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Climate change in the coming centuries will be characterized by interannual, decadal, and multidecadal fluctuations superimposed on anthropogenic trends. Predicting ecological and biogeographic responses to these changes constitutes an immense challenge for ecologists. Perspectives from climatic and ecological history indicate that responses will be laden with contingencies, resulting from episodic climatic events interacting with demographic and colonization events. This effect is compounded by the dependency of environmental sensitivity upon life-stage for many species. Climate variables often used in empirical niche models may become decoupled from the proximal variables that directly influence individuals and populations. Greater predictive capacity, and more-fundamental ecological and biogeographic understanding, will come from integration of correlational niche modeling with mechanistic niche modeling, dynamic ecological modeling, targeted experiments, and systematic observations of past and present patterns and dynamics.

The Neotoma Paleoecology Database is a community-curated data resource that supports interdisciplinary global change research by enabling broad-scale studies of taxon and community diversity, distributions, and dynamics during the large environmental changes of the past. By consolidating many kinds of data into a common repository, Neotoma lowers costs of paleodata management, makes paleoecological data openly available, and offers a high-quality, curated resource. Neotoma’s distributed scientific governance model is flexible and scalable, with many open pathways for participation by new members, data contributors, stewards, and research communities. The Neotoma data model supports, or can be extended to support, any kind of paleoecological or paleoenvironmental data from sedimentary archives. Data additions to Neotoma are growing and now include >3.8 million observations, >17,000 datasets, and >9200 sites. Dataset types currently include fossil pollen, vertebrates, diatoms, ostracodes, macroinvertebrates, plant macrofossils, insects, testate amoebae, geochronological data, and the recently added organic biomarkers, stable isotopes, and specimen-level data. Multiple avenues exist to obtain Neotoma data, including the Explorer map-based interface, an application programming interface, the neotoma R package, and digital object identifiers. As the volume and variety of scientific data grow, community-curated data resources such as Neotoma have become foundational infrastructure for big data science.

Testate amoebae are a group of moisture-sensitive, shell-producing protozoa that have been widely used as indicators of changes in mean water-table depth within oligotrophic peatlands. However, short-term environmental variability (i.e., sub-annual) also probably influences community composition. The objective of this study was to assess the potential influence of short-term environmental variability on the composition of testate amoeba communities in Sphagnum-dominated peatlands. Testate amoebae and environmental conditions, including hourly measurements of relative humidity within the upper centimeter of the peatland surface, were examined throughout the 2008 growing season at 72 microsites within 11 peatlands of Pennsylvania and Wisconsin, USA. Relationships among testate amoeba communities, vegetation, depth to water table, pH, and an index of short-term environmental variability (EVI), were examined using nonmetric multidimensional scaling and correlation analysis. Results suggest that EVI influences testate amoeba communities, with some taxa more abundant under highly variable conditions (e.g., Arcella discoides, Difflugia pulex, and Hyalosphenia subflava) and others more abundant when environmental conditions at the peatland surface were relatively stable (e.g., Archerella flavum and Bullinularia indica). The magnitude of environmental variability experienced at the peatland surface appears to be primarily controlled by vegetation composition and density. In particular, sites with dense Sphagnum cover had lower EVI values than sites with loose-growing Sphagnum or vegetation dominated by vascular plants and/or non-Sphagnum bryophytes. Our results suggest that more environmental information may be inferred from testate amoebae than previously recognized. Knowledge of relationships between testate amoebae and short-term environmental variability should lead to more detailed and refined environmental inferences.

Several regions of the world have recently experienced climate-induced changes in forest composition, highlighting the need to understand the causes, likelihood, and dynamics of abrupt vegetation change. Although few historical examples of climate-induced forest change exist from recent centuries, particularly in humid regions like the northeastern United States, paleoecological records are rich with examples. For example, pollen records from portions of the northeastern United States indicate that eastern hemlock (Tsuga canadensis) and American beech (Fagus grandifolia) abruptly declined in abundance between 500 and 600 yr BP. Concomitant increases in pine (Pinus spp.) and oak (Quercus spp.) occurred. Hypotheses to explain this change have included cooling during the ‘Little Ice Age’ (LIA), Native American activity, drought, and/or fires. To better understand spatiotemporal patterns of forest change and assess potential causes and dynamics, we synthesized regional pollen records and developed two high-resolution, coupled records of vegetation, fire, and drought from bogs in Maine. Results of our synthesis reveal >70% of regional pollen sites recorded shifts in forest composition during this time period. Bog records revealed that forest composition changed a few decades after the onset of drought and regional fires, consistent with increased recruitment of pine and oak during post-disturbance succession. Vegetation changes persisted until European settlement. Our data demonstrate that widespread, long-lasting forest changes were triggered by decadal-to-multidecadal drought and associated fires, highlighting the potential for abrupt, long-lasting forest changes in response to transient climate and disturbance events, particularly when such events occur against the backdrop of more gradual temperature change.

Understanding the processes controlling peatland carbon (C) sequestration is critical to anticipate potential changes in the global C cycle in response to climate change. Although identification of these factors may be relatively straightforward on seasonal timescales, at centennial to millennial timescales complexities arise because of interactions between climate, vegetation, hydrology and long-term ecological processes. To better understand the factors controlling long-term C accumulation, Holocene rates of C sequestration were quantified from three pristine ombrotrophic peatlands in boreal Quebec, northeastern Canada (52°N, 75–76°W). Bulk density and loss-on-ignition analyses, combined with radiocarbon dating and age–depth modelling, were used to estimate long-term apparent rates of carbon accumulation. Past changes in vegetation and water-table depth were obtained from plant macrofossil and testate amoeba analysis. Earliest regional peat accumulation started ~7520 cal. BP, with long-term rates of C sequestration varying between 14.9 and 22.6 g/m2 per yr. High C sequestration rates occurred during the mid Holocene when relatively stable Sphagnum section Acutifolia communities were present, while low rates were found during the cooler late Holocene when Cyperaceae and ligneous vegetation were more dominant. However, C sequestration was highly variable among cores, implying that local topography, geomorphology and hydrology, or disturbance factors such as fire, mediate the influence of climate on C accumulation. Reconstructed water-table depths reveal several dry shifts since 3000 cal. BP, suggesting that episodic cold and dry conditions during the late Holocene may have contributed to lower C sequestration rates. Given the intensity of the water-table shifts at these times, we hypothesize that recurrent episodes of frozen subsurface peat might have intensified surface drying. As projected by climate scenarios, anticipated warmer and wetter conditions may lead to greater stability of hummock Sphagna cover and increased C sequestration potential in boreal peatlands.

Recent high-latitude warming is increasing the vulnerability of permafrost to thaw, which is amplified by local disturbances such as fire. However, the long-term ecological effects and carbon dynamics are not well understood. Here we present a 2200-year record of pollen, plant macrofossils, testate amoebae, and apparent carbon (C) accumulation rates from two peat cores in a collapsescar bog (thermokarst) near Fairbanks, Alaska. A black spruce ecosystem with low apparent C accumulation rates existed on the site during the first ∼1500 years of the record. We identify two thaw events, which are linked to local fires. Permafrost aggraded rapidly following the first thaw, which we attribute to local vegetation feedbacks and a cooler climate. The second thaw event at 525 cal y BP is preceded by a stand-replacing fire, as evidenced by a drastic decline in Picea and an initial increase in Epilobium, Salix, and ericaceous shrubs, followed by a sustained increase in Populus. Locally, the forest does not recover for more than 100 years, and the site has remained permafrost-free for the last 500 years. Following thaw, average apparent C accumulation rates (60 to > 100 g C m -2 y -1 ) are 5—6 times higher than average boreal C accumulation rates, indicating that peat C accumulation rates can remain substantially elevated for much more than a century following thaw. The low apparent C accumulation for the formerly forested, permafrost peat (< 5 g C m -2 y -1 ) may suggest that C accumulation increases substantially following thaw, but it remains unknown whether deep peat C loss occurred immediately following thaw. Well-preserved Sphagnum peat dominates during this period of rapid accumulation, except for an interval from ∼400 to 275 cal y BP which alternates between Sphagnum and vascular plant-dominated peat and wetter, minerotrophic conditions. A decline in Picea pollen during this interval and again ∼100 cal y BP suggests a decrease in suitable substrate for tree growth likely attributable to thermokarst expansion on the collapse-scar margin. These findings suggest that the combined effects of fire and thermokarst will result in a long-term reduction of spruce ecosystems in interior Alaska.

Understanding the role of fire in the Earth system, and particularly regional controls on its frequency and severity, is critical to risk assessment. Charcoal records from lake sediment and fire-scar networks from long-lived tree species have improved our understanding of long-term relationships between fire events and climate. This work has primarily focused on historically fire-prone ecosystems and regions. In the northeastern USA, where wildfire has been relatively infrequent in historical times, fire-risk assessments have incorporated little-to-no pre-historical data and little is known about long-term fire-climate relationships. We developed coupled, high-resolution records of moisture variability and fire from three ombrotrophic peatlands in Maine using testate amoebae and analysis of microscopic charcoal. Water-table depth reconstructions among the three sites were generally coherent, with high-magnitude dry and wet events corresponding within the uncertainty of age-depth models. At all sites, there was a significantly higher probability of fire events during high-magnitude droughts. However, although prolonged droughts were widespread and associated with higher probability of fire, the fire events were rarely synchronous among sites, with the exception of ~550 years before present (yr BP) when all three sites experienced both drought and fire. While fire has been relatively uncommon in the northeastern USA during the past century, our records clearly highlight the potential vulnerability of the region to future drought and fire impacts. Results also demonstrate the utility of coupled records of fire and climate in understanding regional fire-climate dynamics.

We investigated the ecology and paleoecology of testate amoebae in peatlands of the Colombian páramo to assess the use of testate amoebae as paleoenvironmental indicators. Objectives were to (1) identify environmental controls on testate amoebae, (2) develop transfer functions for paleoenvironmental inference, and (3) examine testate amoebae in a Holocene peat core and compare our findings with other proxy records. Results from 96 modern samples indicate that testate amoebae are sensitive to pH and surface moisture, and cross-validation of transfer functions indicates potential for paleoenvironmental applications. Testate amoebae from the Triunfo Peatland in the Central Cordillera provided a proxy record of pH and water-table depth for the late Holocene, and inferred changes were correlated with peat C/N measurements during most of the record. Comparison with a lake-level reconstruction suggests that at least the major testate amoeba–inferred changes were driven by climate. Our work indicates that testate amoebae are useful paleoenvironmental indicators in high-elevation tropical peatlands.

1. European settlement of eastern North America resulted in dramatic changes to ecosystems, although the dynamics and underlying causes of these changes are not always obvious. For example, land clearance likely exposed soils to increased wind erosion, potentially impacting downwind ecosystems indirectly through subsequent enhancement of dust deposition. We hypothesized that otherwise undisturbed wetlands were altered through this indirect disturbance mechanism, increasing nutrient availability and initiating a cascade of ecosystem‐level changes. 2. We tested this hypothesis in a floating kettle peatland located in north‐western Pennsylvania, USA, using an interdisciplinary approach. A series of peat cores were collected along a transect oriented parallel to the dominant wind direction. Palaeoecological techniques were used to identify signatures of upland deforestation and mineral matter deposition within the peat profiles. Elemental analyses were used to reconstruct historic availability of major macronutrients, plant community dynamics were reconstructed using plant macrofossils and tree rings (Pinus strobus) and testate amoebae were used as a proxy for microbial community dynamics. 3. Strong correlations between the concentration of ragweed (Ambrosia) pollen and fine‐grained mineral matter linked upland deforestation to enhanced dust deposition on the peatland surface. Elemental analyses indicated that nitrogen, phosphorus and potassium concentrations increased coincident with dust deposition. Plant communities shifted from Sphagnum dominance to vascular‐plant dominance coincident with enhanced dust deposition, including increased recruitment of P. strobus onto the peatland. Testate amoeba communities shifted towards those adapted to highly variable microenvironmental conditions and likely reflect broader changes in microbial communities. 4. Synthesis. Upland deforestation by European settlers triggered a cascade of ecological changes on a nutrient‐poor peatland by enhancing dust deposition and nutrient delivery on the surface. These results demonstrate that indirect, unintended and often overlooked human disturbances can lead to dramatic structural and functional alterations of carbon‐rich wetland ecosystems, highlighting the potential vulnerability of these systems in human‐dominated landscapes.

The mid-Holocene decline of Tsuga canadensis (hereafter Tsuga ) populations across eastern North America is widely perceived as a synchronous event, driven by pests/pathogens, rapid climate change, or both. Pattern identification and causal attribution are hampered by low stratigraphic density of pollen-sampling and radiometric dates at most sites, and by absence of highly resolved, paired pollen and paleoclimate records from single sediment cores, where chronological order of climatic and vegetational changes can be assessed. We present an intensely sampled (contiguous 1-cm intervals) record of pollen and water table depth (inferred from testate amoebae) from a single core spanning the Tsuga decline at Irwin Smith Bog in Lower Michigan, with high-precision chronology. We also present an intensively sampled pollen record from Tower Lake in Upper Michigan. Both sites show high-magnitude fluctuations in Tsuga pollen percentages during the pre-decline maximum. The terminal decline is dated at both sites ca. 5000 cal yr BP, some 400 years later than estimates from other sites and data compilations. The terminal Tsuga decline was evidently heterochronous across its range. A transient decline ca. 5350 cal yr BP at both sites may correspond to the terminal decline at other sites in eastern North America. At Irwin Smith Bog, the terminal Tsuga decline preceded an abrupt and persistent decline in water table depths by ∼200 years, suggesting the decline was not directly driven by abrupt climate change. The Tsuga decline may best be viewed as comprising at least three phases: a long-duration pre-decline maximum with high-magnitude and high-frequency fluctuations, followed by a terminal decline at individual sites, followed in turn by two millennia of persistently low Tsuga populations. These phases may not be causally linked, and may represent dynamics taking place at multiple temporal and spatial scales. Further progress toward understanding the phenomenon requires an expanded network of high-resolution pollen and paleoclimate chronologies.