Accelerated vegetation succession but no hydrological change in a boreal fen during 20 years of recent climate change

Abstract

Northern mires (fens and bogs) have significant climate feedbacks and contribute to biodiversity, providing habitats to specialized biota. Many studies have found drying and degradation of bogs in response to climate change, while northern fens have received less attention. Rich fens are particularly important to biodiversity, but subject to global climate change, fen ecosystems may change via direct response of vegetation or indirectly by hydrological changes. With repeated sampling over the past 20 years, we aim to reveal trends in hydrology and vegetation in a pristine boreal fen with gradient from rich to poor fen and bog vegetation. We resampled 203 semi‐permanent plots and compared water‐table depth (WTD), pH, concentrations of mineral elements, and dissolved organic carbon (DOC), plant species occurrences, community structure, and vegetation types between 1998 and 2018. In the study area, the annual mean temperature rose by 1.0°C and precipitation by 46 mm, in 20‐year periods prior to sampling occasions. We found that wet fen vegetation decreased, while bog and poor fen vegetation increased significantly. This reflected a trend of increasing abundance of common, generalist hummock species at the expense of fen specialist species. Changes were the most pronounced in high pH plots, where Sphagnum mosses had significantly increased in plot frequency, cover, and species richness. Changes of water chemistry were mainly insignificant in concentration levels and spatial patterns. Although indications toward drier conditions were found in vegetation, WTD had not consistently increased, instead, our results revealed complex dynamics of WTD as depending on vegetation changes. Overall, we found significant trend in vegetation, conforming to common succession pattern from rich to poor fen and bog vegetation. Our results suggest that responses intrinsic to vegetation, such as increased productivity or altered species interactions, may be more significant than indirect effects via local hydrology to the ecosystem response to climate warming.

Spectral Indices of Vegetation Condition and Soil Water Content Reflect Controls on CH4 and CO2 Exchange in Sphagnum‐Dominated Northern Peatlands

“…This result is in line with the idea that microtopography controls the moisture regime and methanotroph/methanogen communities of Sphagnum and thus methane oxidation in northern peatlands (Basiliko et al., 2004). The importance of vegetation and topographic controls on CH 4 exchange emphasizes that, while WT may govern CH 4 fluxes in the short term, ecosystem succession in response to drying, including vegetation change and alteration of hummock‐hollow topography (Kolari et al., 2021; Robitaille et al., 2021) could have more significant effects on long‐term CH 4 exchange and spectral indices of wetness alone would not be sufficient to evaluate changes in CH 4 exchange over longer time periods.…”

Rising temperature modulates pH niches of fen species

“…The interaction between climate and pH niches explains niche shifts reported by previous comparisons between Scandinavia and Western or Central Europe (Graham et al, 2019; Kooijman & Westhoff, 1995; Peterka et al, 2020), or between cold mountains of the Balkans and warmer Central‐European fens (Hájková et al, 2008). The same explanation may apply to observations that the occurrences of some fen mosses had become more limited to the most base‐rich fens during recent decades across Europe (Hájek et al, 2015; Kolari et al, 2021).…”

“…Niche extension of acidophytic and calcium‐tolerant species towards high‐pH habitats is facilitated when nutrient, especially potassium, availability is improved or if the level of alkaline groundwater decreases in warmer climates. Previous studies across Europe (Kooijman, 2012; Kooijman & Bakker, 1995; Hájek et al, 2015; Kolari et al, 2021; Plesková et al, 2016; Vicherová et al, 2017) have indeed demonstrated expansion of Sphagnum teres , S . squarrosum , S .…”

“…Previous studies across Europe (Kooijman, 2012;Kooijman & Bakker, 1995;Hájek et al, 2015;Kolari et al, 2021;Plesková et al, 2016;Vicherová et al, 2017) have indeed demonstrated expansion of Sphagnum teres, S. squarrosum, S. magellanicum or S. recurvum agg. into base-rich fens.…”

“…Acidophytic species growing in mires may hence, to some extent, benefit from the extension of their pH niche in warmer climates. In a pristine boreal rich fen, Kolari et al (2021) found an increase in peat mosses ( Sphagnum recurvum agg., S . magellanicum ) at the expense of brown mosses during the past 20 years when the mean annual temperature in the region had risen by +1.0 °C, while pH had not changed.…”