The effects of drought on Sphagnum moss species and the implications for peatland carbon cycling in a changing climate

Peatlands store more carbon (C) than any other terrestrial ecosystem and as a C sink they are vital to mitigating climate change. The keystone of many peatland ecosystems is Sphagnum, a bryophyte genus of ca. 350 species found on every continent except Antarctica. With climate change, many peatlands face increasing frequency and severity of drought. How Sphagnum responds to, and recovers from, drought will be key to sustaining peatlands over the coming decades.Through a combination of microcosm and field experiments we investigate how different Sphagnum species will respond to short- and long-term drought periods. We detail the effects of drought on Sphagnum C cycling and biochemistry, including photosynthesis, growth, respiration and methane (CH4) fluxes. We show that there are species-specific limits to the ability of Sphagnum to withstand drought and that these align with the adaptations associated with the hummock-hollow microtopography of peatlands. Through this work we identify drought resilience, including a hysteresis between Sphagnum moisture content and C uptake which is delineated by pre-drought and rewetting. We discuss tipping points and determine C sink-source thresholds in Sphagnum which will have vital implications for future peatland C cycling.