Biophysical drivers of seasonal variability in Sphagnum gross primary production in a northern temperate bog

Abstract

Sphagnum mosses are the keystone species of peatland ecosystems. With rapid rates of climate change occurring in high latitudes, vast reservoirs of carbon accumulated over millennia in peatland ecosystems are potentially vulnerable to rising temperature and changing precipitation. We investigate the seasonal drivers of Sphagnum gross primary production (GPP)—the entry point of carbon into wetland ecosystems. Continuous flux measurements and flux partitioning show a seasonal cycle of Sphagnum GPP that peaked in the late summer, well after the peak in photosynthetically active radiation. Wavelet analysis showed that water table height was the key driver of weekly variation in Sphagnum GPP in the early summer and that temperature was the primary driver of GPP in the late summer and autumn. Flux partitioning and a process‐based model of Sphagnum photosynthesis demonstrated the likelihood of seasonally dynamic maximum rates of photosynthesis and a logistic relationship between the water table and photosynthesizing tissue area when the water table was at the Sphagnum surface. The model also suggested that variability in internal resistance to CO2 transport, a function of Sphagnum water content, had minimal effect on GPP. To accurately model Sphagnum GPP, we recommend the following: (1) understanding seasonal photosynthetic trait variation and its triggers in Sphagnum; (2) characterizing the interaction of Sphagnum photosynthesizing tissue area with water table height; (3) modeling Sphagnum as a “soil” layer for consistent simulation of water dynamics; and (4) measurement of Sphagnum “canopy” properties: extinction coefficient (k), clumping (Ω), and maximum stem area index (SAI).

Rapid loss of an ecosystem engineer: Sphagnum decline in an experimentally warmed bog

“…Ecosystem warming can have multiple direct and indirect effects on ecosystem processes, and it can be difficult to tease the different effects apart. The decline in Sphagnum growth and NPP in this experiment might have been a direct result of effects on physiological processes in Sphagnum (Schipperges & Rydin, ; Van Gaalen, Flanagan, & Peddle, ; Walker et al, ) or indirect effects from changes in competition with shrubs for light or nutrients (He et al, ; Malmer, Svensson, & Wallen, ; Turetsky et al, ). Increased mineralization of peat, as would likely occur with warming and lowering of the water table, has been shown to increase growth of vascular plants in peatland ecosystems, leading to increased shading and reduced growth of intolerant Sphagnum (Malmer et al, ).…”

“…Ecosystem warming can have multiple direct and indirect effects on ecosystem processes, and it can be difficult to tease the different effects apart. The decline in Sphagnum growth and NPP in this experiment might have been a direct result of effects on physiological processes in Sphagnum (Schipperges & Rydin, 1998;Van Gaalen, Flanagan, & Peddle, 2007;Walker et al, 2017) or indirect effects from changes in competition with shrubs for light or nutrients (He et al, 2016;Malmer, Svensson, & Wallen, 1994;Turetsky et al, 2012).…”

“…Although we sometimes refer to the different enclosures by their target treatment values (+0, +2.25, +4.5, +6.75, and +9°C), in the regression analysis we used the actual temperature measured at 0.5 m above the hollows and averaged over the period 15 April to 15 October (Table ). We used air temperature above the hummocks in these analyses because it was the metric previously used in models of Sphagnum gross primary production (Walker et al ). To evaluate possible mechanisms by which air temperature affected Sphagnum growth and NPP, we conducted additional analyses in which NPP in 2018 was regressed against VWC of hummocks, maximum depth to water table, and soil temperature at 20 cm depth.…”

Photosynthetic and Respiratory Responses of Two Bog Shrub Species to Whole Ecosystem Warming and Elevated CO2 at the Boreal-Temperate Ecotone

“…To fully understand the integrated response of the plant community to the treatments, measurements of leaf gas-exchange must be combined with other information, such as community composition and leaf area (e.g., Weltzin et al, 2000;McPartland et al, 2019), phenology (e.g., Richardson et al, 2018), profiles of nutrient availability and rooting density (e.g. Iversen et al, 2018), Sphagnum productivity (e.g., Walker et al, 2017), and integrated understory gas exchange (e.g., Hanson et al, 2016).…”

The Multi-Assumption Architecture and Testbed (MAAT v1.0): Code for ensembles with dynamic model structure including a unified model of leaf-scale C3 photosynthesis

“…Comins and McMurtrie, 1993), identify the biophysical factors controlling biological activity (e.g. Walker et al, 2017a), interpolate sparse observations (e.g. Compo et al, 2011), project responses of the Earth System to anthropogenic activity (e.g.…”