Paul J. Morris

Peatlands are important global carbon stores, but it is unclear whether they will continue as carbon sinks under future climates, or degrade, leaking carbon to the atmosphere. We used the DigiBog peatland development model to explore this question for a blanket peatland in Glen Affric, Scotland, investigating how topography and climate have affected net peat accumulation across this complex landscape. DigiBog accurately captured spatial variation in measured modern peat thickness (Lin's Concordance Correlation Coefficient = 0.86). As expected, peat accumulated to greater depths in topographic basins compared to slopes and drainage divides over the Holocene. Simulations of the future suggest the study site will continue accumulating peat until 2100 CE under all Representative Concentration Pathways. However, DigiBog predicts greatest future peat accumulation away from basins, continuing a trend observed over the last c. 2000 years. We also tested Winter's (2000), https://doi.org/10.1111/j.1752‐1688.2000.tb04269.x conceptual model, which proposes that wetland vulnerability to climate change depends on position in the hydrologic landscape, by comparing peat thickness with the Topographic Wetness Index. While correlations were moderate to strong, our finding of greater recent accumulation away from basins contradicts Winter's model's predictions. Additionally, DigiBog shows that thin (<40 cm) hillslope peats persisted throughout the Holocene despite drier climatic phases, rather than experiencing complete loss and re‐establishment cycles. We conclude that process‐based ecohydrological models like DigiBog can complement simpler modeling approaches and should be used when assessing rates and spatial patterns of peat accumulation and loss in response to 21st Century climate change.

The peat swamp forests of the central Congo Basin represent the largest, most carbon‐rich peatland complex in the tropics. The maintenance of peat carbon stocks and accumulation relies on persistent soil saturation, yet the hydrological functions of tropical peatlands remain poorly understood, particularly in the central Congo Basin. We report the first measurements of saturated hydraulic conductivity ( K _sat ) from Congolese peats. We used Mini Disk tension infiltrometers to estimate K _sat at depths of 0 and 10 cm across three study areas in the Democratic Republic of the Congo. K _sat ranged from 1.55 × 10 ⁻⁵ to 5.75 × 10 ⁻⁴ m s ⁻¹ , with a mean of 1.78 × 10 ⁻⁴ m s ⁻¹ . A general linear model revealed significantly higher K _sat at the surface than at 10 cm depth, while K _sat is negatively related to degree of peat humification. However, K _sat did not vary between palm, hardwood and mixed forest types, or with proximity to rivers (a proxy for flooding regime). Although our linear model is not sufficiently skilful to predict K _sat reliably, our findings suggest that a generalisable model for tropical peat K _sat may be attainable with larger datasets that span greater ranges of depths and levels of decomposition. Notably, our surface K _sat values are lower than those from deeper layers in Southeast Asia and the Americas, implying lower depth‐integrated permeability in peats of the central Congo Basin. This may reflect low rainfall and deeper water tables in the region, which would enhance decomposition and reduce pore space more rapidly.

Tropical peatland wildfire incidence has risen in recent decades, driven by drainage for land use and intensified by severe droughts with global climate change. These disturbances have altered vegetation structure, disrupted ecosystem functioning, and increased carbon emissions, particularly in Southeast Asia. However, the long‐term history and characteristics of wildfires in tropical peatlands remain largely unknown. Here, we compiled fifty‐eight macro‐charcoal records from peatlands across the tropics, ranging from lowland forested to montane peatlands, to assess millennia‐scale changes and controlling factors of tropical peatland burning. We divided the datasets into four main sub‐regions: Neotropical, Afrotropical, Indomalayan and Australasian ecoregions to explore regional variability. Tropical peatlands had high burning levels between 0 and 850 ce , followed by a relatively low and stable period until a marked increase during the 20th century. The general trend in tropical peatland burning follows changes in global temperature, and climate variables that control the length and severity of drought events have a notable influence on peat burning before 1900 ce . During the 20th century, regional differences were observed, with declining fire trends in the Neotropical and Afrotropical regions and increasing fire trends in the Indomalayan and Australasian regions. This difference is likely attributable to human activities, and such intervention is also evident in palm swamps and hardwood swamps under similar wet, weakly seasonal climates. With the increase in anthropogenic pressures on peatlands and greater climate variability, future wildfires in peatlands are likely to become more frequent and widespread across all tropical ecoregions. Conservation and sustainable land‐use practices could be used to mitigate and control peatland burning and protect these carbon‐rich sinks.

As climate change increasingly threatens the northern peatland net carbon sequestration function, there is a pressing need to better understand the limits of ecohydrological regulatory mechanisms. This is especially urgent for shallow peatlands (< 40‐cm average peat depth), which consistently experience water stress with greater intensity, frequency and duration than deep peatlands and may represent sentinels for climate change. In this ‘part 2’ paper, we review the peatland hydrological feedbacks originally proposed a decade prior in Hydrological Feedbacks in Northern Peatlands ‘part 1’ (Waddington et al. 2015) to investigate the strength of feedback mechanisms as a function of peat depth. We show that in some hydrogeomorphic and hydroclimatic settings there are differences in hydrophysical properties and vegetation cover between shallow and deep peatlands. These structural characteristics influence the strength of the fast (i.e., function on a timescale of seconds to days) hydrological feedbacks (moss surface resistance and albedo, transmissivity, peat deformation and specific yield). In contrast, the slow feedbacks (i.e., operating on the scale of months to decades) related to vegetation community change and peat decomposition directly impact peatland physical characteristics (patterns and composition of vegetation, bulk density, etc.). We discuss how the vulnerability of shallow peatlands arises from the interactions between regulatory (negative) and destabilizing (positive) ecohydrological feedbacks.

The central Congo Basin contains the world’s most extensive tropical peatland complex, spanning 16.7 million hectares. Until now, radiocarbon dating of basal peats has been limited to 14 samples with poor spatial coverage, and suggested that peat typically initiated during the Holocene. We present 38 new basal dates, improving spatial coverage across the region. Some of the new basal dates are much older than any previous dates, indicating that peat initiated in the central Congo Basin at multiple locations in the Late Pleistocene. Our oldest basal date is 42 300 (41 200–43 800) calibrated years before present, making this one of the world’s oldest extant tropical peatlands, and twice as old as previously believed. The temporal distribution of basal dates suggests that changing climatic wetness has played a role in peat initiation in the region; numerous basal dates correspond with climatically wet phases, whilst few basal dates correspond with dry phases such as the Last Glacial Maximum (LGM). Today we find the central Congo Basin peatlands on wide interfluves between rivers, and on floodplains (mostly of the dendritic left-bank tributaries of the Congo River). We find the oldest basal dates on the floodplains of these left-bank tributaries, indicating a surprisingly high degree of channel stability over many millennia. This contrasts with, for example, peatlands on Amazonian floodplains, which are typically just a few thousand years old. The persistence of peat in the central Congo Basin since before the LGM, likely the most climatically dry period during the last 42 000 yr in this region, suggests that these areas may have played an important biogeographical role as forest refugia during glacial-interglacial cycles.

Shallow peatlands as sentinels of climate change, Sutton, Owen F, Furukawa, Alex K, Moore, Paul A, Morris, Paul J, Waddington, James M

Nascent peatlands represent an emerging, nature‐based carbon sink in the global climate system. A warming climate and changing precipitation regime could drive peat initiation beyond the current latitudinal and altitudinal boundaries of the peatland bioclimatic envelope, through increases in plant productivity and moisture availability, with potential implications for global radiative forcing. However, contemporaneous observations of new peat formation remain scarce. We investigate peat initiation within the deglaciating Rob Roy valley in the Southern Alps, Aotearoa/New Zealand. We find that montane peats have developed across the head of the valley since ∼1949 C.E., coinciding with regional climate warming and glacial retreat. Further, we identify a common ecological succession, characterized by a rise in brown mosses (mainly Bryum ) beginning around ∼1963 C.E. Our findings indicate the potential for wider peat expansion in increasingly warm and wet montane landscapes. However, further bioclimatic modeling is required to elucidate where future peatland developments may occur.

Raised bogs are wetland ecosystems which, under the right climatic conditions, feature patterns of pool hollows and hummock ridges. The relative cover and the spatial arrangement of pool and ridge microforms are thought to be influential on peatland atmosphere carbon gas fluxes and plant biodiversity. The mechanisms responsible for the formation and maintenance of pools, and the stability of these features in response to warming climates, remain topics of ongoing research. We employed historical aerial imagery, combined with a contemporary uncrewed aerial vehicle survey, to study 61 years of changes in pools at a patterned raised bog in central Sweden. We used a pool inheritance method to track individual pools between image acquisition dates throughout the time series. These data show a rapid loss of open-water pool area during the study period, primarily due to overgrowth of open-water pools by Sphagnum. We postulate that these changes are driven by ongoing climate warming that is accelerating Sphagnum colonisation. Open-water pool area declined by 26.8% during the study period, equivalent to a loss of 1001 m² y⁻¹ across the 150-hectare site. This is contradictory to an existing theory that states pools are highly stable, once formed, and can only convert to a terrestrial state through catastrophic drainage. The pool inheritance analysis shows that smaller pools are liable to become completely terrestrialised and expire. Our findings form part of a growing body of evidence for the loss of open-water habitats in peatlands across the boreal and elsewhere.

Permafrost peatlands are vulnerable to rapid structural changes under climatic warming, including vertical collapse. Peatland water budgets, and therefore peat hydraulic properties, are important determinants of vegetation and carbon fluxes. Measurements of hydraulic properties exist for only a limited number of permafrost peatland locations, primarily concentrated in North America. The impacts of thaw‐induced collapse upon properties such as horizontal saturated hydraulic conductivity (K_h), and thus lateral drainage, remain poorly understood. We made laboratory determinations of K_h from 82 peat samples from a degrading Swedish palsa mire. We fitted a linear mixed‐effects model (LMM) to establish the controls on K_h, which declined strongly with increasing depth, humification and dry bulk density. Depth exerted the strongest control on K_h in our LMM, which demonstrated strong predictive performance (r² = 0.605). Humification and dry bulk density were influential predictors, but the high collinearity of these two variables meant only one could be included reliably in our LMM. Surprisingly, peat K_h did not differ significantly between desiccating and collapsed palsas. We compared our site‐specific LMM to an existing, multi‐site model, fitted primarily to boreal and temperate peatlands. The multi‐site model made less skillful predictions (r² = 0.528) than our site‐specific model, possibly due to latitudinal differences in peat compaction, floristic composition and climate. Nonetheless, low bias means the multi‐site model may still be useful for estimating peat K_h at high latitudes. Permafrost peatlands remain underrepresented in multi‐site models of peat hydraulic properties, and measurements such as ours could be used to improve future iterations.