Hummock-hollow Microtopography Research Articles

Distributions of bacterial branched tetraether lipids associated with bacterial communities in hummock and hollow soils from Southern Tibetan peatland

Branched glycerol dialkyl glycerol tetraethers (brGDGTs), special membrane-spanning lipids derived from some bacteria, are widely used as molecular tools in paleoenvironmental studies to reconstruct terrestrial temperature and pH conditions. However, the impact of microtopography discrepancies on the accuracy and applicability of brGDGT-based proxies remains unclear. Here we investigated the distributional patterns of brGDGTs and bacterial communities within the hummock-hollow microtopography in the mid-southern Tibetan peatlands. Our results show that the methylation index of C5-methyl brGDGTs (MBT′5ME) values are significantly correlated with the local mean annual air temperature (MAAT). However, the peat-specific temperature calibration exhibits an underestimation of MAATpeat compared to the present MAAT. We find the MBT′5ME values are significantly lower in the moist hollows than that in the drier hummocks. This discrepancy could be primarily attributed to the shifts in bacterial communities and the physiological adaptation of brGDGT-producing bacteria driven by the high soil water content. These findings highlight the remarkable variability of MBT′5ME values at small spatial scales, which holds profound implications for paleotemperature reconstruction in alpine peatlands.

Long‐term trends in wetland event response with permafrost thaw‐induced landscape transition and hummock development

AbstractNorthwestern Canada's discontinuous permafrost landscape is transitioning rapidly due to permafrost thaw, with the conversion of elevated, forested peat plateaus to low‐lying, treeless wetlands. Increasing hydrological connectivity leads to partial drainage of previously isolated wetlands, which subsequently develop hummock–hollow microtopography. Ultimately, the ecohydrological feedbacks associated with climate‐driven permafrost thaw have led to the expansion of treed wetlands in plateau–wetland complexes. Field research and aerial imagery analyses were conducted at the Scotty Creek Research Station, Northwest Territories, to examine the development of hummock terrain over time and the resultant impacts on the hydrological response of wetlands connected to the basin drainage network. The area of peat plateaus underlain by permafrost declined between 2010 and 2018. The total area of hummock terrain increased in the basin in the same time period, with an overall decrease in the hummock perimeter‐to‐area ratio as small individual hummocks increased in size and aggregated into larger hummock complexes occupied by re‐establishing trees. With the development and expansion of hummock terrain, the tortuosity of flowpaths draining wetlands increased. The average wetland water level recession constants following precipitation events became shorter over the 15 year period of record (2003–2017). The average time of water level rise in response to precipitation events decreased over time, as precipitation was directed quickly to runoff. As permafrost thaw reduces the cover of peat plateaus in exchange for increased wetland area, the presence of treed wetlands appears to be transitioning plateau–wetland complexes into permafrost‐free forest, facilitated by the growth of hummock terrain. Permafrost thaw‐induced wetland transition triggers ecohydrological feedbacks with the potential to alter the availability and sustainability of freshwater resources.

Microtopographic heterogeneity mediates the soil respiration response to grazing in an alpine swamp meadow on the Tibetan Plateau

Soil respiration (Rs) is a major component of terrestrial carbon cycling, which varies greatly with spatial heterogeneity or under grazing. There is scant evidence for the interactive effects of microtopography and grazing on Rs, particularly in alpine swamp meadow ecosystems where hummocks and hollows are common microtopographic features. We conducted a grazing exclusion experiment in an alpine swamp meadow on the central Tibetan Plateau. Seasonal variation in Rs and relevant environmental factors were observed between hummocks and hollows in the free grazing plots and grazing exclusion plots. The growing season Rs was significantly higher in the hummocks than that in the hollows. The responses of Rs to grazing varied greatly with microtopographic heterogeneity, with grazing significantly decreasing Rs in the hummocks but greatly increasing Rs in the hollows. The grazing-induced relative changes in aboveground biomass, belowground biomass, soil organic carbon and microbial biomass carbon were positively correlated with the changes in Rs. Grazing and microtopography caused changes in soil temperature and microbial biomass were the major factors that driving the seasonal variation in Rs. This information stresses the importance of hummock-hollow microtopography in regulating carbon cycling in grazed alpine swamp meadows.

Incorporating Microtopography in a Land Surface Model and Quantifying the Effect on the Carbon Cycle

AbstractNorthern peatlands are a terrestrial carbon store, with an annual sink of 0.1 Pg C yr−1 and a total storage estimate of 547 Pg C. Northern peatlands are also major contributors of atmospheric methane. Most land surface models do not accurately represent peatland carbon emissions, partly because they do not represent the hydrologic cycle and/or microtopography adequately. Interactions between water table depth and microtopography in peatlands influence decomposition and modulate CO2 and CH4 fluxes. A modified version of the land surface component of the Energy Exascale Earth System Model, was recently created to represent the microtopography and hydrology of a raised dome bog in northern Minnesota, USA. In this study, three microtopographic parameters are analyzed in the modified version: hummock height, hummock‐hollow spacing, and percent hollow. Terrestrial laser scanning observations are used to set uncertainty bounds for these parameters. Our model experiment results suggest that carbon‐related quantities of interest (QOI) were typically the most sensitive to hummock height, and those QOI (especially net ecosystem exchange, NEE) were sensitive to interactions between parameters. Furthermore, NEE was most relatively influenced by microtopographic parameters in the model, varying by 35%. We found that increasing hummock height resulted in more C being stored in plant tissue and less in soil organic matter. This coincided with decreases in Sphagnum and increases in Picea and shrub net primary production. These results suggest that future studies may consider extending prognostic capabilities of carbon cycling by incorporating hummock hollow microtopography into earth system models.

Diatom‐inferred microtopography formation in peatlands

AbstractPeatlands, an important carbon pool in terrestrial ecosystems, are often characterised by a hummock–hollow microtopography, which has important implications for hydrologic conditions, biotic community structure and carbon cycling. However, dynamics of microtopography formation are poorly understood. Moss‐inhabiting diatoms are sensitive to water table change and may be used to infer microtopography formation. Sixty‐three surface moss samples were collected from four Sphagnum peatlands in the Changbai Mountains (north‐eastern China), covering a water table gradient of 0–55 cm. Ordination analyses revealed that depth to the water table (DWT) was the determinant of diatom distribution, and its sole effect explained 15% of total variance in diatom composition. Accordingly, a diatom‐based water table transfer function was developed using a weighted averaging model with inverse deshrinking (R2 = 0.82, RMSEP = 5.63 cm with leave‐one‐out cross validation), and applied to diatom records of a Sphagnum hummock profile. Quantitative reconstruction of DWT showed that the height of the Sphagnum hummock increased obviously in the early stage and then remained relatively stable when the reconstructed height was more than 35 cm. The results suggested that annual addition of Sphagnum litter exceeded its decomposition during the early stage of hummock formation, and Sphagnum production and decomposition reached equilibrium when the hummock had grown to a certain height. This is the first study to explore dynamics of hummock height using moss‐inhabiting diatoms, and hence provides a novel tool for quantitative reconstruction of microtopography formation in Sphagnum peatlands.

Plant species diversity of Carex peat mire in Changbai Mountains, China

We investigated the plant species richness both in cespitose Carex mires (C. schmidtii, C. meyeriana) and non-cespitose Carex mire (C. lasiocarpa) in Changbai Mountain. A total of 83 species (36 families, 59 genuses) was recorded in three sites. Among which, 71 species occurred in the C. meyeriana site, 61 species in the C. schmidtii site, and 26 species in the C. lasiocarpa site. The total species number and species richness in the two cespitose Carex mires were much higher than that in the non-cespitose Carex mire, while those on tussocks were much higher than between tussocks in the two cespitose Carex mires. Plant species richness on tussocks was positively related to the height, basal circumference and surface area of the tussocks, suggesting that tussocks were important for plant species diversity in Carex mires. Results of the canonical correspondence analysis indicated that the differences in soil water content, nutrient (soil organic carbon, total N, total P and C/N), and litter layer depth were the main factors influencing the differences of plant community composition on tussocks and between-tussocks. In the cespitose Carex mires, the hummock-hollow microtopography could foster high diversity by increasing surface area and creating multiple micro-habitats. Given its function in maintaining high species diversity, cespitose Carex could be a preferred species for vegetation restoration in degraded peat mires.

The effects of hummock-hollow microtopography on soil organic carbon stocks and soil labile organic carbon fractions in a sedge peatland in Changbai Mountain, China

Hummock-hollow microtopography is common in the northern peatlands of the world, but its effects on soil organic carbon (SOC) components are still poorly understood. In this study, we investigated effects of microtopography on SOC stocks and soil labile organic carbon (LOC) fractions in a sedge peatland in Changbai Mountain in northeast China. We found that SOC and soil LOC fractions had much heterogeneity in microtopography. SOC concentration in hummocks was significantly higher than under hummocks and in hollows. On average, the total SOC stock to a depth of 0.3 m below the ground surface was 19.00 kg C/m2. Fifty six percent of the total SOC stock was stored in soils in and under hummocks, despite the hummock only covering 30% of the total area. Light fraction organic carbon (LFOC), easily oxidizable organic carbon (EOC), microbial biomass carbon (MBC) and dissolved organic carbon (DOC) in hummocks were significantly higher than under hummocks and in hollows. Redundancy analysis (RDA) identified that SOC explained most variations in soil LOC fractions (59.6%), followed by soil total phosphorus (7.4%) and soil water content (6.6%). Our findings indicate that the hummock is an important carbon pool in the sedge peatland. Hummock-hollow microtopography creates heterogeneity in hydrological conditions and soil physicochemical properties, and thus influences SOC stocks and soil LOC fractions at a small scale. This study highlights the importance of microtopography in carbon storage and cycling and has direct implications for the assessment of the carbon sequestration function in northern peatlands.

Hummock-hollow microtopography affects soil enzyme activity by creating environmental heterogeneity in the sedge-dominated peatlands of the Changbai Mountains, China

Soil enzyme activities play a pivotal role in ecosystem processes, such as organic carbon decomposition or nitrogen and phosphorus mineralization. Thus, enzyme activities measurements can provide insights into the rates of ecosystem-level processes in peatlands. Hummock-hollow microtopography is a common feature in boreal peatlands, but its influence on soil enzyme activity is not well understood. We investigated the effects of hummock-hollow microtopography on activities of three soil hydrolases (β-1,4-glucosidase (βG), 4-N-acetylglucosaminidase (NAG), and acid phosphatase (AP)) and one oxidase (peroxidase (PER)), as well as environmental variables in a sedge-dominated peatland in Changbai Mountains, in northeast China. We found that soil enzyme activities had large microtopographical and seasonal heterogeneity. During the growing season, the activities of three soil hydrolases (βG, NAG and AP) in hummocks were 6.6, 4.6 and 5.5 times higher than those in hollows, while the PER activity in hollows was 1.9 times higher than in hummocks. Soil enzyme activities in hummocks exhibited obvious seasonal variation, with the βG, AP and PER presenting minimum values in summer and peak values in spring and autumn. Principal component analysis (PCA) indicated that soil enzyme activity had larger microtopographical heterogeneity than seasonal variations. Water table position explained most variation (74.1%) in soil enzyme activity, followed by soil temperature (5.5%) and pH (3.1%) based on ordination analysis using RDA. We conclude that hummock-hollow microtopography creates heterogeneity in hydrologic conditions, soil temperature and nutrients, and thus influences soil enzyme activity involved in carbon and nutrient cycling in sedge-dominated peatlands. This information stresses the importance of hummock-hollow microtopography in regulating carbon and nutrient cycling in northern peatlands.

Patterns of Denitrification Potential in Tidal Freshwater Forested Wetlands

Limited evidence for spatial patterns of denitrification in tidal freshwater forested wetlands (TFFWs), seemingly due to high spatial variability in the process, is surprising considering the various spatial gradients of its biogeochemical and hydrogeomorphic controls in these ecosystems. Because certain physical environmental gradients may be useful for the prediction of denitrification in TFFWs, we measured denitrification and ecosystem attributes in hummock-hollow microtopography of TFFWs along longitudinal riverine positions (upper, middle, and lower tidal river sites, and nearby upstream nontidal forested floodplains) of the adjoining Pamunkey and Mattaponi Rivers, Virginia. We tested differences by river, site, and plot in denitrification enzyme activity (DEA) and substrate limitations of denitrification potential (DP). The Pamunkey River carries greater river nitrate concentrations, and we found less nitrate limitation of DP and greater soil nitrate in hollows of this river. DEA in tidal hummocks was positively correlated with soil organic matter, nitrogen, and carbon, with the highest rates in lower tidal sites. Hummocks also promoted greater oxygen-controlled substrate limitation of DP, whereby experimental aeration stimulated DP under subsequent inundation more in hummocks than hollows. Additionally, tidal sites had greater DEA than nontidal sites, inferred to be caused by a combination of higher moisture, organic, and nutrient content. Our results indicate that the increasing nitrogen concentrations in these rivers will increase denitrification more on the Mattaponi River by alleviating its greater nitrogen limitation compared to the Pamunkey River, and modification to sedimentation, inundation, or microtopography from sea level rise may alter denitrification gradients in TFFWs and upstream low-elevation nontidal floodplains.

Assessing the peatland hummock–hollow classification framework using high-resolution elevation models: implications for appropriate complexity ecosystem modeling

Abstract. The hummock–hollow classification framework used to categorize peatland ecosystem microtopography is pervasive throughout peatland experimental designs and current peatland ecosystem modeling approaches. However, identifying what constitutes a representative hummock–hollow pair within a site and characterizing hummock–hollow variability within or between peatlands remains largely unassessed. Using structure from motion (SfM), high-resolution digital elevation models (DEMs) of hummock–hollow microtopography were used to (1) examine how much area needs to be sampled to characterize site-level microtopographic variation; and (2) examine the potential role of microtopographic shape/structure on biogeochemical fluxes using plot-level data from nine northern peatlands. To capture 95 % of site-level microtopographic variability, on average, an aggregate sampling area of 32 m2 composed of 10 randomly located plots was required. Both site- (i.e. transect data) and plot-level (i.e. SfM-derived DEM) results show that microtopographic variability can be described as a fractal at the submeter scale, where contributions to total variance are very small below a 0.5 m length scale. Microtopography at the plot level was often found to be non-bimodal, as assessed using a Gaussian mixture model (GMM). Our findings suggest that the non-bimodal distribution of microtopography at the plot level may result in an undersampling of intermediate topographic positions. Extended to the modeling domain, an underrepresentation of intermediate microtopographic positions is shown to lead to potentially large flux biases over a wide range of water table positions for ecosystem processes which are non-linearly related to water and energy availability at the moss surface. Moreover, our simple modeling results suggest that much of the bias can be eliminated by representing microtopography with several classes rather than the traditional two (i.e. hummock/hollow). A range of tools examined herein can be used to easily parameterize peatland models, from GMMs used as simple transfer functions to spatially explicit fractal landscapes based on simple power-law relations between microtopographic variability and scale.

Microtopography Alters Hydrology, Phenol Oxidase Activity and Nutrient Availability in Organic Soils of a Coastal Freshwater Forested Wetland

Hummock-hollow microtopography is a unique feature of wetland ecosystems, but our understanding of its effects on soil carbon and nutrient cycling is limited. We investigated effects of microtopography on hydrology, phenol oxidase activity (POX) and nutrient availability in a freshwater forested wetland of coastal North Carolina. Water table depth (WTD) was measured from September 2012 to August 2013. Ion exchange probes were used to measure nutrient concentrations prior to soil sample collection in August 2013. WTD fluctuated seasonally with maximum and minimum WTD resulting in 92% (September 2012) to 8% (June 2013) of the site in flooded and non-flooded conditions, respectively. Hummocks had greater POX activity (12 ± 2.8 μmol g−1 h−1) compared to hollows (4 ± 0.7 μmol g−1 h−1) and greater concentrations of potassium and sulfur, but lower concentrations of calcium, iron, zinc, boron, and lead. POX was negatively correlated with soil water content. Higher enzyme activity in hummocks likely drives greater rates of carbon and nutrient cycling compared to hollows, consistent with observations that hummocks are hotspots for CO2 fluxes. Microtopography altered site-level hydrologic conditions, phenol oxidase activity and nutrient availability with important implications for understanding carbon and nutrient cycling in forested wetlands and response to changes in hydrology.

Mineral nitrogen and phosphorus pools affected by water table lowering and warming in a boreal forested peatland

AbstractChanges in atmospheric temperature and lowering in water‐table (WT) are expected to affect peatland nutrient dynamics. To understand the response of peatland nitrogen (N) and phosphorus (P) dynamics to warming and drainage in a continental wooded‐bog of hummock–hollow microtopography, we compared three sites: (a) control, (b) recently drained (2–3 years; experimental), and (c) older drained (12–13 years; drained), during 2013. The WT was lowered at experimental and drained sites to 74 and 120 cm, respectively, whereas a warming of ~1 °C was created at one half of the microforms using open‐top chambers. Responses of peat total inorganic nitrogen (TIN = nitrate nitrogen [NO3−‐N] + ammonium nitrogen [NH4+‐N]) and phosphate‐P (PO43−‐P) pools and vegetation C:N ratio, δ13C and δ15N to the experimental treatments were investigated across sites/microforms and over time. Peat TIN available and extractable pools increased with deepening of WT and over time and were greater at hummocks relative to hollows. In contrast, the PO4 pools increased with short‐term drainage but reverted to very close to their original (control) nutrient values in the longer term. The WT and warming driven change in the peat TIN pool was strongly reflected in the vascular vegetation C:N ratio and shrub δ13C and δ15N, whereas moss nutrient dynamics did not vary between sites. Therefore, we suggest that atmospheric warming combined with WT deepening can increase availability of mineral N and P, which then can be reflected in vascular vegetation and hence modify the productivity and ecosystem functioning of the northern midlatitude continental wooded bogs in the long term.

Carbon dioxide flux and net primary production of a boreal treed bog: Responses to warming and water-table-lowering simulations of climate change

Abstract. Midlatitude treed bogs represent significant carbon (C) stocks and are highly sensitive to global climate change. In a dry continental treed bog, we compared three sites: control, recent (1–3 years; experimental) and older drained (10–13 years), with water levels at 38, 74 and 120 cm below the surface, respectively. At each site we measured carbon dioxide (CO2) fluxes and estimated tree root respiration (Rr; across hummock–hollow microtopography of the forest floor) and net primary production (NPP) of trees during the growing seasons (May to October) of 2011–2013. The CO2–C balance was calculated by adding the net CO2 exchange of the forest floor (NEff-Rr) to the NPP of the trees. From cooler and wetter 2011 to the driest and the warmest 2013, the control site was a CO2–C sink of 92, 70 and 76 g m−2, the experimental site was a CO2–C source of 14, 57 and 135 g m−2, and the drained site was a progressively smaller source of 26, 23 and 13 g CO2–C m−2. The short-term drainage at the experimental site resulted in small changes in vegetation coverage and large net CO2 emissions at the microforms. In contrast, the longer-term drainage and deeper water level at the drained site resulted in the replacement of mosses with vascular plants (shrubs) on the hummocks and lichen in the hollows leading to the highest CO2 uptake at the drained hummocks and significant losses in the hollows. The tree NPP (including above- and below-ground growth and litter fall) in 2011 and 2012 was significantly higher at the drained site (92 and 83 g C m−2) than at the experimental (58 and 55 g C m−2) and control (52 and 46 g C m−2) sites. We also quantified the impact of climatic warming at all water table treatments by equipping additional plots with open-top chambers (OTCs) that caused a passive warming on average of ~ 1 °C and differential air warming of ~ 6 °C at midday full sun over the study years. Warming significantly enhanced shrub growth and the CO2 sink function of the drained hummocks (exceeding the cumulative respiration losses in hollows induced by the lowered water level × warming). There was an interaction of water level with warming across hummocks that resulted in the largest net CO2 uptake at the warmed drained hummocks. Thus in 2013, the warming treatment enhanced the sink function of the control site by 13 g m−2, reduced the source function of the experimental by 10 g m−2 and significantly enhanced the sink function of the drained site by 73 g m−2. Therefore, drying and warming in continental bogs is expected to initially accelerate CO2–C losses via ecosystem respiration, but persistent drought and warming is expected to restore the peatland's original CO2–C sink function as a result of the shifts in vegetation composition and productivity between the microforms and increased NPP of trees over time.

Natural Revegetation of Winter Roads on Peatlands in the Hudson Bay Lowland, Canada

Winter roads across subarctic peatlands are increasingly being used to access remote communities and resource development camps, yet relatively little is known on their ability to recover after abandonment. We evaluated the natural recovery of winter roads abandoned within 7 years on peatlands in the Hudson Bay Lowland, Canada. We sampled 5 winter roads of increasing age of abandonment and compared surface elevation, microtopography, active layer thickness, species cover, diversity, and composition between winter road clearances and adjacent undisturbed peatland. No differences in surface elevation and hummock-hollow microtopography were detected between road clearances and adjacent peatlands, but clearances had significantly thinner active layer, which persisted at least 7 years after abandonment. The cover of lichens, bryophytes, and vascular plants returned within 5 years to similar levels as in undisturbed peatlands, although species richness per quadrat remained lower and species composition differed. The limited recovery of black spruce on these peatlands and their slow growth indicates that the full recovery of vegetation structure on these road clearances will take decades. Future research should focus on the restoration of a Sphagnum carpet and on the interactions between a shallower active layer and the revegetation of abandoned winter roads.

Initiation of microtopography in revegetated cutover peatlands

Question: How many years are required for a gradient of microtopography to be initiated in revegetated cutover peatlands and become similar to natural bogs? Location: Newly formed Sphagnum carpets on cutover peatlands that revegetated spontaneously after site abandonment (in Estonia), or following active restoration (in Canada) and on undisturbed natural bogs nearby. Methods: Moss surface height was measured along linear transects above a local reference level (the lowest point for a given transect). Heights of at least 20 cm were associated with hummocks. Frequency distributions of surface height and principal component analyses (separately for Canada and Estonia) were conducted to follow the evolution of microtopography in revegetated sites and their similarity with those of natural peatlands. In Canada, regressions were also performed to estimate the time required for the microtopography in revegetated cutover peatlands to become similar to that found in natural bogs. Results: Only 10–30 yr were needed for microstructures comparable to those in natural bogs to develop on restored peatlands where Sphagnum diaspores have been reintroduced. However, this process may take more than a century in cutover peatlands left to revegetate spontaneously. Conclusions: In cutover peatlands with spontaneous revegetation, hummock–hollow formation starts on bare peat which lacks both plant propagules and viable seed banks, and the initiation of microstructures is probably more akin to the process that occurs naturally. Nonetheless, hummock–hollow microtopography resembling that found in natural bogs without pools appeared, in all of the examined cutover peatlands, over periods that are short in terms of peatland development time-scales. Active peatland restoration could effectively reduce the time required for initiation of microtopography by about 70 yr.

Measuring biodiversity across spatial scales in a raised bog using a novel paired‐sample diversity index

Summary 1. Biodiversity is an inherently spatial phenomenon. It is determined by environmental heterogeneity and by spatially structured ecological processes such as disturbance, competition and dispersal. However, conventional biodiversity indices are based on discrete samples or pooled sets of samples without sufficient consideration of the spatial relationships between the samples. 2. Here we describe a new method for spatial analysis of species diversity, based on a paired‐sample version of the widely used Gini–Simpson diversity index and its numbers equivalent. The index and its numbers equivalent are plotted as a function of lag distance between two samples along spatial and/or environmental gradients. 3. We demonstrate the potential of this approach by applying it to two transects of fine‐scale (5 × 5 cm quadrat) vegetation data from sites with contrasting hydrology within a raised bog, where the location of each quadrat is accurately recorded and the height of the bog surface above the water table is measured using a terrestrial laser scanner. 4. Both transects have similar alpha‐diversity as measured using the Gini–Simpson index, and the transition between alpha‐ and gamma‐diversity occurs at similar length scales, suggesting that species aggregate at similar scales along both transects. However, the transect from the central bog dome has higher gamma‐diversity than that from the bog margin, and shows more marked significant spatial structure at a length scale of 135–140 cm, corresponding to the typical hummock–hollow microtopography at the site. We show that beta‐diversity at both transects can be attributable to both species clustering along the hydrological gradient, consistent with niche partitioning, as well as independent spatial aggregation of species that is not explained by hydrology. 5. Synthesis. The paired‐sample diversity index described here is a potentially useful tool in detecting and attributing patterns of beta‐diversity along both spatial and environmental gradients.

Dealing with microtopography of an ombrotrophic bog for simulating ecosystem-level CO 2 exchanges

Peatlands contain approximately 25% of the global soil carbon (C), despite covering only 3% of the earth's land surface. In order to evaluate the role of peatlands in global C cycling, models of ecosystem biogeochemistry are required, but peatland ecosystems present a number of unique challenges, particularly how to deal with the large variability that occurs at scales of one to several metres. In models, spatial variability is considered either explicitly for each individual unit and the outputs averaged, referred to as flux upscaling, or implicitly by weighting model parameters by the fractional occurrence of the individual units, referred to as parameter upscaling. The advantage of parameter upscaling is that it is much more computationally efficient: a requirement for hemispheric scale simulations. In this study we determined the differences between modelling a raised bog peatland with hummock–hollow microtopography using flux and parameter upscaling. We used the McGill Wetland Model (MWM), a process-based ecosystem C model for peatlands, configured for hummocks and hollows separately and then a weighted mixture of both. The simulated output based on flux and parameter upscaling was compared with eddy-covariance tower measurements. We found that net ecosystem production (NEP) for hollows was much larger than that for hummocks because total ecosystem respiration (TER) for hummocks was greater while gross primary production (GPP) did not differ significantly between the two topographic features. However, despite differences in components of NEP between hummocks and hollows, there was no statistically significant difference between the NEP based on flux and parameter upscaling using the MWM. Both flux and parameter upscaling show equivalent capability to capture the magnitude, direction, seasonality and inter-annual variability. The root-mean-square-errors (RMSE) are 0.66, 0.45, and 0.49 g C m −2 day −1, respectively for GPP, TER and NEP based on the flux upscaling, while 0.67, 0.44, and 0.48 g C m −2 day −1, respectively based on the parameter upscaling. The degree of agreement ( d*) is 0.96, 0.97, and 0.88, respectively for GPP, TER and NEP based on the flux upscaling, while 0.96, 0.97, and 0.89, respectively based on the parameter upscaling. This result suggests that differences in processes caused by peatland microtopography scale linearly, which means an ecosystem-level model set-up ( i.e. parameter upscaling scheme), is sufficient to simulate the C cycling.

Modeling Peat Thermal Regime of an Ombrotrophic Peatland with Hummock–Hollow Microtopography

The theory of conductive heat transfer cannot explain different attenuations of the daily amplitude of peat temperatures (TS) in hummocks (detectable below the 20‐cm depth) and hollows (disappearing above the 10‐cm depth). Large readily drained macropores in the upper fibric peat determine a large air permeability and hence may enhance heat transfer by air convection in porous media, driven by temperature gradients between hummock sides and interiors. In this study, the ecosys model was used to simulate a peat thermal regime at Mer Bleue peatland, Ontario, Canada. It was hypothesized that adding the air‐convective heat transfer to conductive plus water‐convective heat transfers would improve simulations of TS The results for TS, ground heat fluxes, G, and sensible heat fluxes, H, modeled with and without air‐convective heat transfer were tested with continuous hourly measurements from 2000 to 2004 using thermocouples, heat flux plates, and eddy covariance. Simulated air‐convective heat transfer caused an average increase in G and a corresponding decrease in H of ∼20 W m−2 from the simulated conductive plus water‐convective heat transfer. Hastened soil warming in hummocks resulted in better agreement between measured and simulated hummock TS values with (RMSD of 2.23°C) than without air‐convective heat (RMSD of 2.54°C). Enhanced hummock TS caused an indirect increase in hollow TS in the model with (RMSD of 1.68°C) compared to without air‐convective heat (RMSD of 1.82°C). Our results suggest that air convection is probably an important mechanism of heat transfer in peat hummocks and should be included in peatland biogeochemical models.

Microtopography in tidal marshes: Ecosystem engineering by vegetation?

Hummock-hollow microtopography is characteristic of many freshwater wetland systems. It is comprised of elevated, vegetated hummocks and lower elevation hollows; the latter are usually unvegetated, with reducing conditions in sediments unfavorable for plant growth. This microtopography is also often found in interior regions of brackish marshes, where flood duration is high and salinity fluctuations are prominent. Previous investigation showed this spatial patterning to be relatively stable over time and suggested that these microenvironments are produced by the plants themselves. This study investigates the possible mechanisms and controlling factors of this microtopography and considers the effect of different salinity regimes. We examined microtopographic variability of vegetation and sediment biogeochemistry in two interior tidal marshes, a freshwater-oligohaline marsh and a mesohaline marsh, both of which exhibited fine-scale spatial variability. Within a 2-yr period, the freshwater-oligohaline site demonstrated a labile response of both vegetation and sediment chemistry to interannual variability in salinity and sulfide concentrations, whereas the microscale spatial variability of the mesohaline system persisted. Geochronological assessment of the mesohaline marsh, where microtopographic variability was relatively stable, supported the hypothesis that the formation of the hummock-hollow topography is driven by the plants, rather than developing as a result of underlying physical variability. We propose that brackish marsh vegetation alters the sedimentary environment in such a way as to maximize growth under high-stress, variable conditions. The adaptive advantage of this strategy was illustrated in the accretion rates measured at the higher salinity marsh, which were indistinguishable between the interior hummock sediments and those of an adjacent homogeneous bank marsh.

Mortality and growth of trees in peat-swamp and heath forests in Central Kalimantan after severe drought

Lowland forests in Central Kalimantan, Indonesian Borneo, are endangered by land conversion and the increasing frequency of severe drought. Knowledge of the tolerance of tropical trees to drought is urgent for the management of these lowland habitats. The short-term effects of drought on tree demography (mortality and growth) were investigated in an ever-wet riparian peat-swamp forest and a heath forest on coarse sandy soil after the 1997 El Nino Southern Oscillation (ENSO) event. This drought was unusually severe because little rain fell during the following rainy season. However, forest-wide mortality following the drought (1997–1999) was not critically high in the peat-swamp (6.13% yr−1) or heath (4.26% yr−1) forest. In both forests, standing trees frequently died during the dry season following the drought. The riparian peat-swamp forest was not flooded until 1998, after the prolonged drought in 1997. The hummock–hollow microtopography resulted in differential mortality of peat-swamp trees. On tall hummocks, standing death increased two-fold (4.99% yr−1) during the dry season, whereas uprooting decreased by one-third (0.85% yr−1) during the following rainy season. In contrast, tree growth was not affected by hummock height. Common canopy species were concentrated on tall hummocks and died standing more often than did understory species found in hollows, indicating species-specific mortality after the drought. The large stand basal area relative to the forest-wide growth rate in diameter suggested less resilience to drought by peat-swamp (45.6 m2 ha−1 and 0.0186 ln[cm] yr−1) than heath (27.9 m2 ha−1 and 0.0232 ln[cm] yr−1) forest. A single severe drought did not cause dramatic changes in the peat-swamp and heath forests; however, an increasing frequency of droughts similar in severity to that of the 1997 ENSO event may have the potential to alter the community structure and dynamics, leading to a consistent decline in Bornean lowland forests.