Recent Lateral Expansion of Sphagnum Bogs Over Central Fen Areas of Boreal Aapa Mire Complexes

Abstract

We investigated recent changes in spatial patterning of fen and bog zones in five boreal aapa mire complexes (mixed peatlands with patterned fen and bog parts) in a multiproxy study. Comparison of old (1940–1970s) and new aerial images revealed decrease of flarks (wet hollows) in patterned fens by 33–63% in middle boreal and 16–42% in northern boreal sites, as lawns of bog Sphagnum mosses expanded over fens. Peat core transects across transformed areas were used to verify the remote sensing inference with stratigraphic analyses of macrofossils, hyperspectral imaging, and age-depth profiles derived from 14C AMS dating and pine pollen density. The transect data revealed that the changes observed by remote sensing during past decades originated already from the end of the Little Ice Age (LIA) between 1700–1850 CE in bog zones and later in the flarks of fen zones. The average lateral expansion rate of bogs over fen zones was 0.77 m y−1 (range 0.19–1.66) as estimated by remote sensing, and 0.71 m y−1 (range 0.13–1.76) based on peat transects. The contemporary plant communities conformed to the macrofossil communities, and distinct vegetation zones were recognized as representing recently changed areas. The fen-bog transition increased the apparent carbon accumulation, but it can potentially threaten fen species and habitats. These observations indicate that rapid lateral bog expansion over aapa mires may be in progress, but more research is needed to reveal if ongoing fen-bog transitions are a commonplace phenomenon in northern mires.

Inference of future bog succession trajectory from spatial chronosequence of changing aapa mires

“…The overgrowth of acidophilic Sphagnum mosses reported here and in several case studies (Granlund et al, 2022;Kolari et al, 2022;Tahvanainen, 2011) can be a common phenomenon that clearly poses a threat to aapa mire habitat types, but studies so far have focused on exploring the phenomenon, while representative data is lacking on what share of aapa mires is affected. The mosaic pattern of flarks and strings, and the aquatic primary production in flarks are important also for other taxonomical groups than plants.…”

“…Several case studies have shown rapid lateral expansion of bog vegetation across these fen-to-bog gradients (Granlund et al, 2022;Kolari et al, 2022), which makes this case a spatial chronosequence of FBT in a time scale of a few decades in aapa mires. Furthermore, the FBT in this case connects to climate warming and the climate zonation of the focal mire types, as the expanding bog vegetation represents a southern element.…”

“…One mechanism of FBT, as recently recognized in several Finnish aapa mires, is a lateral expansion of Sphagnum-dominated bog margins over central fens (Granlund et al, 2022;Kolari et al, 2022). This phenomenon differs from the lateral expansion of mire vegetation over surrounding dry mineral soil, i.e., paludification (Bauer et al, 2003;Kuhry & Turunen, 2006), and it can rather be considered as a type of hydroseral development, initiating as infilling of flarks by floating Sphagna and eventually leading to terrestrialization as bog ecosystem (Granlund et al, 2022).…”

“…(1) a hummock string and (2) a flark area between strings (lawns in bogs). In a previous study, we used a multiproxy approach including a detailed analysis of dated peat stratigraphies to verify recent FBTs at five out of the 23 sites studied here (Granlund et al, 2022).…”

“…; Charman et al, 2013;Gallego-Sala et al, 2018;Kolari et al, 2021;Loisel et al, 2012;Ma et al, 2022), which can lead to an ecosystem-level change from sedge-dominated fens to Sphagnum-dominated bogs (Kolari et al, 2022;Loisel & Yu, 2013;Magnan et al, 2022;Tahvanainen, 2011). The fen-bog transition (FBT) can alter the climate feedback of mires by increasing carbon sequestration (Granlund et al, 2022;Loisel & Yu, 2013) and by reducing methane emission (Juottonen et al, 2022;Zhang et al, 2021), while it poses threat to the biodiversity of fen habitats. Recently, many studies of high-boreal and subarctic peatlands have indicated increased carbon sequestration and FBT with warming after the Little Ice Age (LIA) and during the 20th century (Granlund et al, 2022;Loisel & Yu, 2013;Magnan et al, 2018Magnan et al, , 2022Piilo et al, 2019;Primeau & Garneau, 2021;Robitaille et al, 2021).…”