reduced rates of CH4 export observed in herbaceous wetland plants, such as Phragmites australis and Typha spp. (Chanton et al., 1993; van Der Nat et al., 1998).

Our results suggest that CH4 is transported through A. glutinosa predominantly by molecular diffusion and released from stem surfaces via lenticels. This assertion is supported by the following observations: the highest rates of stem CH4 emission occurred from the lowest sections of stem and stem CH4 emission rates decreased with increasing height on stems (Fig. 2); there was an absence of measurable CH4 egress through leaves, a lack of or weak stomatal control over stem and whole mesocosm CH4 emission rates (Tables 3, 4), and no distinctive diel patterns in tree-mediated CH4 emissions (Fig. 3; Table 2); the density of stem lenticels related positively, linearly and strongly with rates of stem CH4 flux (Fig. 5; Table 4); and stem CH4 flux strength related positively and linearly with soil water CH4 concentration (Fig. 4a; Table 4). These findings are consistent with observations by Terazawa et al. (2007) of stem CH4 emissions from Fraxinus mandshurica var. japonica during the leafless season and the report by Garnet et al. (2005) of the absence of a mid-morning CH4 emission maxima and a nonhysteretic CH4 emission response curve for T. distichum. Collectively these observations provide compelling evidence for the importance of diffusive transport through stems in driving CH4 transport and emission from trees.

Conclusions

This study provides additional evidence for the capacity of trees to mediate export of significant quantities of soil-produced CH4 to the atmosphere and reinforces the need to include measurements of CH4 fluxes from trees in emission inventories of forested wetlands. It specifically identifies principal mechanisms and controls on CH4 flux from A. glutinosa, demonstrating that stem surfaces dominate CH4 egress and that no measurable quantity of CH4 is emitted from leaves. Consequently, we suggest that upscaling of tree-mediated CH4 emissions from forested wetlands should use the LAI proxy cautiously. Further work is needed to characterise the capacity and mechanisms by which other flood-tolerant tree species may mediate transport of CH4 from soil to the atmosphere in order to quantify more accurately the role of forested wetlands in the global CH4 cycle.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

The work was supported by research studentship funding provided by The Open University (OU) to S.R.P. (via V.G.). We thank A. Dwarakanath for fieldwork assistance, G. Howell, C. Boardman, C. Rooney and S. Green for laboratory assistance, and Y. Araya for assistance in setting up the mesocosm experiment.

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  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information
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Supporting Information

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Please note: Wiley Blackwell are not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing material) should be directed to the New Phytologist Central Office.

FilenameFormatSizeDescription
nph12561-sup-0001-TableS1.docxWord document18KTable S1 Summary of mesocosm CH4 fluxes for different emission pathways from Alnus glutinosa in high water-table treatment mesocosms
nph12561-sup-0002-TableS2.docxWord document16KTable S2 Results of stepwise multiple regression analysis of stem CH4 emissions and whole mesocosm CH4 emissions, and all independent variables measured in the study