Unveiling tipping points in long-term and experimental studies in peatlands

Ecosystems are increasingly prone to climate extremes, such as drought, with long-lasting effects on both plant and soil communities and, subsequently, on carbon (C) cycling. Unveiling past tipping points is a prerequisite for a better understanding of how individual species and entire ecosystems will respond to future climate changes, especially soil moisture. In the first study we identified the response of peatland vegetation to shifts in hydrological conditions over the past 2000 years using plant macrofossils, testate amoebae-based quantitative hydrological reconstructions from seven Polish peat-records (Lamentowicz et al., 2019). Using threshold indicator taxa analysis (TITAN), we discovered that plant community composition strongly converged at a water level of c. 11.7 cm, indicating a community-level tipping-point. We identified 45 plant taxa that showed either an increase or a decrease in their relative abundance between 8 and 17 cm of water level depth. In other the experimental study (Jassey et al., 2018) we investigated the response of plant and soil fungi to drought of different intensities using a water table gradient in peatlands—a major C sink ecosystem. We show that substantial changes in ecosystem respiration, plant and soil fungal communities occurred when the water level fell below a tipping point of 24 cm. As a corollary, ecosystem respiration was the greatest when graminoids and saprotrophic fungi became prevalent as a response to the extreme drought. Graminoids indirectly influenced fungal functional composition and soil enzyme activities through their direct effect on dissolved organic matter quality, while saprotrophic fungi directly influenced soil enzyme activities. In turn, increasing enzyme activities promoted ecosystem respiration. We show that functional transitions in ecosystem respiration critically depend on the degree of response of graminoids and saprotrophic fungi to drought. Our results represent a major advance in understanding the nonlinear nature of ecosystem properties to drought and pave the way towards a truly mechanistic understanding of tipping points in peatlands with use of experiment and palaeoecology.

References

Jassey, V.E.J., Reczuga, M.K., Zielinska, M., Slowinska, S., Robroek, B.J.M., Mariotte, P., Seppey, C.V.W., Lara, E., Barabach, J., Slowinski, M., Bragazza, L., Chojnicki, B.H., Lamentowicz, M., Mitchell, E.A.D., Buttler, A., 2018. Tipping point in plant-fungal interactions under severe drought causes abrupt rise in peatland ecosystem respiration. Glob Chang Biol. 24, (3) 972–986.

Lamentowicz, M., Gałka, M., Marcisz, K., Słowiński, M., Kajukało-Drygalska, K., Dayras, M.D., Jassey, V.E.J., 2019. Unveiling tipping points in long-term ecological records from Sphagnum -dominated peatlands. Biology Letters. 15, (4) 20190043.