The resilience and recovery of two globally important Sphagnum species to drought

Sphagnum mosses are key to peatland functioning and C cycling. They may cover 80- 100% of peatland land area and make up the vast majority of the peat itself. Sphagnum chemistry slows decomposition, enabling organic material accumulation which forms peat; it holds many times its own weight in water, maintaining the wet conditions vital to sustain peatlands. However, projected climate scenarios suggest that drought will increase in both severity and frequency, particularly at latitudes where most peatlands occur. To manage peatlands sustainably, we must understand how Sphagnum responds to and recovers from drought.

We subjected Sphagnum to increasing drought and rewetting periods to determine the limits of recovery as part of a microcosm experiment. We used two co-occurring, globally relevant Sphagnum species, with different microhabitat preferences: S. palustre and S. squarrosum, hypothesising that hummock-forming S. palustre would withstand longer drought periods due to being adapted to growing further from the water table than lawn-forming S. squarrosum. We measured Sphagnum moisture content and net ecosystem exchange (NEE) of CO₂ and methane (CH₄) over manipulated drought periods increasing from one to nine weeks, followed by recovery after rewetting. We also collected hyperspectral data to calculate key vegetation indices from all droughted and rewetted treatments and compared them to controls.

Undroughted S. palustre held more water than S. squarrosum (p< 0.0001): 37.2 ± 2.5 g g^-1 compared to 28.0 ± 2.5 g g^-1. Once droughted, moisture content declined slower in S. palustre, but both species’ moisture content recovered to that of controls within one week of rewetting.

S. palustre (-6.27 nmol g^-1 s^-1) took up 22% more CO₂ than S. squarrosum (-5.13 nmol g^-1 s^-1) across all treatments (p< 0.02). After one week of drought, fluxes between the controls and drought treatments were the same in both species, but after rewetting fluxes in S. squarrosum declined and only recovered to ca. 50% of predrought levels even after 10 weeks of recovery. After two weeks of drought, the S. palustre fluxes did not recover to predrought levels either. In all treatments Sphagnum became net sources of CO₂ after the first week of rewetting, after which they reverted to being net sinks once more.

Hysteresis between Sphagnum moisture content and NEE was delineated by pre-drought and rewetting. During drought, there was a significant asymmetrical relationship between moisture content and NEE around an optimum level of moisture. Optimum moisture was higher in S. squarrosum (18 g g^-1) than in S. palustre (12 g g^-1). After rewetting the relationship between moisture and NEE broke down completely, suggesting permanent damage to photosynthetic apparatus. This is supported by the failure of key vegetation indices to recover after rewetting.

This paper determines species-specific C sink-source thresholds in Sphagnum which have vital implications for future peatland C cycling.