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1. Spring-season flooding is a primary control of vegetation successional trajectories in primary mires

   https://doi.org/10.19189/MaP.2019.BG.393  

   Laine AM, S Frolking (January 2019, Mires and Peat)

   Major regime shifts in mires such as the fen–bog transition and the transition from non-forested to forested peatland are driven by ecohydrological changes. However, little is known about how the magnitudes and/or durations of hydrological shifts relate to these regime shifts. Here we analyse long-term water table data in conjunction with plant community data collected from primary mires on the Finnish coast of the Gulf of Bothnia. These ecosystems represent various stages of drainage: undrained, drained sites with developing tree stands, and unsuccessfully drained sites not supporting tree encroachment. The varying success of drainage provides an ideal field laboratory for investigation of thresholds of water table control on the successional trajectories of primary mire. Our data indicate a likely mechanism for the control of vegetation regime shifts in northern peatlands by water table, with time of year being as important a factor as the magnitude of change. Spring flooding rather than summer water table level appeared to be crucial for controlling state shifts in primary mire vegetation. As the effects of climate change on peatlands are most likely to be mediated by changes in hydrology and water table level, our study indicates a need for more thorough investigation of seasonal variability in the controlling factors. 

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2. Carbon dynamics in high‐Andean tropical cushion peatlands: A review of geographic patterns and potential drivers

   https://doi.org/10.1002/ecm.1614  

   García_Lino, Mary Carolina; Pfanzelt, Simon; Domic, Alejandra I; Hensen, Isabell; Schittek, Karsten; Meneses, Rosa Isela; Bader, Maaike Y (August 2024, Ecological Monographs)

   Peatlands store large amounts of carbon (C), a function potentially threatened by climate change. Peatlands composed of vascular cushion plants are widespread in the northern and central high Andes (páramo, wet and dry puna), but their C dynamics are hardly known. To understand the interplay of the main drivers of peatland C dynamics and to infer geographic patterns across the Andean regions, we addressed the following question: How do topography, hydrology, temperature, past climate variability, and vegetation influence the C dynamics of these peatlands? We summarize the available information on observed spatial and inferred temporal patterns of cushion peatland development in the tropical and subtropical Andes. Based on this, we recognize the following emerging patterns, which all need testing in further studies addressing spatial and temporal patterns of C accumulation: (1) Peatlands in dry climates and those in larger catchments receive higher sediment inputs than peatlands from wet puna and páramo and in small catchments. This results in peat stratigraphies intercalated with mineral layers and affects C accumulation by triggering vegetation changes. (2) High and constant water tables favor C accumulation. Seasonal water level fluctuations are higher in wet and dry puna, in comparison with páramo, leading to more frequent episodes of C loss in puna. (3) Higher temperatures favor C gain under high and constant water availability but also increase C loss under low and fluctuating water levels. (4) C accumulation has been variable through the Holocene, but several peatlands show a recent increase in C accumulation rates. (5) Vegetation affects C dynamics through species‐specific differences in productivity and decomposition rate. Because of predicted regional differences in global climate change manifestations (seasonality, permafrost behavior, temperature, precipitation regimes), cushion peatlands from the páramo are expected to mostly continue as C sinks for now, whereas those of the dry puna are more likely to turn to C sources as a consequence of increasing aridification. 

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

   https://doi.org/10.5194/bg-17-5693-2020  

   Gong, Jinnan; Roulet, Nigel; Frolking, Steve; Peltola, Heli; Laine, Anna M.; Kokkonen, Nicola; Tuittila, Eeva-Stiina (January 2020, Biogeosciences)

   null (Ed.)

   Abstract. Current peatland models generally treat vegetation as static, although plant community structure is known to alter as a response to environmental change. Because the vegetation structure and ecosystem functioning are tightly linked, realistic projections of peatland response to climate change require the inclusion of vegetation dynamics in ecosystem models. In peatlands, Sphagnum mosses are key engineers. Moss community composition primarily follows habitat moisture conditions. The known species habitat preference along the prevailing moisture gradient might not directly serve as a reliable predictor for future species compositions, as water table fluctuation is likely to increase. Hence, modelling the mechanisms that control the habitat preference of Sphagna is a good first step for modelling community dynamics in peatlands. In this study, we developed the Peatland Moss Simulator (PMS), which simulates the community dynamics of the peatland moss layer. PMS is a process-based model that employs a stochastic, individual-based approach for simulating competition within the peatland moss layer based on species differences in functional traits. At the shoot-level, growth and competition were driven by net photosynthesis, which was regulated by hydrological processes via the capitulum water content. The model was tested by predicting the habitat preferences of Sphagnum magellanicum and Sphagnum fallax – two key species representing dry (hummock) and wet (lawn) habitats in a poor fen peatland (Lakkasuo, Finland). PMS successfully captured the habitat preferences of the two Sphagnum species based on observed variations in trait properties. Our model simulation further showed that the validity of PMS depended on the interspecific differences in the capitulum water content being correctly specified. Neglecting the water content differences led to the failure of PMS to predict the habitat preferences of the species in stochastic simulations. Our work highlights the importance of the capitulum water content with respect to the dynamics and carbon functioning of Sphagnum communities in peatland ecosystems. Thus, studies of peatland responses to changing environmental conditions need to include capitulum water processes as a control on moss community dynamics. Our PMS model could be used as an elemental design for the future development of dynamic vegetation models for peatland ecosystems. 

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4. Fresh Air for the Mire-Breathing Hypothesis: Sphagnum Moss and Peat Structure Regulate the Response of CO2 Exchange to Altered Hydrology in a Northern Peatland Ecosystem

   https://doi.org/10.3390/w14203239  

   O’Neill, Ally; Tucker, Colin; Kane, Evan S. (October 2022, Water)

   Sphagnum-dominated peatlands store more carbon than all of Earth’s forests, playing a large role in the balance of carbon dioxide. However, these carbon sinks face an uncertain future as the changing climate is likely to cause water stress, potentially reducing Sphagnum productivity and transitioning peatlands to carbon sources. A mesocosm experiment was performed on thirty-two peat cores collected from two peatland landforms: elevated mounds (hummocks) and lower, flat areas of the peatland (hollows). Both rainfall treatments and water tables were manipulated, and CO2 fluxes were measured. Other studies have observed peat subsiding and tracking the water table downward when experiencing water stress, thought to be a self-preservation technique termed ‘Mire-breathing’. However, we found that hummocks tended to compress inwards, rather than subsiding towards the lowered water table as significantly as hollows. Lower peat height was linearly associated with reduced gross primary production (GPP) in response to lowered water tables, indicating that peat subsidence did not significantly enhance the resistance of GPP to drought. Conversely, Sphagnum peat compression was found to stabilize GPP, indicating that this mechanism of resilience to drought may transmit across the landscape depending on which Sphagnum landform types are dominant. This study draws direct connections between Sphagnum traits and peatland hydrology and carbon cycling. 

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5. The Ecohydrology of Coastal Ghost Forests

   https://doi.org/10.1002/eco.70020  

   Fagherazzi, Sergio; Nordio, Giovanna; Boaga, Jacopo; Cassiani, Giorgio; Michael, Holly A; Pratt, Dannielle; Messerschmidt, Tyler C; Kirwan, Matthew L; Stotts, Stephanie (March 2025, Ecohydrology)

   ABSTRACT Sea level rise and storm surges affect coastal forests along low‐lying shorelines. Salinization and flooding kill trees and favour the encroachment of salt‐tolerant marsh vegetation. The hydrology of this ecological transition is complex and requires a multidisciplinary approach. Sea level rise (press) and storms (pulses) act on different timescales, affecting the forest vegetation in different ways. Salinization can occur either by vertical infiltration during flooding or from the aquifer driven by tides and sea level rise. Here, we detail the ecohydrological processes acting in the critical zone of retreating coastal forests. An increase in sea level has a three‐pronged effect on flooding and salinization: It raises the maximum elevation of storm surges, shifts the freshwater‐saltwater interface inland, and elevates the water table, leading to surface flooding from below. Trees can modify their root systems and local soil hydrology to better withstand salinization. Hydrological stress from intermittent storm surges inhibits tree growth, as evidenced by tree ring analysis. Tree rings also reveal a lag between the time when tree growth significantly slows and when the tree ultimately dies. Tree dieback reduces transpiration, retaining more water in the soil and creating conditions more favourable for flooding. Sedimentation from storm waters combined to organic matter decomposition can change the landscape, affecting flooding and runoff. Our results indicate that only a multidisciplinary approach can fully capture the ecohydrology of retreating forests in a period of accelerated sea level rise. 

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