Responses of vegetation and testate amoeba trait composition to fire disturbances in and around a bog in central European lowlands (northern Poland)
Introduction
Increased fire activity is anticipated by climatic models in the near future, and this alteration will strongly affect ecosystems (EEA Report, 2012). Peatlands are predicted to suffer from global changes and anthropogenic influence, their response to changing climate already being recorded in the boreal and temperate biomes (Dise, 2009). Peatlands located in temperate Central and Eastern Europe may become threatened, even though currently fire activity in this area is not as pronounced as in other latitudes (e.g. boreal or Mediterranean ecosystems; Marlon et al., 2013). As this area of Europe is considered virtually fire-free, it received much less attention compared to areas classified as typical ‘pyromes’ (Archibald et al., 2013). Similarly to Western and Southern Europe the use of fire by humans is recorded since at least the Mesolithic (ca. 11200 years back) or Neolithic (ca. 7500 years back; Clark et al., 1989; Barber et al., 2004; Kuneš et al., 2008; Kothieringer et al., 2015; Vannière et al., 2016); and natural fires have been recorded especially in fire-prone Pinus sylvestris-dominated forests (Adámek et al., 2015). Drying or hydrological instability observed in peatlands in central European lowlands over the last 300 years, as an effect of human activity but also changing climate (Lamentowicz et al., 2015; Marcisz et al., 2015; Gałka et al., 2017b), may trigger an increase in fire activity in the future. During droughts fires can easily spread on dry peatland surfaces, causing carbon emissions (Kettridge et al., 2015; Turetsky et al., 2015). Severe fires are strong ecosystem disturbances that cause hydrological fluctuations and affect local vegetation composition (Kuhry, 1994), especially when smouldering combustion takes place over long periods (Benscoter et al., 2015). The influence of fire on the peatland surface is demonstrated by the change in the microtopography: a reduction of hummock microforms that are much drier than hollows and, therefore, burn first (Sillasoo et al., 2011; Benscoter et al., 2015). Even though ombrotrophic peatland vegetation is, in general, resilient to fire, burning can have an influence on mosses over decades (Magnan et al., 2012). Sillasoo et al. (2011) showed that severe fires can influence ombrotrophic bog vegetation composition typical for dry hummocks, mainly Ericaceae (Tuittila et al., 2007), and that recovery time after fire may take up to 350 years. Therefore, an important additional information for studying local environmental changes is the type of burnt material that can give an idea of the extent and the location of past fires. The importance of charcoal morphotypes for the interpretation of fire data have been shown in previous studies (Umbanhowar and McGrath, 1998; Colombaroli et al., 2014; Feurdean et al., 2017).
The change in the vegetation composition on the peatland surface and dust deposition have an influence on microbial communities, mainly testate amoebae (Fiałkiewicz-Kozieł et al., 2015; Payne et al., 2016); however, microbial response to fire is not well recognized. Out of different groups of microbes inhabiting peatlands, testate amoebae (TA) are especially important as they are top predators in the microbial food web indicating changes in lower food web levels (Jassey et al., 2013). So far, few studies focused on the relationship between TA and fire looking at a long-term response of TA communities to fire (Marcisz et al., 2015), TA functional and morphological traits response to fire (Marcisz et al., 2016), and a short-term TA response to wildfire (Qin et al., 2017). However, more studies are needed to recognize those relationships accurately.
Given the irrelevance of fire today, past fire activity in central European lowlands is under-investigated and, so far, only one study provided high-resolution, contiguous macroscopic charcoal record from peat sediments from Poland and neighbouring countries (Marcisz et al., 2015). Additionally, the analysis of charcoal morphotypes have only been performed on lake sediments (Feurdean et al., 2017). Moreover, there is no palaeoecological study from the area of Poland using numerical analyses to identify fire frequency and the background and peak components of a charcoal record (Higuera et al., 2010). Likewise, no studies so far have used the fire transfer functions based on the European-scale training set for palaeofire reconstructions (Adolf et al., 2018b). As the influence of fire on peatlands in this part of the world is still not well known (Gałka et al., 2013; Marcisz et al., 2015, 2016, 2017), these are crucial analyses that help to understand the nature and effects of fire-regimes in this and similar systems.
Here, we aim to investigate the effects of fire disturbance on wetland vegetation and testate amoebae diversity. To reach this goal we analysed microscopic and macroscopic charcoal and its morphotypes, and used previously analysed pollen, plant macrofossil and testate amoeba data from Gałka et al. (2014) to understand the impact of fire on the peatland ecosystem. We aimed to (1) provide first evidence of statistically significant fire events in Poland (over the last 650 years) by additionally applying novel fire transfer functions; and (2) define the response of local vegetation and testate amoebae (TA) to local fire events. We hypothesized, that fire on the peatland leads to (1) changes in surface vegetation cover favouring vascular vegetation, and (2) change in TA functional diversity – a loss of mixotrophs due to drying and lower light availability, and increase of xenosomes as those possess shells more resistant to mechanical damage.
Section snippets
Study site and sediment sub-sampling
Bagno Kusowo is an ombrotrophic bog located in northern Poland, central European lowlands (53°48′59″N, 16°35′20″E, Fig. 1) (Gałka et al., 2014). Covering an area of 318.82 ha, it is one of the largest ombrotrophic bogs in Poland. Environmental history of the bog was studied before, focusing on vegetation changes, hydrological dynamics, non-contiguous examination of microscopic charcoal, and investigation of volcanic tephra (Gałka et al., 2014, 2017a; Lamentowicz et al., 2015; Marcisz et al.,
Microscopic and macroscopic charcoal
Microscopic charcoal (Fig. 5, S1.1) was abundant along the sequence, showing fluctuations with increases around ca. 1480 CE, 1630–1770 CE, 1835–1935 CE and ca. 2003-present.
Macroscopic charcoal (Fig. 4, Fig. 5, S1.1) was less abundant but more variable: we observed higher influx (MAC) and high background charcoal (BCHAR) values between ca. 1350–1720 CE, low values between ca. 1720–2003 CE, and a rise from ca. 2003 CE-present. Highest MAC occurred at ca. 1650–1720 CE, with a distinct charcoal
Regional fire history and vegetation patterns
The Bagno Kusowo bog development may be linked with the history of Szczecinek city (founded in 1310 CE, located 15 km from the bog) and the region that both witnessed many cataclysms in the last millennium: wars, invasions, plagues and frequent fires (Fig. 5; Gaziński, 2010). Fire activity was rather low until the 16th century, and human pressure was not intensive. However, fire activity slightly increased at ca. 1450–1500 CE, around 1550 CE and at ca. 1580–1600 CE. Dramatic fires (1537, 1540,
Conclusions
We show that peatlands’ vegetation can recover from low-intensity and short-lasting disturbances and, to some extent, advantage “pristine” vegetation cover with Sphagnum communities over the vascular vegetation. Our data suggest that microbial communities in peatlands are highly responsive to disturbances. Testate amoeba trait composition changed substantially, as traits common before disturbance (mixotrophy and proteinaceous shells) recovered only for a short time to greatly decrease
Author contributions
ML and MG conducted field work, provided peat core and palaeoecological data sets (plant macrofossils, testate amoebae and pollen), and dated the samples. KM and WT applied for the FIRECO project to perform additional charcoal analyses. KM performed charcoal analyses, age-depth modelling, statistical analyses and prepared figures. DC and CA helped with statistical analyses and interpretation of charcoal data. KM wrote the manuscript to which all authors contributed with discussions, critical
Data accessibility
Charcoal data produced for this paper will be stored in the Global Charcoal Database (www.paleofire.org).
Acknowledgements
This study was conducted by K.M. during her postdoctoral scholarship – Swiss Government Excellence Postdoctoral Scholarship for the year 2016/2017, project FIRECO 2016.0310 – at the Palaeoecology Section, Institute of Plant Sciences, University of Bern. The research was supported by the National Science Centre (Poland) – grants 2015/17/B/ST10/01656 and NN305 320 436. Authors would like to thank Małgorzata Suchorska (Adam Mickiewicz University, Poznań) and Erika Gobet (University of Bern) for
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