Responses of peatland vegetation to 15‐year water level drawdown as mediated by fertility level

Abstract

Questions Peatland ecosystems are a globally important carbon storage that is predicted to turn into a carbon source due to water level drawdown (WLD) associated with climate change. The predictions assume stable plant communities but how realistic is this assumption? If the vegetation is not stable, what are the nature and rate of changes? Location Peatland complex in Southern Finland. Methods We conducted a water level drawdown (WLD of ~10 cm) experiment over 17 years in three peatland types differing in their fertility. On each peatland type, we included an adjacent forestry drained (FD, with water table ca. 40 cm lower than in control) area for comparison. Results Peatland type had a clear impact on the response to WLD: at the ecosystem level, the two minerotrophic fens underwent rapid species turnover, while the vegetation in nutrient‐poor bog was more resilient to change. In nutrient‐rich sites, WLD initiated tree canopy development and created understorey conditions that strengthened impact of WLD. In nutrient‐poor site, tree establishment was seen only in the FD area. In addition to high nutrient level, high wetness accelerated change at the plant community level, where we found three types of responses: accelerating change, decelerating change, and stability. Succession resulted in an overall loss of community heterogeneity. Conclusions Interaction between hydrology, nutrient availability, and biological factors in boreal peatlands is important: the drop in water table required to achieve the shift from open peatland to forested system is inversely proportional to the nutrient level of the system. The results suggest that predictive models of peatland functions under climate change should consider compositional change for fens and their diverse plant communities but are more realistic for bogs. The response of bog vegetation to climate change may, however, be more dependent on changes in rainfall regime and therefore needs to be further addressed.

Decreased carbon accumulation feedback driven by climate‐induced drying of two southern boreal bogs over recent centuries

“…Loisel & Garneau, 2010;Piilo et al, 2019;. We did not observe any changes in plant functional types, for example, from Sphagnum to shrubs ; thus, we assume that the detected variation in CAR is largely due to variations in moisture and temperature, although changes within one plant functional type, for example, moss community, might alone could still drive changes in carbon accumulation due to different photosynthesis and decomposition rates at the species level (Hajek, Tuittila, Ilomets, & Laiho, 2009;Kangas et al, 2014;Laine, Juurola, Hajek, & Tuittila, 2011;Turetsky, Crow, Evans, Vitt, & Wieder, 2008 (Korrensalo et al, 2018), have the potential to turn to LH habitats; this development has already been predicted in a field experimental study at Lakkasuo (Kokkonen et al, 2019). This potential habitat transition will also stress MTA, as current lawn conditions are generally more appropriate habitats for most of the MTA taxa (e.g.…”

“…However, and more importantly, a net effect on summer moisture balance may be negative, as increased evapotranspiration may result in summertime moisture deficit. Bogs are suggested to be more resistant to drying than fens (Jaatinen, Fritze, Laine, & Laiho, 2007;Kokkonen et al, 2019), as they already regularly experience dry seasons/periods (Thormann, Bayley, & Szumigalski, 1998). Yet, here we evidenced consistent climate-driven water level variations, dry shifts and subsequent changes in biological assemblages in two adjacent bogs under warmer conditions in the past.…”

“…A very recent experimental study of plant community response to a 15 year-long water-table drawdown suggested that fen vegetation is less resilient to water level changes, with these communities experiencing clear species turnover, while bog vegetation appeared to be more resistant (Kokkonen et al, 2019). Considering this potentially slower response time of bog vegetation to changes in the environment, there is a need for studies which capture longer time periods than allowed by field experiments.…”

Modelling the habitat preference of two key <i>Sphagnum</i> species in a poor fen as controlled by capitulum water retention

“…Both microforms are covered by continuous Sphagnum carpet with sparse ground vascular canopies (projection cover of Carex 12% on average), which spread homogeneously over the topography . The annual mean water table was 15.6 ± 5.0 cm deep from lawn surface (Kokkonen et al, 2019). More information about the site can be found in Kokkonen et al (2019).…”

“…The annual mean water table was 15.6 ± 5.0 cm deep from lawn surface (Kokkonen et al, 2019). More information about the site can be found in Kokkonen et al (2019).…”