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Warming is expected to increase the net release of carbon from peatland soils, contributing to future warming. This positive feedback may be moderated by the response of peatland vegetation to rising atmospheric [CO2] or to increased soil nutrient availability. We asked whether a gradient of whole-ecosystem warming (from + 0 °C to + 9 °C) would increase plant-available nitrogen and phosphorus in an ombrotrophic bog in northern Minnesota, USA, and whether elevated [CO2] would modify the nutrient response. We tracked changes in plant-available nutrients across space and through time and in comparison with other nutrient pools, and assessed whether nutrient warming responses were captured by a point version of the land-surface model, ELM-SPRUCE. We found that warming exponentially increased plant-available ammonium and phosphate, but that nutrient dynamics were unaffected by elevated [CO2]. The warming response increased by an order of magnitude between the first and fourth year of the experimental manipulation, perhaps because of dramatic mortality of Sphagnum mosses in the surface peat of the warmest treatments. However, neither the magnitude nor the temporal dynamics of the responses were captured by ELM-SPRUCE. Relative increases in plant-available ammonium and phosphate with warming were similar, but the response varied across raised hummocks and depressed hollows and with peat depth. Plant-available nutrient dynamics were only loosely correlated with inorganic and organic porewater nutrients, likely representing different processes. Future predictions of peatland nutrient availability under climate change scenarios must account for dynamic changes in nutrient acquisition by plants and microbes, as well as microtopography and peat depth. [ABSTRACT FROM AUTHOR]

Bryophytes are largely responsible for globally significant carbon accumulation in peatland ecosystems. This accumulation is primarily caused by the slow decomposition of these mosses, which can be attributed to a combination of intrinsic (chemical) characteristics of decaying mosses and extrinsic (environmental) influences. Here we investigated the importance of intrinsic and extrinsic drivers of moss decomposition in peatlands and hypothesized that the early stages of decay will be driven primarily by intrinsic characteristics, while extrinsic drivers will become more important in the later phases of decomposition. We tested this hypothesis by placing litterbags of three moss types (Sphagnum divinum, Sphagnum angustifolium/fallax and Polytrichum sp.) into hummock and hollow microtopographies in a bog and a lawn microtopography in a nearby poor fen in northern Minnesota, USA. Decomposition was measured over a 5‐year period and confirmed our hypothesis; while intrinsic and extrinsic variables were both important in driving decay rates over time, decay in the first year was primarily predicted by litter type, while extrinsic influences (e.g. peat temperature, moisture, nutrient limitation) that varied at peatland and microtopographic scales became more important by year 5. Though the nutrient (i.e. nitrogen [N] and phosphorus [P]) content of decaying mosses did not explain the variation in decay rates among litter types, analysis of N:P in moss litter suggested that litter‐decaying microorganisms became increasingly P‐limited over time. The majority of litter decomposition studies in peatlands are generally of short duration (< 3 years). Our study suggests that longer‐term studies are needed to reveal the roles of multiple drivers of moss decay. Improving the mechanistic understanding of decomposition processes in peatlands will be critical to developing more robust representations of peatlands and their ecosystem processes in carbon and climate models. [ABSTRACT FROM AUTHOR]

Black ash (Fraxinus nigra) wetlands are widespread, forested landscape features in the western Great Lakes region. However, the future of these ecosystems is threatened due to impending spread of the invasive emerald ash borer (EAB), which results in tree mortality, decreased transpiration, and potential shifts to wetter, non‐forested conditions. The vulnerability to such ecohydrologic shifts likely varies according to local hydrologic regimes controlled by landscape settings, but this site‐dependent vulnerability and our ability to predict it is unknown. Here, we assessed vulnerability potential as a function of site hydrology in 15 undisturbed black ash wetlands from their three most common hydrogeomorphic settings in northern Minnesota: lowland, depression, and transition. Further, we used high‐resolution (1‐cm) surface elevation models to assess spatial variability of water levels at a subset of 10 sites. Although we observed similar ET and groundwater exchange rates among settings, lowland sites were generally drier because of elevated landscape position and greater water level drawdowns (via lower specific yield). We predict that such drier sites will exhibit greater water level increases following EAB‐induced ash mortality, compared to wetter sites where open water evaporation and shallow‐rooted understory transpiration will offset losses in tree transpiration. Moreover, compared to wetter sites, drier sites exhibited minimal microtopographic variation, limiting the number of elevated microsites for tree establishment and eventual canopy recovery after ash loss. These results suggest that site wetness is a simple and effective predictor of black ash wetland vulnerability to hydrologic regime change. To that end, we assessed the ability of common terrain metrics to predict site wetness, providing a potential tool to target vulnerable areas for active management efforts. [ABSTRACT FROM AUTHOR]

Raised roads and their appurtenances (i.e., ditches and culverts) can considerably modify the natural hydrologic processes (e.g., flow direction and runoff spatial distribution) of low-relief watersheds. However, these effects are not well understood and are poorly documented in literature. Also, there is a serious lack of effective approach that can be used to describe these effects in common simulation models, including the Soil and Water Assessment Tool (SWAT). As a result, these effects have been rarely taken into account by the existing model applications. The results from those applications are likely to be misleading for watershed management decisions. The objectives of this study were to (1) develop an approach that enables the incorporation of the aforementioned effects into hydrologic models (e.g., SWAT), and (2) evaluate whether and how a model's simulation performance can be improved by taking these effects into account. The evaluation was conducted for the low-relief Wild Rice River watershed located in northwestern Minnesota by comparing the SWAT-NoRoad model, which is based on a 'conventional flow direction map' derived using the deterministic eight-neighbor (D8) algorithm, with the SWAT-Road model, which is based on a 'corrected flow direction map' generated by using the approach developed in this study. The corrected map provided more realistic flow directions for the regions adjacent to the raised roads than the conventional map. Compared with the SWAT-NoRoad model, the SWAT-Road model had a compatible performance for a calibration period but was more robust and did a better job in reproducing the observed streamflows for a validation period at two U.S. Geological Survey gauging stations, as indicated by the larger values for Nash-Sutcliffe (Ej2>0.60) and determination (R2>0.68) coefficients, as well as from the watershed perspective view, as indicated by the larger values for performance virtue (PVk>0.64). In addition, the spatial patterns of water yield predicted by the SWAT-Road model were more accurate because they are more similar to those of the topographic wet index values than the spatial patterns predicted by the SWAT-NoRoad model. Furthermore, if the raised roads were neglected, the mean daily water yields for the study area would be erroneously predicted by up to 0.6 mm. A reasonable generalization of this study is that hydrologic effects of raised roads are important for accurately simulating watersheds with a low topographic relief and thus need to be taken into account. [ABSTRACT FROM AUTHOR]