mediated CH4 emissions.
Objective 3 was evaluated only in a temperate environment (Chapters 3 and 4). These chapters shed light on the potential global warming feedbacks on tree-mediated CH4
emissions and suggest that increased temperature (Chapter 4) and higher water-table levels (Chapter 3) positively affect tree-mediated CH4 emissions.
Although, water-table depths controlled CH4 production and in turn affected tree-mediated
CH4 transport and release in the mesocosm experiment (Chapter 3), water-table variations
were not a dominant control on stem-CFLt emission rates in situ in the temperate forested wetland (Chapter 4). Water-table depths significantly affected both CH4 production (as
demonstrated by lower pore-water CH4 concentration in hollows; Fig. 4.6) and soil
emissions (as demonstrated by lower CH4 emissions from non-vegetated hollows; Fig. 4.3;
Appendix V and VI). These results demonstrate that soil emissions are more sensitive to water-table fluctuations than stem-CH4 emissions and small changes in water-table depths
(< 14.5 cm) may not significantly impact rates of stem-CH* emissions (discussed further in Chapter 4; section 4.5; Page no: 112). However, large water-table variations that might control CH4 production, CH4 oxidation and ability to transport CH4 by trees due to roots
failing to reach the CH4 production zone, could influence rates of stem-CFLj emissions (as
observed in LW mesocosms; Chapter 3). Although the influence of water-table depths on tree-mediated CH4 emissions were not measured in tropical forested wetland, results
obtained from the temperate forested wetland suggest the influence o f water-table depths on stem-CFLt emissions may be greater in SE Asian forested wetland because water-table depth variations are > 15 cm (difference between dry and wet season) and are the principal
control on CH4 production (Jauhiainen et al., 2005) since temperature variations are
minimal.
A significant decrease in stem-CFLj emissions with decreasing temperature was observed for Alnus glutinosa and Betula pubescens possibly due to temperature affecting CH4
production in soil (Bergman et al., 1998; van Winden et al., 2012), substrate quality and availability (Davidson & Janssens, 2006), and CH4 transport through trees (reduced CH4
transport). Notably, rates of stem-CFE emissions decreased in winter for both tree species in spite of high pore-water CH4 concentrations, suggesting that CH4 transport efficiency
decreased with decreasing temperature probably through temperature control of tree physiological parameters (phenological events such as autumnal leaf loss) and CH4
transport mechanisms (cooler temperature decreasing diffusion rates) resulting in reduced stem-CHj emissions.
A heterogeneous temperature response of CH4 emissions from Alnus glutinosa and Betula
pubescens was observed, with a more pronounced decrease in stem-CFLj flux for Betula pubescens than for Alnus glutinosa with decreasing temperature. A similar trend was
observed when temperature coefficients (Q10), i.e., rate of change in a system with a
temperature increase of 10 °C, were calculated using equation 6.1 for all CH4 transport
pathways (summarised in Table 6.1). The reduced temperature effect on Alnus glutinosa when compared to Betula pubescens is apparent from their temperature responses. It is likely that a number of mechanisms combined to produce such heterogeneous temperature response and are discussed in Chapter 4 (section 4.5; page no: 107-110). The comparison of Q10 coefficients between different CH4 emission pathways also highlights the reduced
significance of temperature on stem-CH4 emissions in general when compared to all other
10
T em p erature response c o e ffic ie n t (Q10) = (Equation 6.1)
Where Ti and T2 are the upper and lower limit of the temperature range (°C), and Yi and
Y2 are the CH4 fluxes at Ti and T2, respectively.
Table 6.1: The Q1 0 coefficients for all CH4 emission pathways studied in temperate
forested wetland.
CH4 emission pathways Q10 coefficients
Hollows 10.5 Hummocks 4.08 Vegetated hollows 14.6 Vegetated hummocks 4.68 Alnus glutinosa 1.53 Betula pubescens 3.03
6.6. Obj.5. To evaluate the role of trees in forested wetland CH4 emissions and
establish an ecosystem-scale CH4 budget by quantifying emissions from wetland-
adapted trees and soil surface components.
Objective 5 was evaluated in Chapters 4 and 5 and the results presented provide conclusive evidence for the importance of tree-mediated CH4 emissions in both tropical and temperate ecosystems. All CH4 transport pathways were quantified in order to evaluate the role of trees in forested wetland CH4 emissions. Although the two forested wetland sites varied
greatly in terms of soil CH4 dynamics, tree-mediated CH4 emissions were found to be significant. Interestingly, these two studies report similar values for tree-mediated CH4
emissions per hectare (5.7 ± 0.6 vs. 6.7 ± 0.7 g ha" 1 d"1; summer emissions from mature
trees from temperate forested wetland compared with emissions reported in Chapter 5 for tropical forested wetland; considering only the emissions from the lowermost 3 m of tree) but differ greatly in ecosystem CH4 contributions (8.8-27% vs. 62-87%). This difference
was not due to differences in tree density, since they were nearly similar between the two ecosystems (2450 vs. 2689 trees ha'1; both young and mature trees in temperate forested wetland vs. only mature trees in tropical forested wetland). Instead, this difference is attributed to the relatively small contributions of non-tree CH4 emission pathways in the tropical forested wetland.
The under-storey of the temperate forested wetland hosted a denser cover of herbaceous plants. These herbaceous plants provide a lower resistance gas transport pathway compared to wetland trees for escape of soil-produced CH4 to the atmosphere, bypassing the aerobic
surface layer. With large land surface cover and higher CH4 flux rates, plant-mediated CH4
emissions contributed substantially to ecosystem CH4 flux. However, such under-storey
vegetation was absent in the tropical forested wetland. Methane emissions from non vegetated hollows also contributed significantly to total ecosystem flux in the temperate forested wetland compared to the tropical forested wetland because CH4 oxidation in the
temperate forested wetland was limited to the top 5 cm of the soil layer due to upwelling hydrology. In contrast, up to 90% of the soil produced CH4 was oxidised in the top 0-50
cm soil layer in tropical forested wetland, resulting in only small quantities o f CH4 being
released at the soil surface (Couwenberg et al, 2010). Under such circumstances, the contribution of tree-mediated CH4 transport pathway will exceed other pathways, which
was the case in the tropical forested wetland studied.
Notably, the observation of young tree CH4 fluxes exceeding that of mature tree fluxes
highlights the possible underestimations of the overall contributions of tree-mediated CH4
emissions estimated in Chapter 5 (tropical forested wetland; Fig. 5.3), since emissions from young trees were not measured in that ecosystem. Furthermore, the two tree species studied in temperate forested wetland although belonging to the same family, Betulaceae, displayed differences in the pattern and magnitude of CH4 emissions. Therefore, while
extrapolating tree-mediated CH4 emissions across ecosystems, tree family can only be used
as a proxy to identify the presence or absence of tree-mediated CH4 emissions and not to
estimate fluxes accurately.