Scaling Sphagnum photosynthesis from leaf to plot in an ecosystem level climate change experiment

Background/Question/Methods

Sphagnum moss is key component of peatland and boreal ecosystems where its persistent detritus has led to vast stores of carbon. Whether these ecosystems will continue to accumulate carbon in response to changing climatic conditions is not known, but significant releases of carbon would create strong feedbacks to climate warming. This study characterizes impacts of temperature, water content (θ) and water potential (ψ) on light-saturated photosynthesis (A_sat) for S. angustifolium and S. magellanicum at individual organism and plot-level scales within an ombrotrophic peat bog associated with the SPRUCE climate change experiment (http://mnspruce.ornl.gov/). These two species differ in their physiological, morphological, and ecological attributes. A specially-adapted moss cuvette was used for organism level gas-exchange while intact Sphagnum peat monoliths were used for scaling to the plot level. A_sat, θ and ψ were assessed initially on well-hydrated Sphagnum capitula, then repeatedly as samples were dried down over several hours. At a larger scale, initially saturated monoliths were dried down in a growth chamber over several months. Peat soil θ and ψ at 10 cm depth was tracked periodically and linked to concurrent observations of surface Sphagnum θ and ψ assessed destructively. Water release curves (θ × ψ) were then used to link leaf-level measurements to monoliths, which could be scaled to the plot-level.

Results/Conclusions

The decrease in A_sat in relation to reduced tissue θ and ψ was more severe for S. magellanicum (hummock species) relative to S. angustifolium (hollow species). Results suggest that the slope of A_sat decline differed between species for θ (p= 0.08) and ψ (p = 0.055), reflecting differences in species leaf anatomy and niche specialization within the landscape. At the plot level, ψ for Sphagnum sp. began dropping with water contents below 800%, declining rapidly for water content <500%. There was a strong linear relationship (r²=0.8) between gravimetric and sensor-based θ within the peat monoliths. θ at depth was a good indicator of surface θ (sigmoidal curve), and thus a good indicator of potential photosynthetic carbon uptake. S. angustifloium and S. magellanicum account for 88% of plant ground cover and 350 gC m^-2y^-1, >15% of total GPP for the site. Sphagnum response to changing climatic conditions will influence community composition and impact the fate of stored C within the SPRUCE peatland. Results are being incorporated into the CLM land surface model to provide more reliable C and water projections for this crucial ecosystem to climate change drivers.