Lots and lots of implications for climate models and GHG budgets.
First published: 06 December 2018 https://doi.org/10.1111/nph.15624
Forest ecosystem methane (CH4) research has focused on soils, but trees are also important sources and sinks in forest CH4 budgets. Living and dead trees transport and emit CH4 produced in soils; living trees and dead wood emit CH4 produced inside trees by microorganisms; and trees produce CH4 through an abiotic photochemical process. Here, we review the state of the science on the production, consumption, transport, and emission of CH4 by living and dead trees, and the spatial and temporal dynamics of these processes across hydrologic gradients inclusive of wetland and upland ecosystems. Emerging research demonstrates that tree CH4 emissions can significantly increase the source strength of wetland forests, and modestly decrease the sink strength of upland forests. Scaling from stem or leaf measurements to trees or forests is limited by knowledge of the mechanisms by which trees transport soil‐produced CH4, microbial processes produce and oxidize CH4 inside trees, a lack of mechanistic models, the diffuse nature of forest CH4 fluxes, complex overlap between sources and sinks, and extreme variation across individuals. Understanding the complex processes that regulate CH4 source–sink dynamics in trees and forests requires cross‐disciplinary research and new conceptual models that transcend the traditional binary classification of wetland vs upland forest.
Forests are a dominant feature of the global carbon (C) cycle and play an important role in regulating climate and climate change (Bonan, 2008; Pan et al., 2011). Research on forests in the context of the global C cycle is focused primarily on carbon dioxide (CO2) dynamics, because the fluxes are large and C sequestration in wood and soil organic matter influence century‐scale projections of radiative forcing (Canadell & Raupach, 2008). Less attention is directed toward forests as sources and sinks of other C trace gases such as methane (CH4). Soils are fairly well characterized in forest CH4 budgets, but trees were only recently recognized as sources or sinks of this important greenhouse gas (Carmichael et al., 2014; Saunois et al., 2016). We review evidence that CH4 dynamics in forests are far more complex than previously believed owing to a combination of plant, microbial, and abiotic processes mediated by living and dead trees.
Methane causes 32–45 times more radiative forcing in a century than CO2 on a mass basis (Neubauer & Megonigal, 2015) and contributes c. 20% of radiative forcing (Denman et al., 2007; Myhre et al., 2013; Neubauer & Megonigal, 2015). Because CH4 is more responsive than CO2 to changes in sources or sinks (Hansen et al., 2013), forest CH4 budgets are a meaningful aspect of management directed at slowing the pace of global climate change (UNFCCC, 2016). A more nuanced understanding of forests is needed across fundamental forest–climate interactions to improve Earth system models and manage forests for climate mitigation (Canadell & Raupach, 2008). It is increasingly clear that forest CH4 cycling is one such interaction.
Despite efforts to constrain and refine the strength of the many sources and few sinks of atmospheric CH4, the global CH4 budget remains highly uncertain (Saunois et al., 2016). The total size of the global CH4 pool is well constrained in the range 539–609 Tg CH4 yr−1, but mismatches between bottom‐up models and top‐down estimates leave considerable uncertainty about individual components (Dlugokencky et al., 2011; Allen, 2016; Saunois et al., 2017).
Wetland ecosystems are the largest natural source of CH4 globally, and forested wetlands are c. 60% of total global wetland area (Matthews & Fung, 1987; Prigent et al., 2007), suggesting that forested wetlands are a significant global source of CH4. Reports of a discrepancy between emissions‐based estimates and satellite‐based estimates of CH4 sources in tropical forests (Frankenberg et al., 2008) sparked new interest in tree surfaces as an overlooked source (Terazawa et al., 2007; Gauci et al., 2010). Most of the research effort on wetland CH4 cycling has been in herbaceous wetland systems, but emerging literature on soil‐ and plant‐mediated CH4 emissions in wetland forests indicates that this source alone may account for 5–10% of global CH4 emissions (Pangala et al., 2017).
Upland ecosystems on freely drained soils are recognized as CH4 sinks in global budgets and have been the focus of studies on CH4 consumption by soils (Le Mer & Roger, 2001; Saunois et al., 2016). Transient periods of CH4 emission have been reported in nominally upland forests, but such emissions are cryptic and easily overlooked (Megonigal & Guenther, 2008). It is now clear that all biological surfaces in upland and wetland forests have the potential to emit or consume CH4 (Carmichael et al., 2014).
The emphasis on wetland forests as net atmospheric CH4 sources and upland forests as net sinks masks the complex interplay of aerobic and anaerobic processes that occur to varying degrees in all forest ecosystems (Fig. 1). The outcome of this dynamic can change the radiative balance of forests over temporal scales of minutes to decades and spatial scales of microsites to biomes. It is perhaps because of the focus on forests as either net sources or net sinks that research on the interrelated processes of CH4 production and oxidation has centered exclusively on just one process or the other. This perspective fundamentally limits our ability to fully represent the dynamic nature of forests in budgets and Earth system models. The goal of this review is to emphasize the common processes that exist across all forested ecosystems in order to advance a holistic understanding of C cycling and the radiative balance of forest ecosystems.
The growing body of literature on CH4 dynamics in forest ecosystems shows that they are far more complex biogeochemical environments than previously believed, and that our previous focus on soil processes alone is insufficient for a rigorous understanding of forests’ greenhouse gas balance and radiative climate forcing. Progress toward this goal will be most effective if we recognize that all CH4‐generating and consuming processes occur in all forest ecosystems regardless of their classification as upland or wetland. Advances in forest ecosystem CH4 dynamics require a new focus on the complex interplay between productive and consumptive processes occurring from the top of the canopy to the subsurface ground water, and their implications for generalized scaling. The subject is ripe for collaborations between people with expertise in plant physiology, soil physics, hydrology, geomorphology and microbial ecology, all of which interact to determine the distribution and activity of microbial communities and abiotic reactions that produce and consume CH4 as a single coupled process (Megonigal et al., 2004; Liu et al., 2015). Of particular importance are collaborations between experts in biogeochemistry, wood anatomy and tree physiology, because they regulate CH4 production and exchange across arboreal surfaces. Indeed, a growing research community with diverse interests in tree CH4 dynamics has developed an agenda for advancing the field (Barba et al., 2018).
Further study is needed to refine ecosystem‐scale estimates, determine the most appropriate scaling metrics, and resolve the distinctions between the arboreal CH4 flux pathways. Whole‐ecosystem studies currently provide the most robust information for global budgeting efforts, but many studies do not distinguish between the three pathways identified here in order to inform mechanistic numerical models. Laboratory studies can isolate specific pathways of CH4 production or consumption, but they often fail to capture the substantial temporal and spatial scales of variation that drive in situ fluxes. In addition to flux measurements, there is a need for thoughtful integration of existing techniques across subdisciplinary boundaries. Until additional integrative empirical studies are conducted, and process‐based models are developed and tested, the contribution of forests to global CH4 dynamics will remain poorly resolved.