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© 2020 The Authors., Fire is a powerful ecological and evolutionary force that regulates organismal traits, population sizes, species interactions, community composition, carbon and nutrient cycling and ecosystem function. It also presents a rapidly growing societal challenge, due to both increasingly destructive wildfires and fire exclusion in fire‐dependent ecosystems. As an ecological process, fire integrates complex feedbacks among biological, social and geophysical processes, requiring coordination across several fields and scales of study. Here, we describe the diversity of ways in which fire operates as a fundamental ecological and evolutionary process on Earth. We explore research priorities in six categories of fire ecology: (a) characteristics of fire regimes, (b) changing fire regimes, (c) fire effects on above‐ground ecology, (d) fire effects on below‐ground ecology, (e) fire behaviour and (f) fire ecology modelling. We identify three emergent themes: the need to study fire across temporal scales, to assess the mechanisms underlying a variety of ecological feedbacks involving fire and to improve representation of fire in a range of modelling contexts. Synthesis: As fire regimes and our relationships with fire continue to change, prioritizing these research areas will facilitate understanding of the ecological causes and consequences of future fires and rethinking fire management alternatives., Support was provided by NSF‐DEB‐1743681 to K.K.M. and A.J.T. We thank Shalin Hai‐Jew for helpful discussion of the survey and qualitative methods.

Wildfire is a ubiquitous disturbance agent in subalpine forests in western North America. Lodgepole pine ( Pinus contorta var. latifolia), a dominant tree species in these forests, is largely resilient to high-severity fires, but this resilience may be compromised under future scenarios of altered climate and fire activity. We investigated fire occurrence and post-fire vegetation change in a lodgepole pine forest over the past 2500 years to understand ecosystem responses to variability in wildfire and climate. We reconstructed vegetation composition from pollen preserved in a sediment core from Chickaree Lake, Colorado, USA (1.5-ha lake), in Rocky Mountain National Park, and compared vegetation change to an existing fire history record. Pollen samples ( n = 52) were analyzed to characterize millennial-scale and short-term (decadal-scale) changes in vegetation associated with multiple high-severity fire events. Pollen assemblages were dominated by Pinus throughout the record, reflecting the persistence of lodgepole pine. Wildfires resulted in significant declines in Pinus pollen percentages, but pollen assemblages returned to pre-fire conditions after 18 fire events, within c.75 years. The primary broad-scale change was an increase in Picea, Artemisia, Rosaceae, and Arceuthobium pollen types, around 1155 calibrated years before present. The timing of this change is coincident with changes in regional pollen records, and a shift toward wetter winter conditions identified from regional paleoclimate records. Our results indicate the overall stability of vegetation in Rocky Mountain lodgepole pine forests during climate changes and repeated high-severity fires. Contemporary deviations from this pattern of resilience could indicate future recovery challenges in these ecosystems.

Floodplain development, land-use, and flooding on the lower Ohio River are investigated with a 3100-year-long sediment archive from Avery Lake, a swale lake on the Black Bottom floodplain in southern Illinois, US. In all, 12 radiocarbon dates show that Avery Lake formed at 1130 BCE (3100 cal. yr BP), almost 3000 years later than previously thought, indicating that the Black Bottom floodplain is younger and more dynamic than previously estimated. Three subsequent periods of extensive land clearance were identified by changes in pollen composition, corresponding to Native American occupations before 1500 CE and the current Euro-American occupation beginning in the 18th century. Sedimentation rates prior to 1820 CE changed independently of land clearance events, suggesting natural as opposed to land-use controls. Comparison with high-resolution paleoclimate data from Martin Lake, IN, indicates that lower Ohio River flooding was frequent when cold-season precipitation originating from the Pacific/Arctic predominated when atmospheric circulation resembled positive Pacific North American (PNA) conditions and the Pacific Decadal Oscillation (PDO) was in a positive mean state (1130 BCE to 350 CE and 1150–1820 CE). Conversely, Ohio River flooding was less frequent when warm-season precipitation from the Gulf of Mexico prevailed during negative PDO- and PNA-like mean states (350 and 1150 CE). This flood dynamic appears to have been fundamentally altered after 1820 CE. We suggest that extensive land clearance in the Ohio River watershed increased runoff and landscape erosion by reducing interception, infiltration, and evapotranspiration, thereby increasing flooding despite a shift to negative PDO- and PNA-like mean states. Predicted increases in average precipitation and extreme rainfall events across the mid-continental US are likely to perpetuate current trends toward more frequent flood events, because anthropogenic modifications have made the landscape less resilient to changing hydroclimatic conditions.

Geosphere-biosphere interactions are ubiquitous features of the Earth surface, yet the development of interactions between newly exposed lithologic surfaces and colonizing plants during primary succession after glaciation are lacking temporal detail. To assess the nature, rate, and magnitude of vegetation influence on parent material and sediment delivery, we analyzed ecosystem and geochemical proxies from lacustrine sediment cores at a grassland site and a forested site in the northern United States. Over time, terrigenous inputs declined at both sites, with increasing amounts of organic inputs toward present. The similarities between sites were striking given that the grassland sequence began in the Early Holocene, and the forested sequence began after the last glacial maximum. Multiple mechanisms of chemical weathering, hydrologic transport, and changes in source material potentially contribute to this pattern. Although there were strong links between vegetation composition and nitrogen cycling at each site, it appears that changes in forest type, or from oak woodland to grassland, did not exert a large influence on elemental (K, Ti, Si, Ca, Fe, Mn, and S) abundance in the sedimentary sequences. Rather, other factors in the catchment-lake system determined the temporal sequence of elemental abundance.

Fire frequency exerts a fundamental control on productivity and nutrient cycling in savanna ecosystems. Individual fires often increase short-term nitrogen (N) availability to plants, but repeated burning causes ecosystem N losses and can ultimately decrease soil organic matter and N availability. However, these effects remain poorly understood due to limited long-term biogeochemical data. Here, we evaluate how fire frequency and changing vegetation composition influenced wood stable N isotopes (δ15N) across space and time at one of the longest running prescribed burn experiments in the world (established in 1964). We developed multiple δ15N records across a burn frequency gradient from precisely dated Quercus macrocarpa tree rings in an oak savanna at Cedar Creek Ecosystem Science Reserve, Minnesota, USA. Sixteen trees were sampled across four treatment stands that varied with respect to the temporal onset of burning and burn frequency but were consistent in overstory species representation, soil characteristics, and topography. Burn frequency ranged from an unburned control stand to a high-fire-frequency stand that had burned in 4 of every 5 years during the past 55 years. Because N stocks and net N mineralization rates are currently lowest in frequently burned stands, we hypothesized that wood δ15N trajectories would decline through time in all burned stands, but at a rate proportional to the fire frequency. We found that wood δ15N records within each stand were remarkably coherent in their mean state and trend through time. A gradual decline in wood δ15N occurred in the mid-20th century in the no-, low-, and medium-fire stands, whereas there was no trend in the high-fire stand. The decline in the three stands did not systematically coincide with the onset of prescribed burning. Thus, we found limited evidence for variation in wood δ15N that could be attributed directly to long-term fire frequency in this prescribed burn experiment in temperate oak savanna. Our wood δ15N results may instead reflect decadal-scale changes in vegetation composition and abundance due to early- to mid-20th-century fire suppression.

Predicting ecosystem resilience is a challenge, especially as climate change alters disturbance regimes and conditions for recovery. Recent research has highlighted the importance of spatially-explicit disturbance and resilience processes to long-term ecosystem dynamics. “Neoecological” approaches characterize resilience mechanisms at relatively fine spatio-temporal resolutions, but results are difficult to extrapolate across broad temporal scales or climatic ranges. Paleoecological methodologies can consider the effects of climates that differ from today. However, they are often limited to coarse-grained spatio-temporal resolutions. In this synthesis, we describe implicit and explicit examples of studies that incorporate both neo- and paleoecological approaches. We propose ways to build on the strengths of both approaches in an explicit and proactive fashion. Linking the two approaches is a powerful way to surpass their respective limitations. Aligning spatial scales is critical: Paleoecological sampling design should incorporate knowledge of the spatial characteristics of the disturbance process, and neoecological studies benefit from a longer-term context to their conclusions. In some cases, modeling can incorporate non-spatial data from paleoecological records or emerging spatial paleo-data networks with mechanistic disturbance/recovery processes that operate at fine spatiotemporal scales. Linking these two complementary approaches is a powerful way to build a complete understanding of ecosystem disturbance and resilience.

The Great Plains experienced extreme fluctuations in precipitation and temperature throughout the Holocene, but these fluctuations have been difficult to quantify systematically across the region. Pollen has long been used as a proxy for reconstructing climate changes, but its power is limited if a region is devoid of modern pollen samples to facilitate comparison with known climate conditions. Here, we present a set of pollen-climate transfer functions developed using weighted-averaging partial least squares to reconstruct mean annual precipitation (MAP), mean temperature of the coldest month (MTcold) and mean temperature of the warmest month (MTwarm). At the foundation of these transfer functions is a new set of 141 modern pollen samples that specifically cover the climate space of the Great Plains. These functions quantify the relationship between pollen assemblages and modern climate in this region (r = 0.928, 0.838 and 0.897 for MAP, MTcold and MTwarm, respectively). We applied these functions to three previously established pollen records taken from lacustrine sites in the region – Fox Lake, MN; Moon Lake, ND; and Kettle Lake, ND – to reconstruct precipitation and temperature at these sites throughout the Holocene. Annual precipitation reconstructed with these transfer functions at Moon Lake and Kettle Lake does not capture a severe, prolonged period of drought during the mid-Holocene as seen in many other proxy records from these sites and from the region. Reconciliation of these results involves both the autecology of Ambrosia and the inability of any pollen surface samples to capture seasonal and short-term variability in precipitation. Thus, the transfer functions correctly reconstruct periodic wet summers and a disturbance factor favouring Ambrosia, but miss the effects of reduced winter precipitation and increased frequency of severe drought. The problem revealed here may be more general to biomes with higher frequency disturbance regimes and more weedy species.

(1) Background: Frequent fire, climate variability, and human activities collectively influence savanna ecosystems. The relative role of these three factors likely varies on interannual, decadal, and centennial timescales. Here, we tested if Euro-American activities uncoupled drought and fire frequencies relative to previous centuries in a temperate savanna site. (2) Methods: We combined records of fire frequency from tree ring fire scars and sediment charcoal abundance, and a record of fuel type based on charcoal particle morphometry to reconstruct centennial scale shifts in fire frequency and fuel sources in a savanna ecosystem. We also tested the climate influence on fire occurrence with an independently derived tree-ring reconstruction of drought. We contextualized these data with historical records of human activity. (3) Results: Tree fire scars revealed eight fire events from 1822&ndash, 1924 CE, followed by localized suppression. Charcoal signals highlight 13 fire episodes from 1696&ndash, 2001. Fire&ndash, climate coupling was not clearly evident both before and after Euro American settlement The dominant fuel source shifted from herbaceous to woody fuel during the early-mid 20th century. (4) Conclusions: Euro-American settlement and landscape fragmentation disrupted the pre-settlement fire regime (fire frequency and fuel sources). Our results highlight the potential for improved insight by synthesizing interpretation of multiple paleofire proxies, especially in fire regimes with mixed fuel sources.

The pace and degree of nutrient limitation are among the most critical uncertainties in predicting terrestrial ecosystem responses to global change. In the northeastern United States, forest growth has recently declined along with decreased soil calcium (Ca) availability, suggesting that acid rain has depleted soil Ca to the point where it may be a limiting nutrient. However, it is unknown whether the past 60 y of changes in Ca availability are strictly anthropogenic or partly a natural consequence of long-term ecosystem development. Here, we report a high-resolution millennial-scale record of Ca and 16 other elements from the sediments of Mirror Lake, a 15-ha lake in the White Mountains of New Hampshire surrounded by northern hardwood forest. We found that sedimentary Ca concentrations had been declining steadily for 900 y before regional Euro-American settlement. This Ca decline was not a result of serial episodic disturbances but instead the gradual weathering of soils and soil Ca availability. As Ca availability was declining, nitrogen availability concurrently was increasing. These data indicate that nutrient availability on base-poor, parent materials is sensitive to acidifying processes on millennial timescales. Forest harvesting and acid rain in the postsettlement period mobilized significant amounts of Ca from watershed soils, but these effects were exacerbated by the long-term pattern. Shifting nutrient limitation can potentially occur within 10,000 y of ecosystem development, which alters our assessments of the speed and trajectory of nutrient limitation in forests, and could require reformulation of global models of forest productivity.

Grassland vegetation varies in composition across North America and has been historically influenced by multiple biotic and abiotic drivers, including fire, herbivory, and topography. Yet, the amount of temporal and spatial variability exhibited among grassland pollen assemblages, and the influence of these biotic and abiotic drivers on pollen assemblage composition and diversity has been relatively understudied. Here, we examine 4 years of modern pollen assemblages collected from a series of 28 traps at the Konza Prairie Long‐Term Ecological Research Area in the Flint Hills of Kansas, with the aim of evaluating the influence of these drivers, as well as quantifying the amount of spatial and temporal variability in the pollen signatures of the tallgrass prairie biome. We include all terrestrial pollen taxa in our analyses while calculating four summative metrics of pollen diversity and composition – beta‐diversity, Shannon index, nonarboreal pollen percentage, and Ambrosia:Artemisia – and find different roles of fire, herbivory, and topography variables in relation to these pollen metrics. In addition, we find significant annual differences in the means of three of these metrics, particularly the year 2013 which experienced high precipitation relative to the other 3 years of data. To quantify spatial and temporal dissimilarity among the samples over the 4‐year study, we calculate pairwise squared‐chord distances (SCD). The SCD values indicate higher compositional dissimilarity across the traps (0.38 mean) among all years than within a single trap from year to year (0.31 mean), suggesting that grassland vegetation can have different pollen signatures across finely sampled space and time, and emphasizing the need for additional long‐term annual monitoring of grassland pollen.

Wildfire activity has been increasing in forests of western North America over the past several decades. However, the biogeochemical effects of changing fire regimes are poorly understood. Here, we utilize sediment records from three subalpine lakes in northern Colorado (Hinman, Gold Creek, and Summit) to investigate the biogeochemical consequences of charcoal-inferred fire events over the past ∼2500 years. We measured element concentrations and stable isotope ratios (δ13C, δ15N) in lake sediments to track past biogeochemical processes. On average, fires were followed by increases in carbon (C), nitrogen (N), and sulfur concentrations in lake sediments, which lasted ∼20 years, while titanium and other metals found in terrestrial mineral material decreased. These changes were only statistically significant (p

Humans have drastically altered the global nitrogen (N) cycle, and these alterations have begun to affect a variety of ecosystems. In North America, N deposition rates are highest in the central US, yet there are few studies that examine whether N availability has been increasing to different tree species in the forests of the region. To determine the species-specific trajectories of N availability in secondary temperate forests experiencing high N deposition, we measured the N concentrations and composition of stable N isotopes in wood of four tree species from six hardwood forest remnants in northern Indiana, USA. Annual nitrogen deposition rates averaged 5.8 kg ha−1 from 2000 to 2008 in this region. On average, wood δ15N values in Quercus alba have been increasing steadily over the past 100 years. In contrast, wood δ15N values have been declining in three other hardwood species – Acer saccharum, Carya ovata, and Fagus grandifolia – over the same time period. The species-specific trends suggest a change in the partitioning of ammonium and nitrate among species, due to an increase in nitrification rates over time. With no apparent net change in wood δ15N over the past century at the stand level, there is currently little evidence for consistent trends in stand-level N availability over time in the Indiana forests.

The density of forested cover in grassland regions has been increasing globally during the past several decades. Although there is some evidence to suggest that climate change is playing a role in this woody encroachment, there is a lack of consensus on both the causes and consequences of this land cover change. To examine the role of climate on tree establishment and growth at a very large spatial scale, we used dendrochronological techniques coupled with spatial analyses of the effects of climate drivers on biotic responses such as establishment and growth. We sampled ring widths of selected large trees and collected establishment dates of trees for four deciduous tree species in the riparian zone of a 119 ha experimental watershed at the Kansas Konza Prairie Biological Station, near the current prairie–forest boundary of North America. Annual tree-ring width is positively correlated with the Palmer Drought Severity Index and growing season precipitation, although winter climate variables also affect de...

Disturbances affect almost all terrestrial ecosystems, but it has been difficult to identify general principles regarding these influences. To improve our understanding of the long-term consequences of disturbance on terrestrial ecosystems, we present a conceptual framework that analyzes disturbances by their biogeochemical impacts. We posit that the ratio of soil and plant nutrient stocks in mature ecosystems represents a characteristic site property. Focusing on nitrogen (N), we hypothesize that this partitioning ratio (soil N: plant N) will undergo a predictable trajectory after disturbance. We investigate the nature of this partitioning ratio with three approaches: (1) nutrient stock data from forested ecosystems in North America, (2) a process-based ecosystem model, and (3) conceptual shifts in site nutrient availability with altered disturbance frequency. Partitioning ratios could be applied to a variety of ecosystems and successional states, allowing for improved temporal scaling of disturbance events. The generally short-term empirical evidence for recovery trajectories of nutrient stocks and partitioning ratios suggests two areas for future research. First, we need to recognize and quantify how disturbance effects can be accreting or depleting, depending on whether their net effect is to increase or decrease ecosystem nutrient stocks. Second, we need to test how altered disturbance frequencies from the present state may be constructive or destructive in their effects on biogeochemical cycling and nutrient availability. Long-term studies, with repeated sampling of soils and vegetation, will be essential in further developing this framework of biogeochemical response to disturbance.

Processes and rates of ecosystem development can be reconstructed using lacustrine sedimentary sequences, but this approach often requires records that contain the start of primary succession. Most lakes in the upper Midwestern U.S. were formed by glaciers at the end of the last Ice Age approximately 11,700 cal yr BP. Devils Lake, Wisconsin is a rare example of a lake from this region whose sediments extend into the Pleistocene and may include the Last Glacial Maximum. Sediment magnetic, geochemical, pollen, and charcoal records were generated from a 10 m core whose basal sediments may be 28,000 years old. Together with a previously published pollen record, these proxies combine to reveal a history of long-term climatic, vegetative and geologic change during the late Pleistocene to Holocene. We identify six sedimentary units that indicate a series of consecutive events rather than a predictable trajectory of ecosystem development at the site. Productivity in the lake was low during the late Pleistocene and increased during the Holocene, as reflected by the sediment lithology, which shows a sudden shift from glacial vivianite-rich and organic-poor clastic-dominated sediments to Holocene diatomaceous sapropels. Several important processes initiated around 17,000 cal yr BP, including the onset of organic matter accumulation and fire in the terrestrial ecosystem. However, the post-glacial landscape was not devoid of vegetation because pollen assemblages indicate that terrestrial vegetation, likely a spruce tundra, survived near the site. A switch to a hardwood forest period during the Holocene also led to a change in the fire regime, with increased frequency of burning. Aquatic ecosystem productivity lagged terrestrial ecosystem productivity throughout the record. Nutrient cycling (as recorded by sedimentary δ15N) was variable but not directional, and appeared to be correlated with climate conditions early in the record, and terrestrial ecosystem processes later in the record. Throughout the Holocene magnetic mineral concentration decreased as productivity increased, and the source of magnetic material shifted from almost exclusively lithogenic to approximately 50% derived from soil or biogenic sources. Magnetic grain size, Ambrosia pollen percentages, and charcoal concentration increased and δ15N decreased in the most recent part of the record, due to anthropogenic influence in the region including agricultural activities.

We provide a synthesis and framework for using lacustrine sedimentary records to study the biogeochemical outcomes of landscape disturbances. Although disturbance regimes can now be effectively reconstructed in sedimentary records, biogeochemical responses to disturbance events are less frequently assessed. Further, there is a lack of consensus on the characteristics of disturbances or ecosystems that would lead to biogeochemical resilience. Both sensitivity (a change in a biogeochemical proxy following a disturbance event relative to a pre-disturbance condition) and complacency (absence of change in a biogeochemical proxy after a disturbance event) have been observed in paleorecords. Here, we discuss the factors that contribute to sensitivity/complacency as well as the short- and long-term biogeochemical effects of terrestrial disturbance agents such as fire and insect outbreaks. We discuss the appropriate strategies for sampling lacustrine sediment cores to assess the biogeochemical outcomes of disturbances and provide a review of the appropriate data scaling techniques for analyzing multiple records in space and time.

Ongoing changes in disturbance regimes are predicted to cause acute changes in ecosystem structure and function in the coming decades, but many aspects of these predictions are uncertain. A key challenge is to improve the predictability of postdisturbance biogeochemical trajectories at the ecosystem level. Ecosystem ecologists and paleoecologists have generated complementary data sets about disturbance (type, severity, frequency) and ecosystem response (net primary productivity, nutrient cycling) spanning decadal to millennial timescales. Here, we take the first steps toward a full integration of these data sets by reviewing how disturbances are reconstructed using dendrochronological and sedimentary archives and by summarizing the conceptual frameworks for carbon, nitrogen, and hydrologic responses to disturbances. Key research priorities include further development of paleoecological techniques that reconstruct both disturbances and terrestrial ecosystem dynamics. In addition, mechanistic detail from disturbance experiments, long-term observations, and chronosequences can help increase the understanding of ecosystem resilience.

Forests cover 30% of the terrestrial Earth surface and are a major component of the global carbon (C) cycle. Humans have doubled the amount of global reactive nitrogen (N), increasing deposition of N onto forests worldwide. However, other global changes—especially climate change and elevated atmospheric carbon dioxide concentrations—are increasing demand for N, the element limiting primary productivity in temperate forests, which could be reducing N availability. To determine the long-term, integrated effects of global changes on forest N cycling, we measured stable N isotopes in wood, a proxy for N supply relative to demand, on large spatial and temporal scales across the continental U.S.A. Here, we show that forest N availability has generally declined across much of the U.S. since at least 1850 C.E. with cool, wet forests demonstrating the greatest declines. Across sites, recent trajectories of N availability were independent of recent atmospheric N deposition rates, implying a minor role for modern N deposition on the trajectory of N status of North American forests. Our results demonstrate that current trends of global changes are likely to be consistent with forest oligotrophication into the foreseeable future, further constraining forest C fixation and potentially storage.

Reconstructing fire regimes and fuel characteristics is an important aspect of understanding past forest ecosystem processes. Fuel sources and fire regimes in the upper Midwestern United States have been shown to be sensitive to regional climatic variability, such as drought periods on millennial timescales. Yet, records documenting the connections between disturbance activity and the corresponding fuel source fluctuations in mesic deciduous forests and prairie/oak savanna in this region are limited. Thus, it has been difficult to provide a framework to evaluate changes in moisture availability on fire activity and the relationships with fuel source fluctuations in this region. We present high-resolution charcoal analyses of lake sediments from four sites in southern Wisconsin (USA) to characterize fire activity and fuel source fluctuation in mesic deciduous forests and prairie/oak savanna over the last 10 000 years. We found that fire occurrence across the four study sites has been asynchronous throughout the Holocene, because of site-specific differences that have strongly influenced local fire regimes. Additionally, we found that during periods of high fire activity the primary fuels were from arboreal sources, and during periods of low fire activity the primary fuels were from non-arboreal sources. However, fluctuations in fuel sources did not always correspond to changes in vegetation, or changes in fire frequency. Copyright # 2014 John Wiley & Sons, Ltd.

Question: As anthropogenic change pushes ecosystems towards historicallynovel states, understanding how landscape disturbances interact with climatevariabilitybecomesincreasinglyimportant.WereconstructedtheHoloceneveg-etation and fire history of a small watershed to investigate the linkages amongdisturbance regimes, biogeochemical cycling and ecosystem structure. Specifi-callyweasked:(i)howdocompositionalandstructuralchangesinforestecosys-tems modify fire regimes; and (ii) how do changes in fuel type and fire regimesimpactbiogeochemicalcycling?Location: Comstock Lake, Wisconsin, USA, a small kettle lake at the modernprairie–forestecotoneintheGreatLakesRegionofNorthAmerica.Methods: We analysed a lacustrine sediment core from Comstock Lake, Wis-consin,forpaleoecologicalproxiesincludingcharcoalquantityandmorphotype,pollen composition, magnetic susceptibility, organic matter concentration andstable carbon and nitrogen isotopes. These paleoecological variables were usedto reconstruct climate-mediated shifts in vegetation, fire disturbance and bio-geochemical cycling during the last 13 200 calendar years before present (cal yrBP).Results: Forest composition in the landscape surrounding Comstock Lake var-ied among coniferous forest, deciduous forest and savanna over the Holocene.Independentofthevegetationchanges,fivedistinctfireregimeswereidentified,ranging from frequent, low-intensity surface fires to infrequent, high-intensitycrown fires. The main feature of our vegetation reconstruction is a shift fromPinus to Quercus forest that occurred between 10 000 and 11 000 cal yr BP.However, changes in fuel source and forest structure were apparently moreimportant than shifts of species composition in determining fire regimes. Weobserved an inverse relationship between non-arboreal fuel sources and bio-mass burning throughout all five fire regimes. The biogeochemical conse-quences of fires were apparent during some, but not all regime transitions.Generally, the sediments were composed of relatively small clastic inputs fromthecatchmentandmostlyautochthonousorganicsources.Conclusion: The location of Comstock Lake on the prairie–forest boundarymeant that regional climate signals during the Holocene were filtered throughvegetation. Both compositional changes in dominant tree taxa and structuralchangesincanopyopennessinfluencedfireregimes.IntroductionVegetation structure, community composition and fuelconditions are important determinants of wildfire recur-rence and fire type. Increasing abundances of fire-proneplant taxa can increase landscape susceptibility to burning(Brooks et al. 2004; Ohlson et al. 2011), change post-firecommunity composition (Johnstone et al. 2004; Griguliset al. 2005) and alter soil properties and biogeochemicalcycling from pre-disturbed conditions (Gillson & Ekblom

Summary Wildfires can significantly alter forest carbon (C) storage and nitrogen (N) availability, but the long-term biogeochemical legacy of wildfires is poorly understood. We obtained a lake-sediment record of fire and biogeochemistry from a subalpine forest in Colorado, USA, to examine the nature, magnitude, and duration of decadal-scale, fire-induced ecosystem change over the past c. 4250 yr. The high-resolution record contained 34 fires, including 13 high-severity events within the watershed. High-severity fires were followed by increased sedimentary N stable isotope ratios (δ15N) and bulk density, and decreased C and N concentrations – reflecting forest floor destruction, terrestrial C and N losses, and erosion. Sustained low sediment C : N c. 20–50 yr post-fire indicates reduced terrestrial organic matter subsidies to the lake. Low sedimentary δ15N c. 50–70 yr post-fire, coincident with C and N recovery, suggests diminishing terrestrial N availability during stand development. The magnitude of post-fire changes generally scaled directly with inferred fire severity. Our results support modern studies of forest successional C and N accumulation and indicate pronounced, long-lasting biogeochemical impacts of wildfires in subalpine forests. However, even repeated high-severity fires over millennia probably did not deplete C or N stocks, because centuries between high-severity fires allowed for sufficient biomass recovery.

Forests store a large amount of terrestrial carbon, but this storage capacity is vulnerable to wildfire. Combustion, and subsequent tree mortality and soil erosion, can lead to increased carbon release and decreased carbon uptake. Previous work has shown that non-constant fire return intervals over the past 4000 years strongly shaped subalpine forest carbon trajectories. The extent to which fire-regime variability has impacted carbon trajectories in other subalpine forest types is unknown. Here, we explored the interactions between fire and carbon dynamics of 14 subalpine watersheds in Colorado, USA. We tested the impact of varying fire frequency over a ~2000 year period on ecosystem productivity and carbon storage using an improved biogeochemical model. High fire frequency simulations had overall lower carbon stocks across all sites compared to scenarios with lower fire frequencies, highlighting the importance of fire-frequency in determining ecosystem carbon storage. Additionally, variability in fire-free periods strongly influenced carbon trajectories across all the sites. Biogeochemical trajectories (e.g., increasing or decreasing total ecosystem carbon and carbon-to-nitrogen (C:N) ratios) did not vary among forest types but there were trends that they may vary by elevation. Lower-elevations sites had lower overall soil C:N ratios, potentially because of higher fire frequencies reducing carbon inputs more than nitrogen losses over time. Additional measurements of ecosystem response to fire-regime variability will be essential for improving estimates of carbon dynamics from Earth system models.

One of the keystone tree species in subalpine forests of the western United States – whitebark pine (Pinus albicaulis, hereafter whitebark pine) – is experiencing a significant mortality event (Millar et al. 2012). Whitebark pine occupies a relatively restricted range in the high-elevation ecosystems in the northern Rockies and its future is uncertain. The current decline of whitebark pine populations has been attributed to pine beetle infestations, blister rust infections, anthropogenic fire suppression, and climate change (Millar et al. 2012). Despite the knowledge that whitebark pine is severely threatened by multiple stressors, little is known about the historic capacity of this species to handle these stressors. More specifically, it is unknown how whitebark pine has dealt with past climatic variability, particularly variation in the type of precipitation (rain vs. snow) available for soil moisture, and how differences in quantity of precipitation have influenced the establishment and growth of modern stands. We propose to study the past responses of whitebark pine to paleoclimatic conditions, which would be useful to park ecologists in developing new conservation and regeneration plans to prevent the extinction of this already severely threatened high-elevation resource. The purpose of this study is to determine in great temporal and spatial detail the demographics of the current stand of whitebark pine trees in the watershed surrounding an unnamed, high-altitude pond (known informally as Whitebark Pine Moraine Pond) located approximately 3.06 miles NW of Jenny Lake in Grand Teton National Park (GTNP). The main objectives of this study were: 1.) To obtain the precise GPS locations of the current stand of whitebark pine trees in the watershed to generate a GIS map detailing their locations. 2.) To obtain increment cores of a subset of the trees in the watershed to estimate age and date of establishment for the current stand of whitebark pines, with particular attention to fire history. 3.) To analyze ring widths from core samples to identify climatic indicators that may influence the regeneration and survival of whitebark pine.

Summary Megaherbivores likely had important influences on past vegetation dynamics, just as they do in modern ecosystems. The exact nature of megaherbivores' role can be studied using a relatively new suite of palaeoecological techniques, including the quantification of fossil spores from Sporormiella and other coprophilous fungi as indicators of megafaunal biomass in sediment records. However, a quantitative linkage of spore abundance with megaherbivore biomass or grazing intensity has been lacking. Konza Prairie Biological Station (Kansas, USA), located in the midcontinent of North America, contains native tallgrass prairie grazed by a herd of bison (Bison bison) in a 1000-ha enclosure, providing an excellent opportunity to test the effects of megaherbivores on grassland community composition and their potential signature in the palynological record. We collected pollen and spores during 2009 and 2010 from a network of 28 modified Tauber traps. The precise locations of the bison herd were recorded using GPS collars; we calculated bison grazing intensity (kg m−2 year−1) to high spatial precision within concentric circles around each trap (radii from 25 to 500 m). Both relative (per cent) and absolute (concentration) abundances of Sporormiella were significantly higher in traps inside the enclosure and were positively correlated with bison grazing intensity. The cut-off for distinguishing between bison-grazed and ungrazed traps was determined to be 2.8% Sporormiella of the total pollen and spore sum, consistent with previous palaeoecological reconstructions. The relationship between Sporormiella abundances and available grazing area around each trap was strongest at short radii (25–100 m), suggesting that spores do not disperse far from their source. Sporormiella should thus be considered a local-scale indicator of megaherbivore presence. Traps in the grazed area had significantly higher percentages of Ambrosia and lower percentages of Poaceae pollen than traps from ungrazed areas. This suggests that the pollen record has the potential to detect the ecological effects of bison grazing on grassland community composition. Synthesis. This study refines the use of Sporormiella as a proxy for local megaherbivore presence, especially in grassland systems. Multiproxy Sporormiella and pollen analyses may help elucidate the past drivers of grassland dynamics, including the possible role of bison in mediating grass–forb interactions during the variable moisture regimes of the last 12,000 years.

Transfer of nutrients from terrestrial to aquatic ecosystems is a natural process with climatic, biotic, and geologic controls. Recently, increasing concern about human manipulation of global nutrient cycles has required a long-term approach to assessing the nutrient status of aquatic systems. Data available in palaeorecords can assess current trophic status, baseline conditions, and long-term processes controlling nutrient fluxes on decadal to millennial timescales. Here, we review three palaeolimnological methods used to reconstruct nutrient cycling: (1) chemical compounds preserved in lacustrine sediment, (2) aquatic biotic indicators (often using a quantitative transfer function), and (3) quantitative empirical sediment flux estimates. The millennial-scale regulation of nutrient cycling by climate and catchment geochemistry leads to a gradual trajectory of dystrophication over the Holocene in many temperate lakes. In many systems, the magnitude of recent anthropogenic changes to nutrient cycling is la...

Fire is a key Earth system process, with 80% of annual fire activity taking place in grassland areas. However, past fire regimes in grassland systems have been difficult to quantify due to challenges in interpreting the charcoal signal in depositional environments. To improve reconstructions of grassland fire regimes, it is essential to assess two key traits: (1) charcoal count, and (2) charcoal shape. In this study, we quantified the number of charcoal pieces in 51 sediment samples of ponds in the Great Plains and tested its relevance as a proxy for the fire regime by examining 13 potential factors influencing charcoal count, including various fire regime components (e.g. the fire frequency, the area burned, and the fire season), vegetation cover and pollen assemblages, and climate variables. We also quantified the width to length (W:L) ratio of charcoal particles, to assess its utility as a proxy of fuel types in grassland environments by direct comparison with vegetation cover and pollen assemblages. Our first conclusion is that charcoal particles produced by grassland fires are smaller than those produced by forest fires. Thus, a mesh size of 120μm as used in forested environments is too large for grassland ecosystems. We recommend counting all charcoal particles over 60μm in grasslands and mixed grass-forest environments to increase the number of samples with useful data. Second, a W:L ratio of 0.5 or smaller appears to be an indicator for fuel types, when vegetation surrounding the site is before composed of at least 40% grassland vegetation. Third, the area burned within 1060m of the depositional environments explained both the count and the area of charcoal particles. Therefore, changes in charcoal count or charcoal area through time indicate a change in area burned. The fire regimes of grassland systems, including both human and climatic influences on fire behavior, can be characterized by long-term charcoal records.

Grassland cover and composition respond to climate and have undoubtedly changed during the Holocene, but quantitative reconstructions from fossil pollen have been vague about spatial scale and taxon-specific cover. Here, we estimate the relevant source area of pollen for sedimentary basins approximately 50 m in radius, and we report pollen productivity estimates for 12 plant taxa in the tallgrass prairies of central North America. Both relevant source area of pollen and pollen productivity estimates were calculated via the Extended R-Value Model. To obtain these estimates, we collected and quantified the pollen found in surface sediment samples from 24 ponds across the study area. Vegetation was surveyed in the field in a 100 m radius around each pond, and vegetation maps from the Kansas Gap Analysis Project (GAP) were used to a radius of 2 km. Pollen fall speeds were calculated according to Stoke’s Law. Pollen assemblages from basins approximately 50 m in radius have a relevant source area of 1060 m in this grassland landscape. Pollen productivity estimates range from 0.02 to over 30 among the 12 taxa: Artemisia, Ambrosia, Asteraceae, Chenopodiaceae, Cornus, Fabaceae, Juniperus, Maclura, Poaceae, Populus, Quercus, and Salix. Woody taxa generally have higher pollen productivity than herbaceous taxa (except for Chenopodiaceae and Ambrosia).

Citation: Berangere Leys and Philip E Higuera and Kendra K McLauchlan and Paul V Dunnette. (2016). Wildfires and geochemical change in a subalpine forest over the past six millennia. Environmental Research Letters, 11(12), 125003.

The pace and degree of nutrient limitation are among the most critical uncertainties in predicting terrestrial ecosystem responses to global change. In the northeastern United States, forest growth has recently declined along with decreased soil calcium (Ca) availability, suggesting that acid rain has depleted soil Ca to the point where it may be a limiting nutrient. However, it is unknown whether the past 60 y of changes in Ca availability are strictly anthropogenic or partly a natural consequence of long-term ecosystem development. Here, we report a high-resolution millennial-scale record of Ca and 16 other elements from the sediments of Mirror Lake, a 15-ha lake in the White Mountains of New Hampshire surrounded by northern hardwood forest. We found that sedimentary Ca concentrations had been declining steadily for 900 y before regional Euro-American settlement. This Ca decline was not a result of serial episodic disturbances but instead the gradual weathering of soils and soil Ca availability. As Ca availability was declining, nitrogen availability concurrently was increasing. These data indicate that nutrient availability on base-poor, parent materials is sensitive to acidifying processes on millennial timescales. Forest harvesting and acid rain in the postsettlement period mobilized significant amounts of Ca from watershed soils, but these effects were exacerbated by the long-term pattern. Shifting nutrient limitation can potentially occur within 10,000 y of ecosystem development, which alters our assessments of the speed and trajectory of nutrient limitation in forests, and could require reformulation of global models of forest productivity.

Both terrestrial and aquatic ecosystems should respond to abrupt climate changes such as those that affected the midcontinent of North America during the Holocene. A variety of paleorecords indicate that the onset of prolonged dry conditions in this region occurred as rapidly as 300 yr during the early Holocene, with a subsequent increase in moisture occurring rapidly in the late Holocene. Here, we report a 9500 yr multiproxy sediment record from Deming Lake, Minnesota, USA, that demonstrates only subtle dynamics during rapid climate changes that caused vegetation in the catchment to shift among pine forest, open grassland, and deciduous forest. The most substantial changes in ecosystem properties immediately followed deglaciation of the landscape, formation of the lake, and initial development of pine forests. In contrast, there were only muted responses to pronounced mid-Holocene climate changes that caused vegetation in the catchment to switch from pine forest to open grassland (ca. 8000 yr B.P.), and then deciduous forest (ca. 5400 yr B.P.). The flux of organic and inorganic terrigenous material, the processing of carbon, and catchment erosion changed rapidly during the most recent shift at 3300 yr B.P. to the modern pine forest. These changes coincided with the onset of meromictic conditions that influenced internal lake dynamics. However, the terrestrial influence on the lake sedimentary record gradually diminished over time, indicating a trajectory of increasing catchment stabilization that was relatively impervious to dramatic regional climate changes. The relative complacency of the Deming Lake record during the late Holocene indicates relative resistance to abrupt climate change at later stages of ecosystem development.

The mid-continent of North America has experienced dramatic and abrupt climate change during the Holocene, but the response of grassland vegetation to past climate change has been difficult to quantify. To improve interpretation of tallgrass prairie vegetation from pollen assemblages, we acquired and analysed a surface sample set collected from 25 small ponds (less than 10 ha surface area) in the largest contiguous remnant of tallgrass prairie in the USA. We compared these tallgrass prairie assemblages to 476 modern pollen samples classified as “prairie” in the North American Surface Sample database. We then compared the surface pollen assemblages with fossil pollen assemblages from sediment cores at two sites in Kansas—Cheyenne Bottoms and Muscotah Marsh—using the modern analog technique. Pollen assemblages in the Flint Hills surface samples were very similar to each other, with an average squared chord distance of 0.19. They were different than other modern grassland pollen assemblages mainly due to higher percentages of pollen from six woody taxa: Carya, Cornus, Juniperus, Juglans, Maclura, and Platanus. Arboreal pollen percentages ranged from 17 to 62 % and did not correlate with woody cover among sites. Cheyenne Bottoms was open grassland for the past 25,000 years, but it did not have many tallgrass prairie analogs. Muscotah Marsh did not have many grassland analogs over the past 30,000 years, possibly due to its position on the prairie-forest border or its surrounding wetland vegetation.

The degree to which microbial communities adjust their decomposition of soil carbon over time in response to long-term increases in temperature is one of the key uncertainties in our modeling of the responses of terrestrial ecosystems to warming. To better understand changes in temperature sensitivity of soil microbial communities to long-term increases in soil temperature, we incubated 27 soils for one year with both short-term and long-term manipulations of temperature. In response to increasing temperature short-term from 20 to 30 °C, respiration rates increased more than threefold on average across soils. Yet, in response to long-term increases in temperature, respiration rates increased approximately half as much as they did to short-term increases in temperature. Short-term Q10 of recalcitrant C correlated positively with long-term Q10 measured between 10 and 20 °C, yet there was no relationship between short-term Q10 and long-term Q10 between 20 and 30 °C. In all, under laboratory conditions, it is clear that there is reduction in the temperature sensitivity of decomposition to long-term increases in temperature that disassociate short- and long-term responses of microbial decomposition to temperature. Determining the fate of soil organic matter to increased temperature will not only require further research on the controls and mechanisms of these patterns, but also require models to incorporate responses to both short-term and long-term increases in temperature.

Droughts are recurrent features in sedimentary records at the prairie–forest ecotone of northern U.S.A. It is therefore important to understand the duration of such events, as well as their severity and consequences beyond the century-scale instrumental record. The existing drought records can be complemented with additional proxies that clearly document the interactions between external factors and the depositional environment. Here we attempt to reconstruct the last millennium drought history of Deming Lake, a small lake at Minnesota's prairie–forest ecotone, using sediment magnetism. In particular, this approach allowed us to explore the interactions between lake and catchment, and test the hypothesis that periods of moderate aridity result in decreased hydrologic sediment transport from the catchment to the lake, as a consequence of reduced erosion and overland flow. Concordantly, during dry episodes we find that the mass fractions of in-lake produced organic matter and biogenic magnetic particles increase relative to allochthonous component fractions. We identify several episodes of aridity at Deming Lake consistent with existing regional drought reconstructions. The most prolonged dry event occurred at the end of the Little Ice Age, suggesting that the second half of this cold interval was drier in the region than previously thought.

It is unclear how the environmental heterogeneity of the prairie biome of North America contributes to the biogeographic ranges of vascular plant species, particularly herbaceous taxa. We examined ...

A major obstacle for predicting the effects of climate and land use changes on global soil carbon (C) stores is the very limited knowledge about the long timescale dynamics of the relatively stable fraction of soil C, which represents the bulk of soil C and the primary determinant of the long-term C balance of terrestrial ecosystems. In this study, we examined how variable topo-edaphic conditions and herds of native migratory ungulates influenced turnover of the stable pool (total minus active fraction) of soil C in grasslands of Yellowstone National Park (YNP). Soil C properties were determined for grasslands located inside and outside long-term ungulate exclosures established 1958-1962 at seven variable topographic positions. Active C pool sizes, estimated with soil laboratory incubations, and soil radiocarbon measures were used to parameterize a process-based model to determine turnover of the stable C pool at the sites. Stable C turnover ranged 37-653 and 89-869 years for 0-10 and 0-20 cm soils, respectively. Among ungrazed communities, there was a trend for stable soil C turnover to slow along topographic gradients of increasing soil moisture, soil C content, and shoot biomass from hilltop to slope-bottom positions. This was likely a result of an increasing amount of support tissue resulting in greater concentrations of lignin and cellulose as shoot biomass increased down slope. In contrast, across the grazed landscape, stable C turnover sped up from hilltop to slope-bottom positions, which was likely a consequence of grazer effects on plant species composition along the topographic gradient. These findings indicated that despite topography playing the primary role in controlling such important site characteristics as soil moisture, soil C content, and plant production in YNP grassland, the long-term turnover of the stable C pool was determined by herbivores. The results demonstrate the important regulatory role of herbivores in controlling the C balance of this semiarid grassland ecosystem.

Soils comprise the largest terrestrial carbon store on the planet. Soil respiration measurements suggest that the more biogeochemically recalcitrant the soil organic matter, the greater the temperature sensitivity of soil respiration.

Understanding the causes of interannual variability in atmospheric pollen concentration is an important but elusive goal for public health and environmental change. We analyzed long-term daily records of pollen counts from urban Kansas City, Missouri, USA collected from 1997 to 2009 for three pollen groups: Ambrosia, Poaceae, and a third group which is mostly composed of arboreal pollen types. The annual pollen index varied from 8,368 to 80,822 over the thirteen-year period. Although Ambrosia pollen is often thought to be associated with droughts and disturbance, years with high Ambrosia pollen were associated with high summer precipitation to the south of Kansas City. Years with high Poaceae pollen were associated with high spring precipitation to the south of the city. In support of the southern influence to Kansas City pollen, Ambrosia and Poaceae pollen mostly arrived on southern winds. In contrast to the other two pollen groups, the arboreal pollen was most associated with growing season precipitation to the east of Kansas City, although it was still highest on days with southern winds. Based on the correlations with climate, the severity of an upcoming allergy season may be predicted with early-season precipitation data, while short-term severity can be forecast from local weather patterns.

The once-extensive tallgrass prairie community of North America has been reduced to small rem- nants, many of which are surrounded by intensive corn (Zea mays) agriculture. We investigated adult corn- rootworm beetles (Chrysomelidae: Diabrotica spp.), important pests of corn, on sunflowers (Asteraceae: He- lianthus spp.) in prairie remnants in southeast Minnesota. Large numbers of beetles invaded the prairie from surrounding corn fields in late summer. D. barberi and D. virgifera were captured on sticky traps in all loca- tions in the prairie, but abundance was much greater near the edge adjacent to corn. We observed D, barberi (but not D. virgifera) feeding extensively on sunflower pollen and occasionally on other flower parts, such as petak. Sunflowers located nearer corn fields sustained morefloral damage than those farther from om. To de- termine the effect of beetle &mage on seed set, we enclosed sunflower heads in bags with either zero, two, or four D. barberi adults. Seed set was reduced in heads enclosed with D. barberi Thus, this agriculturalpest may interfere with the successful reproduction of sunflowers and possibly otherprairie composites thatflower in late summer. Given the small size of mostprairie remnants and the abundance of thisflowerrfeeding beetle in land- scapes dominated by corn agriculture, D. barberi may affect the sustainability ofprairie plantpopulations.

The proportional cover of forest and grassland vegetation, known as landscape openness, has been particularly difficult to reconstruct because of differences in pollen productivity and transport between the two vegetation types. To begin to calibrate landscape openness in eastern North America, we collected 21 samples of surface sediments from small ponds (less than 60 ha) in the Upper Connecticut River Valley of New England, USA. Pollen assemblages from these surface samples were compared with vegetation composition around each pond assessed with three different remote sensing methods: land cover digitized from aerial photographs, the National Land Cover Database and a spectral mixture analysis of vegetation structure using LANDSAT data. A principal component analysis indicates that most of the variation in land cover among sites is captured by pollen data and each of the three vegetation methods. Because spectral mixture analysis contains at least as much vegetation information as the other two techniques, while avoiding the problem of classifying vegetation into categories, it is better for comparison with pollen data and detection of landscape openness. Even though there is a relationship between non-arboreal pollen percentages and a metric of landscape openness, it is inadvisable to use pollen percentages alone to infer landscape openness across regions either in modern or ancient times. Quantitative assessments and reconstructions of plant abundance at a higher taxonomic resolution will require detailed vegetation surveys to correlate with pollen data from surface samples and possibly simulations of modern and ancient landscapes. Open areas in a landscape that is over 75% forested can be detected, allowing for future quantitative reconstructions of landscape openness.

Nitrogen (N) availability, defined here as the supply of N to terrestrial plants and soil microorganisms relative to their N demands, limits the productivity of many temperate zone forests and in part determines ecosystem carbon (C) content. Despite multidecadal monitoring of N in streams, the long-term record of N availability in forests of the northeastern United States is largely unknown. Therefore, although these forests have been receiving anthropogenic N deposition for the past few decades, it is still uncertain whether terrestrial N availability has changed during this time and, subsequently, whether forest ecosystems have responded to increased N deposition. Here, we used stable N isotopes in tree rings and lake sediments to demonstrate that N availability in a northeastern forest has declined over the past 75 years, likely because of ecosystem recovery from Euro-American land use. Forest N availability has only recently returned to levels forecast from presettlement trajectories, rendering the trajectory of future forest N cycling uncertain. Our results suggest that chronic disturbances caused by humans, especially logging and agriculture, are major drivers of terrestrial N cycling in forest ecosystems today, even a century after cessation.

Vegetation composition and fire frequency are tightly linked in North American grasslands and have varied considerably throughout the Holocene in response to different drivers. Yet, detailed records of both long-term changes in grassland vegetation composition and diversity, coupled with fire history, are still relatively sparse. In this study, we examine a sediment core from Fox Lake, Minnesota, using pollen, charcoal, magnetic susceptibility, organic carbon (%C), and silica (%Si) records with the aim of understanding grassland structure and function during the Holocene, particularly in the context of vegetation composition and diversity, erosion, and fire activity. Nonarboreal pollen comprises between 37% and 86% of the assemblage throughout the record with the largest percentages occurring during the mid-Holocene (~8000–4000 yr BP). The pollen record also suggests that at 8200 yr BP, there was an abrupt shift from oak-elm woodland to a more open landscape of grassland or savanna, which remained throughout the mid-Holocene. Additionally, the pollen data suggest that vegetation composition exhibited little change in diversity through time despite recurring fire. Charcoal concentrations varied from 30 to nearly 1200 particles cm−3, indicating changes in relative amount of biomass burned, but the morphotypes of charcoal pieces indicate that woody fuels persisted during the mid-Holocene despite the apparent grassland-dominated landscape. Magnetic susceptibility in the sediment ranges from −0.9 to 22.4 (×10−5 SI) throughout the record, with the biggest increase occurring as the vegetation shifted from woodland to grassland entering the mid-Holocene. Organic carbon ranges from 4.6% to 20.0% and exhibits a slow but steady increase after the 8200 yr BP event. Silica decreases slightly but remains generally high between 20.4% and 22.5%.

Cerrãdo savannas have the greatest fire activity\ud of all major global land-cover types and play a significant\ud role in the global carbon cycle. During the 21st century,\ud temperatures are projected to increase by ∼ 3\ud ◦C coupled\ud with a precipitation decrease of ∼ 20 %. Although these conditions\ud could potentially intensify drought stress, it is unknown\ud how that might alter vegetation composition and fire\ud regimes. To assess how Neotropical savannas responded to\ud past climate changes, a 14 500-year, high-resolution, sedimentary\ud record from Huanchaca Mesetta, a palm swamp located\ud in the cerrãdo savanna in northeastern Bolivia, was analyzed\ud with phytoliths, stable isotopes, and charcoal. A nonanalogue,\ud cold-adapted vegetation community dominated the\ud Lateglacial–early Holocene period (14 500–9000 cal yr BP,\ud which included trees and C3 Pooideae and C4 Panicoideae\ud grasses. The Lateglacial vegetation was fire-sensitive and fire\ud activity during this period was low, likely responding to fuel\ud availability and limitation. Although similar vegetation characterized\ud the early Holocene, the warming conditions associated\ud with the onset of the Holocene led to an initial increase\ud in fire activity. Huanchaca Mesetta became increasingly firedependent\ud during the middle Holocene with the expansion\ud of C4 fire-adapted grasses. However, as warm, dry conditions,\ud characterized by increased length and severity of the\ud dry season, continued, fuel availability decreased. The establishment\ud of the modern palm swamp vegetation occurred at\ud 5000 cal yr BP. Edaphic factors are the first-order control on\ud vegetation on the rocky quartzite mesetta. Where soils are\ud sufficiently thick, climate is the second-order control of vegetation\ud on the mesetta. The presence of the modern palm\ud swamp is attributed to two factors: (1) increased precipitation\ud that increased water table levels and (2) decreased frequency\ud and duration of surazos (cold wind incursions from\ud Patagonia) leading to increased temperature minima. Natural\ud (soil, climate, fire) drivers rather than anthropogenic\ud drivers control the vegetation and fire activity at Huanchaca\ud Mesetta. Thus the cerrãdo savanna ecosystem of the Huanchaca\ud Plateau has exhibited ecosystem resilience to major\ud climatic changes in both temperature and precipitation since\ud the Lateglacial period.

Nitrogen (N) is the limiting nutrient to primary productivity in a variety of temperate forests, and N cycling is undergoing a variety of anthropogenic changes, notably a doubling of reactive N (Nr) on a global scale. Yet, the magnitude of these changes to N cycling has been difficult to document in terrestrial ecosystems, especially in old-growth forests. To determine the trajectory of N cycling and the potential impacts of anthropogenic influences at local scales, we measured the composition of stable nitrogen isotopes (δ15N) in wood from living red pine trees (Pinus resinosa) at a single site in northern Minnesota, USA. A synchronous decline in wood δ15N values began approximately in the 1920s in 17 individual trees at different topographic positions, indicating a common driver. The decline in wood δ15N values corresponded with declines in sedimentary δ15N recorded in lacustrine sediments of the same catchment. Disturbance regime and species composition began to change at the turn of the 20th century with park establishment, providing a likely mechanism of decline in δ15N values toward present. While other mechanisms of this change are possible, we conclude that while there may be consequences of increased influxes of various forms of anthropogenic Nr into terrestrial ecosystems at the global level, these changes are not being expressed at a local level in this temperate forest ecosystem.

Quantifying global patterns of terrestrial nitrogen (N) cycling is central to predicting future patterns of primary productivity, carbon sequestration, nutrient fluxes to aquatic systems, and climate forcing. With limited direct measures of soil N cycling at the global scale, syntheses of the 15N:14N ratio of soil organic matter across climate gradients provide key insights into understanding global patterns of N cycling. In synthesizing data from over 6000 soil samples, we show strong global relationships among soil N isotopes, mean annual temperature (MAT), mean annual precipitation (MAP), and the concentrations of organic carbon and clay in soil. In both hot ecosystems and dry ecosystems, soil organic matter was more enriched in 15N than in corresponding cold ecosystems or wet ecosystems. Below a MAT of 9.8°C, soil δ15N was invariant with MAT. At the global scale, soil organic C concentrations also declined with increasing MAT and decreasing MAP. After standardizing for variation among mineral soils in soil C and clay concentrations, soil δ15N showed no consistent trends across global climate and latitudinal gradients. Our analyses could place new constraints on interpretations of patterns of ecosystem N cycling and global budgets of gaseous N loss. Fil: Craine, Joseph M. Kansas State University. Division of Biology; Estados Unidos Fil: Elmore, Andrew J. University of Maryland Center for Environmental Science. Appalachian Laboratory; Estados Unidos Fil: Wang, Lixing. Indiana University-Purdue University Department of Earth Sciences; Estados Unidos Fil: Augusto, Laurent. INRA. Bordeaux Sciences Agro; Francia Fil: Baisden, Troy. GNS Science. National Isotope Centre; Nueva Zelanda Fil: Brookshire, E.N.J. Montana State University. Department of Land Resources and Environmental Sciences; Estados Unidos Fil: Cramer, Michael D. University of Cape Town. Department of Biological Sciences; Sudáfrica Fil: Hasselquist, Niles. Swedish University of Agricultural Sciences. Forest Ecology and Management; Suecia Fil: Hobbie, Erik A. University of New Hampshire. Earth Systems Research Center; Estados Unidos Fil: Kahmen, Ansgar. Departement of Environmental Sciences - Botany; Suiza Fil: Kaba, Keisuke. Tokyo University of Agriculture and Technology. Institute of Agriculture; Japón Fil: Kranabetter, M. British Columbia (Canadá). Ministry of Forests, Lands and Natural Resource Operations; Canadá Fil: Mack, M. University of Florida. Department of Biology; Estados Unidos Fil: Marin-Spiotta, E. University of Wisconsin. Department of Geography; Estados Unidos Fil: Mayor, J.R. Swedish University of Agricultural Sciences. Department of Forest Ecology & Management; Suecia Fil: McLauchlan, K.K. Kansas State University. Department of Geography; Estados Unidos Fil: Michelsen, A. University of Copenhagen. Department of Biology; Dinamarca Fil: Nardoto, G.B. Universidade de Brasília. Faculdade UnB Planaltina; Brasil Fil: Oliveira, R.S. Universidade Estadual de Campinas. Instituto de Biologia. Departamento de Biologia Vegetal; Brasil Fil: Perakis, S.S. Forest and Rangeland Ecosystem Science Center; Estados Unidos Fil: Peri, Pablo Luis. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Santa Cruz; Argentina. Universidad Nacional de la Patagonia Austral; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Quesada, C. Instituto Nacional de Pesquisas da Amazonia. Coordenação de Dinâmica Ambiental; Brasil Fil: Richter, A. University of Vienna. Department of Terrestrial Ecosystem Research; Austria Fil: Schipper, L.A. University of Waikato. Environmental Research Institute; Nueva Zelanda Fil: Stevenson, B.A. Landcare Research; Nueva Zelanda Fil: Turner, B.L. Smithsonian Tropical Research Institute; Panamá Fil: Viani, R.A.G. Universidade Federal de São Carlos. Centro de Ciências Agrárias; Brasil Fil: Wanek, W. University of Vienna. Department of Terrestrial Ecosystem Research; Austria Fil: Zeller, B. INRA Nancy. Biogéochimie des Ecosystèmes Forestiers; Francia