Vegetation differed significantly between the undisturbed and rewetted sites. When undisturbed sites had oligotrophic raised-bog vegetation, vegetation in rewetted sites was typical to more nutrient rich environmental conditions and higher water table as reported previously (Tuittila et al. 2000; Samaritani et al. 2011; Renou-Wilson et al. 2018). Commonly, the less humified Sphagnum peat has been removed from abandoned milled peatlands, as the mineral-rich substrate supports the establishment and development of more nutrient demanding plant species. Contrary, oligotrophic vegetation is prevailing in bogs where the peat layer is more nutrient-poor and the water level deeper. After rewetting, vegetation establishment is more rapid and species rich in sites with more nutrients (Komulainen et al. 1999; Kozlov et al. 2016). This could have caused the relatively rapid vegetation succession on KõrsaR where peat ash content is reported to be about twice higher (2–3%) than in Hara rewetted sites (about 1–2%; Orru 1995). In KõrsaR, a rather diverse peatland community with a thick Sphagnum mat had developed in about 35 years.
Actually, in KõrsaR a thin layer of new peat – an acrotelm – has formed, which means that the site is functionally (but not structurally) quite similar to a pristine bog. According to results reported by Lucchese et al. (2010), about a 19 cm thick bryophyte layer would be needed in the Bois-des-Bel restored milled peatland in Canada to mitigate summer water level drawdown; this could be reached about 17 years after restoration. Throughout the study period in KõrsaR and HaraRS, the water level stayed inside the moss layer, mainly near the moss surface, therefore not decreasing the moss growth during the summer period. In the rewetted sites with thick moss layer in the current study, the moss layer was looser than in the undisturbed reference sites. This was probably due to the higher water table along with the high abundance of hollow Sphagna in the rewetted sites. Hollow Sphagnum could be affected from extreme droughts to a larger degree due to their larger pore size and less connectivity with the residual peat layer (McCarter and Price, 2015) than the denser Sphagnum cover of undisturbed bogs, therefore making CO2 exchange on rewetted sites more susceptible to drought impacts.
Some PFTs were lacking or had very low abundances in the rewetted sites but were present in the reference sites. We found significantly lower biomass and cover of evergreen shrubs on the rewetted than in the undisturbed sites, similar to results by Soini et al. (2010) and González et al. (2013), and they were absent from the most recently rewetted sites. Hummock Sphagna, which was present in both undisturbed bog sites was completely absent from the rewetted sites. The low occurrence and dying-off of hummock Sphagnum due to high water tables has been reported previously by Soini et al. (2010) and González et al. (2013). In reverse, Karofeld et al. (2015) recorded relatively high cover of hummock Sphagna and the presence of shrubs on restored milled peatland site where those species were dispersed using the moss-layer-transfer technique (Rochefort et al. 2003). Therefore, the application of this technique could lead to a more diverse vegetation composition of restoration sites.
While vegetation differs significantly between the rewetted sites, being more diverse in the older sites, the vegetation in both undisturbed sites with a similar hummock and hollow vegetation pattern did not differ from each other. Hummocks on the two undisturbed sites are typical Calluna-vulgaris-Sphagnum fuscum communities, the most common plant associations in Estonian bogs (Masing 1982), and are comparable to the high hummock communities described by Korrensalo et al. (2018). Lawns in the undisturbed sites belong to the tussocky Eriophorum community or the Sphagnum balticum-Sphagnum rubellum community (Masing 1982), described also by Korrensalo et al. (2018) in an undisturbed bog in central Finland as lawn and high lawn communities. A large variation in vegetation occurred in rewetted, especially in the most recently rewetted site of HaraRS between the measurement plots. However, this could also be caused by the relatively low number of measurement plots in each study site and their positioning on the site. In recovering milled peatlands, vegetation is developing in patterns due to large variations in suitable substrate conditions for plant growth (Tuittila et al. 2000; Purre and Ilomets 2018) and the presence of nurse-plant species (Tuittila et al. 2000; Groeneveld et al. 2007), whereas in undisturbed bogs microtopography explains the largest portion of variation in vegetation composition (Korrensalo et al. 2018; Mežaka et al. 2018).
Sphagnum has been considered a keystone genus of peatland restoration (Rochefort 2000). In the newly rewetted Hara site, Sphagnum was not yet present in the measurement plots, although some patches of lawn Sphagnum (mainly Sphagnum cuspidatum) were present in depressions with high water level. After rewetting, the height of the water table should remain a few centimetres below the peat surface, which leads to optimal conditions for Sphagnum growth and peat accumulation (Beyer and Höper 2015). Sphagnum has relatively high immigration potential (Campbell et al. 2003) and is abundant on the undisturbed plots bordering the rewetted ones, so further colonisation of Sphagna in recently rewetted sites is expected. In both older rewetting sites, Sphagnum had almost total cover. In addition, in the oldest KõrsaR site, lawn Sphagnum species have created some relatively high hummocks and overgrow E. vaginatum tussocks. The AGP and IL of Sphagnum in the rewetted sites was similar to those reported by Ilomets (1982) in Estonian undisturbed peatlands, while we measured about double the production and somewhat higher IL of Sphagna on the undisturbed sites. This probably results from different methods used for growth measurements (Pouliot et al. 2010), variations in weather conditions (Vitt 1990; Bengtsson et al. 2020) and species composition (Lindholm and Vasander 1990; Bengtsson et al. 2020).
4.2. Carbon dioxide fluxes
Both the undisturbed sites and the older rewetted sites were CO2 net sinks during the growing season, while the more recently rewetted site was still a CO2 source. Variations in CO2 fluxes between the rewetted sites are large due to differences in vegetation, weather and water levels — while some sites are important CO2 sinks (Tuittila et al. 1999; Beyer and Höper 2015; Wilson et al. 2016; Lee et al. 2017; Purre et al. 2019a), others could be small CO2 sources (Tuittila et al. 1999; Waddington and Warner 2001; Beyer and Höper 2015; Purre et al. 2019a). Although rewetted sites could be CO2 sources in the first decades after rewetting, they should become a CO2 net sink with time (Samaritani et al. 2011). Similar (Komulainen et al. 1999) or higher (Soini et al. 2010; Strack et al. 2016) CO2 net uptake on rewetted sites as in reference sites has been reported about ten years after rewetting, which is consistent with our results.
NEE in the rewetted sites is rather connected with differences in RECO than photosynthesis (Samaritani et al. 2011; Wilson et al. 2016). Similarly to our results from the Hara rewetted site, lower CO2 net uptake due to higher respiration has been reported from newly rewetted sites than from undisturbed bogs (Urbanová et al. 2012). In reverse, in the studies by Soini et al. (2010), Christen et al. (2016) and Strack et al. (2016), higher Pg compensated for high RECO, therefore leading to a higher CO2 net uptake on a rewetted site, which is consistent with our results from the KõrsaR.
CO2 fluxes and model parameters varied stronger between the measurement plots of the rewetted sites compared to undisturbed sites, as also reported by Soini et al. (2010), Laine et al. (2016) and Strack et al. (2016). This could likely be driven by larger variations in PFT cover in the rewetted sites. Unvegetated plots on rewetted sites remain CO2 sources (Wilson et al. 2016; Purre et al. 2019a) but measurement plots turn from a CO2 source to a sink with increasing plant cover (Strack et al. 2016; Purre et al. 2019a). Respiration on younger sites with still fragmented vegetation cover and lower diversity of plant species is largely influenced by peat temperature and water table depth, whereas those factors have a smaller effect on sites where vegetation has recovered well (Waddington and Warner 2001; Samaritani et al. 2011; Vanselow-Algan et al. 2015). Therefore, it could be expected that the CO2 sink function will increase and be more stable with secondary succession after rewetting, especially as the actual acrotelm is formed with time.
We detected some effect of site status on plant above-ground biomass, which on rewetted sites had a strong positive correlation with photosynthesis, whereas in undisturbed plots the correlation between plant biomass and Pg was insignificant. Similarly to our rewetted sites, Marinier et al. (2004) reported higher photosynthesis in plots with higher AGB, but plots with high AGB have also been reported to have higher RECO (Marinier et al. 2004; Strack et al. 2016; Brown et al. 2017). This was not the case in our rewetted sites, although in the undisturbed sites, RECO and vascular plant biomass had a strong positive correlation. The lack of correlations between the RECO and vascular plant biomass on rewetted milled peatlands is probably due to the domination of heterotrophic respiration on such sites (Wilson et al. 2007b; Järveoja et al. 2016; Purre et al. 2019a). Laine et al. (2016), Strack et al. (2016) and Purre et al. (2019b) also reported interaction between peatland management (undisturbed, rewetted), PFTs and carbon sequestration. According to Järveoja et al. (2016), those correlations depend on water level depth — if the water level is high in restored milled peatlands, bryophyte cover correlates with NEE, Pg and autotrophic respiration, whereas with deeper water table CO2 fluxes correlated with vascular plant cover. Therefore, the different correlations on rewetted and undisturbed sites are consistent with previous studies (Strack et al. 2016) and could be related to differences in water table height and fluctuations on sites with different management.
There are large differences in photosynthetic capacities between PFTs. In the undisturbed sites, we measured higher photosynthesis and maximum photosynthesis rates (Pmax) in the case of higher E. vaginatum cover. Vascular plant, especially graminoid biomass, has a relatively large impact on NEE in comparison with their abundance (Laine et al. 2012; Hassanpour Fard et al. 2020), due to their high photosynthetic capacity (Komulainen et al. 1999; Kivimäki et al. 2008; Urbanová et al. 2012; Strack et al. 2014; Laine et al. 2016). As E. vaginatum was present or abundant on most of the rewetted plots, the lack of correlation between the sedge cover and photosynthesis on the rewetted sites was unexpected. In addition to having high maximum photosynthesis (Pmax), this sedge species also has high light use efficiency (parameter k in the photosynthesis model) (Kivimäki et al. 2008) and high respiration rate (Jordan et al. 2016). Still, in the case of a high water table, rewetted sites with high E. vaginatum cover have a CO2 net sink function, even in unfavourable habitat conditions such as the occasionally lower water table during drought periods (Tuittila et al. 1999).
In the rewetted sites, higher photosynthesis and Pmax were measured with higher evergreen shrub cover. Evergreen shrubs stand out from other vascular plants with low photosynthesis and respiration rates (Laine et al. 2016), while in reverse Korrensalo et al. (2016) reported high maximum photosynthesis rates on evergreen shrubs like A. polifolia, C. vulgaris and V. oxycoccus, which are also present in the undisturbed sites and Kõrsa rewetted site in our study. According to Korrensalo et al. (2016), the Pmax of evergreen shrubs varies between species belonging to the same PFT. Still, the cause of controversies between different studies remains unclear and can be result of a rather low number of measurements that do not cover the whole ecosystem variation.
High photosynthesis in the case of higher evergreen shrub cover in this study could also be connected with higher plant cover and the number of PFTs on the measurement plots in KõrsaR where evergreen shrubs were present. According to Kivimäki et al. (2008), the presence of different PFTs lowers the RECO/Pg ratio, so creating conditions for higher CO2 net uptake as in Kõrsa, while in monostands of E. vaginatum this ratio is higher, which also explains a lower CO2 net uptake, as well as CO2 net emissions from the younger site in this study. According to Hassanpour Fard et al. (2020), the presence of some key species or PFTs either in monostand or in mixed community support the larger carbon accumulation during the growing season than the mixed communities with a different number of PFTs lacking such certain species. Whereas most vascular plants, especially sedges, have high photosynthesis rates during summer when their LAI is highest, the importance of Sphagnum in CO2 sequestration expresses itself during spring and autumn, when LAIvasc is low (Korrensalo et al. 2017).
In the rewetted sites, CO2 net sink function was larger in plots with higher Sphagnum cover. Sphagnum has lower photosynthetic capacities than vascular plants (Laine et al. 2012; Christen et al. 2016; Korrensalo et al. 2016) and also low respiration rates (Waddington and Warner 2001; Laine et al. 2016), and by increasing soil moisture content, a Sphagnum carpet could reduce soil respiration (Waddington and Warner 2001). However, restoring the Sphagnum carpet may not be enough for CO2 sequestering (Samaritani et al. 2011), especially as a newly formed Sphagnum carpet is sensitive to drier conditions (Tuittila et al. 2004). Therefore, constant high water tables are necessary, which support CO2 accumulation of those sites early on after restoration activities (Günther et al. 2017).
Limitations of the study
This paper contributes to the growing but rather sparse knowledge base surrounding peatland restoration, engaging peatlands with different stages after rewetting and also several vegetation variables in addition to CO2 flux measurements. However, some limitations of the study must be taken into account when considering the results. First, the study was conducted at a relatively low number of measurement points in the rewetted sites, especially in HaraRN. This could have affected the statistical analysis results regarding CO2 fluxes as well as the vegetation variables to some extent, especially in case of GLMMs. For each vegetation type in each site, there were two true replicates, and one (HaraRN) or two (all other sites) dominant vegetation types were covered in each study site. Also, the different rewetted milled peatlands or their fields had different time since rewetting, therefore the site conditions could have been affected somewhat the conclusions about the effect of time since rewetted. Still according to GLMMs time since rewetting was the main factor explaining the CO2 flux components on the rewetted study sites.
Second, the study covered only one growing season, so the annual balances of CO2 cannot be derived from this. The CO2 sequestration of the sites presented here are also strongly affected by weather conditions during that year, so they can differ from other years with varying conditions as shown at the Hara rewetted site by Purre et al. (2019a). Also, although all of the sites were open peatland sites, the CO2 exchange and biomass related with scarce tree cover were not accounted for in any of the studied sites. In addition, uncertainties related to flux measurements and reconstructions could affect the source or sink function of the sites during the growing seasons, especially if fluxes are very low and uncertainties higher (Bubier et al. 1999).
Third, the methane emissions, along with dissolved organic carbon and dissolved inorganic carbon, were not measured from the study sites in this paper, as the general aim of the study was to analyse the differences in plant production parameters and PFT composition closely related with the CO2 fluxes. Therefore, the results presented here do not provide information about the full carbon balance of the sites, as methane emissions for such sites have been reported to be high (Strack et al. 2014, 2016; Vanselow-Algan et al. 2015; Beyer and Höper 2015; Günther et al. 2017). Within these limitations, we still hope the paper will be of interest for a wide audience of peatland ecologists.
Although vegetation structure on rewetted milled peatlands approaches this on reference sites with time, some plant functional types present in the undisturbed reference sites, e.g., shrubs, colonise these sites in the later development stages and hummock Sphagnum could be absent even decades after rewetting. Vegetation composition developing with time affects the carbon accumulation of rewetted sites. During the studied growing season, over a decade ago rewetted milled peatlands were carbon sinks similarly to the reference sites, whereas the most recently rewetted site was still a carbon source to the atmosphere. Although graminoids play an important role in the photosynthesis of rewetted sites, as they do in undisturbed reference bogs, the carbon accumulation of rewetted peatlands is related with development of the Sphagnum mat, which is present in the reference sites. A well-developed Sphagnum mat also reflects the development of other environmental variables, of a functioning acrotelm and the development of a C sink function. Thus, a well-developed Sphagnum lawn could be used as an indicator of successful restoration. However, general plant functional type composition can still differ from reference sites in some accounts even several decades after rewetting.