Recovery of Sphagnum from drought is controlled by species-specific moisture thresholds.

As the largest terrestrial carbon (C) store, peatlands are vital to meeting climate targets. Sphagnum, a genus of ca. 350 species, sustains many peatlands through its high water content and chemistry which inhibits decomposition and vascular plant proliferation. However, many peatlands face increased risk of drought due to climate change, and how Sphagnum will respond and recover from drought is unknown. We measured moisture content, CO2 and methane (CH4) flux, and photosynthetic pigments in two species, S. palustre and S. squarrosum, over increasing drought (1-10 weeks) and recovery (1-10 weeks) periods. We identified biomass moisture thresholds of 12 g g- 1 (S. palustre) and 18 g g- 1 (S. squarrosum) below which irreversible damage occurred to photosynthesis. Due to higher moisture retention, and a lower moisture threshold, S. palustre withstood longer drought than S. squarrosum. These species-specific thresholds provide important insight for modelling peatland C sinks and for sustainable peatland restoration.